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Engineering Mechanics
Crack theory and the failure of materials
This unit of work addresses aspects of the following syllabus outcomes
The student:
H1.2 differentiates between properties of materials and justifies the selection of materials, components and processes in engineering,
H2.1 determines suitable properties, uses and applications of materials in engineering.
Source: Stage 6 Engineering Studies syllabus, page 30. Board of Studies NSW, 1999.
Introduction
Failure 
of an engineering component can be initiated by inexact information, or
inadequate consideration and handling of one or more stages of the design
process. The complexity of a typical engineering system design means failure
can manifest itself a number of different ways, the consequences of which vary
from a minor irritation to the catastrophic collapse of the component, product
sales, and even the company. It often results in injury or death to
individuals.
Failure may be manifested in a number of ways:
- Structural inadequacy
- Faults in human-system interaction
- Inadequate management of the design process, or
- Inaccurate assessment of market
wants/needs.(James )
Types of Failure
Failure
of material can be divided into two classes: buckling and fracture.
Buckling causes
a lateral bend when an object is subjected to a compressive load as shown.
Buckling results in a failure of the structure, but not necessarily a catastrophic
failure within the material it is made from.
Types of fracture
Fracture causes a physical separation, or tearing, of the material, through either an internal or external crack. There are two basic types of fracture each of which is illustrated below:
- ductile fracture, and
- brittle fracture.
Fracture occurs due to stress concentrations at flaws like surface scratches, weld strikes and voids.
Ductile 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:
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 when it fractures, 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. Brittle fracture mostly results in catastrophic failure of a structure
and is influenced by: defects, fatigue, stress-corrosion, and hydrogen
embrittlement.
Crack propagation
Crack propagation will occur if the strain energy released provides enough energy required for the crack to grow. Cracks create new surfaces along the edges where the material has split apart. These new surfaces will not form if there is not enough energy available to create them in the material.
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. For example, as the crack grows the amount of material remaining will be less, and so the energy of resistance of the material will also become less.
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.
A force needs to be applied to a structure to initiate fracture. Forces may be caused by temperature variations, chemical reactions, dead, and live loads.
There are three modes of crack propagation, which describe the way forces act, and the type of fracture created. These can occur independently or in combination. The latter is termed mixed-mode cracking.
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Mode I: Opening or tensile Forces act perpendicular to the crack. The crack is pulled open. |
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Mode II: In-plane shear or sliding Forces are parallel to the crack. The crack slides along itself. |
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Mode III: Out-of-plane shear or pushing (pulling) Forces are perpendicular to the crack. The crack tears apart. |
Activity
Read the information contained in the links attached to this page (see failure and buckling above) and then answer the questions set out below.
- List the factors that can influence failure in an engineering component?
- Distinguish between the terms buckling and fracture.
- Describe the characteristics of each form of fracture.
- Explain how are cracks propagated?
- Describe and discuss the factors that contributed to the failure of:
Reference material
Buckling image
Cracking of Dams
Fracture Mechanics
Yielding fracture mechanics
Manifestation of failure and Comet aircraft,
James, M. N., Failure as a Design
Criterion
Gordon, J. E. (1978) Structures: or why
things don’t fall down Part 4). Plenum: New York.
Advanced
Reference for Teachers
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