Failure Modes Of Welds Research Articles

High-Temperature Ultrasound NDE Systems for Continuous Monitoring of Critical Points in Nuclear Power Plants Structures

High temperature pipe cracks are the root of a steam power failure in the EU typically every 4 years, resulting in loss of human life, serious accidents and massive financial losses. According to IAEA’s Reference Technology Database, such an event on a nuclear power plant has an average cost of €120 million, including outage costs, emergency repair costs, insurance and legal costs. Since only one growing crack is needed to cause a major failure, they have to be inspected and monitored thoroughly. Breakdowns at extreme conditions (e.g. 580°C, 400 bar) are a result of two major weld failure modes: a) creep cracks near pipe welds; b) fatigue cracks on pipe welds. Current maintenance practice is to proceed with repairs on a detected crack according to its severity. For cost reasons, cracks that are not judged as severe enough will not be repaired. Crack severity judgement is based on its probability to cause a failure and this probability is derived taking into account the crack size and operational lifetime. More variables such as operating temperature and vibrations may rarely be found in other studies. Recent data from fracture mechanics statistical studies shows this connection between the size of a crack on a nuclear power plant pipe and its probability to lead to a failure. To deal with the above problems two Structural Health Monitoring (SHM) systems have been developed and they are presented in this work. These systems are able to achieve continuous operation for an extended time period at operating temperatures of nuclear power plants. The developed systems employ novel phased array (PA) ultrasonic and ultrasound guided wave (UGW) probes able to withstand and continuously operate even up to 580 °C. The systems are designed to be permanently mounted on superheated steam pipes, at locations of known defects and to continuously monitor their size. However, this supposes that defects will have already been detected by a traditional method during an outage. The PA transducers are placed according to the Time-of- Flight Diffraction (TOFD) technique’s topology, thus creating a novel configuration, while the UGW transducers are placed on a stainless steel ring in a circular array configuration. These configurations can enable continuous tracking of cracks growth with high accuracy, enabling maintenance crews to estimate the severity directly and not through statistics.

Resistance Spot Welding Metallurgy of Thin Sheets of Zinc-Coated Interstitial-Free Steel

The extensive use of galvanized interstitial-free (IF) steels in the automotive industry makes their resistance spot welding (RSW) metallurgy important. In this study, the relationships between microstructure, macrostructure, mechanical performance, and failure mode of resistance spot welds of galvanized IF steels were investigated. In order to characterize the macro- or microstructure, geometry, mechanical performance, and failure mode of the welds, stereographic microscopy, optical microscopy, scanning electron microscopy (SEM), and microhardness techniques were used. The results showed that the heat-affected zone (HAZ) includes ferrite grains that were elongated in the direction of heat transfer from the weld pool boundary to the base metal (BM). In addition, it was found that the nugget microstructure contains lath martensite, bainite, and different ferrite morphologies. Increasing the amount of heat input led to a decrease in martensite phase content in the weld nugget (WN) microstructure. Microhardness test results showed that the hardness of the WN is higher than the HAZ and BM. In the tensile shear tests, interfacial fracture and pullout fracture followed by BM sheet tearing were observed. It was seen that a WN with size $$ 4 \times \sqrt t $$ (t = sheet thickness) does not lead to pullout fracture. Finally, it was found that due to lower electrical resistivity of the steel in contrast to advanced high-strength steels, higher welding currents and longer welding times should be used in order to ensure the formation of large enough WNs and, thus, the satisfactory mechanical performance of the resistance spot welds.

Influence of morphological characteristics on the mechanical properties and failure mechanisms of legacy butt welds

This paper describes an investigation carried out to explore the relationship between the morphological characteristics, mechanical properties, and failure modes of legacy butt welds. A total of 31 tension coupons were cut out of butt-welded tubular structures which had been in service for over 50 years. Monotonic tension tests were conducted to investigate the mechanical behavior and the failure modes of the welds. A digital image correlation (DIC) system was used to monitor the strain distribution on the cut-out surface of the coupons. Results showed that morphological imperfections such as misalignment and plastically deformed regions near the welding zone reduced the capacity and ductility of the weld. Data from the DIC system revealed that fracture at the welding zone initiated from pre-existing imperfections.

