Single Lap Shear Tests Research Articles

Calibration of a Rigid-Trilinear Cohesive Material Law to Describe the Matrix-Fiber Bond Behavior in FRCM Composites

Fiber-reinforced cementitious matrix (FRCM) composites have been increasingly adopted as externally bonded reinforcement (EBR) of existing concrete and masonry members. Being debonding at the matrix-fiber interface one of the most frequent failure mechanisms of externally bonded FRCM, the matrix-fiber bond behavior represents a fundamental aspect for the effectiveness of the external reinforcement. A cohesive material law (CML) that describes the interface where debonding occurs can be used to model the bond behavior observed. In this paper, a rigid-trilinear CML is used to solve the differential equation that governs the bond problem at the matrix-fiber interface of an FRCM composite. The CML adopted has peculiar characteristics that entail for a finite length of the bond stress transfer zone (BSTZ). Furthermore, it allows for a simple and accurate analytical solution of the bond problem. The analytical solution obtained is compared with the results of an experimental campaign comprising single-lap direct shear tests of a polyparaphenylene benzobisoxazole (PBO) FRCM composite specifically designed for masonry substrates. Different calibrations of the rigid-trilinear CML are proposed, also considering the matrix-fiber free end slip.

A Simple and Low-Cost Numerical Model for FRP-Masonry Interface Behavior

In recent years, strengthening with Fiber Reinforced Polymers (FRPs) has emerged as an effective way for the structural upgrading of masonry elements. In such typology of external reinforcement, the bond quality is crucial for the increase of the load bearing capacity. The bond efficacy beyond the elastic limit can be studied analytically or numerically via several different models, where the most important issue to tackle is the reproduction of the typical brittle behavior of the substrate. In this paper, a simple numerical approach which models FRP as elastic and lumps all non-linearity on the FRP/masonry interface is proposed. The non-linear behavior of such interface is modeled in a simplified but effective way integrating numerically the differential equations deduced from equilibrium and compatibility (once that a non-linear constitutive relationship between tangential stress and slip is assumed at the interface). Such integration is carried out by means of a particularly simple forward scheme that requires the estimation of the slip value and its derivatives on specific knot points. A comparison against existing literature indicated that the proposed numerical procedure can adequately reproduce global load-displacement curves in standard single lap shear tests, as well predict the local slip behavior.

Preliminary Results on Natural Aging of GFRP-Reinforced Masonry Components Exposed to Outdoor Environmental Conditions

Fiber reinforced polymers (FRP) have been widely used for strengthening and repair of masonry structures since the 1990s. Although the short-term performance of such systems has been extensively studied, their durability and long-term behavior are still scarcely known. Therefore, a better understanding of the corresponding degradation mechanisms and durability issues is of major interest, especially in cases of externally bonded systems where the materials are prone to undergo a more severe decay. Different accelerated aging tests have been developed in order to simulate weathering conditions in a shorter time: water immersion, freeze-thaw, hygrothermal cycles, salt crystallization, UV radiation, etc. Nonetheless, accelerated aging data cannot be used to make reliable service life estimations without a clear correlation with real environmental processes. In this study, we present the preliminary results (3/12 years) of a real exposure experimental campaign devoted to analyzing the natural aging of masonry components reinforced with externally bonded glass fibers (GFRP). The aging conditions consist of: (a) direct outdoor exposure, i.e. outdoor hygrothermal variation + sun and rain; (b) indirect outdoor exposure, i.e. outdoor hygrothermal variation + sheltered from sun and rain; (c) laboratory conditions as control group. The specimens consist of solid extruded fired-clay bricks strengthened with unidirectional GFRP sheets following the wet lay-up procedure. The aging effects on bond performance are studied by means of single-lap shear tests. Finally, the effect of brick surface treatment and the application of an external render on bond durability is investigated.

