Experimental investigation on transverse low-speed impact behavior of adhesively bonded similar and dissimilar clamped plates

This experimental study investigates the low-speed impact behavior of adhesively bonded similar (Al–Al, St–St) and dissimilar (Al–St, St–Al) plates. The after-impact geometries of the front and back faces of the bonded plates, which were visualized by measuring the displacements, were in good agreement with the simulated surface geometries obtained by using explicit finite element method. The plate stiffness was affective on the deflections of the bonded plates; thus, the bonded Al–Al plates exhibited maximum deflections, contact durations, and minimal contact force levels, whereas the bonded St–St plates had minimum deflections, contact durations, and maximum contact force levels. As the impact energy is increased, the impact forces, durations, and deflections increased naturally; however, the impact force-time histories were not affected evidently. The bonded Al–Al plates can dissipate the impact energy more effectively than the bonded St–St plates. The experimental and simulated contact force-time histories were generally in good agreement. Based on the cross-section photographs of the damaged impact regions the bonded Al–Al plates with low stiffness can deform plastically and dissipate most of the impact energy, and the adhesive layer remains compatible with the deformation of the plates. The interfacial fractures appear along the back plate–adhesive interface for the low impact energy but along both front and back plate–adhesive interfaces and cracks propagated to the back interface to lower interface through the adhesive thickness near the boundaries of the impactor trace. The bonded St–St plates behave more rigid, transmit the impact energy directly to the adhesive layer and the high impact force distributions result severe fractures not only interfacially but also through the adhesive thickness. The color transformations, which are indications of fracture formation and propagation speed in some way, were observed around the adhesive fractures. Although the bonded St–Al and Al–St plates had a fracture mechanism similar to those of the bonded Al–Al plates but the color transformation near the fractures and the crack opening displacement levels were more evident. The existence of a stiffer plate affects considerably the damage formation in the adhesive layer and in the plates, whereas the less stiff plates can dissipate the impact energy by deforming plastically and the adhesive layer experiences less local damages.     

A numerical and experimental study of gap length on adhesively bonded aluminum double-lap joint

The elastic finite element analysis (FEA) and the experimental method were used to investigate the effect of the gap, as well as its length, on the stress distribution in both the mid-bondline and the adherend near the interface along the lap zone of adhesively bonded aluminum double-lap joint. The values of the peak stresses distributed in the mid-bondline were increased a little when an 8 mm length gap was arranged symmetrically around the center of the lap zone. Both peak stresses and stress at the point close to the edge of the gap in the mid-bondline were increased when the gap length was increased, but the increment of the peak stresses was small when the lap length was not greater than 16 mm. The results from the FEA simulation showed that the effect of the gap length on the ultimate load of the joint was small as the gap length was increased. It is supported with the results from the experiments that the ultimate load of the aluminum double-lap joint decreased a little when the gap length was less than 12 mm.     

A numerical study of parallel slot in adherend on the stress distribution in adhesively bonded aluminum single lap joint

The effect of the length and depth of a parallel slot as well as the elastic modulus of the adhesive on the stress distribution at the mid-bondline and in the adherend was investigated using the elastic finite element method. The results showed that the peak stress in mid-bondline decreased markedly when there were two of parallel slots located in the outside of the adherend, corresponding to the middle part of the lap zone and the original low stress in this zone of the joint increases. The peak stress decreased at first, and then increased again as the length of the parallel slot was increased. The stress distribution in the mid-bondline at the position corresponding to the parallel slot decreased significantly as the depth of the parallel slot was increased. The high peak stresses caused by the tensile load occurred close to the edge of the parallel slot in the adherend. Almost all the peak values of stresses at the mid-bondline increased when the elastic modulus of the adhesive was increased. The effect of the parallel slot on the peak stress at the mid-bondline with a low elastic modulus adhesive was negligible, but the peak stress decreased markedly for adhesives with a high elastic modulus.     

