Many materials that exhibit ductile, tough behavior during conventional Mode I testing exhibit reduced toughness when loaded with combinations of Mode 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 to 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 clearly not a factor, such as brass in ammoniacal solutions, cracks loaded in Mode I and III
grow at similar rates. Keywords: mixed-mode loading, stress-corrosion, environment-effects