Single Lap Shear Tests Research Articles

Defects formation during Friction Assisted Joining of metals and semi crystalline polymers

The work aims to identify the key factors (KFs) influencing the product quality in the Friction Assisted Joining process and how these depend on the process parameters. Experimental tests were performed under low and high-speed conditions. The tests were conducted on hybrid joints made of aluminum alloy AA7075 alloy and semicrystalline Polyamide PA66. Friction Assisted Joining experiments were conducted using an instrumented CNC machine equipped with load and position sensors. The temperature variation and distribution were measured through an Infrared Camera. The influence of plunging load, tool rotation speed, plunging rate and dwell time on the quality of the joints was investigated. Single lap shear tests were conducted to determine the mechanical behavior of the joints. The results indicated that the quality of the joints is mainly determined by the processing temperature reached during the joining process. The processability window along with the onset of main adverse phenomena affecting the mechanical behavior of the joints was determined.

Experimental study on Fe-SMA-to-steel adhesively bonded interfaces using DIC

This study aimed to reveal the interface behavior of iron-based shape memory alloy (Fe-SMA) strips bonded to metallic substrates in order to ensure the integrity of such joints. A series of single lap-shear tests was performed on Fe-SMA-to-steel adhesively bonded joints. The test results reveal that the bond strength can reach over 70% of the tensile strength of the Fe-SMA and the cohesive failure mode can be realized. It is identified that three energy dissipation mechanisms occurred during the debonding process, indicating that the experimentally obtained bond-slip relations represent more than the energy dissipation by the failure of the adhesive.

Influence of surface roughness and bondline thickness on the bonding performance of epoxy adhesive joints on mild steel substrates

The bonding performance between adherend and adhesive depends on two main factors including the surface roughness of the adherend and the bondline thickness of the adhesive. This paper investigated how the surface roughness of the adherend substrate and bondline thickness of the adhesive influence the bonding performance of epoxy adhesive joints on A36 mild steel substrates using single lap shear (SLS) tests. The evaluated bonding performances included the bonding strength and the maximum strain of epoxy adhesive joints which were fabricated with four different surface roughnesses and three different bondline thicknesses using a general-purpose epoxy adhesive. The experimental results indicated that rougher steel substrates or thinner epoxy adhesive bondlines yielded larger bonding strengths and higher maximum shear strains. In addition, the morphology of the fracture substrates before and after SLS tests were analyzed using scanning electron microscopy (SEM). The SEM image analyses indicated the improved bonding performance on very rough substrates was because of strong mechanical interlocking. Thinner epoxy adhesive bondlines had larger capabilities of plastic deformation as a result of enhanced toughness.

Evaluation of Single-Lap and Block Shear Test Methods in Adhesively Bonded Composite Joints

Adhesive bonding is increasingly being used for composite structures, especially in aerospace and automotive industries. One common joint configuration used to test adhesive strength is the single-lap shear joint, which has been widely studied and shown to produce significant normal (peeling) stresses. When bonding composite structures, the normal stresses are capable of causing delamination before the adhesive bond fails, providing inconclusive engineering data regarding the bonding strength. An alternative test is the block shear joint, which uses a shorter sample geometry and a compressive-shear loading to reduce normal stresses. Analytical models proposed by Goland and Reissner and Hart-Smith are used to compare the edge-bending moment for the two joint configurations. The stress distributions along the bondline are also compared using finite element analysis. Experimental tests are conducted to evaluate these analyses and the failure modes of each configuration are recorded. Block shear samples demonstrate a joint strength over 100% higher than single-lap shear specimen bonded with the same adhesive material. The lower joint strength measured in single-lap shear is found to be potentially misleading due to delamination of the composite adherend.

The influence of the bonding length on the failure mechanism of the interface between the TRC permanent formwork and cast-in-place concrete

TRC permanent formwork is made of a new type of cement-based material and has wide application prospects in formwork engineering. To promote the large-scale development of TRC permanent formwork and make it widely used in engineering practice, based on the single-lap shear test, this paper analyses the bonding properties between the TRC permanent formwork and cast-in-place concrete at different lengths in terms of the failure mode, load-slip curve, etc., and further elaborates on the fracture mechanics theory. The research shows that when the length of the formwork small, the failure mode of the specimen measured by the single-lap shear test mainly occurs at the textile-matrix interface in the TRC permanent formwork; when the length of the TRC formwork large, the failure mode begins to change to textile fracture. The effective bonding length of the TRC permanent formwork and the cast-in-place concrete is approximately 150 mm. When the TRC formwork length is less than 150 mm, the interface bonding force increases continuously with increasing TRC formwork length. The load-slip is mainly divided into two curves with typical characteristics. The first type of curve has a clear load softening stage, and the whole curve of the second type has a stage where the slip increment is small and the load drops sharply. Finally, the interface fracture energy can better characterize the interfacial bonding properties between TRC permanent formwork and cast-in-place concrete.

