FRACTURE, FAILURE AND FATIGUE

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

4
(No Transcript)
5

• FATIGUE FAILURE
• Comet airliner disasters of 1954

6

• GRIFFITH FRACTURE THEORY
• A A Griffiths 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

17
(No Transcript)
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
25
(No Transcript)
26
(No Transcript)
27
(No Transcript)
28
(No Transcript)
29
(No Transcript)
30
For complex stress loadings Miners Rule
Palmgren-Miner Linear cumulative damage
hypothesis
31
(No Transcript)
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