Weld Failure Research Articles

Fatigue Failure of Flanged Thermowell

A fractured flanged thermowell was received for analysis. The fracture was located at the root of “U” insertion/immersion length which was welded to the flange. The failure of flanged thermowell was attributed to the vortex-induced fatigue. The failure was initiated by the weld failure to withstand the fluctuating load resulted from vortex after years of service. This weld failure then created stress raisers, as evidenced by the ratchet marks and river marks near the outer surface.

A reactive reconfigurable tool for aerospace structures

PurposeThe purpose of this paper is to describe the development, testing and scientific evaluation of a novel, load‐cell‐controlled reactive reconfigurable tooling (RRT) solution. This RRT not only addresses the underlying inherent problems with traditional reconfigurable tools but also potentially expands their use into the area of condition monitoring.Design/methodology/approachThe paper covers the design intent and methodology. The construction and evaluation of both a simple prototype and a fully functional tool are described.FindingsThe tool was successfully demonstrated using friction stir welding (FSW) of fuselage panels as a demanding application and the full functionality of the tool was demonstrated. The condition and process monitoring system was also demonstrated and shown to be able to distinguish both between different types of weld and tool failure conditions.Research limitations/implicationsHaving successfully designed and tested the novel RRT system under the extreme conditions of FSW, it is apparent that there are many more applications and developments that this system could be used for. The same requirements for accurate control of geometry exist in processes such as water jet cutting, trimming and machining. However, there was not sufficient resource or time within the research programme to verify this. One disadvantage of the tool is the cost of the individual load cells and the associated charge amplifiers; however, this cost is offset by the opportunity to use them in a tool condition monitoring function as well.Practical implicationsThe tool developed not only has the potential to provide cost benefits but also time reductions due to the elimination of the need to move large and heavy tools in and out of the FSW machine when part production runs are changed.Originality/valueThe originality of work described is the tool's ability to both adapt and monitor the component being held. This places it considerably beyond the state of the art in large‐scale industrial reconfigurable tooling. The research's value lies in applicability and demonstration for real production parts and processes.

Missile Impact Resistance of a Metal Mesh Roofing System

AbstractThis paper presents the results of impact tests on a roof system constructed with expanded metal mesh welded to a steel frame. The roof system was designed to be installed on a masonry structure that will protect an emergency power generator at a special-needs hurricane shelter during 177 km/h winds. The structure and roof system must meet the windborne debris impact criteria for large missiles from ASTM Standards E1996-09 and E1886-05, which state that the system must resist penetration by a nominal 51×102 mm lumber plank weighing 4.08 kg traveling at 54.7 km/h. Several impacts on the metal mesh were made by firing lumber missiles from an air cannon. The typical damage was mesh deformation and mesh tearing at the impact location and at the welds. The results showed that the tested roof system meets the impact criteria for all tested weld spacings. A smaller weld spacing of 102–229 mm was found to work best in the system, whereas a larger weld spacing of 508 or 762 mm resulted in weld failure that...

Investigation of Conveyor Pulley Shaft Failures

The cause for failures of shafts in a conveyor pulley in iron-making unit at JSW Steel has been investigated. Visual, metallographic, chemical, and fractographic studies were carried out in this study. Fracture studies of the shaft revealed shear failure because of overload. Detailed investigation revealed that the source of overload and cause of failure to be the end disk of the pulley drum, which is not considered a critical component. The disk material complies with the required material specification but failed because of changes in metallurgical characteristics during welding. Severe centerline segregation in the end disk drastically reduced its strength and service life, leading to premature failures. It was found that end disk welding failures led to overloaded bending stresses, thereby causing an imbalance in the shaft rotation and leading to failure.