Groove classification in EBROG FRP-to-concrete joints

This paper is a first attempt to classify the grooves concerning their depth, width, spacing, and direction to provide a variety of selections to be used in the ‘externally-bonded reinforcement on grooves’ (EBROG) method. Twelve classes of grooves were defined in both longitudinal and transverse directions. EBROG specimen joints with different classes of grooves were tested under a single lap-shear test to determine bond resistance, strain field, failure mode, and bond-slip behavior. Three failure modes of debonding, concrete crushing, and concrete cracking followed by debonding were observed in the EBROG joints with different groove classes. For constant groove dimensions, the ultimate load increased with decreasing groove spacing; e.g., for transverse grooves of 2.5 × 2.5 mm (width × height), an increase of about 75% was observed when the groove spacing decreased from 80 to 12 mm. Moreover, bond resistance in EBROG joints increased by up to 141% relative to that of the EBR joints when the groove class of 5 × 5 × 15 mm (width × height × spacing) was used in the transverse direction. Fracture energy was also determined for each groove class, and it was observed that the EBROG joint specimens with the groove class of 5 × 5 × 15 mm in the transverse direction exhibited fracture energies (2.25 N/mm) by more than five times greater than those of EBR joints (0.43 N/mm). The results obtained were exploited to formulate appropriate recommendations for the selection of optimum groove classes for each particular type of EBROG joint.

Study of the Bond Capacity of FRCM- and SRG-Masonry Joints

Fiber-reinforced cementitious matrix (FRCM) and steel-reinforced grout (SRG) have been increasingly applied as externally bonded reinforcement to masonry members in the last few years. Unlike fiber-reinforced polymer (FRP), FRCM and SRG have good performance when exposed to (relatively) high temperature and good compatibility with inorganic substrates, and they can be applied to wet surfaces and at (reasonably) low temperatures. Although numerous studies investigated the mechanical properties and bond performance of various FRCM and SRG, new composites have been developed recently, and their performance still needs to be assessed. In this study, the bond behavior of three FRCM composites and one SRG composite applied to a masonry substrate is investigated. Sixteen single-lap direct shear tests (four tests for each composite) are performed. The FRCM studied comprised one layer of carbon, PBO (polyparaphenylene benzobisoxazole), or alkali-resistant (AR)-glass bidirectional textile embedded within two cement-based matrices. The SRG composite comprised one layer of a unidirectional stainless-steel cord textile embedded within a lime-based matrix. The results show a peculiar bond behavior and failure mode for each composite. Based on these results, the behavior of the carbon and PBO FRCM is modeled solving the bond differential equation with a trilinear cohesive material law (CML).

Static ultrasonic welding of carbon fibre unidirectional thermoplastic materials and the influence of heat generation and heat transfer

Ultrasonic welding is a promising technology to join fibre-reinforced thermoplastic composites. While current studies are mostly limited to fabric materials the applicability to unidirectional materials, as found in aerospace structures, would offer opportunities for joining primary aircraft structures. However, due to the highly anisotropic flow of a molten unidirectional ply undesired squeeze flow phenomena can occur at the edges of the weld overlap. This paper investigates how the fibre orientation in the plies adjacent to the weld line influences the welding process and the appearance of edge defects. Ultrasonic welding experiments with different layups and energy director configurations were carried out while monitoring temperatures at different locations inside and outside the weld overlap. The joints were characterized by single lap shear tests, analysis of corresponding fracture surfaces and microscopic cross-sections. Results showed that the anisotropic flow and the anisotropic thermal conductivity of the plies adjacent to the weld line have a distinct effect on the appearance and location of edge defects. By using energy directors that cover only part of the weld overlap area a new approach was developed to mitigate edge defects caused by the highly directional properties of the unidirectional plies.

