Lap Shear Tests Research Articles

Thermally Controlled Acceleration of Epoxy Resin Curing through Polymer-Bound Imidazole Derivatives with High Latency

Imidazole-containing polymeric accelerators were synthesized through free-radical polymerization and applied in the curing of diglycidyl ether of bisphenol A resin with the hardener dicyandiamide. Reactivity and storage stability were followed via differential scanning calorimetry, and an increased pot life was observed after incorporation into polymers. Depending on the employed monomers and composition, the compatibility with epoxy resin was adjusted and the storage stability thereby improved, while a high reactivity is retained under curing conditions. Microscopy imaging showed the dissolution of the polymeric accelerators in epoxy resin above a critical temperature enabling a high reactivity. The best performing accelerator was then applied in an epoxy adhesive film formulation. The strength of the bonding of stainless-steel test bodies was examined in a lap shear test showing superior long-term stability while maintaining efficient acceleration of this type of polymeric accelerator for adhesive applications.

Effects of Al/Zn interlayer on the solidification path and liquation cracking susceptibility of AZ31/ZK61 dissimilar magnesium alloy resistance spot welding joints

In this study, Al and Zn interlayers are added separately between AZ31 and ZK61 workpieces to investigate the method of eliminating liquation cracking in dissimilar magnesium alloy resistance spot welding (RSW). Calculation of phase diagram technique was used to analyze the effect of Al or Zn element addition on the metallurgical reactions in the nugget. The T-fS curves reveal that the (fS)nugget is higher than (fS)ZK61 during the cooling stage of the AZ31/ZK61 RSW, which triggers the liquation cracking. Adding Al or Zn elements into the nugget can change the T-fS curve of the fusion zone, thereby restraining the liquation cracking at different degrees. Microstructural analysis at the nugget/ZK61 interface shows that the cracks become discontinuous in the AZ31/Al/ZK61 samples, while the cracks are eliminated in AZ31/Zn/ZK61 samples. With the addition of interlayers, the microstructure of the nugget consisted of fully equi-axed dendrites, while the microstructure of the nugget without interlayer consisted of a mixture of columnar and equi-axed dendrites. Lap-shear test of the dissimilar magnesium alloy RSW joints shows that the mechanical properties of the samples can be improved significantly when the liquation cracking is restrained. The AZ31/ZK61, AZ31/Al/ZK61, and AZ31/Zn/ZK61 samples exhibit nugget-drop-off, partial-interfacial-partial-nugget-drop-off, and fully-interfacial failure modes, respectively.

Enhanced and Reusable Poly(hydroxy urethane)-Based Low Temperature Hot-Melt Adhesives.

Poly(hydroxy urethane)s (PHUs) based on 5-membered cyclic carbonates have emerged as sustainable alternatives to conventional isocyanate-based polyurethanes. However, while from the point of view of sustainability they represent an improvement, their properties are still not competitive with conventional polyurethanes. In this work, the potential of PHUs as reversible hot-melt adhesives is discussed. We found that with a judicious choice of reagents (i.e., the dicyclic carbonate and diamine), the detrimental hydrogen bonding between the soft segment of the chains and the pendant hydroxyl groups was partially avoided, thus imparting PHUs with hot-melt adhesion properties (i.e., adhesion at elevated temperatures and cohesiveness at a temperature lower than Tg/Tm). The importance of a balanced hard to soft segment ratio, along with the relevance of the chain extender in the final properties, is highlighted. Addition of aliphatic diamines (HMDA, 1,12-DAD) resulted in rubbery materials, while the employment of cycloaliphatic (CBMA) or aromatic ones (MXDA, PXDA) led to materials with hot-melt adhesive properties. The thermoreversibility of all compositions was assessed by rebonding specimens after lap-shear tests. Lap-shear strength values that were comparable to the virgin adhesives were observed. The breaking and reformation of hydrogen bonding interactions was demonstrated by FTIR measurements at different temperatures, as well as by rheological frequency sweep experiments. In order to mitigate the negative impact of the low molar mass PHUs and to enhance the service temperature of the adhesives, a hybrid PHU was prepared by adding a small amount of an epoxy resin, which acts as a cross-linker. These hybrid PHUs maintain the thermoreversibility displayed by thermoplastic PHUs while providing better adhesion at elevated temperatures. We believe that this work provides some important insights into the design of PHU-based hot-melt adhesives.

