Papers by Keyword: Fatigue Fracture

Abstract: The fatigue characteristic of 2124 aluminum alloy in T851 condition was investigated by means of scanning electron microscopy (SEM) and transmission electron microscope (TEM). And the result shows that, the fatigue life is closely related to the thickness of plates, this is because that different thickness of plates results in the variation of grain size, grain boundaries and substructures of alloy, which thus influence the fatigue behavior of 2124-T851 alloy. The number of grain boundaries along the propagation direction of cracks and the amount of substructures after heat treatment increase with the decline of thicknesses of 2124 alloy plates, while the grain size is reverse. However, no obvious size and dispersion changes for the precipitates. Comparatively, the fatigue resistance performance of 30mm thickness plate is better than the 40mm and 55mm thickness plates, which is due to the more profitable grain size, grain boundary and substructure.

Abstract: Fracture surfaces of two sets of rotor blades failed after strong testing procedures of aircraft engines in the test-stand are investigated by means of SEM. While the process of fatigue fracture in the first set of blades did not reveal any defect features, both the initiation and propagation of fatigue cracks was strongly influenced by casting defects.

Abstract: Fatigue fracture of the intermediate shaft in a transmission was analyzed by finite element method.The solid model has advantages of centrifugal force and spin softening effects over beam models.The results show that the fatigue fracture of the shaft is mainly due to the torsional vibration,the stress stiffening and spin softening have strong influence on the bending vibration medes,and torsional vibration modes do not changed at different speeds.There is accounting for the effects by an adjustment of the stiffness matrix.By using of solid models,the vibration modes of the intermediate shaft were researched,then the model was optimized in order to avoid resonance.

Abstract: Low cycle and high cycle fatigue tests of 35CrMo steel at different pretorsional angles were conducted and cyclic hardening and softening curves, hysteresis loops and S-N curves were obtained of 35CrMo steel after the torsional prestrain. Scanning electron microscopy (SEM) analysis was also made of the fatigue fracture. The results show that: 35CrMo steel features obvious cyclic softening with basically the same law and degree at different torsional prestrains. The area surrounded by the stress-strain hysteresis loop decreases with the increment of the pretorsional angle; the torsional prestain reduces the fatigue life of the materials.

Abstract: The failure shaft for light hydrocarbon pump was analyzed through metallographica test. The fracture appearance was examined through 3-D stereoscopic digital camera and the scanning electron microscope (SEM). The results showed that the chemical composition of the shaft was fit to the requirement of structural steel. There was sulfide inclusion distributed among the white stripe phases. The figure of phases was striation, and the direction of striation was along the direction of the axis that was the machining direction when the shaft was fabricated. The fatigue fracture was the facture characteristic because there was fatigue striation on the fracture surface. The sulfide inclusion and segregation was the main reason of the fracture.

Abstract: Fatigue fracture is the main style of failure in the metal beam under variable load and the formation of cracks is often the omen of breakage. The detection of cracks ahead is of important significance to prevent the production of vast defect and safety accidents. Loading experiments were performed on metal beams with artificial defects, the Acoustic Emission (AE) signals were extracted in the loading process. The methods like tabulation, the experience diagram analysis, distribution analysis and correlation analysis are respectively used to analyze events, ringing counts, energy counts, maximum amplitude, rise time and duration of AE characteristic parameters. Results show that the method of AE parameters analysis is not only able to capture in real time the development of fatigue cracks, but also fix the preliminary position of the cracks. So it is feasible to detect defects of the metal beam by AE technology.

Abstract: In this paper, the test of compact tensile fatigue fracture of 16MnR steel was done after high temperature, and the relation between the proof values Ji at crack initiation of fatigue fracture and fatigue fracture fractal dimension was studied. The experimental results show that the relation between the proof values Ji at crack initiation and the heating temperature. The compact tensile fatigue fracture fractal dimension was changed when 16MnR steel sample was heated to different temperature.

Abstract: The fatigue fractography and surface of A7075 aluminum alloy was investigated by means of optical microscope, scanning electron microscope and technology of metallurgic replica. The results show that the fatigue crack generally initiated in the surface or near the surface of the sample area, such as hole and inclusion. The crack propagation regions under high stress amplitude was characterized by furrow and hollow, and obviously presented fine ripple vein. While lots of fatigue striations and sidesteps with the holes and secondary cracks generated in the crack propagation regions of low stress amplitude. Fatigue transient breaking area is a mixed fracture of brittle and ductile fracture. In the fatigue crack propagation stage, with the increasing of cycle times, secondary cracks gradually merged into primary crack on the path of crack propagation. The dislocation of crack tip,grain boundaries and different grain orientations have great influence on the path of crack growth, which can make cracks bending.

Abstract: Fatigue strength is one of the most important mechanical properties. Durability and reliability of car parts is often defined by their fatigue strength, since most of them are loaded with dynamic, repeating or variable loads and the main type of failure is metal fatigue. Fatigue crack usually starts on the metal surface. The interrelationship of the surface layer together with the characteristics of internal metal volume determine the value of fatigue limit and the coefficient level of stresses’ intensity, which is required for the start of fatigue crack. Various hardening methods of surface have a huge impact on fatigue strength of structural materials. The choice of surface processing method is determined by properties and microstructure of a material, as well as the purpose and working conditions of part’s material. Very often the optimum processing is a combination of several methods, which enables to obtain the required properties (high fatigue strength, wear, etc.). Hardening with high-frequency electric current (HfEC) is widely used process for the surface hardening of steel. The components are heated by means of an alternating magnetic field to a temperature within or above the transformation range followed by immediate quenching. The core of the component remains unaffected by the treatment and its physical properties are those of the bar from which it was machined. Investigation of specific fatigue fracture properties enables to determine the kinetic of processes, related with fatigue crack initiation and propagation. Analysing fractures of machine parts and comparing them with the most specific “mode” fractures it is possible to determine the nature of affecting loads and the reason of part failure. The impact of several combined surfaces processing on the fatigue strength and fractures of carbon steel samples is investigated in the work. The surface was hardened using different processing combinations: by hardening with high frequency electricity currents, rolling by rollers, heating and cooling under different temperatures. Experimentally it was proved that thermal treatment of plastically deformed carbon steel significantly increases fatigue strength. After surface hardening with the given regimes, the microstructure and residual stresses are formed in such way that fatigue crack begins to grow under the hardened sample surface. Samples, in which the fatigue crack initiates inside the sample, have higher fatigue strength than the samples, the fatigue crack of which initiates on the surface.

Abstract: In this paper, the computer picture technology was used for recovering three dimensions shape of 16MnR steel fatigue fracture. The fractal dimensions of the 16MnR fatigue fracture surfaces were calculated after high temperatures. The relation between the fractal dimension of fatigue fracture and heating temperatures, the relation between the fatigue mechanical properties and heating temperatures were researched. The relation between the grain size of 16MnR steel and the fractal dimension of fatigue fracture, and the relation between the grain size of 16MnR steel and the fatigue mechanical properties were discussed.