U-shaped leaf springs, intended to serve as spacers between oil tank floats and the inner walls of the containers, broke while being fitted, or after a short time in use, in the bend of the U. The springs were made of tempered strip steel of type C 88 with 0.84 % C, bent at room temperature, and electroplated with cadmium for protection against corrosion. Each fracture showed seven or eight kidney-shaped cracks. At the origins of these cracks on the concave inner surface of the springs, crater-like depressions and beads of melted and resolidified material were found. Fracture of the springs was caused by stress cracks as a consequence of local hardening. The hardening caused by melting and resolidification, and therefore the cracks in the springs, was the result of a faulty procedure during cadmium electroplating.

The radial-contact ball bearings (type 440C stainless steel and hardened) supporting a computer microdrum were removed for examination as they became noisy. Two sizes of bearings were used for the microdrum and a spring washer that applied a 50 lb axial load on the smaller bearing was installed in contact with the inner ring for accurate positioning of the microdrum. The particles contained in residue achieved after cleaning of the grease on bearings with a petroleum solvent were attracted by a magnet and detected under a SEM (SEM) to be flaked off particles from the outer raceway surface. Smearing, true-brinelling marks, and evidence of flaking caused by the shifting of the contact area (toward one side) under axial load, was revealed by SEM investigation of one side of the outer-ring raceway. The true-brinelling marks on the raceways were found to be caused by excessive loading when the bearing was not rotating or during installation. It was concluded that the bearings had failed in rolling-contact fatigue. The noise was eliminated and the preload was reduced to 30 lb by using a different spring washer as a corrective measure.

Alloy UNS N08800 (Alloy 800) tubes of the steam superheating coils of two hydrocracker charge heaters in a refinery failed prematurely in service. Failure analysis of the tubes indicated that the failures could be attributed to thermal fatigue as a result of temperature fluctuations as well as restriction to movement. Fatigue cracks initiated intergranularly from both the flue gas and steam sides. Enhanced general and grain boundary oxidation coupled with age hardening of the alloy led to the formation of incipient intergranular cracks that acted as sites for the initiation of the fatigue cracks.

Accelerated aging tests on detonator assemblies, to verify the compatibility of gold bridgewire and Pd-In-Sn solder with the intended explosives, revealed an unusual form of corrosion. The tests, conducted at 74 deg C (165 deg F) and 54 deg C (130 deg F), indicated a preferential attack of the gold. To investigate the problem, a matrix of test units was produced and analyzed. Scanning electron microscopy, EDX analysis, and x-ray diffraction techniques were used to determine the extent of the corrosion and identify the corrosion products. The results indicated that the preferential attack of the gold was due to HCN formed by decomposition of the explosive powder at high temperatures. Other associated reactions were also observed including the subsequent attack of the solder by the gold corrosion product and degradation of the plastic header.

A crack was discovered in a cast steel (ASTM A 356, grade 6) steam turbine casing during normal overhaul of the turbine. The mechanical properties of the casting all exceeded the requirements of the specification. When the fracture surface was examined visually, an internal-porosity defect was observed adjoining a tapped hole. A second, much larger cavity was also detected. Investigation (visual inspection and 7500x SEM fractographs) supported the conclusions that failure occurred through a zone of structural weakness that was caused by internal casting defects and a tapped hole. The combination of cyclic loading (thermal fatigue), an aggressive service environment (steam), and internal defects resulted in gradual crack propagation, which was, at times, intergranular-with or without corrosive attack-and, at other times, was transgranular.

Recurring, premature failures occurred in TiN-coated M2 gear hobs used to produce carbon steel ring gears. Fractographic and metallographic examination, microhardness testing, and chemical analysis by means of EDS revealed that the primary cause of failure was a coarse cellular carbide network, which created a brittle path for fracture to occur longitudinally. As the cellular carbide network must be dispersed and refined during hot working of the original bar of material, the hobs were not salvageable. Minor factors contributing to the hob failures were premature wear resulting from lower matrix hardness and high sulfur content of the material, which contributed to lower ductility through increased nucleation sites. It was recommended that the hob manufacturer specify a minimum amount of required reduction for the original bar of tool steel material, to provide for sufficient homogenization of the carbides in the resultant hob, and lower sulfur content.

