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
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
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