The high cycle fatigue property of carburized 20Cr gear steel was experimentally investigated under axial loading with a stress ratio of zero. The test results show that this steel exhibits gradually decreasing S-N characteristics, and the fatigue strength corresponding to 10⁹ cycles is about 455 MPa. Based on the observation of fracture surfaces, the interior inclusion fisheye-induced fracture is the predominant fracture mode in the life regime beyond 10⁵ cycles. The fine granular area (FGA) cannot be found clearly around the inclusion. Based on the evaluation of stress intensity factor ranges (ΔK) at the front of inclusion and fisheye, the interior crack growth rate (da/dN) equation can be characterized by da/dN = 2.39 × 10⁻²¹(ΔK)^12.32. Corresponding to the tested specimen, the predicted maximum inclusion size is about 52.02 μm. The predicted fatigue strength corresponding to 10⁹ cycles by using Wang’s model is slightly higher than the experimental result, but that by using Murakami’s model is relatively conservative.

This study investigates the influence of geometric discontinuities specifically, circular holes – on the fracture behavior of fiber-reinforced polymer composite plates containing edge cracks. Using the Finite Element Method (FEM) as a non-destructive numerical tool, we analyze how variations in hole size and positioning affect the stress distribution σ _yy and the J-integral near the crack tip. The J-integral, used both as a fracture toughness parameter and a crack detection indicator, effectively captures the energy release rate and the severity of crack-tip conditions in anisotropic composite materials. Reference models without holes were first validated before introducing circular holes at varying distances from the crack front. The results reveal that strategically placed holes can reduce local stress concentrations and lower the J-integral values, mitigating the driving force behind crack propagation. These findings highlight the importance of geometric optimization and advanced simulation in improving damage tolerance and fracture resistance of composite structural components. Furthermore, the insights gained are particularly relevant to the aerospace industry, where fiber-reinforced polymer composites are increasingly used in aircraft structures due to their high strength-to-weight ratio and fatigue performance. Understanding fracture behavior under geometrical modifications enhances the structural reliability of such lightweight components under operational conditions.

To study the effect of the variable amplitude on the carburised 17CrNi high-strength steel, very high cycle fatigue tests were performed on smooth specimens under the stress ratios of 0 and −1. After the experiment, the characteristics of the fracture surfaces were investigated by means of an optical microscope and a scanning electron microscope. Circular-formed marks were found around the inclusion inside the fisheye. Finally, based on five damage models, the fatigue life of the carburised 17CrNi high-strength steel was predicted. The comparison of the calculated lives with the experimental data shows that the modified Miner's criterion is the most accurate and it can adequately characterise the fatigue life of the 17CrNi steel.