CALIFORNIA STATE UNIVERSITY, NORTHRIDGE
Low Speed Crash Resilience of a Lightweight Vehicle
A thesis submitted in partial fulfillment of the requirements
For the degree of Master of Science in Mechanical Engineering
By
Andrew Benson
May 2014
The thesis of Andrew Benson is approved:
Dr. Robert Conner
Date
Dr. Vibhav Durgesh
Date
Dr. Robert, Ryan, Chair
Date
California State University, Northridge
ii
Acknowledgments
Thank you to
Dr. Robert Ryan for his insight and guidance
C.S.U.N. for its resources and teachings
Kinemetrics for their funding and patience
All of the editors that made this paper come together
And my parents, Mike and Lynn for their on-going support
iii
Table of Contents
Signature Page . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Abstract . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1 INTRODUCTION
1.1 History . . . . . . . . . . .
1.2 Goal . . . . . . . . . . . .
1.3 Human Impact Resistance
1.4 Implications . . . . . . . .
1.5 Related Areas of Interest .
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2 STRUCTURE
2.1 Design Intent . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.2 Nose Profile . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.3 Mounting structure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
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3 MODELING
3.1 Approach . . . . . . . . . . . . . . . .
3.2 Autodesk InventorT M . . . . . . . . . .
3.3 AnsysT M . . . . . . . . . . . . . . . . .
3.4 Rigid Nose Simulations . . . . . . . . .
3.4.1 Nose Modeling . . . . . . . . .
3.4.2 Material Definition . . . . . . .
3.4.3 Meshing . . . . . . . . . . . . .
3.4.4 Supports and Loading Applied .
3.5 Energy Absorbent Mount Simulations .
3.5.1 Simulation . . . . . . . . . . . .
3.5.2 Mesh . . . . . . . . . . . . . .
3.5.3 Supports and Loading . . . . .
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5 ANALYSIS
5.1 Composite Nose Simulations . . . . . . . . . . . . . . . . . . . . . . . . . . .
5.1.1 Material Selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5.1.2 Filler Material . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
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4 DETAILED DESIGN
4.1 Composite Nose . . . . . . . .
4.1.1 Profile . . . . . . . . .
4.1.2 Honeycomb Placement
4.1.3 Layering Technique . .
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6 TESTING
6.1 Energy Absorbent Mount . . . . . . . . . . . . . . . . . . . . . . . . . . . .
6.1.1 Impact Test with Composite Plate . . . . . . . . . . . . . . . . . . .
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7 RESULTS
7.1 Simulated Complete System Response . . . . . . . . . . . . . . . . . . . . .
7.1.1 Low Speed Impact . . . . . . . . . . . . . . . . . . . . . . . . . . . .
7.1.2 High Speed Impact . . . . . . . . . . . . . . . . . . . . . . . . . . . .
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8 CONCLUSION
References . . . . . . . . . . . . . . .
Appendix A: Analytical Calculations
Appendix B: Uncertainties . . . . . .
Appendix C: Drawings . . . . . . . .
Appendix D: Spring Test Data . . . .
Appendix E: Spring and Carbon Test
Appendix F: Arduino Code . . . . .
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5.2
5.3
5.1.3 Layering Types . . .
5.1.4 Layering Orientation
5.1.5 Resin and Hardener .
Layering Complete Analysis
Energy Absorbent Mount .
5.3.1 Mechanical Design .
5.3.2 Material Selection . .
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. . .
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Data
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List of Figures
1.1
1.2
1.3
Unprotected Recumbent Bicycle [1] . . . . . . . . . . . . . . . . . . . . . . .
Velomobile [2] . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Impact Study [3] . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1
2
3
2.1
2.2
Composite Nose Profile . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Nose Mount . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
6
7
3.1
3.2
3.3
3.4
3.5
3.6
3.7
3.8
3.9
3.10
3.11
3.12
Layering Approach [8] . . . . . . . . . . . . . . . . . . . . . . . . .
Nose Layer Cross sections . . . . . . . . . . . . . . . . . . . . . . .
AnsysT M Pre-Post Composite Analysis Top Level . . . . . . . . . .
AnsysT M Pre-Post Setup . . . . . . . . . . . . . . . . . . . . . . . .
Input Material Properties . . . . . . . . . . . . . . . . . . . . . . .
Weave Orientation . . . . . . . . . . . . . . . . . . . . . . . . . . .
Properties of Composite Form . . . . . . . . . . . . . . . . . . . . .
Mesh Partition . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Meshing of Nose Composite Structure . . . . . . . . . . . . . . . . .
Deformation of Impact . . . . . . . . . . . . . . . . . . . . . . . . .
Meshing of Front Mount (Coil Spring as Energy Absorber) . . . . .
Front Mount Loading Conditions (Coil Spring as Energy Absorber)
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20
4.1
4.2
Honeycomb Placement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Honeycomb Projection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
22
23
5.1
5.2
Layering Orientation Example [11] . . . . . . . . . . . . . . . . . . . . . . .
Spring Pivot Assembly . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
28
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6.1
6.2
6.3
6.4
6.5
6.6
6.7
Impact Energy at Speed . . . . . . . . . . . . .
Spring Testing Apparatus . . . . . . . . . . . .
Arduino Built Accelerometer (Front) . . . . . .
Arduino Built Accelerometer (Back) . . . . . .
Impact Test One Plot (7 M.P.H.) . . . . . . . .
Impact Test Two Plot (4 M.P.H.) . . . . . . . .
Composite with Spring Mount Acceleration Test
7.1
7.2
7.3
7.4
Deflection with Respect to Impact Energy . .
Acceleration and Deformation with Respect to
Deflection at Low Speed Impact . . . . . . . .
Deflection at High Speed Impact . . . . . . .
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36
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. . . . .
Velocity
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44
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46
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List of Tables
1.1
Human Tolerance Limits [4] . . . . . . . . . . . . . . . . . . . . . . . . . . .
4
4.1
4.2
Composite Layering With Insert . . . . . . . . . . . . . . . . . . . . . . . . .
Composite Layering Without Insert . . . . . . . . . . . . . . . . . . . . . . .
24
24
5.1
5.2
5.3
5.4
5.5
5.6
5.7
5.8
5.9
5.10
5.11
Cloth Selection [9] . . . . . . . . .
Filler Material [9] . . . . . . . . . .
Layering . . . . . . . . . . . . . . .
Cloth Orientation . . . . . . . . . .
Resin & Hardener [12] . . . . . . .
Composite Definition Template . .
Composite Trial One . . . . . . . .
Composite Trial Two . . . . . . . .
Composite Trial Three . . . . . . .
Composite Trial Four . . . . . . . .
Energy Absorbent Mount Material
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28
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30
30
31
32
33
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6.1
6.2
6.3
7 M.P.H. Spring Testing . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5 M.P.H. Spring Testing . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5 M.P.H. Spring Testing . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
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Abstract
Low Speed Crash Resilience
of a Lightweight Vehicle
By
Andrew Benson
Master in Mechanical Engineering
Recumbent vehicles are a growing means of transport in todays green energy society. This
need is expanded by the comfort and ability for recumbent tricycles to carry a rider longer
distances without fatigue due to its relaxed seating position.The safety of a recumbent tricycle is an area to be addressed with the rising number of commuters using this means of
transport.While amongst larger and quicker vehicles sharing the same roads a secure safety
shell can prove to be resilient against a low speed collision with another vehicle or other
objects as well as give aerodynamic advantages.
The largest concern to riders is being injured by the acceleration imparted onto the rider as
well as the amount of structure that comes in contact with the body upon impact.
A study done involving the optimization of a lightweight composite structure is evaluated
as to not inhibit the rider in his function while providing safety in collisions. This structure
utilizes the benefits of carbon fiber, honeycomb sub structure and a spring damper mounting
system to lower impact force. This structure is tested and refined using a composite solver
package called AnsysT M pre-post.