The Value of Transitioning from the Lap Shear to an Internal Bond for Testing Plywood*

Abstract The lap shear test has been the standard for bond strength testing in plywood for years. Its goal is to predict the long-term durability of the plywood panels. This test has also been used for root cause analysis by mill quality management teams to identify issues. There are several problems with the test, two significant problems being, (1) the only bond tested is the one that is next to the veneer tested, and (2) the test is highly subjective to the accuracy of the kerfing. This paper will address the first problem, which is the larger issue. During the long-term exposure of the panel, the bond lines most likely to fail are the exposed surface or, more likely, the weakest bond. The lap shear test does not test all the bonds simultaneously, so there is no way to ensure the weakest bond is tested on each sample. The data included in this article clearly showed that there was a difference between the bond lines that would be missed in the standard lap shear test. Lastly, the main bonds tested are in the center of the panel; therefore, the result would be biased and may not be an accurate representation of how the panel would perform in the field. These deficiencies are remedied by shifting to the standard internal bond testing common in other wood products.

Smart behaviour of CNT modified adhesive films for carbon fiber composite repair

Epoxy adhesives films were doped by spraying an aqueous dispersion of carbon nanotubes. Sensing capability of carbon fiber reinforced composite coupons with bonded patch repairs was evaluated; several single lap shear and Mode-I fracture energy tests were conducted and their electrical responses were characterized. It was observed that electrical resistance increases with mechanical strain due to tunnelling effect on CNT percolating networks and increases of electrical resistance associated to crack propagations could be also detected. Once the smart behaviour if the adhesive joints was identified by means of mechanical tests in normalized adhesive joined specimens, panels (400 x 400mm) with square co-bonded patches, made using the same curing cycle based on the repaired material one, were prepared using CNT doped adhesive films. These panels were mechanical tested to determine the effect of the doping treatment on the mechanical behaviour of the repairs, at the same time that electric measurements were carried out to identified local failures during testing. Obtained results showed that structural film adhesives could be surface doped with CNT providing sensing capabilities to be used in structural health monitoring of adhesive repairs in composite structures.

Spatially-resolved chemical and mechanical examination of adhesive mixing efficiency with ATR-IR and Martens hardness

In this work, a new combined approach to investigate the quality of mixed and cured 2-component epoxy adhesives will be proposed. The spatially-resolved attenuated total reflectance fourier transform infrared spectroscopy (ATR-FTIR) on a microscale and Martens hardness were used to investigate the chemical and mechanical properties respectively the performance of mixed 2-component epoxy adhesives. Therefore, the measurement principle was proved on samples with different proportion of the adhesive hardener. The results show that the difference in hardener proportion has a significant effect on the measurable epoxy concentration and the Martens hardness. In a second step the testing methods were applied to differently mixed adhesive samples to investigate the mixing homogeneity. The mixing methods used were hand mixing, a static mixer, an asymmetrical dual centrifuge and a novel mixing method based on power ultrasound. With a spatially-resolved representation of the ATR-FTIR and Martens hardness test results it is possible to distinguish between different mixed samples and to analyze the mixing quality. It was shown that the resolution and the sensitivity of the ATR-FTIR spectroscopy and the Martens hardness method allow to evaluate the mixing homogeneity of the used 2-component epoxy adhesives. Furthermore, the correlation of the non-destructive ATR-FTIR and hardness test results with destructive lap shear test of adhesive joints was investigated. By this, it was shown, that also the mechanical properties of adhesive joints are affected by the proportion of hardener and the used mixing method and correspond with the ATR-FTIR spectroscopy and Martens hardness.

Shear testing and failure modelling of calcium phosphate coated AZ31 magnesium alloys for orthopaedic applications