Enhanced adhesive performance of Ti–6Al–4V/PEEK interface by loading SPEEK on Ti–6Al–4V plates

With the rapid development of the aerospace industry, fiber metal laminates (FMLs) have become crucial composite structures due to their desirable properties. However, FMLs still suffer from poor adhesive performances. In this work, sulfonated poly-ether-ether-ketone (SPEEK) was selected as a modifier to enhance the adhesive performance between Ti–6Al–4V alloy (TC4) plates and PEEK. Anodizing and sandblasting were also utilized as pre-treatments to explore possible synergistic effects with SPEEK modification. The treated TC4 plate surfaces were characterized by Fourier transform infrared spectroscopy (FT-IR), X-ray photoelectron spectroscopy (XPS), contact angle (CA) measurements, and scanning electron microscopy (SEM). Meanwhile, the adhesive performances were evaluated by single lap shear testing. The successful loading of SPEEK on TC4 plates led to enhancement in interlaminar shear strength by 32.11% to reach 40.77 MPa when compared to the value of the unloaded anodized sample. The surface morphologies revealed bonding mechanisms based on mechanical interlocking, molecular entanglement, hydrogen bonding formed between sulfonic and carbonyl groups, and highly enhanced van der Waals forces generated by sulfonic groups. In sum, these findings look promising for resolving issues related to Ti/PEEK laminate interfacial strengths.

Surface engineering of carbon fibre/epoxy composites with woven steel mesh for adhesion strength enhancement

Effects of surface modifications on the adhesive joint of carbon fibre reinforced polymer (CFRP) were investigated. CFRP surfaces were treated with traditional methods, such as acetone cleaning, sanding, grit blasting and peel ply. As a novel type of surface modification, stainless steel 316 wire mesh was co-cured on the CFRP laminate during the manufacturing stage. The surface topography of CFRP adherends was analysed through microscopic imaging. Surface roughness values were measured using a surface roughness tester. Contact angle measurements of probe liquids were performed to calculate surface energy. All adherends were bonded with a low viscosity epoxy adhesive. The single lap shear tests were used to measure the bonding strength of the CFRP joints. The experimental results showed that the highest average shear strength (24.2 MPa) was achieved for metal mesh modified joints. This amounted to a 101.7% increase compared with acetone-cleaned specimens. There was no statistically significant difference in shear strength among joints subjected to traditional surface treatments. Fracture surface images were analysed and correlation between surface treatment and lap shear strength was critically discussed.

Bond between very-high and ultra-high performance fibre reinforced concrete and profiled deck sheeting

The behaviour of steel concrete composite slabs at all load levels is controlled by the shear transfer between the profiled steel deck and concrete slab. To apply new concrete technologies, such as high-strength and fibre-reinforced concretes, to steel concrete composite slabs, it is therefore essential to quantify the interfacial bond-slip properties. In this paper, a testing approach based on single-lap shear tests, commonly applied to quantify interfacial shear properties for external reinforcement, is applied to measure the bond between profiled sheets and concrete. The testing regime consists of 6 trial tests used as the basis for developing the test methodology and 48 tests to quantify the impact of concrete strength (very-high and ultra-high-strength), high volumes of steel micro fibres, and coarse aggregate, on the bond between concrete and dovetailed and trapezoidal profile decks. The results show that the concrete strength and the presence of coarse aggregate have limited impact on the bond properties, however the presence of fibres significantly improves bond strength and toughness for dovetailed profiled decks but has limited influence on trapezoidal decks.

Numerical and experimental identification of fatigue crack initiation sites in clinched joints

In this paper, a study based on experimental and numerical simulations is performed to analyze fatigue cracks in clinched joints. An experimental investigation is conducted to determine the failure modes of clinched joints under cyclic loading at different load amplitudes with single-lap shear tests. In addition, numerical FEM simulations of clinching process and subsequent shear loading are performed to support the experimental investigations by analyzing the state of stresses at the location of failure. An attempt is made to explain the location of crack initiation in the experiments using evaluation variables such as contact shear stress and maximum principal stress.

Shear behaviour and design of cold-formed steel box girders with tubular flanges and sinusoidal corrugated webs

The shear behaviour of cold-formed steel box girders assembled from tubular flanges and sinusoidal corrugated webs was experimentally and numerically investigated in this paper. A total of twelve cold-formed steel box girder specimens were fabricated by self-drilling screws, wherein sinusoidal corrugated webs with two different corrugations were adopted. The fabricated box girder specimens were tested to failure subjected to three-point bending, and the shear performance and the corresponding failure modes were examined. Before testing, the material properties of the employed cold-formed steel grades were acquired by tensile coupon tests, and the mechanical properties of the adopted single-screw connections and double-screw connections were extracted from single lap shear tests. The obtained experimental results of tested box girder specimens were further used to validate finite element (FE) models developed by ABAQUS. Upon validation of the developed FE models, the influences of the height of tubular flanges, the web aspect ratio and the corrugation ratio were evaluated. Based on the obtained experimental and numerical results, a simplified design method for predicting the shear strength of cold-formed steel box girders with tubular flanges and sinusoidal corrugated webs was proposed.

