With the continuous improvement and innovation of fatigue crack testing methods, the research on small fatigue cracks has gradually changed from simple measurement of fatigue crack growth rate to micro analysis of evolution behavior in the process of crack growth. These factors are of great significance to establish a reliable fatigue life assessment method for structures with fatigue cracks, which can be used in engineering practice.
The research work of Li considers the experiments carried out under a wide range of load amplitude and load ratio with or without a certain residence time, and studies the effect of cold creep on the fatigue crack growth rate. In order to further explain the effect of cold creep, a series of finite element results are carried out. Finally, the fracture morphology analysis is carried out to explain the related 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, depending 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. Perform the process of suppressing 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 fracture morphology 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 the fatigue crack growth behavior of commercial pure titanium at high and low load ratios. In addition, combined with the surface morphology of fracture, the characteristics of FCG under pure fatigue and packing fatigue were analyzed. The experimental results show that the FCG rate of commercial pure titanium (cp Ti) increases with increasing temperature and holding time at low loading ratio. Similarly, the holding time increases with the increase of temperature. When the temperature rises to 200 ℃, the packing effect tends to be saturated. At high loading ratio, the FCG rate of cp Ti is also more sensitive to temperature. At high load ratio, the enhancement of FCG rate by holding time is more significant. However, the effect of holding time on FCG rate does not increase at 300 ℃. Da / dN of cp Ti- Δ The K / E FCG curves tend to be close to each other at different loading ratios, which indicates that the electronic modulus is an important factor causing the difference. From room temperature to 300 ℃, the effect of residence time on the FCG behavior of cp Ti is mainly determined by the creep deformation mechanism. At the same time, with the increase of load ratio to 300, the oxidation effect gradually becomes significant. The fracture surface morphology shows that the secondary crack and roughness increase with the increase of temperature or residence time at low load ratio, while the effect of creep deformation on FCG behavior is more obvious at high load ratio, and the plastic deformation gradually becomes significant with the increase of dent.
The small fatigue crack behavior of titanium alloy TC4 under different stress ratios was studied by Hu. At room temperature, the specimens were subjected to axial fatigue at a nominal maximum stress of 370 MPa. The results show that the fatigue crack in TC4 originates from α Mutual harmony β Between phases 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 growth process of TC4 can be divided into three typical stages, i.e. microstructure small crack stage, physical small crack stage and long crack stage. Even though it’s constant σ Under the condition of Max, the stress ratio has a significant effect on the total fatigue life and crack initiation life Δ When r = – 0.1, 0.1 and 0.3, the effect on crack growth rate can not be distinguished.
The fatigue crack growth (FCG) behavior of T6 aged al-4.41cu-0.69mg-0.64si-0.52mn alloy forged at 350 ℃ and 450 ℃ was analyzed by Li. FCG rate test shows that the alloy forged at 350 ℃ shows higher fatigue crack threshold than that forged at 450 ℃（ Δ The FCG rate is lower in the near threshold region, and higher 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 ℃. In Paris and unstable regions, transgranular propagation mainly occurs, while the lower FCG rate of unrecrystallized samples forged at 450 ℃ is attributed to the crack deflection between dislocation wall and sub grain boundary.
Xiao Lei studied the fatigue crack growth performance of fgh97 Powder Superalloy at 650 ℃ by argon atomization and hot isostatic pressing, and compared with EP741NP alloy disk made in Russia by rotating electrode and hot isostatic pressing; The different milling process, oxygen content, grain size and particle size were analyzed γ′ The influence of size and other factors on the crack growth rate. The results show that the fgh97 alloy test plate prepared by argon atomization and hot isostatic pressing process has higher resistance to fatigue propagation than EP741NP alloy plate prepared by Russian rotating electrode milling and hot isostatic pressing process, and the grain size has a greater impact on the crack propagation performance of the test plate. The larger the grain size is, the lower the fatigue crack propagation rate is, γ′ The effect of phase size is the second, and 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 ℃ and 750 ℃. The effects of temperature and stress intensity factor on the fatigue crack growth life and rate were analyzed, and the microstructure of fatigue fracture at different temperatures was observed by SEM. The results show that the crack growth life of gh4742 alloy decreases with the increase of temperature, With the increase of crack growth rate, the intergranular fracture characteristics are more obvious. The fatigue crack growth rate has the same trend as the stress intensity factor.
