LABORATORY 5
Design Failure: Cause & Effect
Polytechnic University
General Engineering Department
6/21/2016
LABORATORY 5
Design Failure: Cause & Effect
Polytechnic University
General Engineering Department
6/21/2016
Outline
• Common Modes of
Failure
• Stress
• Strain
• Ultimate Tensile
Stress (U.T.S.)
• Fracture Stress
• Elasticity
• Plasticity
• Materials
• Limit of Plastic
Strength
• Objectives
• Procedure
• Minimal Design
• Rules of the Competition
• Winners
• Judging
• Price List of Materials
• Recitation Topics
• Report Questions
• Closing
Common Modes of Failure
• Thermal shock
• Corrosion
• Breakage under load (most common)
– Instant fracture
– Delayed response (fatigue)
Thermal Shock
• Result of rapidly increasing or decreasing
a material’s temperature
• Can cause material to crack or shatter
due to stresses created by rapid
expansion or contraction of material
Corrosion
• Occurs when materials are exposed to
various substances for extended periods
of time
– acidic chemicals
– water (rust)
– salt
– air (oxidation)
• Substances react with material and
weaken by “eating away” at material
Breakage Under Load
• Different parts of material have different properties.
• There are zones within the material that can be
subjected to above average stresses, and those
which will reach the plastic region before the
allowable stress for that material.
• Continuously loading and unloading will make this
zone harder and less ductile, eventually fracturing.
• Crack rapidly progress due to plastic deformation in
this region reducing cross-sectional area.
• Cross-sectional area is so drastically reduced that
material will be unable to contain stresses and fail.
• This failure is known as fatigue and is due to change
in microstructure associated with cyclic loading.
Cold Working and Necking
• Cold working
– By exceeding the plastic limit on the first try,
hence obtaining plastic deformation in the
material, it is made harder and stronger.
– This happens as greater stress is now required
to produce a similar strain.
• Necking
– Plastic instability results when the increase in
load carrying capacity due to strain-hardening
is dominated by the decrease in load carrying
capacity due to the reduced cross-sectional
area, causing a rapid increase in length.
Stress ()
• Measure of internal force that resists
change of form in body due to tensile
load
• Calculated from the following formula:
Stress = P
A
A = Applied Force
P = Cross Sectional Area
Cross sectional area
measured perpendicular
Strain (e)
• Measure of the deformation, elongation or
compression, of the material
• Calculated from the following formula:
Strain =  L
L0
 L= Stretch or change
of Length
L0 = Original Length
Ultimate Tensile Stress (m)
• Greatest amount of stress the material
will withstand without failing
• Plastic instability occurs when pass U.T.S.
U.T.S. = Pmax
A0
P = Applied Force
A0 = Cross Sectional
Area
Fracture Stress (f)
• Stress at which the material fails
• Calculated from the following formula:
Fracture
Stress = Pf
A0
P = Applied Force
A0 = Cross Sectional
Area
Elasticity
• Strains produced by stress disappear upon
removal of stress
• Stress is proportional to strain
Hooke’s Law    e
=E*e
E = Young’s Modulus
of Elasticity (slope of
graph in elastic region)
measures stiffness of a
material and constant
for a given material
Plasticity
• Plastic deformation occurs when a material
is loaded to a point beyond elastic region
• When material is unloaded, it would return
along a line
parallel to that
of elastic region
(with slope E) to
a non-zero value
of e, resulting in
permanent
deformation
Materials
• Non-Hookean solids are materials that do not
obey Hooke’s Law, such as rubber
• Ductile (measure of plasticity) materials
– Deform considerably before fracture
– Planes of atoms slip
easily across each other
• Brittle materials (glass)
– No plasticity
– Very difficult to slip
planes of atoms
– Do not deform much;
failure occurs suddenly
Limit of Plastic Strength
Four methods of measuring where we
enter the plastic region:
• Proportional
Limit
• Elastic Limit
• A.S.T.M.
• Yield Point
Proportional Limit
• Examine the stress-strain curve for a
change in slope (i.e. when the slope is no
longer linear)
• Shape of the curve, however, is sensitive
to many different conditions, such as the
machine used, the number of samples
taken, and may not be standard from lab
to lab
Elastic Limit
• Highest stress that can be induced
whereby strains will completely
disappear upon removal of stress
• Done by loading and unloading the
material and measuring its strain
• Only as precise as the number of
measurements and the size of the
intervals between the measurements
• Time consuming
A.S.T.M.
• American Society for Testing of Materials
• Stress which gives a small but specified
plastic strain
• Yield strength: use value of 0.2% strain
or 0.002”
• This tolerance corrects any errors which
may have occurred in proportional limit
• Factor is always applied
Yield Point
• Not to be confused with yield strength
• Used mostly by Civil Engineers who
reinforce concrete structures with mild
steel
• Some alloys
such as mild
steel have
curves as
illustrated:
Objectives
• Prepare design for light-weight container to
protect fragile object such as raw egg from
breaking when dropped from specified height
while keeping minimal design in mind
• Build container from materials provided
• Test container following provided guidelines
• Keep careful notes (signed by instructor) on
design rationale, construction of container,
and results of test; prepare written report
Procedure
• Describe reason for choice of materials
• Plan and sketch design of box
• Construct box
• One partner drops box from
balcony (35 ft to concrete floor)
• Other partner retrieves box
and returns it to balcony
• Repeat test drop with added weight
Minimal Design
• Design that
accomplishes
objective with least
amount of money
• If more than one
egg survives, the
winner of this
competition will be
chosen based on
minimal design.
Rules of the Competition
• Keep record: materials used, amount used, unit
cost, expanded cost, total cost
• Initial sketches of design before construction
• Initial price list before competition
• Each box labeled with team members’ names
• Box shape and size cannot be altered in any
way
• All materials remain inside box
• Box must be completely closed – no “wings”
Judging
Winners determined based on:
• One survivor
• Multiple survivors: team with most
minimal design
• No survivors: no winners
Price List for Materials
Bubble Wrap
$0.50/ft2
Cotton Wrap
$0.60/ft2
Rubber Bands (small) $0.01 each
Rubber Bands (large) $0.02 each
Tape (all kinds)
$0.10/ft2
Styrofoam Pieces
$0.05/6
Staples
$0.01/10
Saran Wrap
$0.02/ft2
Recitation Topics
• Minimal design
• Which materials are good/bad and why
• Common modes of failure
– Breakage under load
– Corrosion
– Thermal Shock
• Material failure
• Strategy: survival or cost
• Rules and testing
Report Questions
• Why does the design fail when the box falls from a
given height?
• Does the height matter? Would you expect
different results if a different height were used?
Explain.
• What is the function of the different materials given
to you to use in construction? Explain the role of
each. Discuss reasons for choice of materials used.
• Does the choice of materials matter? Explain. Can
you think of better materials to use? Discuss.
• Explain why minimal design was emphasized.
Closing
• Write team names on box
• Price list collected before competition and
returned after winner is determined
• Group whose egg survives and has the
most minimal design wins competition
and is exempt from writing a report, but
will be graded on quality of presentation
• Safety: Keep clear of testing area