Failure mode transition in AHSS resistance spot welds. Part I. Controlling factors

Failure mode of resistance spot welds is a qualitative indicator of weld performance. Two major types of spot weld failure are pull-out and interfacial fracture. Interfacial failure, which typically results in reduced energy absorption capability, is considered unsatisfactory and industry standards are often designed to avoid this occurrence. Advanced High Strength Steel (AHSS) spot welds exhibit high tendency to fail in interfacial failure mode. Sizing of spot welds based on the conventional recommendation of 4t0.5 (t is sheet thickness) does not guarantee the pullout failure mode in many cases of AHSS spot welds. Therefore, a new weld quality criterion should be found for AHSS resistance spot welds to guarantee pull-out failure. The aim of this paper is to investigate and analyze the transition between interfacial and pull-out failure modes in AHSS resistance spot welds during the tensile–shear test by the use of analytical approach. In this work, in the light of failure mechanism, a simple analytical model is presented for estimating the critical fusion zone size to prevent interfacial fracture. According to this model, the hardness ratio of fusion zone to pull-out failure location and the volume fraction of voids in fusion zone are the key metallurgical factors governing type of failure mode of AHSS spot welds during the tensile–shear test. Low hardness ratio and high susceptibility to form shrinkage voids in the case of AHSS spot welds appear to be the two primary causes for their high tendency to fail in interfacial mode.

Failure mode of laser welds in lap‐shear specimens of high strength low alloy (HSLA) steel sheets

ABSTRACTIn this paper, the failure mode of laser welds in lap‐shear specimens of non‐galvanized SAE J2340 300Y high strength low alloy steel sheets under quasi‐static loading conditions is examined based on experimental observations and finite element analyses. Laser welded lap‐shear specimens with reduced cross sections were made. Optical micrographs of the cross sections of the welds in the specimens before and after tests are examined to understand the microstructure and failure mode of the welds. Micro‐hardness tests were also conducted to provide an assessment of the mechanical properties in the base metal, heat‐affected and fusion zones. The micrographs indicate that the weld failure appears to be initiated from the base metal near the boundary of the base metal and the heat‐affected zone at a distance away from the pre‐existing crack tip, and the specimens fail due to the necking/shear of the lower left load carrying sheets. Finite element analyses based on non‐homogenous multi‐zone material models were conducted to model the ductile necking/shear failure and to obtain the J integral solutions for the pre‐existing cracks. The results of the finite element analyses are used to explain the ductile failure initiation sites and the necking/shear of the lower left load carrying sheets. The J integral solutions obtained from the finite element analyses based on the 3‐zone finite element model indicate that the J integral for the pre‐existing cracks at the failure loads are low compared to the fracture toughness and the specimens should fail in a plastic collapse or necking/shear mode. The effects of the sheet thickness on the failure mode were then investigated for laser welds with a fixed ratio of the weld width to the thickness. For the given non‐homogenous material model, the J integral solutions appear to be scaled by the sheet thickness. With consideration of the plastic collapse failure mode and fracture initiation failure mode, a critical thickness can be obtained for the transition of the plastic collapse or necking/shear failure mode to the fracture initiation failure mode. Finally, the failure load is expressed as a function of the sheet thickness according to the governing equations based on the two failure modes. The results demonstrate that the failure mode of welds of thin sheets depends on the sheet thickness, ductility of the base metal and fracture toughness of the heat‐affected zone. Therefore, failure criteria based on either the plastic collapse failure mode or the fracture initiation failure mode should be used cautiously for welds of thin sheets.

Corrosion and corrosion-fatigue of AZ31 Magnesium weldments

Variable polarity, gas-tungsten arc welds were made on roll-cast AZ31 magnesium sheet using AZ61 filler metal. Weld and base metal coupons were evaluated in air and buffered saline solution to produce S-N fatigue curves. When tested in air, both welds and base metal demonstrated a fatigue limit, with the weld failing in the heat-affected zone (HAZ) at a reduced fatigue life. In saline solution at low cycle-high stress, there was a notable reduction in fatigue life attributed to crack initiation at corrosion pits. At high cycle-low stress, failure of welds shifted to the base metal where there was severe corrosive attack and reduced load bearing cross-section. Controlled immersion tests confirmed this shift in attack from the HAZ and weld metal to the base metal at long times. Although polarization curve measurements showed the weld metal to be the most reactive (i.e. more negative corrosion potential), the gradual formation of a stable passive film eventu-Bally protects the weld metal and HAZ from further attack. By alloying more Al in the anodic weld metal (e.g. increasing AZ61 filler dilution), its corrosion potential is shifted closer to that of the base metal and corrosive attack is minimized. Higher Al content also results in higher amounts of eutectic β, and smaller grain size and higher hardness in the weld metal, matching that of the base metal.

