Search Results for Transportation applications

A section of clear polymeric tubing failed while in service. The failed sample had been used in a chemical transport application. The tubing had also been exposed to periods of elevated temperature as part of the operation. The tubing was specified to be a polyvinyl chloride (PVC) resin plasticized with trioctyl trimellitate. Investigation included visual inspection, micro-FTIR in the ATR mode, and thermogravimetric analysis. The spectrum on the failed tubing exhibited absorption bands indicative of a PVC resin containing an adipate-based plasticizer. Thermograms of the failed pieces and a reference sample of tubing that performed well showed that the reference material contained a trimellitate-based plasticizer and that the failed material contained an adipate-based material. The conclusion was that the failed tubing had been produced from a formulation that did not comply with the specified material and, as a result, was not as thermally stable as the reference material.

Cracking occurred within the plastic jacket (injection molded from an impact-modified, 15% glass-fiber-reinforced PET resin.) of several assemblies used in a transportation application during an engineering testing regimen which involved cyclic thermal shock (exposing the parts to alternating temperatures of -40 and 180 deg C (-40 and 360 deg F)). Prior to molding, the resin had reportedly been dried at 135 deg C (275 deg F). The drying process usually lasted 6 h, but occasionally, the material was dried overnight. Comparison investigation (visual inspection, 20x SEM views, micro-FTIR, and analysis using DSC and TGA) with non-failed parts supported the conclusion that that the failure was via brittle fracture associated with the exertion of stresses that exceeded the strength of the resin as-molded caused by the disparity in the CTEs of the PET jacket and the mating steel sleeve. The drying process had exposed the resin to relatively high temperatures, which caused substantial molecular degradation, thus limiting the part's ability to withstand the stresses. The drying temperature was found to be significantly higher than the recommendation for the PET resin, and the testing itself exposed the parts to temperatures above the recognized limits for PET.

Failed portions of a 4140 steel axle from a prototype urban transit vehicle were examined to determine the cause of failure. The testing procedures included visual examination, macrofractography, metallography, chemical analysis, and hardness and tensile testing. The analysis showed that a salvage welding repair had resulted in the formation of martensite that developed cracks, leading to fatigue failure of the axle. It was recommended that no weld buildup or repair be permitted on the axles.

A T-bolt was part of the coupling for a bleed air duct of a jet engine on a transport plane. Specifications required that the 4.8 mm diam component be made of AISI type 431 stainless steel and heat treated to 44 HRC. The operating temperature of the duct is 425 to 540 deg C (800 to 1000 deg F), but that of the bolt is lower. The T-bolt broke after three years of service. The expected service life was equal to that of the aircraft. It was found that the bolt broke as a result of SCC. Thermal stresses were induced into the bolt by intermittent operation of the jet engine. Mechanical stresses were induced by tightening of the clamp around the duct, which in effect acted to straighten the bolt. The action of these stresses on the carbides that precipitated in the grain boundaries resulted in fracture of the bolt. Due to the operating temperatures of the duct near the bolt, the material was changed to A-286, which is less susceptible to carbide precipitation. The bolt is strengthened by shot peening and rolling the threads after heat treatment. Avoiding temperatures in the sensitizing range is desirable, but difficult to ensure because of the application.

Material samples collected from failed booster pumps were analyzed to determine the cause of failure and assess the adequacy of the materials used in the design. The pumps had been in service at a power plant, transporting feedwater from a deaerator to a main turbine boiler. Samples from critical areas of the pump were examined using optical and scanning electron microscopy, electrochemical analysis, and tensile testing. Based on microstructure and morphology, estimated corrosion rates, and particle concentrations in the feedwater, it was concluded that cavitation and erosion were the dominant failure mechanisms and that the materials and processes used to make the pumps were largely unsuited for the application.

