Journal of Failure Analysis and Prevention

Published by ASM International
Online ISSN: 1547-7029
Publications
Article
The high-strength aluminum alloy Al–5.5Zn–2.5Mg–1.5Cu (AA 7075) is extensively used in aerospace applications. An injector body of a gas generator for a turbine rotor in a liquid propulsion system was fabricated out of AA 7075 forgings in the T7352 condition, and, subsequent to a qualification hot test, one of the injector discs showed the presence of cracks on the top face of the body. Detailed metallurgical investigation indicated that the failure was caused by stress-corrosion cracking (SCC).
 
Article
A criterion for predicting the workability limits for internal britde failure was developed for cold heading of 1038 steel. The criterion considers internal defects caused by microstructural changes generated by adiabatic shear. This transformation is termed the transformed adiabatic shear band (TASB) phenomena. The defect that develops is the formation ofbritde martensite as a result ofthe temperature rise and fall inside the adiabatic shear band (ASB). In this work, the material is considered to have a TASB defect when the temperature inside the ASB exceeds the phase transformation temperature (AC3). The empirical formulas provided by Andrews[1] were used to determine transformation temperatures. Microhardness testing and etching with 2% Nital and Le Pera etchants were performed on the sectioned specimens to locate and study the TASB. In order to simulate the cold heading process, a drop weight compression test was used and modeled with finite-element analysis (implemented within ABAQUS/Explicit).
 
Article
Silicomanganese grade billets are the most commonly used steels for manufacture of automobile leaf springs. However, Cr-Mn-B grade steel known by trade name of SUP 11A grade is replacing the conventional silicomanganese grades such as 60Si7 or 65Si7 steels because it has become a competitive alternative in the market. Three heats of SUP11A grade spring steel were made through BOF-VAD-CC route and continuously cast into 125×125mm billets. Some of the billets contained blowholes and piping. Rolling of selected billets into 85×15mm flats revealed occasional slivers, seams, and a few shallow hairline surface cracks. A detailed metallurgical investigation was carried out to understand the genesis of these defects. A pearlite-free ferritic microstructure near the cracks combined with the presence of dispersed inclusions resulting from internal oxidation in the vicinity of cracks and the presence of scales within the shallow discontinuous short-length longitudinal cracks indicated that these defects resulted from pre-existing subsurface blowholes lying within 1mm of billet surface. Reduction of the gas content of liquid steel in the mold, optimization of electromagnetic stirring (EMS) current, and control of superheat are some of the broad measures identified to improve the cast quality of SUP 11A spring steel billets and minimize the rejection of rolled flats.
 
Article
The purpose of this investigation was to determine the root cause of the differences noted in the fatigue test data of main rotor spindle assembly retaining rods fabricated from three different vendors, as part of a “Second Source” evaluation process. ARL performed dimensional verification, accessed overall workmanship, and measured the respective surface roughness of the rods in an effort to identify any discrepancies. Next, mechanical testing was performed, followed by optical and electron microscopy, and chemical analysis. Finally, ARL performed laboratory heat treatments at the required aging temperature and follow-up mechanical testing.
 
Article
Spline actuators made of investment cast 17-4 PH (precipitation hardening) stainless steel were found to contain micro-cracks. The cracked actuators were subjected to optical and scanning electron microscopy and hardness testing, which revealed that the failure occurred due to fatigue crack initiation and growth after electrical discharge machining (EDM). The rehardened layer produced by the EDM remained after machining, and the cracks and surface irregularities associated with this layer provided sites for crack initiation and growth, which ultimately caused rejection of parts. Close dimensional tolerances on actuators require post-heat treatment EDM. Thickness of the recast layer was measured to be about 38–55μm, and precipitation in vicinity of the machined surface is a potential source for corrosion. Post-machining polishing by means of fluidized bed granules was employed to remove recast layer and associated precipitates. Test results proved that removal of surface layers improved the microstructure and the resistance to crack formation. The post-EDM polishing and subsequent annual inspections proved that problem was solved. KeywordsElectrical machining–Failure analysis–Microstructure–17-4 PH stainless steel
 
Article
Analysis of failed sections from a 31km long pipeline show that premature failure was caused by microbial-influenced corrosion. This case history summarizes the failure analysis and demonstrates the need for extreme care when using untreated water to hydrotest a pipeline.
 