Mechanical performance of three thickness resistance spot welded low carbon steel

Resistance spot welding is the dominant process for joining sheet metals in automotive industry. Despite the application of three thickness resistance spot welds in this industry, present guidelines and recommendations are limited to two thickness spot welds. Study towards better understanding of weld nugget growth and mechanical properties is the first step to understanding the welding behaviour and developing proper guidelines for the three thickness resistance spot welding. In this paper, weld nugget growth, mechanical performance and failure behaviour of three thickness low carbon steel resistance spot welds are investigated. Macrostrcutural and microstructural investigations, microhardness tests and quasi-static tensile–shear tests were conducted. Mechanical performance of the joint was described in terms of peak load, energy absorption and failure mode. In order to understand the failure mechanism, micrographs of the cross-sections of the spot welded joints during and after tensile–shear are examined by optical microscopy. Unlike two thickness resistance spot welded joint, weld nugget was formed in the geometrical centre of the joint (i.e. centre of the middle sheet). Weld nugget size along sheet/sheet interface was greater than that of along geometrical centre of the joint. Increasing welding time leads to increases in peak load and energy absorption of the joint and transition of interfacial failure mode to pullout failure mode, primarily due to the enlargement of weld nugget size along sheet/sheet interface.

Failure analysis of high temperature superheater tube (HTS) of a pulverized coal-fired power station

This paper presents the failure investigation of high temperature superheater (HTS) tubes. Samples were collected from one of the coal fired power plants in Malaysia, namely, Stesen Janakuasa Sultan Azlan Shah, Manjung (Manjung Power Station). After eight years of non-continuous services of three boilers, welded support-clips were completely separated (detached) from (HTS) tubes, which caused the wall thinning. Collected failed samples were undergone several experimental investigations including visual inspection, thickness measurement, Vickers hardness testing, and microstructure evaluation. The results revealed that some cracks were initiated at the heat-affected zone (HAZ) and propagated partly throughout the weld metal. The estimation on operating temperature and operating hoop stress show indication that the specimen may experienced a hig operating temperature. Other findings confirmed that the detachment of welded support-clips from HTS tubes may also caused by dissimilar metal weld (DMW) failure due to the differences in expansion properties of parent metal and weld metal.

Stress Relief Cracking on the Weld of T/P 23 Steel

T/P 23 weld failures occurred in coal fired supercritical power plants were investigated and possible causes were considered. Susceptibility of stress relief cracking was tested through mechanical properties varying heat treatment conditions in weld and intrinsic stress level in the pipe system was calculated by FEM analysis using displacement data. We also investigated what elements in the filler composition affected stress relief cracking examined. The results showed any serious susceptibility of stress relief cracking in T/P 23 steel. Post weld heat treatment (PWHT) about 740°C, 1hr specified in welding procedure produced good tensile and ductility, and cause reasonable reduction of hardness. Finite element analysis for the system has shown that the stress ranges of inlet header, link pipe and outlet header were 83∼96Mpa, 95∼104Mpa, 40∼41Mpa, respectively. The ratio of maximum stress for the design allowable stress was only 0.565 in link pipe. In the microstructural examination of T/P 23 weld, an interesting result was found that Mo content in the filler composition was significantly involved in the boundary behavior which affected stress relief cracking. Mo content close to the high level of ASME range has shown good microstructural stability and low susceptibility tostress relief cracking.

Static mechanics analysis of no bumping or vibration welding structure applied in high-speed & heavy-load CWR with single inclined surface

Nowadays, the sleeper, railroad bed, welding technology and materials of the high-speed & heavy-load Continuous Welded Rail (CWR) have been improved greatly. However, the orientation relationship between welding surface and rail is still orientation structure, which appeared 185 years ago in UK, namely, the welding surface is normal to the longitudinal direction of the rail. Thus rail welds are subject to vertical pure shear stress, lateral pure shear stress and axial pure tensile stress. Pure shear stress and pure tensile stress are both dangerous stresses, which result in failure of welds under high-speed & heavy-load condition for a long time. When the wheels pass rail welds, it is difficult to avoid vertical bumping and lateral vibration. In order to solve these problems, in this paper we put forward the single inclined welding surface having dip angle α with the lateral rail or dip angle β with the vertical rail and analyze the position geometry and stress state. Thus the vertical bumping and lateral vibration are basically eliminated, then the bearing capacity of the welding surface is significantly improved, and the stress of axial pure tensile and lateral pure shear of the welding surface is reduced.

