Critical Stress Concentration Research Articles

Failure analysis-based mass reduction of an aluminium alloy engine mounting bracket using Design of Experiments approach

In this study, mass reduction of an engine mounting bracket has been performed without changing the failure mode. Firstly, failure modes were determined experimentally using failure loads provided by the vehicle manufacturer. Then critical stress concentration areas and suitable mass reduction regions were determined by using Finite Element Analysis (FEA) for these load cases. A structural optimisation process was carried out using Design of Experiment-Response Surface Methodology (DOE-RSM) to minimise the mass while keeping the failure load as constant as possible. When the stress values obtained from the preliminary structure are compared with the values obtained from the optimised part, it was seen that this target has been mostly achieved. A total mass reduction of 4.31 % was achieved in the new part through failure mode analysis. Furthermore, it was found that a financial saving of 129,000 Euro could also be achieved.

Fatigue lifetime predictions of notched specimens based on the critical distance and stress concentration factors

The theory of critical distances is used for evaluation of effects of notches on load-bearing capacity of notched components under static or cyclic fatigue loading. The theory can also be used to predict the fatigue life of notched components. One of the assumptions for a reliable prediction is that the critical distance is independent of the notch geometry and its stress concentration level. The study shows that the critical distance depends not only on the number of cycles to failure (under fatigue loading), but also on the notch radius. In this case, direct use of the critical distance leads to unreliable predictions. The study quantifies the relation between the model notch (from which the critical distance is determined) and the predicted notch by means of the ratio of their stress concentration factors. The predictions modified by the ratio of stress concentration factors provide results that are in satisfactory agreement with the experimental data. The predictions were made and verified on the basis of fatigue data measured on two stainless steels 1.4306 and 1.4307, and high strength structural steel S690QL. Smooth and notched specimens were tested on an ultrasonic fatigue machine in a symmetrical tension–compression mode. The results were evaluated in the regions of high cycle and very high cycle fatigue.

Развитие CAE системы численного моделирования НДС SIGMA GT

The paper presents the directions and results of developing the Sigma GT software suite designed for numerical modeling of the stress-and-strain state of a rock mass using the finite element method. It describes the process of computational module optimization to ensure the possibility of refined simulation in subareas of the original model. The process of storing coarse grid matrices is upgraded, which optimizes the use of computational resources when calculating the stress-and-strain state of large-scale models. The trial-and-error procedure has been improved up to the system of hierarchical interrelation of different scale models, which allows to actualize the boundary conditions of individual block models when modifying the field model configuration. An extensible library of local models is presented, which makes it possible to quickly generate models of mining system elements with adequate boundary conditions. The expandable functionality of the software product is demonstrated, which allows to accelerate the process of finite element model creation. This tool allows the software user to generate local models of various configurations, minimizing time consumption and possible errors in preparation of the finite element mesh. Application and functionality of tools for post-processing of modeling results, both in 2D and 3D form, are described. Export of stress isolines allows saving the calculation data in the format used at the mining enterprise and combining them with the maps of mine working. 3D visualization of the calculated data contributes to a comprehensive assessment of the stress-and-strain state vector field and identification of the areas with critical stress and strain concentrations under a certain mining development option.

Reduced-order model for RBDO of multiple TMDs on eccentric L-shaped buildings subjected to seismic excitations

L-shaped buildings subjected to ground accelerations undergo coupled lateral and torsional vibrations that can result in critical stress concentrations at their re-entrant corners. Even so, a comprehensive literature review indicates that the Reliability-Based Design Optimization (RBDO) of multiple Tuned Mass Dampers (TMDs) in L-shaped buildings has not been studied yet, to the best of the authors’ knowledge. Indeed, the more refined structural model required to capture the wings motion, rather than assuming shear-framed behavior as performed on asymmetric rectangular buildings, becomes critical in an RBDO framework. Thus, integrating a non-modal Model Order Reduction (MOR) strategy allows for a massive reduction in the computational burden. Emerging from control engineering, these techniques do not form their orthogonal subspace based on modal information. Furthermore, they dispense previous snapshots of the system solution while being suitable for non-classically damped systems. Within this context, this paper presents a novel MOR-based RBDO study of multiple TMDs in L-shaped buildings subjected to seismic excitation. The MOR algorithm connects second-order Krylov subspaces to build the reduced-order model at a specific expansion frequency with a windowing strategy for sweeping the complete frequency domain. Then, it is integrated into the RBDO, aiming to determine bi-directional TMD configurations that minimize the building’s annual failure rate for the serviceability limit state. The application case consists of a 4-story L-shaped building with 2880 active degrees of freedom (DOFs). The results show that a reduction in failure probability of up to 65% can be achieved while reaching a speed-up factor (SUF) of more than 35.

