Failure Of Spot Welds Research Articles

Failure of resistance spot welds: tensile shear versus coach peel loading conditions

This paper aims at investigating the failure behaviour of resistance spot welds under tensile shear (TS) and coach peel (CP) loading conditions. A failure mechanism was proposed to describe both interfacial and pullout failure modes in each loading condition. The mechanisms were confirmed by SEM investigations, examining the cross-sections of the fractured welds to detail the fracture path. The experimental results showed that in the pullout failure mode during TS testing, necking is initiated at the nugget circumference in the base metal, and then failure propagates along the nugget circumference in the sheet, leading to the final fracture, while pullout failure during the CP test occurred by crack initiation and propagation near the weld nugget/heat affected zone boundary. The interfacial failure to pullout failure mode transition in the TS and CP tests was also studied. The critical weld nugget size required to ensure the pullout failure mode was obtained for each loading condition. The critical fusion zone size to ensure pullout failure mode during the TS test was larger than that of the CP test. It was found that the load bearing capacity of the spot welds under CP is significantly lower than that of the TS test.

The Effect of Paint Baking on the Strength and Failure of Spot Welds for Advanced High Strength Steels

The Effect of Paint Baking on the Strength and Failure of Spot Welds for Advanced High Strength Steels

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.

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.

Ductile shear failure or plug failure of spot welds modelled by modified Gurson model

For resistance spot welded shear-lab specimens, interfacial failure under ductile shearing or ductile plug failure are analyzed numerically, using a shear modified Gurson model. The interfacial shear failure occurs under very low stress triaxiality, where the original Gurson model would predict void nucleation and very limited void growth. Void coalescence would therefore be largely postponed. However, using the shear modification of the Gurson model, recently introduced by Nahshon and Hutchinson (2008) [1], failure prediction is possible at zero or even negative mean stress. Since, this shear modification has too large effect in some cases where the stress triaxiality is rather high, an extension is proposed in the present study to better represent the damage development at moderate to high stress triaxiality, which is known to be well described by the Gurson model. Failure prediction and tensile response curves for an interfacial shear failure or a ductile plug failure, are here compared when using either the original Gurson model, the shear modified model, or the extension to the shear modified model. The suggested extension makes it possible to use the shear modified model as a simple way of accounting for damage development under low triaxiality shearing, without further increasing the damage rate in regions of moderate to high stress triaxiality.

A simplified FE model for pull-out failure of spot welds

Abstract A simplified model is developed to simulate the pull-out failure behavior of spot welds in full vehicle crash simulations. The failure is only considered in the base steel which is characterized with the Gurson model. The parameters of the Gurson model are optimized from modeling several different test types of base steel. The weld region is represented by a single solid element, and its properties are optimized from modeling the coach-peel and lap-shear joints. The simplified model is validated by modeling the spot-welded coupon tests. It is indicated that an acceptable time step size for full vehicle crash simulations can be achieved with the model.

An Investigation of Failure Types in High-Strength Steel Resistance Spot Welds

On the shop floor, as in laboratories, destructive testing remains the main means of quality control of spot welds. After hand or mechanized weld destruction, the so-called “plug diameter” is measured and is usually considered a good indicator of the weld quality. However, it turns out that the “plug” failure of spot welds is far from being the rule. Moreover, fracture may occur in different zones of the weld, leading to very different meanings of the plug diameter. Therefore, the plug diameter is most of the time not well-correlated with the weld strength, which is the main value of the spot weld. Partial or full interfacial failures (through the weld nugget) exhibit equivalent mechanical strengths and therefore should not be rejected. Through this work, it is shown that the failure type depends on various parameters (nugget diameter, sheet thickness, loading mode, …), and consequently, it is not an intrinsic property of the steel grade. In this way, recommended quality criteria are based on weld strength, weld diameter (including the interfacial fracture area if any), or absorbed energy independently of failure occurrence. In addition, non-destructive techniques might be key investigation methods and have to be developed further.

On the failure of low carbon steel resistance spot welds in quasi-static tensile–shear loading

Failure behavior of low carbon steel resistance spot welds in quasi-static tensile–shear test is investigated. Microstructure, hardness profile and mechanical performance of the spot welds were studied. Results showed that spot welds are failed in two distinct failure modes: double-pullout and interfacial failure modes. There is a critical fusion zone size beyond which, pullout failure mode is guaranteed. Metallographic examination showed that failure is a competitive process between shear plastic deformation of weld nugget and necking of the base metal. In pullout failure mode, only the grain pattern of the base metal changes significantly and that of the fusion zone and heat affected zone remains unchanged. Strain localization was occurred in the base metal due to its low hardness. Moreover, the experimental results showed that increasing the holding time which increases the hardness of the fusion zone did not affect the peak load. It was concluded that in the pullout failure mode, the strength of the spot welds is not affected by the fusion zone strength. Fusion zone size proved to be the most important controlling factor for the spot welds’ mechanical performance in terms of peak load and energy absorption.

