Dwell Sensitivity Part I. Behavior and Modeling

This paper examines the dwell sensitive behavior of materials subjected to creep-fatigue deformation. A study of published creep-fatigue data on seventeen materials was conducted to investigate the dwell sensitivity for different materials under different hold times applied in tension and compression. The seventeen materials selected were such that their melting temperatures varied from 250 to 1700°C, and the test temperatures ranged from 0.4 to 0.75 of the homologous temperature, where creep and fatigue interact. The dwell sensitivity was found to depend upon the test temperature, period of hold time, and hardening and softening phenomena of the materials subjected to a particular cyclic waveform. Since no previous model exists for the prediction of dwell sensitivity, it is conceptualized in a cycle in terms of hardening (H) and softening (S) and their combinations in tension (T) and compression (C) directions. Thus, for a cycle (TC), there are four possible combinations namely HS, HH, SH and SS. A demonstration of such combinations with H and S was made for a tensile dwell sensitive material where cycles produced tensile hardening and compressive hardening or HH were found to cause dwell sensitivity. However, for a compressive dwell sensitive material, the combinations were tensile hardening and compressive softening or HS. This study on dwell sensitivity attempts to address an important material behavior which is not very well understood and may initiate further research interests.