


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