Superior crack propagation inhibitory effectiveness of MWCNT reinforced SBS toward improving oil/gas well cement integrity

The brittleness and low cracking resistance of oil- and gas-well cement threaten its integrity, specially under the high stress concentrations at bottom-hole conditions. Fortunately, the excellent mechanical properties of carbon nanotubes and elastomers motivate researchers to utilize them as versatile candidates that can positively modify the breakage behaviors of cement-based materials. Herein, we investigated the hybridization effects of multi-walled carbon nanotubes (MWCNTs) and styrene–butadiene–styrene (SBS) on the mechanical properties and breakage behaviors of oil- and gas-well cements. MWCNTs and SBS were synthesized and then, characterized with a wide range of techniques, signifying successful syntheses of these two materials. Afterward, SBS was dry mixed with cement powder and then added to the aqueous suspension of MWCNT prepared via sonication method. The mechanical testing (e.g., uniaxial, triaxial, and Brazilian tests), numerical simulation, and imaging analysis showed that incorporation of the SBS into the cement matrix promoted the ductility and macro-scale cracking resistance. The introduction of MWCNTs improved the cohesion, micro-scale cracking resistance, and strength properties of the cement matrix. Thereupon, the hybridization of 0.075 wt% of MWCNT and 9 wt% of SBS endowed great mechanical behavior and cracking resistance to the conventional oil-well cement in both micro and macro structures. These specifications can maintain cement’s durability under the critical thermal and mechanical stress concentrations at various bottom hole conditions. This improvement contains the increasing of strain energy, tensile strength, cohesion, and Poisson’s ratio by 219.27%, 141.17%, 750% and 100%, respectively and reducing the elastic modulus by 41.67%. Fourier Transform Infrared Spectroscopy (FTIR) analysis exhibited that these enhancements can be attributed to favorable interactions between MWCNT and SBS in the cement matrix.

Improving fatigue performance of metal parts with up-facing inclined surfaces produced by laser powder bed fusion and in-situ laser remelting

• Three-point bending fatigue life significantly improved by in-situ laser remelting. • In-situ laser remelting proved to be applicable to two different materials. • No microstructural defects were observed in remolten zone or interface with bulk. It is well known that the relatively poor surface quality of metal parts produced by Laser Powder Bed Fusion (LPBF) compared to conventionally manufactured parts has a negative effect on their fatigue life. To enhance fatigue life, time consuming post-processing (e.g. machining, polishing) is typically applied to improve the surface quality. Laser remelting is a promising technique for surface quality improvement, applicable to variable shapes. However, laser remelting is traditionally only used as post-process since surfaces inclined with respect to the powder bed are covered with loose powder during building. Recently, the authors have developed a dual laser technique, which enables improving the quality of up-facing inclined surfaces during building. The technique consists of two steps: (1) selectively removing powder from inclined surfaces by laser-induced shock waves, (2) laser remelting newly exposed surfaces. This paper shows a clear improvement in three-point bending fatigue of such treated samples from maraging steel 18Ni-300 and titanium alloy Ti-6Al-4V, compared to as-built state. This performance was quantified by the number of load cycles until failure at a fixed maximum tensile stress level. The improved performance could be linked to reduced surface roughness, total notch depth, critical stress concentration factor and a lower density of surface asperities. The remolten layer exhibited no visible defects and a decreased microhardness, which could beneficially impact the crack nucleation. This technique shows large potential for reducing the time and cost of metal parts produced by LPBF with enhanced fatigue performance by limiting or avoiding the need for surface post-processing.

Algorithm for Designing a Removable Complete Denture (RCD) Based on the FEM Analysis of Its Service Life.

