Lap-shear Specimens Research Articles

Effect of Different Substrate Pre-Treatments on the Resistance of Aluminum Joints to Moist Environments

The goal of this work was to evaluate and compare the performance of different surface pre-treatments applied to aluminium single lap joints exposed to warm moisture. Four different processes were considered: 1) degreasing-roughening; 2) degreasing-roughening and phosphoric acid anodising; 3) degreasing-roughening and conditioning with Picklex®20 (a water-based product for metal surface preparation); 4) degreasing-roughening and phosphoric acid anodising and contaminating the adherend surface with CaCl2 to accelerate the tests. After each pre-treatment, lap-shear specimens were prepared with the open face technique. The half-joints were exposed to warm moisture (50°C, 100% RH) for times up to 4 weeks, then they were completed and subjected to mechanical testing. The results show that anodising yields the best durability, as strength remained practically constant over time; all the other groups of specimens exhibited progressive reduction in strength and wider scatter of the results. A large role is played by interfacial damage of the adherends, since, under increasing exposure time, failure tends to occur as separation at the adhesive-metal interface. The strength decreased dramatically also for the anodised specimens when the surface was contaminated, which evidences the role of the interfacial damage. The interpretation is supported also by scanning electronic microscopy observation and energy-dispersive X-ray analysis.

Comparison of microstructure and mechanical properties of conventional and refilled friction stir spot welds in AA 6061-T6 using filler plate

Friction stir spot welding with refilling by friction forming process (FSSW-FFP) was successfully modified using filler plate. Both of this new refilling technique and conventional friction stir spot welding (FSSW) process were used to weld Al 6061-T6 lap shear specimens and the results were compared. Effects of tool rotational speeds on mechanical and metallurgical properties in both the cases were studied. Static shear strength of refilled weld samples was found to be better than those welded by conventional FSSW process at all tool rotational speeds. This is explained in terms of effective increase in cross-sectional area of weld nugget due to addition of more material from filler plate, thereby eliminating the probe hole. Failure mechanisms were discussed and fracture surfaces were analyzed through scanning electron microscopy (SEM). The hardness profile of the welds exhibited a W-shaped appearance in both the processes and the minimum hardness was measured in the HAZ.

Assessment of the interlaminar behaviour of a carbon fabric reinforced thermoplastic lap shear specimen under quasi-static and tension-tension fatigue loading

Although fusion bonded joints have already been extensively studied, they are only rarely compared to benchmark specimens with the same interlaminar behaviour as the composite material being joined, especially for lap shear geometry. Furthermore, an extensive study of these benchmarks under quasi-static and fatigue loading conditions is almost never considered.In this manuscript, a method of producing such benchmark lap shear specimens is presented for a hot pressed carbon fabric reinforced thermoplastic. The mechanical behaviour of these specimens is then assessed using quasi-static, hysteresis and tension-tension fatigue experiments.It could be concluded that the considered approach worked well for this material and reproducible results and values were found. With respect to the material, no crack growth was visible during the quasi-static experiments, except near the very end of life, whereas significant crack growth was present during fatigue experiments.

A Performance Study of Self-Piercing Riveting of Aluminum and Copper Alloy Sheets

This paper studied the performance of self-piercing riveting (SPR) of aluminum and copper alloy sheet. The SPR process of two copper alloy sheets was analyzed by experimental tests and finite elements stimulation, and good agreements between the simulations and test results of SPR process were obtained. Two different specimen geometries were adopted in experiment: a lap-shear type specimen and a T type specimen. The influence of combination of different materials on the static strength of SPR joints were studied through the tensile-shear and peeling experiments, and the strength of the SPR joints were evaluated by the force-displacement curves of two type specimens. The failure mechanisms corresponding to the static strength were also discussed. On the other hand, the engineering stress - strain curves of two materials have also been studied roughly. These results could provide designer engineers with some application messages of SPR.

Microstructure and Mechanical Properties of Refilled Friction Stir Spot Welding of Commercial Pure Aluminium

Aluminium and magnesium alloys are expected to make considerable contributions in reducing the weight of automobiles as they are increasingly used as an alternative to steel; improving fuel economy and vehicle performance while simultaneously reducing emissions. Higher electrical and thermal conductivities of these materials make them difficult to weld using existing resistance spot welding leading to high energy consumption. Friction stir spot welding has proven to be a better alternative to weld these materials. But a probe hole left behind is the main problem in conventional Friction Stir Spot welding (FSSW). In the present work a new method has been developed to refill the probe hole using an additional filler plate known as Refill Friction Stir Spot welding (RFSSW). This new refilling technique and the conventional FSSW process were both used to weld commercially pure aluminium lap shear specimens and the results were compared. The effect of tool rotational speed on mechanical and metallurgical properties were studied in both cases. Static shear strength of RFSSW weld samples was found to be better than conventional FSSW process at higher tool rotational speed. This is explained in terms of effective increase in cross sectional area of weld nugget due addition of more material from the filler plate thereby eliminating the probe hole.