Experimental study on bond strength of fiber reinforced polymer rebars in normal strength concrete

The bond behavior of reinforcing bars is an important issue in the design of reinforced concrete structures and the use of fiber reinforced polymer (FRP) rebars is a promising solution to handle the problems of steel reinforcement corrosion. This study investigates the bond characteristics of carbon and aramid FRP (CFRP and AFRP) bars embedded in normal strength concrete. A pullout test was performed on 63 normal strength concrete specimens reinforced with FRP and steel rebars with different embedment lengths and bar diameters. The average bond stress versus slip curve is plotted for all specimens and their failure modes are identified. The effects of the embedment length and diameter of an FRP rebar on its bond strength are examined in this work. The bond strengths obtained from the test results are compared with the predictions by the bond strength equation proposed by Okelo and Yuan (2005), and its validity is evaluated.     

Design and analysis of functionally graded adhesively bonded double supported tee joint of laminated FRP composite plates under varied loading

The present research deals with three-dimensional nonlinear finite element analyses for a functionally graded adhesively bonded tee joint made of laminated fiber reinforced polymeric composites when the tee joint is subjected to different types of loadings. The out-of-plane stress components have been evaluated along the interfacial surfaces of bond line of the tee joint. Using the stress values, the failure indices are computed by using Tsai–Wu coupled stress failure criterion in order to predict the location of onset of failures within the interfacial surfaces. Accordingly, critical location is identified based on the magnitude of failure indices for varied load conditions. It has been observed that tee joint under bending load is vulnerable for early failure compared with that when the joint is subjected to tensile and compressive loading. The location of failure is found to be different in tee joint under bending load compared with tensile and compressive loadings. Further, efforts have been made to reduce out-of-plane stress concentration by implementing functionally graded adhesive (FGA) with appropriate smooth and continuous gradation function profile. Further, effects of material gradation function profile with varied modulus ratios on out-of-plane stresses and failure indices are observed along the different interfacial surfaces. Series of numerical simulation result significant reduction in peak values failure index. Based on the present research findings, the FGA is recommended for higher and efficient joint strength. Results also exhibit delayed failure onset and improved structural integrity in the tee joint structure with the use of FGA material.     

Effects of unbalance on the adhesively bonded composites-aluminium joints

This article presents the experimental and numerical results of adhesively bonded hybrid single-lap joint (SLJ) geometry with different configurations of lower and upper adherends subject to a four-point bending test. AA2024-T3 aluminium alloy and carbon/epoxy composites with different lamina numbers and four different stacking angles as adherend and two-part liquid, structural adhesive DP 125 as paste adhesive were used. In the experimental studies, three different types of SLJs were produced using lower material that had a constant thickness of AA2024-T3 aluminium alloy and upper material of composite material that had different numbers of layers and four different stacking sequences ([0], [0/90], [45/?45], [0/45/?45/90]). In the numerical analysis, stress analyses of the SLJs were performed with a three-dimensional non-linear finite element method and the composite adherends were assumed to behave as linearly elastic materials, while the adhesive and aluminium adherend were assumed to be non-linear. Consequently, the change of stacking sequence and thickness of the composite in adhesively bonded SLJs altered the location of the neutral axis in the joint. This situation substantially influences the load-carrying capacity of the joint.     

Experimental investigation of the behavior of variably confined concrete

The behavior of concrete subject to variable levels of confining pressure under concentric axial loading is presented. An extensive experimental investigation of this behavior, using FRP-confined concrete cylinders, is used to develop an understanding of the relationships required to accurately model the behavior of concrete subject to passively induced varying levels of confinement. In particular, the relationship between transverse and longitudinal strains—the dilation relationship—is investigated and a model for this behavior, based on the stiffness of the confining materials, is proposed.Concrete compressive strength is observed to increase with increasing confinement. Axial strain capacity is observed to increase to a greater degree than the compressive strength resulting in a more ductile axial stress-strain behavior for confined concrete as compared to unconfined concrete. The axial stress-strain behavior is also observed to change from parabolic to bilinear as the level of confinement is increased.     

