FRACTURE, FAILURE AND FATIGUE Catastrophic failure in materials resulting from crack development

1. FRACTURE, FAILURE AND FATIGUE • Catastrophic failure in materials resulting from • crack development

2. FRACTURE, FAILURE AND FATIGUE • Catastrophic failure in materials resulting from • crack development • Fracture mechanics • Failure analysis

3. Rail track performance Gauge corner cracking

5. FATIGUE FAILURE • Comet airliner disasters of 1954

6. GRIFFITH FRACTURE THEORY • A A Griffith’s work (around 1920) on the tensile strength of glass fibres is a classic of materials science. • See J E Gordon The new science of strong materials • [several editions] for an excellent popular account

7. Griffith Alan Arnold 1893-1963 Griffith: Phil Trans Roy Soc 1921, v221, 163-198

8. GRIFFITH FRACTURE THEORY • Griffith found • tensile strength of freshly drawn glass fibres is much greater than that of old or aged fibres

9. GRIFFITH FRACTURE THEORY • Griffith found • tensile strength of freshly drawn glass fibres is much greater than that of old or aged fibres • tensile strength decreased with fibre length

10. GRIFFITH FRACTURE THEORY • Griffith found • tensile strength of freshly drawn glass fibres is much greater than that of old or aged fibres • tensile strength decreased with fibre length • tensile strength is very variable from sample to sample … much statistical variance

11. GRIFFITH FRACTURE THEORY • Griffith concluded that • fibres are weakened by microscopic flaws on the surface or the interior of the fibre • … and more generally in brittle materials of any kind

12. GRIFFITH FRACTURE THEORY • Stress concentration around a hole or cavity

13. Uniaxial tensile stress S

14. Uniaxial tensile stress S -S 3

15. GRIFFITH FRACTURE THEORY • Stress concentration around a hole or cavity

16. GRIFFITH FRACTURE THEORY • Stress concentration around a hole or cavity

18. GRIFFITH FRACTURE THEORY • Griffith then considered the stress distribution around a crack • The steps in the argument are: • In a material under stress, stress increases in the vicinity of a crack • The crack may be stable or unstable • The condition for stability can be written down in terms of the strain energy recovered and surface energy of the crack

19. GRIFFITH FRACTURE THEORY • In brittle materials, catastrophic crack propagation • when critical stress is exceeded at crack tip

20. GRIFFITH FRACTURE THEORY • In brittle materials, catastrophic crack propagation • when critical stress is exceeded at crack tip • In non-brittle (TOUGH) materials, local stress concentrations can be relieved by plasticity/yield

21. GRIFFITH FRACTURE THEORY • In brittle materials, catastrophic crack propagation • when a critical stress is exceeded at crack tip • In non-brittle (TOUGH) materials, local stress concentrations can be relieved by plasticity/yield • TOLERANCE OF DEFECTS, FLAWS, CRACKS, • NOTCHES

22. FATIGUE • Failure of materials subject to CYCLIC or REPETITIVE loading

23. S/N curve Wöhler curve

24. Ashby & Jones I, ch 17

30. For complex stress loadings: Miner’s Rule [Palmgren-Miner] Linear cumulative damage hypothesis

32. Fatigue striations in aluminium alloy Callister

33. GRIFFITH FRACTURE THEORY • In brittle materials, catastrophic crack propagation • when critical stress is exceeded at crack tip • In non-brittle (TOUGH) materials, local stress concentrations can be relieved by plasticity/yield

34. GRIFFITH FRACTURE THEORY • In brittle materials, catastrophic crack propagation • when a critical stress is exceeded at crack tip • In non-brittle (TOUGH) materials, local stress concentrations can be relieved by plasticity/yield • TOLERANCE OF DEFECTS, FLAWS, CRACKS, • NOTCHES