Lap Shear Experiments Research Articles

Soft elasticity enabled adhesion enhancement of liquid crystal elastomers on rough surfaces

The fabrication of pressure-sensitive adhesives (PSA) using liquid crystal elastomers (LCE) that are tolerant to substrate roughness is explored in this work. Traditional soft adhesives are designed by maintaining a balance between their cohesive strength and compliance. However, rough surfaces can significantly affect the adhesion strength of PSAs. Lowering the stiffness of the adhesive by reducing the cross-linking density or using additives can improve contact on rough surfaces. But this also decreases the cohesive strength and affects the overall performance of the adhesive. Additive-free LCE-based adhesives are shown to overcome these challenges due to their unique properties. Soft elasticity of LCE and low cross-link density contribute to their high compliance, while moderate cross-linking provides finite strength. The effect of contact time and substrate roughness on the adhesive performance is evaluated using probe-tack, indentation, lap shear, and static loading experiments. The unique combination of properties offered by LCE can lead to the development of roughness-tolerant adhesives, thereby broadening the application scope of PSAs.

Experimental and numerical study on mechanical performance of single shear bolted connections with novel elliptical one-sided bolts

One-sided bolts, which typically employed for connecting components with closed cross section, often present complexities in both construction and installation. Addressing the limitations of conventional one-sided bolts, this study introduces a novel variant distinguished by its elliptical head, offering ease of installation without the need for specialized tools. Due to the special shape of the bolt head, the mechanical properties of the bolt need to be studied in depth. To investigate the shear properties of the novel elliptical one-sided bolts, three sets of single lap shear experiments were conducted. Subsequently, finite element numerical analyses were employed to simulate these experiments, facilitating the development of a reliable finite element model. Furthermore, parametric analyses were undertaken to examine the influence of various parameters on the ultimate shearing capacity and displacement of joints. The findings indicate that the shear capacity of the single lap joint employing novel elliptical one-sided bolts closely rivals that of ordinary hexagonal bolts with equivalent diameters. However, an enhancement in slip resistance capacity is observed in the novel elliptical one-sided bolt joints compared to hexagonal bolts. Notably, the parameters of the bolt exert a more pronounced impact on the ultimate shearing capacity and displacement of joints when contrasted with parameters associated with the plates. The recommended torque coefficient for the novel elliptical one-sided bolt is established as 0.2, and it is recommended to use appropriate torque to ensure the slip performance of the joint during practical application.

Investigation of resistance spot weld failure in tailor hot stamped assemblies

A novel experimental method, dubbed the “Mode I Caiman test”, is introduced to characterize spot weld failure and propagation within spot weld groups. The test configuration comprises a partially welded pair of channel sections subjected to Mode I tensile loading through an opening mode loading applied at the un-welded end of the channels. Experiments were performed considering hot stamped USIBOR® 1500-AS in a fully hardened condition (fully quenched) or in a tailored condition in which the strength of the welded flanged regions was altered using in-die heating (IDH). Both static and dynamic loading conditions are considered. The static loading utilized a conventional tensile frame while the dynamic experiments were performed on an instrumented crash sled at velocities of 7.5 m/s.A high speed thermal imaging camera was utilized to detect the temperature rise associated with plastic work during individual weld failure. This proved to be a rather novel and effective technique to detect weld failure and enabled tracking of weld failure propagation through the weld group for the different parent metal (flange) conditions.The experiments served to demonstrate the complex interaction between weld nugget, heat affected zone (HAZ) and parent metal strengths. For high parent metal strengths (fully quenched, martensitic condition), the welds exhibited interfacial and weld pull-out failure modes with rapid unzipping of the weld group resulting in relatively low energy absorption. In contrast, the lower strength tailored flanges exhibited a more stable (slower) failure propagation through the weld group and considerably higher energy absorption which is largely attributed to greater dissipation through deformation of the flange itself. Numerical simulations of the experiments were performed using spot weld models calibrated from single weld lap shear, cross tension, and coach peel experiments. The models captured the peak loads rather well, but predictions of rate of failure propagation within the weld line exceeded that seen in the experiments, pointing to the need for improved post-failure models for spot weld simulation.In automotive structures in which numerous welds are present, the amount of energy absorbed in weld failure can significantly affect the impact response of the structure. The Mode I Caiman test presents a new opportunity to calibrate the energy release rate of spot weld damage models to improve their accuracy when modelling spot welded connections involving multiple spot welds.

