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Copyright of Acta Materiae Compositae Sinica is the property of Acta Materiea Compositae Sinica Editorial Department and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

The natural fiber-based polymeric composites are being used extensively in various engineering applications, especially in the non-structural parts and components. Although, a large number of primary processing techniques, such as hand-layup and compression molding are available for fabrication of parts, still the secondary processing in terms of joining and machining is inevitable. The joining of composite parts becomes necessary in case of complicated and intricate product designs. Adhesive joining is one of the most commonly used processes for polymer-based composite materials. It is a cheap, easy, and smooth bonding process and does not necessitate the drilling of holes for the purpose of mechanical fastening. In the present experimental investigation, the joint strength of woven fiber mat (sisal, jute, and hybrid) reinforced epoxy composites has been investigated using different joint configurations, namely, single lap, double-strap butt, and scarf joint. The effect of adhesives has also been explored by joining composites with two types of epoxy resins and corresponding hardener. It was observed that the hybrid composites recorded better joining performance for both types of adhesives. Moreover, the Field Emission Scanning Electron Microscopy (FE-SEM) has been used to understand the failure mechanisms during tensile testing of adhesively bonded natural fiber-reinforced composite laminates. The three-dimensional assembly models of adherend specimens were created using the SOLIDWORK V.16 modeling software. ANSYS-V.18.2 WORKBENCH was employed for the analysis of the joint performance. The maximum shear stress and the total deformation results were determined. The finite element analysis (FEA) results were compared with experimental findings and were found to be in good agreement.

Thin-ply composite failure modes also significantly differ from conventional ply composite failure modes, with the final failure mechanism switching from irregular progressive failure to direct fracture characterized by a uniform fracture with the reduction of the ply thickness. When open holes and bolt joints are involved, thin-ply-laminated composites exhibit more complex stress states, damage evolution, and failure modes. Compared to the experimental study of thin-ply-laminated composite-bolted joints, there are few reports about numerical analysis. In order to understand the damage evolution and failure mechanism of thin-ply-laminated composites jointed by single-lap bolt, a progressive damage model based on three-dimensional (3D) LaRC failure criterion combined with cohesive element is constructed. Through an energy-based damage evolution method, this model can capture some significant mechanical characteristics in thin-ply-laminated structures, such as the in situ effect, delamination inhibition, and fiber compressive kinking failure. The comparisons between the numerical predictions and experimental observations are made to verify the accuracy of the proposed model. It is found that the predicted stress-displacement curves, failure modes, damage morphologies, etc., are consistent with the experimental results, indicating that the presented progressive damage analysis method displays excellent accuracy. The predicted stress at the onset of delamination is 50% higher than that of the conventional thick materials, which is also consistent with experimental results. Moreover, the numerical model provides evidence that the microstructure of thin-ply-laminated composite performs better in uniformity, which is more conducive to inhibiting the intra-layer damage and the expansion of delamination damage between layers. This study on the damage inhibition mechanism of thin-ply provides a potential analytical tool for evaluating damage tolerance and bearing capabilities in thin-ply-laminated composite-bolted joints. [ABSTRACT FROM AUTHOR]

In the presented research, a split Hopkinson tension bar (SHTB) was used to measure the mechanical response of multi-material single-lap joints in the high-rate loading regime. High-performance applications require high-quality measurements of the mechanical properties to define safe design rules. Servo-hydraulic machines are commonly used to investigate such small structures, but they are prone to produce oscillation-affected force measurements. To improve force–displacement measurements, an SHTB was chosen to investigate these joints. Three different kinds of joints were tested: multi-material bolted joints, multi-material bonded joints, and multi-material bonded/bolted joints. One substrate of the joints was made of aluminum (Al-2024-T3) and the other one was made of a laminated composite (TC250). A countersunk titanium bolt and a crash-optimized epoxy adhesive (Betamate 1496 V) were used to fasten the joints. A constant impedance mounting device was implemented to limit wave reflections and to improve the signal quality. Quasi-static experiments at a servo-hydraulic machine were performed to compare the data with the respective data from the high-rate loading conditions. The presented research shows that high-quality high-rate tests of multi-material single-lap joints can be achieved by employing an SHTB. With this high-quality measurement, a rate dependency of the mechanical behavior of these joints was identified. The dynamic increase (DI), which is the ratio of a high rate of loading over quasi-static loading, was measured for each of the joint types, where the dynamic increase in the max force was DI = 1.1 for the bolted, DI = 1.4 for the bonded, and DI = 1.6 for the bonded/bolted joints. [ABSTRACT FROM AUTHOR]

