Spot-welded Specimens Research Articles

Fatigue Life Evaluation of Spot Weldment Using GA and FEM

HS40R and EZNCEN are extensively used as components of vehicle bodies because they satisfy environmental standards and improve fuel ratio. Because vehicle body sheets are becoming thinner and stronger, it is difficult to satisfy the design standards of spot welding joints. This research presents the optimization of the welding condition for various welding variables of a spot welding specimen by genetic algorithm (GA). After obtaining the optimum welding condition by use of the genetic algorithm, the fatigue properties of HS40R and EZNCEN spot welded joints are evaluated by using the finite element method (FEM) to determine the durability and stability of a structure.

Fatigue damage model based on the natural frequency changes for spot-welded joints

On the basis of FEM analysis, the variation of natural frequency during crack propagation process was investigated for single-spot and double-spots tensile-shear spot-welded specimens. The crack area was used to define the fatigue damage, with which established the relationship between damage area and natural frequency ratio. The residual fatigue life of specimen undergone a certain fatigue damage was predicted by means of the damage evolution principle of damage mechanics. The results showed that the proposed damage parameter can describe the fatigue process for spot-welded joints.

Spot-welded joints subjected to shear-tension loading. Part 1: Notch stress and strain range predictions and estimates for repeated loading

A three-dimensional finite element model has been used to study the cyclic elastic-plastic stress and strain characteristics of a spot-welded joint subjected to repeated shear-tension loading. Four geometries, having low, medium, and high notch elastic stress concentration factors, are considered in this investigation. Two simple material behaviour models, e.g. elastic-perfectly plastic and kinematic hardening, are assumed. All results have been normalized with respect to material properties so that they can be applied to all geometrically similar spot-welded joints from other materials, which may be represented by the same material models. Normalized maximum steady state strain ranges are plotted as a function of the nominal load for the range of material hardening assumptions and notch elastic stress concentration factors. These plots can be used in the low cycle fatigue life design of such geometrically similar spot-welded specimens. The paper also compares finite element steady state strain range predictions with values estimated using simple notch stress-strain conversion rules for the four geometries and a range of nominal stresses. From this study, a ‘notch plastic flow sensitivity factor’, qp, to be used in conjunction with an intermediate rule, is suggested for estimating the strain range. Finally, a strain range estimate methodology, based on the notch elastic stress concentration factor, coupled with a multiaxial fatigue criterion and an intermediate rule, is then developed for this type of specimen and loading.

Geometric functions of stress intensity factor solutions for spot welds in lap-shear specimens

In this paper, the stress intensity factor solutions for spot welds in lap-shear specimens are investigated by finite element analyses. Three-dimensional finite element models are developed for lap-shear specimens to obtain accurate stress intensity factor solutions. In contrast to the existing investigations of the stress intensity factor solutions based on the finite element analyses, various ratios of the sheet thickness, the half specimen width, the overlap length, and the specimen length to the nugget radius are considered in this investigation. The computational results confirm the functional dependence on the nugget radius and sheet thickness of the stress intensity factor solutions of [Zhang, S., 1997. Stress intensities at spot welds. International Journal of Fracture 88, 167–185; Zhang, S., 1999. Approximate stress intensity factors and notch stresses for common spot-welded specimens. Welding Journal 78, 173s–179s]. The computational results provide some geometric functions in terms of the normalized specimen width, the normalized overlap length, and the normalized specimen length to the stress intensity factor solutions of [Zhang, S., 1997. Stress intensities at spot welds. International Journal of Fracture 88, 167–185; Zhang, S., 1999. Approximate stress intensity factors and notch stresses for common spot-welded specimens. Welding Journal 78, 173s–179s] for lap-shear specimens. The computational results also indicate that when the spacing between spot welds decreases, the mode I stress intensity factor solution at the critical locations increases and the mode mixture of the stress intensity factors changes consequently. Finally, based on the analytical and computational results, the dimensions of lap-shear specimens and the corresponding approximate stress intensity factor solutions are suggested.

The Effects of Post-Weld Cold Working Processes on the Fatigue Strength of Low Carbon Steel Resistance Spot Welds

The investigation on the use of a post-weld cold working process to improve fatigue strength of low carbon steel resistance spot welds is presented. The cold working process generates uniform and consistent large zones of compressive residual stresses in resistance spot-welded low carbon steel structures using a specially designed indentation device. The effect of the indentation process parameters on the mechanical properties of the resistance spot weld was investigated. Comparisons of the mechanical properties and qualitative results between the as-resistance spot-welded specimens and the post-weld cold worked resistance spot-welded specimens have been made in this investigation. Fatigue testing was also conducted to evaluate the effect of post-weld cold working process on the fatigue characteristics of resistance spot welds. Results showed that a significant improvement in the fatigue strength has been achieved through the post-weld cold working process.