Ring-opening metathesis polymerization (ROMP) polymers as structural adhesives and the effects of silane coupling agents on their lap shear properties

In this study, single lap shear experiments with Al2024-T3 substrates were used to evaluate poly(dicyclopentadiene) (pDCPD) for structural adhesive applications. A wide range of monomer-to-catalyst ratios (M:C = 10,000:1 to 90,000:1) and four different silane coupling agents (i.e. hexyl, hexenyl, allyl, and norbornyl trimethoxysilane) were used to understand their effects on the adhesive shear properties of pDCPD. The roles of pDCPD oxidation and the bulk polymer on adhesive properties were also investigated using paint and dynamic mechanical analysis (DMA) measurements. Collectively, the data show that the choice of silane has a significant influence on the lap shear strengths of pDCPD, while the M:C ratio has a smaller but significant influence. Differences in bulk polymer topology as measured by DMA and oxidation of the resin had insignificant effects in the absence of exposed oxygen-rich surfaces that influence catalyst activity. With shear strengths up to 21.6 ± 0.7 MPa for dry conditions and up to 13.0 ± 0.7 MPa for hot-wet conditions (i.e. submersion in deionized water at 60 °C for 2 weeks), we showed that pDCPD-based adhesives have comparable lap shear properties to a well-studied model thermoset epoxy (i.e. DGBA/D230).

Joining of CFRT/Steel Hybrid Parts via Direct Pressing of Cold Formed Non-Rotational Symmetric Pin Structures

In this study, quasi-unidirectional continuous fiber reinforced thermoplastics (CFRTs) are joined with metal sheets via cold formed cylindrical, elliptical and polygonal pin structures which are directly pressed into the CFRT component after local infrared heating. In comparison to already available studies, the unique novelty is the use of non-rotational symmetric pin structures for the CFRT/metal hybrid joining. Thus, a variation in the fiber orientation in the CFRT component as well as a variation in the non-rotational symmetric pins’ orientation in relation to the sample orientation is conducted. The created samples are consequently mechanically tested via single lap shear experiments in a quasi-static state. Finally, the failure behavior of the single lap shear samples is investigated with the help of microscopic images and detailed photographs. In the single lap shear tests, it could be shown that non-rotational symmetric pin structures lead to an increase in maximum testing forces of up to 74% when compared to cylindrical pins. However, when normalized to the pin foot print related joint strength, only one polygonal pin variation showed increased joint strength in comparison to cylindrical pin structures. The investigation of the failure behavior showed two distinct failure modes. The first failure mode was failure of the CFRT component due to an exceedance of the maximum bearing strength of the pin-hole leading to significant damage in the CFRT component. The second failure mode was pin-deflection due to the applied testing load and a subsequent pin extraction from the CFRT component resulting in significantly less visible damage in the CFRT component. Generally, CFRT failure is more likely with a fiber orientation of 0° in relation to the load direction while pin extraction typically occurs with a fiber orientation of 90°. It is assumed that for future investigations, pin structures with an undercutting shape that creates an interlocking joint could counteract the tendency for pin-extraction and consequently lead to increased maximum joint strengths.

Effect of anodic oxidation of 2024-T3 aluminum alloy on interface properties of metal fiber laminates

Interlaminar fracture failure is the main factor restricting the development and application of fiber metal laminates. In this paper, the interlayer bonding properties of 2024-T3 aluminum alloy plates with different surface treatments were compared and studied through lap-shear experiments. Compared with the aluminum plate treated with acid and alkali corrosion, the interlaminar shear strength of the aluminum plate after sulfuric acid anodization increased by 34.3%. After different surface treatments, experimental stressstrain curves of 2024-T3 aluminum alloys reveal the enhancement mechanism and changing trend of interlaminar shear strength.

