Mixed-mode fatigue crack growth thresholds: A personal historical review of work at the National Engineering Laboratory, 1975-1989

Mixed mode fatigue crack growth thresholds in metals were investigated at the National Engineering Laboratory from 1975 to 1989. Re-assessment confirmed that thresholds are determined by the formation and growth of mode I branch cracks at the tip of the initial crack. Lower and upper bounds can be defined, and used for design purposes. For mixed mode I and II loading, the presence of corner points and hence corner point singularities, encourages mode I branch crack formation. Transforming a mixed mode crack into an equivalent mode I crack, as is done some design standards, is not always satisfactory.