The premises upon which prevailing composite toughness theories are based are discussed in the light of observed strength variations in boron-epoxy composites with differing shear strengths of the interracial bond. None of the extant toughness theories (pull-out, debonding, stress redistribution) successfully predicts the work of fracture of the boronepoxy system. However, incorporation of the work to create new surfaces into the total toughness analysis gives better agreement with experiment, and work of fracture predictions for other sytems, such as carbon-polyester, can also be modified. The approach is more generalized than the Outwater/Murphy debonding explanation for toughness, which in the way usually presented only applies when the filament fracture strain is greater than the matrix fracture strain. The present analysis suggests how to tailor the interfacial shear strength in order to obtain a reasonable toughness yet still maintain strengths of the order of the rule of mixtures.
The current American Society of Mechanical Engineers Code procedure for crack arrest in a reactor vessel is based on a static linear elastic fracture mechanics analysis and the assumption that arrest occurs when the crack-tip stress intensification, KI, equals some critical value, KIa—the so-called crack arrest toughness. The paper argues that the conditions for crack arrest in a nuclear pressure vessel, when it is subjected to thermal stresses resulting from a hypothetical loss of coolant accident, can be demonstrated with the current Code method, provided that an appropriate value for KIa can be obtained from laboratory tests. The difficulties in selecting the appropriate value of KIa for this component analysis are highlighted, particular attention being given to the effect of test specimen geometry, crack jump length, and material variability. Against this background, the paper outlines a possible procedure for obtaining the appropriate KIa-value. It is emphasized that the viability of the approach may be highly dependent on the structure's characteristics, and an approach considering kinetic (dynamic) effects may be necessary for other crack arrest problems.
The mechanical properties of weld heat-affected zones (HAZ’s) associated with the heavy section, nuclear quality weldments are evaluated and found to be superior to those of the parent base material. The nil ductility transition temperature (NDTT), Charpy impact and static and dynamic fracture toughness properties of a HAZ associated with a submerged arc weld and one associated with a manual metal arc weld are directly compared with those of the parent base material. It is concluded that the stigma normally associated with HAZ is not justified for this grade and quality of material and weld procedure.
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