Article
A Mazda Miata crankshaft and timing belt pulley bolt failed in service. This caused extensive damage to the engine. The crankshaft and bolt were analyzed to determine the cause of failure. Using visual examination and other means, it was determined that the crankshaft and bolt failed by fatigue. The crankshaft failure initiated at the keyway, while the timing belt pulley bolt initiated in the threads. Inadequate clamping force during installation of the timing belt pulley bolt is thought to have initiated failure. KeywordsCasting–SEM–Shafts–Torsion–Energy dispersive spectroscopy–Failure analysis–Fatigue failure
 
Article
This study deals with the analysis of an ink-producing machine rotor part composed of WC–2Ni–1Co which failed in brittle manner during service. The part was made by powder metallurgy techniques and is being used in ink-grinding machines due to its high hardness and wear resistance. Similar parts had worked satisfactorily for many ink compositions, but the part under investigation failed prematurely. Investigation was considered important because the part is expensive, and other identical components frequently failed after a short service life. Moreover, replacement of the part requires complete dismantling of the machine which reduces the production rate. Spectroscopic analysis, density, optical and scanning electron microscopy, SEM–EDS analysis, fractography, X-ray diffraction, and microhardness measurements were carried out on failed parts to find out the root causes of the failure. Results revealed that the part cracked due to combined effects of selective dissolution of metal binder-caused corrosive action of ink solution and hydrogen-induced deterioration of WC and Ni–Co phases. Localized removal of binder phase left the hard WC phase unsupported. Cracks were found initiating from the root of the machined slot which acted as a stress concentration point and resulted in brittle fracture. KeywordsWC–Co-Brittle fracture-Embrittlement-Fractography-Corrosion-Dissolution-Microhardness
 
Article
This paper reports the details of a test method that uses elements of elastic-plastic fracture mechanics to assess fracture resistance of zirconium (Zr)-2.5 wt.% niobium (Nb) pressure tubes for a pressurized heavy water reactor. The fracture properties were evaluated on curved specimens, and the effect of certain trace elements on the fracture properties was determined. Significant reduction of trace impurities, produced by using four-stage melting practices rather than the conventional two-stage process, was observed to cause considerable improvement in the fracture resistance of the alloy. Scanning electron microscopy (SEM) of the fracture surfaces of the test specimens confirmed this observation.
 
Article
Characterization of macrostructure, microstructure, hardness, precipitate distribution, residual stress, and cyclic deformation behavior of 2024-T351 friction stir welded joints has been conducted. Inhomogeneous microparameters governing the nonuniform residual stresses and cyclic strength are discussed. The cyclic strength of the weld microregimes is controlled by grain size and distribution of precipitates achieved during the weld process. The comprehensive information of micro-and macromechanics is used to assist in understanding the mechanism that governed the fatigue crack initiation, propagation, and life of the welded joints.
 
Article
Duplex stainless steel (DSS) grades are used in pulp mills for their superior properties and resistance to general corrosion. However, stress corrosion cracking (SCC) of DSS equipment has been experienced in different pulp mills. The susceptibility of DSS grades to SCC can be mainly attributed to the various heating processes involved during the manufacturing of industrial equipments, especially welding. It is generally understood that heating cycles during welding may affect the dual microstructure (ferrite/austenite ratio) of the steel, making it more prone to cracking in aggressive environments such as chlorides and caustics and further exposure to high temperatures. Welded 2205 DSS failed in white liquor (mainly NaOH+Na2S) was examined for SCC crack morphology and microstructure. Heat-treated 2205 DSS samples were tested in simulated white liquor to see the effect of microstructure on SCC susceptibility. Austenite is more susceptible to SCC than ferrite, but the SCC susceptibility primarily depends on the composition of the alloy and the chemistry of the exposure environment.
 
Article
The susceptibility of austenitic stainless steels to the formation of two distinct weld defects, solidification cracking and lack of penetration, is related to the chemical composition of the base and filler material. The propensity for cracking is determined primarily by the solidification mode and the amount of residual tramp elements such as phosphorous and sulfur. High sulfur levels can lead to weld centerline cracking and heat affected zone (HAZ) cracking while very low sulfur levels (less than ∼50 ppm) in types 304L and 316L are associated with lack of penetration weld defects and a distinct loss in puddle control during fusion welding. A calculated Creq to Nieq ratio of 1.52 to 1.9 is recommended to control the primary mode of solidification and prevent solidification cracks in type 304L while the Creq/Nieq ratio of 1.42 to 1.9 is recommended for type 316L stainless steel. A lower limit of 50 ppm sulfur is recommended to avoid possible lack of penetration. These ranges should be validated by welding trials for specific weld processes and applications.
 
