Based on load separation theory, the load separation parameter Spb method is an effective approach for estimating the J-resistance curve from records of load versus displacement directly, using one sharp cracked specimen and an additional reference blunt cracked specimen. However, the effect of the reference blunt cracked specimen on J-resistance determination was not explicitly considered in past work. In this paper, a modified load separation parameter Spb method was developed to eliminate this effect, and then a unique estimation of instantaneous crack length for one sharp cracked specimen could be obtained. Furthermore, a forced blunting calibration method was also adopted to determine the instantaneous crack length in the load inseparable region, referring to a normalization method. Experiments on steam turbine rotator steel Cr2Ni2MoV were carried out to estimate J-resistance curves using an unloading compliance method. By removing unload and reload data from load-displacement records, the J-resistance curve for the same sharp cracked specimen was estimated using the modified separation parameter Spb method. The results indicate that the modified Spb method completely eliminates the effect of the reference blunt cracked specimen on the instantaneous crack length determination of the sharp cracked specimen. However, different J-resistance curves in a small range of crack extension are present when different blunting coefficients are used in the blunting line equation. The J-resistance curve obtained from the modified Spb method agrees well with that obtained from the compliance method.
A theoretical model of semi-elliptic surface crack growth based on the low cycle strain damage accumulation near the crack tip along the cracking direction and the Newman-Raju formula is developed. The crack is regarded as a sharp notch with a small curvature radius and the process zone is assumed to be the size of cyclic plastic zone. The modified Hutchinson, Rice and Rosengren （HRR） formulations are used in the presented study. Assuming that the shape of surface crack front is controlled by two critical points： the deepest point and the surface point. The theoretical model is applied to semi-elliptic surface cracked A1 7075-T6 alloy plate under cyclic loading, and five different initial crack shapes are discussed in present study. Good agreement between experimental and theoretical results is obtained.
针对无法直接获得载荷(P)-加载线位移(load line displacement,LLD)对应的塑性功导致无法直接完成基于直通型紧凑拉伸试样(front face compact tension,FFCT)试样材料断裂韧性测试的问题,该文基于迭代理念将适用于台阶型试样(load line compact tension,LLCT)的规则化法推广应用于FFCT试样,得到基于FFCT试样评估延性材料断裂韧性的可行方法。结合汽轮机转子钢和迭代方法,验证新方法准确性,最后将迭代方法应用于核反应堆材料A508-III钢的J阻力曲线测试中,有助于解决基于1/2FFCT试样获取断裂韧性的难题。