Experimental studies in weld nugget strength of resistance spot-welded 316L austenitic stainless steel sheet

Resistance spot welding is one of the oldest and effective joining processes which generates heat by sending an electrical current through the sheets at controlled time and pressure to develop a joint at the interface of the sheets. As the demand for spot welding is drastically increased due to the increase in manufacturing of biomedical and automotive components, a detailed study on the quality of weld generated during the resistance spot welding is required. The quality of spot weld is assessed from the tensile shear strength and weld nugget diameter. An attempt is made through this research work, to study the quality of weld developed by the spot welding process of 1.6-mm-thick 316L-type austenitic stainless steel sheets (1.6 + 1.6 mm). The effect of significant process parameters namely heating time and welding current on ultimate tensile shear strength, weld nugget diameter, and failure modes is studied extensively. The critical weld nugget diameter is predicted by utilizing the proposed analytical model to ensure the tearing-type failure under a tensile shear test. Metallographic and SEM fractograph examinations are carried out to characterize the resistance spot welds. Based on the results, it is inferred that the weld nugget size, i.e., diameter and fusion depth, is the controlling factor of ultimate tensile shear strength and the resulting spot weld failure modes.

Laser welding of CP Ti to stainless steel with different temporal pulse shapes

CP Ti and stainless steel sheets were laser welded by using a pulsed wave Nd:YAG laser welding system. The effect of pulse profiles used in laser welding was studied by investigating weld appearance, weld geometry, microstructure, hardness variation, joint strength and failure mode of welds. Weld quality was strongly affected by the temporal pulse profile adopted in laser welding. In comparison with the use of a normal rectangular pulse profile, stronger welds with a better homogeneity and a complex fracture mode were achieved by using a ramp-down pulse profile. This quality enhancement was contributed from the less degree of intermixing between two welding materials in melting pools.

Failure modes of gas metal arc welds in lap-shear specimens of high strength low alloy (HSLA) steel

The failure modes of gas metal arc welds in single lap-shear specimens of high strength low alloy (HSLA) steel are investigated in this paper. Notched lap-shear specimens of gas metal arc welds were made. Quasi-static test results showed two failure locations for the welds. The specimens cut from coupons with shorter weld lengths failed near the weld root whereas the specimens cut from coupons with longer weld lengths failed near the weld toe. Scanning electron and optical microscope images of the failure surfaces and cross sections showed that the gas metal arc welds failed in a ductile necking/shear failure mode. Micro-hardness tests were conducted to provide an assessment of the mechanical properties of the base metal, the heat affected zone, and the weld metal. In order to understand the failure modes of these welds, finite element models were developed with the geometric characteristics of the heat affected zones designed to match the micrographs of the cross sections for the long and short welds. Three-dimensional finite element analyses were conducted with consideration of micro void nucleation and growth. The distributions of the void volume fraction near the welds shown from the finite element analyses are consistent with the failure modes observed in the experiments. Further finite element analyses were conducted in order to understand the effects of the geometric characteristics of the heat affected zone. The results showed that the geometric characteristics of the heat affected zone have a negligible effect on the load–displacement results; however, the geometric characteristics of the heat affected zone are key factors for the resulting failure location. Finally, finite element analyses were conducted in order to understand the effects of the weld metal geometry. The results indicate that the weld penetration and the weld toe angle have negligible effects on the load–displacement response, the peak load, and the predicted failure locations.

Fatigue Behavior of Swept Spot Friction Welds in Lap-Shear Specimens of Alclad 2024-T3 Aluminum Sheets

Failure modes of swept spot friction welds in lap-shear specimens of alclad 2024-T3 aluminum sheets are first investigated based on experimental observations. Optical and scanning electron micrographs of the welds before and after failure under quasi-static and cyclic loading conditions are examined. Experimental results show that the failure modes of the welds under quasi-static and cyclic loading conditions are quite different. Failure modes of swept spot friction welds depend considerably on the weld geometry, microstructure, and load amplitude. A fatigue crack growth model based on the paths of the dominant kinked fatigue cracks is developed to estimate the fatigue lives of the spot friction welds. The global and local stress intensity factors for finite kinked cracks, the stress intensity factors for finite transverse cracks, and the Paris law for fatigue crack propagation are used. The fatigue life estimations agree well with the experimental results.

