Fracture mechanics is the study of the nucleation and the propagation of cracks in materials, which includes both stable and unstable crack growth. The subject allows for the quantification of the relationships between material properties, stresses, the presence of crack-producing flaws, and crack propagation mechanisms. In modern materials science, fracture mechanics is an important tool used to improve the performance of mechanical components. Fracture mechanics is used in fitness-for-service assessments, which analyze flaws to determine which are safe and which are liable to propagate and cause failure of a flawed structure.
Fracture toughness is a material property which measures a material’s resistance to crack extension when an existing crack is present and is subjected to monotonic loading. There are various fracture toughness parameters that are measurable and used in engineering analysis. In practice, these parameters are generally narrowed down to 3 types: KIC, JIC, and CTOD.
Accu-Test Labs exhibits technical excellence in the preparation of specimens, specimen notch placement, precise fatigue crack growth to target crack lengths and specific microstructures, and ultimately the determination of these fracture toughness parameters.
KIC testing evaluates the fracture toughness in the predominately linear-elastic plane strain condition and must have sufficient specimen thickness to meet the plane strain condition. KIC is a measure of the critical stress intensity factor, K, under Mode I loading (the crack-tip opening under tensile loading normal to the crack plane) which causes sudden brittle fracture. Accu-Test Labs performs KIC in accordance with ASTM E399.
JIC testing evaluates the fracture toughness in the elastic-plastic condition and is much less restrictive on valid specimen thickness than KIC testing. It is used to evaluate materials which exhibit ductile tearing behavior. JIC is a measure of the critical J-parameter, which is a measure of the energy required to propagate a crack under Mode I loading. Accu-Test Labs performs JIC testing in accordance with ASTM E1820 using the unloading compliance method to produce a tearing resistance curve and evaluating for the initiation of stable tearing.
Crack-Tip Opening Displacement (CTOD) testing measures the critical opening displacement of the crack-tip which drives crack extension. It is used for materials which display some plastic deformation before sudden brittle fracture occurs. CTOD is the most common fracture toughness parameter of choice for evaluating weldments in the oil & gas industry. Accu-Test Labs performs CTOD testing in accordance with BS 7448 Part 1 & 2, ISO 12135, ISO 15653, and ASTM E1820. Accu-Test Labs commonly performs CTOD testing as a part of weld qualification testing in accordance with industry standards such as API 1104 Appendix A, CSA Z662 Annex K, and DNV-OS-F101.