Acoustic emission method to study fracture (Mode-I,II) and residual strength characteristics in composite-to-metal and metal-to-metal adhesively bonded joints

Failure behaviour of two types of adhesively bonded joints (composite-to-metal, metal-to-metal) has been studied under failure modes (Mode I: double cantilever beam (DCB) and Mode II: three-point end notch flexures (3-ENF)) using acoustic emission (AE) technique. The bonded specimens were prepared using two types of adhesive bond materials with three variations of adhesive bond quality. The effect of the presence of interfacial defects along the interface on the residual strength of the joint has also been studied. It was possible using the maximum AE amplitude method to select the AE events of mechanical significance. However, it proved difficult to propose a definitive AE trait for the mechanical phenomena occurring within specific AE event signals, for all adhesive types, bond qualities, and substrate configurations, therefore, all specimen combinations. There was a notable shift in spectral energy proportion as the AE source of mechanical significance varied along the specimen length for specimen combinations. However, it was difficult to confirm this distinctive trait for all specimen combinations due to difficulty in confirming the location and exact mechanical source. The proposed measurement technique can be useful to assess the overall structural health of a bonded system and may allow identification of defects.     

Experimental investigation of the shear dynamic behavior of double-lap adhesively bonded joints on a wide range of strain rates

The main concern of this work is the mechanical characterization of adhesively bonded assemblies under dynamic shear loading ranging from quasi-static (10⁻⁴ s⁻¹) up to high (10⁴ s⁻¹) strain rates. The double-lap shear sample is proposed and a bonding procedure is established. The assemblies are made of steel substrates bonded with an epoxy adhesive. Two surface treatments of the substrates are considered: ethanol and sand shooting. The shear strength and the failure strain are measured by taking into account the testing setups accuracy and the non-uniform distribution of the stress and strain fields in the overlap region. The sensitivity of the strength and the failure strain to the strain rate is highlighted; it is found that the failure strain decreases and the shear strength increases with the strain rate until reaching a maximum value then it drops for very high strain rates.     

Influence of multi-walled carbon nanotubes on creep behavior of adhesively bonded joints subjected to elevated temperatures

Polymeric materials are prone to creep loading. This paper is aimed to study the effect of multi-walled carbon nanotubes (MWCNTs) on creep behavior of adhesively bonded joints. Neat and MWCNTs-reinforced adhesively bonded joints were manufactured and tested under creep loading at elevated temperatures. Two MWCNT weight percentages of 0.1 and 0.3 were used for reinforcing the single lap joints (SLJs) and the joints were tested at different temperature and load levels. The results showed that 0.1 wt% of MWCNTs resulted maximum improvements in creep behavior of adhesive joints. Adding 0.1 wt% of MWCNTs into the adhesive layer caused maximum reductions of 57%, 60% and 47% in the steady-state creep rates of the joints tested at 30, 40 and 50°C, respectively. Furthermore, 0.1 wt% of MWCNTs resulted maximum reductions of 29%, 33% and 37% in the creep strains corresponding to a specific creep loading time and maximum reductions of 23%, 45% and 49% in the elastic strains corresponding to the time at which creep loading started.     

Stress analysis of aluminium plates one-sided adhesively bonded reinforced with square composite patches using state space method

Based on state space method, a 2-D model is established for metallic plates with composite patches as reinforcement, the governing equations for the model are derived, and the stress and strain distributions are analyzed. By means of MATLAB, the structure deflection, the adhesive peel stress, and the shear stress under tensile load are figured out. In order to validate the analytical model, the finite element analysis (FEA) is also applied and a FEA model is developed by ABAQUS. Both models give pretty identical results. In the end, using the analytical model, the interlaminar stress and in-plane stress distributions through the thickness of the patch are calculated, which are known to be key contributions to composite failure, and the damage mode is briefly analyzed as well.     

