TWI Industrial Member Report Summary 142/1981
By O L Towers
The shape of the fatigue cracks in fracture toughness test specimens can often cause the fracture test result to be invalid according to the rather arbitrary restrictions of the testing standards. A review of crack front shape measurements for nearly 700 three-point bend specimens mainly sampling ferritic structural steels has indicated a rejection rate of up to 17% according to KIc test procedures, and 3% according to crack opening displacement (COD) procedures. Relaxation of the KIc standards restrictions on crack front shape is clearly desirable, and the results of recent published work for aluminium alloys indicates that relaxation would not cause undue errors. This work, however, appears to conflict with the results obtained from a computer model of curved crack fronts in a compact tension specimen. This uncertainty leads to the suggestion 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 DD19:1972 to those of the current COD testing standard, BS 5762 : 1979, is presented.