Overload failure curve and fatigue behavior of spot-welded specimens

The mechanical behavior of a spot-welded specimen is generally approached in angles of overload and fatigue failures. The primary issue in an overload failure is to establish an overload failure criterion. Fatigue failure of spot-welded specimens can be dealt with a fracture parameter, since a spot-weld forms a singular geometry of external crack type. In this work, we express the limit loads in terms of base metal yield strength and specimen geometries. We then present a master overload failure curve for a single spot-welded specimen in a mixed-mode load domain. The coordinates of the domain are normalized by the limit loads of single spot-welded specimens. Recasting the load vs. fatigue life relations experimentally obtained, we attempt to predict the fatigue life of various spot-weld specimens with a single parameter denoting the equivalent stress intensity factor. This crack driving parameter is demonstrated to successfully describe the effects of specimen geometry and load type in an inclusive manner. The suggested fatigue life formula for a single spot-weld can be used in the assessment of spot-welded panel structures as the fatigue strength of multi-spots is eventually determined by that of each single spot-weld.

Non-linear and failure behaviour of spotwelds: a “global” finite element and experiments in pure and mixed modes I/II

The paper deals with joint element model used in crashworthiness simulations. The first part of the paper is dedicated to the formulation of a new “global” finite element for spotweld modelling. The mechanical behaviour of the joint is elastic–plastic type and damage is taken into account to model the failure of the welded area. The second part of the paper concerns a new experimental procedure for joint strength analysis in pure and mixed modes I/II and for joint model characterisation. Experiment is based on Arcan principle and results are compared to open literature. In the last part of the paper, the parameters of the new joint model are identified using experiments and used for several shapes of spot-welded specimens. The model predicts reasonably the elastic–plastic part of the response but is unable to predict the post-peak response observed especially in the case of pure shear.

Failure loads of spot weld specimens under impact opening and shear loading conditions

Failure loads of spot weld specimens are investigated under impact combined loading conditions. A set of test fixtures was designed and used to obtain failure loads of mild steel spot weld specimens under combined opening and shear loading conditions. Three different impact speeds were applied to examine the effects of separation speed on failure loads. Micrographs of the cross-sections of failed spot welds were obtained to understand the failure processes in mild steel specimens under different impact combined loads. The experimental results indicate that the failure mechanisms of spot welds are very similar for mild steel specimens at various impact speeds. These micrographs show that the sheet thickness can affect the failure mechanisms. For 1.0 mm specimens, the failure occurs near the base metal in a necking/shear failure mode. For 1.5 mm specimens, the failure occurs near the heat-affected zone in a shear failure mode. Based on the experimental results, the effects of the inertia force, the separation speed, and the loading angle on the failure loads of spot welds are investigated. Failure criteria are proposed to characterize the failure loads of spot welds under impact combined opening and shear loads for engineering applications. The failure load can be expressed as a function of the tensile strength of the base metal, the nugget size, the sheet thickness, the maximum separation speed, the loading angle, and empirical coefficients for a given welding schedule.

Failure Loads of Spot Weld Specimens Under Impact Opening and Shear Loading Conditions

Failure Loads of Spot Weld Specimens Under Impact Opening and Shear Loading Conditions

Overload analysis and fatigue life prediction of spot-welded specimens using an effective J-integral

This paper proposes an integrated approach for predicting the fatigue life of spot-weld specimens. The approach is independent of specimen geometry and loading type. We establish finite element (FE) models reflecting the actual specimen behavior observed from the experimental load-deflection curves of four types of single spot-welded specimen. Using established FE models, we first evaluate J-integral as a fracture parameter for describing the effects of specimen geometry and loading type on the fatigue life. It is confirmed, however, that J-integral concept alone is insufficient to produce the generalized relationship between load and fatigue life of spot-welded specimens. On this ground, we introduce another effective parameter J e composed of J I, J II, J III. The mechanism of the mixed mode fracture, which incites the use of J e, is discussed. The parameter J e is demonstrated to more sharply define the relation between load and fatigue life of spot-welded specimens. The crack surface displacement method is adopted for decomposition of J.