Magnesium orthopaedic fracture fixation devices can potentially provide significant clinical benefits, such as the elimination of secondary surgeries for device removal due to in-vivo resorption and reduced stress shielding due to reduced device stiffness. However, development, approval, and clinical adoption of magnesium devices has been hindered by the excessively high rates of in-vivo corrosion such that the structural integrity of the device can be catastrophically reduced before fracture healing occurs. Coating of devices with calcium phosphate coatings has been shown to significantly reduce corrosion rates, while enhancing osseointegration. However, the adhesion strength between the CaP coatings and magnesium substrates has not been previously investigated. Clinical insertion of fracture fixation devices such as intramedullary nails and k-wires will impose significant shear loading on the coated surface of the implant. If the effective shear strength of the coating-device interface is not sufficiently high, the coating will be damaged and removed during device insertion. In the current study a bespoke experimental-computational approach is developed to provide a new understanding of the relationship between coating thickness, surface roughness, and effective shear strength of the CaP coating- Mg substrate interface. Nine test cases were created by adjusting either the deposition time (3 thickness values) or the surface treatment of the Mg alloy using SiC paper (3 roughness values) and double-lap shear testing was performed for these coating configurations. Strain development in the Mg substrates was monitored using strain gauges, and failure stress was determined for each configuration. Test results revealed that the effective shear strength of the coating-substrate interface is significantly higher for coatings on the rougher substrate surfaces when compared to those on smoother surfaces. Coating thickness was not found to significantly influence the effective shear strength over the range considered in this study (0.37–1.34 μm). Micro-scale finite element models of lap-shear tests were constructed using experimental profilometry data. Simulations of rough coating-substrate interfaces reveal that significant localised compression occurs at the coating-substrate interface in regions of large asperities. A novel cohesive zone formulation has been developed to simulate compression induced shear hardening, and the resultant simulations are found to accurately predict the significantly higher effective shear strength measured experimentally for rougher coatings compared to smoother Mg substrate surfaces.

Influence of bond interface over the lap-shear performance of 3D printed multi-material samples

Multi-material 3D printing offers new possibilities regarding product development, allowing design freedom and multiple materials choices in terms of colour and polymer type. Material extrusion technologies are among the most popular options for multi-material printing due to their low equipment cost and various thermoplastic materials. However, polymers’ compatibility and bond interface must be considered for multi-material components. Material Extrusion creates the parts layer by layer, and each layer is characterised by multiple lines of extruded thermoplastic at a defined width. Therefore, regardless of the 3D model’s surfaces, they are composed of numerous lines of material and voids. Depending on the 3D Printing process setup, the bonding mechanism between materials can be influenced due to the different characteristics of horizontal and vertical contact interfaces. For this reason, this paper aims to study the influence of process parameters over horizontal interface through lap-shear tests for multimaterials samples made of acrylonitrile butadiene styrene (ABS), acrylonitrile styrene acrylate (ASA), and polycarbonate (PC). The results show that bond interface strength can be improved by creating ways for the mechanical interlock of the materials.

Adhesive properties of thiol-acrylate-epoxy composites obtained by dual-curing procedures

Composite materials were prepared using a thiol-acrylate-epoxy dual-curing system filled with Boron Nitride (BN) agglomerates. The adhesive properties obtained with two different acrylate ratios (r a ) were tested to characterize differences and applicability of final thermosets. The effect of bondline thickness, BN wt% and curing procedure on the adhesive strength was evaluated for both acrylate ratios r a by means of lap-shear tests. Materials with viscous intermediate (r a = 0.2) results were observed to have the best performance reaching a maximum of almost 5 kN with 20 wt% of BN. On the other hand, materials with an intermediate gelled network (r a = 0.5) showed a high accuracy in dimension control but lower values of adhesive strength were obtained. A decrease of adhesive strength with the increase of thickness was found for both r a , while the increase of BN wt% lead to higher adhesive strength. The beneficial effect of dual-curing processing on the adhesive properties of liquid intermediate materials was also experimentally demonstrated.

Investigating on the influence of multi‐walled carbon nanotube and graphene nanoplatelet additives on residual strength of bonded joints subjected to partial fatigue loading