Copyright of Journal of Natural Fibers is the property of Taylor & Francis Ltd and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

This paper presents experimental and numerical investigations in the single-lap, single-bolt joints of carbon fiber reinforced bismaleimides (CF/BMI) composites. In order to guide the engineering design, an optimum configuration of composite joints is determined. Effects of stacking sequence and geometry dimensions on composite joints under tensile loading are discussed to select an optimum configuration. The optimum configuration is validated by the experimental method. Quasi-static tensile tests are conducted according to ASTM D5961 to evaluate the joint stiffness, joint strength, and bearing behavior. Finite element (FE) models considering progressive failure and delamination failure are established to investigate the joint stiffness, bearing strength, damage mechanisms, and evolution of delamination damage. Results show that bearing failure accompanied with delamination caused by secondary bending is the main failure mode, indicating an expected non-catastrophic failure mode.

In the presented research, a split Hopkinson tension bar (SHTB) was used to measure the mechanical response of multi-material single-lap joints in the high-rate loading regime. High-performance applications require high-quality measurements of the mechanical properties to define safe design rules. Servo-hydraulic machines are commonly used to investigate such small structures, but they are prone to produce oscillation-affected force measurements. To improve force–displacement measurements, an SHTB was chosen to investigate these joints. Three different kinds of joints were tested: multi-material bolted joints, multi-material bonded joints, and multi-material bonded/bolted joints. One substrate of the joints was made of aluminum (Al-2024-T3) and the other one was made of a laminated composite (TC250). A countersunk titanium bolt and a crash-optimized epoxy adhesive (Betamate 1496 V) were used to fasten the joints. A constant impedance mounting device was implemented to limit wave reflections and to improve the signal quality. Quasi-static experiments at a servo-hydraulic machine were performed to compare the data with the respective data from the high-rate loading conditions. The presented research shows that high-quality high-rate tests of multi-material single-lap joints can be achieved by employing an SHTB. With this high-quality measurement, a rate dependency of the mechanical behavior of these joints was identified. The dynamic increase (DI), which is the ratio of a high rate of loading over quasi-static loading, was measured for each of the joint types, where the dynamic increase in the max force was DI = 1.1 for the bolted, DI = 1.4 for the bonded, and DI = 1.6 for the bonded/bolted joints.

Experimental and finite element research was conducted on the bolted interference fit of a single-lap laminated structure to reveal the damage propagation mechanism and strength change law. A typical single-lap statically loading experiment was performed, and a finite element damage prediction model was built based on intralaminar progress damage theory. The model was programmed with a user subroutine and an interlaminar cohesive zone method. The deformation and damage propagation of the specimen were analyzed, and the failure mechanism of intralaminar and interlaminar damage during loading was discussed. The effect of secondary bending moment on load translation and damage distribution was revealed. The experimental and simulated load-displacement curves were compared to validate the developed model's reliability, and the ultimate bearing strengths under different fit percentages were predicted. An optimal percentage was also recommended. [ABSTRACT FROM AUTHOR]

