TWI Industrial Member Report Summary 247/1984
By A P Smith, O L Towers, and I J Smith
Background
LFatigue cracks in fracture toughness test pieces have shapes which are, at times, unacceptable to the requirements of the fracture toughness testing standards. For example, it was found that 17% of a sample of 677 specimens reviewed at The Welding Institute would be rejected to the KIC test procedures of BS 5447: 1977 because of the bowed fatigue crack shapes obtained. This rejection rate is clearly undesirably high. The standards' restriction on fatigue crack shape have up to now been necessarily arbitrary, and it is possible that the high rejection rates referred to above are unnecessary. In this report, numerical modelling of the effect of fatigue crack front curvature on the distribution of the mode I stress intensity factor, KIc at different positions across the specimen thickness is described. The modelling is performed for two of the most common geometries used for fracture toughness tests, namely the compact tension and tbree-point bend specimens. Both finite element and boundary integral equation methods have been used to analyse the compact tension specimen and comparison is made between the results obtained using the two techniques. In the modelling of the three-point bend specimen, only the finite element method was adopted and the results obtained using this method are compared for both specimen geometries. The results are then assessed in relation to the restrictions on fatigue crack shape in the current standard for measuring KIc. The KI values obtained from the two-dimensional models implicit in the calculation of fracture toughness in current standards tend to overestimate the value computed at the specimen mid-thickness when the fatigue cracks have significant bowing. The implications of this are discussed.orem ipsum