AbstractAdhesively bonded joints in engineering structures, especially automotive body structures, inevitably experience long or limited fatigue loading conditions. In this article, the shear strength of steel bonded lap joints reinforced by multi‐walled carbon nanotube (MWCNT) and graphene nanoplatelet (GNP) were evaluated after being subjected to limited fatigue loading cycles. For this purpose, three specimen groups of neat, MWCNT, and GNP‐reinforced joints (with 0.2, 0.5, 1.0, and 2.0 wt% contents) were prepared. After performing the static lap shear tests, fatigue testing of each specimen group was conducted under a maximum fatigue load of 50% of its average static failure load until complete separation of the substrates. Accordingly, limited fatigue loadings were applied to each specimen group for 30%, 50%, and 70% of its total fatigue life, and subsequently, a second‐round static testing was performed to evaluate the residual strength of the joints. The results revealed that the maximum static shear strength improvements of MWCNT and GNP‐reinforced joints were associated with incorporating 1.0 and 0.5 wt% particle contents. Moreover, for 0.2 and 0.5 wt% contents, the static strength of the joints reinforced by GNPs was 3.4% and 3.9% greater than that of MWCNTs reinforced joints. On the contrary, for 1.0 and 2.0 wt% filler contents, specimens including MWCNTs had 9.1% and 18.6% higher strength than GNP‐reinforced ones. The highest fatigue life enhancements of 34.1% and 31% were obtained by adding 1.0 and 0.5 wt.% of MWCNT and GNP particles to the adhesive, respectively. It was concluded that, for 0.2 and 0.5 wt% contents, nanoparticle type did not have a noticeable effect on the residual strength of the joints after being subjected to fatigue loading for 30% of its total life. The specimens subjected to 50% and 70% of their total fatigue life indicated a minimum loss in the shear strength for the joints reinforced by dispersing 1.0 wt% of MWCNTs.

Low-temperature operating adhesive films capable of in-situ use of moisture with outstanding water-resistant capacity

ABSTRACT Adhesives with properties of low-temperature and water-resistance are important in manufacturing. In this work, dopamine hydrochloride (DA), containing catechol groups, was introduced into the polymer by covalent grafting to obtain water-resistant films. Films with low-temperature and water-resistance properties were obtained through layer-by-layer assembly of branched polyethyleneimine (bPEI) and chemically cross-linked hyaluronate (HA)-DA (denoted HA-D). The film surfaces were wetted with moisture generated from low-temperature pretreatment, and the resulting films, named as bPEI/HA-D multilayer films, could adhere strongly by manual application of pressure of about 0.8 MPa. High transmittance (≥78%) was confirmed by ultraviolet–visible absorption spectroscopy, and the adhesion was determined by lap shear tests. Results indicated that the bPEI/HA-D multilayer films had excellent adhesive strengths and were insensitive to temperature, when stored at −20°C (1.08 ± 0.54 MPa), in 100% relative humidity (0.54 ± 0.22 MPa), under ambient conditions (0.99 ± 0.29 MPa), which suggests their potential applications in optical fields and special environments, such as underwater or at low temperature.

Sensor Fusion and On-Line Monitoring of Friction Stir Blind Riveting for Lightweight Materials Manufacturing

Abstract This research focused on developing a hybrid quality monitoring model through combining the data-driven and key engineering parameters to predict the friction stir blind riveting (FSBR) joint quality. The hybrid model was formulated through utilizing the in situ processing and joint property data. The in situ data involved sensor fusion (force and torque signals) and key processing parameters (spindle speed, feed rate, and stacking sequence) for data-driven modeling. The quality of the FSBR joints was defined by the tensile strength. Furthermore, the joint cross-sectional analysis and failure modes in lap shear tests were employed to confirm the efficacy of the proposed model and development of the process–structure–property relationship.

Polyethylene FSSW/Adhesive hybrid single strap joints: Parametric optimization and FE simulation

In this paper, the mechanical behavior of single strap joints was investigated under tensile loading. Four sets of specimens were fabricated e.g., adhesively bonded, friction stir spot welded (FSSW), adhesively bonded-FSSW (AB-FSSW), and hybrid joints. The failure load of specimens subjected to lap shear tests was employed to compare the load carrying capacity of the samples. L9 Taguchi design of experiment was used and the effects of FSSW parameters e.g. tool rotational speed (RS), plunge rate (PR), and dwell time (DT) on the failure load of the specimens were investigated. It was found that the welding parameters of RS = 1600 rpm, PR = 8 mm/min, and DT = 30 s leads to the highest failure load (FL) of FSSW joints, and applying the adhesive increased the joints failure load capacity up to 80%. By comparing the Scanning Electron Micrographs (SEM) images of the FSSW joints, it was revealed that a specimen with the maximum failure load has a homogeneous microstructure. The finite element method was developed to estimate the failure load of different joint types. It was observed that the numerical results are in good agreement with the experimental data with errors of less than 6%.

Catalyst-Free Synthesis of Lignin Vitrimers with Tunable Mechanical Properties: Circular Polymers and Recoverable Adhesives.

