Search Results for improper materials

The fan drive support shaft, specified to be made of cold-drawn 1040 to 1045 steel, fractured after 2240 miles of service. It was revealed by visual examination of the shaft that the fracture had initiated near the fillet at an abrupt change in shaft diameter. The cracks originated at two locations approximately 180 deg apart on the outer surface of the shaft and propagated toward the center. Features typical of reversed-bending fatigue were exhibited by the fracture. A tensile specimen was machined from the center of the shaft and it indicated much lower yield strength (369 MPa) than specified. It was disclosed by metallographic examination that the microstructure was predominantly equiaxed ferrite and pearlite which indicated that the material was in either the hot-worked or normalized condition. An improvement of fatigue strength of the shaft by the development of a quenched-and-tempered microstructure was recommended.

This article focuses on manufacturing-related failures of injection-molded plastic parts, although the concepts apply to all plastic manufacturing processes It provides detailed examples of failures due to improper material handling, drying, mixing of additives, and molecular packing and orientation. It also presents examples of failures stemming from material degradation improper use of metal inserts, weak weld lines, insufficient curing of thermosets, and inadequate mixing and impregnation in the case of thermoset composites.

Six cases of failure attributed to microbiologically influenced corrosion (MIC) were analyzed to determine if any of the failures could have been avoided or at least predicted. The failures represent a diversity of applications involving typical materials, primarily stainless steel and copper alloys, in contact with a variety of liquids, chemistries, and substances. Analytical techniques employed include stereoscopic examination, energy dispersive x-ray spectroscopy (EDS), temperature and pH testing, and metallographic analysis. The findings indicate that MIC is frequently the result of poor operations or improper materials selection, and thus often preventable.

Plastic product failures are directly attributed to one of the following four reasons: omission of a critical performance requirement, improper materials specification, design error, and processing/manufacturing error. Therefore, product failures can be minimized or eliminated if all of these parameters are comprehensively examined during the design process. This article focuses on all of these factors, except processing-related failures, which are outside the design and engineering domain. It is dedicated to the identification and avoidance of common problems associated with the selection and designing of plastic parts. The article provides information on the material-related design criteria that depend on the applications, environmental conditions of use, and performance requirements. It discusses physical properties of plastics based on their relevance to real-world environmental conditions. The most-common design problems related to design considerations are also covered.

A large portion of the four-hole Lane plate disintegrated and consisted mainly of corrosion products after remaining in the body for 26 years. Transformation structures and carbides were exhibited by the plate which was made from chromium steel. Minimal corrosion was exhibited by the soft austenitic 304 stainless steel used to make the screws. The corrosion products of the plate were revealed by microprobe analysis to impregnate the surrounding tissues. Improper material selection was concluded to be the reason for the general corrosion behavior.

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.

Several torsion bars had failed in a projectile weaving machine and were analyzed to determine the cause. Specimens prepared from the damaged components were subjected to visual inspection, hardness testing, chemical analysis, and metallurgical evaluations. The failed torsion bars had been fabricated from spring steel which, according to stress calculations, did not have sufficient torsional strength. Examination of the damaged parts confirmed the finding, revealing that all fractures started at a shoulder radius in an area of high stress concentration. Based on the investigation, the shoulder radius should be increased to alleviate stress and the working torsion angle of the bar should be decreased to improve safety factors.

A stainless steel pipe transferring hot white liquor solution of sodium hydroxide and sodium sulfite, developed leaks adjacent to the welds within four years of service. The stainless steel pipe was AISI type 304 and welded with E308 weld electrodes. The service temperature was 190 deg C (375 deg F) and the solution contained approximately 700 ppm chlorides. Liquid penetrant inspection of the pipeline showed the leaks were numerous and confined adjacent to the welds. A metallographic specimen from the circumferential weld showed the cracks initiated at the inside surface. In addition to the base metal, SCC also had initiated at a notch at the weld root due to improper welding procedures. Failure was attributed to chloride-induced SCC with secondary contributory factors, including improper welding procedures. It was recommended that the pipeline be replaced with a material more resistant to SCC. The candidate materials are commercial grade unalloyed titanium or Inconel 600, which have superior resistance to SCC compared to austenitic stainless steels.

