Introduction to Engineering
Bike Lab #4 – 1
Introduction
Agenda
 Strength & Reliability
Strength And Reliability
Failure, Fatigue
How can engineers design a safe
structure?
Answer: Design for Reliability
Engineers must know the loads that will be
applied to the structure.
 Engineers must know the strength
properties of the materials used for the
structure.
Answer: Design for Reliability
Engineers must be able to design the
structure such that, at every point, the
stress due to the maximum predicted
loads that will be applied to the structure
will not cause the structure to fail
Design for Reliability in other engineering
disciplines:
 Maximum information flow rate on a
computer network (Computer and
Electrical Engineers).
 Maximum loading rate for water treatment
facility (Civil Engineers and FABE).
 Other
Two Recent Engineering Failures
The crash of
the Concorde
Assume: The crash was caused by debris
from the tire that punctured the fuel tank.
Two Recent Engineering Failures
The crash of the Concorde
•
•
•
Could this kind of loading have been
predicted?
Could the design be such that the fuel
tanks could be protected?
Could a stronger material have been
used for the fuel tanks?
Two Recent Engineering Failures
Natural Gas
Line Explodes
in New
Mexico
11 campers died when a 30-inch line exploded
200 yards from where they had their tents. The
pipe was installed in 1950.
Two Recent Engineering Failures
Natural Gas Line Explodes in New Mexico
• What caused the pipe to fracture?
• Was it designed correctly?
• Was the material manufactured with no
defects?
• Did corrosion change the material strength
over the years?
An Engineering Failure During WWII The Liberty Ships
From 1941 to 1945, 2710 Liberty ships were built.
By February 1946, 362 ships, over 13% of the fleet,
had suffered at least one major fracture.
An Engineering Failure During WWII The Liberty Ships
Some were lost
at sea, but one
named “The
Schenectady”
broke in two 24
hours after
launching while
tied up near the
outfitting dock.
Approaches for Safe Design
 “Perfect world” approach: The applied load is
known exactly, the strength is known exactly.
 STRUCTURE STRENGTH > MAX APPLIED
LOAD
 How do engineers know the strength of a
specific material?
Use data from handbook.
Measure the strength in the lab.
Consider an
example of 1/8”
diameter, ordinary
zinc-plated steel
screws. Results
from tensile testing
of 75 screws
provide the
following data:
Number of Samples
Approaches for Safe Design
20
15
10
5
0
700
750
800
850
900
Failure Load (Pounds)
Is the “perfect world” approach acceptable?
What value would you use based on this data?
Factor of Safety (F.S.) Approach
Expected Structure Strength
F.S. =
Expected Maximum Load
F.S. > 1
 The factor of safety accounts for uncertainty in
material strength, loads, design flaws,
manufacture flows, etc.
 Most components are designed with a factor of
safety approach. F.S. for a bridge can be 3 - 5.
Factor of Safety (F.S.) Approach
What are the advantages of this
approach?
 What are the disadvantages of this
approach?
Risk in Design
Perfect world: NO RISK since the actual load
never exceeds the designed strength.
Frequency of occurrence
Applied load
Structure
designed
maximum
strength
Load
Risk in Design
Real world: Uncertainties in design and in actual
loading conditions introduce risk
Frequency of occurrence
RISK OF FAILURE
Structure
designed
maximum
strength
Applied
load
Load
How do things fail ???
Overload: The applied load in a single
event is higher than the material strength.
An Example is the Concorde crash. The
force applied to the fuel tanks by the flying
pieces of rubber was more than required
to puncture the tank.
How do things fail ???
 Fracture: A crack exists in the structure. The
sharp crack creates a stress concentration that
causes the crack to propagate under a load that
would otherwise be safe. Cracks grow by a
process of Fatigue.
 Microscopic cracks exist in engineering
materials (typically at the surface). During
repetitive low amplitude loading the cracks grow.
Once a crack reaches a critical length, the
component fractures
How do things fail ???
Fatigue is the cause of most
mechanical failures.
In a good design, fracture of one
component will not cause a catastrophic
failure of an entire structure. An example
on the next slide
Aloha Flight 243
Aloha Flight 243 from Hilo to Honolulu on
April 28, 1988
Aloha Flight 243
The 19 year-old aircraft had
taken off and landed 89,680
times prior to the accident.
Each time the cabin was
pressured like a balloon.
The repetitive load on the
skin caused fatigue in the
skin between the rivets.
Due to the good design, the fracture was limited and
the rest of the structure was strong enough to keep the
airplane flying.
Fatigue
Low stress
Crack closed
Crack closed
High stress
Region of very
high stress
Time
Crack open
Stress
Da crack growth
Low stress
Typical Fatigue Fracture Surface
Fast crack
growth
Beach
marks
Slow crack
growth
Crack origin
Analysis Of Fatigue
S - N CURVES
The aluminum alloy has
no endurance limit.
S-N Curves:
Experimental curves that
are used by engineers to
predict the number of
times a component can
be loaded to a certain
level of stress.
Endurance Limit:
The stress amplitude for
which a material has
infinite life (more than a
million cycles).
Part II - Assessment of Fork Design
Your goal in this lab is to observe the stress
conditions in the front forks of your bicycle
under actual field conditions, and to make
judgments about the adequacy of the fork
design.
In Lab
 Two riders (the lightest and heaviest in your
group) will ride the bikes.
1. Set up the data logger as you did for Bike
Lab #3, except set the acquisition rate to 1
reading every 2 seconds
2. View the data collected by the datalogger in
real time by clicking on "Receive, Real
Time".
In Lab
3. Record the voltage signal for the unloaded
bike.
Initial signal =
volts
4. Record the voltage signal for each of the 2
riders.
 Rider 1:
weight =
 Rider 2:
weight =
lbs., signal =
volts
lbs., signal =
volts
In Lab
5. Reset the datalogger to have an
acquisition rate of 50 Hz.
6. Take the bicycle outside and have both
riders ride the bike for about 45 seconds
each.
 Leave the bike sit unloaded between
riders for a few seconds so the data will
clearly show when you switched riders.
In Lab
7. Bring the bikes back to the lab. Upload
your data from the datalogger to the PC
and save it to disk.
8. Import the data into Excel and save it as
a spreadsheet.
After Lab
1. In the spreadsheet create a plot of stress
vs. time for the experimental data. Label
and add a legend to your plot.
2. Find the maximum stress observed for
each rider. What percent of the yield
stress for your bike frame is this
maximum value?
After Lab
3. How many times larger is this dynamic
stress than the stress observed when the
rider was sitting still?
4. Prepare a paragraph evaluating the fork
design of your bicycle. Do you think it is
over-designed, under-designed, or just
right?
After Lab
5. Consider the weight of the rider, the life
expectancy of the bike, and the yield
stress of the bike frame material.
6. What rider weight limit would you impose
for this fork design?
After Lab
Prepare a team Lab Report using the
standard format given and include the
following:
 Plot of stress vs. time.
 Calculations including answers to questions 2
and 3 above.
 Answer to question 4.
 First page of spreadsheet (don’t include all
pages of data)
Assignment
 Re-read Bike Lab #4 procedure