Showing papers in "Practical Failure Analysis in 2002"
TL;DR: In this article, a procedure to evaluate and optimize a polymer-modified asphalt (PMA) was developed, where two asphalt cements and two styrene-butadiene-styrene (SBS) copolymers were mixed at ten concentration levels.
Abstract: Polymers are increasingly being used to modify asphalt and enhance highway pavement performance. This paper reports the development of a procedure to evaluate and optimize a polymer-modified asphalt (PMA). Two asphalt cements and two styrene-butadiene-styrene (SBS) copolymers were mixed at ten concentration levels. The engineering properties and morphologies of the binders were investigated using a dynamic shear rheometer, scanning electron microscopy (SEM), and other rheological techniques. The morphology of the PMA was characterized by the SBS concentration and the microstructure of the copolymer. Polymer modification increased the elastic responses and dynamic moduli of asphalt binders. As the SBS concentration increased, the copolymer gradually became the dominant phase, accompanied by a change in engineering properties. Results from SEM demonstrated that, up to 6% concentration, good compatibility exists between SBS and asphalt binder. The modified binders show either a continuous asphalt phase with dispersed SBS particles or a continuous polymer phase with dispersed asphalt globules, or two interlocked continuous phases. The optimum SBS content was determined based on the formation of a critical network between asphalt and polymer.
62 citations
TL;DR: Fractography is critical to failure analysis of metals and polymers as discussed by the authors, and the key fractographic features for metals and plastics are described in this paper, including stress raisers, microstructure, material defects, and thermomechanical history.
Abstract: Fractography is critical to failure analysis of metals and plastics. Fractography of plastics is a relatively new field with many similarities to metals. Using case histories, various aspects of failure analysis and fractography of metals and plastics are compared and contrasted. Failure modes common to both metals and plastics include ductile overload, brittle fracture, impact, and fatigue. Analogies can also be drawn between stress-corrosion cracking (SCC) of metals and stress cracking of polymers. Other metal/plastic failure analogies include corrosion/chemical aging, dealloying/scission, residual stress/frozen-in stress, and welds/knit lines. Stress raisers, microstructure, material defects, and thermomechanical history play important roles in both types of materials. The key fractographic features for metals and plastics are described in this paper.
52 citations
TL;DR: The specific approach can vary depending on when and where the inspection is made, the nature of the failure, and time constraints, but it should be planned carefully to preserve evidence.
Abstract: Failure Conditions When gears fail, there may be incentive to repair or replace failed components quickly and return the gear system to service. However, because gear failures provide valuable data that may help prevent future failures, a systematic inspection procedure should be followed before repair or replacement begins. The failure investigation should be planned carefully to preserve evidence. The specific approach can vary depending on when and where the inspection is made, the nature of the failure, and time constraints.
15 citations
TL;DR: In this paper, the bendability of two Al-Mg-Si heat-treatable alloys was studied and compared with the performance of a non-heat-treatingable Al Mg alloy.
Abstract: The bendability of two Al-Mg-Si heat-treatable alloys was studied and compared with the performance of a non-heat-treatable Al-Mg alloy. A semi-guided wrap-bend tester consistent with ASTM E290 was used to obtain minimum bend radii and produce consistent bend radii for analysis of fracture mechanisms. Bending failure in these alloys was based on a surface roughening, or orange peel, process where the outer surface grains separated and produced depressions on the surface of the material. These depressions acted as notches that increased local stresses and eventually caused failure. The fracture was intergranular in nature, with a jagged crack progressing through the thickness of the material. Several factors that affect the bendability of AA6xxx alloys are quantified in this study. Critical elements regarding natural aging, artificial aging, deformation, and composition are discussed.
13 citations
TL;DR: Recommendations to improve the fixation implant design are suggested, and the potential benefits of the substitution of titanium or a titanium alloy for the stainless steel are discussed.
