Analysis of fatigue fracture morphology of low speed and heavy load gears

Fatigue fracture is the surface formed by the fatigue fracture of the test sample or component during the test or use. Fatigue fracture includes the unrecoverable deformation of specimen or part before fracture under the influence of loads, material defects and environmental media, and the whole process from fatigue crack initiation to propagation to final fracture. It is the key part of fracture process research. The color, morphology, smoothness, crack location and propagation path of fatigue fracture are affected by many factors, such as the test environment, stress state, material properties and defects, which are related to time. The observation and analysis of fatigue fracture can trace the process of fracture, infer the cause of fracture and determine the nature of fracture. Therefore, it is of great significance to quantitatively describe the characteristics of fatigue fracture, to establish the characteristics of fatigue fracture, to analyze the mechanical properties and to solve the relationship between various parameters in the fracture process, and to deeply understand the nature of fracture.

In recent years, with the development of characterization methods such as electronic technology and the popularity of scanning electron microscope, energy dispersive spectrometer and transmission electron microscope, the high-power microscopic observation of metal fatigue fracture morphology and the characterization of microstructure are becoming more and more easy.

Qin Chao, Liu Xinling and Liu Lele used fractal theory to simulate and identify metal fracture; The micro mechanism of fatigue damage of advanced nickel base superalloy and the micro characteristics of high cycle fatigue fracture of 5A06 aluminum alloy were studied by using scanning electron microscope and energy spectrum analysis. The results show that the crack will deflect and the deflection path will become more complex with the change of stress level, and the fatigue striation spacing will continue to increase.

Yin et al. Used EBSD to study the effect of twinning and de twinning in the process of fatigue crack initiation and propagation. After the EBSD calibration and microscopic observation of the fatigue fracture, it was found that the residual twins produced by twinning and de twinning resulted in the lamellar structure in the fatigue source region of the fatigue fracture.

Tan et al. Studied the high cycle fatigue fracture morphology of AZ31 magnesium alloy rolled plate. The strip like lamenas were observed in both the fatigue source region and the fatigue propagation region. The experimental analysis showed that this was due to a {101 ̅ 2} There is a secondary {101} in the twin ̅ 2} The morphology of fracture surface after twin fracture.

Jiao studied the fracture behavior of high pressure die casting hypereutectic (HPDC) Al Si alloy using high resolution laboratory CT and synchrotron X-ray tomography, with special attention to the effect of HPDC microstructure. The results show that the microstructure of the alloy is mainly composed of primary silicon particles (PSPS), aluminum dendrites, copper rich phases and pores. Most of the rough PSP, copper rich phase and pore are located in the center of the sample. The rapid solidification of HPDC results in the heterogeneity of microstructure. Copper is enriched at the boundary of the solid-liquid interface, resulting in the formation of large-scale dendritic arms. Pores are formed in dendrites under high applied stress. The microcracks originate from the pores and further connect the copper rich phases, resulting in intergranular fracture. As an obstacle, PSP causes stacking dislocation in the phase interface. In the area where a large amount of PSP is enriched, the PSP is broken rather than stripped from the matrix, indicating that the intergranular fracture of PSP occurs. The microcracks and PSPS originating around the hole tend to converge to the main crack, thus reducing the energy required for crack propagation.

Zhao used css280i-20w electro-hydraulic servo universal testing machine with microscope observation system to measure the low cycle fatigue life of TC25 titanium alloy smooth specimens under various stress ranges at room temperature. Based on the analysis of low cycle fatigue stress-strain hysteresis loop of TC25 titanium alloy, a more simple Manson coffin formula is derived by experimental characteristics and stress-strain constitutive model, and the relationship between fatigue life and stress range is drawn. The stress fatigue life curve of TC25 titanium alloy is obtained by linear regression analysis. Finally, the fracture surface morphology of TC25 was studied by jsm-6360 scanning electron microscope, and the fatigue fracture mechanism of low cycle fatigue was studied. The results show that the plastic deformation mainly occurs in the accelerated fracture stage, and various shear lips are observed on the fracture surface, which indicates that the shear stress leads to the ultimate fracture of TC25 titanium alloy. In the process of low cycle fatigue fracture, splitting nucleation leads to the formation of fatigue crack initiation zone, and the fatigue crack growth shows a mixed growth mode of transgranular and intergranular cracks. In the final fracture zone, the fracture surface of low cycle fatigue is TC25 titanium alloy, which is a typical semi brittle fracture mode.

Through the observation and analysis of the micro morphology of the fatigue fracture obtained from the fatigue crack growth test, the research on fatigue crack growth and fatigue life can be more in-depth and comprehensive.

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