Despite the significant technical/scientific progress achieved in the lasts years, design and modelling of adhesive joints is still an active research area. Among different approaches, the finite element method (FEM) constitutes one of the most useful, precise and powerful calculation tools for the analysis and design of bonded joints. Through a proper definition of the models, the FEM allows the determination of accurate fields of stresses and strains in the adhesive and in the adherents even for complex joint geometries or load histories. Moreover, the influence of geometrical parameters (joint configuration, thicknesses, fillets, etc) on the stress and strain levels in critical regions of the adhesive can be easily evaluated. In this way, different designs can be analysed to optimize the performance and reliability of the joints. The definition of the material models clearly constitutes one of the most relevant aspects of these numerical methodologies. The prediction capability of the FE simulations and the precision of the results will strongly depend on the material definition and the accuracy level of the material models to reproduce the mechanical behaviour of the adhesive. This is especially critical for adhesives that present markedly non linear behaviour. This work presents a study of the application of FE models for the analysis of joints with flexible adhesives. The work is focused on the characterization of elastomeric-like adhesives and the definition of proper material models to correctly reproduce their mechanical response. The operation of the models is checked through a numerical/experimental study of different bonded joints configurations. As result, it has been evidenced that a proper selection/adjustment of hyperelastic models can lead to accurate FE predictions for different joints configurations. In addition, special care must be taken when assuming incompressible hyperelastic adhesive behaviours since in some cases this hypothesis could lead to erroneous results. [ABSTRACT FROM AUTHOR]

Adhesive bonding is increasingly used in automobile, marine and aeronautical structures. The dynamic response of adhesively bonded joints is therefore a main concern. This paper deals with the harmonic response of single-lap joints. A closed-form analytical solution is derived to account for the case of joints with dissimilar substrates. The transfer functions predicted by the analytical model match well the transfer functions predicted by a two-dimensional finite element analysis, and so do the natural frequencies. The numerical and analytical models show that the natural frequencies are sensitive to the order of substrates. Mainly, fixing the end of the stiffer substrate leads to natural frequencies that are mostly higher than those which are obtained by fixing the end of the softer substrate. [ABSTRACT FROM AUTHOR]

Abstract Due to manufacturing variations of composite parts, assembly gap and consequent shimming widely occur in the airframe skin-structure. To guarantee the safety and reliability, the aircraft manufacturers expect to fully understand the effect of assembly gap and shimming on the loading behavior of composite skin joints. One of the main concerns is the effect of interface gap and shimming on the elastic stress state at the hole due to skin joints involve significant stress concentration and composite parts present brittleness and limited stress relief ability. But this topic has not yet been reported so far. In this paper, the effect of joining interface condition (including both shimming and interface gap) on three-dimensional layer-by-layer stress field at the countersunk hole of single-lap countersunk composite joints was fully and systematically investigated by employing a three-dimensional FE model. Shimming conditions include material type and shim thickness, and interface gap conditions include gap span and gap thickness. Some previously unrevealed conclusions were concluded. This study could contribute to improving the reliability of composite airframe, and benefit both design and manufacture in aircraft industry. [ABSTRACT FROM AUTHOR]

This paper presents the effect of geometric parameters on the behavior of bolted glass fibre reinforced polymer (GFRP) pultruded plates for civil engineering applications. After a literature review, results of an experimental analysis investigating the behavior of GFRP-to-steel single-lap bolted connections are presented. Then, a finite element analysis validated by experimental data is used to evaluate the effects of the end-distance, side-distance, pitch, and plate properties on the strength and failure mode of the connection. A critical examination of geometric recommendations proposed in design references is presented. Bearing failure caused by contact of the bolt on the GFRP plate is usually defined as the preferred failure mode. With highly orthotropic plate, this type of failure was found to be less likely to occur when loading is applied in the pultruded direction. The investigation showed that the minimum end-distance and pitch-distance recommended by design references usually produce a connection with the maximum capacity. However, it was found that the minimum side-distance recommended by these references does not necessarily lead to the maximum capacity for one bolt and for two bolt in a column connections. [ABSTRACT FROM AUTHOR]

Through an experimental study, this paper describes the behavior of single-lap bonded and bolted-bonded connections for configurations with minimum geometric parameters proposed in design references. Two types of multi-material connections are considered: glass fiber reinforced polymer (GFRP)-steel and aluminum-steel. At first, the behavior of bonded connections using methacrylate and epoxy adhesives is evaluated. Then, the contribution of adhesive in bolted connections is investigated. Test results show that on bonded joints, failures mostly occur at the substrate to adhesive interface. Sanding the GFRP plate was found to improve the connection strength. Despite their lower elastic modulus, methacrylate adhesives with larger capacity to undergo plastic deformation provide better strength than other methacrylate and epoxy adhesives. For bolted-bonded joints, the adhesive was found to improve the elastic behavior and the strength of GFRP-steel joints while its effect for aluminum-steel joints was not apparent due to reduced bonded surface and the high strength performance of the bolted plates. [ABSTRACT FROM AUTHOR]

