Wedge type creep damage in low cycle fatigue
- PDF / 692,756 Bytes
- 8 Pages / 594 x 774 pts Page_size
- 61 Downloads / 206 Views
I.
INTRODUCTION
CREEP-FATIGUE interaction is a complex phenomenon. The interaction between cyclic loading and creep-cavitation damage can occur by several mechanisms, and we believe that it is essential to understand each one of them separately before fatigue failures at high temperatures can be predicted with reliability. Phenomenological models for creep-fatigue have been developed for a considerable time, 1'2 but the first mechanistic model was proposed by Tomkins and Wareing a which has been fairly conclusively verified by experiment.4'5'6 The mechanism pertains to fatigue crack propagation at high temperature. When the crack opening displacement becomes greater than the displacement required to accommodate cavity linkage in grain boundaries, then the crack growth per cycle increases radically and becomes intergranular. This is an example of creep-fatigue interaction where creep-damage can lead to an acceleration of the "low temperature" mode of fatigue failure; that is, crack initiation and crack propagation. In a preceding paper 7 we have shown that creep-fatigue failure is also possible by cumulative creep damage. In this instance failure occurs by cavitation damage in general, rather than by the growth of a fatigue crack. In developing models for this type of creep-fatigue failure we must distinguish between cavitation of the 'r' type and the 'wedge' type. The difference between them is illustrated in Figure 1. Both involve the nucleation and growth of microcavities in the boundary except that grain boundary sliding is a necessary condition for 'wedge' cavitation. We also found that imbalanced cycle shapes promoted this mode of creepfatigue failure. Tension hold cycles favored 'r' type cavitation, while unsymmetrical strain-rate cycles favored "wedge" type of damage. This paper is devoted to a simple analysis of the wedge damage failures. The limitations of the analysis presented here need to be emphasized. We are dealing only with creep damage. Environmental effects are not considered. Also, the model is limited to wedge cavitation. The relative susceptibility of S. BAIK, Postdoctoral Associate, and R. RAJ, Associate Professor, are both with the Department of Materials Science and Engineering, Bard Hall, Cornell University, Ithaca, NY 14853-0121. Manuscript submitted February 20, 1981. METALLURGICAL TRANSACTIONS A
t
t
T
GE C R A C K
' CAVITIES
Fig. 1 - - A schematic of the similarity and difference between 'r' and wedge microcrack damage. Both involve the nucleation and growth of small, discrete cavities, but the 'r' cavities grow uniformly across the grain facet under the influence of the tensile stress normal to the facet, while a wedge crack propagates from one triple junction to an adjacent one under the influence of grain boundary sliding.
different materials to 'r' and wedge cavitation differs considerably. Copper alloys and oxide dispersion strengthened alloys are prone to 'r' cavitation, while precipitation strengthened 'alloys can fail by wedge cavitation as well. The reason for this diff
Data Loading...
