Fracture toughness is used in Engineering Critical Assessment (ECA) to assess the fitness-for-service of engineering structures with respect to avoidance of fracture. Differences in the values of fracture toughness measurements on the same specimen using different methods could result in a structure being considered safe or not. It is therefore important that the estimation of failure criteria, such as critical flaw size, does not result in over-conservative design, while still ensuring structural integrity.
There are three parameters used to quantify fracture toughness: - stress intensity factor, J‑integral and crack tip opening displacement (CTOD). CTOD is a measure of the physical opening of an original crack tip in a standard fracture toughness test specimen at the point of stable or unstable crack extension. The CTOD concept was first proposed by Alan Wells using notched tension bars. In the early days, a ‘COD meter’ had been used to measure CTOD.It was placed at the bottom of a sawn notch and the opening of the crack could be measured directly. Modern techniques introduce a fatigue pre-crack in fracture toughness specimens to mimic an actual crack. Displacement data is obtained by measuring the displacement of the load or the opening of the crack mouth (CMOD) from which CTOD is inferred.
Current standards-based procedures – such as BS 7448-1, ISO 12135, and ASTM E1820 - specify methods to determine fracture toughness, including determination of the critical CTOD for the material under the application of slowly increasing loading on the specimen. The fracture test procedure and methodology is well established and is similar between standards. A clip gauge is often used to measure the displacement data from the opening of the crack mouth due to its consistency and simplicity. However, despite the similar testing methods, different standards give different CTOD estimation equations due to the different assumptions used in the formulation of the equations.
Recently, researchers at the Japanese Welding Engineering Society (JWES) have suggested a modification to the BS 7448-1 equation based on experimental results and finite element modelling. It is important that CTOD is estimated as accurately as possible, as it would lead to different flaw size predicted. To investigate the actual CTOD for the validation of the different CTOD equations, the physical crack on the fracture toughness specimen was replicated and measured. It could then be decided which equation is most accurate for the determination of CTOD. Inclusive to the validation of the available equations, the effect of different parameters involved in the determination of CTOD were investigated to improve the accuracy of the equation.