Enhancing water resistance of interface between starch films and acrylated epoxidized soybean oil coating

Starch-based materials are a promising biobased polymer for packaging applications. To improve the water resistance of starch films, composite systems consisting of hydrophobic acrylated epoxidized soybean oil (AESO) coating and oxidized starch were prepared. To enhance the interfacial bonding between the starch film and AESO coating (AC), surface modification of oxidized starch with polyethyleneimine (PEI) was carried out. PEI modified AC coated oxidized starch films demonstrated significantly reduced water adsorption at the exposed cross-section, while only marginal improvement was observed when unoxidized starch AC coated with PEI treatment was used. Lap shear experiments demonstrated that the surface treatment of PEI on oxidized starch resulted in more than 650% improvement of bonding strength with AESO coating compared to untreated oxidized starch. This improvement of interface adhesion and water resistance is due to the new interface created through the formation of ionic bonds between PEI and oxidized starch, as well as the chemical bonds between the hydrophobic AESO and PEI molecules.

Coupled experimental and simulative investigation of the influence of polymer moisture content on the strength of amino-silane-mediated aluminum polyamide 6 joints

Amino-silane coupling agents are used to improve adhesion to polyamide-based composites. This paper aims to study the influence of polyamide 6 moisture content on the interfacial strength between the polyamide and amino-silane-coated aluminum. A coupled experimental and simulative approach is applied that enables the interfacial strength to be taken separately from the moisture-dependent mechanical behavior of polyamide 6. Lap joints of polyamide 6 and aluminum, functionalized by 3-(2-aminoethylamino)propyl trimethoxy silane, are tested at three different polyamide moisture conditions. Afterwards, numerical simulations of the lap shear experiments are carried out to identify the parameters for a mechanical model of the interface. Subsequently, the dependence of polymer's properties on the moisture content is investigated by different sets of parameters of an applied inelastic material model. Finally, an analysis of the interfacial parameters proves and characterizes the dependence of the joint strength on polyamide moisture content. It is found that the lower the moisture content of the polyamide, the higher the interfacial strength.

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.

Shear failure in supported two-dimensional nanosheet van der Waals thin films

Liquid-phase deposition of exfoliated 2D nanosheets is the basis for emerging technologies that include writable electronic inks, molecular barriers, selective membranes, and protective coatings against fouling or corrosion. These nanosheet thin films have complex internal structures that are discontinuous assemblies of irregularly tiled micron-scale sheets held together by van der Waals (vdW) forces. On stiff substrates, nanosheet vdW films are stable to many common stresses, but can fail by internal delamination under shear stress associated with handling or abrasion. This “re-exfoliation” pathway is an intrinsic feature of stacked vdW films and can limit nanosheet-based technologies. Here we investigate the shear stability of graphene oxide and MoSe2 nanosheet vdW films through lap shear experiments on polymer-nanosheet-polymer laminates. These sandwich laminate structures fail in mixed cohesive and interfacial mode with critical shear forces from 40 to 140 kPa and fracture energies ranging from 0.2 to 6 J/m2. Surprisingly these energies are higher than delamination energies reported for smooth peeling of ordered stacks of continuous 2D sheets, which we propose is due to energy dissipation and chaotic crack motion during nanosheet film disassembly at the crack tip. Experiment results also show that film thickness plays a key role in determining critical shear force (maximum load before failure) and dissipated energy for different nanosheet vdW films. Using a mechanical model with an edge crack in the thin nanosheet film, we propose a shear-to-tensile failure mode transition to explain a maximum in critical shear force for graphene oxide films but not MoSe2 films. This transition reflects a weakening of the substrate confinement effect and increasing rotational deformation near the film edge as the film thickness increases. For graphene oxide, the critical shear force can be increased by electrostatic cross-linking achieved through interlayer incorporation of metal cations. These results have important implications for the stability of functional devices that employ 2D nanosheet coatings.

Lap shear test for solder materials: Local stress states and their effect on deformation and damage

This work presents a detailed analysis of stress states and different strain measures of two lap-shear sample designs. The stress- and strain distribution are discussed for both sample designs through numerical simulation. Additionally in-situ measurements of lap-shear experiments are performed to compare the large strain and damage behavior of the different sample designs. Despite the use of in-situ video-extensometer strain measurements, a deviation among elastic moduli from tensile- and lap-shear experiments is observed. Finite element analysis reveals that inhomogeneous strains due to boundary effects are the reason for the observed moduli deviation. A strain correction method is presented to correct video-extensometer measurements. The parameters of the correction function were obtained from numerical simulation and are provided for several sample dimensions. To illustrate the effectiveness of the proposed strain correction method, the correction function is applied to experimental lap-shear test data. The corrected shear moduli compare well with shear moduli calculated from tensile tests reported in literature.

