Material Cyclic Creep Characteristics Analysis The three-stage characteristic test results of cyclic creep show that the relationship between N and the logarithmic coordinates is generally as shown by the curve. This curve can be called a three-stage schematic variable characteristic curve of the cyclic creep cycle creep characteristic curve. It can be seen that the cyclic creep of the material can generally be divided into three stages. In the first phase, each stress cycle produces a significantly larger creep variable, which is called the fast creep phase. Although the creep generated in each stage of the second stage is smaller than the first stage, it is basically stable and lasts for a long time. This stage is called the stable creep stage. After the material undergoes a certain cycle of stress cycles, its damage accumulation is close to its capacity limit, and it enters the third stage of cyclic creep. During this phase, as the stress cycle increases, the creep variable no longer remains stable, but increases faster and faster until it breaks. This phase is called the unstable creep phase.
Comparing the cyclic creep curves of different materials, it is found that the cyclic creep of steels with lower carbon content (35CrMnSi, 35SiMnB and ML35CrMo) has a complete three-stage characteristic, and the creep variable generated in the first stage is much larger than the second stage. Therefore, rapid creep is the main stage of cyclic creep of such materials. The first stage of the steel with higher carbon content (60Si2Mn, 55CrSi and 55SiMnVB) is not obvious or even appears, only the last two stages occur, and the third stage only takes several weeks to ten weeks to break, so it is stable. Creep is the main stage of cyclic creep of this type of material.
The mathematical model of cyclic creep is used to regress the N of the main stages of cyclic creep under different strengths and different cyclic stress amplitudes for each test steel and the data is as follows: 0Nc. The corresponding parameters 0, c and the linear correlation coefficient r thus obtained are as shown. It can be seen that the correlation coefficient r of each curve is above 0.939. It can be considered that the relationship between the cyclic creep variable and the cycle N of the main stage of the cyclic creep of the test steel does have the relationship determined by the formula (1).
Let N = 1 in equation (1), get 0, and see 0 is the cyclic creep variable produced by the material after the first cycle of stress cycle. When c is constant, the larger the 0, the larger the cyclic creep of the material at a particular week. After 0 is determined, the larger c is, the faster the cyclic creep of the material increases with the cycle. Therefore, 0 and c can be referred to as cyclic creep coefficient and cyclic creep index, respectively, which respectively cite the magnitude and speed of cyclic creep of the material under certain cyclic stress. The larger the 0 and c, the larger The worse the ability of the material to resist cyclic stress, the more likely it is to cause cyclic creep, and the corresponding micro-plastic failure tendency of engineering parts is more obvious.
Factors affecting the cyclic creep properties of materials 3.1 The carbon content of chemical components also has a significant impact on the size and trend of cyclic creep of materials. According to the data, when the test steel reaches a considerable hardness (HRC4647) and the cyclic stress amplitude is about 1200 MPa, the cyclic creep coefficient of 35SiMnB is 0.1310-5, respectively, and the corresponding cyclic creep index c is 0.46, respectively. Presenting an increasing trend, their cycle creep curves are compared. It can be seen that as the carbon content decreases, the size and trend of cyclic creep of the general medium carbon low alloy structural steel will increase.
Effect of carbon content on cyclic creep curve 35SiMnB; strength and stress level In order to investigate the influence of material strength on cyclic creep performance, 55CrSi samples were treated according to different heat treatment processes, and their hardness was controlled to be HRC42, HRC47 and HRC50, respectively. The cyclic creep test was carried out at a stress amplitude of 1427 MPa, and the results were as shown. It can be seen that under the same cyclic stress amplitude, the higher the strength of the material, the smaller the cyclic creep variable.
The test results of the influence of the magnitude of the cyclic stress amplitude experienced by the material on the cyclic creep performance show that the smaller the cyclic stress amplitude, the smaller the cyclic creep variable, regardless of the strength level of the material. When the cyclic stress amplitude is very low, the material will undergo many times of influence on the cyclic creep curve. Cyclic creep will occur after the cycle of the cycle. It can even be considered that cyclic creep does not occur (when N>104, 10- 5) In order to observe the cyclic creep process of the material at low stress level, this study also controlled the cyclic creep test of 55CrSi (HRC47) with the control of 0.650.2. The result showed that the sample did not creep after the cycle of 2.28104 weeks. (<10-5), fracture occurred after 2.29104 weeks cycle, and the fracture showed high cycle fatigue fracture characteristics. At this point, failure of the material should be considered fatigue failure, not microplastic failure.
Pre-strain treatment The high-strength bolt steel ML35CrMo sample was subjected to 1% pre-strain treatment and then subjected to cyclic creep test at a stress amplitude of 1220 MPa. The results were compared with the results of direct cyclic creep test without pretreatment. It can be seen that the cyclic creep curve of the sample after the pre-strain treatment is located below the un-strained treatment, which indicates that the pre-strain treatment can significantly suppress or slow down the occurrence of cyclic creep. Therefore, the high-strength bolts are pre-tightened in the plastic zone during assembly, which can effectively reduce the cyclic creep of the material. This is because pre-tightening in the plastic zone is equivalent to performing a cold work strengthening, and multiple tightening is equivalent to multiple strengthening. The effect of improving the bearing capacity of the bolt in the subsequent service and the factors affecting the cyclic creep behavior of the material and the microscopic mechanism of cyclic creep have yet to be further studied.
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