A damage zone model for the failure analysis of adhesively bonded joints

The design of structural adhesively bonded joints is complicated by the presence of singularities at the ends of the joint and the lack of suitable failure criteria. Literature reviews indicate that bonded joint failure typically occurs after a damage zone at the end of the joint reaches a critical size. In this paper, a damage zone model based on a critical damage zone size and strain-based failure criteria is proposed to predict the failure load of adhesively bonded joints. The proposed damage zone model correctly predicts the joint failure locus and appears to be relatively insensitive to finite element mesh refinement. Results from experimental testing of various composite and aluminium lap joints have been obtained and compared with numerical analysis. Initial numerical predictions indicate that by using the proposed damage zone model, good correlation with experimental results can be achieved. A modified version of the damage zone model is also proposed which allows the model to be implemented in a practical engineering analysis environment. It is concluded that the damage zone model can be successfully applied across a broad range of joint configurations and loading conditions.     

In vitro degradation behavior of carbon fiber-reinforced PLA composites and influence of interfacial adhesion strength

In the present study, carbon fiber-reinforced polylactide (C/PLA) composites with different interfacial conditions were prepared to determine the influence of interfacial adhesion strength (IAS) on in vitro degradation behavior of the C/PLA composites. Pure PLA and untreated and treated C/PLA composite samples were immersed in phosphate buffered saline (PBS; pH 7.4, 37 ± 0.5^°C) for predetermined time periods. These samples were removed at each degradation time, measured to analyze molecular weight loss, weighed to assess water uptake and mass loss, and mechanically tested to obtain bending strength, modulus, and IAS. The matrixes in the C/PLA composites showed higher water uptake and lower mass loss in comparison with the pure PLA. Further, the PLA matrix in the treated composite absorbed less water and lost less mass and molecular weight than its counterpart in the untreated composite. Mechanical tests confirmed that the treated C/PLA composite exhibited a slower rate of decrease in bending strength, modulus, and IAS than the untreated one. The differences in degradation behavior between two composites can only be attributed to the difference in interfacial conditions because all other parameters were kept constant. The loss of bending strength and modulus was mainly caused by the interface degradation of the C/PLA composites. It can be concluded from our in vitro observations that the IAS had an obvious influence on the degradation characteristics of the C/PLA composites. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 82: 150–158, 2001     

Use of silane coupling agents to enhance the performance of adhesively bonded alumina to resin hybrid composites

Silane coupling agents were employed to improve the adhesion of vinyl-ester to alumina (Al₂O₃). Shear test by compression loading (ASTM D905) was used to study dry and wet adhesion. Scanning electron microscopy (SEM) was used to study the uniformity of silane coatings and the fracture modes after shear testing. Results showed that the adhesion and durability of the sandwiched alumina/vinyl-ester systems were significantly improved by certain silane surface treatment for most of the systems.     

Damping of carbon fibre-reinforced plastics plates with holes

ABSTRACT

This paper mainly deals with the specific damping capacity (SDC) of carbon fibre-reinforced plastics (CFRP) plates with holes of different sizes, positions and quantity control. Through establishing a three-dimensional damping matrix and using the finite element analysis (FEA) and Matlab, numerical calculations were conducted on the SDC of CFRP plates with same layering scheme, and a vibration test as well as a damping analysis was implemented on the CFRP test pieces with middle holes. By comparing the calculation and experimental results, good accuracy was found in predicting the modal frequency as well as SDC of the CFRP vibrations through the three-dimensional damping matrix analysis model. By comparing the modal damping of plates with different hole sizes, it could be found that as the hole size increases, the frequency showed a slowly changing tendency. The SDC of the plates with holes could reach 170% of that of the plates without holes.     