The working fluid of a hypersonic wind tunnel is freon 14 heated in molten-metal-bath heat exchangers. The coils of the heaters have failed several times from various causes. They have been replaced each time with a stainless steel deemed more appropriate, but they continue to fail. In this case study, the history of failures is traced, the causes are analyzed, and recommendations are made for future design and maintenance. Coils fabricated from AISI 316 should provide satisfactory service life if reasonable precautionary measures are observed during maintenance and testing.

Cam crack failures are a common occurrence in cam-follower systems often caused by excessive loading or inappropriate operating conditions. An investigation into such a failure was conducted to assess the effect of cam crack damage on the dynamic behavior of cam-follower systems. It was shown both theoretically and experimentally that a cracked cam causes an overall reduction in stiffness. To further probe the effect, investigators derived an analytical formula expressing the time varying stiffness of a cam-follower system. They also succeeded in quantifying the relationship between crack size and stiffness, showing that cracks have an amplitude modulating effect.

A trunnion bolt that was part of a coupling in a metropolitan railway system failed in service, causing cars to separate. The bolt had been in service for more than ten years prior to failure. Visual examination showed that the failure resulted from complete fracture at the grease port and surface groove located at midspan. Drillings machined from the bolt underwent chemical analysis, which confirmed that the material was AISI 1045 carbon steel, in accordance with specifications. Two sections cut from the bolt were subjected to metallographic examination and hardness testing. The fracture origin was typical of fatigue. The ultimate tensile strength of the bolt was in excess of requirements. Wear patterns indicated that the bolt had been frozen in position for a protracted period and subjected to repeated bending stresses, which resulted in fatigue cracking and final complete fracture. It was recommended that proper lubrication procedures be maintained to allow free rotation of the bolts while in service.

This article discusses the most common thermal analysis methods for thermosetting resins. These include differential scanning calorimetry, thermomechanical analysis, thermogravimetric analysis, and dynamic mechanical analysis. The article also discusses the characterization of uncured thermosetting resins as well as the curing process. Then, the techniques to characterize the physical properties of cured thermosets and composites are presented. Several examples of stress-strain curves are shown for thermosets and thermoplastic polymers.

A coupling in a line-shaft vertical turbine pump installed in a dam foundation fractured after a very short time. The coupling material was ASTM A582 416 martensitic stainless steel. Visual, macrofractographic, and scanning electron microscopic examination of the coupling showed that the fracture was brittle and was initiated by an intergranular cracking mechanism. The mode of fracture outside the crack initiation zone was transgranular cleavage. No indication of fatigue was found. The failure was attributed to improper heat treatment during manufacture, which resulted in a brittle microstructure susceptible to corrosion. The crack initiated either by stress-corrosion or hydrogen cracking. It was recommended that the couplings in the system be examined for surface cracking and, if present, corrective measures be taken.

A segment of a stainless steel clad bottom cone of an acid sulfite pulping batch digester failed from severe corrosion loss. The digester was fabricated of 19 mm ( 3 4 in.) low-carbon steel with 3.8 mm (0.15 in.) type 317L stainless steel cladding. The manufacturing method for the cladding was unknown. Visual and metallographic analyses indicated that the failure was from transgranular stress-corrosion cracking (TGSCC), which caused extensive cracking and spalling of the cladding and was localized in a segment of the bottom cone. The remainder of the digester cladding was unaffected. The TGSCC was attributed to high, locked-in residual stresses from the cladding process. It was recommended that the bottom cone replacement segment be stress relieved prior to installation.

This article reviews generalized test methodologies for fatigue characterization of polymers and examines fatigue fracture mechanisms in different engineering plastics. It provides detailed micromechanistic images of crack-tip processes for a variety of semicrystalline and amorphous engineering polymers. The article describes fracture mechanics solutions and approaches to the fatigue characterization of engineering polymers when dealing with macroscale fatigue crack growth. It includes mechanistic images for high-density polyethylene, ultrahigh-molecular-weight polyethylene, nylon 6, 6, polycarbonate, and polypropylene. The article describes the micromechanisms of toughening of plastics and uses a macroscale approach of applying fracture mechanics to the fatigue life prediction of engineering polymers, building on the mechanistic concepts. It also describes the factors affecting fatigue performance of polymers.