(1) Background: The paper addresses the computer simulation and prediction of the service life of the base of removable complete dentures (RCDs) under typical loads caused by biting and chewing food. For this purpose, the finite element method (FEM) was used. It is assumed that various blocks of teeth, such as incisors, canines, premolars and molars, are subjected to cyclic impacts during a human life. (2) Methods: Both symmetric and asymmetric mastication (two- and one-sided loads, respectively) cases were considered. The load level was assumed to be 100 N, which corresponds to the average muscular compression force of typical human jaws. (3) Results: The FEM analysis of the stress–strain state evolution for RCDs under cyclic loads was carried out. Maps of equivalent lines were drawn for the denture base in terms of its durability. A multi-axial criterion was implemented to determine the number of cycles prior to failure by the mechanism of a normal opening mode crack. The FEM-based assessment of the service life of RCDs enabled us to establish the critical stress concentration areas, thereby allowing for further planning for the correction of an occlusal scheme or teeth inclinations. As a result, the service life of RCDs under cyclic loading can be improved. (4) Conclusions: An algorithm for designing RCDs in the case of edentulism based on the FEM simulation using commercial software as part of the procedure is proposed.

Elastic solutions for stresses in compliance-tailored adhesive anchors

Joining of structures via adhesive anchors is of theoretical and practical engineering importance, including anchors that support concrete ceilings in urban infrastructure. Such anchors typically fail due to stress concentrations at the loaded and/or embedded ends, captured by the well-known ‘shear-lag’ model. Herein, such anchors are revisited by considering elastic properties variation of the adhesive along the embedment length in discrete steps, to reduce critical stress concentrations, and thereby minimize the propensity of failure. Initially, a closed-form solution is developed for a system with single-step variation in adhesive compliance along the embedment length (henceforth referred to as double-adhesive bondline), which agrees well with Finite Element simulations. The simplest double-adhesive tailoring is found to reduce the maximum shear stress by 43% while maintaining the super-critical bondlength characteristics of such designs. The theoretical framework thus developed is extended to systems comprising an arbitrary number of discrete adhesives along the embedment length considering, fixed and free boundary conditions of the embedded-end, to allow for parametric evaluation of the adhesive compliance tailoring for optimal stress reduction (maximum shear stress reduces by 46% for triple-adhesive bondline) while maintaining critical-length characteristics. The adhesive tailoring could be effectively applied to anchors with critical-length characteristics by employing a facile double-adhesive bondline with the compliant adhesive near the loaded-end and stiffer adhesive near the embedded-end. The particular case of the well-known Boston tunnel anchor problem is analyzed as an exemplary demonstration of the approach.

In Silico Finite Element Analysis of Implant-Supported CAD-CAM Resin Composite Crowns.

The aim of this study was to evaluate the mechanical behavior of an implant-supported crown made using computer-aided design and computer-aided manufacturing (CAD-CAM) resin composite (RC) blocks in the posterior region. Four commercially available CAD-CAM RC blocks were used in this study: Cerasmart 300 (CS300; GC, Tokyo, Japan), Katana Avencia P Block (KAP; Kuraray Noritake Dental, Niigata, Japan); KZR HR3 Gamma Theta (HR3; Yamakin, Osaka, Japan), and Estelite P block (ESP; Tokuyama Dental, Tokyo, Japan). Katana Zirconia STML (ST; Kuraray Noritake Dental) was used as the control group. The elastic moduli of each material were determined by a three-point bending test. After the CAD models were designed, two different loading scenarios (oblique, vertical) were created. 3D finite element analysis was conducted with the prepared models. The elastic modulus of the material utilized for the implant restorations did not cause any change in the stresses transmitted to the implant or peripheral bone. An important difference was detected in the abutment-crown junction area. The minimum von Mises value at the abutment-crown interface was obtained in ST, which has the closest elastic modulus to the titanium abutment. The 3D finite element model designed in this study was used to demonstrate that implant-supported crowns fabricated with four different CAD-CAM RCs showed no critical stress concentrations in the bone or implant under all loading conditions. These results suggest that CAD-CAM RC blocks could be used as an alternative material for implant-supported restorations in the posterior region in terms of stress distribution.

Estimate of Coffin-Manson Curve Shift for the Porous Alloy AlSi9Cu3 Based on Numerical Simulations of a Porous Material Carried Out by Using the Taguchi Array.

In real engineering applications, machine parts are rarely completely homogeneous; in most cases, there are at least some minor notch effects or even more extensive inhomogeneities, which cause critical local stress concentrations from which fatigue fractures develop. In the present research, a shift of the Coffin–Manson εa–N material curve in a structure with random porosity subjected to dynamic LCF loads was studied. This allows the rest of the fatigue life prediction process to remain the same as if it were a homogeneous material. Apart from the cyclic σ–ε curve, which is relatively easy to obtain experimentally, the εa–N curve is the second most important curve to describe the correlation between the fatigue life N and the strain level εa. Therefore, the correct shift of the εa–N curve of the homogeneous material to a position corresponding to the porous state of the material is crucial. We have found that the curve shift can be efficiently performed on the basis of numerical simulations of a combination of five porosity-specific geometric influences and the associated regression analysis. To model the modified synthetic εa–N curve, five geometric influences of porosity by X-ray or μ-CT analysis are quantified, and then the porosity-adjusted coefficients of the Coffin–Manson equation are calculated. The proposed approach has been successfully applied to standard specimens with different porosity topography.