Recommended Design Fatigue Factors for Reassessment of Piles Subjected to Dynamic Actions From Driving

Calculated probabilities of fatigue failure depend on the analysis procedure used for design. Calculated probabilities of a fatigue failure also depend on long term stress ranges due to loading and uncertainties associated with this. In order to ensure the consistent safety level for assessment of fatigue failure, the design fatigue factors (DFFs) to be used for fatigue design should be dependent on the analysis procedure and premises used. In the present paper, an assessment of appropriate DFFs for piles subjected to dynamic actions from pile driving has been performed by probabilistic analysis based on: uncertainty with respect to dynamic cyclic stress during pile driving, and fatigue capacity of circumferential welds in piles. Accumulated probabilities of fatigue failures in pile butt welds are presented. An assessment of uncertainties involved in calculation of stress ranges during pile driving has been performed. It is shown that the uncertainty in loading when driving records are known is lower than that estimated on the basis of soil data. Thus, in order to obtain consistent safety levels, different DFFs should be used when calculated stress ranges are derived based on soil data only, as compared with the actual stress ranges and number of blows determined from driving records. The results from probabilistic analyses together with recommended design fatigue factors are presented in this paper.

Estimating main steam piping circumferential weldment creep life consumption

A high energy piping (HEP) program is important for the safety of plant personnel and reliability of the piping systems. These piping systems have been designed considering allowable stresses with some safety margin at 100,000 hours. Unfortunately, many of the main steam (MS) piping systems evaluated by the author have operated beyond 350,000 hours, for which the ASME B31.1 safety margin may no longer exist. Since creep and fatigue is a typical failure mechanism, the probability of HEP failures increases with unit age.The MS piping system is one of the most critical HEP systems. Weldment failures are typically due to a combination of high temperature creep and fatigue. The circumferential weld stresses for MS piping systems operating in the creep regime should include a weldment performance factor that is both multiaxial and time dependent. Application of the actual field stresses, considering malfunctioning supports, to weldment life consumption has resulted in more accurate weldment creep – fatigue rankings and remaining life estimates. This paper demonstrates that the evaluation of actual piping stresses and variations in the actual pipe wall thickness can have a significant effect on the estimates of remaining lives of the girth weldments.

Main Steam Piping Girth Weldment Stresses and Life Consumption Considering Malfunctioning Supports

A high energy piping (HEP) program is important for the safety of plant personnel and reliability of the generating units. HEP weldment failures have resulted in serious injuries, fatalities, extensive damage of components, and significant lost generation. Since creep/fatigue is a typical failure mechanism, the probability of HEP failures increases with unit age. The main steam (MS) piping system is one of the most critical HEP systems. Weldment failures are typically due to a combination of high temperature creep and fatigue. Industry best practices include (1) the evaluation of historical operating conditions; (2) examinations of critical weldments to reveal nondestructive examination (NDE) indications, microstructural material damage, and detailed geometry data; (3) hot and cold walkdowns to document the field piping system behavior and anomalies; (4) simulation of as-found piping displacements to estimate actual stresses; (5) ranking of critical weldments; (6) recommendations for support repairs and adjustments; (7) recommendations for future examinations; and (8) remaining life estimates at critical weldments. Appropriate examinations, condition assessments, and recommendations for corrective actions are provided as a cost-effective life management process to maintain the piping system integrity. This paper provides examples demonstrating that the girth welds ranked below the top five to six welds are subject to significantly less applied stress and have substantially more creep/fatigue life than the top ranked welds. Hanger adjustments, along with selective identification, NDE, and possible repairs of top ranked welds provide substantially greater life to MS piping systems. Some fitness-for-service and risk-based programs for MS piping system girth weldments recommend a stress evaluation using typical pressure vessel or boiler tube calculations, in which the hoop stress is the principal stress. In some cases, the effective weldment stresses can be more than 50% above the hoop stress, resulting in the estimated remaining lives less than 15% of the life estimates using the hoop stress methodology. Some HEP life management programs may vaguely discuss using the principal stress based on a finite element analysis of the piping system. These principal stress values may be based on a conventional as-designed piping stress analysis. In the majority of the as-found piping stress analyses performed by the author, the maximum as-found stresses are substantially greater than the maximum conventional as-designed piping stresses. In the example case study, the maximum effective weldment stress was more than three times greater than the estimated as-designed piping stress at the same location. This paper illustrates than an as-designed piping stress analysis will typically overestimate the life of an HEP system and typically not predict the locations of maximum creep/fatigue damage.