Biobased circular materials are alternatives to fossil-based engineering plastics, but simple and material-efficient synthetic routes are needed for industrial scalability. Here, a series of lignin-based vitrimers built on dynamic acetal covalent networks with a gel content exceeding 95% were successfully prepared in a one-pot, thermally activated, and catalyst-free “click” addition of softwood kraft lignin (SKL) to poly(ethylene glycol) divinyl ether (PDV). The variation of the content of lignin from 28 to 50 wt % was used to demonstrate that the mechanical properties of the vitrimers can be widely tuned in a facile way. The lowest lignin content (28 wt %) showed a tensile strength of 3.3 MPa with 35% elongation at break, while the corresponding values were 50.9 MPa and 1.0% for the vitrimer containing 50 wt % of lignin. These lignin-based vitrimers also exhibited excellent performance as recoverable adhesives for different substrates such as aluminum and wood, with a lap shear test strength of 6.0 and 2.6 MPa, respectively. In addition, recyclability of the vitrimer adhesives showed preservation of the adhesion performance exceeding 90%, indicating a promising potential for their use in sustainable circular materials.

Enhanced Long-Term Reliability of Seal DeltaSpot Welded Dissimilar Joint between 6061 Aluminum Alloy and Galvannealed Steel via Excimer Laser Irradiation.

Structural-adhesive-assisted DeltaSpot welding was used to improve the weldability and mechanical properties of dissimilar joints between 6061 aluminum alloy and galvannealed HSLA steel. Evaluation of the spot-weld-bonded surfaces from lap shear tests after long-term exposure to chloride and a humid atmosphere (5% NaCl, 35 °C) indicated that the long-term mechanical reliability of the dissimilar weld in a corrosive environment depends strongly on the adhesive–Al6061 alloy bond strength. Corrosive electrolyte infiltrated the epoxy-based adhesive/Al alloy interface, disrupting the chemical interactions and decreasing the adhesion via anodic undercutting of the Al alloy. Due to localized electrochemical galvanic reactions, the surrounding nugget matrix suffered accelerated anodic dissolution, resulting in an Al6061-T6 alloy plate with degraded adhesive strength and mechanical properties. KrF excimer laser irradiation of the Al alloy before adhesive bonding removed the weakly bonded native oxidic overlayers and altered the substrate topography. This afforded a low electrolyte permeability and prevented adhesive delamination, thereby enhancing the long-term stability of the chemical interactions between the adhesive and Al alloy substrate. The results demonstrate the application of excimer laser irradiation as a simple and environmentally friendly processing technology for robust adhesion and reliable bonding between 6061 aluminum alloy and galvannealed steel.

Ultrasonic Welding of Nickel with Coarse and Ultrafine Grained Structures

Solid state joints of samples of coarse-grained (CG) and ultrafine-grained (UFG) nickel have been obtained for the first time using spot ultrasonic welding (USW). The UFG structure in disk-shaped samples was processed by means of high-pressure torsion (HPT). On the basis of lap shear tests, the optimal values of the clamping force resulting in the highest values of the joint strength are determined. The microstructures in the weld joints obtained at optimal parameters of USW are characterized by scanning electron microscopy. It is shown that during ultrasonic welding of coarse-grained nickel, a thin layer with an UFG microstructure is formed near the weld surfaces. The bulks of sheets retain the CG microstructure, but a significant dislocation activity is observed in these regions. During USW of samples having an UFG initial microstructure, significant grain growth occurs. Fine grains are observed only along the welding interface. An average lap shear strength of 97 MPa was obtained by welding the UFG samples, which was approximately 40% higher than the strength of samples processed by welding coarse-grained sheets (70 MPa). It is concluded that higher strength weld joints can be obtained by using sheets with the UFG structure as compared to the CG sheets.

Structural performance of cold-formed steel box girders with C-section flanges and sinusoidal corrugated webs

This paper introduces a novel type of cold-formed steel box girder assembled from C-section flanges and sinusoidal corrugated webs, and the structural performance of such box girders was experimentally and numerically investigated. A total of seven cold-formed steel box girder specimens were fabricated by self-drilling screws and were tested subject to three-point bending. Prior to testing, the initial geometric imperfections of the sinusoidal corrugated webs were accurately determined using the 3D optical scanning technique. The structural behaviour of the tested box girder specimens, including load versus displacement curves and corresponding failure modes, was obtained and reported herein. The load-carrying properties of the adopted self-drilling screw connections were examined by separate single lap shear tests. Elaborate finite element (FE) models of the cold-formed steel box girders were developed by ABAQUS and were further validated against the obtained test results. Upon validation of the numerical models, the influences of key parameters including the number of screws, the corrugation ratio a/q, and the initial geometric imperfections were discussed in detail. It has therefore been recommended that the double-screw connections and appropriate corrugation ratios be adopted for design of cold-formed steel box girders with C-section flanges and sinusoidal corrugated webs.