Improving single-lap joint load bearing by bioinspired interlocking patterns on substrates

This study aims to evaluate the effect of adding a pattern of conical holes on the substrate surface to improve mechanical interlocking of the bonded joint. The holes allowed the formation of micro pillars in the adhesive layer, promoting mechanical interlocking and considerably increasing the resistance of the joints. The single-lap shear tests were performed according to the ASTM D1002 standard. The results showed that the proposed solution presents a viable alternative to improve the resistance of bonded joints, promoting an increase of up to 99.84% in the final displacement and final resistance of the tested specimens. The finite element analysis also showed a reduction up to 18.11% in the stress peaks in the singularity point of the adhesive layer. Significant changes in the stress distribution around the micro pillars also was observed. Failures also have changed from an almost entirely adhesive condition to extremely fragmented cohesive failures.

Characterization of adhesive joints under ballistic shear impact

In this paper, we study the ballistic impact strength and energy absorption of single lap joints in predominantly shear (Mode II) loading. The behavior of two adhesives, namely a methacrylate and an epoxy, with steel adherends is presented. The developed ballistic shear test is analogous to the static single lap-shear joint test and uses an apparatus that is commonly used in ballistic penetration tests. The testing approach is coupled with a non-linear finite element parametric study to estimate the mechanical properties of the adhesives under ballistic loading. The finite element stress analysis results indicate that the shear failure stresses of both adhesives are much greater under ballistic loading than under static loading. In addition, the shear ballistic tests also show greater strength and energy absorption than the adhesive joints tested in Mode-I dominant ballistic impact. Though both the epoxy and methacrylate adhesives used in this study have similar static shear strengths, the shear strength of the epoxy under ballistic shear was found to be significantly higher than that for the methacrylate.

Experimental characterization of the textile-to-mortar bond through distributed optical sensors

TRM (Textile Reinforced Mortar) and FRCM (Fiber Reinforced Cementitious Mortar) strengthening materials are highly heterogeneous composites involving domains that have dramatically different mechanical properties (i.e., inorganic matrix and fabric textile). Maximizing their exploitation ratio involves achieving a better bond between textile and inorganic matrix which shifts the failure mode from textile sliding to textile rupture or cohesive failure modes. So far, the local bond behaviour between fiber bundles and inorganic matrix has been analysed indirectly through the evaluation of the global performance of TRM/FRCM materials subjected to tensile tests or single lap shear tests. In this article, the authors adopted distributed fiber optic sensors directly installed to textile bundles to track the strain evolutions of PBO-TRM strengthening materials. The strain evolutions were used, for the first time, (i) to understand the behaviour of TRM coupons subjected to tensile tests and (ii) to experimentally calibrate the interfacial tangential bond-slip law in flat masonry pillars strengthened with TRMs. The results allowed a better insight of the response of TRM materials not possible with traditional sensors and an accurate characterization of their bond performance. The interfacial tangential stress-slip law was then adopted in analytical models to predict the global performance of TRM materials providing satisfactory results compared with experimental outcomes.

Bond behaviour between flax-glass hybrid fibre reinforced epoxy composite and laminated veneer lumber joints

In this paper, single-lap shear tests were performed to investigate the bond behaviour between flax-glass hybrid fibre reinforced polymer composite (HFRP) and laminated veneer lumber (LVL) joints. Epoxy was used as polymer matrix of the HFRP and adhesive to bond LVL and HFRP to be joints. Digital image correlation (DIC) technique was used to measure the strain distribution of the LVL-HFRP joints for determining their bond-slip relationship. The effects of fibre fabric sequence of the HFRP and bond length on average shear strength, maximum shear stress, bond-slip relationship and fracture energy of the LVL-HFRP joints were evaluated. The test results showed that the bond length had a significant influence on the average shear strength, i.e., as the bond length increased, the average shear strength of the joints decreased. The fibre fabric stacking sequence of HFRP also affected the bond behaviour of the joints significantly at a bond length of 200 mm. It was found that the specimens of LVL bonded with flax fabric layer of HFRP had higher average shear strength, maximum shear stress and interfacial fracture energy than the LVL joints bonded with the glass fabric layer of HFRP.

