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.

This article offers an overview of fatigue fundamentals, common fatigue terminology, and examples of damage morphology. It presents a summary of relevant engineering mechanics, cyclic plasticity principles, and perspective on the modern design by analysis (DBA) techniques. The article reviews fatigue assessment methods incorporated in international design and post construction codes and standards, with special emphasis on evaluating welds. Specifically, the stress-life approach, the strain-life approach, and the fracture mechanics (crack growth) approach are described. An overview of high-cycle welded fatigue methods, cycle-counting techniques, and a discussion on ratcheting are also offered. A historical synopsis of fatigue technology advancements and commentary on component design and fabrication strategies to mitigate fatigue damage and improve damage tolerance are provided. Finally, the article presents practical fatigue assessment case studies of in-service equipment (pressure vessels) that employ DBA methods.

This article describes some of the welding discontinuities and flaws characterized by nondestructive examinations. It focuses on nondestructive inspection methods used in the welding industry. The sources of weld discontinuities and defects as they relate to service failures or rejection in new construction inspection are also discussed. The article discusses the types of base metal cracks and metallurgical weld cracking. The article discusses the processes involved in the analysis of in-service weld failures. It briefly reviews the general types of process-related discontinuities of arc welds. Mechanical and environmental failure origins related to other types of welding processes are also described. The article explains the cause and effects of process-related discontinuities including weld porosity, inclusions, incomplete fusion, and incomplete penetration. Different fitness-for-service assessment methodologies for calculating allowable or critical flaw sizes are also discussed.

Welded connections are a common location for failures for many reasons, as explained in this article. This article looks at such failures from a holistic perspective. It discusses the interaction of manufacturing-related cracking and service failures and primarily deals with failures that occur in service due to stresses caused by externally applied loads. The purpose of this article is to enable a failure analyst to identify the causative factors that lead to welded connection failure and to identify the corrective actions needed to overcome such failures in the future. Additionally, the reader will learn from the mistakes of others and use principles that will avoid the occurrence of similar failures in the future. The topics covered include failure analysis fundamentals, welded connections failure analysis, welded connections and discontinuities, and fatigue. In addition, several case studies that demonstrate how a holistic approach to failure analysis is necessary are presented.

The structural collapse of an iron-ore bucket-wheel stacker reclaimer at the beginning of operation was investigated by means of mechanical tests, microstructural characterization, and computational structural analysis. The mechanical failure was a consequence of a brittle fracture by cleavage. The crack followed the heat-affected zone of a welded joint connecting a rectangular hollow section member and a plate flange. The main factors contributing to failure were related with a combination of design-in and manufacturing-in factors like high load-strength ratio at the point of failure, local stress concentration as a result of geometry restrictions, and weld defects. This particular section was responsible for the load transfer between the front tie member and the boom extremity, and its failure was the main cause of the catastrophic failure of the equipment.

During the pre-test inspection following the stress calculation check on a 7-ton capacity Scotch derrick crane, it was noted that threads on the back stay anchorage bolts were of unusually fine pitch (11 tpi) and that the machined faces of the nuts showed irregular pits or depressions disposed in an annular manner. When sectioned, the nuts showed a surprising method of construction. The nuts for the bolts had been made by using conventional pipe couplings inserted into sleeves made from hexagonal bar and the coupling secured to the sleeve by welding at each outer face. The ends of the sleeve bore were chamfered to form a weld preparation. After welding, the faces were machined which resulted in the removal of most of the weld metal and revealed a pronounced lack of penetration. All bolts used to anchor derrick crane back stays should be designed in accordance with the recommendations of British Standard 327:1964 (Clauses 10 and 18).

