Create Presentation
Download Presentation

Download

Download Presentation

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

460 Views
Download Presentation

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

- - - - - - - - - - - - - - - - - - - - - - - - - - - E N D - - - - - - - - - - - - - - - - - - - - - - - - - - -

**FRACTURE, FAILURE AND FATIGUE**• Catastrophic failure in materials resulting from • crack development**FRACTURE, FAILURE AND FATIGUE**• Catastrophic failure in materials resulting from • crack development • Fracture mechanics • Failure analysis**Rail track performance**Gauge corner cracking**FATIGUE FAILURE**• Comet airliner disasters of 1954**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**Griffith Alan Arnold**1893-1963 Griffith: Phil Trans Roy Soc 1921, v221, 163-198**GRIFFITH FRACTURE THEORY**• Griffith found • tensile strength of freshly drawn glass fibres is much greater than that of old or aged fibres**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**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**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**GRIFFITH FRACTURE THEORY**• Stress concentration around a hole or cavity**Uniaxial tensile stress**S -S 3**GRIFFITH FRACTURE THEORY**• Stress concentration around a hole or cavity**GRIFFITH FRACTURE THEORY**• Stress concentration around a hole or cavity**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**GRIFFITH FRACTURE THEORY**• In brittle materials, catastrophic crack propagation • when critical stress is exceeded at crack tip**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**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**FATIGUE**• Failure of materials subject to CYCLIC or REPETITIVE loading**S/N curve**Wöhler curve**For complex stress loadings:**Miner’s Rule [Palmgren-Miner] Linear cumulative damage hypothesis**Fatigue striations in aluminium alloy**Callister**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**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