Engineering Studies
Home > Engineering Studies > Civil Structures > Crack Theory
Engineering Mechanics
Crack theory and the failure of material
Suggested answers
- List the factors that can influence failure in an engineering component?
- Structural inadequacy
- Faults in human-system interaction
- Inadequate management of the design process
- Inaccurate assessment of market wants/needs
- Distinguish between the terms buckling and fracture.
- Buckling causes a lateral bend when an object is subjected to a compressive load as shown.
- Fracture causes a physical separation, or tearing, of the material, through either an internal or external crack.
- Describe the characteristics of each form of fracture.
- Ductile fracture is characterised by
plastic deformation that precedes failure of the part. Ductility is usually
understood to mean the ability of a material to accept large amounts of
deformation (mainly tensile) without fracture. It is the antithesis of
brittleness.
Ductile fracture may be influenced by the following factors:- transition temperature (or Nil Ductility Transition temperature),
- inclusions,
- and strain hardening.
- Brittle fracture occurs with little or no
gross plastic deformation occurring in the component. It is characterised by
the very small amount of energy, which is absorbed, and by the crystalline
appearance of the surfaces of the fracture (break). Once a fracture has been
initiated in a large plate, for example, it will be propagated (multiplied) at
a velocity, approaching that of the speed of sound in the material. Such
fractures often have a ‘chevron’ (inverted ‘V’ shape)
appearance due to the formation of the crack (fissure) front inside the
material, which moves ahead of the surface crack. Brittle fracture mostly
results in catastrophic failure.Brittle fracture is influenced by: defects,
fatigue, stress-corrosion, and hydrogen embrittlement.
- Ductile fracture is characterised by
plastic deformation that precedes failure of the part. Ductility is usually
understood to mean the ability of a material to accept large amounts of
deformation (mainly tensile) without fracture. It is the antithesis of
brittleness.
- How are cracks propagated?
Crack propagation will occur if the strain energy released provides enough energy required for crack to grow. Cracks create new surfaces along the edges where the material has split apart. These new surfaces will not form if the material does not have enough energy.
If G is the energy required for crack growth, and R is a material's resistance to crack growth, the condition for a crack to begin is G = R. As the crack grows the resistance of the material will vary.
Cracks will continue to grow while the change in energy is equal to the change of resistance in the material.
If G < R the crack will not continue to grow.
If G > R the crack will grow in an unstable manner until the material fails.
- Describe and discuss the factors that contributed to the failure of:
- comet airliners, and
- liberty ships.
The fatigue crack was associated with the stress concentrations of the ‘square’ rear ADF window cutout.
Causes:- New technology introducing new load cases – high altitude flight for turbojet engines requiring cabin pressurisation.
- Mismatch between service loads and fatigue test procedures.
- Possible contribution from out-of-plane bending loads.
Failures could be attributed to the:- all-welded construction which eliminated crack-arresting plate boundaries which are present in riveted joints,
- presence of crack-like flaws in welded joints performed by inexperienced operators pressed into service by the exigencies of the programme, and
- use of materials whose low resistance to crack advance (toughness) was further reduced by low temperatures (NDT temperature).
|