An Analytical Study for Debonding in Single-lap Shear Test by Considering the Residual Strength

External bonding of composite reinforcements has become an accepted method for strengthening the existing structures, e.g., FRCM to concrete. In the present manuscript, the shear lag model is combined with two different bond-slip laws to investigate the mechanics of debonding of composite-to-substrate joints by considering the residual strength. Boundary conditions of the problem are written based on a so-called pull-push test, while the obtained equations can also be used for the pullout test. Results illustrate the considerable effect of friction in mode II debonding, even at starting the process. Despite the simplicity of equations and differences in bond-slip laws, it is demonstrated that both the models predict the debonding load with reasonable accuracy. Finally, some simple expressions – generalizing the ones without residual friction – are proposed to calculate the effective bond length.

Development of Multi-Functional Hybrid Carbon-Based Nano-Reinforced Epoxy Adhesives

In an effort to expand the insulating behavior of adhesives, incorporated nano-sized fillers, such as multi-walled carbon nanotubes (MWCNTs) and graphene nanoplatelets (GNPs), are usually selected. Including both MWCNTs and GNPs into polymers is assumed to have complementary influence (synergy), providing a new research area. Nevertheless, limited studies have been carried out towards this hybrid direction, as it is challenging to achieve a uniform distribution of both fillers into the polymer matrix. In this work, the addition of MWCNTs and GNPs into the epoxy adhesives has been studied to increase their thermal and electrical conductivity without diminishing their mechanical properties. Three types of nano-reinforced adhesives were developed by using: 1) 2%wt. MWCNTs, 2) 8%wt. GNPs and 3) 1%wt. MWCNTs and 8%wt. GNPs. The production of nano-reinforced adhesives was achieved by using a three-roll milling technique, while during the experimental characterization single lap shear tests, thermal and electrical conductivity measurements were performed. According to the results, the introduction of nano-particles caused significant increases in electrical and thermal conductivity. MWCNTs in content of 2%wt. showed the highest improvement in the electrical conductivity (9 orders of magnitude), while GNPs in content of 8%wt. recorded the highest increase (207%) in the thermal conductivity of nano-reinforced adhesives. Finally, it was observed that the hybrid system successfully contributed to the development of a multi-functional epoxy adhesive with improved thermal and electrical properties without significantly compromising its mechanical properties.

Experimental calibration of the bond-slip relationship of different CFRP-to-timber joints through digital image correlation measurements

Nowadays, the use of the Digital Image Correlation (DIC) technique has spread and it is being used in several engineering areas to measure displacements. The available data obtained from the DIC measurement to evaluate the bond performance between a Carbon fibre Reinforced Polymer (CFRP) externally bonded to a timber substrate is scarce. From the existing data obtained with other materials, this contactless technique revealed to be quite useful but its accuracy with other well-established techniques, such as the use of electric strain gauges is not well understood yet. Therefore, the current work aims to study the accuracy of 2D DIC measurements with the measurements obtained from the use of strain gauges within a low-cost perspective. To that end, several CFRP-to-timber bonded joints were tested under the single-lap shear test and different bonding techniques were considered as well. Some flaws intrinsically derived from the DIC measurements that complicate the bond assessment, such as the fluctuations in the generated displacements field, are identified, and to bypass this problem a new methodology is proposed. This new methodology is based on two different closed-form solutions that, after defining the local and global bond behaviours of different CFRP-to-timber bonded joints, allowed to eliminate the fluctuations found from the DIC measurements, facilitating the estimation and the comprehension of the full debonding process of the CFRP-to-timber joints, which was achieved with a good proximity to the homologous debonding process derived from the strain gauge measurements.

A tailored heating element for resistance welding of complex shaped joining surfaces

To obtain a larger freedom of design for structural components, a single wire heating element which is capable of welding large and complex shaped surfaces has been developed. The processing window for welds of endless-carbon-fiber-reinforced polyamide 6 (CF-PA6) using the developed heating element is investigated, varying the main process parameters electrical power, electrical energy and joining pressure. The influence of these process parameters on the quality of the welded joints is judged by single and double lap shear tests and by the analysis of micrographs. As a result, recommendations for the processing parameters are derived. The climate conditions of the specimens are varied to analyze the impact of moisture on the shear-transmitting joints whereby an accelerated method for conditioning of PA6 specimens is used. Furthermore, a sensor-free method for monitoring the temperature of the welding process is presented and compared with a classical measuring method.