Results from this study conclude that the structure is capable of providing safety for
the rider in sudden impacts up to 25 mph both by limiting maximum deceleration and by
maintaining structural integrity. This structure will act in two parts, the first by the mounts
absorbing energy up to their limit of 5150 foot pounds and secondly the structure of the
nose will protect the rider in larger impacts.
viii
Chapter 1
INTRODUCTION
1.1
History
Recumbent Bicycles are a variation of the upright bicycle designed to give the rider the
advantage of a lower aerodynamic profile by placing the rider in a laid back position along
with a more relaxed riding position as to accomplish longer rides while avoiding exhaustion.
There are many variations on this design including general size, number of wheels and steering systems. For the purpose of this research a single passenger recumbent tricycle tadpole
design will be focused on as a means of transport built to travel distances ranging between
one and twenty miles. The tadpole design described is shown in Figure 1.1.
To date recumbent tricycles are a growing means of transport maximizing rider comfort, economy and relative speed, but a rising concern is the safety of being encapsulated in
a lightweight trike which will be addressed in this research. The area of emphasis in this
project is to focus on the improved safety of being struck by a vehicle or other object while in
transit without sacrificing the performance of the vehicle which includes top speed, handling
and visibility.
A standard recumbent tricycle is configured to place the rider in a relaxed reclined position as to increase stamina and aerodynamic performance. The riders feet are set in front as
to operate the crank system of propulsion which sits at the leading-most part of the vehicle
shown in Figure 1.1. The construction of these trikes while variable, rests on a common point
of having an open cockpit riding position to remain light weight and simplistic to benefit
speed and travel but lacking in the safety necessary to join current automobiles on crowded
city roads.
Figure 1.1: Unprotected Recumbent Bicycle [1]
1
To address the issues the common commuter has with today’s tricycles, an open cockpit
trike built for sport and enclosed to the weather was designed and labeled a velomobile which
provides the same comforts as the previously stated trike along with minor protection from
low speed impacts against other vehicles on the road.
The velomobile design is as diverse as its trike counterpart differing in protection, coverage
and general purpose. Figure 1.2 depicts a fully enclosed velomobile that allows storage space
for trips to the store among with the safety features previously discussed. For the purposes
of this research velomobiles of the light weight category providing minor protection, such
as low speed impacts against other vehicles, will be focused upon as short trip commuting
vehicles.
Figure 1.2: Velomobile [2]
1.2
Goal
The need of a low speed crash resilient human powered vehicle is met by including a
light weight safety enclosure to surround the rider providing protection from an impact of
a low speed vehicle crash classified at twenty five miles per hour or less without harm to
the internal rider space.This structure will not impede performance, feasibility, visibility or
maneuverability of the vehicle in any noticeable way.
To remain lightweight human powered vehicle’s bodies are commonly made of composite
materials providing a location to mount and reinforce the safety structure. This study will
include testing honeycombs, foams, rubbers and polymers for their ability to absorb impact,
weight contribution and compatibility with bonding to the composite body. The ability for
this structure to survive an impact will determine the survivability of the given crash by the
occupant inside. The structure needs to be tailored within a safe margin of what a human
can survive.
2
1.3
Human Impact Resistance
In order to determine the structure design, the need of the human body must be first
understood. Extensive research has been done by Shanahan [3] and the U.S. Air Force separately to determine the impact limits a human can undergo.
The first study done by Shanahan [3] focused on lateral acceleration and the duration
a subject can undergo. The results depicted in Figure 1.3 demonstrates the importance of
an impact acceleration as opposed to an acceleration from an external source that may last
for several seconds causing greater fatigue. A study done by the California Department of
Motor Vehicles defines the standard impact time of a vehicle crash can be averaged to 0.1
seconds placing it just after the decline of human tolerance. From this chart we can see that
a 40 G head on lateral impact is survivable but would be very discomforting for the rider.
Figure 1.3: Impact Study [3]
A similar study was done by the U.S. Air Force providing acceleration limits in all three
reference axes [4]. This study differs from Shanahan’s [3] results as the impact was not varied
in its duration but focused on the pure impact. The longevity of this impact was clearly
stated in the paper at 0.1 seconds. It can then be concluded that this data is valid and useful
for research on impact testing on humans due to its proximity to the duration a standard
vehicle crash. The results from this test are displayed in Table 1.1.
3
Table 1.1: Human Tolerance Limits [4]
Direction of Accelerative Force Occupant’s Inertial Response
Head-ward (+Gz)
Eyeballs Down
Eyeballs Up
Tail-ward (-Gz)
Lateral Right (+Gy)
Eyeballs Left
Eyeballs Right
Lateral Left (-Gy)
Back to Chest (+Gx)
Eyeballs Out
Chest To Back (-Gx)
Eyeballs In
Tolerance Level
20-25 G
15 G
20G
20 G
45 G
45 G
The results of the research done by the Air Force shows a strong bias in human tolerance
to frontal or rear impact as opposed to side impacts. The results on vertical acceleration (z)
will be overlooked for this application as all data is being related to a vehicle traveling in a
fixed plane. It can also be noted that a restrained body has no preference to frontal or rear
impact.
1.4
Implications
The purpose of this research and development of the structure designed for a recumbent
tricycle is to provide the daily commuter with a means of safety as the rider travels. This
structure is specifically focused on creating a light weight body to provide protection not
only from frontal impact below the speed of twenty five M.P.H. but provide a shelter from
the elements as well.
As mentioned previously the recumbent tricycle was originally developed in order to provide
a rider with more comfort and longevity which can be expanded to include in-town commutes
to work or stores. With the current state of roads in the U.S., this ability can not be fully
harnessed because trikes are forced to share the road with much larger and faster automobiles.
This forces the need for a protective layer between the trike and other heavier vehicles
by making an integrated structure around the current trike design. Doing so will allow
passengers to utilize this economic and environmentally friendly technique of transportation.
1.5
Related Areas of Interest
Applications and inspiration for this study include automobile bumper design, racing
car safety structure and lightweight vehicles such as the SmartCarT M . Historically safe
structures in the past went hand in hand with heavy structures creating the inertia necessary
to absorb a collision.
Elements of this study are attributed to earlier work done to further the research in slow
speed impacts of light weight vehicles [5]. This area is important as modern day transportation efficiency is becoming more of a selling point and vehicles are becoming smaller and
4
lighter [6]. This trend must not compromise weight savings for safety and must continue
to create a structure that can withstand the high inertia impacts of older vehicles with the
weight saving materials of modern cars.
Most larger cars of today have crumple zones built into the car body due to thier size.
With the size reduction mentioned earlier, crumple zones become too compact to be controlled by the material thickness and must be re-directed to an energy absorbent mounting
point or crumple area [7]. This is most easily depicted in side impacts to concept race cars or
small economic cars which are narrow and thin walled to save weight and aid aerodynamics
by sliming the flow profile. This slim structure demands the need for an externally stiff
structure that can withstand impact along with a routed absorbent mounting structure.
5
Chapter 2
STRUCTURE
2.1
Design Intent
Design of the nose structure was needed to keep the rider safe in a collision up to a speed
of 25 M.P.H.. This structure was chosen to be a two stage system to reduce the deceleration
imparted on the rider as discussed previously. These two stages are an energy absorbent
mount that will absorb and slow the vehicle safely up to a 20 M.P.H. impact; and a hard
nose structure that can withstand crumpling or structural failure up to 25 M.P.H. This two
stage system allows for a light weight system that is simplistic in nature as to avoid failure,
and a low profile as to not expand the overall frontal area of the vehicle. Lastly the structure
needs to be placed out of the way of the rider as to not interfere with performance.
This structure will be designed and implemented to perform at the 2014 C.S.U.N. H.P.V
competition. All aerodynamic and structural mention of the vehicle and its supporting
components are part of the design implemented by the 2014 H.P.V.C. team working in
collaboration with this project.
