TWI Industrial Member Report Summary 157/1981
By O L Towers and S J Garwood
Existing defect assessment procedures are largely based on the concepts used to describe cleavage fracture in ferritic steels and void coalescence, or tearing, where linear elastic fracture mechanics is applicable. Current requirements have led to the extension of these procedures in order to make defect assessments in materials which fail by a tearing mechanism accompanied by extensive plasticity. The use in these procedures of toughness values which describe the point of initiation of tearing provides extremely conservative predictions of critical flaw sizes when compared with service experience for materials of high tearing resistance. To provide a more realistic, but safe, flaw size evaluation it is suggested that, under specified conditions, it is possible to make use of the geometry-dependent toughness value derived from the deflection at maximum load in laboratory bend tests. This paper discusses the conditions leading to a critical event by a tearing mechanism of crack growth, and explains how the maximum load point experienced with stable crack extension represents instability under load-controlled conditions, i.e. deadweight loading. The conditions necessary to ensure that conservative measurements of this maximum load toughness are made in the laboratory relative to structural situations are outlined. The use of maximum load toughness values in the well-established crack opening displacement (COD) design curve defect assessment procedure is then discussed. The recently proposed J-integral design curve is presented in a slightly simplified form and the compatibility with the COD curve is demonstrated. The use of the design curve assessment procedures is thus extended to provide estimates of allowable defect sizes for materials behaving in a fully ductile manner.