A newly installed pipeline leaked during cleaning prior to hydrotest at a pressure of approximately 400 psig. The intended hydrotest pressure was 750 psig. The pipeline was constructed from spiral-welded API 5L-X65 HSLA steel and was intended for seawater injection. Analysis included nondestructive testing, metallography, and scanning electron microscopy. Based on the results, the failure was attributed to transit fatigue, caused during highway transportation. Cracks along the toes of the weld from both the outside and inside surfaces, the transgranular nature of cracking, and the presence of fatigue striations all supported transit fatigue as the damage mechanism.

This article provides the framework for the investigation of bridge failures. It explains the types of bridge loading and presents the regulatory provisions for bridges. Some bridge failures in the U.S. that resulted in significant changes in bridge manufacturing, design, regulation, and/or maintenance are also discussed. In addition, the article provides information on traffic damage and fatigue cracking that result in bridge failures. The need for steels with better fracture toughness in bridge design is also discussed.

Upon arrival at the erection site, an AISI type 316L stainless steel tank intended for storage of fast breeder test reactor coolant (liquid sodium) exhibited cracks on its shell at two of four shell/nozzle fillet-welded joint regions. The tank had been transported from the manufacturer to the erection site by road, a distance of about 800 km (500 mi). During transport, the nozzles were kept at an angle of 45 deg to the vertical because of low clearance heights in road tunnels. The two damaged joints were unsupported at their ends inside the vessel, unlike the two uncracked nozzles. Surface examination showed ratchet marks at the edges of the fracture surface, indicating that loading was of the rotating bending type. Electron fractography using the two-stage replica method revealed striation marks characteristic of fatigue fracture. The striations indicated that the cracks had advanced on many “mini-fronts,” also indicative of nonuniform loading such as rotating bending. It was recommended that a support be added at the inside end of the nozzles to rigidly connect with the shell. In addition to avoiding transport problems, this design modification would reduce fatigue loading that occurs in service due to vibration of the nozzles during filling and draining of the tank.

Failure of carbon-manganese steel wheel studs caused by improper tightening of the inner wheel nuts resulted in separation of a dual wheel assembly on a heavy truck. The benchmark pattern observed on the fracture surfaces of the studs evidenced fatigue cracks emanating from multiple origins around the circumference. There was no indication that any microstructural characteristics of the material contributed to the failure. Inclusions that were present were small and relatively few in number. Failure to check the torque of the inner wheel nuts as per the manufacturer's recommendation caused the inner wheel nuts to loosen during break-in and lose the required clamping force. The development and promotion of educational programs on proper wheel tightening procedures was recommended.

A 2 ft. diam 20 ft. long cylinder with a wall thickness of 1 in. used for the transportation of a compressed gas failed by cracking. The cylinder was forged in a low ally steel. The working pressure was 3000 psi and it had been in service for about seven years. A longitudinal crack, about 2 in. long, developed at the approximate mid-length of the vessel, and allowed slow de-pressurization. Subsequent examination by radiography and ultrasonic means indicated the crack was associated with an irregularly shaped, laminar type of defect located within the wall of the vessel. It was concluded that failure of this vessel resulted from the development of a radial crack orientated in the axial direction. This appeared to have originated on the bore surface in a region where the laminar defect closely approached this surface. The defect was introduced during the manufacture of the vessel, probably originating as a secondary pipe in the ingot which was subsequently displaced and forced into the wall of the vessel during the piercing operation.

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 is necessary. 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.

A bracket which formed part of the carrier of a chain conveyor system used to transport components through a continuous oven fractured. A brittle crack originated on the inside of the right-angled bend, the surface having oxidized subsequently. The remaining portion of the fracture resulted from fatigue. Shallow oxidized regions adjacent to the inside surface of the bend indicated pre-existing cracks. A sulphur print on the edge of the bracket showed the material was rolled from a rimming steel ingot. The general appearance of the fracture, and the fact failure took place where embrittlement had developed following plastic straining and service at a temperature of 260 deg C (500 deg F) suggested that failure resulted from strain-age embrittlement.