2.2
Nose Profile
The nose profile chosen for this analysis was designed based on a combination of aerodynamics and clearance to allow the rider to properly pedal without interference. The profile
shown in Figure 2.1 depicts the nose structure that will be analyzed and optimized further
in this study. This structure will be designed to function under a low speed impact of up
to 25 M.P.H. . This structure will be the second in a two part safety system to protect the
rider.
Figure 2.1: Composite Nose Profile
6
Figure 2.1 shows the frontal section of the full fairing enclosing the tricycle. This section
is isolated for ease of study and construction an was actually manufactured as an individual
pice so it could move independently of the rest of the fairing.
2.3
Mounting structure
To allow for a safe enclosure around the rider a dynamic mounting structure will be designed to properly absorb the oncoming energy. The design intent of this system is to absorb
energy from an impact up to 25 M.P.H. until full compression of the structure.
Figure 2.2 depicts the lever arm design of the front mount used for holding the fairing
nose in place. The spring is set on a lever arm as to create a linear movement from from any
angle directed at the nose structure.
Figure 2.2: Nose Mount
The design of this front mount is optimized for light weight and simplistic as to avoid
failure. Structures being tested in the simulations are discussed in section 5.1.3 including
energy absorbent materials such as nomex and aluminum honeycomb, rubber and springs.
These materials take the place spatially where the coiled spring is shown in Figure 2.2. This
alteration carried with it a change in support structure which can be simplified in simulation
and detailed only for the chosen material type. Detailed drawings for this mounting structure
is provided in the appendix.
7
Chapter 3
MODELING
3.1
Approach
To remain lightweight the vehicle’s body is commonly made of composite material which
provides a location to mount and reinforce the structure. This study will include the testing
of honeycombs, foams, rubbers and polymers for their ability to absorb impact, weight
contribution and compatibility with bonding to the composite body. The layering of this
composite material will build the strength of the system as demonstrated later in this study.
Figure 3.1 gives a general case of how composites are layered as to create an ideal structure
including number of layers, orientation of fiber direction in each layer and placement of filler
materials between layers.
Figure 3.1: Layering Approach [8]
3.2
Autodesk InventorT M
AutodeskInventorT M employs a user based environment to graphically built 3D models
of any desired structure. AutodeskInventorT M is a vertically integrated system used to
add, subtract or sweep material to the desired shape. This format is most desirable for
the application being studied because a vertically integrated design means that additional
features or trials can be run using a suppressed version of the final construct. For instance,
in testing wheel arches for the effect on the simulated results a quick simulation can be run
between the full model and a fall back model of the wheel arches suppressed to see their net
8
effect. A second strong advantage using AutodeskInventorT M is its ability to save file types
in common formats such as .step or .iges. These formats are what allows the translation of
not only the given solid from AutodeskInventorT M but transfers vital information such as
part mates and contact regions where the laws of physics will automatically be applied in
AnsysT M .
3.3
AnsysT M
AnsysT M is a powerful numerical solver that was used due to its abilities in F.E.A. (Finite
Element Analysis) given the mechanical nature of this project. The details of the simulation
are noted later in this study, however the basic format of the program is a parceled interface
with sub windows and subroutines that define each given detail of the simulation separately
and combined using the master window. This interface is ideal for the application at hand
due to its ease in ability for running multiple trials. Because each parameter is defined in
its own subroutine and because subroutines can be shared between simulations it is possible
for example, to share one geometry through several parallel trials all with different material
input routines. This parallel path technique allows for quick convergence on a desired output
for the system and can create an opportunity to pinpoint a specific tolerance as opposed to
trial and error.
3.4
Rigid Nose Simulations
The purpose of the following simulations of this design is to support the theoretical
calculations and research done on choosing material types. Each material type has been
chosen for its benefits in both strength and weight savings as well as other characteristics
mentioned later. The simulation of the combination of these elements will allow an insight
into how they function together. Simulations will be run at low speed impact and high speed
impact speeds, while varying the composite structure in thickness, layers and tolerance to
ensure the optimal outcome and protection when struck.
3.4.1
Nose Modeling
To acquire the shape shown previously, AutodeskInventorT M was again utilized for its
modeling capabilities. This structure shown in Figure 3.2 was constructed using a series
of planar sketches set apart by the lofting distance and swept using a built in feature to
construct the solid 3D profile. The benefit of this method is that the clearance of the profile
can easily be altered if needed, given a change in rider size or position, by simply only altering
the cross section in the location the clearance problem arises.
9
Figure 3.2: Nose Layer Cross sections
In this particular application the structure was lofted to a zero thickness element meaning
the profile seen only exists in a single plane with no thickness. This is to aid the ability
for AnsysT M to non-graphically place the material properties talked about next onto the
surface just created, melding the ability for computational material design with the geometric
manipulability of AutodeskInventorT M .
The material to be studied will be placed upon the geometry needed to properly fit the
rider and give an aerodynamic advantage. The profile of this shape was determined by
sketching clearance lines around the moving profile of the rider fixed atop the frame. The
geometry was then shaped to alleviate any aerodynamically stagnate elements, creating a
complex 3D geometry to place the following material studies onto as shown in Figure 2.1.
3.4.2
Material Definition
To properly describe the system at hand it is very crucial to properly define the problem
and materials of the structure. Setting up a simulation on AnsysT M is no different in that
the results returned are only as valid as the material properties and models input initially.
AnsysT M has a specific composite solver subroutine embedded as an add-in called Pre-Post.
This additional software allows the user to define a composite as a heterogeneous material
10
to the necessary level to properly simulate the system. This process involves starting with
a zero thickness surface to define the geometry of the part being modeled. Then each layer
of cloth is defined with its given properties including weave direction and density. This is
followed by inputing the type of resin and hardener used and their properties to create a
bonded solid,including any intermediate layers of foam or honeycomb whose strengths and
bonding strengths are characterized to create a complete system.
The AnsysT M environment divides each subroutine into a separate graphical category
for organization showing the pre composite setup which includes all composite material
properties, loading condition with its given restraints and forces applied, and meshing detail followed by the post or measured results of the composite simulation. This final post
graphic gives results in locations defined by the Pre step process which defined where to place
Rosettes (strain deflection measurement devices) that will give the results for this study. The
complete overview of this process is depicted as a graphical user interface or G.U.I. in Figure
3.3 depicting a model simulated in both a static structural simulation, as well as a transient
structural simulation to determine the difference of a time dependent loading on composite
material and its support structure. These simulations share the Pre and Post setup depicted
by the connecting wires in between and are applied to the meshed geometry shown in Figure
3.4.
The AnsysT M Pre-Post add-in opens its own navigation window allowing the user to
define each property step by step. A design tree on the left provides a quick look at the
structure being created and layered together, where the right shows the geometric profile
along with a default mesh and depicts any strain measurement devices that are attached
shown by the independent axis.
11
Figure 3.3: AnsysT M Pre-Post Composite Analysis Top Level
As depicted in Figure 3.3 the setup begins with defining all materials that will be used
in the simulation including any mounting materials and honeycomb substructures. These
values include stress-strain data from the material supplier as well as yield stress (σy ), Modulus of Elasticity (E) and shear yield stress (γy ) among others to get a clear understanding
of the material in a non-thermal or electromagnetic environment [8][9][10].
One benefit of working with AnsysT M is there are no pre input materials to choose from
in this procedure, all material data must come from a supplier or physical testing. For the
purpose of this simulation material data was taken from the appropriate manufacturer’s data
sheets and input to the level of detail needed for an impact load test. This excludes any
thermal or electromagnetic effects as well as life cycle or environmental effects.
12
Figure 3.4: AnsysT M Pre-Post Setup
Fabrics are then defined by what can be stacked on top of the zero thickness surface
that was previously created. In this case bi-weave was defined and characteristics such as
density and weave orientation were input as variables to simulate the manufacturer’s material. Resin and honeycomb are also classified in this category so they can be layered on the
surface correctly and done as a stack up.