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.

Modeling of failure mode of laser welds in lap-shear specimens of HSLA steel sheets

Failure mode of laser welds in lap-shear specimens of high strength low alloy (HSLA) steel sheets is investigated in this paper. The experiments for laser welds in lap-shear specimens under quasi-static loading conditions are briefly reviewed first. The experimental results showed that the laser welds failed in a ductile necking/shear failure mode and the ductile failure was initiated at a distance away from the crack tip near the boundary of the base metal and heat affected zone. In order to understand the failure mode of these welds, finite element analyses under plane strain conditions were conducted to identify the effects of the different plastic behaviors of the base metal, heat affected zone, and weld zone as well as the weld geometry on the ductile failure. The results of the reference finite element analysis based on the homogenous material model show that the failure mode is most likely to be a middle surface shear failure mode in the weld. The results of the finite element analysis based on the multi-zone non-homogeneous material models show that the higher effective stress–plastic strain curves of the weld and heat affected zones and the geometry of the weld protrusion result in the necking/shear failure mode in the load carrying sheet. The results of another finite element analysis based on the non-homogeneous material model and the Gurson yield function for porous materials indicate that the consideration of void nucleation and growth is necessary to identify the ductile failure initiation site that matches well with the experimental observations. Finally, the results of this investigation indicate that the failure mode of the welds should be examined carefully and the necking/shear failure mode needs to be considered for development of failure or separation criteria for welds under more complex loading conditions.

Failure modes of friction stir spot welds in cross-tension specimens of dissimilar aluminium sheets

Failure modes of friction stir spot welds in cross-tension specimens of dissimilar aluminium 5754-O and 7075-T6 sheets are investigated. Experimental results show that the failure loads of friction stir spot welds in cross-tension specimens are much lower than those in lap shear specimens. Optical micrographs of the 5754/7075 and 7075/5754 welds before and after failure under quasistatic loading conditions are examined to understand the failure modes of the welds in cross-tension specimens. The micrographs show that the 5754/7075 and 7075/5754 welds in cross-tension specimens mainly fail from the fracture along the interfacial surface and then from the fracture surface through the upper sheet material. The failure modes of the welds in cross-tension specimens are quite different from those in lap shear specimens. Finally, the paper presents discussion of the load carrying capacities and fatigue data of the welds in lap shear and cross-tension specimens based on the linear elastic stress intensity factor solutions.

Some issues on creep damage modelling of welds with heterogeneous structures

Welded components are used widely in power plants. Many of these components operate at high temperatures and under high pressures. Defects or micro cracks may be found in the vicinity of the welds. These micro cracks or voids can grow under creep cracking conditions. These cracks are often found in a narrow zone close to the interface between the parent material and the heat-affected zones (HAZ), known as the type IV region, or in some cases, in the weld metal. Welds are metallurgically complex, with heterogeneous structures within the weld metal and the HAZ. It is important to have knowledge of the initial metallurgical features and the associated material property variations so that it may be possible to accurately model the possible failure modes of welds using, for example, finite element (FE) numerical modeling techniques. This paper describes a procedure related to the determination of the high temperature creep properties for each sub-region of welds from experimental results, and it also describes the subsequent failure modeling of welds, using a creep damage mechanics approach in FE analyses. Typical examples, for P91 welds in power plant, are used to illustrate the applications.

Some issues on structural integrity analysis of P91 welds in power plants subjected to high temperature creep*

ABSTRACTWelds are metallurgically complex, with heterogeneous structures within the weld metal and heat‐affected zones. In order to carry out a structural integrity assessment, it is important to have knowledge of the initial metallurgical features and the associated material property variations so that it is possible to accurately model all the possible failure modes of welds using, for example, numerical techniques. This paper describes some fundamental issues concerning a holistic process for high temperature performance and failure prediction assessment of power plant welds. This includes welding process simulation and residual stress determination, microstructural evolution and the formulation of creep damage mechanics constitutive equations including weld metal anisotropy. Typical examples, from specific case studies for P91 welds, are used to illustrate the application. Future requirements for development of high temperature assessment methods for welds are suggested.

