Lifetime prediction in creep-fatigue environment
The creep-fatigue interaction has been studied and innovative mathematical models are proposed to predict the operating life of aircraft components, specifically gas turbine blades subject to creep-fatigue at high temperatures. The historical evolution of the creep-fatigue lifetime prediction is given in order to place the present study in the context. A literature review of the life estimation under creep-fatigue environment is presented.
The Effect of Geometry on Crack Formation
Publisher Summary This chapter discusses the effect of geometry on crack formation. To properly assess the fatigue integrity of components, the local material behavior must be understood, both from the point of view of the number of cycles required to initiate a crack (the crack formation life) and the subsequent crack growth, while the crack is still within the influence of the geometry of a stress concentrator. A fundamental approach necessitates the ability to determine precisely the division between nucleation and Stages I and II crack propagation. To estimate the crack formation life of a component, it is necessary to relate the conditions at the critical region to known material behavior and allow for influencing factors. If it is assumed that the conditions in a component can be represented by tests on plain specimens of the type used in obtaining fatigue data, this may provide the basis for assessing crack formation life. Two possible approaches are at present available for evaluating the strain at a concentration feature, namely, the Neuber and the Hardrath-Ohman or modified Stowell methods. The accuracy of the Neuber and modified Stowell methods is found to depend upon the shape of the cyclic stress strain curve and the degree of plastic strain.
Instability Effects in Thermal Fatigue
This paper considers shape changes which develop progressively in hourglass specimens subjected to fully reversed cyclic plastic strain applied in a variety of mechanical and thermal waveforms. While the strain at the minimum diameter is controlled, off-center locations subjected to cyclic plastic strain undergo ratcheting. Two major off-center shape changes have been observed: barreling and necking. The paper discusses the reasons for these changes.