As an example of how a material is defined, a spreadsheet of raw data is uploaded using
a wizard built into AnsysT M and exported showing the values in Figure 3.5. These values
depict the modulus of elasticity (E), Poisson’s ratio (νN ) and Shear modulus (GN ) in all
three directions.
13
Figure 3.5: Input Material Properties
Stack-ups and sub-laments are any combination of the fabrics that were previously described allowing the user for instance to layer 3 bi-weave fabrics together or to include a
layer of honeycomb in between that stack. A sublayer is similar but depicts the resin and
hardener types that are added.
A sample of a stack-up procedure is shown in Figure 3.6 where a two layer stack-up is
created out of two bi-weave fabrics. They are oriented in a 45, -45 deg fashion as to maximize the homogeneous ability to absorb stress impacts from any direction. This particular
stack-up was used in the nose where stresses were just as likely to flow in one direction as the
other as opposed to the mid or rear of the nose that was primarily uniform in compression
longitudinally carrying the impact taken from the front.
An infinite number of layers can be added to this process including honeycomb and
separate types of fabrics in any orientation. This is beneficial in calibrating a composite
structure to tailor the exact need for the design intent. A trial and error process combined
with a preset change in variable tool built into AnsysT M allows the user to run multiple
arrays of design alterations at once to gain a trend to converge on a solution much more
quickly than by solving each case individually.
14
Figure 3.6: Weave Orientation
As a graphical representation of what has been built, AnsysT M puts together a separate
G.U.I. window to display the properties of the components that were input previously. Figure 3.7 depicts in red each fabric layer including three orientated bi-weave components both
above and below a honeycomb center which is depicted to scale. The Green section depicts
the material boundaries showing where the transitions from composite to honeycomb lie.
The leftmost yellow rectangle depicts all solids that are bonded together which in this case
is one continuous bonded structure.
The radial plot to the right is a representation of what has been defined for the properties
previously. This plot as per its legend defines the Modulus of Elasticity in X and Y axes
relative to the fabric and its relative orientation there of. It also depicts the shear modulus
in a similar fashion which is 90 degrees offset from its counterpart due to the symmetric
nature of the weave orientation in this design.
15
Figure 3.7: Properties of Composite Form
Lastly in the pre process a series of Rosettes was applied to the surface to determine
local stresses. This can be between layers, in shear, bending and any location about the
geometry. Two locations were selected on the body, one on the layer of carbon innermost to
the honeycomb to observe shear force and the second on the right rear mounting point on
the outermost layer to observe deformation in the body.
3.4.3
Meshing
Meshing is a process used commonly by numerical solvers by breaking a complex geometry
part into small geometric shapes. These shapes are then mathematically coupled and solved
as individual parts for stiffness and deflection being combined in the end for a global result
of the effect on the part or assembly as a whole. These elements come in many shapes and
sizes limited only by the user’s imagination to fit specific needs. A smaller mesh size means
having more, or smaller elements filling the part allowing for a more refined definition of how
the stress is passing though the part. Having more elements also increases the computational
time linearly with elements and to the power of two in relationship to the area being covered.
16
Refinements to a standard mesh include techniques such as sizing, sweeping and shaping,
amongst others not used here. Sweeping is a technique that places a planar mesh surface
in the thickness of the part shown in Figure 3.8. This in turn simulates two separate parts
joined together to further the accuracy in the thickness direction of the part being studied.
Shaping can be utilized much like swapping coordinate systems, for instance a hexagonal
mesh is beneficial for round parts and a triangular mesh is better for planar parts. This can
also get more complex with the introduction of trapezoids and other simple polygons and
can be found to be more beneficial for a given part by trial and error.
Figure 3.8: Mesh Partition
As a coarse control on the solver type a box containing the command ”Large Deflections”
can be turned on which does two important things to the system. First, it swaps the standard
solving formula for a non-linear formula accounting for the strain hardening, Poisson’s ratio
effects and time dependent effects of the system. The second is the refinement of the mesh
at each interval of the solving stage. As the solver progresses past the elastic limit of the
part the solver will refine the mesh in these areas to gain a detailed evaluation because this
area is so sensitive to loading changes at this time.
This particular model had no abrupt geometry changes such as sharp corners or small
raised sections so meshing was not a complication in the simulation. Element sizes of 0.05
inches were kept surrounding the solid with refinement down to 0.001 inches within the
frontal impact area and mounting points. For each trial mid sided nodes where forced on
and large deflections kept off as it was seen to give extraneous data. Square elements were
set to preferred but not forced due to the need for triangular elements in the honeycomb and
surrounding layers. Each layer was swept uniformly and kept a uniform mesh although minor
refinements were needed at the honeycomb layer during large contours to avoid contacting
nodes.
17
Figure 3.9: Meshing of Nose Composite Structure
3.4.4
Supports and Loading Applied
The model simulation was constrained in all six axes (rotationally and transitionally) at
three mounting points, all considered to be immovable. This does not directly reflect reality
as the mounts are designed to absorb energy and move, but this assumption simplifies the
simulation. Physically it corresponds to the case were the mount has reached maximum
travel, which is the most demanding on the structure and will be focused on in this study.
A simulation of the vehicle crashing into an object was modeled by fixing the front most
node of the nose as well as the rear face of the nose to simulate its attachment to the rest
of the fairing. An dynamic load was applied equal to the momentum of the weight of the
vehicle and rider traveling at a known speed depicted in Figure 3.10. This was done over a
range of speeds from 5 to 20 M.P.H. and rider weights of 100 to 250 pounds at a 0.1 second
impact time (discussed earlier). To determine a best fit application as far as material and
layering these simulated impacts were simulated by applying a 735 ft-lb load relating to a
10 M.P.H. collision and a 2930 ft-lb load relating to a 20 M.P.H. collision.
18
Figure 3.10: Deformation of Impact
3.5
Energy Absorbent Mount Simulations
In a similar fashion to the simulation of the composite nose structure, the mounting hardware was simulated using AnsysT M software for its non-linearity capabilities and the ability
for superior mesh control to depict the proper sizing for the energy absorbent material. The
following materials were used to create the energy absorption needed including, aluminum
honeycomb, nomex honeycomb, spring steel, coil spring, rubber and plastic. These materials were analyzed for their ability to absorb the given load while remaining lightweight in
construction.
3.5.1
Simulation
A 3D Computer Aided Design (C.A.D.) model was made, defining each material type including the fixture setup and coil over spring assembly. All modeling was done in AutodeskInventorT M
and imported to AnsysT M as a complete mechanical package with mates and connections
intact. Each of these systems was chosen for their individual benefits and comparability with
one another to accomplish the desired structure.
3.5.2
Mesh
Mesh for these simulations was run at 0.05 inch standard square mesh keeping mid sided
nodes. Refinement was necessary to the coil elements of the spring to define its non-linear
movement. The spring was swept using layered nodes though the front and back connection
19
refined to 0.001 inch triangles as shown as the spring compresses to give an accurate representation of the distributive forces traveling though the mounts. Range deflections were left
on for this process because the intent was to observe non-linear behavior in Figure 3.11.
Figure 3.11: Meshing of Front Mount (Coil Spring as Energy Absorber)
3.5.3
Supports and Loading
One end of the support structure was fixed in six axes rotationally and translationally to
simulate the junction to the frame while the opposing end was imparted with a force equal
to that of the collision scenario, mentioned in the previous section.
Figure 3.12: Front Mount Loading Conditions (Coil Spring as Energy Absorber)
20
Chapter 4
DETAILED DESIGN
4.1
Composite Nose
The composite nose structure is defined in three ways including profile, layering technique
and honeycomb placement. These three elements fully define the structure that will be
implemented and was simulated to support the specified load of a low speed impact. Each
of these factors will be modeled and optimized to best fit the application to the low speed
impact.