Thermal effects on the performance of ultrasonically welded CF/PPS joints and its correlation to the degree of crystallinity at the weldline

The effect of temperature on the lap shear strength (LSS) and failure mechanisms of ultrasonically welded carbon fibre reinforced polyphenylene sulphide (CF/PPS) joints was investigated, correlating the weld performance to the crystallinity degree of PPS at the weldline. The single-lap shear tests were carried out at temperatures ranging from –50 °C to 120 °C on three series, one with amorphous and two with semi-crystalline weldline. The overall trend was decreasing LSS with increasing temperature and the largest LSS reduction was observed above the glass transition temperature. Fractographic analysis revealed that the main failure mechanism at –50 °C was matrix fracture while fibre/matrix debonding became more pronounced with increasing temperature. It was demonstrated that higher degree of crystallinity of PPS at the weldline was beneficial at high temperatures (90 °C and 120 °C) most likely due to the higher fibre/matrix interfacial strength compared to amorphous PPS. The amorphous weldline was shown to be advantageous at -50 °C, probably due to the higher toughness and ductility of amorphous PPS.

Performance and failure assessment of adhesively bonded non-crimp fabric carbon fiber/epoxy composite single lap joints

Carbon fiber-reinforced plastic (CFRP) composite materials have experienced increased use for load-bearing structures in automobiles owing to their high specific mechanical properties. However, critical to widespread adoption of CFRPs are the development of robust joining methods with appropriate surface preparation to maximize joint strength, as well as an improved understanding of failure mechanisms to enable computational modeling for supporting design. In the present study, an adhesively bonded non-crimp fabric CFRP single lap joint (SLJ) was investigated to quantify the effect of the composite adherend surface treatment and stacking sequence on the lap shear strength, and to assess the corresponding failure mechanisms. Single lap shear test results revealed that the highest lap shear strength was achieved by abrading the surface with sandpaper, relative to an unprepared surface or one treated by grit blasting. Similarly, an adherend with a higher effective flexural longitudinal modulus, achieved through stacking sequence, increased the lap shear strength owing to reduced Mode I loading on the adhesive joint. Distinct failure processes were observed for CFRP adherends with different stacking sequences, where the locations of intra-ply and inter-ply cracks varied as a result of variations in the in-plane and out-of-plane stresses. Notwithstanding, the final failure event for all SLJ specimens was failure of the thin layer of matrix in the 0° ply of the adherends. Three-dimensional finite element analyses of the SLJ specimens were employed to determine the local stress components driving the observed failure mechanisms within the plies of the CFRP adherends. The results of this study provide a recommended surface treatment to maximize adhesive joint strength for CFRP adherends, along with a comprehensive assessment of failure mechanisms, to inform the design and application of CFRP materials in lightweight structures.

Selected machine learning approaches for predicting the interfacial bond strength between FRPs and concrete

Accurately predicting the interfacial bond strength (IBS) between concrete and fiber reinforced polymers (FRPs) has been a challenging problem in the evaluation and maintenance of reinforced concrete (RC) structures strengthened by FRP laminates. In this work, we employ three different machine learning (ML) approaches, including a multiple linear regression (MLR), a support vector machine (SVM), and an artificial neural network (ANN), to establish the correlation between influencing variables and the IBS and then to predict the IBS. Two datasets containing the single-lap shear test results for FRP laminates externally bonded on concrete prisms and externally bonded on the grooves of concrete prisms are used to train the three selected ML approaches and test their performances. The predicted results show that SVM-ML has the best accuracy and efficiency among the three approaches. By comparing it with the existing empirical IBS models, we find that the developed SVM-ML method shows the same or better prediction accuracy on both datasets. Furthermore, based on the SVM model, the relationships between influencing variables and the IBS are analyzed through partial dependence plots. In addition, the stacking strategy of ML is adopted to further improve the prediction accuracy for IBS. The ML method explored in this work is proven to be feasible and efficient for predicting the IBS of RC structures strengthened by FRP laminates.

Effects of thermal expansion coefficients discrepancy on the CFRP and steel bonding

The long-term durability of steel structures strengthened by externally bonded carbon-fiber-reinforced polymer (CFRP) plates or sheets is a concern due to the discrepancy in the thermal expansion coefficients (CTE) between CFRP plate and steel. In the present study, the single-lap shear test was applied to study the variation of the bonding performances between CFRP plates and steel using different types of adhesive after 150, 300, and 500 temperature cycles from −20 °C to 40 °C. The result shows that the temperature cycle led to an insignificant reduction in the bond performance between the CFRP plate and steel. After 500 temperature cycles, the decrease in bond strength is less than 10%. Finally, the distribution of thermal shear stress under temperature cycles is simulated by the finite element method, which is consistent with the analytical solution in published literature.