Article
The room temperature burst pressure of 316L stainless steel burst discs exhibited increases of about 10% over 90 days. This increase may be associated with a strain-aging phenomenon requiring the presence of carbon since tensile property instability in worked austenitic stainless steels has been reported.[1–5] The cold worked material directly beneath the score root on the burst disc could undergo the strain aging process, thus causing the observed increase in burst strength. Characterization and analysis were therefore undertaken to identify the controlling phenomena in the small heterogeneous volume that controls rupture of the burst disc. Optical metallography and magnetic measurements confirmed the presence of martensite. Nanoindentation hardness measurements were correlated with finite element simulation of the as-formed mechanical properties. A representative portion of the microstructure was then recreated through cold rolling, and subjected to real-time and accelerated thermal aging treatments and mechanical activation analysis. Saturation of strengthening was observed, and a low temperature martensite reversion anneal was found to prevent or reverse the aging process. The results are consistent with previous observations of strain aging, although in this instance the effects are observed over a 10,000-fold greater aging time. Aging mechanisms are discussed, incorporating the phenomenologies of activation enthalpy and aging kinetics. A model explaining the sensitivity of aging rate to extreme cold work-induced dislocation densities and cold work-induced vacancy content is proposed.
 
Article
Several boiler superheater tubes showed circumferential cracking at weld seams after 2years in noncontinuous service (several shutdowns). On-site inspection revealed that several tubes were cracked and leaked; while many others were cracked, however, the severity was less pronounced. Two types of superheater tubes samples were collected: one with butt-welded tubes and the other with fillet-welded sleeve. The latter was found to be out of the boiler fireplace, and the sleeve was used as tubing support to the boiler shell. Detailed investigation showed that the butt-welded tubes contained circumferential fatigue cracks that initiated from the internal surface. The cracks initiated in the heat-affected zone and propagated as a result of tube vibration. The variations in the tube internal diameter and tube wall thickness are expected to play a role in tube fatigue failure. On the other hand, tubes with fillet-weld sleeve showed circumferential cracking as a result of fatigue crack initiation from weld defects on the tube external surface. The high vibration during several unscheduled shutdowns in addition to several other factors such as variations in tube inside diameter, wall thickness, and weld defects resulted in the initiation and propagation of fatigue cracks and premature failure. White deposits, similar to those observed when boiler tubes failed by caustic exposure, were seen in the vicinity of the tube cracks. Therefore, it was difficult to confirm whether the boiler tubes failed because of the fatigue cracks or because of the caustic salts (pH control chemical).
 
Article
Early in the shuttle Columbia crash investigation, item 33767 was one of several “Pathfinder” components selected from the Columbia debris that exhibited damage patterns similar to those observed on the left wing airframe components, the components in which initial failure was thought to have occurred. “Pathfinder Analysis” sought to answer academic questions regarding the maximum heat attained and heating direction/duration and identify debris imposed on this fuselage section during Columbia breakup and re-entry. Traditional failure analysis techniques provided useful information on debris constituents and damage sequence and were successful in identifying heat effects, such as the presence of large thermal gradients across the component, and the existence of several failure modes that included hot tensile failure, hot bending failure, and rapid overload fracture.
 
Article
A bent Ni-Cu Monel 400 alloy tube, which operated as part of a pipeline in a petrochemical distillery installation, failed by through-thickness cracking. The pipeline was used to carry a stream of gaseous hydrocarbons containing hydrochloric acid (HCl) into a reaction tower. The tower provided a caustic solution (NaOH) to remove HCl from the stream, before the latter was directed to a burner. Metallographic examination showed that the cracks were intergranular and were frequently branched. Although nominal chemical composition of the component was found within the specified range, electron dispersive analysis by X-ray (EDXA) indicated significant segregation of sulfur and chlorine on grain boundaries. Failure was attributed to hypochlorous-acid (HClO)-induced stress-corrosion cracking (SCC). The HClO was formed by the reaction of HCl with atmospheric O2, and the oxygen entered the tube during shutdowns/startups of the installation. Residual stresses, originating from the in situ bend forming of the tube during assembly of the line, provided a driving force for crack growth, and the segregation of sulfur on grain boundaries enhanced the susceptibility of the material to cracking.
 
Article
Alloy 430 stainless steel tube-to-header welds failed in a heat recovery steam generator (HRSG) within one year of commissioning. The HRSG was in a combined cycle, gas-fired, combustion turbine electric power plant. Alloy 430, a 17% chromium (Cr) ferritic stainless steel, was selected because of its resistance to chloride and sulfuric acid dewpoint corrosion under conditions potentially present in the HRSG low-pressure feedwater economizer. Intergranular corrosion and cracking were found in the weld metal and heat-affected zones (HAZs). The hardness in these regions was up to 35 HRC, and the weld had received a postweld heat treatment (PWHT). Metallographic examination revealed that the corroded areas contained undertempered martensite. Fully tempered weld areas with a hardness of 93 HRB were not attacked. No evidence of corrosion fatigue was found. Uneven temperature control during PWHT was the most likely cause of failure.
 
