Huang believes that the dispersion of fatigue crack growth behavior should be accurately characterized in the structural integrity assessment based on probabilistic fracture mechanics. In his research work, ten groups of crack growth tests of superalloys were carried out under the same conditions. According to the statistical results of the experimental data, the fatigue crack growth behavior of gh420li is quite dispersed. In the same Δ The crack growth rate (DA / D, n) at k is lognormal distribution. According to the random number generation principle of lognormal distribution variables, a random crack growth model based on mean value and coefficient of variation (COV) is derived. Because the average value of Ln (DA / DN) and cov can be expressed as Δ Therefore, the random crack growth rate is determined by Δ K and standard normal distribution of random variables. Finally, the random model is used to calculate the fatigue crack growth of compact tensile specimens. The comparison between the predicted results and the experimental results proves the effectiveness of the proposed model.
Cort é s. R the fatigue and crack growth behaviors of Inconel 718 and AL6XN alloys bonded with ernifecr-2 fillers were evaluated by gas tungsten arc welding. The W-type properties of the dissimilar welded joints of integral IN718 and AL6XN alloy and in718-al6xn were compared ö Hler curve. Although the Vickers microhardness curves show that there is a soft zone in the heat affected zone of IN718 matrix material, the fatigue crack nucleates in the fusion zone near AL6XN and propagates on the weld. Although there is an unmixed zone at the AL6XN / weld metal interface, the welded joint and AL6XN base material still have similar high cycle fatigue behavior. The fatigue crack growth test results show that the weld metal has the highest growth rate due to the presence of brittle second phases such as Laves and NBC. The fatigue crack growth rate obtained from the experimental data is compared with the measurement results of the distance between the stripes on the crack surface, both of which are very close.
In the research work of Li, the fatigue crack growth behavior of commercial pure titanium (cp Ti) under different load levels was studied considering the plastic deformation degree at the crack tip. The results show that there are great differences in the stages of stable crack growth under different loading conditions. At the same crack growth rate, when the crack growth rate is low, the strain energy and plastic zone at high load ratio are higher than those at low load ratio. However, when the crack growth rate is high, the difference between the strain energy and the plastic zone is much smaller. Under high load amplitude, more plastic strain energy will be consumed and more plastic deformation will occur at the crack tip. it seems that Δ K and Δ J can’t describe these phenomena, but the method proposed in this paper Δ Jack and Δ Jac ‘may be more suitable to describe these phenomena on the fracture surface. Under different loading conditions, at the later stage of crack propagation, the fracture morphology is pit. The data under all conditions follow Paris law, and parameters are used in the rapid growth phase Δ Jac ‘has the same coefficient and exponent.
Yang carried out a series of fatigue tests on C45 steel specimens with local surface cracks. The effect of carburizing treatment under different carburizing depth was discussed under cyclic tensile and torsional loads. The surface crack growth path and crack growth rate were discussed. The effects of additional static torsion under cyclic tensile load and additional static tension under cyclic torsional load were analyzed. The results of Carburized Materials and non carburized materials were compared. The results show that carburizing can effectively prevent crack growth and improve fatigue life. However, the improvement of surface depth on fatigue life is limited to a certain range, and larger surface depth may reduce the fatigue strength of materials. For research materials, carburizing has more direct effect on crack growth under cyclic tensile load than cyclic torsional load.
Tang analyzed and studied the fatigue crack growth behavior of base metal (BM), weld metal (WM) and heat affected zone (HAZ) in 06cr19ni10 stainless steel MIG welded joints. The results of fatigue crack growth test show that a new fatigue crack begins at the crack tip of an existing crack and then propagates perpendicular to the direction of periodic fatigue load. The results show that the original mixed mode crack transforms into mode I crack. WM sample has the largest fatigue crack growth rate, followed by HAZ sample and BM sample. In order to explain the difference of fatigue crack growth behavior of three different types of specimens, the microstructure, fracture morphology and residual stress of BM, HAZ and WM were studied and discussed. Metallographic observation shows that the average grain size of HAZ is larger than that of metal phase. Morphology analysis shows that WM has the largest fatigue stripe width, followed by HAZ and BM. It is also found that the dent depth in WM is shallower than that in HAZ and BM, which indicates that the plasticity of the material is poor. The results of residual stress analysis show that there are a lot of tensile residual stresses in WM and HAZ.