Sensitivity of double cantilever beam test to surface contamination and surface pretreatment

The double cantilever beam (DCB) test is one of the commonly used destructive tests to qualify structural adhesives joints. By measuring the force and the corresponding crack growth of the mode I—loaded specimen, the adhesive fracture energy, GIC, can be determined. Many parameters like adhesive joint thickness, mechanical properties of the adhesive, substrate thickness and stiffness and production quality influence the results of the DCB test.In this article the influence of surface modification due to contamination and surface pretreatment on the results of DCB tests is investigated. Different amounts of hydraulic oil as a typical surface contamination have been applied to a carbon fiber reinforced plastic (CFRP) substrate. Three different surface pretreatments of the CFRP (peel ply, teflon foil and atmospheric pressure plasma) have been investigated. The surfaces have been characterized by contact angle measurements and atomic force microscopy (AFM). Results of DCB tests are analyzed in relation to the fracture surface and compared with tensile strength results of single lap shear specimens.

Bisphenol E cyanate ester as a novel resin for repairing BMI/carbon fiber composites: Influence of cure temperature on adhesive bond strength

The increasing use of polymer matrix composites (PMCs) in structural applications created demand for advanced repair techniques to fix internal delaminations in PMCs. One of a variety of repair techniques is injection repair, which involves injecting a low viscosity resin directly into the damaged area and subsequently curing the resin to heal the damage. In this study, bisphenol E cyanate ester (BECy) was investigated as a potential resin for injection repair of bismaleimide-carbon fiber based composite panels for aircraft. Temperature sensitive repair applications required a technique that avoided the high temperature post-cure of the injection repair resin. Modulated differential scanning calorimetry (MDSC) experiments were used to examine the degree of cross-linking and the glass transition temperature (Tg) of under-cured injection repair resin. The chemistry of cross-linking in under-cured BECy was studied by Fourier transform infrared spectroscopy (FTIR). Lap shear tests of the under-cured injection repair resin on composite substrates revealed the influence of change in isothermal under-cure temperature on the bond strength. Temperature dependent dynamic mechanical analysis disclosed the significance of sub-Tg relaxations on the adhesive properties of the under-cured resin. Post-fracture surface analysis of the lap shear specimens, performed using Scanning Electron Microscope (SEM) micrographs, indicated a mixed mode of fracture in the form of a combination of resin and resin–composite interface failure.

Failure Mode and Fatigue Behavior of Ultrasonic Spot Welds with Adhesive in Lap-Shear Specimens of Magnesium and Steel Sheets

Failure Mode and Fatigue Behavior of Ultrasonic Spot Welds with Adhesive in Lap-Shear Specimens of Magnesium and Steel Sheets

Fatigue Behavior of Self-Piercing Rivets and Clinch Joints in Lap-Shear Specimens of Aluminum Sheets

Fatigue Behavior of Self-Piercing Rivets and Clinch Joints in Lap-Shear Specimens of Aluminum Sheets

The Stability of Different Titanium-PEEK Interfaces against Water

Fibre Metal Laminates (FML) consisting of alternating stacked layers of polymer matrix composites and metallic foils are considered for structures with high fracture toughness and good impact resistance in aeronautic applications. The properties of thermoplastic Fibre Metal Laminates composed of titanium and carbon fibre reinforced polyetheretherketone (Ti/CF-PEEK laminates) are under investigation at DLR. The adhesion between the polyetheretherketone (PEEK) matrix and titanium degrades by the influence of humidity. Physical, chemo-physical and chemical surface pre-treatments of the titanium layers were tested to improve the long-term behaviour of the interface. To compare the different surface treatments, lap shear specimens were prepared and partly exposed to hot water (80°C). Lap shear tests were conducted to determine the degradation of the initial strength by the influence of water. Concerning the physical pre-treatment, the laser pre-treatment offers the highest magnitude of humidity resistance because of the magnitude and kind of surface roughness. Concerning the chemo-physical pre-treatment, the anodization offers reduced initial bonding strength and reduced humidity resistance caused by the created oxide layer. Concerning the chemical pre-treatment, the usage of adhesion promoter causes enhanced initial bonding strength but also reduced humidity resistance.