Article
The genesis of failure of 6.1 mm thick electric resistance welded (ERW) API 5L X-46 pipes during pretesting at a pressure equivalent to 90% of specified minimum yield strength (SMYS) was investigated. Cracks were found to initiate on the outer surface of the pipes in the fusion zone and propagate along the through-thickness direction. The presence of extensive decarburization and formation of a soft ferrite band within the fusion zone may have contributed to the nucleation of the cracks. Crack propagation was aided by the presence of exogenous inclusions entrapped within the fusion zone. Analysis of these inclusions confirmed the presence of Fe, Si, Ca, and O, indicating slag entrapment to be the most probable culprit.
 
Article
This paper describes the remote ultrasonic (UT) examinations of a high-level radioactive waste (HLW) storage tank at the Savannah River Site in South Carolina. The inspections, carried out by E.R. Holland, R.W. Vande Kamp, and J.B. Elder, were performed from the contaminated, annular space of the 46-year-old, inactive, 1.03 million gallon waste storage tank. A steerable, magnetic wheel wall crawler was inserted into the annular space through small (6 in., or 150 mm, diameter) holes/risers in the tank top. The crawler carried the equipment used to simultaneously collect data with up to four UT transducers and two cameras. The purpose of this inspection was to verify corrosion models and to investigate the possibility of previously unidentified corrosion sites or mechanisms. The inspections included evaluation of previously identified leak sites, thickness mapping, and crack detection scans on specified areas of the tank. No indications of reportable wall loss or pitting were detected. All thickness readings were above minimum design tank-wall thickness, although several small indications of thinning were noted. The crack detection and sizing examinations revealed five previously undetected indications, four of which were only partially through-wall. The cracks that were examined were found to be slightly longer than expected but still well within the flaw size criteria used to evaluate tank structural integrity.
 
Article
Metal dusting can cause a significant amount of expensive damage to ammonia, hydrogen, carbon monoxide and methanol plants. One of the means to limit this development has been to design the equipment parts, considered subject to metal dusting attack with Alloy 601. This will typically mean parts subjected to a reformed gas at 600°C (1,110°F). The risk of stress relaxation cracking of Alloy 601 at exactly this temperature has not previously been reported, but it must be considered a major risk that has to be dealt with in the design phase.
 
Article
The theoretical basis of detecting wire rope anomalies using permanent magnetic field has been fully established. The local faults (LF) such as broken wires and loss of metal area (LMA) signals are provided by nondestructive evaluation instruments. These signals represent the electronic equivalent of the mechanical anomalies present in the wire rope. The saturating magnetic field of the instrument makes the anomalies visible to the magnetic sensors placed around the rope. The condition of a haulage rope (construction 6X19) in a monocable continuously moving passenger ropeway has been studied using this nondestructive method, and the results are presented in this paper.
 
Article
Aluminum alloys are frequently preferred materials for aerospace applications due to their high-specific density, high-specific stiffness, and ease of fabrication. In one such application, adaptors used in the torroidal shaped water tank of liquid propulsion system, were made of an AFNOR 7020 (Al–4.5Zn–1.5Mg) aluminum alloy extrusion. These pressurization adaptors, in T6 temper condition, were initially shrink fitted to the openings provided in water tank main body and later circumferentially welded, manual TIG, to configure the tank. Four such adaptors were welded at different locations of a water tank. During one of the qualification tests, cracks were noticed near to the weld fusion line of one of the adaptor. Detailed metallographic investigation on the cracked adaptor revealed that cracking was due to combined effect of locked-in stresses in the material and anodic dissolution of solute rich phases present at elongated grain boundaries of the HAZ: a typical case of Stress Corrosion Cracking (SCC).
 
Article
Two out of three struts of a drilling tool’s 6.75″ Flow Diverter failed during operation. This failure resulted in invasion of mud into the electronic module of the tool. During drilling, high stick slip was observed. Measurement While Drilling (MWD) signal was good and Logging While Drilling (LWD) data was used to pick the oil/water contact point. At the end of the run, cracks were observed on the radii of the struts. Failure investigations were performed to identify the cause of the cracks. Material characterization, microstructural examinations, and fractography by SEM technique revealed that fatigue under compression forces was the cause of the cracking of the struts. Finite elemental analysis (FEA) was also used to determine the magnitude and the area of stress concentration. Failure mechanism has been explained. A few issues related to integrity of the material have also been raised.
 