Aromatic thermosetting copolyester foam core and aluminum foam face three-layer sandwich composite for impact energy absorption

In this work, we introduce a three-layer sandwich composite structure having an aromatic thermosetting copolyester (ATSP) foam core with two aluminum foam face layers joined together by an in situ generated foaming mechanism. The ATSP foam core was synthesized on-site between the aluminum foam layers via a heat-induced polycondensation reaction. Upon curing, the ATSP foam core adhered to the aluminum foam layers through an interfacial compatibility-enabled chemical bonding. Lap shear experiments demonstrated that the bond strength clearly surpassed the tensile performance of the bare aluminum foam parts. Drop-weight impact tests showed that the three-layer sandwich structure could absorb four times the impact energy as compared to the bare aluminum foam of the same overall thickness.

Fabrication of a thermoplastic matrix composite stiffened panel by induction welding

In this work, the experimental and numerical study of induction welding devoted to the fabrication of a composite stiffened panel, representative of a typical aeronautic sub-component, is presented. A thermoplastic matrix composite, polyphenylene sulfide (PPS) reinforced with carbon fibers, is used. The influence of the fundamental process parameters, such as generator power, distance between induction coil and laminate, coil geometry and laminate lay-up on the heating rate and the heat distribution was analyzed applying finite element simulations. The model was validated through the comparison of experimental and model results obtained in static experiments. Optimized parameters for composites welding were found out, and the mechanical properties of the welded joints were evaluated by single lap shear and pull-off experiments. Finally, a prototype panel made of a flat laminate stiffened with four “L” shaped stringers is fabricated by continuous induction welding, exploiting modeling and experimental results. A C-scan of the panel was also performed.

Feasibility of iron-based shape memory alloy strips for prestressed strengthening of concrete structures

Near-surface mounted reinforcement (NSMR) is a strengthening method for concrete structures, such as buildings or bridges. NSMR involves strips or bars that are glued into grooves in the cover of the concrete. In this paper, a feasibility study is presented that uses iron-based shape memory alloy (Fe-SMA) strips instead of fiber-reinforced polymer (FRP) strips for NSMR. SMAs can more easily be prestressed than FRP. Because prestressing of SMAs does not require any mechanical jacks and anchor heads, the additional openings on the concrete surface beside the grooves that are needed to clamp the NSMR are significantly smaller.The recovery stresses (i.e., the prestresses) were investigated in a tensile testing machine combined with a climate chamber. The temperature of the strips was increased up to 160°C to provoke the phase transformation in the SMA. The bond behavior of the Fe-SMA strips glued into a groove with cement-based mortar was studied in lap-shear experiments using a 3D image-correlation measurement system. The result was compared with the bond behavior of CFRP strips glued with epoxy. Finally, two concrete bars with lengths of 70cm were each reinforced with an Fe-SMA strip. After the concrete was cured, the Fe-SMA strips were activated (i.e., prestressed) by resistive heating, and the prestressing effect on the concrete bar was measured on the concrete surface using a mechanical strain gauge.The study demonstrated the general feasibility of Fe-SMA strips in prestressed NSMR. The recovery stresses were in the range of 250–300MPa. A sufficient bond behavior was observed. Concrete bars could be successfully prestressed with a centrally embedded Fe-SMA strip.

Interlaminar behavior of infrared welded joints of carbon fabric-reinforced polyphenylene sulfide

The use of fiber-reinforced thermoplastics for structural applications is continuously increasing and therefore load-bearing joints cannot be avoided. Because most well-established joining techniques for metallic structures are not directly applicable to composites, and because thermoplastics are difficult to bond adhesively because of their chemical inertness, another solution is found, namely fusion bonding. This study assesses the use of infrared welding for a carbon fabric–reinforced polyphenylene sulfide (PPS). After a short description of the welding setup, the welding parameters such as heating time, contact pressure, and consolidation time are optimized using lapshear experiments. Two welding procedures are considered, one with and one without prior consolidation of PPS on the specimens. It can be concluded that the infrared process proves interesting for the material under study and that the process with prior consolidation of PPS yields reproducible results. POLYM. COMPOS., 2012. © 2012 Society of Plastics Engineers

Simple stiffness tailoring of balsa sandwich core material

A concept for improving the shear stiffness properties of balsa core material for sandwich structures is presented. The concept is based on utilization of the strongly orthotropic properties of the balsa wood, applying an appropriate transverse layup sequence. The effective core material shear modulus is modeled using basic laminate theory. This is subsequently validated through sandwich beam bending and lap shear experiments. Compared to the standard balsa core systems, a substantial increase in the shear stiffness is demonstrated, whereas the transverse stiffness is reduced. The concept is suitable for mass production, using standard plywood fabrication technology.