The impeller of a 4 ft. diam extraction fan driven by a 120 hp motor at 1,480 rpm. disrupted suddenly. The majority of the vanes had become detached where they were welded to the plates. At other locations, separation of the vanes was accompanied by tearing of the adjacent plate, failure being initiated at the weld fillets of the inner end of the vanes. An unusual feature was that the blades disclosed regions having a pronounced striated and stepped appearance. The etched microstructure was typical of a low carbon rolled plate having the usual banded appearance. A cross section through the fillet welds and zone showed lamellar tearing, which confirmed that failure had occurred in weld metal adjacent to the fusion face of the fillet to the vane. Results of the investigation indicated that the primary cause of failure of the impeller was the development of fatigue cracks from the unwelded roots of the fillet welds, by which the vanes were attached to the supporting plates. The impeller would have shown increased resistance to fatigue crack initiation if the T joint between the vanes and plates had been of the full penetration type.

A riveted 0.25% carbon steel oil-storage tank in Oklahoma was dismantled and reassembled in Minnesota by welding to form a storage tank for soybean oil. An opening was cut in the side of the tank to admit a front-end loader. A frame of heavy angle iron was welded to the tank and drilled for bolting on a heavy steel plate. The tank was filled to a record height. In mid-Jan the temperature dropped to -31 deg C (-23 deg F), with high winds. The tank split open and collapsed. The welding used the shielded metal arc process with E6010 electrodes, which could lead to weld porosity, hydrogen embrittlement, or both. At subzero temperatures, the steel was below its ductile-to-brittle transition temperature. These circumstances suggest a brittle condition. Steps to avoid this type of failure: For cold conditions, the steel plate should have a low carbon content and a high manganese-to-sulfur ratio and be in a normalized condition, low-hydrogen electrodes and welding practices should be used, all corners should be generously radiused, the welds should be inspected and ground or dressed to minimize stress concentrations, postweld heating is advisable, and radiographic and penetrant inspection tests should be performed.

A truck-mounted hydraulic crane had a horizontal thrust bearing with one race attached to the truck and the other to the rotating crane. The outside race of the bearing was driven by a pinion gear, and it is through this mechanism that the crane body rotated about a vertical axis. The manufacturer welded the inner race to the carrier in a single pass. After several years of service, the attachment weld between the bearing inner race and the turntable failed in the area adjacent to the heat-affected zone. The fracture zone where there was the greatest tension was heavily oxidized. In the zone where the bearing was in compression, there was a clean surface indicating recent fracture. Finally, there were areas where the weld did not meet AWS specifications for convexity or concavity. These areas were weak enough to allow fatigue cracks to initiate. Recommendations to prevent reoccurrence of the failure include the use of bolts in lieu of welding, a welding schedule that reduces the propensity of lamellar tearing, and the use of an alloy that precludes lamellar tearing. However, if abuse of the crane was the primary cause of failure, none of these recommendations would have prevented deterioration of the machine to an extent that would have rendered the failure improbable.

A pipe in a chip conveyor cracked at the toe of an exterior fillet weld connecting a flange to the pipe. The chip conveyor consisted of several spool sections. Each section was made up of a length of low-alloy steel pipe and two flanges, which were welded to each end. The composition specified for the pipe steel was 0.25C-0.98Mn-3.52Ni-1.34Cr-0.24Mo, which approximates a 9300 steel with high molybdenum. Investigation supported the conclusion that the conveyor pipe failed by brittle fracture, which was attributed to the stresses induced in forcing the circular flange over the elliptical section of the pipe. The toe of the weld and the adjacent undercut were stress raisers that determined the point of major crack origin. Under residual stress, the internal point of incomplete fusion also initiated additional cracks. Recommendations included ensuring a proper fit between an elliptical flange and pipe end to eliminate the cracking.