4.1.1
Profile
The profile of this nose structure can be defined by a series of cross sections and two
guiding line profiles shown in Figure 2.1. These cross sections for accuracy are spaced at one
inch increments over the span of the nose and lofted using the 3D modeling capabilities of
AutodeskInventorT M .
Each profile curve was shaped around a figure of the moving pedal assembly and a riders
legs as to both allow proper clearance for the rider and also not create an excess of room as
to impart a larger structure than is needed. The culmination of the nose is set to a specific
cross section as to match that of the rest of the body in the fairing. This mating feature
must match in cross section as well as angular approach to create a seamless transition.
4.1.2
Honeycomb Placement
The location of the honeycomb reinforcement was an iterative process done by running a
matrix of profiles in AnsysT M to determine the optimal design. The width and angle of each
section was altered over a given range to result in the following structure. The honeycomb
sublayer can be seen highlighted in black from a rear view of the nose. These thin strips
were decided upon over a blanket sheet across the whole structure. A large sheet would have
been stronger but heavier and also very difficult to manufacture. Each strip is kept under
4 inches allowing the contour of the nose to only effect the honeycomb in two axes and not
require a sheet to conform to a complex three dimensional solid.
21
Figure 4.1: Honeycomb Placement
The honeycomb structure was defined and tested as shown in Figure 4.2. This projection
from a frontal view was warped on the structure similar to how one would lay a two dimensionally cut honeycomb strip on a three dimensional curved surface. This definition will be
used to cut each component of the honeycomb structure before placement and adhesion to
the surface.
22
Figure 4.2: Honeycomb Projection
4.1.3
Layering Technique
The layering technique as previously discussed has been optimized for the loading conditions and to meet the specifications of rider protection. There are two layering profiles
including those where the additional honeycomb structure is incorporated and that where
the structure is simply layered carbon.
The first of the composite layering cases is that with the honeycomb substructure included
which is shown in Table 4.1. This structure has been tested to perform optimally under the
loading conditions of a low speed impact at a variation of the angle imparted upon the
composite nose according to analysis done in section 5.2 .
23
Table 4.1: Composite Layering With Insert
..................................................
-45
Carbon Bi- Weave
..................................................
45
..................................................
0.25 in
Honeycomb Nomex
..................................................
45
..................................................
-45
Carbon Bi- Weave
..................................................
The second composite profile case has no honeycomb sublayer. This section is defined by
the following composite layering technique in Table 4.2. This profile was settled upon for its
manufacturability and does not require switching the weave orientations specifically for the
coverage of the honeycomb. This uniformity will allow for equal coverage of the cloth and
make the structure more easily manufacturable.
Table 4.2: Composite Layering Without Insert
..................................................
-45
Carbon Bi- Weave
..................................................
45
Carbon Bi- Weave
..................................................
45
Carbon Bi- Weave
..................................................
-45
Carbon Bi- Weave
..................................................
24
Chapter 5
ANALYSIS
5.1
Composite Nose Simulations
To design a structure to meet the specifications mentioned previously a series of simulations was used to both compare material choices as well as optimize the placement of the filler
material. All of the simulations for this application were done using AnsysT M mechanical
package combined with the additional pre-post composites package. To find the best fit to
the specifications the topics to be verified from the simulations were material type, including
material weave, layering, orientation, resin additive, filler material including honeycomb and
foam, and lastly filler material placement. By moving step by step through each of these
categories a constructive element of the nose can begin to take shape. For example, it is
impossible to know the correct placement of the honeycomb substructure until the weave
type has been determined.
For the purposes of studying the effect each element has on the final outcome, each trial
listed below was tested on a standard twelve inch by twelve inch square sample. This sample
was devoid of curvature or pre-stressed geometry as to gain a complete understanding of the
benefits for each addition mentioned.
5.1.1
Material Selection
Material choices for the nose structure were chosen for their strength to weight ratio
while also taking into account costs of materials and fabrication. Seven material choices
listed in Table 5.1 were categorized for these traits and were imported into AnsysT M with
their material properties to show the Factor of Safety (F.O.S.) under a 100 pound load using
the 3 layers, a flat geometry, and epoxy resin to create comparable cases.
This 100 pound load does not have any reference to the impact specifications stated
earlier but does create a common basis as to compare all structures in their elastic zones.
If a larger than 100 pound force was imparted on a single layered carbon sample, large
deformations and failure would cause the results to be invalid.
25
Table 5.1: Cloth Selection [9]
Material
Weave
Cost per Yd
Fiberglass E Glass
Bi-Weave
Fiberglass E Glass
Uni-Weave
Bi-Weave
Fiberglass S Glass
Fiberglass S Glass
Uni-Weave
Kevlar
Twill-Weave
Kevlar
Plain Bi-Weave
Twill-Weave
Carbon Fiber
Carbon Fiber
Plain Bi-Weave
Carbon Fiber
Uni-Weave
11.45
6.27
24.15
8.85
43.95
43.95
14.95
14.95
14.95
Density
(Oz per Sq Yd)
3
2
9
5
5
5
5.7
5.7
9
Load (lb)
FOS
100
100
100
100
100
100
100
100
100
6.28
2.35
7.84
2.65
13.22
13.49
12.9
14.83
3.84
The conclusion of this trial shows how carbon fiber bi-weave will be the best option for
creating a resistant structure to meet the specifications of the vehicle. This is determined due
to carbon bi-weave’s high factor of safety, its relatively low density and its affordable cost.
Twill-weave carbon would have been a second choice over Kevlar due to its affordability, as
well as its added stiffness which is not listed on this table.
5.1.2
Filler Material
Filler Material was evaluated in a similar way by using a constant geometry to evaluate
the different materials using a single layer of carbon bi-weave with epoxy resin top and
bottom for support. Six filler materials were evaluated for strength, weight and cost shown
in Table 5.2. The samples were loaded using AnsysT M software with a compression load of
100 pounds for each material simulated at 0.25 in thickness.
Table 5.2: Filler Material [9]
Material
Vinyl Foam
Divinymat Foam
Polyisocyanurate Foam
Nomex Honeycomb
Aluminum Honeycomb
Zoot Foam
Density (Oz per Sq Yrd) Cost per Sq Yrd FOS
3
29.92
12.4
3.8
55.95
14.9
2
44.95
2.1
3
112.24
25.5
10
185.11
48.5
6.7
43.48
23.8
The conclusion of this test reveals that single layered nomex material simulated at .25
inch thick is most desirable for this application. In this study it can be said that aluminum
honeycomb has a higher factor of safety, meaning it will accept more of the impact before
failure. However, further study at impact speeds denoted in the scope of this study showed
26
the aluminum honeycomb to be unnecessary, because the nomex could handle the entirety
of the load being imparted in an accident. This negated the strength difference and made
the nomex more desirable for the purposes of weight savings.
5.1.3
Layering Types
Layering techniques were analyzed using the geometry built up previously with bi-weave
carbon fiber and honeycomb sub layer. A simulation was then run to determine the correct
number of layers on top and below the honeycomb insert. Four separate trials were tested
and studied for F.O.S. at a 13 mile per hour impact, input as a dynamic energy input of
1250 ft-lbs, to determine the effectiveness of each sub-layer addition in Table 4.3. The energy
impact testing on these simulated flat samples is a representation of the force exerted in a
low speed collision and can be directly related to the overall design of the structure.
Table 5.3: Layering
Layers Above Layers Below
Weight of
Honeycomb
Honeycomb Cloth (lb/yd)
1
1
0.57
2
.855
1
2
2
1.14
2
3
1.425
3
1.71
3
4
2.04
3
4
4
2.28
Cos per Sq Yrd
FOS
120
180
240
300
360
420
480
0.0032
0.0155
0.7511
1.1542
3.5176
7.7182
16.9354
The resultant of this trial depicts the profile with three top and bottom layers to be most
effective by being far enough from a F.O.S. of one to remaining safe in terms of alterations
in manufacturing and simulation error while remain light and efficient. Moving forward with
the overall study the 2-3, 3-3, and 3-4 layering patterns will all be implemented to see their
ability on a whole scale depicted in section 5.2.
