Durability

Transcript Durability

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

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. Increasing failure rate (IFR) show the effect of accumulated damage (metal fatigue, cumulative environmental exposure, etc.)

Random Failure (CFR) 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

Failure modes discovered and fixed during product testing

Failure Rate

Some “extreme-duty” customers (<10%) may experience early wear out Random failures cannot be avoided. ( They are unrelated to time-in-service) - Minor accidents - Severe road hazards - Misuse or abuse Majority of wear out failures (>>90%) occur outside the 10yr/150K mile target

Product Development Testing (DFR) J#1 Random Failure (CFR) 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

“Robust Testing”

“Front-load” the discovery of failure modes using CAE and laboratory tests

Failure Rate “Design for Strength”

Failure may be unavoidable. If vehicle fails, it must fail safely (within reasonable limits)

“Low-occurrence loads” “Design for Fatigue”

Identify and design against all potential failure modes related to repeated duty cycles

“Common-occurrence loads” Product Development Testing (DFR) J#1 Random Failure (CFR) 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

Salt Bath

Proving Ground Testing

Hilly Terrain for Powertrain Rough Road Track Dynamic Loads Average length of the circuit: 5 - 6 miles Average speed: 30-55 mph Proving Ground Miles: 10,000 Equivalent 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

Fixed Force ,F Fixed Force ,F F N 0 Cycles, N

S-N Curve for Metals

S-N Curve for SAE 1010 Steel

50 45 40 35 30 25 20 15 10 5 0 1.E+00 1.E+01 1.E+02 1.E+03 1.E+04 1.E+05 1.E+06 1.E+07

Fatigue Life, Cycles

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

 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 Crack Nucleation Method Stress-Life Crack Size < 0.1 mm Surface Finish Influence Strong Microcrack Growth Strain-Life Macrocrack Growth Crack Propagation 0.1 – 1 mm >1 mm Moderate None

Stress Cycle

m σ t – max tensile stress σ m = (σ t + σ c )/2 σ m σ m σ m = 0 if σ < 0 if σ > 0 if σ t t t = σ c < σ c > σ c Time σ c – 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 N 0 When Mean Stress/Ultimate Strength = 0.2, then N = 0.1 N 0 When Mean Stress/Ultimate Strength = 0.4, then N = 0.05 N 0 When Mean Stress/Ultimate Strength = -0.2, then N = 10 N 0 When Mean Stress/Ultimate Strength = -0.4, then N = 100 N 0

Actual Service Loads & Histogram

Time Load Histogram Cycles

S 1 S 2 S 3

Fatigue Damage Calculation

S 4 S 5 Stress Histogram S 6 N 1 N 2 N 3 N 4

50 45 40 35 30 25 20 15 10 5 0 1.E+00 1.E+01 1.E+02

N 5 N 6

1.E+03 1.E+04

Cycles

1.E+05 1.E+06 1.E+07

Cycles S-N Curve for Metal Damage D And D < 1 6

Σ N(σ i )/N i 1

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

• • • •

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

Rebound Rebound High Low speed speed damping (N.sec/m) damping (N.sec/m

1000 2000

Cut - Off - Speed (Rebound) m/s

1.5

Jounce Jounce High Low speed speed damping (N.sec/m damping (N.sec/m

750 2000

Cut - Off - Speed (Jounce) m/s

Sprung corner wt Unsprung weight

400 kg 40 kg

Road Profile Tire Lift-off Tire Stiffness Tire lift-off

200 N/mm 21.582 mm

Rim Contact Tire Compression Rim Stiffness(N/mm) Rim contact (mm)

2000 75

Rebound Bumper Rate (N/mm)

200

Rebound Wheel Rate (N/mm)

Rebound Clearance (mm)

100

Whl speed Whl Deflection Jounce Wheel Rate (N/mm)

Jounce Bumper Rate (N/mm)

200

Jounce Clearance (mm)

Jounce/Rebound Clearance

Jounce Clearance Fender Tire Small Car 50 mm Large Car 90 mm Big SUV 120mm Truck 150mm

Suspension Loads

Parameters that affect Dynamic 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

Stress Calculation

Shock Absorber Tube Cross-section with area A Shock absorber load from suspension load calculation P max Peak stress = P max /A