Article
The quality advantages of continuous casting (concast) have enabled the production of a wide range of billets for various end applications, including IS:7887 Gr. 3 billets for the fastener industry. This paper discusses the influence of the internal quality of these concast billets on the processing of a wide range of products for the fastener industry. Internal soundness, inclusion volume fraction, and cleanliness were found to have a strong influence on the cracking susceptibility of self-tapping screws and nuts. A high incidence of hairline cracks on nut surfaces was found to be due to a combination of a high volume fraction of inclusions (0.54%) and the presence of complex manganese (Mn) (aluminate-silicate)-type inclusions.
 
Article
The observation that carrier panel fasteners recovered from the Columbia appeared to have fractured with minimal macroscopic ductility led to concern that stress-corrosion cracking (SCC) or other age-related degradation processes in the A286 bolts may have been the cause of failure during re-entry. To assess this hypothesis, several recovered fasteners having a brittle fracture appearance and several fasteners that fractured in a more ductile mode were analyzed. The results provided the information necessary to discount the “stress-corrosion” hypothesis and conclude that the fasteners failed by some unidentified tensile load after being weakened by high-temperature grain growth and liquid aluminum attack.
 
Article
In industrial applications failures of mechanical parts made of carbon and alloyed steels may develop either during heat treatment steps or final finishing operations. Such failures have high impact costs for manufacturers, since heat treated steel products, in general, are high value products which increase in value with each step in the production process until the final life-cycle manufacturing steps are completed. This work highlights the selection of steels to avoid premature ruptures developing during either the heat treatment steps or finishing operations with emphasis on the role of banding in the failure process. Failure does not have to involve fracture but may simply imply a decrease in performance of surface treated components as consequence of surface properties, even in the presence of correct heat treatment parameters. The root causes for banding in steels, are described in literature, and banding has major effects on final product properties (and causal relationships). Therefore, the causes of banding are studied and classified. Conclusions suggest that microstructural defects such as (micro)segregation bands and other defects such as slag and oxides inclusions are developed in the early fabricating cycle steps and can cause premature failure of either semi-finished or finished products. KeywordsSegregation-Heat treatments-Toughness-Banding-Anisotropy-Martensite-Transformation plasticity-Failure-Austenite
 
Article
Three basic modes of failure are known for oil tanks: manufacturing defects, mechanical damage, and corrosion. Most of the tanks currently in use and manufactured in or prior to the 90s, are subject to leaking caused by internal corrosion. Corrosion-induced leakage always results in environmental damage by underground and/or aboveground oil contamination. Considering the costs related to decontamination and to replacement of residential tanks, the possibility of corrosion should be carefully addressed. To accomplish this task, the most common issues related to tank corrosion must be recognized and understood. This article is an overview of the corrosion mechanisms in aboveground residential tanks made of non-coated mild steel. KeywordsPitting corrosion-Oil tanks-Water-Heating oil
 
Article
The high stress wear behavior of steels is dependent on a number of experimental factors, such as load, sliding distance, speed, abrasive size, and so forth. With the aid of computer-based regression analysis, this study investigates the role of each experimental variable on wear rate. Correlations are formed between the predicted and experimentally observed values.
 
Article
Increases in failures due to internal board shorting by conductive filament formation have driven glass and laminate manufacturers to consider screens and qualification tests to assess the hollow fiber concentration of circuit cards. This article describes the hollow fiber problem, methods to deter conductive filament formation, a hollow fiber screening technique, and the impact of next generation technology on conductive filament failure mechanism.
 
Article
A double-walled, hemispherical metal beam exit window made of alloy 718, used at the Los Alamos Neutron Science Center (LANSCE), developed a crack during service, leading to leakage of coolant. The window had been exposed to radiation damage from 800 MeV protons and a cyclic stress from 600 MPa tensile to near zero induced by numerous temperature cycles calculated to be from 400 to 30 °C. The window was activated to > 200 Sv/h. It was determined, through analysis using remote handling techniques and hot cells, that the crack initiated near a spot weld used to affix thermocouples to the window surface. In addition to analysis of the crack, some of the irradiated material from the window was used to measure mechanical properties. Hot cell techniques for preparation of samples and testing have been developed at both Forschungszentrum Juelich (FZJ) and Los Alamos National Laboratory (LANL). Important and useful data on radiation effects can be obtained from components used in service because they have experienced true operating conditions of radiation, strain, and temperature.
 
Article
On 3 February 2003, the space shuttle Columbia and its crew were lost during re-entry from a successful scientific mission. The physical root cause of the space shuttle Columbia accident was determined to be a breach in the thermal protection system (TPS) on the leading edge of the left wing. After a thorough investigation of the incident, the Columbia Accident Investigation Board (CAIB) set forth their recommendations which are focused on three major areas: the accident, why the accident occurred, and a look ahead.
 