A roadarm for a tracked vehicle failed during preproduction vehicle testing. The arm was a weldment of two cored low-alloy steel sand castings specified to ASTM A 148, grade 120–95. A maximum carbon content of 0.32% was specified. The welding procedure called for degreasing and gas metal arc welding; neither preheating nor postheating was specified. The filler metal was E70S-6 continuous consumable wire with a copper coating to protect it from atmospheric oxidation while on the reel. Analysis of the two castings revealed that the carbon content was higher than specified, ranging from 0.40 to 0.44%. The fracture occurred in the HAZ , where quenching by the surrounding metal had produced a hardness of 55 HRC. Some roadarms of similar carbon content and welded by the same procedure had not failed because they had been tempered during a hot-straightening operation. Brittle fracture of the roadarm was caused by a combination of too high a carbon equivalent in the castings and the lack of preheating and postheating during the welding procedure. A pre-heat and tempering after welding were added to the welding procedure.

The sudden collapse of a tower crane on a building site resulted in severe injuries to the driver. Failure took place at the upper portion of the foundation or lowermost section. The mast sections were constructed from four main corner angles welded to end frames also made from angle sections which were gusseted and fitted with additional doubling plates in the corners where the jointing bolts were fitted. It was evident that the collapse was due to failure of the welds attaching the corner angles to an end frame. Many of the welds at the locations where failure occurred were of poor quality. The corner angles appeared to have been cut slightly shorter than the required dimensions. This was compensated in one case by the use of a weld build-up and in the other three by make-up pieces attached by welds of insignificant dimensions.

Tie bars of a dragline excavator each consisted of a rectangular section steel bar to which eye-pieces, to facilitate anchorage, were attached by butt-welds. Failure of one weld in each bar after seven years of service allowed the boom to fall and become extensively damaged. The appearance of the fracture faces of the two welds showed partial-penetration joints. Failure in each bar had taken place through the weld metal. The presence of built-in cracks introduced zones of stress concentration and the fluctuating loads to which the ties were subjected in service served to initiate fatigue cracks. While the partial-penetration type of weld may be tolerated in a component subjected to bending stresses it is undesirable in one that is required to withstand fluctuating tensile stresses.

A heat exchanger failed five years after going into service in an ammonia synthesis plant. Its container, made of Cr-Mo alloy steel (Material No. 1.7362), operated in an environment that did not exceed 400 deg C or 600 atm of hydrogen partial pressure. X-ray examination revealed a fissure in one of the welded seams, which according to microscopic examination, originated in the base material of the container. Higher magnification revealed a narrow zone adjacent to the weld seam permeated with intergranular cracks, the result of hydrogen attack. It also showed the structure to be completely martensitic. Thus, the failure was due to hardening of the base material during welding, and recommendation was made to temper or anneal the welded regions to reduce the effects of hydrogen under pressure.

A large pressure vessel designed for use in an ammonia plant failed during hydrostatic testing. It was fabricated from ten Mn-Cr-Ni-Mo-V steel plates which were rolled and welded to form ten cylindrical shell sections and three forgings of similar composition. The fracture surfaces were metallographically examined to be typical for brittle steel fracture and associated with the circumferential weld that joined the flange forging to the first shell section. Featureless facets in the HAZ were observed and were revealed to be the fracture-initiation sites. Pronounced banding in the structure of the flange forging was revealed by examination. A greater susceptibility to cracking was interpreted from the higher hardenability found within the bands. Stress relief was concluded to have not been performed at the specified temperature level (by hardness and impact tests) which caused the formation of hard spots. The mode of crack propagation was established by microstructural examination to be transgranular cleavage. It was concluded that failure of the pressure vessel stemmed from the formation of transverse fabrication cracks in the HAZ fostered by the presence of hard spots. It was recommended that normalizing and tempering temperatures be modified and a revised forging practice explored.

