The Role of Fracture Mechanics in Design Technology

Fracture mechanics has in recent years become an independent discipline that deals with determining the conditions under which machine or structural elements attain uncontrollable failure by crack propagation. A knowledge of these conditions can assist the designer to safeguard structures against catastrophic fracture. In contrast to the conventional approach, which does not account for flaws initiated in the material by manufacturing procedures, overloads, or fatigue loadings, fracture mechanics [1] assumes that all materials contain cracks from which failure starts. This concept has been used successfully for high-strength/low-toughness materials design and for structures that exhibit brittle behavior. Obtained from laboratory specimens loaded symmetrically with respect to the crack plane is a critical stress intensity factor parameter K_1c. It is a characteristic of the material commonly referred to as the fracture toughness value. When machine elements are subjected to combined loading, where symmetry does not exist, the direction of crack initiation is no longer known as an a priori. The condition of crack instability can then be predicted from the strain energy density factor S whose critical value, S_c, is related to K_1c from uniaxial tension tests by the relation S_c = (1 − 2ν)K_1c²/4 π μ, with ν being the Poisson’s ratio and μ the shear modulus of elasticity. Numerous numerical examples involving press fit, rotating disk, thermally stressed pipe, pressure vessel, etc., are presented to show how fracture mechanics can be used for estimating the load that a member can sustain without causing unstable fracture. The results are compared with those obtained from the conventional design approach whenever possible.