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