A simple and effective resin pre-coating treatment on grinded, acid pickled and anodised substrates for stronger adhesive bonding between Ti-6Al-4V titanium alloy and CFRP

Adhesive bonding between carbon fibre reinforced polymer (CFRP) and titanium alloy is extensively employed to achieve lightweight and adequate strength in aircraft structures. In this study, acid pickling, grinding and anodising treatments were applied to the Ti-6Al-4V titanium substrates, and the CFRPs were grinded to compare the performances. A special technique named resin pre-coating (RPC) was also used to improve substrate wetting. The single lap shear tests results showed that the bond strengths of grinded and anodised specimens were higher than that of the acid pickled specimens. The RPC treatment further increased the bond strengths of all the specimens by 29.9%–51.5% and appeared to be more effective on rough surfaces. A combination of the NaOH anodising and RPC treatment yielded the highest bond strength of 22.0 MPa, which was 105.3% and 70.1% higher than the bond strengths of acid pickled and grinded specimens, respectively, and the failure mode changed from adhesive failure to CFRP delamination failure. These surface treatments are simple and thus suitable for aerospace and aviation industries. • Ti alloy was treated by grinding, acid pickling and anodising for adhesive bonding. • A special technique called resin pre-coating was used to improve surface wetting. • The bond strength between Ti alloy and CFRP was increased by up to 105.3%.

Dynamic and static interfacial bonding properties of CFRP–concrete subjected to freeze–thaw cycles

Dynamic and static single-lap shear tests of 144 carbon-fiber-reinforced polymer (CFRP)–concrete specimens subjected to freeze–thaw cycles were completed. Static and dynamic single-lap shear tests of CFRP–concrete specimens subjected freeze–thaw cycles were conducted, and a total of 32 groups of specimens with different strength grades, numbers of freeze–thaw cycles, and loading rates were tested. The corresponding dynamic and static interfacial bonding properties were carefully analyzed based on the failure characteristics, strain distributions on the CFRP surface, interfacial shear stress, interfacial average peak shear stress, interfacial fracture energy, interfacial shear strength, effective bonding length, and interfacial bond-slip curves. This study finds that the resistance to the freeze–thaw cycles of concrete was the most important factor affecting the static and dynamic properties of the CFRP–concrete interface, as well as the deterioration effect of freezing and thawing on the concrete was more severe than the effect of the erosion of the glue layer. Further analysis showed that the dynamic properties of the CFRP–concrete interface decreased as the number of freeze–thaw cycles increased. The freeze–thaw cycle could cause the embrittlement of the interface and decrease its dynamic energy absorbing ability. Finally, the refined finite element method with the concrete-damaged-plasticity (CDP) model was proposed to reveal the damage process of the CFRP–concrete interface.

Bond behaviour and thermal stability of flax/glass hybrid fibre reinforced polymer–timber structures connected by polyurethane

In this paper, the bond behaviour between timber and flax-glass hybrid fibre reinforced polyurethane polymer (HFRP) joints was investigated. The HFRP and timber (termed as HFPR-timber) joints were bonded by polyurethane adhesive. The influence of fibre layer sequences of HFRP on the shear strength and fracture energy of HFRP-timber joints was investigated with a single-lap shear test. Based on the tensile test results of FRP and combined with results from the single-lap shear test, the strain efficiency of the HFRP-timber bond was also calculated. In addition, Fourier transformed infrared spectroscopy (FTIR) was performed to analyse the surface chemical reaction between polyurethane adhesive and the flax fibre. Thermogravimetric analysis (TGA) was also conducted to evaluate the thermal stability of the HFRP, timber and their joints. Furthermore, scanning electronic microscopy (SEM) was used to study the microstructure of the HFRP-timber joints. The results showed that the fibre sequence in the HFRP affected the bond behaviour between the HFRP and timber remarkably. The highest shear strength, strain efficiency, and fracture energy were obtained, when the glass fibre layer was placed next to the timber substrate. FTIR and SEM studies showed that the flax and glass fibres in the FRP composites were not fully saturated by the polyurethane. The TGA results showed that the combination of glass and flax fibres in HFRP led to a higher thermal stability in comparison to flax FRP alone.

