Fracture of any material (be it a recently acquired child's toy or a nuclear pressure vessel) is generally an undesirable happening, resulting in economic loss, an interruption in the availability of a desired service, and, possibly, damage to human beings. Besides, one has good, technical reasons to do fracture testing: to compare and select the toughest (and most economical material) for given service conditions; to compare a particular material's fracture characteristics against a specified standard; to predict the effects of service conditions (e.g., corrosion, fatigue, stress corrosion) on the material toughness; and to study the effects of microstructural changes on material toughness. One or more of these reasons for fracture testing may apply during the design, selection, construction, and/or operation of material structures. There are two broad categories of fracture tests; qualitative and quantitative. The Charpy impact test exemplifies the former, and the plane-strain fracture toughness (KIc) test illustrates the latter. We describe briefly important tests in both of these categories.
We saw in Chapter 7 that stress concentrations, like cracks and notches, are sites where failure of a material starts. It has been long appreciated that the failure of a given material in the presence of a notch is controlled by the material's fracture toughness. Many tests have been developed and standardized to measure this “notch toughness” of a material. Almost all are qualitative and comparative in nature.