Joints In Composite Laminates Research Articles

WITHDRAWN: A critical review on influencing parameters for adhesively bonded joints in composite laminates for structural applications

This review article critically examines the influencing parameters that play a pivotal role in the performance of composite laminated adhesive bonded joints for structural applications. Adhesive bonding has emerged as a promising joining technique for composites, offering numerous advantages such as weight reduction, improved fatigue endurance, and enhanced load-bearing capacity. However, achieving optimal adhesive bonding requires a comprehensive understanding of the key parameters that influence bond strength and durability. The review begins by outlining the fundamental principles of adhesive bonding in composite laminates and discusses the importance of interfacial strength between the adhesive and composite substrates. It delves into the various adhesive materials and their properties, highlighting the significance of selecting the right adhesive for specific applications. The impact of environmental conditions, including temperature, humidity, and exposure to aggressive media, on adhesive bond performance is thoroughly assessed. Special attention is given to the influence of surface preparation techniques, such as mechanical abrasion, chemical treatments, and surface coatings, in promoting adhesion and mitigating potential failure modes. Moreover, the review presents an in-depth analysis of the effects of joint geometries, such as overlap length, bond line thickness, and joint configurations, on the mechanical strength of composite laminated adhesive bonded joints. In conclusion, this comprehensive review article synthesizes the existing knowledge on influencing parameters for composite laminated adhesive bonded joints and identifies critical areas for future research and development. By gaining insights into these influencing factors, researchers, engineers, and practitioners can enhance the performance and reliability of composite structures in various structural applications.

Finite Element Analysis of Short-edge Distance of Fastener Holes in Composite Laminates

In the production process of aircraft structures, due to design defects or the accumulation of tolerances in part manufacturing and assembly, structural short margins appear at the mechanical joints of composite laminates. In this paper, the analytical method is used to analyze the hole edge stress of the composite material fastener, and the finite element analysis of the short edge distance of the fastener hole of the composite laminate structure is carried out. The hole edge stress and residual net of the structure after the short edge distance. The stress distribution of the cross-section is analyzed to discuss the changes of the stress and strain at the edge of the hole after the short margin of the composite laminate to determine the effect on the strength of the aircraft.

Experimental investigations of bolted, adhesively bonded and hybrid bolted/bonded single-lap joints in composite laminates

The work presented in this paper covers an experimental study on the tensile property of the single-lap joints in carbon-fiber reinforced bismaleimide (BMI) resin composite laminates. This kind of composites are widely used in the aerospace industry for its high-temperature resistance. Three configurations of joints were involved in this paper, which were bolted, adhesively bonded and hybrid bolted/bonded single-lap joints. Effect of stacking sequence and width to diameter (W/D) ratio of the composite laminates were discussed on the tensile property of the single-lap joints. It was found that the stacking sequence and W/D ratio were important in the design of the joints. The hybrid joint had the highest tensile strength, but the secondary bending was serious. The test results indicated that the bolts in the hybrid joints could enhance the tensile property after the adhesive failure. The adhesively bonded joint achieved the lowest strength, and the failure mode was brittle failure.

Strength enhancement of bonded composite laminate joints reinforced by composite Pins

This paper presents an experimental and numerical investigation in the static strength enhancement of composite laminate Single Lap bonded Joints (SLJ), reinforced by pins made of Uni-Directional (UD) fibre reinforced plastic composite materials. Bonded lap joint specimens were experimentally tested in tension to obtain the failure loads and failure modes. The specimens were subsequently benchmarked against the hybrid version of the joint resulted from the introduction of composite Pins. The Pin reinforcement enhanced the hybrid single lap joint strength by an average of 19.1% increase. Numerical models generated were used for correlation with the experimental results. Numerical and experimental results observation indicated that increased strength of the hybrid bonded/Pinned joint was partly attributed to the load sharing between the adhesive and the Pin past the adhesive failure initiation as well as to the enhanced out-of-plane bending stiffness after the Pin introduction on the lap joint. Numerical investigations were performed as well with hybrid SLJ reinforced by composite pins versus designs employing metallic Pins. The simulations showed that for the investigated lap joint design parameters, the hybrid metallic pin joint failed at a higher failure load. Nevertheless, the hybrid joint utilizing the composite Pin could benefit from the enhanced corrosion resistance properties. In the case of applying a larger composite Pin diameter and/or rearranging the fibre orientation in the Pin, the hybrid SLJs could potentially achieve higher strength characteristics before the adhesive bond ultimate failure in relation to the steel Pin, as well as resulting to additional weight saving up to 46.9%.

