Strength prediction of bolted joints in graphite/epoxy composite laminates

A parametric finite element analysis was conducted to investigate the effect of failure criteria and material property degradation rules on the tensile behaviour and strength of bolted joints in graphite/epoxy composite laminates. The analysis was based on a three-dimensional progressive damage model (PDM) developed earlier by the authors. The PDM comprises the components of stress analysis, failure analysis and material property degradation. The predicted load–displacement curves and failure loads of a single-lap single-bolt joint were compared with experimental data for different joint geometries and laminate stacking sequences. The stiffness of the joint was predicted with satisfactory accuracy for all configurations. The predicted failure load was significantly influenced by the combination of failure criteria and degradation rules used. A combination of failure criteria and material property degradation rules that leads to accurate strength prediction is proposed. For all the analyses performed, the macroscopic failure mechanism of the joint and the damage progression were also predicted.     

Hybrid titanium–CFRP laminates for high-performance bolted joints

This paper presents an experimental and numerical investigation of the mechanical response of bolted joints manufactured using new hybrid composite laminates based on the substitution of CFRP plies with titanium plies. The local hybridization of the material is proposed to increase the efficiency of the bolted joints in CFRP structures. Two modeling strategies, based on non-linear finite element methods, are proposed for the analysis of the bolt-bearing and transition regions of the hybrid laminates. The bolt-bearing region is simulated using a three-dimensional finite element model that predicts ply failure mechanisms, whereas the free-edge of the transition region is simulated using plane stress and cohesive elements. The numerical and experimental results indicate that the use of hybrid composites can drastically increase the strength of CFRP bolted joints and therefore increase the efficiency of this type of connection. In addition, the numerical models proposed are able to predict the failure mechanisms and the strength of hybrid composite laminates with a good accuracy.     

Experimental and numerical investigation on bolted joint in glass–fiber reinforced composites

In this paper, vacuum assisted resin injection technique was employed to prepare the composite laminates with different plies ways. The macro-mechanical performances of the bolted joint of the composite laminates were investigated by experimental and finite element simulation. The influence mechanism of different parameters on the joint performance and failure models of the composite laminates were analyzed. A VUMAT subroutine was developed, according to the failure criteria and corresponding stiffness degradation criteria, to describe the progressive damage process of bolted joint composite laminates through finite element analysis method. The results indicated that the failure strength of composite laminates would be improved with the increasing of tightening torque, and the strength of three-ply-way hybrid composite laminates is higher than that of two-way hybrid laminates. The comparisons between numerical simulation results and experimental results showed that the developed subroutine can effectively predict the macroscopic response of bolted joint glass–fiber reinforced composite laminates.     

分析复合材料螺栓连接接头静强度的一种方法

根据复合材料机械连接区逐点逐层破坏的物理本质,采用了每层破坏单元刚度退化和应力空间二阶张量破坏准则,用有限元素法,计算了T300/648碳纤维复合材料21种不同铺层情况的接头强度,分析了它们的破坏模式和破坏过程并与试验结果进行了比较.      

三维六向编织复合材料单剪搭接连接结构在拉伸载荷下的力学性能

通过试验测试与数值模拟相结合的方法对三维六向编织复合材料的螺栓连接性能进行了研究。首先,通过拉伸试验对不同侧向约束螺接方式连接件的连接强度进行了测试。测试结果表明:单搭连接结构的二次弯曲现象明显,连接强度与侧向约束有一定的关系,使用垫片可有效提高连接强度,螺栓拧紧力矩增加对连接强度影响不大;连接结构的破坏模式包括挤压破坏和拉伸破坏,在孔径较小时其主导破坏模式是挤压破坏。随后,基于测试中发现的破坏模式,建立了基于点应力准则的分析模型,并使用升温法实现螺栓拧紧力矩的施加。通过数值结果与试验结果的比较验证了分析模型的可靠性。最后,利用得到验证的分析模型,分析了单搭连接的二次弯曲现象,获得了侧向约束面积、螺栓拧紧力矩及连接平板厚度对单搭单螺栓连接结构力学性能的影响规律。分析结果表明:当侧向约束应力增加时,连接强度表现为先增加后降低的规律。     

Subcritical damage mechanisms of bolted joints in CFRP composite laminates

A detailed experimental programme is presented that was conducted in order to establish a data base for strength and subcritical damage mechanisms of bolted joints in CFRP composite laminates. Single fastener double-shear tensile tests for various joint geometries were performed for a range of cross-ply and quasi-isotropic lay-ups of HTS40/977-2 CFRP material system. Penetrant enhanced X-ray radiography was used to define the subcritical damage locations which are of great importance when modelling the failure response of the joints. It is suggested that the subcritical damage planes can be modelled using cohesive zone elements (CZEs) in order to develop physically based strength prediction methods for bolted joints in CFRP laminates.     

