Search Results for flanged beams

A supplementary axle, which was used as an extension to a highway-trailer tractor to increase its load-bearing capacity, failed in service. The rolled steel channel extensions that secured the axle assembly to the tractor main-frame I-beams fractured transversely, with the crack in each instance initiating at a weld that joined the edge of the lower flange to the support bracket casting. The cracks propagated through the flange on each side until the effective cross-sectional area had been reduced sufficiently to bring about sudden and complete fracture of the remaining web and upper flange. Fatigue fracture was caused by a combination of high bending stresses in the bottom flanges of the channels due to the heavy load being carried, concentration of stresses due to the rapid change in section modulus of the channel at its point of attachment to the support-bracket casting, and brittleness of the high-hardness HAZ of the weld associated with the abnormally high carbon content in the central part of the channel. Welding of channel edges contributed to harmful gradients in section moduli and should be avoided in future assemblies.

Several mercury diffusion pump stages in the Tritium Purification process at the Savannah River Site (SRS) have been removed from service for scheduled preventive maintenance. These stages have been examined to determine if failure has occurred. Evidence of fatigue around the flange portion of the pump has been seen. In addition, erosion and cavitation inside the throat of the venturi tube and corrosion on the other surface of the venturi tube has been observed. Several measures are being examined in an attempt to improve the performance of these pumps. These measures, as well as the noted observations, are described. Six stages [two machined (MP) and four electron beam (EB) welded] from the mercury diffusion pumps operating in the Tritium Purification process at SRS have been analyzed to determine their condition after nine months of usage. Several cracks were found around the necked region of the two MP stages. The EB welded stages, however, seemed to perform better in service; only two of four stages showed cracking. The cracking is caused by fatigue that has been enhanced by high stresses and tritium in the flange area. The EB welded stage appears to be a step in the right direction. Since the EB weld is a shrink fit, the surface is in compression, thereby eliminating crack propagation. In addition, shot peening has been employed to produce a compressive material surface since fatigue usually originates at the surface. Pitting was observed down the throat of the venturi. This pitting was caused by cavitation and erosion along the length of the venturi tube. Corrosion and pitting was seen on the exterior walls of the diffuser tubes. Stress-corrosion cracks were observed emanating from these corrosion pits. The corrosion likely occurred from the chloride ions present in the process cooling water. Shot peening is now being used in an attempt to place the outside of the diffuser tube in compression to eliminate the stress-corrosion cracking.

This article illustrates the defects, which result because of poor-quality welds in the bridge components. The cracks resulting from the use of low fatigue strength details are also discussed. The article describes the effect of out-of-plane distortion in floor-beam-girder connection plates, multiple-girder diaphragm connection plate, and tied-arch floor beams.

An I-beam of IS-226 specification—I-section dimensions of 450 x l50 x 10 mm (17.7 x 5.9 x 0.4 in.) and a length of 12.41 m (40.7ft)—was flame cut into two section in an open yard near these a coast under normal weather conditions. After approximately 112h, the shorter section of he I-beam split catastrophically along the entire length through the web. Detailed investigation revealed segregation of high levels of carbon, sulfur and phosphorus in the middle of the web and high residual stresses attributed to rolling during fabrication. Flame cutting caused a change in the distribution of the residual stresses, which, aided by low fracture toughness due to the poor quality of the beam, resulted in failure. It was recommended that segregation be avoided in cast ingots used for I-beam manufacture by implementing a better quality-control procedure.

Cold cracking of structural steel weldments is a well-documented failure mechanism, and extensive work has been done to recognize welding and materials selection parameters associated with it. These efforts, however, have not fully eliminated the occurrence of such failures. This article examines a case of cold cracking failure in the construction industry. Fortunately, the failure was identified prior to final erection of the structural members and the weld was successfully reworked. The article explains how various welding parameters, such as electrode/wire selection, joint design, and pre/postheating, played a role in the failure. Human factors and fabrication practices that contributed to the problem are covered as well.

Two sand-cast low-alloy steel equalizer beams (ASTM A 148, grade 105-85) designed to distribute the load to the axles of a highway truck broke after an unreported length of service. Normal service life would have been about 805,000 km (500,000 mi) of truck operation. Investigation (visual inspection, chemical analysis, tensile testing, unetched 65x and 1% nital etched 65x magnification) supported the conclusions that the steel was too soft for the application – probably due to improper heat treatment. Fracture of the equalizer beams resulted from growth of mechanical cracks that were formed before the castings were heat treated. Recommendations included the following changes in processing: better gating and risering in the foundry to achieve sounder castings; better shakeout practice to avoid mechanical damage; better inspection to detect imperfections; and normalizing and tempering to achieve better mechanical properties.

