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 Griffith: Phil Trans Roy Soc 1921, v221,

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 -S-S

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

Ashby & Jones I, ch 17

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