Durability
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Definition of durability and reliability, warrantee
Examples of durability – structural failure, malfunction, rust
Bathtub curve
Durability evaluation: lab test, proving ground, fleet, analysis
Proving ground correlation
Structural fatigue failure – hair clip example
S-N curve
S-N curve for metals
Load histogram/load signal
Damage calculation
Suspension load estimation
Suspension parameters
Road surfaces
Assignment
System design
Reliability & Durability
• Reliability: System is unreliable when it malfunctions or
fails unexpectedly, examples of unreliability:
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A new car will not start after 3 months of purchase
Window does not roll down after 6 months
Power lock does not work within a month
Rattling noise within 2 months
• Durability: System is durable when it performs or does
not fail beyond its expected life, examples of durability:
– A car does not need any repair during warranty period of 3 years
– A car is still on the road after 10 years
– A car is still on the road after 200,000 km
Types of Failures
• Early or Infant Mortality Failures: These are mostly due
to manufacturing defects and has a decreasing failure
rate. Examples: Electronic modules not working, window
does not open due to interference fit, etc.
• Durability Failures: These are mostly due to wear and
tear or fatigue failures and has an increasing failure rate.
Examples: Wearing of brake pads, wearing of shock
absorbers, tire wear, body rust, muffler rust damage, etc.
• Random Failures: These are random in nature and occur
due to accidents abuse or misuse and has a constant
failure rate.
Typical Failure Rate During Product
Life Cycle
Failure
Rate
Decreasing failure
rate (DFR) indicates
manufacturing defects
resulting in early
failures
Infant Mortality
(DFR)
Constant failure rate (CFR) indicates
failures that happen at random.
They are unrelated to wear and may
happen due to accidents, abuse or
misuse.
Random Failure (CFR)
Increasing failure rate
(IFR) show the effect
of accumulated
damage (metal fatigue,
cumulative
environmental
exposure, etc.)
Wear out Failure
(IFR)
“Useful Life”
Time in Service
• The rate at which failures occur is typically characterized by the “bathtub
curve”
• The three regions of the curve indicate distinct failure modes
Ideal Failure Rate in Vehicle Life Cycle
Some “extreme-duty”
customers (<10%)
may experience early
wear out
Failure
Rate
Failure modes
discovered and fixed
during product testing
Random failures cannot be avoided.
(They are unrelated to time-in-service)
- Minor accidents
- Severe road hazards
- Misuse or abuse
Product Development
Testing (DFR)
Random Failure (CFR)
J#1
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Majority of wear out
failures (>>90%) occur
outside the 10yr/150K
mile target
Wear out Failure
(IFR)
“Trouble-Free Life” Target Time in Service
(10 yr/150K Miles for 90% of customers)
The intent of PD is that all potential failures modes that we design against are discovered
and fixed before Job #1.
We accept that we cannot possibly design for every single customer. Therefore we define
the usage spectrum corresponding to 90% of the customers as our target for wear out
failures.
Potential Failure Modes and Their Relationship to
Strength and Fatigue Requirements
Failure
Rate
“Design for Fatigue”
“Design for Strength”
“Robust Testing”
Failure may be unavoidable. If
vehicle fails, it must fail safely (within
reasonable limits)
“Front-load” the
discovery of failure
modes using CAE and
laboratory tests
Identify and design against all
potential failure modes related to
repeated duty cycles
“Common-occurrence loads”
“Low-occurrence loads”
Random Failure (CFR)
Product Development
Testing (DFR)
J#1
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Wear out Failure
(IFR)
“Trouble-Free Life” Target
(10 yr/150K Miles)
Time in Service
The “Fatigue Requirements” cover the usage spectrum of 90% of the customers
The “Strength Requirements” cover “extreme duty” customers as well as “random” events.
Failures are possible, and the intent is to develop fail-safe designs.
During product development, laboratory tests at component and system levels are employed as
early as possible to “front-load” the discovery of strength and fatigue failure modes (as opposed
vehicle tests in the proving ground)
Methods of Durability Testing
• FE & fatigue analysis of complete body/chassis
system subject to duty cycle
• Lab testing of the vehicle
• Vehicle testing on the proving ground
• Vehicle fleet testing on public roads
Laboratory Testing
Proving Ground Testing
Salt
Bath
Hilly Terrain for Powertrain
Rough Road Track
Average length of the circuit: 5 - 6 miles
Average speed:
30-55 mph
Proving Ground Miles:
10,000
Equivalent Miles:
150,000
Dynamic
Loads
Proving Ground Description
• Rough Road Track for Structural Durability includes: road with pot
holes, speed bumps, curb, cobblestone, twist ditch, etc.
• Powertrain Durability Track includes: 1% - 5% uphill and downhill
roads
• Dynamic Loads Track includes: Roads with ability produce 0.8 –
1.0G lateral acceleration
• Salt Bath Track includes: Muddy terrain and salt spraying facility
Description of Fatigue Failure
Force ,F
Force ,F
Fixed
Load
Fixed
F
N0
Cycles, N
S-N Curve for Metals
S-N Curve for SAE 1010 Steel
50
45
(Engg.) Stress Range, KSI
40
35
30
25
20
15
10
5
0
1.E+00
1.E+01
1.E+02
1.E+03
1.E+04
Fatigue Life, Cycles
1.E+05
1.E+06
1.E+07
Notes of Fatigue Life
 Endurance Limit (EL) is the same as Fatigue Limit (FL). EL is more
commonly used in U.K. and for Steel; FL is used in the U.S. for all materials.
