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Bruce Mayer, PE Registered Electrical & Mechanical Engineer BMayer@ChabotCollege

Engineering 45. Material Failure (2). Bruce Mayer, PE Registered Electrical & Mechanical Engineer BMayer@ChabotCollege.edu. Learning Goals.1 – Failure. How Flaws In A Material Initiate Failure How Fracture Resistance is Quantified How Different Material Classes Compare

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Bruce Mayer, PE Registered Electrical & Mechanical Engineer BMayer@ChabotCollege

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  1. Engineering 45 MaterialFailure (2) Bruce Mayer, PE Registered Electrical & Mechanical EngineerBMayer@ChabotCollege.edu

  2. Learning Goals.1 – Failure • How Flaws In A Material Initiate Failure • How Fracture Resistance is Quantified • How Different Material Classes Compare • How to Estimate The Stress To Fracture • Factors that Change the Failure Stress • Loading Rate • Loading History • Temperature Last Time

  3. Learning Goals.2 – Failure • FATIGUE Failure • Fatigue Limit • Fatigue Strength • Fatigue Life • CREEP at Elevated Temperatures • Incremental Yielding at <y Over a Long Time Period at High Temperatures

  4. Fatigue Defined • ASTM E206-72 Definition The Process of PROGRESSIVE LOCALIZED PERMANENT Structural Change Occurring in a Material Subjected to Conditions Which Produce FLUCTUATING Stresses and Strains at Some Point or Points Which May Culminate in CRACKS or Complete FRACTURE After a Sufficient Number of Fluctuations

  5. Caused by Load-Cycling at <y Brittle-Like Fracture with Little Warning by Plastic Deformation May take Millions of Cycles to Failure Fatigue Failure • Crack Initiation Site(s) • “Beach Marks” Indicate of Crack Growth • Distinct Final Fracture Region • Fatigue Failure Time-Stages

  6. Recall Fatigue Testing (RR Moore Tester) compression on top specimen counter motor bearing bearing flex coupling tension on bottom s s max S s m s time min Fatigue Parameters • Stress Varies with Time; Key Parameters • m  Mean Stress (MPa) • S  Stress Amplitude (MPa) • Failure Even thoughmax < c • Cause of ~90% of Mech Failures

  7. More Fatigue Parameters • σmax = maximum stress in the cycle • σmin = minimum stress in the cycle • σm = mean stress in the cycle = (σmax + σmin)/2 • σa = stress amplitude = (σmax - σmin)/2 • Δσ = stress range = σmax - σmin = 2σa • R = stress ratio = σmax/σmin

  8. Fatigue (Endurance) Limit, Sfat in MPa Unlimited Cycles if S < Sfat S = stress amplitude case for unsafe steel (typ.) S fat safe 3 5 7 9 10 10 10 10 N = Cycles to failure S = stress amplitude case for Al (typ.) unsafe safe 3 5 7 9 10 10 10 10 N = Cycles to failure Fatigue Design Parameter • Some Materials will NOT permit Limitless Cycling • i.e.; Sfat = ZERO

  9. Fatigue Cracks Grow INCREMENTALLY during the TENSION part of the Cycle Math Model for Incremental Crack Extension typ. 1 to 6 increase in crack length per loading cycle Factigue Crack Growth Opening-Mode (Mode-I) Stress Intensity Factor • Example: Austenitic Stainless Steel

  10. Impose a Compressive Surface Stress (to Suppress Surface cracks from growing) S = stress amplitude near zero or compressive, m moderate tensile, m larger tensile, m N = Cycles to failure Improving Fatigue Performance • Method 1: shot peening • Method 2: carburizing (interstitial) • Remove Stress-Concentrating sharp corners better bad bad better

  11. Creep Deformation • Creep Defined HIGH TEMPERATURE PROGRESSIVE DEFORMATION of a material at constant stress.  High temperature is a relative term that is dependent on the material(s) being evaluated. • For Metals, Creep Becomes important at Temperatures of About 40% of the Absolute Melting Temperature (0.4Tm)

  12. In a creep test a constant load is applied to a tensile specimen maintained at a constant temp. Strain is then measured over a period of time Typical Metallic Dynamic Strain at Upper-Right Creep: ε vs t Behavior • Stage-1 → Primary • a period of primarily transient creep. During this period deformation takes place, and StrainHardening Occurs

  13. Stage-II → Steady State Creep a.k.a. Secondary Creep Creep Rate, dε/dt is approximately Constant Strain-Hardening and RECOVERY Roughly Balance Stage-III → Tertiary Creep Creep: εvs t Behavior cont.1 • a reduction in cross sectional area due to necking, or effective reduction in area due to internal void formation • Creep Fracture is often called “Rupture”

  14. Most of Material Life Occurs in this Stage Strain-Rate is about Constant for Given T & σ Work-Hardening Balanced by Recovery The Math Model Secondary Creep • Where • K2  A Material-Dependent Constant • σ The Applied Stress • n  A Material Dependent Constant • Qc  The Activation Energy for Creep • R  The Gas Constant • T  The Absolute Temperature

  15. Occurs Along Grain Boundaries g.b. cavities 100 2 0 applied stress Stress, ksi 10 data for S-590 Iron 1 12 16 20 24 28 3 24x103 K-log hr L(10 K-log hr) temperature function of applied stress 1073K Ans: tr = 233hr time to failure (rupture) Creep Failure • Estimate Rupture Time • S590 Iron, T = 800 °C, σ= 20 Ksi • The Time-to-Rupture Power-Law Model

  16. P Al2014-T6 σm =5 ksi 0.60” P WhiteBoard Work • Problem 8.17 • Ø 0.60” 2014-T6 Al Round bar • Cyclic Axial Loading in Tension-Compression • Design Life, N = 108 Cycles • σmean = 5 ksi • S-N per Fig 8.34 • Find Loads: Pmax, Pmin • See NEXT Slide

  17. S-N Data for 2014-T6 Al 19.5 ksi

  18. Creep Test Instrument

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