An experimental study on the effect of joining interface condition (including shimming and interface gap) on bearing response of single-lap, countersunk composite-aluminum bolted joints are presented. The specimens consisted of a T700/3068 carbon/epoxy laminate with quasi-isotropic lay-up and an Aluminum alloy 7075T651 substrate. Bearing stress/bearing strain behavior were obtained according to ASTM standard. Both solid shim and liquid shim were considered and a comparison was made for them. 3D Digital Image Correlation was utilized to evaluate the effect of shimming on the surface strain distribution and out-of-plane deformation of the joints. One focus of the study was to investigate the effect of interface gap on the bearing performance of composite bolted joints. The interface gap was designed and characterized by variable parameters, i.e., thickness and span. It is found that compared to liquid-shim series, specimens with solid shim gain a little better bearing performance because of higher joint bending stiffness that benefits from the higher tensile modulus of solid peelable fiberglass shim. The presence of interface gap significantly weakens the bearing performance of single-lap, countersunk composite-aluminum joints by making the countersunk hole losing support from aluminum plate at the shear plane, and meanwhile intensifying the loading eccentricity of single-lap joints. [ABSTRACT FROM AUTHOR]

Through experimental and numerical studies, this research work aims to provide directions on the optimal geometric configuration for single-lap and double-lap bolted connection between aluminum alloy 6061-T6 and steel. From experimental test results, the effects of different geometric parameters on the joint strength were discussed. These parameters include the end-distance, the side-distance, the pitch-distance, the plate thickness and the joint eccentricity. Then, the experimental results were compared to predicted results using design references and geometric recommendations proposed by design references were critically examined. The experimental study was complemented by finite element (FE) analysis to extend the study to a larger range of parameters. In addition to the analysis of the geometric parameters listed above, the effects of the gage-distance on the joint strength were studied in the FE analysis. The experimental and finite element results show that a careful selection of geometric parameters can result in the high improvement of the connection strength and failure mode. Limiting the side-distance to the minimum recommended value was found to limit the strength of a connection with two bolts in a column to that of the one-bolt connection. In most cases, bearing was found to govern the strength of the connections. The calculated bearing strengths were found to underestimate significantly the connection strength. Based on these analyses, maximum geometric parameters beyond which there is no further increase of the joint capacity were evaluated and optimum geometric parameters were proposed. [ABSTRACT FROM AUTHOR]

An experimental study on the effect of bolt-hole clearance and bolt torque on single-lap, single-bolt, countersunk composite joints are presented. The specimens were manufactured from carbon fiber/epoxy unitapes with quasi-isotropic lay-ups. Bearing stress/bearing strain behavior, bearing strength and joint stiffness were obtained according to ASTM standard D5961. The interaction between bolt-hole clearance and bolt torque on bearing response was evaluated by varying multiple parameters. One focus was to evaluate the surface strain distribution and the out-of-plane deformation of the joints by using a commercial 3D Digital Image Correlation (DIC) system. It is found that there is interaction between bolt-hole clearance and bolt torque on joint stiffness loss and 2% offset bearing strength. Furthermore, bolt-hole clearance intensifies the surface strain concentration and the out-of-plane deformation of the joints. Bolt torque alleviates the surface strain concentration, but has little effect on the out-of-plane deformation. [ABSTRACT FROM AUTHOR]

