Aiming at the base metal (BM) zone, heat affected zone (HAZ) and fusion zone (FZ) of the weld, the tip plastic zone and initiation behavior of microscopic short cracks (MSCs) in the toe of cross-welded joints are studied. Based on the theory of crack tip plastic zone and the fatigue index parameter (FIP), the shape and boundary size of the plastic zone at the tip of MSCs as well as the initiation life were calculated. The results show that the plastic zone was bone-like under plane stress and butterfly-like under plane stress; when the crack length increased from 2 μm to 30 μm, the boundary size of the plastic zone at the MSC tip increased by 15 μm and 2 μm under the action of plane stress and plane strain in the X-axis direction, respectively. As the grain gradient decreased from 0.5 to 0.1, the boundary size of the plastic zone at the tip of the MSC increased about 50%. MSC initiation occurs first in the BM zone, and life of initiation was about several hundred thousand cycles, which was consistent with the experimental result. The MSC expansion rate in BM zone was the fastest, with the average expansion rate about 2.5 × 10⁻³ μm/cycle; the MSC expansion rate in the FZ and HAZ was about 3.1 × 10⁻⁴ μm/cycle.

Through the comparison of fatigue properties of components made of composite materials and high-strength structural steel materials, this study proves that composite materials can replace traditional steel materials used in the production of mechanical structural components. The focus of this study was a low-speed, high-torque radial piston motor mounted on a roadheader. According to different theories, the motor block was designed using a composite material made of carbon fiber, a classic high-strength structural steel, and an aluminum alloy. The thickness of the motor cylinder obtained by theoretical calculation was verified by finite-element numerical simulation technology, and the fatigue phenomenon caused by the time change of the piston cylinder pressure was considered. The results showed that the stress results of the numerical simulation verify the rationality of the theoretical calculation of the cylinder size. In terms of safety factors, the motor cylinder made of composite materials was close to the motor cylinder made of high-strength structural steel, and the difference between the static safety factor and fatigue safety factor was only 0.8 and 0.86. The weight of the motor cylinder made of composite material was reduced from 32 N to 7 N compared with steel material, which was about 78% lighter. This is of great significance for improving the use efficiency of equipment and reducing fuel costs.