Mechanism and Control of Rockburst Induced by Draining Spatial Islands and Squaring

A small-pillar gob-side roadway showed rockburst appearance during the mining of a gob-side working face located in the Shaanxi-Inner Mongolia mining area. This study examines the 2202 gob-side working face of a coal mine in Inner Mongolia as a case study. A stress evolution model was built for the static-stress spatial islands formed by drainage regions and goafs based on the spatial relationships between drainage regions and goafs. The average microseismic frequency and energy of the high-stress zone of spatial islands were at least 1.37 times of those of other zones, validating the presence of spatial islands. The dynamic and static load effects of working face squaring were obtained based on the evolution of the stope roof as well as changes in microseismic data. Microseismically active zones were advanced to 200 m–300 m on working faces. The rockbursts induced by high static loads and dynamic and static loads formed by spatial islands and squaring were calculated. According to calculation results, the critical stress concentration value under high static loads was 3.27; the critical static stress concentration value under dynamic and static loads was 1.81. The superposition of drainage boundary stress, goaf lateral stress, and lead stress might reach the critical stress concentration under dynamic load disturbances, causing rockbursts. A stress adjustment scheme was established, including overall hydraulic fracturing of the external roof of the drainage region, reduction of stoping speed, and pressure relief of large-diameter boreholes. The stress adjustment scheme was implemented on-site and supplemented by monitoring and early warning methods to safely advance by the first squaring region.

Stress analysis of finite orthotropic plates loaded by a bolted joint

Bolted joints are commonly used to connect safety-critical parts in aeronautics, which can be composite laminates. However, drilling holes also means introducing stress raisers. For narrow connections, fatal tension failure mode is likely to be triggered. Accurate stress assessment methods are crucial to create safe and lightweight optimal designs. These tools can be based on analytical means beneficial in terms of efficient computation and shall be scope of this paper. In this context, a plate model idealising orthotropic composite laminates without bending extension coupling is chosen. This type of laminates is commonly used in industry contexts. Special consideration is given to finite plate dimensions. Their impact on the critical stress concentrations is studied for different lay-ups of the composite laminate. This further allows to investigate the effects of material orthotropy. Validation to Finite Element analyses shows good agreement for commonly used lay-ups and plate dimensions.

Ex situ Synthesis and Characterization of High Strength Multipurpose Bacterial Cellulose-Aloe vera Hydrogels.

The innate structural and functional properties of bacterial cellulose (BC) have been greatly improved by developing its composites with other materials for its applications in different fields. In the present study, BC-Aloe vera (BCA) gel composite with high tensile strength was ex situ developed and characterized for its potential applications in environmental and medical fields. FE-SEM micrographs showed the impregnation of Aloe vera gel into the fibril network of BC. The dry weight analysis showed the addition of 40 wt.% Aloe vera contents into the BC matrix. The addition of Aloe vera resulted in a 3-fold increase in the mechanical strength of BCA composite. The critical strain or stress concentration points were accurately identified in the composite using a three-dimensional digital image correlation (3D-DIC) system. The BCA composite retained water for an extended period of up to 70 h. The BCA composite effectively adsorbed Cu, Co, Fe, and Zn metals. Moreover, the BCA composite supported the adhesion and proliferation of MC3T3-E1 cells. The findings of this study suggest that the developed BCA composite could find multipurpose applications in different fields.