The Effect of Process Parameters on Weld Quality in Resistance Spot Welding Dual-Phase Steels

In order to reduce vehicle weight and improve crash performance, dual-phase (DP) steels are increasingly used for lightweight auto-body. the high cooling rates in Resistance spot welding (RSW) process will make the nugget very brittle and result in interfacial fracture modes. In this study, a servo resistance welding system is firstly introduced to control process parameters during the welding process. The influences of different process parameters (weld current, weld time and electrode force) on the weld strength, nugget diameter, facture modes and weld indentation are analyzed. Finally, an on-line weld failure mode inspection method is proposed based on servo gun’s feedback characteristics.

Effects of processing time on strengths and failure modes of dissimilar spot friction welds between aluminum 5754-O and 7075-T6 sheets

The effects of the processing time on the strengths and failure modes of two types of dissimilar spot friction welds between aluminum 5754-O and 7075-T6 sheets are investigated by experiments. Dissimilar 5754/7075 and 7075/5754 spot friction welds made at different processing conditions were tested under lap-shear loading conditions. The experimental results indicate that the failure loads of both types of welds in lap-shear specimens increase when the processing time increases for the given ranges of the processing time. The optimal processing times to maximize the failure loads of the 5754/7075 and 7075/5754 welds under lap-shear loading conditions are identified. The maximum failure load of the 7075/5754 welds is about 40% larger than that of the 5754/7075 welds. Selected optical and scanning electron micrographs of both types of welds made at different processing times before and after failure are examined. The micrographs show different weld geometries and different failure modes of the welds made at different processing times. The failure modes of the dissimilar 5754/7075 and 7075/5754 spot friction welds are quite complex and appear to strongly depend on the geometry and strength of the interfacial surface between the two deformed sheet materials.

Failure modes and fatigue life estimations of spot friction welds in lap-shear specimens of aluminum 6111-T4 sheets. Part 1: Welds made by a concave tool

Failure modes of spot friction welds made by a concave tool in lap-shear specimens of aluminum 6111-T4 sheets are first investigated based on experimental observations. Optical and scanning electron micrographs of the welds before and after failure under quasi-static and cyclic loading conditions are examined. The micrographs show that the failure modes of the welds under quasi-static and cyclic loading conditions are quite different. Under quasi-static loading conditions, the failure mainly starts from the necking of the upper sheet outside the weld. Under cyclic loading conditions, the experimental observations indicate two types of fatigue cracks. One type initiates and grows into the lower sheet outside the stir zone and another type initiates from the bend surface of the upper sheet outside the weld. The dominant fatigue cracks for the final failures of the welds are the kinked fatigue cracks growing in the lower sheet outside the stir zone. Based on the experimental observations of the paths of the dominant kinked fatigue cracks, a fatigue crack growth model is then adopted to estimate the fatigue lives of the spot friction welds. The global stress intensity factors and the local stress intensity factors for kinked cracks based on the recent published works for resistance spot welds and the Paris law for crack propagation are used in the fatigue crack growth model. The fatigue life estimations based on the fatigue crack growth model with the global and local stress intensity factors as functions of the kink length and the experimentally determined kink angles agree well with the experimental results.

Failure modes and fatigue life estimations of spot friction welds in lap-shear specimens of aluminum 6111-T4 sheets. Part 2: Welds made by a flat tool

