Durability• 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:– 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
• The rate at which failures occur is typically characterized by the “bathtub curve”
• The three regions of the curve indicate distinct failure modes
Time in Service
Infant Mortality(DFR) Random Failure (CFR) Wear out Failure
(IFR)
“Useful Life”
Constant failure rate (CFR) indicates failures that happen at random. They are unrelated to wear and may happen due to accidents, abuse or misuse.
Decreasing failure rate (DFR) indicates manufacturing defects resulting in early failures
Increasing failure rate (IFR) show the effect of accumulated damage (metal fatigue, cumulative environmental exposure, etc.)
Failure Rate
Ideal Failure Rate in Vehicle Life Cycle
Time in ServiceJ#1
Product Development Testing (DFR)
Random Failure (CFR) Wear out Failure(IFR)
“Trouble-Free Life” Target(10 yr/150K Miles for 90% of customers)
Random failures cannot be avoided. (They are unrelated to time-in-service)
- Minor accidents- Severe road hazards- Misuse or abuse
Failure modes discovered and fixed during product testing
Some “extreme-duty” customers (<10%) may experience early wear out
Majority of wear out failures (>>90%) occur outside the 10yr/150K mile target
Failure Rate
• 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
Time in Service
Failure Rate
J#1
Random Failure (CFR)
Wear out Failure(IFR)
“Trouble-Free Life” Target(10 yr/150K Miles)
“Design for Strength”Failure may be unavoidable. If
vehicle fails, it must fail safely (within reasonable limits)
“Low-occurrence loads”
“Robust Testing”“Front-load” the
discovery of failure modes using CAE and
laboratory tests
“Design for Fatigue”Identify and design against all potential failure modes related
to repeated duty cycles
“Common-occurrence loads”
• 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)
Product Development Testing (DFR)
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
Rough Road Track
Hilly Terrain for Powertrain DynamicLoads
Salt Bath
Average length of the circuit: 5 - 6 milesAverage speed: 30-55 mphProving Ground Miles: 10,000Equivalent Miles: 150,000
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
Force
Fixed Fixed
,F
,F
Lo
ad
Cycles, N
F
N0
S-N Curve for Metals
0
5
10
15
20
25
30
35
40
45
50
1.E+00 1.E+01 1.E+02 1.E+03 1.E+04 1.E+05 1.E+06 1.E+07
(En
gg
.) S
tres
s R
an
ge,
KS
I
Fatigue Life, Cycles
S-N Curve for SAE 1010 Steel
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.
For steel structures, a fatigue crack needs to be 1 mm long before it propagates; scratches and nicks don’t grow.
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
Notes on Fatigue Life
Stress Cycle
Cyc
lic
Str
ess,
σ
Time
σt – max tensile stress
σc – max compressive stress
σm = (σt+ σc)/2
σm = 0 if σt = σc
σm < 0 if σt < σc
σm > 0 if σt > σc
m
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
Actual Service Loads & Histogram
Cyc
lic
Lo
ad
Time
Lo
ad
Cycles
Load Histogram
Fatigue Damage Calculation
Cycles
S1 S2S3
S4S5
S6
N1 N2 N3 N4 N5 N6
0
5
10
15
20
25
30
35
40
45
50
1.E+00 1.E+01 1.E+02 1.E+03 1.E+04 1.E+05 1.E+06 1.E+07
Str
ess
Cycles
Str
ess
Stress Histogram
S-N Curve for Metal
Damage D = Σ N(σi)/Ni
And D < 1
1
6
Process to Evaluate Structural Durability
Road Surface,Speed and
Number of Passes
Suspension LoadHistogram forComponents
ComponentStress
Histogram
Damage Calculation from Material
S-N Curve
Durability Road Surface
• 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 CalculationRebound Low speed damping (N.sec/m)
Rebound High speed damping (N.sec/m
Cut - Off - Speed (Rebound) m/s
Jounce Low speed damping (N.sec/m
Jounce High speed damping (N.sec/m
Cut - Off - Speed (Jounce) m/s
1000 2000 1.5 750 2000 1
Sprung corner wt 400 kgUnsprung weight 40 kg
Road Profile
Rim Stiffness(N/mm) 2000Rim contact (mm) 75
Tire Stiffness 200 N/mm
Rebound Bumper Rate (N/mm)
Rebound Wheel Rate (N/mm)
Rebound Clearance (mm)
Jounce Wheel Rate (N/mm)
Jounce Bumper Rate (N/mm)
Jounce Clearance (mm)
Tire lift-off 21.582 mm 200 50 100 45 200 80
Tir
e L
oad
Tire Compression
TireLift-off
Rim Contact W
hl
Lo
ad
Whl Deflection
Sh
ock
Lo
ad
Whl speed
Jounce/Rebound Clearance
Tire
FenderJounceClearance
Small Car 50 mmLarge Car 90 mmBig SUV 120mmTruck 150mm
Suspension Loads
• 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
Parameters that affect Dynamic Loads*
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
Cycles
S1 S2S3
S4S5
S6
N1 N2 N3 N4 N5 N6
0
5
10
15
20
25
30
35
40
45
50
1.E+00 1.E+01 1.E+02 1.E+03 1.E+04 1.E+05 1.E+06 1.E+07
Str
ess
Cycles
Str
ess
Stress Histogram
S-N Curve for Metal
Damage D = Σ N(σi)/Ni
And D < 1
1
6
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