Field failures, traced to internal cracks that were initiated from gross nonmetallics, were encountered in the upset portion of forged 4118 steel shafts. Ultrasonic inspection was thought to be the best method for detection from the location of these cracks, their orientation, and the size of the shaft. A longitudinal beam was sent in from the end of the shaft. The shaft was observed to have a radially drilled oil hole 9 mm in diam. Since there was a variation in flaw orientation, testing of the shaft was desired from both the long and short end. The rejection level was set at 20% of full screen and was based on the size of flaws observed when the shafts were cut up. The inclusions were considered to be rejectable if the size was larger than 20 mm diam. Similar flaws were observed in larger shafts, but no flaws were observed once the shafts were sectioned. It was interpreted that the flaw signals were false and had happened when a portion of the beam struck the oily surface of the longitudinal oil hole. The problem was solved by removing the oil film from the longitudinal oil hole.

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.

The flanged bearing bush carrying the drive shaft of a feed pump suddenly fractured after about two years of service. The chemical composition was normal for high chromium ledeburitic cast steel, which was corrosion and wear resistant as well as refractory. For unknown reasons the rotating shaft came into direct contact with the flange. Mechanical friction caused a rise in temperature on both contact surfaces. This mutual contact lasted long enough for the temperature in the contact zone to exceed 1200 deg C, at which the flange material became softened or molten. As a result, considerable structural changes took place on the inner wall of the flange. Thermal stresses and excessive mechanical loads due to smearing of the flange material then led to fracture of the flange.

Several 6063-T6 aluminum alloy extension ladders of the same size and type collapsed in service in the same manner; the extruded aluminum alloy 6063-T6 side rails buckled, but the rungs and hardware remained firmly in place. The ladders had a maximum extended length of 6.4 m (21 ft) with a recommended maximum angle of inclination of 75 deg (15 deg from vertical). Investigation (visual inspection, hardness testing, metallographic examination, stress analysis, and tensile tests) supported the conclusion that the side rails of the ladders buckled when subjected to loads that produced stresses beyond the yield strength of the alloy. Recommendations included increasing the thickness of the flange and web of the side-rail extrusion.

The side supporting flange of the bottom platen of an 800 ton hydraulic press fractured after 9 x 10's cycles under a maximum load of 530 tons. The platen material specified in the design was cast steel 52. Metallographic examination of the fracture surface indicated that the platen had failed in fatigue as a result of a high stress concentration in a sharp 0.6 mm (0.02 in.) radius fillet. Stress analysis and fracture mechanics predictions revealed that there was also danger of fatigue failure for platens with the design radius of 10 mm (0. 4 in.) if the press operates at 800 tons. It was recommended that the remaining life of similar presses be assessed periodically controlling the cracks, their dimensions, and their propagation rates. An increase in the radius of the fillet was also recommended.

Thermal and transformation stresses, resulting from welding, adding up with operational stresses can result in failure. Examples involving the crankshaft of a shaft-drive to produce artificial waves in a swimming pool, the joint bar of a dredger cast out of a running non-alloyed steel with 39 kg/sq mm tensile strength, which had been strengthened by welding plate strips on both sides had fractured in service; an axle tube out of 40 Mn 4 after DIN 17 200 from a paper fabrication machine, which had three short longitudinal slits distributed uniformly over its surface; welding to repair worn out bearing or fits, and a broken rear axle tube of a bus are described.

This article briefly reviews the general causes of weldment failures, which may arise from rejection after inspection or failure to pass mechanical testing as well as loss of function in service. It focuses on the general discontinuities observed in welds, and shows how some imperfections may be tolerable and how the other may be root-cause defects in service failures. The article explains the effects of joint design on weldment integrity. It outlines the origins of failure associated with the inherent discontinuity of welds and the imperfections that might be introduced from arc welding processes. The article also describes failure origins in other welding processes, such as electroslag welds, electrogas welds, flash welds, upset butt welds, flash welds, electron and laser beam weld, and high-frequency induction welds.

Distortion failure occurs when a structure or component is deformed so that it can no longer support the load it was intended to carry. Every structure has a load limit beyond which it is considered unsafe or unreliable. Estimation of load limits is an important aspect of design and is commonly computed by classical design or limit analysis. This article discusses the common aspects of failure by distortion with suitable examples. Analysis of a distortion failure often must be thorough and rigorous to determine the root cause of failure and to specify proper corrective action. The article summarizes the general process of distortion failure analysis. It also discusses three types of distortion failures that provide useful insights into the problems of analyzing unusual mechanisms of distortion. These include elastic distortion, ratcheting, and inelastic cyclic buckling.

Distortion often is observed in the analysis of other types of failures, and consideration of the distortion can be an important part of the analysis. This article first considers that true distortion occurs when it was unexpected and in which the distortion is associated with a functional failure. Then, a more general consideration of distortion in failure analysis is introduced. Several common aspects of failure by distortion are discussed and suitable examples of distortion failures are presented for illustration. The article provides information on methods to compute load limits, errors in the specification of the material, and faulty process and their corrective measures to meet specifications. It discusses the general process of material failure analysis and special types of distortion and deformation failure.

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.