Application of K_(residual) Measurements to Fracture Toughness Evaluations

摘要

Residual stresses are self-equilibrating internal stresses that exist in materials and structures due to a number of factors including thermal-mechanical processing, machining, forming, or welding. These stresses are of critical importance, particularly in structural materials for fatigue-crack growth and fracture design, where residual stresses can bias the material property measurements, leading to either unconservative or overconservative design data. The driving force due to residual stress is referred to as the residual stress-intensity factor (K_(residual)) and can be used to understand and prevent bias during material characterization for material expected to contain residual stresses. The cut-compliance method can be employed to measure the K_(residual) based on Schindler's method, where new crack surface area is created by electrical discharge machining (EDM). However, the cut-compliance test is destructive and, as a result, residual stress effects on a property such as fracture toughness would require two tests, one for residual stress and one for fracture toughness, along with an assumption that the residual stress is the same for both measurements. The new crack-compliance method is similar to the cut-compliance method, but the new surface area is created through fatigue-crack growth due to cyclic loading. The advantage of the crack-compliance method is to produce K_(residual) data on the same specimen as the property data. This crack-compliance method is based on the same standard compliance data used to calculate crack length, with a simple extension of the method for calculating the K_(redidual) value. The method has been demonstrated in the past with good success for fatigue-crack growth, and this paper will extend the concept to fracture toughness. The approach is to apply the crack-compliance method during the precrack phase of a fracture toughness test to estimate K_(residual) at the end of the precrack. Then a simple superposition model is used to correct measured fracture toughness.