A study of high temperature crack growth under creep conditions with emphasis on the effects of cavitation damage.

摘要

The high-temperature creep crack growth behavior of pure Cu, Cu-Sn alloy, and pre-damaged Cu has been investigated. The purposes of this study have been to understand the mechanisms of creep crack growth; to prove theoretical predictions with experimental studies; and eventually to predict the time to failure of high-temperature structural materials.;High-temperature crack growth experiments have been conducted under various loading conditions with a computer-based data acquisition and control system. A characterization study of Cu-Sn alloy has shown that a singular stress field controls creep crack growth only in the early stages of crack growth, and that cavitation damage affects crack growth even in its early stages. The results also indicate that under extensive damage, crack growth occurs in a progressive manner, not by the singular stress field but by a damage gradient.;To see the effect of damage on creep crack growth, the creep crack growth behavior in pre-damaged Cu has been studied. The crack growth rate of pre-damaged Cu was much higher than that of undamaged Cu. However, the basic crack growth behavior of pre-damaged Cu with the given initial damage level is similar to that of undamaged Cu. The result shows that in the beginning, the coupling of damage and creep deformation characterizes crack growth, while later, the crack tip stress singularity decreases with increased damage and creep damage have more influence on crack growth.;A model to describe creep crack growth in pre-damaged Cu has been developed. It is based on cavity growth by coupled diffusional and creep processes under the damage- modified stress field. Favorable correlations of model-predicted results with experimental data suggest that the stress distribution ahead of a crack tip is influenced by damage and that cavity growth and coalescence limit creep crack growth.;High-temperature crack growth experiments have been carried out under cyclic loading conditions. Time-dependent creep processes dominate the fatigue crack growth behavior; the C*-integral characterizes the crack growth rate under the given cyclic loading conditions. A fractographic study confirms the view of controlling mechanisms.