Fundamentals of Durability Page 1
Your single provider of solutions System simulation solutions 3D simulation solutions Test-based engineering solutions Engineering services - Deployment services Troubleshooting interventions Development support from concept to final validation Sharing know-how and best practices Extensive trainings From troubleshooting to Design-Right-First-Time Page 2
Durability Agenda Durability Basics Fatigue, Stress, Strain Load Determination Measurements, Multi-Body Simulation Loads and Damage S-N Curve, Cycle Counting Load Characterization Establishing Durability Targets: Superposition, Extrapolation Fatigue Life Predictions Infinite Life, Stress Life, Strain Life Accelerated Testing & Analysis RP-Filter, Mission Synthesis Page 3
Durability Engineering in Product Development Troubleshoot Validate Cost of Change Engineer Concept Detail Drawing Prototype Production Field Failure Page 4
Durability Why is it important? Warranty Costs High rate of return is large liability Example: Heavy truck Competitive Advantage Reputation for reliability Example: Longest lasting appliance, safest aircraft Performance Over-engineering reduces performance Example: Fuel economy on heavy car Page 5
The Durability Process Acquisition Analysis Simulation Shaker testing Sign-off Page 6
Energy Wind turbine blade failure / structural failure There is a general trend upward in accident numbers over the past 10 years. Blade failure 24 accidents in 2009 Structural failure 15 accidents in 2009 Page 7
Civil construction 2007 Minneapolis' I-35 Bridge Collapse (2007) Kenneth Russell, professor MIT, suspects metal fatigue could be a contributing factor The bridge was very near to the fatigue limit and had gone through many cycles," he says. Page 8
Turbine Blade Failure Page 9
Wall Page 10
Helicopter Page 11
Tacoma Narrow Bridge Page 12
What is fatigue? Versailles rail crash (1842) Page 13
What is fatigue? Woehler (1870) railroad axles Cyclic stress range Can be more important than peak stress Page 14
Famous People In Fatigue Sir Robert Hooke British (1635-1703) Hooke s Law of Elasticity in 1660 Goodman English (1869-1942) Goodman s Rule in 1899 MA Miner English (1915-1978) Miner s Rule in 1945 August Wohler German (1819-1914) Wohler curves in 1867 Richard Von Mise Austrian (1883-1953) Theory of Plasticity 1913 Tatsuo Endo Japan (1925-1989) Rainflow Counting in 1968 Restricted Siemens AG 2013 All rights reserved. Smarter decisions, better products.
Durability Agenda Durability Basics Fatigue, Stress, Strain Load Determination Measurements, Multi-Body Simulation Loads and Damage S-N Curve, Cycle Counting Load Characterization Establishing Durability Targets: Superposition, Extrapolation Fatigue Life Predictions Infinite Life, Stress Life, Strain Life Accelerated Testing & Analysis RP-Filter, Mission Synthesis Page 16
Dynamic versus Static Failures Cyclic Fatigue refers to gradual degradation and eventual failure that occurs under loads which vary with time, and which are lower than the static strength of the metallic specimen, component or structure concerned. The Static Strength is the load which causes failure in one application. Page 17
Static Strength Aircraft Wing Bending Test Bend till Break Page 18
Dynamic (Cyclic) Loading Dynamic Loads below Static Strength vary with time Page 19
Definitions Durability is the ability of something to perform its function longlasting and repeatedly. Failure is Industry specific. For example: Crack growth versus crack initiation Page 20
Page 21
Crack Growth Fatigue Failures in Real Life Aloha airlines flight #243 April 28 th, 1988 Maui, HI Fatigue failure occurred due to repeated pressurization of the cabin causing a small crack to rupture in the fuselage, killing a stewardess. 22 Unrestricted copyright LMS Siemens International AG 2014-2010 Page 22 Siemens PLM Software
Page 23
Definitions Durability is the ability of something to perform its function longlasting and repeatedly. Failure is Industry specific. For example: Crack growth versus crack initiation Fatigue is the progressive and localized structural damage that occurs when a material is subjected to cyclic loading. Stress and strain are used to calculate fatigue damage. Damage Measure of fatigue. When = 1 by Miner s Rule, failure occurs. Fatigue Life Inverse of damage (Example: 0.5 damage, is fatigue life of 2) Page 24
Stress σ = F n /A F n How to reduce Stress? Either: Increase Area Reduce Force A * Normal Stress Page 25
Stress σ = F n /A F n How to reduce Stress? Either: Increase Area Reduce Force A * Normal Stress Page 26
Stress σ = F n /A F n Either: Increase Area Reduce Force * Normal Stress Reduced crosssectional area causes stress concentration A Effective Crosssectional area Page 27
What about Simulation? Finite Element Models are used F n When Fn=1: referred to as a Static Unit Load Case Stress is calculated at each element as opposed to a predetermined location because real geometry is more complex than this... * Normal Stress Page 28
Strain l o = original length Page 29
Strain Strain: ε = dl / l o dl = change in length l o = original length Page 30
Stress and Strain: Hooke s Law Young s Modulus σ = E ε E = Young s Modulus F n A Stress E is slope l o = original length Strain Page 31
Famous People In Fatigue Sir Robert Hooke British (1635-1703) Hooke s Law of Elasticity in 1660 Goodman English (1869-1942) Goodman s Rule in 1899 MA Miner English (1915-1978) Miner s Rule in 1945 August Wohler French (1819-1914) Wohler curves in 1867 Richard Von Mise Austrian (1883-1953) Theory of Plasticity 1913 Tatsuo Endo Japan (1925-1989) Rainflow Counting in 1968 Restricted Siemens AG 2013 All rights reserved. Smarter decisions, better products.