Tensile behavior of hybrid multi-bolted/bonded joints in composite laminates

In composite structures, the strength of a standard single-lap joint with multiple bolts at best matches the strength predicted by the standard open-hole tension (OHT) test, which is about 50% of the tensile strength of the unnotched material. Although bonded joints do not have such limitation, they carry other drawbacks. The advantages of bolting and bonding may be combined in hybrid/bonded-bolted (HBB) joints. This study investigates HBB joints using carbon and glass-fiber reinforced composites with up to three bolts. It is found that multi-bolt specimens with or without adhesive fail in net-tension at the outer bolts like in OHT tests. However, HBB joint is not anymore limited by the OHT strength. The addition of the adhesive increases the strength of a three bolts joints by 70% for cross-ply laminates and 30% for quasi-isotropic laminates. The synergy between the bolts and the adhesive in the HBB system is interpreted by the fact that the outer bolts limit peel stresses and concurrently, the adhesive reduces the stress concentration around the bolts. This is particularly important for the cross-ply configuration where the stress concentrations around the holes are high. Other features observed suggest that for multi-bolted HBB joint, only external bolts are needed. Such joint configuration combines the safety provided by the bolts and the efficient load transfer provided by the adhesive.

Influence of material and manufacturing technology on the failure behavior of composite laminate bonded joints

The purpose of this work is to evaluate the influence of co-lamination vs. co-bonding on the failure behavior, and namely the fracture toughness, of carbon fibre reinforced (CFR) composite laminate joints in order to assess comparatively their performance. Since the strength of the laminate and ply texture are parameters affecting the strength of the joint, the comparison is extended to two different types of CFR pre-preg fibers, a satin T1100 with 2573 Nanoalloy® epoxy resin supplied by Toray and a twill T700 with ER450 toughened epoxy resin supplied by CIT, Toray group, representative of two different fields of application, racing and automotive, respectively.

The Effect of Fastener Type on the Connection Performance of Bolted Joints in Composite Laminates

An experimental investigation was conducted to determine the effect of fastener type on the connection performance of double lap-type single bolted composite joints. The joints with blind bolts and high lock bolts were tested separately by static tests and dynamic tests. ASTM D 5961 was adopted by static tests, and tension-compression reversed force/stress ratio, R=-1, was selected to evaluate the fatigue tests. The appropriate levels of fatigue stress were determined by the ultimate bearing strength of the fastener structure obtained from the static tests. The bearing stress and the fatigue life (S-N) date of different fastener types of specimen were presented and the relationship between fastener types and fatigue life were obtained. The experimental results showed that the connection performance of the blind bolted joint was better than that of the high-locking bolted joint. The ultimate bearing strength of the blind bolted joints was 1.9% higher than that of the high lock bolted joints, and the fatigue life of the blind bolted joints was 1-2 orders of magnitude higher than that of the high-locking bolted joint under the same cyclic stress.

A modified Arcan test for mixed-mode loading of bolted joints in composite structures

A novel modified Arcan test rig has been developed for testing the combined loading performance of bolted joints in composite laminates. The fixture was designed to complement existing bearing test standards (ASTM D5961) and pull-through test standards (ASTM D7332) for fastened joints in composite laminates. The new mixed-mode loading fixture fills a crucial niche, as interaction between pull-through and bearing failure modes can potentially reduce the overall joint performance and lead to anti-conservative designs.The test fixture was benchmarked against ASTM standards and was found to reliably reproduce pull-through failure behaviour observed during D7332-B testing. The comparison with D5961-B was less favourable, but the test specimens and configurations differ considerably. Work is underway to benchmark the new rig against D5961-C, which is a more comparable specimen configuration.Two mixed-mode test programs are reported in which joints were loaded at mixed-mode ratios between pure bearing and pure pull-through. It was found that the ‘failure’ load was well predicted by a quadratic mode-mixing rule. Conversely, the ‘maximum’ joint load was considerably over-predicted by a quadratic mode-mixing rule which could lead to anti-conservative predictions, particularly for dynamic systems in which joint damage is considerable.Based on the success of the preliminary testing, there is a strong case for further development and benchmarking of the new test fixture. There is scope for this fixture to be added to the suite of accepted test standards for bolted joints in composite structures.