Abstract: Failures of four different 300-series austenitic stainless steel biomedical fixation implants were examined. The device fractures were observed optically, and their surfaces were examined by scanning electron microscopy. Fractography identified fatigue to be the failure mode for all four of the implants. In every instance, the fatigue cracks initiated from the attachment screw holes at the reduced cross sections of the implants. Two fixation implant designs were analyzed using finite-element modeling. This analysis confirmed the presence of severe stress concentrations adjacent to the attachment screw holes, the fatigue crack initiation sites. Conclusions were reached regarding the design of these types of implant fixation devices, particularly the location of the attachment screw holes. The use of austenitic stainless steel for these biomedical implant devices is also addressed. Recommendations to improve the fixation implant design are suggested, and the potential benefits of the substitution of titanium or a titanium alloy for the stainless steel are discussed.
12 citations
12 citations
TL;DR: Fretting fatigue can occur when repeated loading on a structure or part causes repetitive relative movement at contacting metallic surfaces as discussed by the authors, which may cause local metal loss and impact the initiation and/or propagation of fatigue cracks.
Abstract: Fretting can occur when repeated loading on a structure or part causes repetitive relative movement at contacting metallic surfaces. The fretting process may cause local metal loss and impact the initiation and/or propagation of fatigue cracks. There are some features of fretting-fatigue cracking that are unique. Several of these features are illustrated in three case histories described in this paper: the failure of a splined papermaking refiner shaft, the failure of two coal-pulverizer shafts, and the cracking of a crankshaft flange from a ship’s engine. Fretting fatigue usually results in recognizable damage to at least one of the contacting surfaces. The fretted areas are roughened and, in steel, are usually decorated with reddishbrown deposits. Cracks may be initiated in the damaged region but are located close to the boundary between the damaged areas. Cracking normally starts at an angle of less than 90° to the surface. The geometric stress concentrations present on the component may be overridden because of fretting, and cracks may initiate on previously smooth surfaces. Some cracks may initiate and grow to only a shallow depth before ceasing propagation.
10 citations
TL;DR: In this paper, Alloy 430 stainless steel tube-to-header welds failed in a heat recovery steam generator (HRSG) within one year of commissioning in a combined cycle, gas-fired, combustion turbine electric power plant.
Abstract: 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.
9 citations
TL;DR: In this paper, the genesis of failure of 6.1 mm thick ERW API 5L X-46 pipes during pretesting at a pressure equivalent to 90% of specified minimum yield strength (SMYS) was investigated.
Abstract: 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.
8 citations
TL;DR: In this article, the role of each experimental variable on wear rate was investigated with the aid of computer-based regression analysis, and correlations were formed between the predicted and experimentally observed values.
Abstract: 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.
7 citations
TL;DR: In this article, failure of hydraulic couplings used to connect pipes in a naval vessel is described and used to illustrate some of the general procedures for failure analysis, and the primary cause of failure was probably the unanticipated presence of high-frequency stress cycles with very low amplitudes, possibly due to vibration, resonance or acoustic waves transmitted through the hydraulic fluid.
Abstract: Failures of various types of hydraulic couplings used to connect pipes in a naval vessel are described and used to illustrate some of the general procedures for failure analysis. Cracking of couplings, which were manufactured from nickel-aluminum-bronze extruded bar, occurred in both seawater and air environments. Cracks initiated at an unusually wide variety of sites and propagated in either longitudinal or circumferential directions with respect to the axis of the couplings. Fracture surfaces were intergranular and exhibited little or no sign of corrosion (for couplings cracked in air), and there was very limited plasticity. Macroscopic progression markings were observed on fracture surfaces of several couplings but were not generally evident. At very high magnifications, numerous slip lines, progression markings, and striations were observed. In a few cases, where complete separation had occurred in service, small areas of dimpled overload fracture were observed. It was concluded from these observations, and from comparisons of cracks produced in service with cracks produced by laboratory testing under various conditions, that cracking had occurred by fatigue. The primary cause of failure was probably the unanticipated presence of high-frequency stress cycles with very low amplitudes, possibly due to vibration, resonance, or acoustic waves transmitted through the hydraulic fluid. Secondary causes of failure included the presence of high tensile residual stresses in one type of coupling, undue stress concentrations at some of the crack-initiation sites, and overtorquing of some couplings during installation. Recommendations on ways to prevent further failures based on these causes are discussed.