A novel ytterbium-169 brachytherapy source and delivery system for use in conjunction with minimally invasive wedge resection of early-stage lung cancer

Purpose To describe a novel source–delivery system for intraoperative brachytherapy in patients with early-stage lung cancer that is readily adaptable to a video-assisted thoracoscopic surgery approach and can be precisely delivered to achieve optimal dose distribution. Methods and Materials Radioactive ytterbium-169 ( 169Yb) was sealed within a titanium tube 0.28 mm in diameter and then capped and resealed by titanium wires laser welded to the tube to serve as the legs of a tissue-fastening system. Dose simulations were performed using Monte Carlo computer code (Los Alamos National Laboratory, Los Alamos, NM) to mimic the geometric and elemental compositions of the source, fastening apparatus, and surroundings. Results Five test source capsules were subjected to a tensile load to failure. Failure in each capsule occurred in the wire of the fastener leg; there were no weld failures. Monte Carlo simulations and subsequent dose measurement showed the perturbation by the source legs in the deployed (bent over) position to be small (4–5%) for 169Yb and much less than that for iodine-125 (32%). Conclusion We have developed a 169Yb brachytherapy source–delivery system that can be used in conjunction with commercially available surgical stapling instruments, facilitates the precise placement of brachytherapy sources relative to the surgical margin, assures the seeds remain fixed in their precise position for the duration of the treatment, overcomes the technical difficulties of manipulating the seeds through the narrow surgical incision associated with video-assisted thoracoscopic surgery, and reduces the radiation dose to the clinicians.

Evaluation of creep damage in a welded joint of modified 9Cr–1Mo steel

This paper aims to evaluate the creep damage of modified 9Cr–1Mo steel under 600 °C operating conditions, using constitutive equations based on the continuum damage mechanics. The accumulation of voids over a long period is believed to contribute to the formation of Type IV cracking, which in turn leads eventually to the failure of weldment under conditions of higher temperatures and lower stresses. Specimens of base metal, a simulated fine-grained heat affected zone, and a thin (thick) welded joint were kept under stress from 80 to 160 MPa at 600 °C. During the creep tests of thick plate welded joint specimens, the application of stress was suspended several times, and the creep damage as indicated by the void distribution was examined quantitatively using a laser microscope. The combined effect of the equivalent creep strain and the stress triaxial factor was considered and introduced into the constitutive equations with the aid of a finite element method. The logarithms of m and 1 / λ in the continuum damage mechanics equations were determined to have a linear correlation with the ratio of the applied stress to the yield stress for homogeneous materials. In this way, the damage distribution and evolution in the fine-grained heat affected zone were evaluated successfully.

Root cause failure analysis of a division wall superheater tube of a coal-fired power station