Article
A metallurgical and mechanical failure analysis was applied as part of a vehicle accident reconstruction of a multi-vehicle collision. One of these vehicles was a coal-hauling tractor-trailer. Examination of the trailer involved in the incident revealed a fatigue fracture to a primary lateral stiffener, along with a significant misalignment of the stiffener. Stress and fatigue analysis indicated that the misalignment severely degraded the fatigue life of the stiffener. Evaluation of the structural dynamics of the trailer after the fatigue fracture indicated decreased lateral stability. The decreased stability caused by fracture of the lateral stiffener allowed rollover of the trailer to occur while negotiating a curve. The failure sequence developed in this investigation proved consistent with all physical damage observed on the trailer and with witness accounts of the incident. The failure scenario developed in this investigation is compared with other conclusions made by other investigators to show that those conclusions are not consistent with all of the available evidence.
 
Port side main landing gear uplock roller. Note shadowing effect on journal surface, slag deposits on center of part, and erosion on flanges.
Port side main landing gear strut and tire
Port side lower carrier panel tiles displaying eroded and slumped regions of tile, as well as slag deposition and directionality of damage
Article
The space shuttle Columbia launched from the Kennedy Space Center (KSC) in January of 2003. During ascent, between one and three pieces of material—likely insulating foam from the external tanks—impacted the leading edge of the left side of the orbiter. Upon re-entry back to earth, the Columbia began to disintegrate, leaving an enormous primary debris field stretching over eastern Texas and western Louisiana. Tens of thousands of volunteers were mobilized to help with the recovery of the Columbia remnants. Once the debris was delivered to KSC, several hundred scientists, engineers, and technicians helped analyze the debris and identify its original location on the orbiter. A Materials and Processes Team performed extensive failure analysis and chemical identification to help determine the most likely breach location resultant from the strike that occurred during liftoff, and the path that the impinging plasma generated during re-entry followed once it penetrated the wing of the Columbia. A combination of qualitative and quantitative analytical methods, ranging from radiographic nondestructive examination (NDE) and X-ray diffraction to scanning electron microscope with energy-dispersive spectroscopy (SEM/EDS) and electron probe microanalysis (EPMA), were used to help determine the breach location and the plasma path within the wing itself.
 
Article
Aircraft accidents are usually complex, and hence their investigation requires expertise from several fields. The challenge for an investigating team is to analyze the bits and pieces of information gathered in the course of the investigation through background information, interrogation, and material evidence and stitch them into a descriptive picture to arrive at the possible cause(s) of the accident. During this process, the investigating team is extremely dependent on laboratories where failed components/structures are subjected to detailed study. The laboratory findings are the decisive factors in ensuring a successful investigation. Any mistakes committed at this stage can, apart from wasteful work, lead to confusion and wrong direction of investigation. This paper presents an investigation that in the author’s opinion should have been conclusive, but remained inconclusive. The shortcomings in the investigation are discussed.
 
Article
Acoustic Emission (AE) technology applied to condition monitoring is gaining acceptance as a useful complimentary tool. This article demonstrates the use of traditional AE parameters, the Enegry Index and Kolmogorov–Smirnov test (KS-test) to detect, locate, and monitor natural defect initiation and propagation in a conventional rolling element thrust bearing. To undertake this task a special purpose test-rig was built to allow for accelerated natural degradation of a bearing race. It is concluded that sub-surface initiation and subsequent crack propagation can be detected using a range of data analysis techniques on AE’s generated from natural degrading bearings. The article also investigates the source characterization on AE signals associated with such a bearing while in operation.
 
Article
Vibration monitoring of rolling element bearings is possibly the most established diagnostic technique for rotating machinery. The application of Acoustic Emission (AE) for bearing diagnosis is gaining ground as a complementary diagnostic tool, however, limitations in the successful application of the AE technique have been partly due to the difficulty in processing, interpreting and classifying the acquired data. Furthermore, the extent of bearing damage has eluded the AE diagnostician. The investigation reported in this paper was centered on the application of the Acoustic Emission technique for identifying the size of a defect on a radially loaded bearing. An experimental test-rig was designed such that defects of varying sizes could be seeded onto the outer race of a test bearing. It was concluded that AE can provide an indication of the defect size, thus allowing the user to monitor the rate of degradation on the bearing.
 