Bond-slip models for the interface between steel fabric reinforced cementitious matrix and concrete substrate

Composite materials for strengthening existing structural members represent a valid alternative to the traditional strengthening techniques. In particular, Fabric Reinforced Cementitious Matrix (FRCM) composites have been successfully employed to strengthen existing reinforced concrete and masonry structures.Among the different FRCM composites available on the market, the attention of the authors was recently focalized on systems composed of high tensile strength steel cords embedded in a cementitious matrix (systems also well known as Steel Reinforced Grout, SRG).The main failure mechanism of FRCM is represented by the debonding at the matrix-fiber interface. Therefore, the study of the bond behavior of FRCM composites is a key topic to develop reliable design procedures. In this paper, the applicability of different existing bond-slip interface models for FRP-concrete interface to SRG system is studied. The different parameters characterizing the models are calibrated by using an experimental database available by the authors and including results of direct single-lap shear tests on concrete prisms bonded to SRG strips. All the models are calibrated using a classical technique which minimizes the difference between the measured and computed interfacial shear stress values at different slip levels. The results indicate that all the models predict relatively well the slope of the ascending branch of the shear stress-slip curve, but they give substantially different descending branch profiles.The calibrated models are, then, compared with some other laws proposed in the literature for PBO and basalt FRCM systems; some of them are also implemented within a numerical procedure developed by the authors for the estimation of both the maximum axial stress at debonding and the corresponding effective bonded length of the SRG strip. The comparison with the experimental data has allowed for verifying the accuracy of the calibrated bond-slip models.

Formation of micro-mechanical interlocking sites by nanoscale sculpturing for composites or hybrid materials with stainless steel

Abstract

Adhesives for increasing the bonding strength of in situ manufactured metal-composite joints

In this present work, the potential of metallic parts, locally reinforced with a continuous glass fibre reinforced thermoset material, pre-impregnated with an epoxy matrix (prepreg), was evaluated by differential scanning calorimetry (DSC), single-lap shear tests and 3-point bending tests of a metal-composite hybrid hat profile. This technology is evaluated regarding an automotive use case, the DSC experiments in combination with moulding trials have proven curing times below 30 s for a moulding temperature of 180°C. A bonding strength of 13.5 MPa was characterized for a co-cured fibre-reinforced plastic (frp) onto a metallic joining partner. By additionally introducing an epoxy glue film as a bonding agent, which is co-cured together with the frp, the bonding strength can be increased significantly up to 25.4 MPa at the expense of the curing time. The mechanical tests on the hybrid hat profile have shown an increase of energy absorption compared with non-reinforced hat profiles. Here, also an additional glue film extends the performance regarding a co-cured plastic reinforcement without glue film. The influence of the storage conditions of the uncured prepreg materials on the mechanical performance was evaluated by a simulated physical ageing at elevated temperatures, followed by a mechanical characterization of the bonding strength and part performance. Also the effect of different testing temperatures and testing velocities on the capability of the metal-composite hybrid part is illustrated.

Bond behavior and ultimate capacity of notched concrete beams with externally-bonded FRP and PBO-FRCM systems under different environmental conditions

The influence of environmental conditions on the bond behavior and ultimate capacity of strengthening composite systems applied to concrete substrates is investigated. Two different systems are analyzed, namely fiber-reinforced polymer (FRP) system with carbon sheet and epoxy resin (CFRP), and fiber-reinforced cementitious matrix (FRCM) system with polybenzoxole (PBO) grid and cement-based mortar. The experimental campaign comprises three-point bending tests on 24 notched beams with the strengthening system being externally bonded onto the bottom face of the beam in order to selectively study the bond behavior with no tensile contribution of concrete. In addition to classical curing process in air at environmental conditions, two wet curing procedures are studied, namely partial immersion in water at 30 °C and 50 °C and 100% relative humidity for a period of 31 days. Results in terms of load-displacement curves, failure modes and ultimate capacity are critically analyzed so as to highlight the influence of the conditioning factors (humidity and temperature) on the bond behavior at the concrete-composite interface of the two strengthening systems comparatively. For the relatively short duration of the partial immersion process considered in this experimental campaign, the FRCM system is not sensitive to environmental conditioning, whereas the FRP system is only sensitive to curing temperatures close to the glass transition temperature of the resin/primer. For the PBO-FRCM system, an indirect method to evaluate the stress-global slip curve is elaborated in this study through an analytical procedure based on kinematics considerations, which allows a comparison between results from the proposed notched beam test setup and results from alternative single-lap or double-lap shear tests that are more widely used in the literature.