Case studies in engineering failure analysis最新文献

An oil pipeline embedded in an underground trench had failed. Through the accident investigation we found that there was a perforation at the leak point of the pipeline. Macroscopic observation revealed that some pits collectively located at the exterior surface of the failed pipeline. In addition, raised ridges and circumferential cracks were observed inside the large pits with stereoscope and scanning electron microscope. After careful analysis it is concluded that the leakage of the pipeline was mainly caused by the liquid impingement erosion.

This paper mainly investigates the fracture problem of U71Mn rail flash-butt welding joint. Fracture surface morphology, microstructure and micro hardness are analyzed by using the scanning electron microscopy (SEM/EDS), the optical microscope (OM) and the micro Vickers hardness tester (Vickers-tester). The analysis results show that the welding joint is fatigue fracture, and the fracture surface morphology is the cleavage fracture characteristics. The metallographic morphology, inclusions and micro-hardness near the fracture surface are all in the normal levels. On the other side, the free solidification microstructure which extended from the outside to inside in the joint of the left side of the rail web and the rail head is the crack source of the rail welding joint fatigue fracture. Under the action of bending stress, the crack firstly generates in this area, and gradually extended to the rail web, to final fracture.

The analysis of an abnormal crack of a forging plate is presented in this work. The crack was found after forging process. Macro-analysis, SEM, composition inspection, metallographic analysis, inclusion analysis, EPMA and EDS were implemented. SEM shows that cleavage fracture is the main feature of the fracture surface, according to which it can be decided that the fracture is brittle fracture. XRF and carbon and sulphur analyzer indicate regular composition condition of the plate. EPMA and EDS suggest phosphorus segregation spread a lot around the crack. The results indicate that: brittleness caused by phosphorus segregation around crack zone is the main reason that cracks generate in the 12Cr13 steel during forging. As all we know, phosphorus has deleterious effect on toughness of steel because of its enrichment on grain boundaries, which can weaken the bond strength of grain boundary. Its existence should be avoided and microstructure of the steel should be homogeneous when good plasticity and toughness are wanted.

A corroded tee pipe belonging to a 10-in new separator water line outlet installed in an offshore oil well drilling rig was investigated. The configuration of the large corroded pit had the shape of an imperfect horse's hoof with a completely corroded interior and a honeycomb-like cavity. There is a badly corroded pit at the welded seam at the joint connecting the tee pipe and flange. The material strength of the tee pipe meets the requirement of ASTM-A234 Gr. WPR, but its chemical composition does not meet the stipulated requirements. A deformed streamline structure or twin crystal in the ferrite phase can be seen near the surface or sub-surface of the perforated corrosion puncture edge. The micro-hardness is also different from that of the original material. The SEM results show that the puncture appears to have been a mode of quasi-cleavage fractures with secondary cracks along the direction of the crystal grain; thus, the failure has the features of mechanical and chemical corrosion. The corroded surface contains high amounts of O, C, and N, as well as S, Cl, Si, Na, Mg, Al, K, Ti, etc., all of which are corrosion products caused by sea water. The penetrating puncture hole at the turn of the tee pipe is likely the result of cavitation erosion accompanied by chemical/electrochemical corrosion, and the corrosion on the seam connecting the tee pipe and flange is likely the result of electrochemical corrosion.

Failure of a cracked steering wheel is studied in this paper. Steering wheel, mounted on a speed boat, had cracks emanating from one of the fastener holes until final fracture occurred. Failure analysis, combining experimental and numerical techniques, was performed. Experimentally, fasteners torque moments were measured, visual inspection performed and material type determined (aluminum alloy AA 6061). Additionally, scanning electron microscopy examination was employed to characterize the microstructure of the fractured surface. Using finite element analysis, stress analysis of a cracked steering wheel was conducted. Stress intensities of uncracked and cracked steering wheel were compared to find out about stress concentration points. Possible causes of crack occurrence include excessive fastener torque moment, fretting between fastener and hole combined with poor machining that left marks that serve as potential crack initiation points. Obtained results are valuable for predicting fracture behavior of the cracked steering wheel and can be taken as a reference for design and exploitation process of such component.

The bridges experience increasing traffic volume and weight, deteriorating of components and large number of stress cycles. Therefore, assessment of the current condition of steel railway bridges becomes necessary. Most of the commonly available approaches for structural health monitoring are based on visual inspection and non-destructive testing methods. The visual inspection is unreliable as those depend on uncertainty behind inspectors and their experience. Also, the non-destructive testing methods are found to be expensive. Therefore, recent researches have noticed that dynamic modal parameters or vibration measurement-based structural health monitoring methods are economical and may also provide more realistic predictions to damage state of civil infrastructure. Therefore this paper proposes a simple technique to locate the damage region of railway truss bridges based on measured modal parameters. The technique is discussed with a case study. Initially paper describes the details of considered railway bridge. Then observations of visual inspection, material testing and in situ load testing are discussed under separate sections. Development of validated finite element model of the considered bridge is comprehensively discussed. Hence, variations of modal parameters versus position of the damage are plotted. These plots are considered as the main reference for locating the damage of the railway bridge in future periodical inspection by comparing the measured corresponding modal parameters. Finally the procedure of periodical vibration measurement and damage locating technique are clearly illustrated.

A failure incident of heat-exchange tubes in ammonia evaporators, which suffered from unexpected wall thinning after only one-year service with respect to their original design lifetime of fifteen years, was reported and carefully analyzed. After overall inspection, many tube walls in the evaporators were found to experience severe degradations at both sides with distinct corroded defects and general cracking of corrosion layers. Thus, comprehensive investigations including external appearance, microscopic morphology and chemical composition were carried out by using a series of characterization methods. The analysis results demonstrated that the unexpected wall thinning of tubes was primarily ascribed to multiple corrosion factors including uniform corrosion, pitting and interaction behavior between them. Relative failure mechanisms were discussed in detail and prevention measures were also proposed for ammonia evaporators under similar operating condition.