Two oil-pump gears broke after four months of service in a gas compressor that operated at 1000 rpm and provided a discharge pressure of 7240 kPa (1050 psi). The compressor ran intermittently with sudden starts and stops. The large gear was sand cast from class 40 gray iron with a tensile strength of 290 MPa (42 ksi) at 207 HRB. The smaller gear was sand cast from ASTM A536, grade 100-70-03, ductile iron with a tensile strength of 696 MPa (101 ksi) at 241 HRB. Analysis (metallographic examination) supported the conclusion that excessive beam loading and a lack of ductility in the gray iron gear teeth were the primary causes of fracture. During subsequent rotation, fragments of gray iron damaged the mating ductile iron gear. Recommendations included replacing the large gear material with ASTM A536, grade 100-70-03, ductile iron normalized at 925 deg C (1700 deg F), air cooled, reheated to 870 deg C (1600 deg F), and oil quenched. The larger gear should be tempered to 200 to 240 HRB, and the smaller gear to 240 to 280 HRB.

A failed tapered-ring sprocket locking device consisted of an assembly of four tapered rings that are retained by a series of cap screws. The middle wedge-shaped rings were pulled closer as the screws were tightened forcing the split inner ring to clamp tightly onto the shaft. One of the wedge-shaped middle rings fractured prior to having been fully torqued, preventing the sprocket from being locked to the shaft. “Woody” fracture features, as a result of decohesion between a high volume fractions of manganese sulfide stringers and the matrix, was revealed during examination. The material was revealed by chemical analysis to be resulfurized grade of carbon steel (SAE type 1144, UNS G11440) which has enhanced longitudinal tensile properties but low transverse properties. It was observed that when the fastening screws were torqued, a significant hoop stress was placed on the middle rings and it caused the failure at the large inclusion present at the minimum section thickness zone of the middle ring. It was concluded that since the material contained a high volume fraction of these inclusions, the material choice was not appropriate for this application. A nonresulfurized grade of low-alloy steel was suggested as recommendation.

A portion of the roof of a single story building collapsed during a thunder storm. A failure analysis was conducted to determine whether this structural failure was due to improper design, substandard construction materials, faulty erection, or extreme weather conditions. The failure analysis consisted of an onsite inspection, macrofractographic examination of the fractures where the girders were welded to the columns, macrofractographic examination of the fractured trusses, metallographic examination of the girder and truss materials, chemical analysis of the low-carbon steel girder and truss materials, and mechanical testing of the truss material. It was concluded that substandard structural components in combination with faulty construction was responsible for this service failure.

This article focuses on failure analyses of aircraft components from a metallurgical and materials engineering standpoint, which considers the interdependence of processing, structure, properties, and performance of materials. It discusses methodologies for conducting aircraft investigations and inspections and emphasizes cases where metallurgical or materials contributions were causal to an accident event. The article highlights how the failure of a component or system can affect the associated systems and the overall aircraft. The case studies in this article provide examples of aircraft component and system-level failures that resulted from various factors, including operational stresses, environmental effects, improper maintenance/inspection/repair, construction and installation issues, manufacturing issues, and inadequate design.

A plastic bracket exhibited relatively brittle material properties, which ultimately led to catastrophic failure. The part had been injection molded from a medium-viscosity polycarbonate resin and had been in service for a short duration prior to the failure. Investigation (visual inspection and analysis using micro-FTIR in the ATR mode) revealed the spectrum showed changes in the relative intensities of several bands, as compared to the results representing the base material. A spectral subtraction was performed, and the results produced a good match with diphenyl carbonate, which is a common breakdown product produced during the decomposition of polycarbonate. The conclusion was that the most likely cause of the molecular degradation was improper drying and/or exposure to excessive heat during the injection molding process that in turn caused the material degradation.

Gears in a strip mining dragline failed in service. The material was identified as a low-alloy (NiCrMoV) steel. SEM analysis indicated that the initial fracture and subsequent fractures resulted from impact or a suddenly applied load. Mechanical testing indicated that the gears had low impact strength. Failure was attributed to low toughness caused by the absence of, or improper, heat treatment. Casting defects identified during metallographic examination were determined to be the fracture initiation site, but were considered less significant than the low as-received impact strength of the material. It was recommended that the equipment manufacturer implement an appropriate heat treatment to meet the impact requirements of the application.

