4.1 Establishment of S-N curve of rotary table
The EBZ160 Roadheader machine rotary table is designed for large low-alloy steel castings. It is made of Zg35CrMo with C content of 0.3%–0.37%, Si content of 0.3%–0.5%, Mn content of 0.5%–0.8%, P content of less than 0.035%, Cr content of 0.8%–1.2%, and Mo content of 0.2%–0.3%. Its yield limit is above 510 MPa, strength limit is 740–880 MPa, and the overall strength is relatively high.
The basic S-N curve describing the fatigue properties of the material is given by the symmetric cyclic fatigue test at R=-1. Approximate estimates can also be made by consulting the relevant manual based on material strength limit in Refs. [23, 24].
Through considerable experience and experimental accumulation, the following empirical relationship has been derived for the general use of metal materials:
The fatigue limit R=-1 under bending load could be estimated as:
The fatigue limit R=-1 under axial tensile pressure load could be estimated as:
The fatigue limit R=-1 under torsional load can be estimated as: .
The S-N curve generally has a form , where m and C are constants related to materials, loading forms, etc.
For metal materials, the number of cycles corresponding to the fatigue limit is generally 107. As the S-N curve describes long life fatigue and is not suitable for the following N＜1000, the fatigue stress of N=103 was assumed to be . In this study, the strength limit of ZG35CrMo is 740 MPa, that is: N1=103, .
The area where the roadheader is known to be weak in the left and right rotary cutting process is mainly the inner side of the truncated arm and the hinge ear of the rotary, as well as the hinge of the rotary table and the rotary hydraulic cylinder. These areas are mainly subjected to tensile loads.
The fatigue analysis of the EBZ160 rotary table was performed using the AWE fatigue module. The Poisson ratio was 0.3, yield limit was 510 MPa, strength limit was 740 MPa, and the elastic modulus E was 2.1×1011 Pa. A set of data was calculated by the final S-N curve relational formula in Table 1.
After importing this set of data into AWE, the software automatically generated curve 10, as shown in the following figure:
In the generated linear form and single logarithmic form S-N curve, the whole curve is not smooth with N=108. From the AWE module, a one-dimensional interpolation of various stress states in the form of a set of input data was performed, and the S-N curve of the linear form of ZG35CrMo was obtained, as shown in Figure 11. The S-N curve of the single logarithmic form of ZG35CrMo obtained by interpolation is shown in Figure 12.
4.2 Fatigue analysis
According to the previous static analysis results, the loading mode of the maximum stress was selected to load the rotary. The stress cloud diagram of the rotary is shown in Figure 13, with a maximum stress of 502.96 MPa, which is located inside the hinged ear of the rotary and the cutting part.
The results of fatigue analysis of the rotary using AWE show that the number of cycles in the most critical areas is 15,615 times, as shown in Figure 14.
In the process of cutting and digging of the EBZ160 roadheader machine, the cutting part cuts a section according to the class S type from bottom to top at a rate of about 1 m at a time. When the coal rock of the roadway is hard, the EBZ160 roadheader machine cuts in the maximum output state, and fatigue crack appears in the inner side of the hinged ear when the excavation reaches 7807 m.
At 75% of the maximum output state, the maximum stress of the rotary was 377.22 MPa, which was identified inside of the hinge ear of the rotary and the cutting part. Using AWE to analyze the fatigue of the rotary table, the number of cycles at the most critical areas was 2.557×105 times. When the cutting operation was performed with a maximum output state of 75%, the excavation operation of the roadheader machine could reach 127900 m before the appearance of fatigue crack on the inner side of the hinged ear.
At 50% of the maximum output state, the maximum stress of the rotary was 251.48 MPa, which was identified on the inside of the hinged ear of the rotary table and the cutting part. In the AWE fatigue analysis, the number of cycles at the most critical areas was 1.6384×107 times. No fatigue failure is expected in the rotary table of the roadheader machine when the cutting operation is carried out with a maximum output state of 50%.
From the data of the EBZ160 roadheader, the cutting output load is generally about 50% of the maximum output load. Therefore, fatigue damage is not likely to occur in the rotary table. Although the maximum output load may be reached occasionally, it will not cause fatigue damage because this situation lasts only for a short time. Even assuming that the roadheader machine is operated with a maximum output load of 75%, the above analysis shows that the roadheader can still continue excavating up to 127900 m before the appearance of fatigue cracks. According to the current known EBZ160 roadheader machine, with a maximum monthly excavation of 1000 m, it can last for more than 10.66 years before the appearance of fatigue damage. This greatly exceeds the normal length of service of the roadheader machine.
Therefore, the rotary table does not face the risk of fatigue damage with the roadheader machine under normal working loads and normal service life.