Fatigue life prediction of multi-spot-welded panel structures using an equivalent stress intensity factor

Fatigue failure of spot-welded panel structures can be evaluated by means of a fracture parameter, since a weld-spot has an external crack type geometry. Recasting the load vs. fatigue life relations experimentally obtained from single-spot-welded specimens, we predict the fatigue life of spot-welded specimens with a parameter denoted the equivalent stress intensity factor ( K e). This crack driving parameter describes the effect of material ductility, specimen geometry and loading type on the fatigue failure in an inclusive manner. The proposed fatigue life formula for a single spot weld is used for assessment of multi-spot panels, as the fatigue strength of multi-spots is eventually determined by that of each constituent single spot. In this study, we consider three types of multi-spot seat belt anchors as analysis models. We obtain the effective crack driving parameter K e using finite element analyses. The K e concept is then confirmed to successfully predict the fatigue life of multi-spot panels represented by auto seat belt anchors here.

Fracture mechanics solutions to spot welds

Notch stress, stress intensity factors and J-integral at a spot weld are generally expressed by structural stresses around the spot weld. The determination of these parameters are then simplified as determining the structural stresses that can be calculated by a spoke pattern in finite element analysis. Approximate stress formulas for structural stress, notch stress and equivalent stress intensity factor are given for common spot-welded specimens. With the aid of the formulas, test data in terms of the original load can be easily transformed into the data in terms of the structural stress, notch stress or equivalent stress intensity factor at the spot weld. The formulas also facilitate the transfer of test data across different specimens. A measuring method is given for lap joints. The strain gauge technique developed for the tensile-shear specimen shows that all the structural stress, notch stress, stress intensity factors and J-integral at the spot weld can be determined by two strain gauges attached only to the outer surface of one sheet. The results presented here should be helpful for the analysis and testing of spot welds and for developing measuring methods for spot welds.

Performance of shell elements in modeling spot-welded joints

The numerical performance of general shell elements in modeling spot-welded joints is investigated. Finite element meshes composed of purely three-dimensional (3D) elements and purely general shell elements are used to analyze the stress and deformation fields in a symmetric coach-peel spot-welded specimen and their solutions are compared in detail. It is found that a properly refined finite element mesh of general shell elements can produce stress and deformation solutions comparable to those generated by a similarly refined mesh of 3D solid elements. The findings of this paper have direct implications to the analysis of sheet metal structures with spot-welded joints (such as automotive structures).

An Experimental Investigation of Limit Loads and Fatigue Properties of Spot Welded Specimens

The study on the mechanical behavior of a spot-welded specimen is largely divided into the quasi-static overload failure analysis and the fatigue failure prediction. The main issue in an overload analysis is to examine the critical loads, thereby providing a generalized overload failure criterion. As the welding spot forms a singular geometry of an external crack type, fatigue failure of spot-welded specimens can be evaluated by means of a fracture parameter. In this study, we first present the limit loads of 4 representative types of single spot-welded specimens in terms of the base metal yield strength and specimen geometries. Recasting the load vs. fatigue life relationships experimentally, obtained here, we then predict the fatigue life of spot-weld specimens with a single parameter denoted the equivalent stress intensity factor. This crack driving parameter is demonstrated to successfully describe the effects of specimen geometry and loading type in a comprehensive manner. The suggested fatigue life formula for a single spot weld can play a key, role in the design and assessment of spot-welded panel structures, in that the fatigue strength of multi-spots is eventually determined by the fatigue strength of each single spot.

Spot Weld Fatigue Life Prediction of Auto Set Belt Anchors Using $K_e$

As the welding spot forms a singular geometry of an external crack type, fatigue failure of spot-welded specimens can be evaluated by means of a fracture parameter. Recasting the load vs. fatigue life relationships experimentally obtained, we predicted the fatigue life of spot-weld specimens with a single parameter denoted the equivalent stress intensity factor. This crack driving parameter is demonstrated to successfully describe the effects of specimen geometry and loading type in a comprehensive manner. The suggested fatigue life formula for a single spot weld can play a key role in the design and assessment of spot-welded panel structures, in that the fatigue strength of multi-spots is eventually determined by the fatigue strength of each single spot. We therefore attempt to evaluate the effectiveness and validity of in predicting the fatigue life of auto seat belt anchor panel. We first establish finite element models reflecting the actual mechanical behavior of 3 types of seat belt anchor specimens. Using finite element models elaborately established, we then obtain the effective crack driving parameter composed of its ductility -dependent modal components. It is confirmed that the concept successfully predicts the fatigue life of multi-spot welded panel structures represented by auto seat belt anchors here