复合材料单钉接头三维逐渐损伤破坏分析

针对纤维增强复合材料螺栓双盖板接头, 发展了面内静拉伸三维逐渐损伤模型。并对损伤累积过程中出现的四种基本损伤机理类型(纤维断裂、纤维-基体剪切、基体开裂、分层) 及其之间的相互关联性进行了分析模拟, 并能成功预测其接头层合板静拉伸强度、破坏模式及损伤与扩展的整个过程。同时用参数化设计方法, 对任意铺层、任意尺寸单钉接头进行建模, 使分析工作更加方便。用本模型计算的结果与文献[ 4 ]提供的9 种不同铺层尺寸接头实验结果进行了比较分析, 对比结果非常令人满意。      

Influence of stamping on the compressive behavior and the damage mechanisms of C/PEEK laminates bolted joints under severe conditions

Bolted joint tests have been performed in order to evaluate the influence of stamping on the behavior of thermoplastic-based woven-ply laminates subjected to structural loadings under severe service conditions (120 °C after hygrothermal aging). Compressive tests have been carried out on carbon fabrics reinforced PolyEtherEtherKetone (PEEK) laminates to investigate fibers buckling due to changes induced by stamping on the non-planar interply structure of woven-ply laminates. As compressive strength decreases by 13% in stamped laminates, it facilitates the plastic buckling of 0° and ±45° oriented fibers due to compressive loads in bolted joints. Contrary to double-lap joints, stamping does not affect the strength of single-lap joints, as the geometry of single-lap joints is non-symmetric. Stamping modifies the damage mechanisms of PEEK-based laminates under bolt-bearing loadings, such as the failure of stamped bolted joints is dominated by bearing failure mode.     

Application of the point stress criterion to the failure of composite pinned joints

An analysis was performed to evaluate the bearing strength of pin-loaded composite joints using a two parameter characteristic curve model. This model involves determination of characteristic dimensions in tension and compression and based on this model, a two-dimensional stress analysis was used to determine the stress distribution around the fastener hole. In this analysis, characteristic dimensions in tension and compression were evaluated using the point stress failure criterion and joint bearing failure evaluated using the Yamada-Sun failure criterion. Results were compared with available experimental data for joints made from AS4/3501-6 graphite epoxy composite laminates and good correlation observed when evaluated as function of edge distance to hole diameter. However, the analysis yields conservative results when joint strength is evaluated as a function of plate width to hole diameter.     

A three-dimensional progressive damage model for bolted joints in composite laminates subjected to tensile loading

A three-dimensional progressive damage model was developed to simulate the damage accumulation and predict the residual strength and final failure mode of bolted composite joints under in-plane tensile loading. The parametric study included stress analysis, failure analysis and material property degradation. Stress analysis of the three-dimensional geometry was performed numerically using the finite element code ANSYS with special attention given to the detailed modelling of the area around the bolt in order to account for all damage modes. Failure analysis and degradation of material properties were implemented using a set of stress-based Hashin-type failure criteria and a set of appropriate degradation rules, respectively. In order to validate the finite element model, a comparison of stress distributions with results from analytical models found in the literature was carried out and good agreement was obtained. A parametric study was performed to examine the effect of bolt position and friction upon damage accumulation and residual strength.     

