Papers by Keyword: Fatigue Fracture

Abstract: The study of fracture surfaces of materials is very important to understand failures in engineering elements, especially in structural products. However, it is not easy to evaluate fatigue fracture surfaces by using ordinary image analysis techniques. In this paper, we propose a new analyzing method to evaluate fatigue fracture surfaces from a view point of Homology.

Abstract: Fluid cylinder as an important fracturing equipment with stable and reliable performance is commonly applied in oil and gas fields, but it fractures frequently and its working life is generally low, which has seriously affected the fracturing process efficiency. In this paper, we used the stereo microscope, scanning electron microscope (SEM), direct reading spectrometer, metallurgical microscope and bending fatigue testing machine to study the fatigue failure mechanism of steel E4330 for fluid cylinder. The fracture analysis, hardness test, metallographic observation, bending fatigue test and pre-corrosion fatigue test were carried out. The result shows that the main cracking form of the fluid cylinder is fatigue cracking, the cleavage crack initiated near the intersection line and extended friable due to the corrosion and impact wear effects from the fracturing fluid and the high stress of the fluid cylinder; the fluid cylinder material E4330 contains a variety of alloy elements, and its organization is the uniform sorbite which can meet the working requirements. It has been observed that the fatigue limit of the material is 607MPa, and at a higher stress, the fatigue life would be nearly halved in the corrosive environment. As the fatigue life would greatly reduce due to the corrosion effects from the fracturing fluid, we should control the working stress and make measures for anti-corrosion.

Abstract: Split-sleeve cold expansion processing was employed on the 7050-T7451 aluminum alloy plate. Fatigue lives were compared according different expansion, then the relationship of fatigue life and expansion was analyzed. Residual stresses were measured with different expansion, and the fatigue fractograph was analyzed by SEM. The results show that the split-sleeve cold expansion can obtain longer life compared with the non-strengthen hole. When over the optimum expansion, fatigue life began to decrease. The maximum fatigue life increased to 2.92 times with 4.1% expansion. The maximum values of radial residual stresses grew with expansion. The depths of residual compressive stresses were more than 6mm with 2.6% and 4.1% expansion. The fatigue fractograph shows mixed transgranular fracture.

Abstract: Failure causes on a condensate pump shaft were discussed by means of macroscopic observation, chemical composition analysis, metallurgical microstructure inspection, mechanical property tests and stress analysis. The experimental results showed that the improper material with disqualified chemical composition, improper microstructure and poor mechanical properties acted as inherent factor. In addition, structure design that led to stress concentration contributed to the failure as external factor. Finally, fatigue fracture happened at the reduce section of the pump shaft.

Abstract: Fracture morphology analysis is an important way to understand material fracture process, find the reason of fracture and improve its performance. Nickel based powder metallurgy (P/M) superalloy is an important material which is widely used in the modern aeronautic and astronautic industry. In order to determine the fracture characteristic morpholgies and mechanisms of the P/M superalloy fractured at different conditions, the fracture surfaces of the high temperature low cycle fatigue,high cycle fatigue, high-low cycle complex fatigue, tension and creep specimens from the P/M superalloy were studied by scanning electron microscopy and stereo-microscopy. The mechanisms and influencing factors of high temperature fatigue crack initiation and propagation were emphatically analyzed, and creep and tension fracture mechanisms of the P/M superalloy were also discussed. The application of the fracture surface analysis for the fracture failure analysis of the P/M superalloy turbine disk were introduced finally.

Abstract: The paper is concerned with an analysis of utility properties of selected aluminium alloys, namely EN AW 6082 (AlSi1MgMn), EN AW 6061 (AlMg1SiCu) and EN AW 7075 (AlZn5.5MgCu) with different technology of hot processing. The alloys were hot processed to reach T3 condition (solution annealing, cold forming and natural aging) and to reach T4 condition (solution annealing and natural aging). The following parameters were subject to evaluation: microstructure and sub-structure, mechanical properties (strength and plastic characteristics stated in Table. 1) as well as fatigue properties of these alloys assessed during alternative symmetrical stress in torsion. The paper was aimed to study the affect of processing technology of three aluminium alloys on their utility properties in the conditions of tensile stress and cyclic stress in torsion.

Abstract: A repeat premature gear tooth breakage occurred in a load gearbox of a 40MW steam turbine. Factographic examination indicated that the breakage was a complex fatigue fracture. The oil deposit on the fracture surface has been applied as an auxiliary fractographic method. The root cause of the failure was due to improper heat treatment. The insufficient surface hardness and shallow case resulted in case/core separation through main fatigue crack propagation. The very coarse detrimental tempered low carbon martensites in core resulted in cleavage fracture in final fast fracture. The fractographic morphology of the butterfly has been revealed. Hertzian stress and non-metallic inclusions are not the necessary condition of the butterfly formation. The microcracks within the butterflies did not actively play the role of the fatigue rupture.

Abstract: This paper illustrates a technique that may be used to evaluate the risk of structural failure of each aircraft in a fleet when a crack has been detected in a particular member aircraft, or the risk of failure for that member has become too high. When a crack is detected, the calculated risk of failure for other aircraft in the fleet will increase significantly, and the aircraft operators need to decide which aircraft should be temporarily grounded for unscheduled inspection and which ones be allowed to fly. The proposed method applies the Bayesian inference to update the risk assessment by updating the equivalent initial flaw size distribution, which is one of the key inputs for risk analysis. To illustrate the method, a hypothetical fleet aircraft is considered and the single flight probability of failure of each aircraft in the fleet is revised after the occurrence of a failure in one fleet aircraft.

Abstract: The aim of this paper is to provide an alternative methodology, high reliability, in order to monitor, exploiting non-invasive techniques, railway track. In this particular case study, the presence of structural defects is assessed through an innovative system. The proposed approach provides the use of rotating magnetic field. Within this framework, the Eddy Current Techniques (ECTs) have greatly increased their importance for their capacity to detect the magnetic field variations caused by the presence of material alterations. Whilst the researches in this field propose various solutions to the detection of flaws (in surface and subsurface), a still open problem concerns the difficult detection of defects due to the geometrical complexity of the structure and the particular orientations of the crack. Particularly, this contribution proposes an approach based on Finite Element (Finite Element Method, FEM) for the modeling of railway tracks, in order to obtain a rapid and precise assessment about their integrity.

Abstract: Fatigue fracture occurred on one blade when a certain aero-engine was under test run. The fracture is observed to measure the crack length and the average distance between fatigue striations. The Paris formula is employed to calculate the fatigue propagation life of the two main fatigue propagation regions observed on the fracture. The results show that the rate of crack propagation present a tendency of regular increase, and the fatigue propagation life takes a very small ratio of the total fatigue life of the blade.