Strain-rate dependence in spot welds: Non-linear behaviour and failure in pure and combined modes I/II

When modelling assemblies for structural crashworthiness computations, fasteners are usually only characterised by their tension and shear strengths. However, for large deformations potentially leading to the failure of the assembly, joints are often loaded in combined modes, and the usual macro models fail to predict the assemblies' non-linear behaviour and rupture. In this paper, an advanced experimental procedure is proposed for testing and modelling spot-welded plates in pure and combined modes I/II under quasi-static and dynamic loading conditions. Obtained using a hydraulic jack and the Split Hopkinson Pressure Bar technique, the experimental results make it possible to analyse the mechanical strength (and the displacement rate dependence) of spot-welded plates. The non-linear behaviour and failure of experimental spot welds were proved to be strain-rate dependent in pure and combined mode I/II loading conditions. Our analysis showed that ultimate strengths in pure modes I and II would be particularly strain-rate dependent. Based on these results, a strain-rate dependent model was developed for ultimate loads in pure opening mode (tensile load) and presented in this paper. A computational approach for building a model for ultimate loads in pure shear mode is also discussed.

Application of Spot Weld and Sheet Metal Failure Prediction to Non-Linear Transient Finite Element Analysis of Automotive Structures

Application of Spot Weld and Sheet Metal Failure Prediction to Non-Linear Transient Finite Element Analysis of Automotive Structures

Behavior of Thin-Walled Square Tube and Tubular Hat Sections Subjected to Low Velocity Impact Loading

In this paper, the prediction and comparison of the behavior of thin-walled prismatic structures (square tube, top-hat and double-hat sections) in absorbing energy using theoretical and numerical analysis are presented. Equations to predict crushing length and dynamic mean crushing force of top-hat and double-hat sections were applied for material mild steel St37 and the effect of spot weld positions are also figured out. For comparison, an explicit non-linear commercial finite element code LS-DYNA was used to predict the response of the structures subjected to axial crushing. It was found that results of numerical methods and theoretical prediction have good agreement. Assuming that the failure of spot-weld is neglected, mean dynamic crushing force of double-hat section is 90% higher compared to that of square tube.

Predicting the failure of ultrasonic spot welds by pull-out from sheet metal

A methodology for determining the cohesive fracture parameters associated with pull-out of spot welds is presented. Since failure of a spot weld by pull-out occurs by mixed-mode fracture of the base metal, the cohesive parameters for ductile fracture of an aluminum alloy were determined and then used to predict the failure of two very different spot-welded geometries. The fracture parameters (characteristic strength and toughness) associated with the shear and normal modes of ductile fracture in thin aluminum alloy coupons were determined by comparing experimental observations to numerical simulations in which a cohesive-fracture zone was embedded within a continuum representation of the sheet metal. These parameters were then used to predict the load–displacement curves for ultrasonically spot-welded joints in T-peel and lap-shear configurations. The predictions were in excellent agreement with the experimental data. The results of the present work indicate that cohesive-zone models may be very useful for design purposes, since both the strength and the energy absorbed by plastic deformation during weld pull-out can be predicted quite accurately.

The robustness of dynamic vehicle performance to spot weld failures

Spot welds are the dominant joining method in the vehicle assembly process. As the automated assembly process is not perfect, some spot welds may be absent when the vehicle leaves the assembly line. Furthermore, spot welds are highly susceptible to fatigue, so that a substantial number may fail during the vehicle lifetime. The scope of this paper is twofold. First, the impact of spot weld quality and design on a vehicle's functional performance is reviewed, addressing strength and stiffness, NVH and durability as performance attributes. The overview briefly covers both experimental tests and predictive finite element modeling approaches. Second, an industrial robustness study is presented that assesses the effect of spot weld failure on dynamic vehicle characteristics. Damaged models are generated automatically by breaking a subset of the vehicle's spot welds, using a (weighted)-uniform selection probability. Monte Carlo simulations are then used to assess the scatter on dynamic vehicle characteristics.

One shot failure modes in spot welded structures

This article focuses on modes of one shot failure of spot welds used to join metal sheets. One shot here means unique static or dynamic load events. After a careful examination of all the failure mechanisms that can occur, the authors outline provisional criteria to establish the likelihood of each of the possible mechanisms in a probabilistic sense. Experimental data from the scientific literature are integrated with other data obtained from special tests on the materials and technologies typical of the automobile industry.

