The Influence of Biaxial Loading on the Assessment of Structures with Defects

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Abstract

Assessments of structures with postulated or existing defects are generally carried out using standards and engineering assessment procedures. Assessments of this type involve comparing an applied force, in this case the crack driving force, with a material property, which in this case is the materialâ€Â™s resistance to fracture, its fracture toughness. The crack driving force J can be calculated directly or implicitly by using a failure assessment diagram. Assessments can be based on either the initiation of the growth of a crack or, when dealing with ductile fracture, on an amount, e.g. 2 mm, of ductile tearing. Material fracture toughness values are obtained by testing high constraint specimens such as deeply cracked compact tension specimens and single edge notched bend specimens under uniaxial loading conditions. The high constraint of the test specimens provides conservative measurements of the fracture toughness for most applications. However, this assumption of conservatism is not necessarily applicable where there are biaxial loading conditions. The literature concerning assessments of such components mainly discusses whether uniaxial loading conditions provide conservative estimates of fracture toughness. Crack driving forces under biaxial loading can be overestimated, leading to a loss of conservatism. Conversely, biaxial loading could be beneficial and thus an approach that is consistently conservative has implications for the cost and time involved in the consequences of prematurely assessing or predicting the failure of a structure or component. This research considers the effects of biaxial loading on all the parameters involved in the integrity assessment of structures, components and specimens with defects. These parameters include the crack driving force, material fracture toughness, internal stresses and limit loads. It will address their relative effects on the determination of failure when compared with the assumption of uniaxial loading. The methods used will be analytical, using the equations and theories of standard solid mechanics, fracture mechanics and existing advice in R6 and the literature, and numerical using finite element analyses. Experimental, analytical and numerical work in the literature will be assessed and discussed and their outcomes compared with the findings of this research. The overall aim is to provide more explicit advice on the assessment of defects in components under biaxial loading in the R6 procedure.

Keyword(s)

FAD; R6; biaxial; constraint; limit load

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