5.1.4
Layering Orientation
Orientation of the layers was then separately simulated to distinguish between Table 5.4
that depict orientations such as 0 deg, 90 deg and 45 deg. This simulation was run under
the same conditions as the layering profile test using the 3-3 layering solution.
Orientation of each layer is done by placing the fibers in the direction defined by the
angle from parallel to the longitudinal axis of the part. An example of these layer angles is
defined in Figure 5.1.
27
Figure 5.1: Layering Orientation Example [11]
By simulating each layer as defined above a beneficial structure can be determined by
analyzing the cases shown in Table 5.4.
Table 5.4: Cloth Orientation
Layer One Layer Two Layer Three Layer Four
0
0
0
0
45
0
0
0
45
0
-45
45
0
90
90
90
45
90
-45
45
90
90
90
90
Layer Five Layering Six FOS
0
0
3.51
45
0
3.94
0
-45
4.24
0
90
3.12
90
-45
3.28
90
90
3.25
The determination of this trial was that the orientations that best fit this application is
the 45,0,-45,45,0,-45 pattern. This is based on having the highest F.O.S. out of the group. It
should be noted however that the range of F.O.S. values is such that each layering technique
will be acceptable for this structure. Elements not taken into account in this simulation were
manufacturability of the cloth orientation or cost of wasted material in cutting 45 angles.
If these factors outweigh the benefit the simulation shows a separate method can then be
chosen.
5.1.5
Resin and Hardener
Lastly the final variable to be simulated was the bonding agent. The resin and hardener
combinations were tested for strength, weight, cost and manufacturing ability. The types of
28
resin that were tried, under the same conditions and loading as the layering and orientation
trials, are documented in Table 5.5.
Table 5.5: Resin & Hardener [12]
Resin
Polyester
Aluminum Filled Epoxy
Urethane
Polyester no Sag
Epoxy
Density
( lb per cubic ft)
51
112
60
80
72
Cost per Sq Yrd Gel Time (min) FOS
279.95
289.95
299.95
449.95
389.95
15
100
25
10
45
3.27
4.44
3.94
3.48
4.24
Resultants from this study show aluminum filled epoxy having the highest impact resilience however due to its high density it was found that standard epoxy resin would work
best for this application. This resin has the largest strength addition while remaining moderately light in comparison. The second benefit of this resin type not mentioned is its universal
availability. Since it is a common material it is possible to purchase many different gel and
hardening times in terms of complementary hardeners to improve the manufacturing options
and help control quality of the final product.
5.2
Layering Complete Analysis
Once an optimum structure was determined using the methods described above it was
then advantageous to test the result and variations thereof on the complex nose geometry
described previously. This loading scenario is different from that of the previous trials because, unlike the static loaded planar samples described above, the following cases will be
tested using the transient loading element of AnsysT M projected upon the nose geometry.
The difference in the transient studies is that the input field for the loading of the part is an
energy applied over a time rather than a static weight.
The composites that will be projected upon the nose geometry include the previously
found best option with minor variations thereof to observe the true result of each addition or
subtraction over the entire structure. Meshing profiles defined in each trial are only found in
the locations corresponding to honeycomb placement and are not found universally though
the structure.
The way the layering alterations are defined in this study is by cross section. A template
depicting the properties of each element in the cross section (major fiber angle, layer thickness
and material) is shown in Figure 5.6.
29
Table 5.6: Composite Definition Template
1st
Layer Angle
..................................................
Material Type
..................................................
Filler
Thickness
(in defined areas)
Filler Type
..................................................
2nd
Layer Angle
Material Type
..................................................
The first composite trial utilized three layers of carbon bi-weave as described in Table
4.1 . This layering orientation along with other permutations discussed later were simulated
under the same loading conditions as used in the previous section corresponding to 13 mile
per hour impact. the maximum deflection and maximum stress are shown below.
Table 5.7: Composite Trial One
..................................................
-45
Carbon Bi- Weave
..................................................
0
Carbon Bi- Weave
..................................................
45
Carbon Bi- Weave
..................................................
M axDef lection = 0.984in
M axStress = 48KSI
The results from this simulation were as expected from a layering technique without a
honeycomb sub-structure and showed a very high stress rate causing failure of the structure
and a deflection not acceptable by the original specification.
The second simulation tested the honeycomb sub-structure with a single layer top and
bottom which is one layer less than the previously determined optimum.
30
Table 5.8: Composite Trial Two
..................................................
0
Carbon Bi- Weave
..................................................
0.25 in
Honeycomb Nomex
..................................................
0
Carbon Bi- Weave
..................................................
M axDef lection = 0.394in
M axStress = 33KSI
The results were greatly improved from that of the previous simulation however the max
deflection is significantly larger than the two layered system found in section 5.1.4 .
31
Table 5.9: Composite Trial Three
..................................................
-45
Carbon Bi- Weave
..................................................
45
..................................................
0.25 in
Honeycomb Nomex
..................................................
45
..................................................
-45
Carbon Bi- Weave
..................................................
M axDef lection = 0.216in
M axStress = 23KSI
This structure shows vast improvement from the initial three layers of carbon showing a
low stress rate at the given load. The deformation of the structure when loaded is also under
one inch which when combined with the two inches of travel the mount has determines that
no part of the structure will come into contact with the rider.
Lastly a trial with an additional layer was simulated to see the benefits of adding an
additional layer to get a proper trend line from the trial process.
32
Table 5.10: Composite Trial Four
..................................................
-45
..................................................
0
Carbon Bi- Weave
..................................................
45
..................................................
0.25 in
Honeycomb Nomex
..................................................
-45
..................................................
0
Carbon Bi- Weave
..................................................
45
..................................................
M axDef lection = 0.078in
M axStress = 15KSI
The results of the final trial show an additional benefit of the third layer however the
reduction is much under the specification and is considered over designed.
Trial four was calculated analytically after being determined to be the most beneficial structure to check the validity of the AnsysT M results. Using the equations in appendix A strain
and shear normal to the loading surface were calculated at 0.156 and 0.0 respectively. Congives a max deflection
verting this to a comparable value of deflection by the equation = ∆L
L
analytically of 0.069 inches in the normal direction. This result depicts the validity of the
solutions given by AnsysT M with a 13 percent difference in calculated deflection.
5.3
Energy Absorbent Mount
Determining the functionality of the energy absorbent front mount, in comparison to the
work done on the nose, is relatively concise. This study tested the effectiveness of multiple
types of material and their absorption capabilities. The mechanical setup was first derived
from a standard linkage system which uses a lever arm to reposition the absorbent material
to a centrally mounted place while not interfering with the rider’s performance as shown in
Figure 5.2.
33
5.3.1
Mechanical Design
The mechanical mounting structure was designed to be simplistic, cheap and effective for
absorbing the most amount of energy before the energy was transfered to the nose protecting
the rider. It was necessary to relocate the absorbent feature below the rider’s leg movements
to keep the center of mass low as well as not to create a contact point where the heel of
the rider could contact the mount. For these reasons a standard pivot mount was chosen to
allow the energy absorber to be mounted via two brackets under the pedal boom arm shown
in Figure 5.2 and transfer the energy up towards the nose acting as a static mount (as well
as dynamic in the event of a crash).
The two inch lever arm was chosen for the specific alignment tolerance of the final spring
that was chosen for this design. Due to manufacturing tolerances the spring can angularly
rotate 1.5 degrees which translates into 0.125 inch of translation between mounting brackets
allowing for flexibility in alignment. Due to this constraint it can be calculated that anything
over 1.73 inches lever arm shown as the yellow mount in Figure 5.2 will result in a large
enough rotation angle as to keep the shock in linear compression. A two inch arm was then
specified for a safety factor and accommodation for swapping out shocks.