Ultraviolet-patterned acrylic pressure-sensitive adhesives for flexible displays

Acrylic pressure-sensitive adhesives (PSAs) are extensively used to fasten display layers. Although PSAs must show sufficient adhesion for application to existing displays, PSAs must utilize stretching and recovery to mitigate the effects of deformation for application to flexible displays. Therefore, in this study, an ultraviolet (UV)-patterned acrylic PSA was prepared by incorporating a region showing variable degrees of crosslinking achieved using a contrasting UV-patterned film. The gel fraction was measured to confirm the degree of crosslinking, and the adhesion was measured by peel strength, pull-off, and lap shear tests. Dynamic mechanical analysis (DMA) stress relaxation was used to measure the recovery. The highest gel fraction was obtained at a crosslinker content of 1 part per hundred resin (phr) and a UV dose of 1600 mJ/cm2. The low-crosslink-density area was optimized using the 50% gray patterned film. The acrylic PSA prepared with 0.001 phr of 2,5-bis(5-tert-butyl-benzoxazol-2-yl)thiophene (BBT), a light-emitting compound used to visualize the pattern formation, showed the clearest pattern. Although the adhesion of the UV-patterned acrylic PSA initially decreased, it gradually increased for the 2 mm pattern. However, the shear adhesion was maintained without appreciably changing. Although the acrylic PSA could not withstand certain deformation conditions during UV patterning, it was stretchable at a pattern size of 4 mm, and the recovery increased to approximately 60% at a pattern size of 2 mm. The results confirmed that the UV-patterned PSA is suitable for application to flexible displays.

On the performance of birch tar made with different techniques

Birch tar is one of the oldest adhesives known in human history. Its production has been discussed in the framework of early complex behaviours and sophisticated cognitive capacities. The precise production method used in the Palaeolithic remains unknown today. Arguments for or against specific production pathways have been based on efficiency or process complexity. No studies have addressed the question whether birch tar made with different techniques is more or less performant in terms of its properties. We therefore investigate the adhesive performance of birch tar made with three distinct methods: the open-air condensation method and two variations of underground structures that approximate the double-pot method in aceramic conditions. We use lap-shear testing, a standard mechanical test used for testing the strength of industrial adhesives. Tar made in 1 h with the condensation method has a shear strength similar to, although slightly higher than, tar made underground if the underground process lasts for 20 h. However, tars from shorter underground procedures (5 h) are significantly less strong (by a factor of about 3). These findings have important implications for our understanding of the relationship between the investment required for Palaeolithic birch tar production and the benefits that birch tar represented for early technology. In this regard, the simple and low-investment open-air condensation method provides the best ratio.

Mechanical Properties and Failure Mechanisms of Refill Friction Stir Spot Welds

Refill friction stir spot welding (RFSSW) is an innovative solid-state welding technology for aluminum structures. The presented study aimed to evaluate the mechanical properties of refill spot welds and their failure mechanisms with the use of industrial test standards. The mechanical properties of refill spot welds were compared with those of rivet joints with comparable joint sizes. Static load tests indicated that RFSSW coupons demonstrate higher ultimate shear strengths but slightly lower ultimate tension strengths than those of rivet coupons. Fatigue test results indicated that both RFSSW coupons and rivet coupons demonstrate comparable performances during low-load-level fatigue lap shear tests but RFSSW coupons outperform rivet coupons during high-load-level fatigue lap shear tests. The failure mechanisms of refill spot welds were characterized in terms of external loading, parent metal properties, and weld properties. Refill spot weld failures included parent metal tensile failures, nugget pullouts, and interfacial failures. A refill spot weld may demonstrate one or a combination of these mechanical failures. Although the mechanical tests of refill spot welds demonstrated promising results with predictable failure mechanisms, the metallurgical evolution involved in RFSSW remains a subject to study.