Yang carried out a series of fatigue tests on C45 steel samples with local surface cracks. The effects of carburizing treatment under different carburizing depths under cyclic tensile and torsional loads are discussed, the surface crack propagation path and crack propagation rate are discussed, the effects of additional static torsion under cyclic tensile load and additional static tension under cyclic torsional load are analyzed, and the results of Carburized Materials and non carburized materials are compared. The results show that carburizing can effectively prevent crack propagation and improve fatigue life. However, the improvement of surface depth on fatigue life is limited to a certain range, and a larger surface depth may reduce the fatigue strength of the material. For the studied materials, carburizing has a 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 metal inert gas (MIG) welded joints of 06crl9nil0 stainless steel. The results of fatigue crack growth test show that a new fatigue crack starts at the crack tip of an existing crack and then propagates perpendicular to the direction of periodic fatigue load. This observation shows that the original mixed mode crack changes to mode I crack. WM specimen has the largest fatigue crack growth rate, followed by HAZ specimen and BM specimen. In order to explain the differences of fatigue crack propagation behavior of three different types of specimens, the metallographic structure, fracture morphology and residual stress of BM, HAZ and WM were studied and discussed. Metallographic observation shows that the average grain size of heat affected zone is relatively 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 relatively shallow than that in HAZ and BM, indicating that the plasticity of the material is poor. The analysis results of residual stress analysis show that a large number of tensile residual stresses appear in WM and HAZ.
Chang’s research work considers the tests carried out under a wide range of load amplitude and load ratio under a certain or no certain residence time, and studies the influence of cold creep on the fatigue crack growth rate. In order to further illustrate the effect of cold creep, a series of finite element results were carried out. Finally, fracture morphology analysis is also carried out to explain the relevant crack growth behavior. The results show that the fatigue crack growth rate increases or decreases with a certain degree of holding load and residence time, which depends on the value of load amplitude and load ratio. This finding is the same as that in fatigue life. The discussion shows that the effect of creep on fatigue crack growth can be attributed to the competition between creep stress relaxation and creep strain, and the process of inhibiting and promoting crack growth. The fatigue creep interaction diagram based on analysis is described, through which the interaction between cold creep and fatigue can be well understood. Finally, the crack morphology analysis of dislocation accumulation is further studied and discussed, which is consistent with the finite element results of local creep behavior. This study will provide support for a clear understanding of the effect of cold creep on fatigue crack growth behavior.
Su studied the effects of temperature and residence time on fatigue crack growth behavior of pure titanium under high load ratio and low load ratio. In addition, combined with the surface morphology of fracture, the characteristics of FCG under pure fatigue and pressure maintaining fatigue are analyzed. The experimental results show that under low load ratio, the FCG rate of commercial pure titanium (cp Ti) increases with the increase of temperature and pressure holding time. Similarly, the increase of holding time increases with the increase of temperature. When the temperature rises to 200 ° C, the pressure maintaining effect tends to be saturated. At high load ratio, the FCG rate of cp Ti also shows increased sensitivity to temperature. Under high load ratio, the enhancement effect of pressure holding time on FCG rate is more significant. However, the effect of holding time on FCG rate will not increase at 300 ° C. From room temperature to 300 ° C, under different load ratios, the effect of residence time on FCG behavior is mainly determined by creep deformation mechanism. At the same time, with the load ratio increasing to 300, the oxidation gradually becomes significant, and the fracture surface morphology shows that under the condition of low load ratio, the secondary crack and roughness increase with the increase of temperature or residence time, while under the condition of high load ratio, the effect of creep deformation on FCG behavior is more obvious, and the plastic deformation gradually becomes significant with the increase of dent.
Hu studied the small fatigue crack behavior of titanium alloy TC4 under different stress ratios. At room temperature, the unilateral notch tensile specimen is subject to axial fatigue under the condition of nominal maximum stress of 370mpa. The results show that the fatigue crack in TC4 is from α Between phase and P phase or α The interface within the phase begins. More than 90% of the total fatigue life is consumed in the stage of small crack initiation and propagation. The crack propagation process of TC4 can be divided into three typical stages: microstructure small crack stage, physical small crack stage and long crack stage. Although at constant σ Under the condition of Max, the stress ratio has a significant effect on the total fatigue life and crack initiation life, and its effect on the crack growth rate can not be distinguished when r = – 0.1, 0.1 and 0.3.
Li analyzed the fatigue crack growth (FCG) of al-4.41cu-0.69mg-0.64si-0.52mn alloy forged at 350 ° C and 450 ° C. FCG rate test showed that the dielectrics forged at 350 ° C showed a higher fatigue crack threshold than the alloys forged at 450 ° C（ Δ KTH), and has a lower FCG rate in the near threshold region, while it shows a higher FCG rate in Paris and unstable regions. The inspection of crack path shows that the near threshold region is mainly intergranular propagation, and more crack deflection at the grain boundary helps to reduce the FCG rate of recrystallized samples forged at 350 ° C. In Paris and unstable regions, transcrystalline propagation mainly occurs, while the lower FCG rate of non recrystallized samples forged at 450 ° C is attributed to the crack deflection between dislocation wall and sub grain boundary.
Xiao Lei studied the fatigue crack growth performance of Powder Superalloy fgh97 prepared by argon atomization pulverization + hot isostatic pressing process at 650 ° C, and compared it with EP741NP alloy disk formed by Russian rotating electrode pulverization + hot isostatic pressing process; The effects of different pulverizing processes, oxygen content, grain size and / size on crack growth rate are analyzed. The results show that the fgh97 alloy test disk prepared by argon atomization powder preparation + hot isostatic pressing process has higher fatigue propagation resistance than the EP741NP alloy disk prepared by Russian rotating electrode powder preparation + hot isostatic pressing process, and the grain size has a great influence on the crack propagation performance of the test disk. The larger the grain size, the lower the fatigue crack propagation rate, γ’ The effect of phase size is the second, and the oxygen content in a certain range has little effect on it.
Zhang Baoning studied the fatigue crack growth behavior of gh4742 alloy at room temperature, 700 ° C and 750 ° C. by analyzing the effects of temperature and stress intensity factor on fatigue crack growth life and rate, and observing the micro morphology of fatigue fracture at different temperatures by scanning electron microscope, the results show that, The crack growth life of the alloy decreases with the increase of temperature, while the crack growth rate increases, and the intergranular fracture characteristics are more obvious. The fatigue crack growth rate has the same development trend as the stress intensity factor.