Ensemble Machine Learning-Based Approach for Predicting of FRP–Concrete Interfacial Bonding

Developments in fiber-reinforced polymer (FRP) composite materials have created a huge impact on civil engineering techniques. Bonding properties of FRP led to its wide usage with concrete structures for interfacial bonding. FRP materials show great promise for rehabilitation of existing infrastructure by strengthening concrete structures. Existing machine learning-based models for predicting the FRP–concrete bond strength have not attained maximum performance in evaluating the bond strength. This paper presents an ensemble machine learning approach capable of predicting the FRP–concrete interfacial bond strength. In this work, a dataset holding details of 855 single-lap shear tests on FRP–concrete interfacial bonds extracted from the literature is used to build a bond strength prediction model. Test results hold data of different material properties and geometrical parameters influencing the FRP–concrete interfacial bond. This study employs CatBoost algorithm, an improved ensemble machine learning approach used to accurately predict bond strength of FRP–concrete interface. The algorithm performance is compared with those of other ensemble methods (i.e., histogram gradient boosting algorithm, extreme gradient boosting algorithm, and random forest). The CatBoost algorithm outperforms other ensemble methods with various performance metrics (i.e., lower root mean square error (2.310), lower covariance (21.8%), lower integral absolute error (8.8%), and higher R-square (96.1%)). A comparative study is performed between the proposed model and best performing bond strength prediction models in the literature. The results show that FRP–concrete interfacial bonding can be effectively predicted using proposed ensemble method.

Joining sheets made from dissimilar materials by hole hemming

This research work presents a new joining process based on the hemming process for attaching sheets made from dissimilar materials with very different mechanical properties. The process is termed ‘hole hemming’ and consists in producing a mechanical interlock between pre-drilled holes which can be made anywhere on the sheets. The process is carried out in a two-stage operation including flanging the hole of an outer sheet and bending the flange over the hole of an inner sheet. First, the joining stages and the required tools are designed. Then, the joining of DP780 steel and AA6061-T6 aluminium alloy sheets, which are applied to manufacture lightweight structures in the automotive industries, is investigated using finite element analysis. Results show that the hole hemming process is able to successfully join these materials without fracture. The hole-hemmed joint withstood the maximum forces of 2.5 and 0.5 kN in single-lap shear and peel tests, respectively, and failed with hole bearing mode which is known as a gradual failure mode. The results demonstrate the applicability of the hole hemming process for joining dissimilar materials.

Vol4 num2-18 Optimization of direct dissimilar joining metal – thermoplastic composite for automotive and aerospace applications

The transport industry is dominated by the challenge on lightening the weight due to environmental restrictions and cost reduction. The use of different materials and multi-material designs provide a great opportunity to develop products capable of meeting this challenge. Critical aspects to be considered in multi-material manufacturing are surface preparation and dissimilar joining. The multi-material joining metal-composite thermoplastic (CTP) is critical for different aspects such as low adhesion due to a low chemical affinity or corrosion problems in the case of the use of carbon fiber reinforced composites. On the other hand, laser texturing is one of the most promising techniques to create surfaces with controlled roughness, manufacturing in a single step and improving metal-CTP adhesion, without the need to use adhesives. This work, developed within the framework of the European COMMUNION project (GA680567), addresses the optimization of the direct joining process between different high-performance metals and thermoplastic composite cintas. The study deals with surface preparation using laser texturing, the study of different joining strategies and the characterization of joints using different techniques and Single-Lap Shear tests. Depending on the texture, type of materials or manufacturing strategy, the strength of the joint is improved by 50-200%, achieving structural strength values (> 10MPa) in the optimized joints.

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.

Carbon textile reinforced mortar to masonry bond: An experimental investigation

The utilization of high-performance composite materials for the repair and strengthening of existing masonry structures is a common practice among the engineering community at present. To this point, textile reinforced mortars (TRM) have been recognized to be an efficient technique for the strengthening of the unreinforced masonry. TRMs comprise a textile fibre material impregnated within an inorganic matrix that is externally bonded to the masonry surface. A key parameter that controls the effectiveness of any externally bonded system is the bond between the mortar, the reinforced fibres, and the substrate; this has been often identified as the weak link in TRM strengthening. The effect of the strengthening mortar on the bond strength is not widely investigated. However, in some studies, the strengthening mortar defines as one of the crucial parameters to the TRM to masonry bond behavior. Hence, a number of experimental studies have been conducted to highlight the bonding performance of TRM. However, results are in many cases contradicting due to the inherent complexity of the problem that stems from the uncertainties characterizing the constituent material structural properties. Therefore, more experimental results are required to inform the investigation of the TRM to masonry bond mechanics. In this work, preliminary results from an experimental study on the masonry to TRM bond by means of single-lap shear tests with regards to 150mm bond lengths are presented. The parameter of the investigation was the type of mortar matrices.