Failure Mechanism of Laser Welds in Lap-Shear Specimens of a High Strength Low Alloy Steel

In this study, the failure mechanism of laser welds in lap-shear specimens of a high strength low alloy (HSLA) steel under quasi-static loading conditions is examined based on the experimental and computational results. Optical micrographs of the welds in the specimens before tests were examined to understand the microstructure near the weld. A micrographic analysis of the failed welds in lap-shear specimens indicates a ductile necking/shear failure mechanism near the heat affected zone. Micro-hardness tests were conducted to provide an assessment of the mechanical properties of the joint area which has varying microstructure due to the welding process. A finite element analysis was also carried out to identify the effects of the weld geometry and different mechanical properties of the weld and heat affected zones on the failure mechanism. The results of the finite element analysis show that the geometry of the weld protrusion and the higher effective stress–plastic strain curves of the heat affected and weld zones result in the necking/shear failure of the load carrying sheet. The deformed shape of the finite element model near the weld matches well with that near a failed weld. A finite element analysis based on the Gurson yield function with consideration of void nucleation and growth was also carried out. The results of the finite element analysis indicate that the location of the material elements with the maximum void volume fraction matches well with that of the initiation of ductile fracture as observed in the experiments.

Fatigue Behavior of Swept Spot Friction Welds in Lap-Shear Specimens of Alclad 2024-T3 Aluminum Sheets

Failure modes of swept spot friction welds in lap-shear specimens of alclad 2024-T3 aluminum sheets are first investigated based on experimental observations. Optical and scanning electron micrographs of the welds before and after failure under quasi-static and cyclic loading conditions are examined. Experimental results show that the failure modes of the welds under quasi-static and cyclic loading conditions are quite different. Failure modes of swept spot friction welds depend considerably on the weld geometry, microstructure, and load amplitude. A fatigue crack growth model based on the paths of the dominant kinked fatigue cracks is developed to estimate the fatigue lives of the spot friction welds. The global and local stress intensity factors for finite kinked cracks, the stress intensity factors for finite transverse cracks, and the Paris law for fatigue crack propagation are used. The fatigue life estimations agree well with the experimental results.

Comparative study of fatigue behaviour in dissimilar Al alloy/steel and Mg alloy/steel friction stir spot welds fabricated by scroll grooved tool without probe

Aluminium alloy A6061-T6 or magnesium alloy AZ31 sheet was welded to steel sheet by a friction stir spot welding technique using a scroll grooved tool without a probe. The material flow in the nugget of the Mg/steel weld was less than that in the Al/steel one. The Al/steel weld exhibited higher static tensile–shear strength than the Al/Al weld, while the strengths of Mg/steel and Mg/Mg welds were comparable. Tensile–shear fatigue tests were performed using lap shear specimens of both dissimilar and similar welds. The dissimilar welds exhibited nearly the same fatigue strengths as the similar ones. The effective nugget size in the dissimilar welds was defined as the area where Al or Mg alloy remained on the steel side after static fracture. When the fatigue strengths of dissimilar welds were evaluated based on the effective nugget size, the normalised fatigue strengths of Al/steel and Mg/steel welds were comparable.

Hygrothermal aging of composite single lap shear specimens comprised of AF-555M adhesive and T800H/3900-2 adherends

Fiber reinforced resin matrix composites and structural adhesives have found increased usage on commercial and military aircraft. These structural materials offer weight savings without sacrificing strength and mechanical performance. Since service history and long-term aging performance of these relatively new material systems are not well established, a long-term environmental aging study was undertaken. Adhesive bonds were prepared by secondary bonding of 3M™ Scotch-Weld™ AF-555M between pre-cured, unidirectional composite adherends comprised of Toray™ T800H/3900-2 prepreg. Single lap shear specimens (SLS) were fabricated and subsequently aged in an unstressed configuration at 82°C (180°F) and 85% relative humidity for up to 772 days. The aging conditions are more severe than those expected to occur in service and were selected in order to observe some changes within a reasonable exposure time. The apparent shear strengths were measured periodically at both room temperature and 82°C (180°F), and failure modes were determined and compared to control specimens that had been stored for equal periods of time at room temperature under low humidity. With hygrothermal aging, apparent shear strengths decreased and some changes in the failure mode were observed relative to those of the control specimens. However, in spite of the severe aging conditions, no adhesive failures indicative of interfacial failures were observed. SLS specimens that were hygrothermally aged, subsequently dried, and then tested did not exhibit complete recovery of unaged apparent shear strengths indicating some irreversible changes.

The significance of lap-shear testing of wood adhesive bonds by means of Volkersen’s shear lag model

Lap-shear specimens of varying geometry were bonded with two different adhesives and tested. At the same time, the stress distribution along the adhesive bond lines of the examined specimens was modelled by means of Volkersen’s equation. A good agreement was found between apparent shear strength and a stress concentration factor derived from the model. Highly significant effects of adhesive stiffness were found at short overlap length and high specimen thickness.