Effect of clamping force on the delamination onset and growth in bolted composite laminates

Clamping force is a key element that alters the mechanism and sequence of failure in bolted joints of composite laminates. The mode of failure in bolted joints can be controlled by geometrical parameters and the preferred fail safe mode of failure is ‘bearing’ which generally consists of matrix cracks, delamination and fibre microbuckling. Three-dimensional (3-D) pinned (without clamping force) and bolted (1 kN clamping force) joint models were developed in [0/90]_s carbon fibre reinforced plastic (CFRP) laminates to show the clamping force effect on the onset and growth of delamination. It is shown that delamination was resulted from the shear stress components (Mode II & III) at the interface and the contribution of the out-of-plane component (Mode I - opening), so the clamping force, was negligible without modelling the in-plane failure modes and their coupling with delamination, which will be considered in future work.     

T800碳纤维增强复合材料双剪单钉连接的拉伸试验及强度估算

为系统地研究T800碳纤维增强复合材料螺栓连接的力学性能,首先,对T800碳纤维增强复合材料双剪单钉连接进行了面内准静态拉伸试验,探讨了铺层比例、铺层顺序、螺栓直径以及固化工艺对复合材料螺栓连接刚度和2%偏移挤压强度的影响;然后,根据试验结果得到了T800碳纤维增强复合材料螺栓连接的应力集中减缓因子;最后,结合相应铺层比例的无缺口层合板的应力集中减缓因子和拉伸强度,建立了复合材料连接最终挤压强度的工程算法。结果表明:当螺栓断裂时,连接的最终挤压强度由螺栓剪切强度和螺栓直径/板厚比决定;连接存在挤压和剪切2种失效形式,与±45°铺层比例有关;工程算法的计算结果与试验结果吻合良好。所得试验结果和工程算法可为T800碳纤维增强复合材料螺栓连接的初步设计提供理论依据和技术支持。      

Experimental and numerical analysis of aluminum-aluminum bolted joints subject to an indentation process

The increasing interest of the industry (especially automotive, aviation and marine) in the fastener joints (riveted, bolted, etc.) between metallic materials, has re-opened the study on the possibility to improve the performance of the drilled structure using plastic deformation processes.Indentation process, performed before the drilling operation, creates circumferential compression stresses around the hole which increase significantly the mechanical performance of the drilled structures.In this paper, static and the fatigue performances of aluminum–aluminum (AW 6082-T6) single-lap bolted joints are studied. In particular, the study compares the mechanical strength of only drilled single-lap bolted joints (OD specimens) and single-lap bolted joints subject to an indentation process (IP specimens). In order to determine the cycles to failure and the corresponding Wöhler diagram, several fatigue tests are performed. The analyses allow to determine the mechanical performance and the failure mode of the analyzed joints.Several numerical analysis, conducted in ANSYS environment on three-dimensional models of the single-lap joint, are focused on the evaluation of the residual stress on the indented plate and, in particular, to compare the stress distribution on both type of analyzed joints.     

A global bolted joint model for finite element analysis of load distributions in multi-bolt composite joints

This paper presents the development and validation of a global bolted joint model (GBJM), a highly efficient modelling strategy for bolted composite joints. Shell elements are used to model the composite laminates and the bolt is represented by a combination of beam elements coupled to rigid contact surfaces. The GBJM can capture effects such as bolt–hole clearance, bolt-torque, friction between laminates, secondary and tertiary bending in the laminates as well as the load distribution in multi-bolt joints. The GBJM is validated using both three-dimensional finite element models and experiments on both single- and multi-bolt joints. The GBJM was found to be robust, accurate and highly efficient, with time savings of up to 97% realised over full three-dimensional finite element models.     

STRENGTH PREDICTION AND DAMAGE MECHANISMS OF GLASS/EPOXY WOVEN LAMINATES WITH CIRCULAR HOLES

Abstract— The notched strengths of four woven laminates (two orthotropic, one quasi-isotropic and one square symmetrical) under monotonic uniaxial loading, predicted with the point and average stress criteria, the two parameter criterion, the progressive degradation model, and the initial and improved point and minimal strength models, have been respectively compared with the experimental data. The damage mechanisms for the laminates were tentatively studied. The results show that, for minimal experimental information, the notched strengths predicted by the improved minimal strength models are the most precise and the progressive degradation model can numerically illustrate the damage mechanisms in detail. The damage mechanisms of the woven laminates were found to be very different from those of non-woven laminates.     