All-aromatic liquid crystalline thermosets as high temperatures adhesives

In this paper we will describe the synthesis and characterization of a novel series all-aromatic liquid crystal thermosets (LCTs) for high temperature (>100 °C) adhesive applications. Our ester-based oligomers, end-capped with reactive phenylethynyl functionalities, were synthesized using a standard one-pot melt condensation technique. The obtained reactive oligomers exhibit low melting temperatures and low melt viscosities, which resulted in improved flow and surface wetting. The adhesive properties of our LCTs based on 4-hydroxybenzoic acid, hydroquinone and isophthalic acid were investigated using standard (ASTM) lap-shear experiments conducted at room temperature (25 °C) and at elevated temperatures, i.e. 150, 200 and 250 °C. Our LCTs were used on Al2024, MS and Ti6Al4V substrates and we found lap-shear values of 11, 12 and 16 MPa for Al, MS and Ti, respectively. For the Al2024 substrate the lap-shear strength remained constant up to 150 °C and dropped to 4 MPa at 200 °C. A post-mortem inspection of the fracture surface, using high-resolution scanning electron microscopy (HRSEM), showed that there was no adhesive failure at the metal substrate at room temperature or at elevated temperatures. The lap-shear values of our polymers are significantly higher as reported for other commercial available LCPs such as Vectra™.

A cyclic viscoplastic and creep damage model for lead free solder alloys

Abstract The reliability of microelectronic components under cyclic thermomechanical loading is an important problem especially for new leadfree solder alloys. To investigate the low cycle fatigue strength of solder joints, material models are required, that can describe the constitutive inelastic deformation and damage behavior of solder materials. Such models form the basis for advanced numerical analyses by the finite element method. In the present contribution an appropriate material model that combines the viscoplastic constitutive model of Chaboche-type with the damage law of A.C.F. Cocks for porous creep will be introduced. The algorithm is reported for an implementation as a user defined material subroutine into the FEM-code ABAQUS®. The necessary parameters of the material model are identified using results of miniaturized double lap-shear experiments and tensile tests for a Sn96Ag3Cu1 solder alloy at various temperatures. The comparison of experimental and numerical results shows a good agreement with respect to strain rate sensitivity, relaxation and damage behavior of the investigated solder material. Finally, some numerical applications to surface mounted microelectronic devices are presented.

Effects of Loading Speed and Shear Prestrain on Adhesive Fatigue Strength in Single-Lap Joint

The adhesive fatigue strength was investigated by performing repeated tensile lap shear experiments of an aluminum single-lap joint bonded with highly ductile acrylic adhesive. In load-controlled fatigue tests, progressive transverse shear deformation of the adhesive layer took place, and it led to the final fracture of the joint. The fatigue strength becomes higher with increasing loading speed, especially at low-cycle fatigue region. From experiments on shear-prestrained specimens, it was found that the prestrain does not affect so much the fatigue life. In order to discuss the behavior of progressive shear strain accumulation (viscoplastic ratcheting) of adhesive under cyclic loading, the numerical simulation of ratcheting was conducted using a constitutive model of elasto-viscoplasticity for the adhesive.

Viscoplastic Behavior of Acrylic Adhesive in Butt-Joint at Various Temperatures under Complex Loading : Experimentation and Modelling

The effects of temperature and strain rate on flow stress of a highly ductile acrylic adhesive were investigated by performing tensile lap shear experiments on an adhesively bonded single-lap joint, as well as torsion experiments on a tubular butt-joint at temperatures ranging from 10 to 40oC at various shear strain rates. The flow stress decreases considerably with decreasing strain rate and with temperature rise. The stress-strain responses under multi-axial stress conditions were also examined by performing combined tension-torsion experiments on the butt-joint. A constitutive model of temperature-dependent elasto-viscoplasticity that describes multi-axial stress-strain behavior of the adhesive is presented.