Article
Over the last four to five decades, applications of fiber-reinforced plastics (FRP) have seen tremendous growth. In spite of the complexity of their behavior, development of composite pressure chambers has significantly contributed to the continuing increase in the strength-to-weight ratio of a significant amount of hardware and testing techniques to evaluate that hardware are continually being developed. This article presents an overview of the developments toward burst pressure prediction of FRP pressure chambers using acoustic emission (AE) technique. Burst strength prediction methods and related research in this context are also briefly discussed. The historical background of FRP pressure chamber/vessel testing using AE is also covered in this review. KeywordsAerospace fasteners–Catastrophic failure–Characterization–Composite materials–Non-destructive testing
 
Article
Concrete walkways are relatively low cost alternatives to bridges and culverts. They provide good and dependable ways across streams and rivers. This article discusses failures of concrete walkways in selected locations in Nigeria. The selected locations in Katsina state were visited before raining season started and during raining season. Concrete walkways across selected rivers (streams) were examined and their failures were assessed. Soil samples at the location of the walkway were collected and classified. The study revealed that earlier failures of culverts/bridges led to the construction of these concrete walkways in 1985 because the walkways were cheaper and required lower technology than construction of bridges. The soils at the site were mainly sandy–loamy soil with an average of 75.5% sand, 9.85% clay, and 14.65% silt. Failures of the concrete walkways were due to rapid and varied flow in the middle of the river channel, the presence of a hydraulic jump, the lack of resistance to turbulent flow, the presence of impulse forces, high Froude number (F r), and the lack of cohesion between the soil and concrete materials. It was then concluded that provision must be made against turbulent flow and cohesive materials must be provided in the construction of walkways to prevent failure of walkways.
 
Article
Precipitation-hardened stainless steel of 15-5PH grade was used for the fabrication of forward cylinder used in advanced linear actuator of aircraft. The component had undergone a series of qualification tests. During one of the accelerated fatigue tests, an external leak was observed after experiencing more than 1,50,000 cycles of fluctuating stress. Subsequent to disassembly, the leak was found to be because of the hair line cracks in the forward cylinder. The cracked component was subjected to a detailed metallurgical investigation to understand the cause of failure. The failure analysis revealed that the fracture initiated at the hard chrome layer and propagated under cyclic loading. This premature failure of the component was attributed to the surface roughness and the presence of the hard chrome layer at the corner contour of the component. This article brings out the details of the investigation carried out and the results achieved. Keywords15-5PH stainless steel–Hard chrome layer–Effect of surface roughness on fatigue–Fatigue failure
 
Article
Determination of the failure mechanisms of mechanical devices is the key to the design of reliable products. This paper reports an investigation on creep and fatigue failure of microelectromechanical (MEMS) thermal actuators. Finite element modeling is used to predict thermomechanical behavior of actuators under low to moderate voltage differences. The modeling results are compared with experimental results to evaluate the models. Two probable failure modes associated with thermal actuators, that is, fatigue and creep, are investigated, and it is found that creep is the dominant failure mechanism. The creep behaviors of several U-shape and double hot arm thermal MEMS actuators are examined, and their deformation-time curves are obtained numerically and experimentally. The curves follow a typical three-stage creep curve usually observed in metals. The creep life cycles of the devices are compared on the basis of their stress and temperature distributions. This study shows that actuators with the maximum temperature occurring at the location where the high stress is induced have shorter life spans than those experiencing the high stress away from the maximum temperature location. It is concluded that the double hot arm actuators with equal length have longer creep life than the U-shape (single hot arm) actuators.
 
Article
This paper reports on an investigation in which several standard Microelectromechanical Systems (MEMS) elements consisting of thermal actuators, inchworm drives, and comb drives were subjected to vibration loading representative of the environment seen in space applications. Finite-element analysis of the MEMS devices showed that sufficient margins existed under the expected environmental loading. Vibration testing, however, resulted in several failures in the devices, and analysis showed that progressive failure initiated from large local displacements. Debris transport and entrapment was another source of failure leading to shorting of thermal actuators. The results illustrate the importance of debris control and packaging design for reliable MEMS operation. Suggestions for improving the reliability of MEMS devices through practical layout and packaging guidelines are made.
 
Article
Adaptors used in check valve inlet pressure sensors were fabricated from austenitic stainless steel. Recently, during pretest operation and during hydrogen pressurization of an engine, a leak was noticed through the adaptor. Subsequent to disassembly, the leak was attributed to a crack in the adaptor. Detailed metallurgical investigations were carried out to understand the cause of cracking. Synergistic effect of chloride ions present in marine environment, the presence of residual stresses and sensitized grains of the material led to intergranular stress corrosion cracking. This article highlights details of investigation. KeywordsAustenitic stainless steel–Sensitization–Anodic dissolution of chromium-depleted region–Failure analysis
 
