Fatigue fracture toughness of metals

Abstract

A B S T R A C T The paper considers the peculiarities of fatigue crack propagation and final fracture of metals under cyclic loading. It is shown that the value of the fatigue fracture toughness of steels in an embrittled state is appreciably lower than that of the fracture toughness under static loading. A model of the transition from stable to unstable fatigue crack propagation is justified.Keywords crack jumps; fatigue; fracture toughness. I N T R O D U C T I O NThe analysis of fractures of machine parts and structures has revealed that in most cases, their fracture is due to material fatigue, which is known to be responsible for the initiation and propagation of fatigue cracks in the course of cyclic loading. Such cracks ultimately lead to complete failure of a component. In the literature, there are many reported investigations related to the fatigue crack development. The peculiarities of the fatigue crack growth at the stage preceding final fracture are the least studied. Knowledge of this behaviour is of particular importance when final fracture of a component is by a brittle mode and with a fatigue crack of small size. In this case, an important characteristic is the fatigue fracture toughness, which is determined directly under cyclic loading. The paper presents a brief account of the results of studying this problem. First experimental results were published in Refs [1,2], a generalization of the results of investigations of many years was performed in Refs [3,4] and the results of studying this problem by other authors are presented in Refs [5][6][7] and in other works. E X P E R I M E N T A L P R O C E D U R E A N D M A T E R I A L SInvestigations involved testing of compact tension specimens, 7.5-150 mm in thickness, on electrohydraulic machines of various capacities. The machines were equipped with systems for specimen cooling and heating that provided variation of the test temperature from 77 to 623 K. The specimen preparation, crack growing procedures and Correspondence: V.T. Troshchenko. E-mail: vtt@ipp.kiev.ua processing of results were in conformity with the accepted standards. Cyclic loading of specimens was performed at a constant load, with the loading frequency varying in the range from 10 to 15 Hz. The stress intensity factors were calculated with the use of the formulaewhere P is the load, a is the crack size, t is the specimen thickness and w is the specimen width. The plane strain conditions and the size of the plastic zone at the crack-tip were determined by the following formulae:2r y = 1 3πwhere σ 0.2 is the material offset yield strength. The materials under study were different heat-resistant steels used for manufacturing high-pressure vessels, including those for nuclear reactors, high-strength chromemolybdenum steels, ductile austenitic steel and titanium alloys. The mechanical properties and fracture toughness characteristics of some of the materials studied are shown in Table 1. Heat-resistant steel II was subjected to a special heat treatment that simulated radiation embritt...