The tensile performance of adhesively bonded CFRP joints has been investigated experimentally. In this study, overlap length, adherend thickness, adherend width and scarf angle were chosen as design parameters. All load–displacement curves are linear, except that the thicker single-lap joints behave slightly nonlinearity due to the bending effect caused by eccentric loading. The lap shear strength is not directly proportional to overlap length, adherend thickness, adherend width and scarf angle for the brittle adhesive studied in the paper. The major failure mode includes adhesive shear failure and adherend delamination failure, sometimes accompanying with some fiber pull-out. Finally, the lap shear strength of three different lap types with similar bonding area ( W =25 mm, L =10 mm, θ =5.71°) and adherend thickness (0.96 mm) was analyzed. It is found that the double-lap joint has the highest ultimate failure load. However, when considering the lap region weight, the scarf-lap joint is the most efficient. [ABSTRACT FROM AUTHOR]

Accurately and efficiently predicting the load sharing of multi-bolt thick laminate joints is necessary to quicken the optimization of the large-scale structures over various design variables, and a two-dimensional (2D) finite element method (FEM) is introduced to meet such a demand. The deformation contributions of the joint zone are analyzed and calculated separately, including the shearing deformation of the fasteners shank, the bending deformation of the fasteners shank, and the bearing deformation of the fasteners and joint plates. These deformations are all transferred and incorporated into the components of the fastener's flexibility. In the 2D finite element model, the flexibilities of the beam elements and bush elements are used to simulate different components of the fastener's flexibility. The parameters of the beam elements which include the bending moment of inertia and intersection area, and the parameters of the bush elements which include the stiffness in different directions, are all obtained through equalizing the fasteners flexibilities. In addition, the secondary bending effect introduced by the single-lap joints is also taken into account to verify the flexibilities of the fasteners in practical application. The proposed FEM is testified to be more accurate than the traditional 2D FEMs and more efficient than the three-dimensional (3D) FEM in solving load sharing problem of multi-bolt single-lap thick laminate joints. With the increase of joint plates' thickness, the advantages of the proposed method tend to be more obvious. The proposed 2D FEM is an effective tool for designing bolted joints in large-scale composite structures. [ABSTRACT FROM AUTHOR]

A damage zone method based on 3D finite element analysis was proposed to predict the failure loads of single-lap bonded joints with dissimilar composite-aluminum materials. To simulate delamination failure, interply resin layers between any two adjacent orthotropic laminas of composite adherend were assumed with a thickness of one-tenth of a composite lamina. Geometrically nonlinear effects due to the large rotation of the single-lap joint were included in the analysis. Analysis also considered the material nonlinearity of the aluminum adherend due to the stress exceeding yield level. Based on the experimental observation that the failure modes of the specimens were dominated by delamination and debonding, the Ye-criterion was applied to account for the out-of-plane failure of composite adherend and the Von Mises strain criterion was applied for the adhesive layer. The failure indices were multiplied to the predicted damage zone as a weight factor and the calculated damage zones were divided by an area or volume considering the joint geometry. Predicted failure loads show deviation within 18% from experimental results for nine different bonding lengths or adherend thicknesses. [ABSTRACT FROM AUTHOR]

Carbon composite-to-aluminum single-lap adhesive joints with six different bonding lengths were tested under tension. The main objective of this study is to investigate the relationship among the failure loads, modes, and bonding lengths of the joints. It is shown that the final failure modes of the joints are caused by delamination in the composite adherend, and that the delamination patterns vary depending on the bonding length. It was also observed that the failure loads are affected by the delamination patterns. The study's conclusion is that to achieve the maximum strength of the adhesive, the joints should be designed to have strong resistance to delamination. [ABSTRACT FROM AUTHOR]

Abstract: Failure process, mode and strength of unidirectional composite single lap bonded joints were investigated experimentally with respect to bonding methods, that is, co-curing with or without adhesive and secondary bonding. The co-cured joint specimen without adhesive had the highest failure strength. Progressive failures along an adhesive layer occurred in the secondary bonded specimen. In the co-cured specimen with adhesive film, delamination failure occurred and the joint strength was lower than that of secondary bonded specimens. Delamination failure did not occur in the secondary bonded specimen because of early crack growth and progressive failure in the adhesive layer. Therefore, The failure strength of composite bonded joint is not always proportionate to adhesion strength of adhesive due to the weakness of delamination in composite materials. The influences of surface roughness, bondline thickness and fillets in the secondary bonded specimens were also studied. [Copyright &y& Elsevier]