TL;DR: In this article, a wire rope in one such auto tensioning system suffered premature failure due to fatigue cracks initiating at nonmetallic inclusions near the surface of individual wire strands in the rope.
Abstract: Wire ropes, pulleys, counterweights, and connecting systems are used for auto tensioning of contact wires of electric railways. A wire rope in one such auto tensioning system suffered premature failure. Failure investigation revealed fatigue cracks initiating at nonmetallic inclusions near the surface of individual wire strands in the rope. The inclusions were identified as Al-Ca-Ti silicates in a large number of stringers, and some oxide and nitride inclusions were also found. The wire used in the rope did not conform to the composition specified for AISI 316 grade steel, nor did it satisfy the minimum tensile strength requirements. Failure of the wire rope was found to be due to fatigue; however, the ultimate fracture of the rope was the result of overload that occurred after fatigue failure had reduced the number of wire strands supporting the load.
TL;DR: In this article, a failure analysis of 37.5 mW gas turbine third stage buckets made of Udimet 500 superalloy was performed, showing that the bucket experienced repetitive integral tip shroud fractures assisted by a low temperature (type II) hot corrosion.
Abstract: This article presents a failure analysis of 37.5 mW gas turbine third stage buckets made of Udimet 500 superalloy. The buckets experienced repetitive integral tip shroud fractures assisted by a low temperature (type II) hot corrosion. A detailed analysis was carried out on elements thought to have influenced the failure process:
The most probable cause of the bucket damage was the combination of increased stresses due to corrosion-induced thinning of the tip shroud and unfavorable microstructures in the tip shroud region.
TL;DR: In this article, both residual and applied stresses are converted to stress intensity factors independently and combined using the superposition principle, and the fatigue crack propagation rates are predicted using the weight function method combined with the three-component model.
Abstract: In this research, both residual and applied stresses are converted to stress intensity factors independently and combined using the superposition principle. The fatigue crack propagation rates are predicted. Experiments using two different loading modes, constant applied stress intensity factor (SIF) range, and constant applied load modes are done for samples with and without initial tensile residual stresses. The samples with initial tensile residual stresses exhibit accelerations of the crack propagation rates. The results show that the weight function method combined with the three-component model provides a good prediction of fatigue crack propagation rates in tensile residual stress fields.
TL;DR: The sector shaft is a critical component of an automotive power assisted recirculating ball steering gear and one that is vulnerable to fracture due to loads transmitted through the steering linkage under certain conditions as discussed by the authors.
Abstract: The sector shaft is a critical component of an automotive power assisted recirculating ball steering gear and one that is vulnerable to fracture due to loads transmitted through the steering linkage under certain conditions. This paper describes conditions under which such fractures can occur and the characteristics that distinguish the most frequently observed fractures.
TL;DR: In this article, varying widths of low carbon steel sheets were punched stretched under laboratory conditions, and thinning during punching was measured at various locations along the steel sheets, showing that thinning is a function of the strain path followed by the sheet.
Abstract: Thinning during forming is often considered a failure criterion in the metal forming industry. It is believed that a critical amount of thinning takes place in a sheet metal before failure. In this study, varying widths of low-carbon steel sheets were punch stretched under laboratory conditions. Thinning during punch stretching was measured at various locations along the steel sheets. These measurements demonstrated that thinning during forming is not constant, but that it is a function of the strain path followed by the sheet. Hence, thinning should not be used as a failure criterion during forming of sheet metals.
TL;DR: In this paper, the rolling contact fatigue behavior of sintered and heat treated steel rollers was investigated using a fatigue test machine designed and fabricated for that purpose, which was found to depend on the porosity present in the material.