A missile detached from a Navy fighter jet during a routine landing on an aircraft carrier deck because of a faulty missile launcher detent spring. Visual inspection of Inconel 718 detent spring assembly revealed that four of the nine spring leafs comprising the assembly were plastically deformed while two of the deformed leafs did not meet minimal hardness or tensile requirements. Liquid penetrant testing revealed no cracks or other surface discontinuities on the leaf springs. Material sectioned from the soft spring leafs was heat-treated according to specifications in the laboratory. The resultant increase in mechanical properties of the re-heat-treated material indicated that the original heat treatment was not performed correctly. The failure was attributed to improper heat treatment. Recommendations focused on more stringent quality control of the heat-treat operations.

Numerous protective covers, used in conjunction with an electrical appliance, failed during assembly with the mating components. The failures were traced to a particular production lot of the covers and occurred during insertion of the screws into the corresponding bosses. The parts had been injection molded from an ABS resin to which regrind was routinely added. Inspection of both the failed covers and retained parts, which exhibited normal behavior during assembly, included visual inspection, micro-FTIR in the ATR mode, and analysis using DSC. The FTIR results indicated the presence of contaminant material exclusively within the ABS resin used to mold the failed covers, and the thermograms suggested contamination with a PBT resin. Further TGA analysis showed the contamination was estimated to account for approximately 23% of the failed cover material. The conclusion was that the appliance covers failed via brittle fracture associated with stress overload. The failures, which occurred under normal assembly conditions, were attributed to embrittlement of the molded parts, due to contamination of the ABS resin with a high level of PBT. The source of the PBT resin was not positively identified, but a likely source appeared to be the use of improper regrind.

An integral coupling and gear (Cr-Mo steel), used on a turbine-driven main boiler-feed pump, was removed from service after one year of operation because of excessive vibration. Spectrographic analysis and metallographic examination revealed the fact that gritty material in the gear teeth (found at visual inspection) was composed of the same material as the metal in the coupling. Beach marks and evidence of cold work, typical of fatigue failure, were found on the fracture surface. Chips remaining in the analysis cut were difficult to remove, indicating a strong magnetic field in the part. Evidence found supports the conclusions that failure of the coupling was by fatigue and that incomplete demagnetization of the coupling following magnetic-particle inspection caused retention of metal chips in the roots of the teeth. Improper lubrication caused gear teeth to overheat and spall, producing chips that eventually overstressed the gear, causing failure. Because the oil circulation system was not operating properly, metal chips were not removed from the coupling. Recommendations included checking the replacement coupling for residual magnetism and changing or filtering the pump oil to remove any debris.

A two-section extension ladder, made from 6061-T6 aluminum alloy extrusions and stampings that were riveted together at each rung location and at the ends of side rails, broke in service after having been used at the sites of several fires by the fire department of a large city. The fracture surfaces were examined visually and by optical (light) stereomicroscopy. Material testing showed a sample to be within the specified material limits for aluminum alloy 6061. Microscopic examination showed no significant differences in microstructure or grain size among the four T-sections, and thickness measurements at various locations indicated that thicknesses were well within standard industry tolerances for aluminum extrusions in this size range. However, hardness testing of the four T-sections showed that in two, hardness was considerably lower than the acceptable hardness for the T6 temper and were within the range for 6061-T4 (acceptable hardness, 19 to 45 HRB). This indicated they had been naturally aged at room temperature after solution heat treatment instead of artificially aged as per specs. Edge cracking in two of the T-sections was the result of improper conditions during extrusion of the T-sections; however, this condition was not a primary cause of failure.

Mechanical springs are used in mechanical components to exert force, provide flexibility, and absorb or store energy. This article provides an overview of the operating conditions of mechanical springs. Common failure mechanisms and processes involved in the examination of spring failures are also discussed. In addition, the article discusses common causes of failures and presents examples of specific spring failures, describes fatigue failures that resulted from these types of material defects, and demonstrates how improper fabrication can result in premature fatigue failure. It also covers failures of shape memory alloy springs and failures caused by corrosion and operating conditions.