A nozzle in a wastewater vaporizer began leaking after approximately three years of service with acetic and formic acid wastewaters at 105 deg C (225 deg F) and 414 kPa (60 psig). The shell of the vessel was weld fabricated from 6.4 mm (0.25 in.) E-Brite stainless steel plate and measured 1.5 m (58 in.) in diameter and 8.5 m (28 ft) in length. Investigation (visual inspection, chemical analysis, radiography, dye-penetrant inspection, and hydrostatic testing of all E-Brite welds, 4x images, 100x/200x images electrolytically etched with 10% oxalic acid, and V-notch Charpy testing) supported the conclusion that failure of the nozzle weld was the result of intergranular corrosion caused by the pickup of interstitial elements and subsequent precipitation of chromium carbides and nitrides. Carbon pickup was believed to have been caused by inadequate joint cleaning prior to welding. The increase in the weld nitrogen level was a direct result of inadequate argon gas shielding of the molten weld puddle. Two areas of inadequate shielding were identified: improper gas flow rate for a 19 mm (0.75 in.) diam gas lens nozzle, and contamination of the manifold gas system. Recommendations included changes in the cleaning and welding process.

Several tubes in a tube bundle in an evaporator used to concentrate an acid nitrate solution failed by leakage. The feed to the evaporator contained about 6% nitrate, and the discharge about 60% nitrate. The tube bundle was comprised of type 309S (Nb) stainless steel drawn-and-welded tubes expanded and welded into two type 304L stainless steel tube sheets. The tubes failed by crevice corrosion. The failed tubes were defective as-received, and the establishment of concentration cells within the longitudinal cracks in the seam welds led to ultimate corrosive penetration of the wall. There was no evidence of crevice corrosion or any localized penetration of tubes that had sound welds. The leaking type 309S (Nb) welded tubes should be replaced with seamless tubes of type 304L stainless steel to minimize the areas requiring welding and to provide maximum weldability for the tube-sheet joints.

Welds in two CMo steel catalytic gas-oil desulfurizer reactors cracked under hydrogen pressure-temperature conditions that would not have been predicted by the June 1977 revision of the Nelson Curve for that material. Evidence of severe cracking was found in five weld-joint areas during examination of a naphtha desulfurizer by ultrasonic shear wave techniques. Defect indications were found in longitudinal and circumferential seam welds of the ASTM A204, grade A, steel sheet. The vessel was found to have a type 405 stainless steel liner for corrosion protection that was spot welded to the base metal and all vessel welds were found to be overlaid with type 309 stainless steel. Long longitudinal cracks in the weld metal, as well as transverse cracks were exposed after the weld overlay was ground off. A decarburized region on either side of the crack was revealed by metallurgical examination of a cross section of a longitudinal crack. It was concluded that the damage was caused by a form of hydrogen attack. Installation of a used Cr-Mo steel vessel with a type 347 stainless steel weld overlay was suggested as a corrective action.

A detailed failure analysis was conducted on an ammonia refrigerant condenser tube component that failed catastrophically during its initial hours of operation. Evidence collected clearly demonstrated that the weld between a pipe and a dished end contained a sharp unfused region at its root (lack of penetration). Component failure had started from this weld defect. The hydrogen absorbed during welding facilitated crack initiation from this weld defect during storage of the component after welding. Poor weld toughness at the low operating temperature facilitated crack growth during startup, culminating in catastrophic failure as soon as the crack exceeded critical length.

Two cases of failure of centrifuge baskets were investigated. The first involved a centrifuge running at approximately 1000 rpm. The basket was constructed from a perforated sheet of stainless steel rolled into a cylinder and joined by a single vee longitudinal weld. Detailed examination showed the weld had not completely penetrated the full depth of the section. The fracture faces showed a gradually progressing fatigue crack developing from a notch, formed by the lack of penetration, at the root of the weld. Microscopic examination of the parent plate showed it was a typical titanium stabilized austenitic steel. It is probable that had the basket been subjected to a periodic inspection by a competent person, this failure would not have occurred. The second case concerned a continuous duty centrifuge operating at 2200 rpm. Fracture had occurred at the circumferential weld attaching the stainless steel skirt to the basket rim and also in the region of the vertical weld which was made when the skirt was formed into a cone. Stress-corrosion cracking of the skirt material, which contained residual stresses due to cold-rolling, had been caused by the presence of sodium chloride.