 Rule of Thumb for Fatigue Design:
- 5 to -10% Stress => +100% Life
To increase Fatigue Life, increase the strength of the part without inflicting
surface damage. Fatigue begins at stress concentrators which are most
frequently located on surfaces
 Low cycle Life is dominated by Ductility and Plastic Behavior;
High cycle Life is dominated by Strength and Elastic Behavior.
The crossover point on the S-N Curve is called “Transition Fatigue Life”.
The higher the hardness of the steel (lower ductility),
the lower the Transition Fatigue Life.
Notes on Fatigue Life
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For steel structures, a fatigue crack needs to be 1 mm long before it
propagates; scratches and nicks don’t grow.
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To resist Crack Nucleation (Initiation), make the part stronger;
To resist Crack Propagation, select a more ductile material.
Physics
Method
Crack Size
Surface Finish
Influence
Crack Nucleation
Stress-Life
< 0.1 mm
Strong
Microcrack Growth
Strain-Life
0.1 – 1 mm
Moderate
Macrocrack Growth
Crack Propagation
>1 mm
None
Stress Cycle
Cyclic Stress, σ
m
σm = (σt+ σc)/2
σt – max tensile stress
σm = 0 if σt = σc
σm < 0 if σt < σc
σm > 0 if σt > σc
Time
σc – max compressive stress
Notes on Fatigue Life
 Variability in Loading is much more critical for accuracy in
estimating Fatigue Life, than variability in Material Strength.
 Mean Stress Effect - Tensile Mean Stresses reduce Fatigue Life or
decrease the allowable Stress Range.
 Compressive Mean Stresses increase Fatigue Life or increase the
allowable Stress Range.
If the Fatigue Life corresponding to Zero Mean Stress is N0
When Mean Stress/Ultimate Strength = 0.2, then N = 0.1 N0
When Mean Stress/Ultimate Strength = 0.4, then N = 0.05 N0
When Mean Stress/Ultimate Strength = -0.2, then N = 10 N0
When Mean Stress/Ultimate Strength = -0.4, then N = 100 N0
Cyclic Load
Actual Service Loads & Histogram
Time
Load
Load Histogram
Cycles
Fatigue Damage Calculation
S2
S3
Stress
S1
S4
Stress Histogram
S5
N1 N2 N3 N4
S6
N5
N6
Cycles
50
S-N Curve for Metal
45
40
Stress
35
30
25
20
6
Damage D = Σ N(σi)/Ni
15
1
10
5
And D < 1
0
1.E+00
1.E+01
1.E+02
1.E+03
1.E+04
Cycles
1.E+05
1.E+06
1.E+07
Process to Evaluate Structural Durability
Road Surface,
Speed and
Number of Passes
Suspension Load
Histogram for
Components
Component
Stress
Histogram
Damage Calculation
from Material
S-N Curve
Durability Road Surface
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Severe pothole – 1 pot hole per how many miles (N)
Severe bump - 1 bump per how many miles (N)
Cobble stone - 1 cobblestone per how many miles (N)
Etc.
Pothole dimensions, speed, no. of occurrence
Bump dimensions, speed, no. of occurrence
cobblestone dimensions, speed, no. of occurrence
No. of Occurrences = Warranty mileage/N
Suspension Load Calculation
1000
Sprung
corner wt
Unsprung
weight
Rebound
High
speed
damping
(N.sec/m
2000
Cut - Off Speed
(Rebound)
m/s
1.5
Jounce
Low speed
damping
(N.sec/m
Shock Load
Rebound
Low speed
damping
(N.sec/m)
750
Jounce
High
speed
damping
(N.sec/m
2000
Cut - Off Speed
(Jounce)
m/s
1
400 kg
Whl speed
40 kg
Whl Load
Tire
Lift-off
Tire Load
Road
Profile
Rim
Contact
Whl Deflection
Tire Compression
Rim Stiffness(N/mm)
Rim contact (mm)
Tire
Stiffness
Tire lift-off
200 N/mm
21.582 mm
2000
75
Rebound
Bumper
Rate
(N/mm)
200
Rebound
Wheel
Rate
(N/mm)
50
Rebound
Clearance
(mm)
100
Jounce
Wheel
Rate
(N/mm)
Jounce
Bumper
Rate
(N/mm)
45
200
Jounce
Clearance
(mm)
80
Jounce/Rebound Clearance
Jounce
Clearance
Fender
Tire
Small Car
Large Car
Big SUV
Truck
50 mm
90 mm
120mm
150mm
Suspension Loads
Parameters that affect Dynamic Loads*
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Tire Stiffness / Size
Vehicle Weight / Weight Distribution
Jounce / Rebound Travel (J/R Bumper Height)
Jounce / Rebound Bumper Properties
Shock-Absorber Parameters
Unsprung (Wheel, Spindle, Axle, Suspension) Mass
Spring Stiffness
Stress Calculation
Shock Absorber Tube Cross-section with area A
Shock absorber load from suspension load calculation Pmax
Peak stress = Pmax/A
Fatigue Damage Calculation
S2
S3
Stress
S1
S4
Stress Histogram
S5
N1 N2 N3 N4
S6
N5
N6
Cycles
50
S-N Curve for Metal
45
40
Stress
35
30
25
20
6
Damage D = Σ N(σi)/Ni
15
1
10
5
And D < 1
0
1.E+00
1.E+01
1.E+02
1.E+03
1.E+04
Cycles
1.E+05
1.E+06
1.E+07
Procedure
• Design durability road event, geometry, speed and number of
occurrences
• Calculate maximum shock absorber load from spreadsheet for
each road profile
• Construct load and stress histogram
• Assume material S-N curve from internet
• Calculate damage
• If damage is > 100%, use different material or area