Figure 5.2: Spring Pivot Assembly
5.3.2
Material Selection
Selecting the proper material for the energy absorbing device was a matter of applying
the material properties into a transient study using AnsysT M . This study was conducted
much the same as the composite structure, however the Pre-Post process was replaced by
simple material properties such as modulus of elasticity and yield strengths in shear and
compression. The geometry of each material sample was then uploaded into AnsysT M via
AutodeskInventorT M and applied with a force coresponding to a 13 M.P.H. impact or a
1250 ft-lb load. The results of this test determined the energy absorption of each material as
well as its percent compression equated by taking before and after lengths of the material.
34
The compression limit was determined by the change in length over the original length at
the time when the imput energy no longer was diffused by the absorbent material. It was
also tabulated if the material had reached its full compression length at the given force or
not. Table 5.11 shows these results in detail.
Table 5.11: Energy Absorbent Mount Material
Material
Rubber
Plastic
Honeycomb
Coiled Spring
Energy Absorbed (Foot pounds) % Compression
1380
10
1090
7
2230
82
3375
65
From this data it can be inferred that the material with the largest energy absorption
capacity is the coiled spring. It is shown that the coiled spring provides a large amount
of absorption and travel distance. It can be noted that honeycomb is also comparable to a
coiled spring in its energy absorption and could be used in parallel with the spring to provide
the maximum protection for the rider.
35
Chapter 6
TESTING
6.1
Energy Absorbent Mount
To validate the energy absorption of the specified spring system a setup was made to
represent a 7 miles per hour (M.P.H.) impact with the front mount. This test of three incremental spring stiffnesses with respect to thier ability to absorb the impact. The apparatus
utilized two sets of weights, one free falling and one sliding towards the mount to increase
the momentum of the crash sled depicted in Figure 6.1. The impacts speeds being tested are
4 and 7 M.P.H. against the hard fixed mount. These speeds are relatively low in comparison
to the discussion above however due to spatial limits of the apparatus a small scale test was
done to extrapolate and validate data gained from simulation. These speeds directly relate
to impact energies simulated at 117 and 360 ft-lbs respectively.
To relate a full size human powered vehicle impacting a wall at speed to the energy at
impact a kinetic energy was calculated for the full size vehicle assuming an average rider and
vehicle weight of 220 lbs. Figure 6.1 depicts the relationship of how much energy must be
absorbed by the structure on impact at any given speed.
Figure 6.1: Impact Energy at Speed
Weights chosen for the falling mass and the weighted sled were calculated to give the
36
proper amount of kinetic energy when colliding with the shock apparatus to simulate a low
speed collision. By adding to the falling mass the kinetic energy of the sled will increase
while mass added to the sled will increase the inertia of the system transferred during impact.
The proper loading for this simulation was to have 70 lbs hanging from the falling mass and
100 lbs on the weighted sled. This can be directly related to a simulation done at an impact
speed of 7 mph or an impact energy of 456 ft-lbs.
Figure 6.2: Spring Testing Apparatus
The data from the testing apparatus was taken using a mounted accelerometer and data
acquisition device utilizing an Arduino. Figure 6.3 and 6.4 depict the setup used which is
a proto board glued to the reverse side of an Arduino Uno. This proto board includes a 3
axis accelerometer, a switch for starting and ending the trial, a voltage converter to utilize a
separately mounted 9 volt battery as a power source and a secure digital (S.D.) card reader
and writer to store data that can be transfered to Excel for reduction.
37
Figure 6.3: Arduino Built Accelerometer (Front)
The top side of the Arduino is programmable using an open source software to collect
data in the proper manner while the reverse side contains all sensors and data storage units.
The acceleration of the unit is originally stored as an analog voltage signal output converted
internally using the internal code and exported as an acceleration.
Figure 6.4: Arduino Built Accelerometer (Back)
The raw data exported from trials for each at the three shocks, when placed on the same
plot, describe the inherent differences in the impact resistance in each spring, and quantifies
38
the acceleration transmitted to the structure supporting the shock.
Table 6.1: 7 M.P.H. Spring Testing
Spring
1
2
3
Falling
Mass (lbs)
70
70
70
Weighted
Sled (lbs)
100
100
100
Height
Energy
(ft)
In (ft/lbs)
6.52
456
6.52
456
6.52
456
Compression (in)
1.7
2
1.3
G’s
(Max)
4.4
<10
6.1
Table 6.1 describes the real time data taken from the mounted accelerometer and the
amount of compression obtained from photographic evidence. It can be seen that the max
accelerations shown in Figure 6.5 are tabulated in Table 6.1.The error in this G’s measured
was calculated to be +/- 0.05 and the error in the compression +/- 0.125 inches defined in
Appendix B. This shows how the chosen shock structure is of benefit to the system as a
whole and minimizes the maximum acceleration felt by the rider.
Plotting the cropped raw data from each trial graphically depicts the difference between
each spring system.
Figure 6.5: Impact Test One Plot (7 M.P.H.)
39
A secondary trial was tested to better relate the comparison of shocks since the first trial
resulted in a bottomed out spring. This trial is conducted with a simulated 5 mile per hour
collision input using the same configuration. The results for this trial are seen below in Table
6.2.
Table 6.2: 5 M.P.H. Spring Testing
Spring
1
2
3
Falling
Mass (lbs)
28
28
28
Weighted
Sled (lbs)
40
40
40
Height
Energy
(ft)
In (ft/lbs)
6.52
183
6.52
183
6.52
183
Compression (in)
0.9
1.3
0.7
G’s
(Max)
2.45
1.74
2.75
This test depicted a similar result as the previous run while allowing the limit of the
second spring to show in Figure 6.5. It remains clear that spring 3 is the most beneficial
spring for this application due to its higher energy absorbency while taking roughly the same
amount of time to settle out and return back to an unloaded state.
Figure 6.6: Impact Test Two Plot (5 M.P.H.)
40
The conclusion of this testing shows that spring three most closely fits the specifications needed for the support absorption device. the maximum acceleration imparted are
significantly less than the other two springs that were tested
41
6.1.1
Impact Test with Composite Plate
The impact tests were represented for spring 3 while replacing the solid plate with a
carbon fiber nomex sandwich. The goal was to see if the presence of the sandwich would
absorb some of the impact and reduce the maximum measured accelerations. Comparisons
of the maximum accelerations are shown in Figure 6.7 and Table 6.3
Figure 6.7: Composite with Spring Mount Acceleration Test
Table 6.3: Max G Loading Impact Test
Case
Wood Plate
5 M.P.H.
1.75
7 M.P.H.
4.28
Carbon/Nomex Plate
1.41
3.84
It is clearly shown from Table 6.3 that the composite/nomex reduced the amount of
energy transfered to the rider. This is shown true for both tested collisions at 5 and 7
M.P.H.
42
Chapter 7
RESULTS
7.1
Simulated Complete System Response
Once each individual component was chosen for its strengths and ability to meet specifications the system as a whole was tested. By swapping the hard mount points previously
modeled into the nose for the mounting structure with the shock system, the complete system can be analyzed. Similar meshing and constraint procedures were utilized in previous
simulation trials. To fully test the performance of the system, trials were run though a
variation of impact energies. This variation of energy will depict the trend of the system as
it impacts a solid barrier at velocities up to 17 M.P.H., assuming a typical H.P.V. and rider
mass.
To test the spacial safety of the rider the inward motion of the nose structure deflecting
in was calculated as a function of the impact energy. This test will not only validate the
final deflection limit of the structure at the highest speed but will allow insight into at which
speed each element previously discussed is valid. Each of the following data points in Figure
7.1 is a solution given by AnsysT M at a straight angle impact load placed at the point where
the mounting structure affixes to the composite nose.
