Static Lap Shear Strength Research Articles

Effect of PMMA Coupling Layer in Enhancing the Ultrasonic Weld Strength of Novel Room Temperature Curable Acrylic Thermoplastic to Epoxy Based Composites

The joining of composites can be performed in an extremely short time with more energy-efficient ultrasonic welding techniques. The current research investigated the performance optimization of ultrasonic welding of carbon/Elium® composite to carbon/epoxy composite using a polymethyl methacrylate (PMMA) coupling interlayer. The weld strength was quantified by static lap shear strength (LSS) testing. A new methodology was used by creating a PMMA coupling layer on the epoxy composite adherend to achieve an improved interphase and thus enhance the weld properties. The LSS of Elium (EL)-Epoxy (EP) _0.25_0.25 was found to be 190% higher compared to that of EL-EP, confirming the effectiveness of the strategy used for creating an interlayer thermoplastic coupling layer. The time required for welding was optimized to be 2s as compared to 10 min required for adhesive bonding. Scanning electron microscopic images of epoxy and PMMA/Elium matrix interphase were observed to have a rough surface and remained largely unaffected by welding. There was an interphase change further away from the interphase to a rougher texture. There was little to no effect on the penultimate layer on the weld strength, as no interphase change could be observed after welding. Fractography investigation revealed shear cusps, matrix plastic deformation, fiber imprints, fiber pull-out, and good adhesion between matrix and fiber, features seen for configuration with maximum LSS. The current research findings present a way to join Elium® with epoxy composites that could be used in applications that require a selective strengthening, such as in sporting goods and consumer products. Furthermore, a detailed investigation is ongoing to use different filler particles and coupling layers to reach the maximum welding performance.

The influence of loading, temperature and relative humidity on adhesives for canvas lining

The structural conservation of canvas paintings may require lining, a process in which a secondary canvas is adhered to the reverse of the damaged original canvas to provide additional support. Choosing the optimum adhesive or canvas for lining is challenging. Comprehensive data on thermal and mechanical behaviour of different adhesives to enable the conservator to make informed choices for their treatment purposes is scarce. Hence, in this study, four prevalently used adhesives for lining are chosen and their thermal and mechanical behaviour, such as the glass transition and melting temperatures, static lap shear strength and creep resistance, are compared. Thermal properties of the different adhesives are characterised using differential scanning calorimetry (DSC). Furthermore, the effect of temperature cycles (25, 35, and 45°C at a fixed relative humidity of 48%) on the creep behaviour of lined canvases is evaluated. Lap shear and creep experiments are performed on lined canvas mock-ups. The four adhesives tested are: studio formulations of an animal glue-wheat flour paste, as well as a beeswax-damar resin mixture; a patented formula based on an ethylene vinyl acetate copolymer mixture (BEVA 371 O.F.™); and a mixture of two industrially produced acrylic copolymers (Plextol™ D541 and K360). The results demonstrate the remarkable effect of temperature on the creep behaviour of lined canvases, which can be related to their thermal stability.

Fatigue properties of self‐piercing riveted multi‐rivet joints in steel and aluminum sheets

AbstractSelf‐piercing riveting (SPR) is an important joining technology for connecting steel and aluminum sheets. In this paper, AA6111 aluminum alloy and DP780 high‐strength steel were adopted to study the influence of fatigue on remaining static strength and energy absorption properties on self‐piercing riveting multi‐rivet joints. The results showed that energy absorption capacity of the specimens decreased significantly after high cycle fatigue. Fatigue reduced the remaining static lap shear strength of riveted specimens. Scanning electron microscopy (SEM) was used to analyze the cross section of fatigue specimens fractured by static tension. The results showed that fretting wear was found at the contact area between rivet and aluminum sheets. Fatigue bands and fatigue cracks appeared in fatigue specimens after high cycle fatigue, while those with low cycle fatigue specimens did not appear. Small cracks weaken the strength of the aluminum sheet, resulting in the static tensile strength of the riveted specimen with high cycle fatigue is lower than that of other fatigue specimens.

Influence of Corrosion of Self-Piercing Riveted High Strength Aluminium Alloy Joints with Button Cracks on the Mechanical Strength

For high strength aluminium alloys, such as AA6008T61 and AA6111 PFHT, when they are joined as the bottom material by self-piercing riveting (SPR), they tend to crack at the joint buttons. These cracks, especially those penetrated to the rivets, may cause galvanic corrosion problem with the steel rivet. In this paper, the mechanical strength of four stack/die combinations with different joint button cracking severity was studied before and after salt-spray corrosion test. The results showed that corrosion on the severely cracked joints was worse than that on the joints with small cracks and the corrosion on the stack with the same top and bottom material was less than that on the stack with different top and bottom materials. For all joints the static lap shear strength after corrosion was improved, but the static T peel strength after corrosion was slightly reduced or did not have obvious change. The results also showed that after corrosion the lap shear fatigue strength of the specimens with severe cracks did not have obvious change, but the lap shear fatigue strength of the specimens with small cracks slightly reduced.

