Chapter 14: Brittle Fracture & Impact Testing

The text provides a comprehensive overview of notched-bar impact testing, specifically detailing the Charpy and Izod tests used to measure the energy absorbed during fracture and to determine the ductile-to-brittle transition temperature, a vital parameter for designing safe structures. It examines the distinct transition temperature behaviors of different crystal structures, noting that while face-centered cubic metals often retain toughness at low temperatures, body-centered cubic metals typically exhibit a sharp transition from ductile to brittle behavior. A significant portion of the chapter analyzes metallurgical factors influencing notch toughness, such as the detrimental effects of carbon, phosphorus, and oxygen, contrasted with the beneficial effects of manganese, nickel, and grain refinement. The discussion extends beyond standard laboratory tests to large-scale fracture testing methods, including the Pellini drop-weight test and the explosion-crack-starter test, which help define the Nil Ductility Temperature (NDT). These concepts are integrated into the Fracture Analysis Diagram (FAD), a design tool that correlates stress, temperature, and flaw size to identify safe operating limits like the Crack Arrest Temperature (CAT). Furthermore, the summary covers complex failure mechanisms such as temper embrittlement in alloy steels, which occurs due to impurity segregation at grain boundaries, and environment-sensitive fractures including hydrogen embrittlement, stress-corrosion cracking, and liquid-metal embrittlement. Finally, the chapter addresses material behavior under very rapid loading rates, exploring the propagation of elastic and plastic stress waves, shock waves, and the phenomenon of spalling caused by wave reflection.