TWI Industrial Member Report Summary 146/1981
By O L Towers
Fracture mechanics based toughness testing requires that the specimens contain fatigue cracks to ensure that the sharpest type of structural defect is modelled. The testing standards impose restrictions on the fatigue cracking loads because experiments have shown that if the loads are too high the resulting
toughness might be unconservative. In this report the basis of the restrictions of two KIc testing standards and a COD testing standard are reviewed with reference to published and experimental data. The current 'restrictions of the COD standard have some inconsistencies for the subsidiary, B x B, testpiece and are impractical for small section thicknesses. Suggestions for amendments are made. The KIc standards' restrictions on fatigue cracking load, PF, may be too stringent for some materials but insufficiently so for others. This difference in sensitivity appears to be a function of cyclic softening. Experimental data for 76 x 38mm section specimens of a structural steel and other published data indicate that the present fatiguing conditions specified in the COD testing standard, BS 5762:1979, are sufficiently stringent. A parallel study to assess the effect of high R-ratio, i.e. high minimum load during the fatigue cycle, produced inconclusive results. The removal of the present restriction on R-ratio would be desirable because high R-ratio can help in producing straight fatigue cracks in as-welded specimens. However, this cannot be recommended at present because of uncertainties as to the effect of R-ratio during fatigue on fracture toughness.