Papers by Keyword: Fatigue Fracture

Abstract: A failure analysis has been conducted on a diesel engine crankshaft used in a commercial vehicle, which is made from 42CrMo forging steel, and the crankshaft was after induction hardening process. The fracture occurred between the 4th journal and the 4th crankpin, fracture section indicates that fatigue is the dominant mechanism of failure for this crankshaft. In order to find the failure causes, the dimension of journal boss fillet, material chemical compositions, surface hardness and depth of hardness layer were evaluated. Chemical compositions of crankshaft material, surface hardness and the depth of hardness layer are within the specified range, however the dimension of journal boss fillet is less than specified, and the transition zone of hardness layer was found on the journal boss fillet. Heavy segregation and many non-metallic inclusions aggregate zones were found from metallographic, these would lead to the reduction of endurance bending strength. FEA analysis results suggest that the dimension of journal boss fillet have an important effect on the stress and stress amplitude of the crankshaft. So the disqualification machining and material property are the main causes for the crankshaft fatigue fracture.

Abstract: The tensile properties and fatigue properties were tested, and the fatigue fracture and damage behaviore of Al-5Zn-2Mg high strength aluminum alloy was investigated by means of optical microscope (OM) and scanning electron microscope (SEM). The results show that the grain of the Al-5Zn-2Mg high strength aluminum alloy is elongated after rolling deformation. The tensile strength of the Al-5Zn-2Mg aluminum alloy is 470MPa. The fatigue life is 6.4×104 cycle in R=0.1, f=99Hz, and σmax=210MPa. The average rate is 0.08μm per cycle from micro crack formed to 8mm of fatigue crack length. The fatigue crack forms in the surface. There are the fatigue striations in the fatigue crack propagation area. The crack velocity is about 2μm per cycle in propagation area. The damaged microstructure with the fatigue loading is loose, that indicates the micro cracks progressively emerge.

Abstract: In order to find the cause of the cracked intermediate axis in a transmission,the characteristic of the bending vibration，torsional vibration and bending-torsional coupling vibration were studied through the analysis of the nature frequency and modal shape based on prestress.The results show that the fatigue fracture of the axis is mainly due to the resonant torsional frequency and bending-torsional coupling vibration and It is basically demonstrated by experiment.It has been found that the fundamental frequency is increase with the increase in spin axis velocity because of the centrifugal load i.e.prestress.The effets can be accounted for by an adjustment of the stiffness,and the dynamic equations are derived.In the end,it is made the optimized design on the axis,the low inherent frequencies are optimized in order to avoid resonance.The problem of the cracked intermediate axis has been solved.

Abstract: Basing the foundation on the cutting experiments of 2.25Cr-1Mo-0.25V steel which is the material of hydrogenated cylindrical shell, the dynamic mechanical characteristics of the excessively heavy-duty cutting were analyzed. Then through researching the influence that the dynamic mechanical characteristics had on tools failure in limited heavy-duty cutting process, and the model of dynamic shearing force in cutting area was established. However, the experimental results showed that the dynamic shear-flowing-stress in cutting area had great influence on tools fatigue, and the mainly damaged form of the heavy-duty cutting tool was shearing fracture. Analyzing the theory comprehensively, the critical condition of tools fracture which was under the terms of extreme loading had been established finally.

Abstract: The cause of the fatigue fracture of the coal compactor roller shaft is analyzed in this paper. By means of investigating the macro-fracture and micro-fracture analysis for the shaft roller, and use ANSYS software to simulate the fatigue stress to find out the reasons of the roller shaft fracture and thus provides theoretical support for further optimizing the design of the roller shaft structure.This lied a solid foundation for how to improve the designation of roller shaft and how to improve the lifespan of the roller shaft

Abstract: Abstract: The fatigue behavior of cast steel ZG20SiMn under strain control mode was investigated, and the influence of imposed total strain amplitude on the cyclic deformation and fracture behavior of the cast steel ZG20SiMn was determined. The experimental results reveal that during fatigue deformation, the cast steel ZG20SiMn may exhibit the cyclic strain hardening and stable cyclic stress response behavior, which mainly depends on the imposed total strain amplitude. The relationship between plastic and elastic strain amplitudes with reversals to failure shows a single linear behavior, and can be well described by the Coffin-Manson and Basquin equations, respectively. At different total strain amplitudes, the fatigue cracks initiate at the free surface of fatigue specimens and propagate for the cast steel ZG20SiMn.

Abstract: The fatigue parameters of welded joints made of FV520B were obtained considering the real operating situations. The fatigue fracture types were divided into two groups. The scanning electron microscope (SEM) was applied to carefully study the fatigue fracture surfaces so as to investigate the fatigue failure mechanisms. The microstructures of the base metal and the weld seam were studied by the metallographic microscope (MM), SEM and TEM in support of the dissimilar mechanical property analysis. The present paper concluded that the fatigue microcracks mainly initiated from the surface near the weld toe and inclusions in the weld seam, the heat-affected zone (HAZ) disappeared due to the postweld heat-treatment processes, and that the fatigue resistance of the base metal was better than the weld seam.

Abstract: The fracture of U71Mn rail welded butt joints on flash welding was analyzed by using visual observation, metallographic microscope, SEM and EDS from macro to micro.The results show that, the transverse fracture is rail oval flaw and the fracture mechanism of the rail is fatigue-crack propagation. The fatigue cracking is caused by silicate-non-metallic inclusions which are residues in the welded joints after flash welding.

Abstract: Load distribution of the flexspline under alternating dynamic loads is complicated, and it’s difficult to measure the stress of the flexspline by experiment. To study the intensity of the flexspline, operating process of a harmonic drive is simulated based on the contact theory and the explicit algorithm of the finite element method. The parameterized finite element grid of the whole harmonic drive model is realized by Matlab, and then the assembly process of the vave generator is simulated by ABAQUS. Meanwhile, the distribution of the flexspline stress and the load characteristics of the flexspline tooth during dynamic transmission process of a harmonic drive are analyzed. The fatigue fracture position on the flexspline is forecasted according to the simulation results at last.

Abstract: Since the fatigue crack propagation process from initial size till final fracture is affected by lots of random factors, it is difficult to evaluate the fatigue reliability. Based on reliability theory, the first order second moment method ( JC method) is adopted to analyze and compute the fatigue reliability. To account for the uncertainties of material resistance, the parameters in the deterministic fatigue crack growth rate equation and material fracture toughness are taken as random variables with Normal distribution or Log-Normal distribution. Consequently, the limit state equation of fatigue crack growth is derived. The fatigue reliability index at any moment is calculated iteratively through JC method. As a computation example, the curve of fatigue crack growth reliability index with time is presented.