Search Results for total surface area

Proper stress analysis during component design is imperative for accurate life and performance prediction. The total stress on a part is comprised of the applied design stress and any residual stress that may exist due to forming or machining operations. Stress-corrosion cracking may be defined as the spontaneous failure of a metal resulting from the combined effects of a corrosive environment and the effective component of tensile stress acting on the structure. However, because of the orientation dependence in aluminum, it is the residual stress occurring in the most susceptible direction that must be considered of primary importance in material selection for design configuration. A Navy UH-1N helicopter main rotor blade grip manufactured from a 2014-T6 aluminum alloy forging failed because of a design flaw that left a high residual tensile stress along the short transverse plane; this in turn provided the necessary condition for stress corrosion to initiate. A complete failure investigation to ascertain the exact cause of the failure was conducted utilizing stereomicroscopic examination, scanning electron microscopy, metallographic inspection and interpretation, energy-dispersive chemical analysis, physical and mechanical evaluation. Stereomicroscopic examination of the opened crack fracture surface revealed one large fan-shaped region that had propagated radially through the thickness of the material from two distinct origin areas on the internal diam of the grip. Higher magnification inspection near the origin area revealed a flat, wood-like appearance. Scanning electron microscopy divulged the presence of substantial mud cracking and intergranular separation on the fracture surface. Metallographic examination revealed intergranular cracking and substantial leaf separation along the elongated grains parallel to the fracture surface. Chemical composition and hardness requirements were found to be as specified. The blade grip failed due to a stress corrosion crack which initiated on the inner diam and propagated in the short transverse direction through the thickness of the component. The high residual tensile stress in the part resulting from the forging and exposed after machining of the inner diam, combined with the presence of moisture, provided the necessary conditions to facilitate crack initiation and propagation.

One percent Cr-Mo low alloy constructional steel is widely used for high tensile applications, e.g., for manufacture of high tensile fasteners, heat treated shafts and axles, for automobile applications such as track pins for high duty tracked vehicles etc. The steel is fairly through hardening and heat treatment does not present any serious difficulty. Care is still required in processing to avoid decarburization. In an application of track pins for tracked vehicles, bars about 22 mm diam were required in heat treated and centerless-ground condition prior to induction hardening of the surface. Indifferent results were obtained in induction hardening; cracks were noticed, and patchy hardness figures were obtained on the final product in several batches. Metallographic examination of transverse sections through the defective areas showed decarburization to varying degrees, i.e., from partial to total decarburization. Observations suggested the defects originated at the stages of ingot making and rolling. This was apparently the reason for complete decarburization of the area with original surface defect which opened up further in the oxidizing atmosphere of the furnace with low melting clinkers from scale and furnace lining filling up the crevice of the original defect.

Drastic reduction in the service life of a production gearbox was observed. Within the gearbox, the axial load on a bevel gear (8620 steel, OD 9.2 cm) was taken by a thrust-type roller bearing (3.8 cm ID, 5.6 cm OD) in which a ground surface on the back of the bevel gear served as a raceway. Spalling damage on the ground bearing raceway at five equally spaced zones was disclosed by inspection of the bevel gear. The bearing raceway was checked for runout by mounting the gear on an arbor. It was found that the raceway undulated to the extent of 0.008 mm total indicator reading and a spalled area was observed at each high point. The presence of numerous cracks that resembled grinding cracks was revealed both by magnetic-particle inspection and microscopic examination. Spalling was produced by nonuniform loading in conjunction with grinding cracks. As corrective measures, the spindle of the grinding machine was reconditioned to eliminate the undulations and retained austenite was minimized by careful heat treatment.

The plate used to treat a pseudarthrosis in the proximal femur was investigated for reasons of non-progress of healing. Fatigue cracks were revealed on the top surface of the small section of the plate at the fifth screw hole. The plate was found to be heavily loaded by comparison of intensity of these structures, compared to results of systematic crack-initiation experiments. It was revealed by fatigue bending tests that the fatigue life of plates with asymmetrically arranged holes is at least as long as for plates with holes situated in the center. Fatigue began at the large section only after a fatigue crack begins to propagate into the small plate section. A large secondary crack which had developed parallel to the main crack in the center of the surface was revealed. The fifth hole was situated at the transition between the supporting bone and the defect and hence stress concentration was revealed to be high.