Coal-fired boiler tubes are generally exposed to high internal pressure and high temperature of steam and flue gas. The common cause of any metallurgical failure of a superheater tube is due to the tube metal temperature higher than that as originally specified [1]. Tube metal temperature may increase gradually over many years due to the growing oxide scales inside tube or elevate rapidly caused by loss of internal steam or water-coolant flow. Internal pressurized tubes are critical components in water-tube boiler and steam superheater elements. Tubes in such application are vulnerable to high temperature upset condition, undergoing severe creep deformation or even final rupture. Therefore, boiler tubes in power plants have finite life because of prolonged exposure to high temperature, stress, aggressive environment, corrosive degradation, etc. However, uses of suitable boiler tube material in thermal power plants are required to ensure that the materials are safely used under higher temperatures and pressures for a long period of operation [2]. Many works concerning the failure problems in water-tube boiler have been reported. The North American Electric Reliability Council (NERC) reported that the coal-fired boilers are among the highest economic risk components in any other power plant. By far, the greatest number of forced outages in all types of boiler is caused by failures [3]. Elimination of boiler tube failure could save the electric power industry about $5 billion a year [4]. Metallurgists from French, Inc. [1] published data of the top 10 causes of failures where creep (long-term overheating) is 23.4%, followed by fatigue (13.9%) (thermal 8.6%, corrosion 5.3%), ash corrosion (12.0%), hydrogen damage (10.6%), weld failures (9.0%), high temperature (short-term overheating) (8.8%), erosion (6.5%), oxygen pitting (5.6%), caustic attack (3.5%) and stress corrosion cracking (2.6%). In general, 30% of all tube failures in boilers and reformers are caused by creep [5–7].

Prediction of creep crack growth properties of P91 parent and welded steel using remaining failure strain criteria

Old grades of creep resistant materials such as P11 and P22 have been studied in depth and data and prediction models are available for design and fitness for service assessment of creep rupture, creep crack growth, thermo-mechanical fatigue, etc. However, as the 9%Cr material is relatively new, there is relatively limited data available and understanding with respect to quantifying the effect of variables on life prediction of components fabricated from P91 is more difficult. Since grade P91 steel was introduced in the 1980s as enhanced ferritic steel, it has been used extensively in high temperature headers and steam piping systems in power generating plant. However, evidence from pre-mature weld failures in P91 steel suggests that design standards and guidelines may be non-conservative for P91 welded pressure vessels and piping. Incidences of cracking in P91 welds have been reported in times significantly less than 100,000 h leading to safety and reliability concerns worldwide. This paper provides a review and reanalysis of published information using properties quoted in codes of practice and from recent research data regarding the creep crack growth of P91 steel, and uses existing models to predict its behaviour. Particular areas where existing data are limited in the literature are highlighted. Creep crack growth life is predicted based on short-term uniaxial creep crack growth (CCG) data. Design and assessment challenges that remain in treating P91 weld failures are then addressed in light of the analysis.

Analysis of cross-weld creep rupture data on the ½Cr½Mo¼V type IV zone

For the ferritic steel ½Cr½Mo¼V, the operating experience of medium and long term creep failures in power station welds has been of cavitation and cracking in the heat affected zone (HAZ) adjacent to the parent material, in what is referred to as the Type IV zone. This experience has led to the generation of uniaxial cross-weld creep rupture data on ½CrMoV weldments and to the development of life assessment procedures such as R5 Volume 7, which uses the rupture strength of the Type IV zone to calculate the life of power station components. Recently, the ECCC Working Group 3A has conducted a collation of the UK and German cross-weld creep rupture data on ½CrMoV weldments. Creep failure in ½CrMoV cross-weld specimens occurs in a variety of weldment zones; typically in the parent, the Type IV zone or the weld metal. Post test examination of the specimens has enabled those tests that failed in the Type IV zone to be identified and a creep rupture data assessment has been performed to derive a new model for the rupture strength of the Type IV zone.

Fatigue performance of resistance spot welds in three sheet stack-ups

This paper describes the fatigue characteristics of spot welds for three equal thickness sheet stack-ups of a Dual Phase (DP600) welded to itself and a Mild steel welded to itself under tensile shear loading. The experiments were designed to investigate the effects of electrode tip geometries, surface indentation levels, and base metal strengths on fatigue life of the tensile shear spot welds. The electrode tip geometries were B-nose for DP600 and Mild steel and E-nose for DP600. Surface indentation levels were 10%, 30% and 50% of the sheet thickness. To obtain the goal, geometric factors were precisely controlled, especially the diameters of spot welds. Welding schedules varied to obtain proper surface indentation levels while keeping the same weld sizes for the different materials and electrode geometries. The fatigue test results showed scatter in the plot of the maximum applied loads versus cycles to failure of spot welds due to weld sizes and base metal strengths. Thus, a normalized structural stress parameter was proposed and correlated well to the fatigue test results.