Abstract: Powder-metal-processed bearings and gears are finding increasing application because of their economical and technical advantages. The residual pores from the sintering operatives act as lubricant pockets and dampen sound and vibration. However, porosity also decreases the mechanical strength and reduces the life of components fabricated by powder processing relative to similar wrought components. The rolling contact fatigue behavior of sintered and heat treated steel rollers was investigated using a fatigue test machine designed and fabricated for that purpose. The powder-metal-processed and the wrought steel rollers that were tested had similar composition and hardness and were mated against wrought steel rollers of high hardness. The contact stress versus number of cycles to failure data showed that the wrought steel had a very high endurance limit under rolling contact fatigue compared to the sintered steels investigated. Rolling contact fatigue behavior was found to depend on the porosity present in the material. Large surface peeling failures and pitting type fatigue failures were observed in the sintered and hardened steels, while only pitting type failures were observed in the wrought steels
TL;DR: In this article, the authors found that cavitation erosion was the responsible failure mechanism, not corrosion as might be expected, and that the failure of a heat exchanger occurred due to specification of a plain carbon steel that did not survive service in the SO2 vapor environment.
Abstract: Critical heat exchanger components are usually manufactured from durable steels, such as stainless steel, which exhibit good strength and corrosion resistance. Failure of a heat exchanger occurred due to specification of a plain carbon steel that did not survive service in the SO2 vapor environment. However, failure analysis showed that cavitation erosion was the responsible failure mechanism, not corrosion as might be expected.
TL;DR: In this article, the authors describe cold cracking failure specifically related to the construction industry, and this particular failure was successfully identified prior to final erection of the structural member, and the weld was successfully reworked.
Abstract: Cold cracking of structural steel weldments is a well-characterized, well-documented, and well-understood failure mechanism. Extensive effort has been put forth to recognize the welding and materials selection parameters that are conducive to cold cracking; however, these engineering efforts have not fully eliminated the occurrence of such failures. This article describes cold cracking failure specifically related to the construction industry. This particular failure was successfully identified prior to final erection of the structural member, and the weld was successfully reworked. These actions potentially prevented a serious catastrophic event that could have occurred have occurred either later in the construction process or possibly during the use of the building. Individual welding parameters, such as electrode/wire selection, joint design, and pre/postheating, played a role in the failure, and a number of human factors relating to the actual fabrication practices also contributed to the failure process.
TL;DR: In this article, the authors used quantitative fractography as a tool to extract information, including striation spacing and size of the stretched zone, in order to make a direct correlation with fracture mechanic concepts.
Abstract: After a quick-release fitting of an ejection seat broke, an investigation was performed to determine the manner and cause of crack propagation Most fractography-based investigations aim to characterize only qualitative characteristics, such as the fracture orientation and origin position, topology, and details of interactions with microstructural features The aim of this investigation was to use quantitative fractography as a tool to extract information, including striation spacing and size of the stretched zone, in order to make a direct correlation with fracture mechanic concepts
TL;DR: In this paper, the distribution of fragment masses was shown to follow a Weibull-type form M (n)=M0 [1 − exp (−Bnλ] for RDX-based military explosives and a bimodal distribution when commercial explosives were used.
Abstract: Twelve pipe bombs were exploded, and the fragments were collected and weighed. The distribution of fragment masses was shown to follow a Weibull-type form M (n)=M0 [1 − exp (−Bnλ)] when RDX-based military explosives were used and a bimodal distribution when commercial explosives were used. The constant, B, was a function of the mass of fragments recovered, making its use inappropriate in a forensic context where complete collection of the fragments would not be possible. For RDX-based explosives with added Mg or Al powder, the value of the constant, λ, was reduced compared to that of the RDX-based explosive without metal powder additions. An alternative fragment distribution formula, log [(100 M (N))/M0]=FWDM [m(N)/ M0], can also be used to distinguish between explosives, although the correlation coefficient is inevitably somewhat lower and the scatter between the results of nominally identical tests can be greater. The coefficient, FWDM, has a dependence on the mass of fragments collected, restricting its utility in forensic contexts.
TL;DR: In this paper, the authors present case histories that, over a span of approximately 25 years, used vibration analysis to troubleshoot a wide range of problems in rotating and non-rotating equipment.
Abstract: Vibration analysis can be used in solving both rotating and nonrotating equipment problems This paper presents case histories that, over a span of approximately 25 years, used vibration analysis to troubleshoot a wide range of problems
TL;DR: In this article, 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 was presented.
Abstract: 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.
