Case studies in engineering failure analysis最新文献

As in situ natural composites with silicon phase acting as the reinforcing phase, Al-Si alloys are among most commonly used aluminum alloys in automotive applications (i.e. engine component). Silicon contributes to the strength of Al-Si alloys through load transfer from the Al matrix to the hard (rigid) Si phase in the microstructure (load-carrying capacity). Casting parameters (i.e. solidification rate, elemental segregation, secondary dendrite spacing…) as well as the size and distribution of the microstructural constituents in Al-Si alloys (i.e. morphology of Si particles, intermetallic compounds, secondary dendrite spacing) contribute directly to the mechanical response and failure (or fracture) behavior of the alloy within the service. In hyper-eutectic Al-Si alloys (i.e. B390.0), distribution of coarse pre-eutectic Si particle mainly contribute to stress concentration, crack initiation and propagation during the actual service condition. In the present paper, the parameters contribution to the formation of the circumferential cracks in clutch housings made of die cast hyper-eutectics B390.0 Al-Si alloys are assessed through optical microscopy and scanning electron microscopy. Casting variable, cooling rate, their effect on the cracks as well some of the possible causes are also discussed in detail.

This article describes the findings of a detailed investigation into the failure of a secondary super heater tube in a 140 MW thermal power plant. Preliminary macroscopic examinations along with visual examination, dimensional measurement and chemical analysis were carried out to deduce the probable cause of failure. In addition optical microscopy was a necessary supplement to understand the cause of failure. It was concluded that the tube had failed due to severe creep damage caused by high metal temperature during service. The probable causes of high metal temperature may be in sufficient flow of steam due to partial blockage, presence of thick oxide scale on ID surface, high flue gas temperature etc. rupture.

This is a case study regarding frequent forced outages in an oil-fired power generating station due to failure of fire resistant fluid (FRF) piping of material ASTM A-304. This analysis was done to find out the most probable cause of failure and to rectify the problem. Methods for finding and analyzing the cracks include nondestructive testing techniques such as visual testing (VT) and dye penetrant testing (PT) along with that periodic monitoring after rectification of problem. The study revealed that pitting and pit to crack transitions were formed in stainless steel piping containing high pressure (system pressure 115 bars) fire resistant fluid. However, after replacement of piping the pitting and cracking reoccurred. It was observed that due to possible exposure to chlorinated moisture in surrounding environment pitting was formed which then transformed into cracks. The research work discussed in this paper illustrates the procedure used in detection of the problem and measures taken to solve the problem.

This research paper deals with the investigation of causes of failure of the first stage gas turbine blade of 30 MW gas turbine having tip cracks at the trailing as well as leading edge. This blade is made of nickel based super alloy IN738LC, having directional solidified grain structure and contains aluminide coating (Pt–Al₂) by diffusion process. The main focus of the paper is evaluation of tip crack at the trailing edge and further degradation of blade coating. The investigations included the visual observations, material composition analysis, scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS). During examination of the failed blade, it is found that the blade surface is completely degraded due to overheating. Corrosion pits are formed on the blade surface, these pits act as a notch to produce stress concentrations and cracks is initiated due to fatigue which further propagate due to hot corrosion. It is also reported that the Al-Pt coating is completely degraded, heavily oxidized. From these investigations, it is concluded that the failure of gas turbine blade takes place due to the combined effect of surface degradation caused by overheating, oxidation, hot corrosion and degradation of coating heavily oxidized.

The Forced Draft Fan (FDF) blade in a 300 MW coal fired power plant that experienced catastrophic failure has been investigated. There were two main locations of the blade damage, namely damage at the root of the blade and the other one is at the third of the blade height. The FDF blade has been run for 5 years and before its failure, the FDF experienced high vibration (14 mm/s). The forced draft fan is an axial flow fan horizontally in front of the boiler and the fan is single stage. Visual inspection, metallographic analysis, chemical composition and hardness test were carried out to find the cause of the failure. It is concluded that the material of the blade is cast Al-Si alloy (A356.0) that fits the requirements for FDF blade application, the failure of the third of the blade height is owing to the external particles collide to the leading edge of the blades causing erosion and notch. That notch acted as initial crack. The failure at the root blade was caused by broken fragments of the others damaged blades entered in between casing (stator) and the blade (rotor) so they obstructed the blade rotation.

On July 11, 2012 an eastbound Norfolk Southern train derailed 17 cars within the city limits of Columbus, OH. Three of the cars that derailed were carrying over 86,000 gallons of denatured ethanol. Once breached, the ethanol in the tank cars ignited, fueling a large fire. The derailment led to the evacuation of over 100 people and cost over $1.2 million. This paper will detail the on-scene response and failure analysis performed by the NTSB Materials Laboratory, focusing on the recovered rail that contained 25 transverse detail fractures.

In this study, a low-cycle fatigue experiment was conducted on printed wiring boards (PWB). The Weibull regression model and computational Bayesian analysis method were applied to analyze failure time data and to identify important factors that influence the PWB lifetime. The analysis shows that both shape parameter and scale parameter of Weibull distribution are affected by the supplier factor and preconditioning methods Based on the energy equivalence approach, a 6-cycle reflow precondition can be replaced by a 5-cycle IST precondition, thus the total testing time can be greatly reduced. This conclusion was validated by the likelihood ratio test of two datasets collected under two different preconditioning methods Therefore, the Weibull regression modeling approach is an effective approach for accounting for the variation of experimental setting in the PWB lifetime prediction.

The formula SAE is a competition among students worldwide, where they are challenged to design, build and test a small formula-type racing car, following rules and specifications defined by the Society of Automotive Engineers (SAE). In the present analysis, a premature failure of a half-shaft of the transmission system of a racing car occurred after circa 100 km of use. The fractured component was analyzed by means of metallographic, chemical and mechanical analysis, fractography and finite element analysis. The results show that the alloy steel bar used for the half-shaft did not follow specifications, with a consequent lower strength and resulting in a material with insufficient loading capacity and fatigue resistance. As a consequence, the fracture process was a confluence of torsional fatigue crack propagation and overload ductile fracture through the spline section of the half-shaft.

Failure analysis was carried out on a tube-to-tubesheet welded joint of a shell-tube heat exchanger to confirm its failure mechanism. The collected evidence suggests that the failure of the tube-to-tubesheet welded joint was induced by fatigue. Under the morphology analysis, the fracture surface exhibit obvious fatigue crack propagation traces. Fatigue striations were observed clearly in the propagation zone. Detecting results showed that chemical composition and mechanical properties of the tubes was consistent with the standard requirements for the 304 stainless steel. The metallographic test showed that the microstructure of both the tube and tubesheet were normal. However, serious defects were found at the tube-to-tubesheet welded joint. The fatigue crack initiated from the defects at the tube-to-tubesheet welded joint. The periodic load might come from the resonant vibration of the tube or the changes in temperature and pressure of the fluid inside the heat exchanger. Bad welding and unsuitable expansion position of the tubes at the tube-to-tubesheet might give rise to the formation of initial cracks.

Three cases of trilayered fixed dental prostheses fractured in vivo are fractographically analyzed to identify causes of failure. In all cases fractures initiated from a preexistent crack/defect in the zirconia framework produced previous to the fusion firing of a lithium disilicate veneering ceramic. In two cases zirconia framework pre-cracks were found to have been infiltrated with the intermediate fusion glass that diffused therein during sintering. This report highlights the susceptibility of zirconia to grinding damage and its role in limiting the lifetime of zirconia-based fixed dental prostheses.