Analysis of bolted–bonded composite single-lap joints under combined in-plane and transverse loading

A semi-analytical solution method is developed for stress analysis of single-lap hybrid (bolted/bonded) joints of composite laminates under in-plane as well as lateral loading. The laminate and bolt displacements are based on the Mindlin and Timoshenko beam theories, respectively. For the adhesive, the displacement field is expressed in terms of those of laminates by using the shear-lag model. The derivation of the governing equations of equilibrium of the joint is based on the virtual work principle, where the kinematics of each laminate are approximated by local and global functions and the bolt kinematics is assumed in terms of cubic Hermitian polynomials. The capability of the present approach is justified by validation and demonstration problems, including the analysis of bolted and bonded joints and hybrid joints with and without considering a disbond between the adhesive and laminates.     

An evaluation of failure criteria for matrix induced failure in composite materials

This paper reports on work being undertaken in the Cooperative Research Centre for Advanced Composite Structures Ltd. (CRC-ACS) to develop improved techniques for predicting the failure of composite materials. The procedures being investigated include a maximum strain criterion for fibre failure. For failure of the resin a new approach, which includes determination of the residual stresses due to manufacturing, is being trialed. This work closely parallels the new criteria proposed by Gosse and Hart-Smith [AIAA/CRC-ACS text on composite materials, submitted for publication] and we have subsequently replaced a simple stress criterion for matrix failure with their proposals based on strain invariants. The new procedures are applied to the failure of laminates in bolted joints with complex steered fibre patterns. Thermal residual stress was included to predict the matrix failure of T-section laminates under loads that open the angle between the flanges and the web. Here a transverse tension stress criterion was used.     

Fretting fatigue behavior and life prediction of automotive steel bolted joint

Fatigue tests of bolted joints of SAPH400 automotive steel plate were carried out. Effect of groove on fretting fatigue strength was investigated by introducing various geometries of grooves at contact edge. The fretting fatigue strength was improved by introducing groove: the fatigue strength increased with increasing groove depth. As the next step, the applicability of the tangential stress range–compressive stress range diagram to the bolted joints was investigated using the tangential stress range–compressive stress range diagram obtained from conventional laboratory-type SAPH400 steel specimens. The result showed that the fretting fatigue strength of actual component, i.e. the bolted joint could be successfully predicted based on the tangential stress range–compressive stress range diagram.     

接头局部增强的复合材料层合板螺栓连接试验与数值模拟

采用真空辅助成型工艺（VARI）制备了四种局部增强的复合材料层合板螺栓连接试件，通过试验及数值模拟对其力学性能进行了研究。数值研究中将复合材料层合板连接件的拉伸作为一个准静态问题，运用ABAQUS的显示分析算法及所编写用户材料子程序VUMAT对连接件进行了三维渐进失效模拟，同时在有限元模型中采用内聚力单元模拟了层合板与所设增强层的界面分层失效。数值计算结果与试验结果取得了较好的一致，验证了本文中数值方法的有效性。研究结果表明，不同的局部增强方案对复合材料螺栓连接性能的影响较大，设置[0/90/0/90]_S铺层的内置纤维增强层能显著提高层合板的螺栓连接性能。     

Numerical analysis of damage progression and strength of countersunk composite joints

A numerical investigation is conducted into the damage progression and strength of bolted joints between fibre-reinforced composite laminates using countersunk fasteners. Experimental tests were previously conducted on a bearing test specimen and countersunk fastener single-lap joints. In this work, computational models are developed for Abaqus/Explicit, with continuum shells employed to model in-plane ply failure. The bolt-nut assembly is modelled with rigid elements, and the models account for bolt torque and frictional contact. The material properties required in the computational model are determined from standard tests, with the compression fracture toughness of composite plies calibrated against experimental data from the bearing test. The analysis approach captures the load-carrying capability of all configurations, and provides reasonable accuracy in predicting damage patterns. The effects of bolt torque, clearance and countersink height ratio are investigated, and the analysis results compare well with experimental findings. Furthermore, the analysis provides rich insight into the damage progression and joint behaviour at the ply level, with the in-plane and through-thickness damage patterns mapped for increasing applied load. Delamination is incorporated using a cohesive element layer at the start of the countersunk region, though has minimal influence on damage progression and load-carrying capability, which agrees with the experimental results.