복합재료 적층판 기계적 체결부 파손시험 및 점진적 파손해석에 대한 연구

복합재료 적층판 기계적 체결부(ASTM D5961 Proc. A, B)에 대하여 치구 설계에서 시험 결과의 해석까지 전체 과정을 제시하였다. 복합재료 적층판 기계적 체결부를 유한요소법을 사용하여 분석하였으며 시험 결과와 비교하였다. 시험편의 파손 거동을 분석하기 위해 점진적 파손해석 방법을 유한요소법에 적용하였다. 시험 파손 하중을 예측하기 위해 3가지 파손이론(최대 응력, 최대 변형률, Tsai-Wu)을 FEM에 적용하였다. 기계적 체결부의 일반적인 변수들을 검토하였으며 주요 변수에 대하여 베어링 강도 차이를 비교하였다. In this paper, the total procedure for composite laminate mechanical joint (ASTM D5961 Proc. A, B) from fixture design to test analysis was showed. Composite laminate mechanical joints were analyzed using the FEM(Finite Element Method) and compared to test results. A progressive failure analysis was applied to FEM to analyze the failure behavior of test specimens. Three failure theories - maximum stress, maximum strain, and Tsai-Wu were applied to FEM to predict test failure load. General parameters for composite laminate joints were reviewed and the differences of bearing strength were compared with major parameters.

Effect of the geometry in the strength of single-lap adhesive joints of composite laminates under uniaxial tensile load

Adhesively bonded joints are widely used in structures manufactured with composite laminates. Single-lap adhesive joints are the common most used due to their simple geometry and structural efficiency. In this study, a 2D numerical model has been developed in Abaqus/Standard to evaluate the influence of variations in the geometry of the adherends and the adhesive on the mechanical strength of a single-lap joint subjected to uniaxial tensile load, using the Cohesive Zone Model (CZM). Different geometrical configurations of single-lap joints have been studied, such as adherend recessing and chamfering of the adherends and adhesive. The objective is to analyse the effect of these variations with respect to the mechanical strength of the adhesive joint. In this sense, the vertical displacement of the adherends, the peak peel stress, the distribution of peel stress on the overlap length, and the failure load have been studied, identifying the chamfer in the adherends and adhesive as the most influential parameter governing the strength of the adhesive joint.

Design-oriented strength of mechanical joints in composite laminate structures and reliability-based design factor

Mechanical joints may act as bottlenecks in the design process of composite structures. Fitting factors need to be multiplied with the applied load condition to assure reliability. For more efficient design, this paper proposes a brand-new design-oriented strength: knee point strength for the mechanical joints in composite laminates. The knee point strength is derived from the stiffness change rate for the bearing stress–strain behavior in a bearing test. The strength can be correlated with the internal bearing damage; this is verified by micrography about the bearing test specimen and finite element analysis. The probabilistic design and analysis method was applied to acquire the reliability-based fitting factor. An approach combining finite element damage analysis and a stochastic technique was adopted to analyze the probabilistic properties for the design-oriented strength of the bolted joints in CFRP laminates. The probabilistic and deterministic designs were compared and discussed. Then a proper value of the fitting factor was proposed.

Structural behaviour modelling of bolted joints in composite laminates subjected to cyclic loading

Abstract The structural behaviour of bolted joints of composite laminates for aerospace applications was modelled comparing the shape, amplitude and phase of stress–strain cycles. This study proposes a model for the bolted joints resulting in a typical load–displacement curve, under cyclic loading, significantly affected by hysteretic effects. From the data gathered through the experimental activities, a constitutive relationship between strain and stress was proposed, starting from simple physical models. The assumption of a rigid shift between the laminates was used to correlate load and displacement curves in the different phases of the load cycle. The hysteretic behaviour was attributed to friction phenomena and interpreted using damping coefficients characterizing the global dynamic response of the structural joint.

Prediction of Fatigue Life for Single Fastener Joints in Composite Laminates

A progressive damage method of predicting the fatigue life of mechanically fastened joints in fiber-reinforced plastics was established, which is integrated fatigue material property degradation models. The equations of virtual work based on the theory of time increment were deduced to analyze stress-strain states under fatigue loading conditions. The three-dimensional Hashin-type fatigue failure criteria were introduced into the method to detect damage for diverse damage modes. The criteria of the structure catastrophe and the sudden material property degradation rules including the correlation among four basic damage mechanisms were also established. A software module of progressive damage analyses for bolted composite laminates is compiled, which is convenient for the application in engineering. The fatigue life, failure initiation, propagation and catastrophic failure of composite bolted joints under tension-tension fatigue loading conditions are predicted by using the fatigue progressive damage method established in the paper. An excellent agreement is found between data obtained from this study and the experiment.