歯科領域レーザー治療のトピックス

Soldering has been chiefly used for jointing dental prosthetic materials. Titanium is very difficult to solder because it is highly susceptible to oxidation. The recent development of an infrared heater soldering system is expected to make titanium soldering easier. However, because this soldering system uses a solder different from the conventional solder, it may cause problems in the oral cavity such as low resistance to corrosion. Titanium soldering requires prior fixation with the investment material. Therefore, the author recently attempted laser welding of titanium. In this attempt, normal pulse Nd-YAG lasers (M801A and M802, NEC) were used. The weld strengths of titanium plate specimens obtained by two different welding techniques were compared. With a laser beam (70mm in focal distance, 0.8mm in diameter), two titanium plates (40×10×3mm), superimposed each other 2mm at the end, were welded either by lap seam welding or by spot welding under the following settings: pulse width, 4ms (for spot welding) or 3ms (for lap seam welding); output, 12.5J/P (for spot welding) or 6.3J/P (for lap seam welding). The load required to tear off the joint by pulling for the spot-welded specimen was 90kgf in average, while that for the lap-seam-welded specimen was more than 120kgf in average, where the breakage of the base metal often arose in the proximity of the weld. A major factor hampering the utilization of titanium for prosthesis is the difficulty in processing. If laser welding is introduced to titanium, the dental application of titanium will widen.

Welding of Titanium using Normal Pulse Nd-YAG Laser

Soldering has been chiefly used for jointing prosthetic materials. Titanium is very difficult to solder because it is highly susceptible to oxidation. The recent development of an infrared heater soldering system is expected to make titanium soldering easier. However, because this soldering system uses a solder different from the conventional solder, it may cause problems in the oral cavity such as low resistance to corrosion. Titanium soldering requires prior fixation with the investment material. Therefore, the authors recently attempted laser welding of titanium.In this attempt, normal pulse Nd-YAG laser (ML-2310A, Miyachi Technos Co.) was used. Laser welding was first used to join 2 titanium cast crowns together and then 2 round tita nium rods (2mm in diameter) together under the following settings: voltage, 300V; pulse width, 6ms (for crowns) or 5ms (for round rods); output, 7J (for crowns) or 6J (for rods). The round rods were rotated during welding. The focus spot diameter was 0.6mm. Laser was applied to the materials while spraying argon gas onto them.In both cases, laser welding resulted in glossy and strong union. Because the spot diameter was small, the heat-affected area was very narrow. Therefore, direct welding on a working model was possible, without necessitating fixation with an investment material. In case of round rods, it was possible to hold one end of both rods between fingers during laser welding. Because laser welding is a kind of sweating, it is expected to involve no risk for reduction of anti-corrosion resistance inthe welded area.The weld strengths of titanium plate specimens obtained by two different welding techniques were compared. With a laser beam (70mm in focal distance, 0.8mm in diameter), two titanium plates (40×10×3mm), superimposed each other 2mm at the end, were welded either by lap seam welding or by spot welding. The load required to tear off the joint by pulling for the spot-welded specimen was 56kgf in average, while that for the lap-seam-welded specimen was more than 106kgf in average, where the breakage of the base metal often arose in the proximity of the weld.A major factor hampering the utilization of titanium for prosthesis is the difficutly in processing. If laser welding is introduced to titanium, the dental application of titanium will widen.

Stress intensity factors for spot welds between plates of unequal thickness

The stress intensity factors for spot welds between plates of unequal thickness are derived as a basis for fatigue strength assessments for spot-welded specimens and components. As a first step, the structural stress state at the weld spot edge is determined, in this case by the finite element method. Then the stress state is decomposed, edge point by edge point, into symmetrical and antisymmetrical components of membrane, bending, transverse shear and longitudinal shear stresses. Different decomposition modes can be used, on the basis of stresses or resultant forces. A mixed decomposition mode which deletes those components which cause no stress singularity is preferred. The internal stress or force states are transferred to a strip model with slits representing the cross sectional contour of the weld spot edge (extended to a strip plate model for longitudinal shear loading). The resulting equivalent stress intensity factor is determined, applying the J integral on a path identical to the outside contour of the model. To deal with the antiplane shear loading case connected with K III , a J ∗ integral is defined analogous to the J integral. The boundary element method for plane and antiplane stress fields is used to determine K I , K II and K III . The dependence of the stress intensity factors on the thickness ratio of the two strips is stated by simple formulae, which are derived on the basis of the J and J ∗ integral solutions. The fundamental structure of the formulae, “structural stress multiplied by square root of thickness” is maintained including a coefficient which depends on the thickness ratio. The content of the formulae is illustrated in relevant diagrams. Comparisons with values from the literature in special cases of the strip model showed complete correspondence. The generally derived procedure is applied to the tensile shear and cross tension specimen with a thickness ratio of 4.0. The change of the maximum stress intensity factors versus a thickness ratio of 1.0 is small. The maximum stress intensity can be assessed on the basis of a simple engineering formula.