Durability Definition of durability and reliability warrantee Examples

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 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. 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 • • Majority of wear out failures (>>90%) occur outside the 10 yr/150 K mile target Wear out Failure (IFR) “Trouble-Free Life” Target Time in Service (10 yr/150 K 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 • • • Wear out Failure (IFR) “Trouble-Free Life” Target (10 yr/150 K 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. 0 G lateral acceleration • Salt Bath Track includes: Muddy terrain and salt spraying facility

Description of Fatigue Failure Force , F Fixed Load Fixed F N 0 Cycles, N

S-N Curve for Metals (Engg. ) Stress Range, KSI S-N Curve for SAE 1010 Steel Fatigue Life, Cycles

Notes of Fatigue Life Ø 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. Ø 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 Ø 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 Ø For steel structures, a fatigue crack needs to be 1 mm long before it propagates; scratches and nicks don’t grow. Ø 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 Ø Variability in Loading is much more critical for accuracy in estimating Fatigue Life, than variability in Material Strength. Ø Mean Stress Effect - Tensile Mean Stresses reduce Fatigue Life or decrease the allowable Stress Range. Ø 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

Cyclic Load Actual Service Loads & Histogram Time Load Histogram Cycles

Fatigue Damage Calculation Stress S 1 S 2 S 3 S 4 N 1 N 2 N 3 N 4 S 5 N 5 Stress Histogram S 6 N 6 Cycles Stress S-N Curve for Metal 6 Damage D = Σ N(σi)/Ni 1 And D < 1 Cycles

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

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

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 Pmax Peak stress = Pmax/A

Fatigue Damage Calculation Stress S 1 S 2 S 3 S 4 N 1 N 2 N 3 N 4 S 5 N 5 Stress Histogram S 6 N 6 Cycles Stress S-N Curve for Metal 6 Damage D = Σ N(σi)/Ni 1 And D < 1 Cycles

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