TL;DR: In this article, a method to model creep failure of polycrystalline materials based on a real microstructure taken from an optical microscope is presented, where the creep failure is simulated in 304 stainless steel and the simulation is based on Norton's creep law.
Abstract: This paper outlines a method to model creep failure of polycrystalline materials based on a real microstructure taken from an optical microscope. The creep failure is simulated in 304 stainless steel and the simulation is based on Norton’s creep law. By treating the grain boundaries and the grains differently and adopting the void nucleation process proposed by Shewmon, the creep strain energy density can be used as a failure criterion. The result of the simulation confirmed the results of conventional methods used in a high-temperature remnant life assessment. The intermediate results of the simulation process allow calculation/monitoring of stiffnesses degradation as the material undergoes creep failure.
TL;DR: In this paper, severe pitting corrosion of a carbon steel tube was observed in the air preheater of a power plant, which runs on rice straw firing, and approximately 1450 tubes were removed from Stage 3 of the pre-heater due to corrosion and local bursting.
Abstract: Severe pitting corrosion of a carbon steel tube was observed in the air preheater of a power plant, which runs on rice straw firing. Approximately 1450 tubes were removed from Stage 3 of the preheater (air inlet and flue gas outlet) due to corrosion and local bursting. Samples from Stage 2 (where corrosion was low) and Stage 3 (severe corrosion) were taken and subjected to visual inspection, scanning electron microscopy (SEM), X-ray diffraction (XRD), microhardness measurement, and chemical and microstructural analysis.
TL;DR: In this paper, the investigation of corrosion damage to hot rolled steel during transit requires metallurgical, chemical, and corrosion knowledge, as well as knowledge of non-destructive techniques and sampling procedures.
Abstract: The investigation of alleged corrosion damage to hot rolled steel during transit requires metallurgical, chemical, and corrosion knowledge. Familiarity with non-destructive techniques and sampling procedures are of assistance. A complete record of shipment history is also required, including the purchasing specifications and observations and photographs taken during surveys enroute. A frequent conclusion of such investigations is that the alleged corrosion is of no significance or did not occur during the voyage.
TL;DR: The Marine Forensics Panel of the Society of Naval Architects and Marine Engineers (SNAME) as mentioned in this paper was created to investigate shipwrecks of the German battleshipBismarck and the passenger shipsTitanic andLusitania.
Abstract: The Marine Forensics Panel of the Society of Naval Architects and Marine Engineers (SNAME) grew out of analyses of the shipwrecks of the German battleshipBismarck and the passenger shipsTitanic andLusitania. It is now co-sponsored by five other professional societies on both sides of the Atlantic. A number of ship losses have been investigated, and lessons have been learned relevant both to marine technology and history.
TL;DR: In many cases, proper investigation of a failure involves several disciplines, and the failure analyst must recognize and use all relevant expertise to identify all the causes for a failure, regardless of the size of the damages caused by a failure.
Abstract: In many cases, proper investigation of a failure involves several disciplines, and the failure analyst must recognize and use all relevant expertise to identify all the causes for a failure. This is true irrespective of the size of the damages caused by a failure, which governs the amount of time and effort that is usually put into investigating it. The analyst must keep in mind the interdisciplinary nature of failure analysis, no matter the size of the project.
TL;DR: In this paper, high contact resistance was observed in SIG contacts in a railway signaling operation after a few thousand operations of the relay, which was discovered to be a result of loose dust/foreign particles residing between the two contacts.
Abstract: Silver impregnated graphite (SIG) contacts are used in high safety applications of railway signaling, traffic signaling, and numerous other applications due to high silver content that eliminates the risk of welding in the presence of graphite. High contact resistance (>200 mΩ) was observed in SIG contacts in a railway signaling operation after a few thousand operations of the relay. The high contact resistance was discovered to be a result of loose dust/foreign particles residing between the two contacts. Silver had been preferentially removed from the contacting surfaces during switching operations by an abrading effect of the dust/foreign particles, leaving only graphite on the contact area. Wear was accentuated by the presence of porosity and low hardness of the contacts, leading to improper interaction between the mating surfaces.