Effect of the aspect ratio of the pre-existing rectangular adhesion failure on the structural integrity of the adhesively bonded single lap joint

Non-linear three dimensional (3-D) finite element analyses (FEA) of the single lap joints (SLJs) having pre-existing rectangular adhesion failure in the interface of the strap adherend and the adhesive have been carried out. The effect of the size, the shape and the aspect ratio of the pre-existing rectangular adhesion failure on (i) the strength, (ii) the interfacial stresses and (iii) the strain energy release rates (SERRs) in the vicinity of the adhesion failure front have been presented in this research work. The SLJ is subjected to uniformly applied tensile load. The adherends are made with very high strength steels and the adhesive is a commercially available AV119. The analyses of the adhesion failure propagation have been carried out by sequentially releasing the constraints of the nodes ahead of the pre-existing adhesion failure front in finite element model. The SERR values in the vicinity of the adhesion failure fronts are computed using the virtual crack closure technique (VCCT) for assessment of the structural integrity of the SLJ. The strength of the SLJ, the interfacial stresses, and the three modes of strain energy release rates (SERRs) have been found to be significantly affected by the shape and size of adhesion failures. The SERRs and interfacial stresses along the rectangular adhesion failure front are compared with the corresponding values around the circular adhesion failure front of same area, pre-existing in the SLJ. It is observed that the circular and rectangular adhesion failures of the same area will have dissimilar growth rate and the mode II is the dominant failure mode. The total strain energy release rate and the failure strength, computed from the 3-D FEA of the SLJ is in good agreement with the experimental fracture toughness of the AV119 adhesive and the experimentally obtained failure loads, respectively.     

Experimental and theoretical investigation on the postcracking inelastic behavior of synthetic fiber reinforced concrete beams

A realistic method of analysis for the postcracking behavior of newly developed structural synthetic fiber reinforced concrete beams is proposed. In order to predict the postcracking behavior, pullout behavior of single fiber is identified by tests and employed in the model in addition to the realistic stress-strain behavior of concrete in compression and tension. A probabilistic approach is used to calculate the effective number of fibers across the crack faces and to calculate the probability of nonpullout failure of fibers. The proposed theory is compared with test data and shows good agreement. The proposed theory can be efficiently used to predict the load-deflection behavior, moment-curvature relation, load-crack mouth opening displacement (CMOD) relation of synthetic fiber reinforced concrete beams.     

Failure analysis of adhesively bonded tubular joints of laminated FRP composites subjected to combined internal pressure and torsional loading

Abstract

Finite element analysis has been carried in the present research to study individual and combined effect of internal pressure and torsional loading on stress and failure characteristics in case of an adhesively bonded Tubular Single Lap Joints (TSLJ) made of laminated Fiber reinforced polymer (FRP) composite materials. Effect of changing torsional load magnitude on an internally pressurised adhesively bonded TSLJ on interlaminar stresses and onset of different joint fracture modes (adhesion and cohesion failures) has also been studied in the present analysis. Three dimensional stress analysis of the adhesively bonded TSLJ has been carried out through suitable ANSYS Parametric Design Language (APDL) of ANSYS 14.0. Tsai-Wu coupled stress criterion has been used for predicting the onset of joint failures in the TSLJ. It has been observed that stresses (σ_r, σ_θ, σ_z, τ_rz) induced within the joint region under pure internal pressure loading are least affected through introduction of a torsional loading in the TSLJ. However, the stresses (τ_rθ and τ_θz) which are considered to be significant under pure torsional loading get tremendously enhanced due to the varying torsional loading. The interface between the outer tube and adhesive of the TSLJ has been observed to be the most critical bondline interface which is prone to undergo adhesion failure towards the free edges under pure internal loading conditions. However, under pure torsional loading conditions it tends to fracture through adhesion failure towards the clamped edge of the TSLJ. Under combined torsional and internal pressure loading the joint fails towards the clamped edge of the along the critical path which happens to be within the bondline interface, indicating predominance of torsional loading over the pure internal pressure loading. A comparative study based on the magnitude of failure index revealed that torsional loading marginally affects the joint failure as the internal pressure loading improves the compactness of the bonded joint hence improving the resistance of the TSLJ against initiation of joint fractures.