Considerations in selecting laboratory scale test specimens for evaluation of fracture toughness

Abstract

The assessment of defects in large steel structures requires a trustworthy evaluation of the material’s toughness. This toughness is not only a material property but is also influenced by the loading conditions and geometry; the so-called constraint. The resulting representative value is referred to as the apparent toughness. The evaluation of apparent fracture toughness in a flawed structure is preferentially performed through laboratory scale testing, as full scale tests are both expensive and often challenging to perform. Several laboratory scale test specimens are available, among which a Single Edge Notch Bending specimen, Single Edge Notch Tensile specimen, Double Edge Notch Tensile specimen and Centre Cracked Tensile specimen. Each of these specimens has its own specific constraint. Therefore, the selection of an appropriate test specimen is of primary importance for limiting the conservatism and avoiding potential unconservatism with respect to full scale behaviour. This paper provides a general framework to select an appropriate test specimen based on detailed finite element simulations of both the full scale structure and the laboratory scale test specimens. These finite element calculations allow for a characterization of the crack tip stress fields in both situations. Different theoretical frameworks are available for this characterization; theQ -parameter is considered in this paper. To demonstrate the applicability of this procedure, an example case is presented for circumferentially oriented defects in pressurized pipelines under longitudinal tension. It is concluded that the presented framework allows for efficiently selecting a laboratory scale test specimen, which enables to evaluate the apparent fracture toughness for a given large scale structure. The obtained toughness can thus be incorporated in analytical flaw assessment procedures, reducing the degree of conservatism. This in turn allows an economically effective design.