Failure modes of spot friction welds made by a flat tool in lap-shear specimens of aluminum 6111-T4 sheets are investigated based on the approach presented in Part 1. Optical and scanning electron micrographs of the welds before and after failure under quasi-static and cyclic loading conditions are examined. The micrographs show that the microstructure, geometry, and the failure modes of the welds made by the flat tool are quite different from those of the welds made by a concave tool. Under quasi-static loading conditions, the failure mainly starts from cracking near the boundary of the stir zone close to the upper sheet surface inside the weld. Under cyclic loading conditions, the experimental results indicate three types of fatigue cracks. The first type initiates and grows near the boundary of the stir zone close to the upper sheet surface inside the weld as that under quasi-static loading conditions. The second type initiates and grows into the lower sheet outside the stir zone. The third type initiates from the bend surface of the upper sheet outside the weld. Under low-cycle loading conditions, the dominant kinked fatigue cracks are the first type growing near the boundary of the stir zone. Under high-cycle loading conditions, the dominant kinked fatigue cracks are the second type growing in the lower sheet outside the stir zone. Based on the experimental observations of the paths of the dominant kinked fatigue cracks, the fatigue crack growth model presented in Part 1 is then adopted to estimate the fatigue lives of the spot friction welds made by the flat tool. The fatigue life estimations based on the fatigue crack growth model with the global and local stress intensity factors as functions of the kink length and the experimentally determined kink angles agree well with the experimental results. A comparison of the experimental results suggests that the failure strengths of spot friction welds under quasi-static loading condition can cautiously be used as references to select a tool and the corresponding processing parameters. However, fatigue tests must be performed to validate the performance of spot friction welds under cyclic loading conditions.

Effects of fusion zone size and failure mode on peak load and energy absorption of advanced high strength steel spot welds under lap shear loading conditions

This paper examines the effects of fusion zone size on failure modes, static strength and energy absorption of resistance spot welds (RSW) of advanced high strength steels (AHSS) under lap shear loading condition. DP800 and TRIP800 spot welds are considered. The main failure modes for spot welds are nugget pullout and interfacial fracture. Partial interfacial fracture is also observed. Static weld strength tests using lap shear samples were performed on the joint populations with various fusion zone sizes. The resulted peak load and energy absorption levels associated with each failure mode were studied for all the weld populations using statistical data analysis tools. The results in this study show that AHSS spot welds with conventionally required fusion zone size of 4 t cannot produce nugget pullout mode for both the DP800 and TRIP800 welds under lap shear loading. Moreover, failure mode has strong influence on weld peak load and energy absorption for all the DP800 welds and the TRIP800 small welds: welds failed in pullout mode have statistically higher strength and energy absorption than those failed in interfacial fracture mode. For TRIP800 welds above the critical fusion zone level, the influence of weld failure modes on peak load and energy absorption diminishes. Scatter plots of peak load and energy absorption versus weld fusion zone size were then constructed, and the results indicate that fusion zone size is the most critical factor in weld quality in terms of peak load and energy absorption for both DP800 and TRIP800 spot welds.

The Effect of Grain Boundary Cavities on the Tertiary Creep Behavior and Rupture Life of 1.25Cr-0.5Mo Steel Welds

In general, the ultimate creep failure mode of welds fabricated from low alloy ferritic steels can be considered to be Type IV cracking resultant from coalescence of grain boundary damage. Thus, observation of grain boundary feature using a replication technique has been one of the most common practices for the remnant life assessment of welds operated at high temperatures. The experimental results in the present work and those found in previous works, however, prove that the feature of grain boundary damage highly depends upon testing conditions. Therefore, the relationship between the extent of damage and remaining life would not necessarily be effective for the practical use if it were obtained under the extremely accelerated laboratory conditions. In contrast, strain rate measurement predicted the rupture life with reasonable accuracy independently of the susceptibility to grain boundary cavitaion. The increase in the strain rate due to the presence of creep cavities was not observed in the experiments using heat treated specimens, which had the microstructure expected at the Intercritical HAZ in whole gauge length. This fact suggests the effectiveness of strain rate measurement for the life assessment and the necessity of reconsidering the physical meaning of grain boundary damage.

Creep analysis of pressurized circumferential pipe weldments—a review

This paper reviews work related to the high-temperature creep analysis of pressurized circumferential pipe weldments. It is important to define the problem and thus metallurgical features correctly; the identification of material microstructural-property variations within the heat-affected zone (HAZ) and the failure modes of welds are briefly included, as well as in-service experience of pipe welds. Experimental methods, including model and full-size component testing, are summarized and examples of typical tests results are described. Material constitutive equations, which can be used in describing the creep deformation of and failure mechanisms in welds, are briefly described. Numerical modelling using finite element (FE) methods, covering a range of approaches and analyses, taking account of the effects of material properties, pipe geometry, weld dimensions and system loading, on the stresses and failure behaviour of pipe weldments, are summarized. Typical results are presented to illustrate the potential uses and limitations of the FE methods.