Papers by Keyword: Fatigue Crack

Abstract: This study examines the fatigue crack propagation behaviour of AISI 4140 steel subjected to multi-austempering heat treatments. Tensile test and fatigue crack propagation (FCP) specimens were prepared in accordance with ASTM E8 and ASTM E647, respectively. Multi-austempering was carried out by heating the specimen to an austenite temperature for 10 min using an induction heating coil. The specimen was then immersed in a salt bath for each isothermal transformation time of 60 min at three austempering temperature levels from 312°C to 412°C with a temperature increase of 50°C. Tensile and fatigue crack growth tests were performed on both annealed and multi-austempered specimens. It is observed that the multi-austempering heat treatment significantly improves the tensile properties and the FCP properties of AISI 4140 steel. Microstructural observations indicate that the bainitic phase and the retained austenite increase the tensile strength and reduce the fatigue crack propagation rate (da/dN). It is found that the bainitic structures are an effective barrier in reducing fatigue crack propagation as the fatigue loading cycle increases

Abstract: Compressor valve spring failure and rapid fracture occurred in a petrochemical enterprise. To find the cause of the failure, and to ensure the safe, stable, and continuous operation of the device, the failure of the spring is analyzed. In this paper, through the macro inspection, chemical composition analysis, metallographic analysis, scanning electron microscopy analysis, energy spectrum analysis, hardness analysis, and other tests, it is concluded that the main reason for the spring fracture is the fatigue fracture caused by unqualified materials. Suggestions are given to avoid similar problems in the future, and it is hoped that this failure analysis will provide valuable experience for similar failure problems in petrochemical enterprises.

Abstract: During a routine overhaul of an aero engine, a joint of the fuel transmitting pipe was observed leaked. To investigate the cause (s), failed transmission pipe was subjected to detailed investigations utilizing multiple testing techniques to find out the root cause (s) such as radiography, dye penetration testing, optical microscopy, scanning electron microscopy and energy dispersive spectroscopy. It was identified that the welding defects i.e., lack of fusion caused initiation and propagation of the subsurface fatigue cracks under vibrational condition of the aircraft. When the crack length reached a threshold level, it resulted in fuel leakage.

Abstract: This paper demonstrates a simple technique to detect vibration-induced fatigue cracks using a hybrid method by vibration and acoustic emission techniques. A thin aluminum plate of 6082-T6 was excited using a vibration shaker to achieve a bending mode where the maximum stress exhibited at the plate mid-span. To simulate crack formation, a sharp notch was created. This systematic setup allows mode I crack propagation through plate thickness. The development of cracks over time changed the natural frequency of the plate which leads to the reduction of vibration amplitudes. This experimental technique facilitates the identification of acoustic emission waves during the onset of damage in the presence of noise due to dynamic motion. The effect of crack development on Lamb waves was investigated. The acoustic emission signals were cross-correlated with a Gaussian window of a central frequency of 250kHz. The results show a reduction in the fundamental wave A₀, whilst an increase in S₀ wave amplitudes at some stages during crack extension. The current experimental work can be an alternative technique for vibration-induced fatigue test evaluation.

Abstract: Based on the Paris model of fatigue crack growth theory, the fatigue crack growth behaviour of Center Crack Tension (CCT) specimens of laser melting deposited 12CrNi2 alloy steel is studied by extended finite element method (XFEM). The crack growth rules and fatigue life are analyzed by experiment and finite element simulation. The experimental results are in good agreement with the finite element results, which verifies the accuracy of XFEM method to simulate the fatigue crack growth behaviour of laser melting deposited 12CrNi2 alloy steel components. Based on this, the effects of initial crack direction and load amplitude on fatigue crack growth behaviour are discussed. The results indicate that even if the initial crack direction is different, the crack will finally propagate in a direction perpendicular to the load. With the increase of the load amplitude, the fatigue life of the specimen with initial crack decreases exponentially.

Abstract: To know the fatigue damage state of X80 pipeline steel which is used for the drilling riser and prevent the occurrence of fatigue fracture accidents. Pulsating-impact-fatigue tests were conducted in laboratory, magnetic memory signals were measured during the whole fatigue process. The distribution of stress and magnetic field vector with different crack depth was analyzed by using finite element (FE) method. During fatigue crack initiation stage, magnetic signals increased slowly with fatigue loading cycles because of the effect of stress-magnetization. When fatigue crack appeared, magnetic signals were mainly affected by magnetic flux leakage (MFL) which was induced by fatigue crack. Magnetic memory characteristic parameters had a good linear relationship with fatigue crack depth. Magnetic memory signals were able to reflect the change of fatigue damage status.

Abstract: Surface defect is the main form of axle failure. In this paper, the cause of surface defect on wheel seat of a certain type of dynamic axle is analyzed systematically. In the test, the macroscopic chemical component of inspection axle was determined using spectrometer. And the chemical composition near the crack was detected by energy disperse spectroscopy and X-ray photoelectron spectroscopy analysis. The microstructure of the crack area was observed by optical microscope, while the crack microscopic characteristic was characterized using scanning electron microscope and confocal laser scanning microscope. The study found that, the defect is transverse crack, which was distributed circumferentially along the wheel seat. Further more, the typical features of wear or friction and iron oxide was observed on the wheel seat near the crack area. It was concluded that, due to surface presure and the existence of stress concentration, which the wheel and axle interference fit resulted in, wear abrasion and fretting fatigue initiated fatigue crack.

Abstract: It is known that fatigue crack path has an influence on the fatigue life of the structural components. In this study, a method of predicting the crack path is developed based upon superposition of a stress intensity factor (Green’s function solution) for the crack problem subjecting to a pair of concentrated loads. This method has a benefit to give us a complete closed crack path solution, and it is easy to predict the complex crack path in comparison with a numerical prediction by finite element (FE) analysis.

Abstract: Stage-I-fatigue-cracks are used as highly localized dislocation sources with well-known Burger’s vectors to study the interaction between dislocations and grain boundaries. This interaction in the plastic zone is of particular interest to understand the fluctuating crack growth in the very short crack regime. In the case of a blocked slip band the dislocations pile up at the grain boundary causing a local stress concentration. The resulting local stress distribution is calculated based on measurements of the dislocation density distribution in the plastic zone. For this purpose the slip line profiles were measured by AFM, the dislocation density distribution was determined and the dislocation-free zone model of fracture (DFZ) was validated. With this it is possible to quantify the grain boundary resistance and to combine geometric and stress approach for grain boundary resistance against slip transfer.

Abstract: Railway axles are subjected to cyclic amplitude loading which can lead to fatigue failure. For safe operation of railway axles a damage tolerance approach taking into account a possible defect in railway axle is often required. Because of different operation regimes of trains (fast/slow ride, ride on straight track, on curved track, over switches etc.) the load amplitude of axle is not constant. The variability of load is defined by a load spectrum, which is determined experimentally by measuring of load in service conditions. Even though the load spectrum is measured on several hundreds or thousands of operation kilometres, the railway axles are in operation much longer time (often tens of years). Therefore, some load amplitudes higher than ones measured in the test can occur during a long-term axle service. The contribution presented deals with the effect of extension of load spectrum by rare high load amplitudes, which can occur during long-term operation, on residual fatigue lifetime of railway axles.