Effects of mixed-mode I/III loading on environment-induced cracking

Many materials that exhibit ductile, tough behavior during conventional Mode I testing exhibit reduced toughness when loaded with combinations of Modes I and III. A minimum in the energy for crack initiation and stable crack growth has been demonstrated for ductile metals with mixed-mode loading with crack planes at angles between 40 degrees and 60 degrees relative to Mode I loading. Internal hydrogen further reduces these energies in a ferritic/martensitic steel with the effect being relatively equal in pure Mode I and mixed-mode loading conditions. Hydrogen effects can rationalize a number of observations of Mode I crack growth rates being faster than Mode III rates for environment-induced cracking. Where hydrogen is not a factor, cracks loaded in Mode I and mixed-Mode I/III grow at similar rates. (C) 2001 Elsevier Science Ltd. All rights reserved.