Analysis of the failure modes and design improvement of an eccentrically loaded connecting rod for a double front axle steering linkage prototype

Failure analysis and optimal redesign study of the connecting rod prototype to be used in the multi-axle steering linkage of an 8x8 Aircraft Rescue and Fire Fighting (ARFF) vehicle is reported. At the prototyping stage, the field tests showed that the connecting rod experiences a plastic deformation during a zero-speed steering manoeuvre. In order to predict the possible causes of the failure, Finite Element (FE) analyses were applied to the complete steering linkage which contains the failed rod design. Primary FE analyses showed that there are critical stress concentration regions on the rod. It was also seen that the maximum stress values at these regions are influenced by the tyre steering angle and the slope of the road surface. Subsequently, critical structural parameters having an influence on the mechanical behaviour of the part were chosen, and a parametric base model was built. Then, using DOE-RSM (Design of Experiments – Response Surface Methodology) based optimisation study, optimal values were selected within the applicable variation range. Nonetheless, by considering the form of the structure together with the loading type, it was decided that the elastic stability should also be taken into account. Although a sufficient improvement was obtained against the overload failure, the elastic stability limit remained exceeded according to the American Institute of Steel Structures (AISC) criteria. Thus, the eccentricity of the connecting rod was also decreased by employing a particular kinematic arrangement in order not to disturb the Ackermann’s rule-based synchronisation ratio of the linkage. Compared to the failed design, it was seen that von-Mises stress can be reduced by up to 66%. The results of this study showed that, factors such as wheel steering angle and road inclination-related axle load variation greatly influence the design load of the steering linkage. Field tests also indicated that, suggested design improvements such as increase in thickness and eccentricity reduction meet the strength requirements of the system.

Influence of weld geometry on stress concentration factor distributions in tubular joints

Fatigue design of welded tubular joints relies on an extensive number of parametric equations that allow calculation of the stress concentration factors at characteristic locations around the weld circumference. These equations were developed with finite element models that either do not include a weld geometry or include a simplified representation thereof. This paper presents a new approach for modeling American Welding Society D1.1 standard compliant welds with different geometries. The generated finite element models are used to study the effect of different weld geometries on the stress concentration factor distributions in tubular joints. The results show that the geometry of the weld has a strong influence on the magnitude of the critical stress concentration factor, the shape of the stress concentration factor distribution and the expected failure location. Finally the authors highlight the need for more comprehensive guidelines regarding the determination of complete stress concentration factor distributions from finite element simulations.

CONVEX (CONtinuously Varied EXtrusion): A new scale of design for additive manufacturing

This study introduces a new microscale design approach, CONVEX, based on the idea of CONtinuously Varying EXtrusion widths of deposited filaments in material extrusion additive manufacturing (MEAM). The CONVEX design approach breaks free from the traditional 3D printing workflow of filling a CAD-model volume with stacked extruded filaments of constant width and height. Instead, the geometry of each filament is explicitly designed over its entire length. The concept may disrupt a wide range of applications in both the short and long term. In the least ambitious short-term implementation of CONVEX, its principles can be integrated into 3D printing slicing software to allow a geometric form to be fitted by streamlined filaments, with constantly varying widths as required to match the overall geometry (referred to as “streamlined slicing”). The use of continuous streamlined filaments, as opposed to frequently changing the number of filaments, improved the quality of manufactured parts by eradicating voids and defects, which are known to cause critical stress concentrations in specimens for tensile testing. At the other end of the scale, in the most disruptive implementation of the CONVEX design approach, entire new material structures and product types will be enabled, with feature size-scales perhaps an order of magnitude lower than that permitted by present design rules. This will enable new innovative metamaterials and is particularly appropriate for high-value applications such as advanced filtering, tissue engineering, drug delivery, microfluidics or electronics, where the geometries of individual extruded filaments (or deliberate inter-filament pores) form the functional design geometry. To prove the technical feasibility of CONVEX, this study includes rigorous experimental work to characterise how instantaneous changes to MEAM process parameters (e.g. speed, acceleration, extrusion rate and retraction) enable highly controllable and dynamic variation of the width of extruded filaments (from 75 % to 250 % of nozzle diameter). The concept is proven for multiple materials, layer heights, extrusion temperatures, nozzle sizes, and for both Bowden and direct-drive printer types. The Bowden printer was found to be an order of magnitude less responsive to changes in extrusion rate than the direct-drive printer. Case studies demonstrate the CONVEX design approach, which has already enabled break-through micro-mechanical research for MEAM. Industrial implications are discussed along with the potential for translation to other manufacturing processes.

PPG-Terminated Tetra-Carbamates as the Toughening Additive for Bis-A Epoxy Resin.