Influence of local surface texture by tool impression on the self-piercing riveting process and the static lap shear strength

To increase fuel efficiency and reduce emission, aluminium alloys have been increasingly used in automotive body-in-white structures for lightweighting. Due to its advantages over some of the more traditional joining technologies, self-piercing riveting (SPR) has been widely used for these lightweight structures. Research has showed that friction is a very important factor that influences both the riveting process and joint strength for SPR, but these influences have not been fully understood. In this paper, an innovative method was used to modify the local surface of the top sheet around the rivet piercing location with different impression tools (the central pin, the small ring, the medium ring and the large ring) and garnet particles to study the influence of local frictions on rivet inserting process, joint features and static lap shear strength. The results showed that the local surface textures on the aluminium sheet did not have significant influence on the rivet inserting process based on displacement-force curve analysis. The local surface textures could slightly change the joint features, especially the rivet head height, but overall this influence was not significant. The lap shear tests showed that ring impressions with garnet particles and central pin impressions on the bottom surface of the top sheet increased the static lap shear strength of the SPR joints. The results confirmed that the critical location for local friction influence on the lap shear strength was around the rivet leg piercing location.

Influence of aluminium sheet surface modification on the self-piercing riveting process and the joint static lap shear strength

Self-piercing riveting (SPR) has been widely used in automotive as one of the major joining technologies for aluminium structures due to its advantages over some of the more traditional joining technologies. Research has shown that friction is a very important factor that influences both the riveting process and the joint strength for SPR, but these influences have not been fully understood. In this paper, AA5754 sheets with different surface textures, such as original with solid wax, hot water washed, sandpaper ground and grit blasted, were used to study the influence of friction on therivet inserting process, joint features and static lap shear strength. The results of joint features and rivet setting displacement-force curve showed that hot water wash and sandpaper grinding on aluminium sheet did not have significant influence on the rivet inserting process and joint features; however, for joints with grit-blasted substrates, the rivet -setting forces were higher at the beginning, and a middle section of the curve and the joint features, such as interlocks and minimum remaining bottom material thickness (Tmin), were clearly altered. The lap shear tests showed that hot water washing can slightly increase the lap shear strength, sandpaper grinding increased the static lap shear strength further and grit blasting increased the static lap shear strength the most.

The influence of fatigue on the stiffness and remaining static strength of self-piercing riveted aluminium joints

Self piercing riveting (SPR) is one of the major joining technologies for aluminium structures due to its advantages over some of the more traditional joining technologies. In this paper, the mechanisms of crack initiation and growth during fatigue and the influence of fatigue on the stiffness and remaining static strengths of SPR joints in both lap shear and T peel configurations were studied. The results showed that cracks could initiate and develop from different locations on the substrate materials depending on load levels and test types. Fatigue increased the remaining static lap shear strength and stiffness of specimens due to the increased friction force at the top/bottom sheet interfaces around the tip of punched hole through fretting; however, fatigue reduced the remaining static T peel strength of specimens due to crack initiation and development; T peel fatigue at high load levels also increased the stiffness of specimens due to geometry change through large plastic deformation.

Fatigue performance characterisation of resistance-welded thermoplastic composites

This study investigates the fatigue performance of resistance-welded thermoplastic composites. Lap shear specimens consisting of unidirectional carbon fibre/poly-ether-imide (CF/PEI), unidirectional carbon fibre/poly-ether-ketone-ketone (CF/PEKK) and 8-harness satin weave fabric glass fibre/poly-ether-imide (GF/PEI) composites were resistance-welded using a metal mesh heating element. The specimens were fatigue-tested at various percentages of their static lap shear strengths at a load ratio R = 0.1 and frequency f = 5 Hz. The fatigue performances of resistance-welded semi-crystalline (PEKK) and amorphous (PEI) composites were compared and the failure modes of the specimens were described. The stiffness degradation was monitored during the tests in order to evaluate the damage accumulation in the specimens. Linear stress-life ( S– N) curves were obtained for all three materials when plotted on a semi-log scale. Interlaminar failure modes, involving tearing of the heating element and damage to the adherends were observed. The indefinite fatigue lives of CF/PEKK and CF/PEI welded specimens were obtained at 25% of their static lap shear strengths. The indefinite fatigue life of the GF/PEI welded specimens was obtained at 20% of the static lap shear strength.

Smart cure cycle with cooling and reheating for co-cure bonded steel/carbon epoxy composite hybrid structures for reducing thermal residual stress

Abstract In this work, a smart cure cycle with cooling and reheating for co-cure bonded steel/carbon epoxy composite hybrid structures was developed to reduce the fabricational thermal residual stress between the steel and carbon epoxy composite material. The thermo-mechanical properties of the high modulus carbon epoxy composite were measured by a Differential scanning calorimetry (DSC) and rheometer to obtain the optimal time to apply the cooling operation. The static lap shear strength of the co-cure bonded steel/composite lap joints and tensile strength of the composite specimen were measured to investigate the effect of cure cycle on the thermal residual stress. Also, the deflection of the hybrid structures was measured to measure the actual cure temperature with respect to various cure cycles. From the experiments, it was found that the smart cure cycle with cooling and reheating not only reduced the fabricational thermal residual stress but also improved the strength and dimensional accuracy of the hybrid structures.