The mechanics of creep crack growth in a magnox alloy

Magnox AL80 has been used for a study of creep crack propagation. A number of variables have been considered such as specimen geometry, notch root radius, material thickness, creep prestrain and stress level. The work has covered the material behaving under two values of the creep exponent, n=3.5 and n=7, according to the stress level. As well as observing initiation times and crack growth rates, scribed grids have been used to examine the near crack tip strain levels and distribution. Strong evidence was found for crack advance being displacement controlled, however it was shown that the COD approach should be considered geometry dependant. The summation of Esub(xx) and Esub(yy) provided the most successful description of crack advance as it produced a single value that described propagation in all the cases considered. The strain distributions indicated that σsub(yy) was related to distance from a point ahead of the crack tip by the exponent -(1/n+1) and that σsub(xx) is proportional to σsub(yy). The constraint stresses were evaluated. Initiation time results are given. Crack growth was found not to obey either the empirical kappa or σ relationships but was reviewed in context of the observed strain behaviour. (author)