We synthesized PPG-terminated tetra-carbamates as a new toughening additive for epoxy thermosets through facile addition reaction of hexamethylene diisocyanate (HDI) with poly(tetra-methylene glycols) (PTMG) and poly(propylene glycols) (PPG). The effects of prepared tetra-carbamates on the rheological behavior of neat epoxy resin were studied along with the various cured properties of their modified epoxy systems. Four carbamate groups (–NHCOO–) endow the prepared additives not only with good intramolecular interactions, but also with optimal intermolecular interactions with epoxy polymers. This results in the suitable miscibility of the additives with the epoxy matrix for the formation of the typical biphasic structure of microparticles dispersed in the epoxy matrix via polymerization-induced microphase separation. The impact strength and critical stress concentration factor (KIC) of cured modified epoxy systems with the additives are significantly higher than those of unmodified epoxy systems, without sacrificing the processability (Tg) and flexural strength. The toughening mechanism is understood as a synergism combination among the phase separation mechanism, the in situ homogeneous toughening mechanism, and the particle cavitation mechanism.

Computational analysis via FEM of Tirefond screws used in the fastening system of railroads aiming to avoid a recurrent failure case

This work presents a computational evaluation of a Tirefond screw used in Brazilian railroads to support the plate on the wood sleepers. This component has been studied because nowadays it is manufactured by a common low carbon steel, and due to the increase of trains speed and the axle load transported it has been failing with a short life time, causing significant economic losses to the companies that control the Brazilian railways. In this context, static and dynamic analysis were performed using the Finite Element Method (FEM) in order to better understand the recurrent failure causes. The obtained results highlighted the critical stress concentration points where the component fatigue life is low, explaining the premature failures. Based on the generated information, it was possible to propose and to evaluate the effects of geometric and materials change on the Tirefond screw fatigue performance. The actual geometry was evaluated but replacing the actual steel by a carbonitrited SAE 1015 and by a quenched and tempered SAE 4340. Geometric changes as the increase of the screw diameter and elimination of the continuous diameter decrease as it moves away from the screw head were tested. The best solution that guaranteed a fatigue life higher than 106 cycles for all critical positions along the screw structure were the combination of the use of a SAE 4340 steel in a higher diameter screw and the use of a carbonitrited SAE 1015 steel with a continuous 22 mm diameter.

Synthesis and characterization of zirconia toughened alumina ceramics prepared by co-precipitation method

Abstract Undoped and 3 mol% yttrium doped ZrO 2 –Al 2 O 3 composite powders with partially stabilized ZrO 2 (PSZ) content varying from 0 to 30 wt% were prepared by a co-precipitation route using inorganic precursors Al(NO 3 ) 3 , ZrOCl 2 and Y(NO 3 ) 3 . The precipitates were characterized by DTA and subsequently calcined at 1200 °C for 4 h to achieve fine grained composite powders. The calcined powders were characterized by FTIR and XRD. In order to enhance the sinterability, the calcined powders were wet milled in a high energy ball mill. Powders were uniaxially pressed to form pellets and sintered at 1600 °C for 5 h to achieve greater than 96% relative density. Microstructural analysis of the sintered compacts revealed the uniform distribution of the zirconia particles among the alumina matrix. It was also observed that the faceted intergranular zirconia grains were present at the grain boundaries and junctions in the alumina matrix. Vickers indentation was carried out at 1 kgf load for hardness and 2 kgf load for estimating the critical stress concentration factor (K c ). Microscopic studies of the indented samples showed that cracks were propagating around the grain boundaries. Highest K c ∼8.40 ± 0.4 MPa√m and hardness ∼16.31 ± 0.58 GPa was obtained for the 30 wt% PSZ-Al 2 O 3 composite. The sintered density and critical stress intensity factor (K c ) achieved were compararble to that achieved earlier by hot press and SPS.

Overloading effect on the fatigue strength in resistance spot welding joints of a DP980 steel

Dual phase steel DP980 sheets were joined by resistance spot welding (RSW) process. Mechanical resistance of the welds was characterized by microhardness, tensile shear and fatigue tests. A significant hardness decreases was observed in the RSW lap joints with respect to the base material, which was attributed to phase transformations during the heating and cooling of the steel. Fatigue Wöhler curves using a fixed load ratio of 0.1 were obtained. It was found that the spot weld at the nugget interface close to the fusion zone induced a critical stress concentration factor, which decreased the fatigue life of the joints in the as-welded condition. Failure of the welds was initiated at the interface between welded sheets. Two predominant fatigue fracture modes were observed associated with mode I/III cyclic loading, which were correlated with the fatigue crack initiation and propagation stages. Compressive residual stresses were induced by a loading-unloading cycle on the spot welds, which tends to increase the fatigue life of the joints when compared to the as-welded condition.