Impact Toughness Testing of Metals...

Impact Toughness Testing of Metals Metals undergo dynamic fracture under rapidly applied loads which are generally produced by impact or by explosive detonation. In comparison to quasi-static loading, dynamic conditions involve loading rates which are higher than those encountered in conventional tensile testing or fracture mechanics testing. Dynamic fracture includes two cases namely (i) a stationary crack subjected to a rapidly applied load, and (ii) a rapidly propagating crack under a quasi-stationary load. In both the cases the material at the crack tip is strained rapidly and, if rate sensitive, can offer less resistance to fracture than at quasi-static strain rates. As an example, values for dynamic fracture toughness are lower than those for static toughness as experienced in the testing of low carbon steels at different temperatures. Many structural components are subjected to high loading rates in service. They also are to survive high loading rates during accident conditions. This makes high strain rate fracture testing is of interest and components are to be designed against crack initiation under high loading rates or designed to arrest a rapidly running crack. Also, since dynamic fracture toughness is normally lower than static toughness, more conservative analysis requires consideration of dynamic toughness. Measurement and analysis of fracture behaviour under high loading rates is more complex than under quasi-static conditions. There are also several different test methods which are used in the evaluation of dynamic fracture resistance. Test methods based on fracture mechanics produce quantitative values of fracture toughness parameters which are useful in design. However, several qualitative methods are also been used in the evaluation of impact energy to break a notched bar, percent of cleavage area on fracture surfaces, or the temperature for nil ductility or crack arrest. These qualitative tests include methods such as...