TABLE OF CONTENTS It is intuitively obvious that a large crack is more severe than a small crack. A basic requirement for the study of the behaviour of cracked bodies is to express this numerically. The presence of a crack dominates the stress field in the vicinity of the crack tip, and some results are not intuitively obvious.
The key concept of stress intensity factor, for Mode I and for Mode II, arises from a two dimensional linearly elastic analysis for a straight crack. This follows the usual methods of elastic stress analysis in which strains and distortions are assumed to be small, and conditions of equilibrium and compatibility must be satisfied (Gere and Timoshenko 1991). The use of a two dimensional analysis simplifies the mathematics, and also simplifies some descriptions. For example, what is meant by crack length is unambiguous. Mode III is not possible in two dimensions, so for this mode a quasi two dimensional anti plane analysis is used.
Stress intensity factors may be used to characterise the mechanical properties of cracked specimens in just the same way that stresses are used to characterise the mechanical properties of uncracked specimens. They help to quantify the rather elusive concept of a material s toughness. One convenient definition is this: resistance to crack propagation (including fatigue crack propagation). For example the fracture toughness, K c, of a metallic material may be defined as the value of the Mode I stress intensity factor, K I, for...
