TABLE OF CONTENTS The term fracture toughness refers to the fracture resistance of the material. It can be evaluated by various more or less quantitative tests [FRA 07]. However, more specifically, fracture toughness concerns the resistance to crack propagation. It is measured by the energy needed to increase the crack area by one unit. An essential question is to know whether this quantity is a characteristic of the material, independent of the shape and size of the crack itself and of the part in which it is located. This is indeed the case if the zone close to the crack tip, where the stresses and strains are maximum, is autonomous.
It is known that this is the case in linear elasticity, the asymptotic stress and strain fields at the tip of a perfectly sharp crack depending on the loading on the part, on the crack size and on their shape through a single parameter: the stress intensity factor K. Under those conditions, provided the stress intensity factors are the same, the crack tip sees identical environments in a test piece and in any part. The test results are transferable. The measurement of fracture toughness should not raise great problems. However, the difficulty arises from the necessity of avoiding too large perturbations with respect to the ideal conditions of linear elastic fracture mechanics; these conditions are never perfectly achieved because irreversible deformations at the crack tip are unavoidable. The standards specify conditions to be respected in order that the linear elasticity...