43
Figure 7.1: Deflection with Respect to Impact Energy
Observing the trend of this figure a distinct range is shown where the energy absorbent
spring mount structure is functioning as designed. It is clear to see the linear trend of the
deflection imparted onto the spring for deflections less than 2 inches following the formula
of E = F ∗ D. The definition of the energy relates the effective force on the rider to the
distance traveled by the crush structure. The sharp change in slope in the plot for deflections
greater than 2 inches represents the travel of the spring as it reaches its maximum travel
and the composite structure in turn begins to compress. The benefit of the spring mount
can now be fully observed in its ability to fully absorb the energy from low speed collisions
without damaging the composite structure. From Figure 7.1 energy corresponding to the 2
inch deflection is roughly 1100 ft-lbs, which based on the impact energies shown in Figure
6.1, corresponds to a 13 M.P.H. impact. Similarly the energy of 1600 ft-lbs corresponding to
the full deflection of the nose structure corresponds to an impact velocity of 16 M.P.H. These
velocities define the workable range of the nose/mount structure. Higher velocity impacts
will likely result in a rider injury.
The results shown in Figure 7.2 are arguably the most important in terms of determining
the safety limit of the nose/mount system. Relating the acceleration of the impacted system
44
to the velocity traveled at the given impact defines the ultimate safety of the rider. As
discussed previously the deceleration limits of the human body in an impact are well defined
and understood to cause the most bodily harm aside from objects colliding with the body
itself. To this extent a trend of rider safety can be displayed showing the riders risk of injury
at each velocity. As defined previously, a 10 g load and less is deemed non harmful to human
occupants. At the velocity corresponding to full structure compression at 16 M.P.H., the
maximum acceleration is calculated as 4.8 G’s.
As a second note of safety the deformation of the structure is of interest because if the
rider is struck by pieces of the structure , direct injury will result. A combination between
the deformation of the system not contacting the rider and the acceleration being well within
the lethal limit will define the overall system as safe.
Figure 7.2: Acceleration and Deformation with Respect to Velocity
Further inferred from this study is the acceleration variation as felt by the rider in relation
to the frame during the simulated change in velocity. As shown from Figure 7.2 there are
four distinct regions. Firstly is the compression of the spring mount denoted by an second
order curve from zero M.P.H. till the velocity and energy is large enough to fully compress
the spring. This section culminates into the point where the spring has fully compressed
creating a step in the plot, altering the acceleration transfer rate instantaneously. The
following curve from the point where the spring has bottomed out until the full compression
of the honeycomb follows a similar but more sensitive curve as the spring. Since the material
is more dense and less able to compress it increases the acceleration load quicker per velocity
45
increase than its spring mount counterpart. Immediately following the full compression of
the honeycomb is the failure of the composite shell. This effect is shown in a higher order
curve due to the elongation and large strain rate of the system at this point, the system has
very little resistance to dissipate any incoming energy and will not return to proper working
order after the impact. This can be shown in the system becoming completely stiff, not
allowing any further deflection and maxing out the incoming acceleration at 11.39 G’s after
passing the impact speed limit of 25 M.P.H.
7.1.1
Low Speed Impact
A low speed impact as defined in the previous section is 15 M.P.H. The simulation of
the nose structure was combined with that of the mounting structure giving a complete
structural system analysis. Evaluation of the setup was done in 25 equal increments for
impact energies corresponding to zero to 17 M.P.H. to determine if and when a failure in the
structure will occur. This test was designed to simulate the composite and mount structure
together under the max loading of the mount to test its compression and absorption abilities.
Results from this test shown in Figure 7.3 represents a graphical representation of a single
data point in Figures 7.1 and 7.2.
Figure 7.3: Deflection at Low Speed Impact
Results from this simulation show a very similar result as seen by the mounting structure
simulated alone. This is shown in the deflection equivalents of two simulations of .894 inches
at 17 M.P.H. for the mount alone and .907 inches at 17 M.P.H. for the complete unit. This
difference is due to the compression of the honeycomb at the point of contact.
This figure shows the graphical representation of the deflection on the nose structure.
It clearly shows the movement of the nose is biased towards a downward motion which is
46
beneficial in a failure mode as to allow the structure to remain around the rider in the event
of a failure.
7.1.2
High Speed Impact
Tested at an input energy corresponds to of 25 M.P.H. , the graphical results of the
nose structure can be seen below. The loading condition for this was segmented into 25
equal parts from zero to 25 M.P.H. as to observe the step by step loading evaluation of the
composite. It can be shown that the deformation of the structure is linear due to the shock
absorbing its complete amount of energy and reaching its limit of 2 inch travel. By following
the result up until the full load is applied it is seen that the system including mounting
and nose structure remain under the yield point and the total deflection of the unit can be
determined at 2.125 in including the 2 in of travel given by the on board shock. A graphical
representation of this simulated impact is shown in Figure 7.4.
Figure 7.4: Deflection at High Speed Impact
The figure shows a graphical representation of the deflection the nose encounters at an
impact of 15 M.P.H. It is clear to see how the front of the nose deforms most as would
be expected. Secondly is the apparent bubbling deformation of the lower section which is
shown to be the highest deformation point. This visually is much larger in scale than it is in
an actual impact for clarity purposes, however it points out the failure mode of this structure.
Failure of the structure can be described as a buckling force of the flat under structure
of the enclosure as the nose is compressed and the flat section begins to shorten and buckle
under the load. This area is a beneficial failure mode of the system because in the event of
47
an over stress the buckling will occur outwards and below the rider, allowing the bulk of the
structure protecting the rider to remain intact. This will protect the rider from shrapnel and
keep the system from collapsing in on him as all mounts will remain valid and structural.
48
Chapter 8
CONCLUSION
In this study a compliant structure was designed to provide safety to the rider of a recumbent tadpole design tricycle during local commuting in or around town. This design was fit
to meet specific design specifications in safety, aerodynamics, sizing and weight. AnsysT M
was used to analyze the structure’s ability to absorb impact energies. This structure and
mount system protects the rider from injury during impacts at speeds up to 17 M.P.H, and
impacts up to 13 M.P.H. can be absorbed solely by the energy absorbent mount.
Calculation of acceleration experienced by the structure as a function of impact energy
indicates that for impact up to 17 M.P.H., the rider will experience an acceleration of less
than 10 G’s and thus will remain safe.
49
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°
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50
Appendix A: Analytical Calculations
Material Constants to Analytically check the calculations done by Ansys are done using
the following equations. The material represent the following.
ET = Young’s Modulus of ply along transverse fiber direction
σT = Stress along transverse fiber direction
T = Strain along transverse fiber direction
γLT = Shear strain along transverse fiber direction
τLT = Shear stress along transverse fiber direction
µ = Poisson’s ratio
GLT = Shear Modulus
51
Representative element subjected to uniaxial stress in transverse direction
ET =
σT
T
T = f T Vf + mT Vm
γLT =
1
EL
− µT L
ET

S=
τLT
GLT
− µETTL
1
EL
0
0

0
0 
GLT
Translational strain though the fiber orientation can be calculated using a matrix of the S
values calculated at each point.
 
 
x
S1,1 S1,2 S1,3
P
 y  = S2,1 S2,2 S2,3  A
τxy
S3,1 S3,2 S3,3
0

52
Appendix B: Uncertainties
Uncertainty of the measured trials were calculated using the known limitations of the
measurement devices available.
Accelerometer uncertainty was measured by placing it in free fall and measuring the
variation in acceleration over 10 trials.
δA = +/ − 0.05G0 s
Spring compression measurement was done graphically and varried depending on
photogrophy quality and angle.
δB = +/ − 0.125inches
A = Accelerometer Measurement in G’s
B = Spring Compression Measurement
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Appendix C: Drawings
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Appendix D: Spring Test Data
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Appendix E: Spring with Carbon Test Data
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Appendix F: Arduino Accelerometer Code
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