Failure Modes of Friction Stir Spot Welds in Lap-Shear Specimens of Dissimilar Advanced High Strength Steels under Quasi-Static and Cyclic Loading Conditions

Failure Modes of Friction Stir Spot Welds in Lap-Shear Specimens of Dissimilar Advanced High Strength Steels under Quasi-Static and Cyclic Loading Conditions

Laser Surface Pre-Treatment of CFRP for Adhesive Bonding in Consideration of the Absorption Behaviour

The use of carbon fibre-reinforced plastics (CFRP) is growing significantly in all areas of lightweight construction. Thermosets like epoxy still build a major part of deployed matrix material. A suitable joining technology for CFRP is adhesive bonding. However, the bonding performance is well reduced because of contaminations. To achieve full bond-strength, adhesion of the adhesive to the components is essential. One innovative method for surface pre-treatment is laser radiation. Former investigations show that an important parameter is the absorption. As a result of that, a UV laser (wavelength of 355 nm) and a CO2 laser (wavelength of 10600 nm) were used to pre-treat specimens manufactured from 120°C curing epoxy. The focus is to achieve a selective removal of resin without impairing the fibres. Lap-shear specimens have been pre-treated, bonded with an one component epoxy film adhesive and tested. The laser pre-treated specimens achieve the same bond strength as references prepared by manual abrading. Furthermore, the mechanisms of interaction between laser radiation and matrix material as well as fibres are discussed considering the different heat deposit and absorption behaviour of the emitted radiations.

Prediction of pad cratering fracture at the copper pad – Printed circuit board interface

Pad-crater fracture was characterized in terms of the critical strain energy release rate, Jci, measured at various mode ratios, ψ. Specimens were prepared from lead-free chip scale package-PCB assemblies and fractured at low and high loading rates in various bending configurations to generate a range of mode ratios. The specimens tested at low loading rates all failed by pad cratering, while the ones tested at higher loading rates fractured in the brittle intermetallic layer of the solder. The Jci of pad cratering increased with the phase angle, ψ, but was independent of surface finish (OSP and ENIG) and reflow profile (time above liquidius 60s and 120s). The generality of the J=Jci(ψ) failure criterion to predict pad-cratering fracture was then demonstrated by predicting the fracture loads of single lap-shear specimens made from the same lead-free assemblies.

Thermal bonding of microfluidic devices: Factors that affect interfacial strength of similar and dissimilar cyclic olefin copolymers

Thermal bonding of the same polymer is an important method for obtaining good thermal sealing between two polymeric substrates in microfluidic devices. It is worthwhile to use different grades of the same polymer which have different glass transition temperatures ( T g) because no change in microchannel profile and dimensions would occur in the higher T g substrate during thermal bonding. However, apart from using different grades of a polymer with different T g, there is no fundamental basis for the selection of suitable substrate pairs. Thus an in-depth study of the thermal bond strength of different grades of TOPAS cyclic olefin copolymer (COC) was conducted. The strength development and chain inter-diffusion across the bonded interface between different TOPAS grades was studied, and the influence of molecular weight ( M w ) and bonding temperature was considered. Bonding in both symmetric (between two substrates of the same COC grade) and non-symmetric interfaces (between two substrates of different COC grades) was compared. Bonding was conducted at both below and above the T g. The thermal bond strengths were assessed from lap shear specimens prepared using a range of bonding temperature, time and pressure. Thermal bonding between 5013 and 6015 polymer substrates at 125 °C for 6 min time under 2 MPa pressure was found to be the most favorable conditions at which no deformation in the geometry of the COC microchannel has been observed. The failure surfaces of the specimens were characterized by scanning electron microscopy (SEM). Through the above, a basis for selecting different grades of TOPAS to obtain thermally bonded COC microfluidic devices with high strength was developed. The principles established are believed to be applicable to other polymeric microfluidic chips.

Strategies for the Analysis of the Behavior of an Adhesive in Bonded Assemblies

Experimental and numerical analyses of the mechanical behavior of bonded joints can be made particularly difficult by the influence of edge effects. Therefore, understanding the stress distribution in an adhesive joint can lead to improvements in adhesively-bonded assemblies. Such an analysis is proposed in the case of usual single lap shear specimens. Stress singularities can contribute to the initiation and propagation of cracks in the adhesive. Thus, in order to obtain reliable experimental data to analyze the nonlinear behavior of an adhesive in an assembly, tests which strongly limit the influence of stress singularities must be proposed. The design and the abilities of such a device for shear tests are presented. Moreover, some experimental results obtained using a modified Arcan fixture, which has been designed to strongly limit edge effects, are presented in the case of monotonic and complex history loadings. Furthermore, a 2D non associated elasto-visco-plastic model is proposed to accurately describe the experimental behavior under tensile-shear monotonic loadings. An extension of this model is also proposed to represent relaxation type effects under shear loadings.