Stress and Strain: Hooke s Law Young s Modulus σ = E ε E = Young s Modulus Ultimate Strength Stress Yield Strength E is slope Rupture Strain Time Lapsed Video Page 33
Static Stress and Strain Relationship A - Red Fixed Area (Engineering Stress) B - Blue Changing Area (True Stress) 1. Necking occurs, applied load decreases 2. Plastic Deformation Begins 3. Fracture Occurs 4. Strain hardening region 5. Necking Page 34 1 Ultimate Strength 2 Yield Strength 3 Fracture Strength
Materials Terms Creep: is a time-dependent deformation of a material while under an applied load that is below its yield strength Hardness: is the resistance of a material to localized deformation Toughness: the ability of a metal to deform plastically and to absorb energy in the process before fracture Yield strength or yield point: of a material is defined in engineering and materials science as the stress at which a material begins to deform plastically. Ductility: is a solid material's ability to deform under tensile stress The following list ranks metals from the greatest ductility to least: gold, silver, platinum, iron, nickel, copper, aluminum, zinc, tin, and lead The ductility of steel varies depending on the alloying constituents. Increasing levels of carbon decreases ductility Brittle: A material when subjected to stress, it breaks without significant deformation (strain) Ultimate tensile strength (UTS), often shortened to tensile strength (TS) or ultimate strength, is the maximum stress that a material can withstand while being stretched or pulled before necking. Point at which load on specimen decreases Page 35
Graphical Representation of Material Terms Page 36
Some Material Properties and Failure Modes Page 37
Definition of the Stress Ratio R σ stress ratio R= σ lower σ upper 0 σ m = 0 R = -1 σ l = 0 R = 0 σ u = 2 σ l R = 0.5 σ u = 0 R = - σ l = 2 σ u R = 2 t Page 38
Stress Ratio R Plotted On the Haigh Diagram Page 39
Dynamic Stress/Strain Test Page 40
Dynamic Fatigue: Crack initiation and crack growth Page 41
What influences fatigue? Geometry Material Loads Fatigue Fatigue Life Page 42
What influences fatigue? Geometry: Load Configuration Notch Severity Local Stress State Material : Surface Finish Residual Stresses Basic Properties Loads Load level Uni-axial/multi-axial Constant/variable amplitude Fatigue Fatigue Life Page 43
Applied loading vs. structural strength Optimal Design Minimal Overlap Affordable Cost probability density LOADS Material + Geometry (structural strength) more scatter less scatter strength criterion failure: loading > strength Page 44
Sources of Fatigue Scattering load usage strength typical load ratios of 10% / 90% probability manufacturing geometry 1.02 failure probability: 10% 50% 90% life material - controlled 1.15 - different welds 1.45 loads (car) 2.00 Today, customer usage is the most important source of fatigue scattering. Page 45
When were your durability test schedules established? Applied loading vs. structural strength Wider - Over-Design - More expensive probability density LOADS more scatter Material + Geometry (structural strength) less scatter strength criterion Page 46
When were your durability test schedules established? Applied loading vs. structural strength probability density NOT SO GREAT DESIGN! LOADS Material + Geometry (structural strength) more scatter less scatter strength criterion Page 47