A commercial aircraft wheel half, machined from an aluminum alloy 2014 forging that had been heat treated to the T6 temper, was removed from service because a crack was discovered in the area of the grease-dam radius during a routine inspection. Neither the total number of landings nor the roll mileage was reported, but about 300 days had elapsed between the date of manufacture and the date the wheel was removed from service. The analysis (visual inspection, macrographs, micrographs, electron microprobe) supported the conclusions that the wheel half failed by fatigue. The fatigue crack originated at a material imperfection and progressed in more than one plane because changes in the direction of wheel rotation altered the direction of the applied stresses. Recommendations included rewriting the inspection specifications to require sound forgings.

Type 316L (UNS S31603) austenitic stainless steel piping was installed as part of a storm-sewer treatment collection system in a manufacturing facility. Within six months of start-up, leaks were discovered. Investigation (on-site current flow testing, visual inspection, water tests, and 5x/10x images etched in ASTM 89 reagent) supported the conclusion that the pitting in the austenitic stainless steel pipe was believed to be caused by damage to the passive layer brought about by a combination of MIC, high chloride levels, and high total dissolved solids. The low-flow and stagnant conditions present in the piping were primary contributors to the pit progression. Recommendations included replacing the pipe. Several alloys, nonmetallic materials, and lining materials were proposed for coupon testing to determine which would operate best in an environment with high levels of aerobic bacteria.

Component slippage in the left-side final drive train of a tracked military vehicle was detected after the vehicle had been driven 13,700 km (8500 miles) in combined highway and rough-terrain service. The slipping was traced to the mating surfaces of the final drive gear and the adjacent splined coupling sleeve. Specifications included that the gear and coupling be made from 4140 steel bar oil quenched and tempered to a hardness of 265 to 290 HB (equivalent to 27 to 31 HRC) and that the finish-machined parts be single-stage gas nitrided to produce a total case depth of 0.5 mm (0.020 in.) and a minimum surface hardness equivalent to 58 HRC. Investigation (visual inspection, low-magnification images, 500X images of polished sections etched in 2% nital, spectrographic analysis, and hardness testing) supported the conclusion that the failure occurred by crushing, or cracking, of the case as a result of several factors. Recommendations included reducing the high local stresses at the pitch line to an acceptable level with a design modification. Also suggested was specification of a core hardness of 35 to 40 HRC to provide adequate support for the case and to permit attainment of the specified surface hardness of 58 HRC.

Nodular cast iron crankshafts and their main-bearing inserts were causing premature failures in engines within the first 1600 km (1000 mi) of operation. The failures were indicated by internal noise, operation at low pressure, and total seizing. Concurrent with the incidence of engine field failures was a manufacturing problem: the inability to maintain a similar microfinish on the cope and drag sides of a cast main-bearing journal. Investigation supported the conclusion that the root cause of the failure was carbon flotation due to the crankshafts involved in the failures showing a higher-than-normal carbon content and/or carbon equivalent. Larger and more numerous cope side graphite nodules broke open, causing ferrite caps or burrs. They then became the mechanism of failure by breaking down the oil film and eroding the beating material. A byproduct was heat, which assisted the failure. Recommendations included establishing closer control of chemical composition and foundry casting practices to alleviate the carbon-flotation form of segregation. Additionally, some nonmetallurgical practices in journal-finishing techniques were suggested to ensure optimal surface finish.

This article commences with a description of the prosthetic devices and implants used for internal fixation. It describes the complications related to implants and provides a list of major standards for orthopedic implant materials. The article illustrates the body environment and its interactions with implants. The considerations for designing internal fixation devices are also described. The article analyzes failed internal fixation devices by explaining the failures of implants and prosthetic devices due to implant deficiencies, mechanical or biomechanical conditions, and degradation. Finally, the article discusses the fatigue properties of implant materials and the fractures of total hip joint prostheses.

The quantitative characterization of fracture surface geometry, that is, quantitative fractography, can provide useful information regarding the microstructural features and failure mechanisms that govern material fracture. This article is devoted to the fractographic techniques that are based on fracture profilometry. This is followed by a section describing the methods based on scanning electron microscope fractography. The article also addresses procedures for three-dimensional fracture surface reconstruction. In each case, sufficient methodological details, governing equations, and practical examples are provided.