Maximum entropy method for probabilistic bearing strength prediction of pin joints in composite laminates

The mechanical properties of composite materials exhibit a much wide range of scatter than that of metallic materials, even under the same loading condition. A general computational framework for probabilistic strength prediction of pin joints in composite laminates using maximum entropy method is proposed and developed in this study. The deterministic progressive failure analysis of composite pinned joint is based on a two-dimensional ABAQUS model, which takes into account large deformation theory and the non-linear shear behavior of composite material. Uncertainties arising in the mechanical properties and ultimate strengths of one lamina are modeled as random variables and considered in the process of probabilistic strength prediction. Point estimate method is used to estimate the first four statistical moments of bearing strength of composite pinned joint. Incorporating all those statistical moments, a maximum entropy method is used to approximate its probability distribution to avoid the pitfall encountered in Edgeworth expansions, which is a popular tool to approximate a probability distribution. An example of T300/QY8911 composite plate is given to demonstrate the proposed computational framework. Comparisons between the proposed framework and Edgeworth expansions based method are also performed to illustrate the advantage of the proposed framework.

The effect of interference fit size on the fatigue life of bolted joints in composite laminates

An experimental investigation was conducted to determine the effect of interference fit size on fatigue life of double lap-type single bolted composite joints. Tension–compression reversed force/stress ratio, R=−1, was selected to evaluate the fatigue tests. The joints with blind bolts of four interference fit sizes, 0% (neat fit), 0.5%, 1.8%, 3% were tested separately. The appropriate levels of fatigue stress were determined by the ultimate bearing strength of the fastener structure obtained from the static tensile tests. The bearing stress and the fatigue life (S–N) date of different interference fit sizes specimen were presented and the relationship between interference fit sizes and fatigue life were obtained. The experimental results show that the interference fit can improve fatigue life of bolted joints compared to neat fit. For the composite material studied in this paper, the joints with 3% interference fit has the best fatigue performance in lower bearing stress (less than 540MPa), however in higher bearing stress (more than 660MPa), the best size of interference fit is 1.8%.

An Experimental Investigation on the Bearing Failure Load of Multi-Pin Joints in Composite Laminates

In this paper, an experimental study has been carried out to investigate the failure load and the failure mode of multi-pin joints of glass fibre reinforced composite laminates which have the stacking sequence of [0/±45/90/0]S subjected to a traction force. The laminates were manufactured by vacuum assisted resin infusion (VARI) and tested with a special fixture. The number of pins was set from 3 to 5, while there were two different arrang styles for each condition. The test results showed that the failure loads were directly affected by the number of pins and arrang styles.The ultimate failure loads increased with the number of pins.

Study on Bearing Behaviors of Bolted Joints in Composite Laminates

An experimental investigation was conducted to determine the bearing stress of single-lap double bolted composite joints. The bearing stress of a group of specimen was presented and the relationship between the stress and strain was obtained. The experimental results show that the damage of the bolted joints was a progressive process; and the ultimate bearing stress depends not only on the laminates’ strength but also on the bolt strength. A finite element model was created based on the bolted joint specimen to simulate the loading – displacement response. The numerical results verified the experiment results qualitatively.

A semi-analytical method to predict net-tension failure of mechanically fastened joints in composite laminates

A Finite Fracture Mechanics model is proposed to predict the net-tension strength of composite mechanically fastened joints. A modification of coupled stress–energy criteria is suggested to take properly into account the effect of the R-curve in determining the crack length corresponding to the unstable crack propagation. The proposed model works for quasi-isotropic laminates and it requires as input the longitudinal strength and the fracture toughness of the laminate. The predictions obtained show good agreement when compared with the experimental results.

Progressive failure analysis of bolted joints in compositelaminates

A three-dimensional progressive failure analysis methodology was developedto predict the strength of double lap bolted joints in [0°/90°/±45°]2scarbon fibre reinforced plastic laminates. An experimental programme was conductedto verify and validate the proposed computational model. Good agreement wasobtained between the experimental data and predictive model. A parametricstudy was conducted for varied clamping torque and friction coefficient values.The well known effect of those variables on the joint strength was captured.Although the in-plane mode of failure of each individual layer around thefastener hole was predicted, X-ray radiographs have shown that delaminationfailure is particularly dominant around the washer’s outer edge. Atpresent, the proposed model does not account for delamination onset and propagation.Future work will involve implementing cohesive zone elements in regions ofinterest to capture this interlaminar cracking, a work in which the authorsare currently engaged in.

An experimental study of Flat-Joggle-Flat bonded joints in composite laminates

Flat-Joggle-Flat (FJF) bonded joints in composite laminates are designed to eliminate eccentricity by the presence of joggle so that loads remain in-plane and any bending effect is avoided thereby the strength of the joint is increased. An experimental study carried out on FJF bonded joints showed a 90% increase in failure load over flat joints.