When bulging occurred in mortar tubes made of British I steel during elevated-temperature test firing, a test program was formulated to evaluate the high-temperature properties (at 540 to 650 deg C, or 1000 to 1200 deg F) of the British I steel and of several alternative alloys including a maraging steel (18% Ni, grade 250), a vanadium-modified 4337 gun steel (4337V), H19 tool steel, and high-temperature alloys Rene 41, Inconel 718, and Udimet 630. All the alloys evaluated had been used in mortar tubes previously or were known to meet the estimated minimum yield strength. The alloys fall in this order of decreasing strengths: Udimet 630, Inconel 718, Rene 41, H19 tool steel, British I steel, 4337V gun steel, and maraging steel. When cycled between room temperature and 540 to 650 deg C (1000 to 1200 deg F), only Udimet 630, Inconel 718, and Rene 41 retained yield strengths higher than the minimum. Also, these three alloys maintained high strengths over the tested range, whereas the others decreased in yield strength as cycling progressed. Analysis showed Inconel 718 was considered best suited for 81-mm mortar tubes, and widespread industrial use ensured its availability.

A steel splice plate in a power transmission line tower cracked while in service. Metallographic analysis indicated the presence of a white hard martensite layer near the crack, which occurred in the heel of the plate. Mechanical property tests revealed localized hardening in the area of the crack, supporting the metallurgical findings. A substantial deterioration of the Charpy impact toughness of the material in the heel region was also observed which is believed to have caused the initiation and propagation of the cracks leading to the failure.

This study examines the role of calcium-precipitating bacteria (CPB) in heat exchanger tube failures. Several types of bacteria, including Serratia sp. (FJ973548), Enterobacter sp. (FJ973549, FJ973550), and Enterococcus sp. (FJ973551), were found in scale collected from heat exchanger tubes taken out of service at a gas turbine power station. The corrosive effect of each type of bacteria on mild steel was investigated using electrochemical (polarization and impedance) techniques, and the biogenic calcium scale formations analyzed by XRD. It was shown that the bacteria contribute directly to the formation of calcium carbonate, a critical factor in the buildup of scale and pitting corrosion on heat exchanger tubes.

A heavily worked 304 stainless steel wire basket recrystallized and distorted while in service at 650 deg C (1200 deg F). This case study demonstrates that heavily cold worked austenitic stainless steel components can experience large losses in creep strength, and potentially structural collapse, under elevated temperature service, even at temperatures more than 300 deg C (540 deg F) below the normal solution annealing temperature. The creep strength of the recrystallized 304/304L steel was more than 1000 times less than that achievable with solution annealed 304H. These observations are consistent with limitations (2000 Addendum to ASME Boiler and Pressure Vessel Code) on the use of cold worked austenitic stainless steels for elevated temperature service.

A pressure vessel failed causing an external fire on a nine-story coke gasifier in a refinery power plant. An investigation revealed that the failure began as cracking in the gasifier internals, which led to bulging and stress rupture of the vessel shell, and the escape of hot syngas, setting off the fire. The failure mechanisms include stress relaxation cracking of a large diameter Incoloy 825 tube, stress rupture of a 4.65 in. thick chromium steel shell wall, and the oxidation of chromium steel exposed to hot syngas. The gasifier process and operating conditions that contributed to the high-temperature degradation were also analyzed and are discussed.

This article reviews the abrasive and adhesive wear failure of several types of reinforced polymers, including particulate-reinforced polymers, short-fiber reinforced polymers (SFRP), continuous unidirectional fiber reinforced polymers (FRP), particulate-filled composites, mixed composites (SFRP and particulate-filled), unidirectional FRP composites, and fabric reinforced composites. Friction and wear performance of the composites, correlation of performance with various materials properties, and studies on wear-of failure mechanisms by scanning electron microscopy are discussed for each of these types.

Several compressor diaphragms from five gas turbines cracked after a short time in service. The vanes were constructed of type 403 stainless steel, and welding was performed using type 309L austenitic stainless steel filler metal. The fractures originated in the weld heat-affected zones of inner and outer shrouds. A complete metallurgical analysis was conducted to determine the cause of failure. It was concluded that the diaphragms had failed by fatigue. Analysis suggests that the welds contained high residual stresses and had not been properly stress relieved. Improper welding techniques may have also contributed to the failures. Use of proper welding techniques, including appropriate prewelding and postwelding heat treatments, was recommended.