Lakes in zinc die castings are areas encompassed by irregular lines or waves on flat or slightly contoured surfaces which are intended to look uniform. The laked areas have to be removed by polishing before the castings can be plated. This adds considerably to the overall cost of production. Castings examined were of an automobile name-plate holder with two flat sides of approximately 113 sq cm. All castings produced during a trial showed laking defects, the number and position varying from casting to casting. It was found that formation of metal waves and lakes depended primarily on the design of the gate and runner system and operating conditions. High flow efficiencies, with adequate feeding to all sections of the die, and short cavity fill times are desirable.

This article covers the three most popular techniques used to characterize the very outermost layers of solid surfaces: Auger electron spectroscopy (AES), X-ray photoelectron spectroscopy (XPS), and time-of-flight secondary ion mass spectrometry (TOF-SIMS). Some of the more important attributes are listed for preliminary insight into the strengths and limitations of these techniques for chemical characterization of surfaces. The article describes the basic theory behind each of the different techniques, the types of data produced from each, and some typical applications. Also discussed are the different types of samples that can be analyzed and the special sample-handling procedures that must be implemented when preparing to do failure analysis using these surface-sensitive techniques. Data obtained from different material defects are presented for each of the techniques. The examples presented highlight the typical data sets and strengths of each technique.

JIS SCM435 steel bolts that connected the slewing ring to the base carrier on a truck crane failed during the lifting of steel piles. The bolts were double-ended stud types and had been in operation for 5600 h. Failure occurred in the root of the external thread that was in contact with the first internal thread in the slewing ring. Examination of plastic carbon replicas indicated that failure was the result of fatigue action. Failure was attributed to overloading during service and increased stress concentration on a few bolts due to nonuniform separations around the slewing ring. A design change to achieve equal separation between bolt holes was recommended.

This article provides information on the chemical characterization of surfaces by Auger electron spectroscopy (AES), X-ray photoelectron spectroscopy (XPS), and time-of-flight secondary ion mass spectrometry (TOF-SIMS). It describes the basic theory behind each of these techniques, the types of data produced from each, and some typical applications. The article explains the strengths of AES, XPS, and TOF-SIMS based on data obtained from the surface of a slightly corroded stainless steel sheet.

An aircraft engine in which an in-flight fire had occurred was dismantled and examined. A bracket assembly fabricated from 2024 aluminum, one of several failed components, was of prime interest because of apparent heat damage. Scanning electron microscopy was used to compare laboratory-induced fractures made at room and elevated temperatures with the bracket failure. The service failure exhibited grain separation and loss of delineation of the grain boundaries due to melting. SEM revealed deep voids between grains and tendrils that connected grains, which resulted from surface tension during melting. Microscopic examination of polished, etched section through the fractured surface verified intergranular separation and breakdown of grain facets. The absence of any reduction of thickness on the bracket assembly at the point of fracture, along with evidence of intense heat at this point, indicated that little stress had been applied to the part. Comparisons of the service failure and laboratory-induced failures in conjunction with macroscopic and metallographic observations showed that the bracket assembly failed because an intense, localized flame had melted the material.

Two A6 tool steel (free machining grade) shafts, parts of a clamping device used for bending 5.7 cm OD tubing on an 8.6 cm radius, failed simultaneously under a maximum clamping force of 54,430 kg. The shaft was imposed with cyclic tensile stresses due to the clamping force and unidirectional bending stresses resulting from the nature of operation. Nonmetallic oxide-sulfide segregation was indicated by microscopic examination of the edge of the fracture surface. Both smooth and granular areas were revealed on visual examination of the fracture. The shaft was subjected to a low overstress as the smooth-textured fatigue zone was relatively large compared with the crystalline textured coarse final-fracture zone. The fatigue crack was nucleated by the nonmetallic inclusion that intersected the surface and initiated in the 0.25 mm radius fillet at a change in section due to stress concentration. To minimize this stress concentration, a larger radius fillet shaft at the critical change in section was suggested as corrective measure.

A crack was detected in one arm of the right-hand horizontal brace of the nose landing gear shock strut from a large military aircraft. The shock strut was manufactured from a 7049 aluminum alloy forging in the shape of a delta. A laboratory investigation was conducted to determine the cause of failure. It was concluded that the arm failed because of the presence of an initial defect that led to the initiation of fatigue cracking. The fatigue cracking grew in service until the part failed by overload. The initial defect was probably caused during manufacture. Fleet-wide inspection of the struts was recommended.