Research engineer, The Welding Institute, Research Laboratory, Abington Hall, Abington, Cambridge,
The shapes of fatigue cracks in fracture toughness test specimens have been reviewed for a sample of approximately 680 three-point bend specimens. Up to 17% of this sample would be rejected for plane strain fracture toughness for Mode I loading KIc test procedures because of fatigue crack front shape. The corresponding rejection rate to crack tip opening displacement (CTOD) procedures is 3%. The results of published work for aluminum alloys indicate that relaxation of the KIc testing standards' restrictions on shape would not cause undue errors. This indication, however, appears to conflict with a computer model of curved crack fronts in a compact tension specimen. The suggestion is therefore made that the calculation procedures for fracture toughness determinations should be made more appropriate to the curved crack fronts usually observed, before the limits quoted in the standards are relaxed. To reduce the number of crack length measurements the removal of one limit on crack front shape is suggested. Finally, experimental justification for the change from the limits on crack front shape of British Standard Draft for Development DD19 ) to those of the current CTOD testing standard, British Standard Methods for Crack Opening Displacement (COD) Testing BS 5762 (1979), is presented.
Paper ID: JTE11584J