A parametric study of the bending crash performance of empty and metal foam-filled box-beams

The effect of spot welding failure, flange location, sheet metal thickness, glue presence in foam-wall contact area, and foam filling on the bending crash performance and the collapse mechanism of empty and foam-filled box-beams is investigated. It is observed that the flange location in box- beams controls spot welds force, the energy absorption capacity of box-beams, and collapse behavior of the sections. As a result, the definition of spot weld failure criterion is inevitable in most cases. A survey on the effect of slight aluminium foam filling (12% mass increase) shows that the specific energy absorption value (SEA) and the energy dissipation performance of a filled section can even compete with a thicker beam. Also, the collapse pattern of sections and the number of wrinkles for many box-beams are obviously changed in the presence of foam and glue.

Influence of spot-weld failure on crushing of thin-walled structural sections

Structural effectiveness differences have been observed in a recent study on the progressive axial collapse of thin-walled structural sections when made from different classes of steels (mild steel, interstitial-free rephosphorized high-strength steel and high-strength low-alloyed steel). A higher effectiveness was observed for spot-welded top-hat sections made from a mild steel than for similar sections made from a high-strength steel. For square sections, the structural effectiveness was not affected by the steel classes. It is anticipated that this observation applies not only for spot-welded top-hat and square sections, but for other joined and unjoined thin-walled structures as well. The part and full failure of spot-welds, during the axial collapse of the thin-walled structural sections, is one possible explanation for the above inconsistency. This is investigated experimentally in this article using peel tests on spot-weld samples under quasi-static and dynamic conditions. Despite having a lower material strength, the mild steel spot-weld samples exhibited a higher peak force and similar energy absorption during failure when compared with a high-strength steel, both under quasi-static and dynamic loadings. The potential contribution to the mean crushing force during progressive axial collapse is estimated from the experimental results and comparisons are made with deformed thin-walled structural sections from a recent experimental study. Possible implications for the determination of the mean crushing force from analytical and numerical models are identified and discussed.

A general failure criterion for spot welds under combined loading conditions

The circumferential failure mode of spot welds is investigated under combined loading conditions. Failure mechanisms of spot welds under different loading conditions are first examined by the experimental observations and a plane stress finite element analysis. An approximate limit load analysis for spot welds is then conducted to understand the failure loads of spot welds under combinations of resultant forces and resultant moments with consideration of the global equilibrium conditions only. The approximate limit load solution for circumferential failure is expressed in terms of sheet thickness, nugget diameter and combinations of loads. Failure contours are generated for spot welds under opening and shear loading conditions. The results indicate that failure contours become smaller when the ratio of the sheet thickness to the nugget diameter increases. Based on the approximate limit load solution, a general quadratic failure criterion for spot welds under combined three resultant forces and three resultant moments is proposed with correction factors determined by fitting to the experimental results of spot welds under combined loading conditions. The failure criterion can be used to characterize the failure loads of spot welds with consideration of the effects of sheet thickness, nugget diameter and combinations of loads. Experimental spot weld failure loads under combined opening and shear loading conditions and those under combined shear and twisting loading conditions are shown to be characterized well by the proposed failure criterion. Finally, a simplified general failure criterion for spot welds under three resultant forces and three resultant moments is proposed by neglecting the coupling terms of the resultant forces and moments for convenient use of the failure criterion for engineering applications.

A force-based failure criterion for spot weld design

This paper suggests a very simple, force-based formula that combines four failure modes into one dimensionless equation to govern spot weld failure under general static loading conditions. The four failure modes are shear, rotation, normal and peel. The normal separation mode and the peel mode are corresponding to mode I (opening mode). The tensile/shear mode is mode II (sliding mode), and the in-plane rotation mode is mode III (tearing mode). Test coupons and test fixtures are designed and tested to establish and verify this equation. To further verify this equation, a long difficult to understand automotive spot weld failure problem was studied. Applying finite element-calculated resultant loads to the proposed formula resulted in analytical values that correlated very well with the long time field observed spot weld failures. This analytical prediction reasonably explained the spot weld failure mechanism and provided good design directions to improve the durability of the auto structure.

Failure of spot welds under in-plane static loading

Under in-plane loading conditions, two independent modes contribute to the failure of a spot weld: the in-plane shear mode and the in-plane rotational mode. In this work, the failures of both modes under large static load are examined individually. To study the combined failure of these two modes, two special test coupons are designed. The first coupon contains one spot weld. The second coupon contains five spot welds. Tests conducted in this work show that a very simple force-based failure criterion can be used to predict the failure of a spot weld under large in-plane combined static loads. Current multiaxial failure theory cannot explain this combined failure. This force-based spot weld failure criterion fits current automotive industry needs for body shell finite element application very well.