Article
This investigation characterizes five surgical stainless steel piercings and one niobium piercing that caused adverse reactions during use, culminating with the removal of the jewelry. Chemical composition shows that none of the materials are in accordance with ISO (International Organization for Standardization) standards for surgical implant materials. Additionally, none of the stainless steel piercings passed the pitting-resistance criterion of ISO 5832-1, which implies that %Cr + 3.3 × %Mo >26. Under microscopic examination, most of the jewelry revealed the intense presence of linear irregularities on the surface. The lack of resistance to pitting corrosion associated with the poor surface finishing of the stainless steel jewelry may induce localized corrosion, promoting the release of cytotoxic metallic ions (such as Cr, Ni, and Mo) in the local tissue, which can promote several types of adverse effects in the human body, including allergic reactions. The adverse reaction to the niobium jewelry could not be directly associated with the liberation of niobium ions or the residual presence of cytotoxic elements such as Co, Ni, Mo, and Cr. The poor surface finish of the niobium jewelry seems to be the only variable of the material that may promote adverse reactions.
 
Article
Stainless steel fasteners are often chosen to replace carbon steel fasteners in many engineering applications. These fasteners offer some advantages over their carbon steel counterparts—most notably, increased corrosion resistance. However, stainless steel fasteners have a tendency to gall, or cold weld, when in contact with a mating stainless steel fastener of similar composition when the proper precautions are not taken. Galling can vary in severity, and may be only a nuisance when minor, but lead to fastener failure in the extreme. This paper describes the galling of a threaded stainless steel ejector rod from a laser guided training round (LGTR) used by the U.S. Navy. Also presented are the precautions that should have been taken to avoid galling.
 
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A failure analysis case study is presented for a two-piece aerosol containing tetrafluoroethane, commonly referred to as Refrigerant 134a. A gentleman was preparing to recharge the air conditioning system of an automobile when the bottom exploded off the aerosol container, propelling the body of the aerosol container like a rocket, which hit the man in the eye and blinded him in that eye. The aerosol was never connected to the air conditioner, therefore backpressure from the air conditioner (AC) compressor was ruled out as a cause for the explosion. The objective of the study was to determine why the aerosol exploded. Several recently developed test methods were used, including two types of heat-to-burst tests and a puncture chamber to measure the pressure-versus-temperature behavior of aerosols. More common test methods were also used, such as water bath pressure tests, hydro pressure burst tests, pneumatic pressure burst tests, hardness measurements, weight measurements, metallography, scanning electron microscopy (SEM), energy-dispersive spectroscopy (EDS), and an accident scenario recreation. A semi-empirical correlation between the hardness and weights of the container bottoms was used to determine the explosion temperature and/or pressure. This semi-empirical correlation agrees in principle with an analysis of the explosion pressures using finite-element analysis (FEA). The root cause for the explosion was determined to be a lack of strength of the bottom of the two-piece aerosol coupled with heating the aerosol to temperatures significantly above room temperature.
 
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This paper explains how three-piece aerosol cans made from tin-plated steel are designed and manufactured, how they behave structurally, and provides information on doing failure analyses of aerosol cans. Particular emphasis is placed on the metallurgical aspects of the side-seam welding process used to create the body cylinders, the process of attaching the ends, and explains how three-piece aerosol cans made from tin-plated steel behave structurally. The paper also addresses a number of representations and conclusions presented in a paper by M. Fox and R. Hastings entitled “Pressurized 3-Piece Steel Container Explosions and Failure Mechanisms” published in the June 2003 issue of Practical Failure Analysis. Aerosol cans may fail, although not necessarily explode, by internal or external corrosion, by mechanical abuse (puncture, for example), or by being overheated. Understanding the design, manufacture, and structural behavior of these pressure vessels will aid in the formulation of conclusions as to the root causes of aerosol can failures.
 
Article
This paper presents a continuation of an open public discussion about the failure analysis of a relatively simple mechanical device: the three-piece steel aerosol container. In the June 2003 issue of Practical Failure Analysis, M. Fox and R. Hastings published a paper on the failure of aerosol containers. In August 2004, R.C. Daehn and J.J. Blum published a paper in the Journal of Failure Analysis and Prevention that was critical of the Fox-Hastings article. The present manuscript addresses some of the criticisms of the June 2003 paper, using the methods of the August 2004 paper. Also discussed is the fact that not many things are manufactured perfectly 100% of the time and the observation that quality-assurance tests do not always represent failure mechanisms in the real world.
 
Top-cited authors
David A Roberson
  • University of Texas at El Paso
Angel Torrado
Mohamed Haddar
  • Ecole Nationale d'Ingénieurs de Sfax
Tahar Fakhfakh
  • University of Sfax
Todd Letcher
  • South Dakota State University