Class 8 - Durability

8/10/2019 Class 8 - Durability

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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


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Types of Failures

Early or Infant Mortality Failures: These are mostly due

to manufacturing defects and has a decreasing failurerate. 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 constantfailure rate.


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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


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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 (>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 discoveredand 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 outfailures.


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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 StrengthFailure may be unavoidable. If

vehicle fails, it must fail safely (within

reasonable limits)

Low-occurrence loads

Robust TestingFront-load the

discovery of failure

modes using CAE and

laboratory tests

Design for FatigueIdentify 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 opposedvehicle tests in the proving ground)

Product Development

Testing(DFR)


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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


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Proving Ground Testing

Rough Road Track

Hilly Terrain for PowertrainDynamic

LoadsSalt

Bath

Average length of the circuit: 5 - 6 miles

Average speed: 30-55 mph

Proving Ground Miles: 10,000

Equivalent Miles: 150,000


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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


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Description of Fatigue Failure

Force

Force

Fixed Fixed

,F

,F

Load

Cycles, N

F

N0


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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

(Engg.)

StressRange,

KSI

Fatigue Life, Cycles

S-N Curve for SAE 1010 Steel


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Notes of Fatigue Life

Endurance Limit (EL)is the same as Fatigue Limit (FL). EL is morecommonly used in U.K. and for Steel; FL is used in the U.S. for all materials.

Rule of Thumbfor 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 Lifeis dominated by Ductility and Plastic Behavior;

High cycle Lifeis 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.


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For steel structures, a fatigue crack needs to be 1 mm long before it

propagates; scratches and nicks dont grow.

To resist Crack Nucleation (Initiation), make the part stronger;

To resist Crack Propagation, select a more ductile material.

Physics Method Crack Size Surface FinishInfluence

Crack Nucleation Stress-Life < 0.1 mm Strong

Microcrack Growth Strain-Life 0.11 mm Moderate

Macrocrack Growth Crack Propagation >1 mm None

Notes on Fatigue Life


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Stress Cycle

CyclicStress,

Time

tmax tensile stress

cmax compressive stress

m= (t+ c)/2

m = 0 ift= cm< 0 if t< cm> 0 if t> c

m


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Notes on Fatigue Life

Variability in Loading is much more critical for accuracy inestimating 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


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Actual Service Loads & Histogram

CyclicLoad

Time

Loa

d

Cycles

Load Histogram


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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 kg

Unsprung

weight 40 kg

Road

Profile

Rim Stiffness(N/mm) 2000

Rim 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-of 21.582 mm 200 50 100 45 200 80

TireLoad

Tire Compression

Tire

Lift-off

Rim

Contact WhlLoad

Whl Deflection

ShockLoad

Whl speed


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Jounce/Rebound Clearance

Tire

FenderJounce

Clearance

Small Car 50 mm

Large Car 90 mm

Big SUV 120mm

Truck 150mm


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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*


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Stress Calculation

Shock Absorber Tube Cross-section with area A

Shock absorber load from suspension load calculation Pmax

Peak stress = Pmax/A


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Fatigue Damage Calculation

Cycles

S1 S2 S3

S4S5

S6

N1 N2N3 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

Stress

Cycles

Stress

Stress Histogram

S-N Curve for Metal

Damage D= N(i)/Ni

And D < 1

1

6


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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

Class 8 - Durability

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