Despite extensive aircraft landing gear design analyses and tests performed by designers and manufacturers, and the large number of trouble-free landings, aircraft users have experienced problems with and failures of landing gear components. Different data banks and over 200 failure analysis reports were surveyed to provide an overview of structural landing gear component failures as experienced by the Canadian Forces over the last 20 years on more than 20 aircraft types, and to assess trends in failure mechanisms and causes. Case histories were selected to illustrate typical problems, troublesome failure mechanisms, the role of high strength aluminum alloys and steels, and situations where fracture mechanics analyses provided insight into the failures. The two main failure mechanisms were: fatigue occurring mainly in steel components, and corrosion related problems with aluminum alloys. Very few overload failures were noted. A number of causes were identified: design deficiencies and manufacturing defects leading mainly to fatigue failures, and poor materials selection and improper maintenance as the principal causes of corrosion-related failures. The survey showed that a proper understanding of the failure mechanisms and causes, by thorough failure analysis, provides valuable feedback information to designers, operators and maintenance personnel for appropriate corrective actions to be taken.

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 4340 steel piston engine crankshaft in a transport aircraft failed catastrophically during flight. The fracture occurred in the pin radius zone. Fractographic studies established the mode of failure as fatigue under a complex combination of bending and torsional stresses. SEM examination revealed that the fracture origin was a subsurface defect-a hard refractory (Al2O3) inclusion—in the zone close to the pin radius. Chemical analysis showed the crankshaft material to be of inferior quality. It was recommended that magnetic particle inspection using the dc method be used to cheek for cracks during periodic maintenance overhauls.

A 52000 bearing steel valve guide component operating in the fuel supply system of a transport aircraft broke into two pieces after 26 h of flight. The valve guide fractured through a set of elongated holes that had been electrodischarge machined into the component. Analysis indicated that the part failed by low cycle fatigue. The fracture was brittle in nature and had originated at a severely eroded zone of craters in a hard, deep white layer that was the result of remelting during electrodischarge machining. It was recommended that the remaining parts be inspected using a stereoscopic microscope and/or a borescope.

This article provides a discussion on the structural ceramics used in gas turbine components, the automotive and aerospace industries, or as heat exchangers in various segments of the chemical and power generation industries. It covers the fundamental aspects of chemical corrosion and describes the corrosion resistance characteristics of specific classes of refractories and structural ceramics. The article also examines the prevention strategies that minimize corrosion failures of both classes of materials.

A carbon steel piping cross-tee assembly which conveyed hydrogen sulfide (H7S) process gas at 150 to 275 deg C (300 to 585 deg F) with a maximum allowable operating pressure of 3 MPa (450 psig) ruptured at the toe of one of the welds at the cross after several years of service. The failure was initially thought to be the result of thermal fatigue, and the internal surfaces exhibited the “elephant hide” pattern characteristic of thermal fatigue. However metallographic failure analysis found that this pattern was the result of corrosion rather than thermal fatigue. Corrosion caused failure at this location because the weld was abnormally thin as fabricated. Thus, failure resulted from inadequate deposition of weld metal and subsequent wall thinning from internal corrosion. It was recommended that the cross-tee be replaced with a like component, with more careful attention to weld quality.

During routine quality control testing, small circuit breakers exhibited high contact resistance and, in some cases, insulation of the contacts by a surface film. The contacts were made of silver-refractory (tungsten or molybdenum) alloys. Infrared analysis revealed the film to be a corrosion layer that resulted from exposure to ammonia in a humid atmosphere. Simulation tests confirmed that ammonia was the corrodent. The ammonia originated from the phenolic molding area of the plant. It was recommended that fumes from molding areas be vented outside the plant and that assembly, storage, and calibration areas be isolated from molding areas.

This article focuses on the corrosion-wear synergism in aqueous slurry and grinding environments. It describes the effects of environmental factors on corrosive wear and provides information on the impact and three-body abrasive-corrosive wear. The article also discusses the various means for combating corrosive wear, namely, materials selection, surface treatments, and handling-environment modifications.