3钉带衬套复合材料/金属接头拉伸疲劳性能试验研究

带衬套沉头螺栓连接已经在复合材料连接结构中得到了一定的应用,需要对其疲劳性能进行研究.本研究对一种单搭接3钉带衬套碳纤维复合材料/钛合金沉头螺栓连接接头进行了静态拉伸试验,测量了接头的载荷-位移曲线、拉伸极限强度与条件挤压强度.在此基础上,确定67％接头极限载荷为拉伸疲劳最大载荷,按应力比为0.1的循环载荷对接头进行疲劳试验,并与对应的无衬套接头进行了对比,研究了衬套对该接头疲劳性能的影响.结果表明,衬套的引入改善了应力分布情况,使结构疲劳寿命延长了98.4％.同时,静态拉伸试验中发生层合板的钉孔挤压以及净截面拉伸破坏,疲劳试验中发生钛合金板的拉伸疲劳破坏,部分无衬套接头还发生了螺栓疲劳破坏.经分析发现,两类材料的疲劳性能表现差异较大,复合材料/金属机械连接接头的疲劳破坏模式会因载荷水平的不同而发生变化,在低于一定载荷水平下容易出现金属结构的破坏.     

复合材料层板单钉沉头螺栓连接结构拉伸性能EI北大核心CSCD

通过ABAQUS软件建立三维累积损伤有限元模型,计算复合材料层板单钉沉头螺栓连接结构的条件挤压载荷,并对其损伤机理与破坏过程进行分析;选用适合该类结构层板的损伤判据与衰减准则进行验证。计算获得的初始挤压破坏载荷与实验结果吻合良好,说明所建模型的有效性。在此基础上,分析了接触面摩擦、螺栓刚度和连接金属板刚度等因素对连接结构拉伸性能的影响。结果表明:接触面摩擦因数和螺栓刚度对结构拉伸性能的影响较为明显,金属板刚度的影响很小。     

复合材料单钉接头疲劳累积损伤破坏分析

基于时间增量原理 , 推导了层合板接头疲劳加载累积损伤应力2应变分析的虚功方程。同时 , 引入Hashin三维疲劳失效准则进行材料的损伤判定 , 并结合建立的疲劳加载材料退化模型、 4种基本损伤机制相互关联作用的材料性能退化方法及复合材料接头最终失效判据 , 建立了层合板接头疲劳载荷作用下三维累积损伤分析的寿命预测方法。最后 , 对层合板接头拉2拉疲劳载荷作用下的损伤累积扩展与失效规律进行了仿真分析 , 并与试验结果进行了对比 , 结果表明 : 本文中建立的寿命预测方法能够很好地预测层合板接头的寿命以及损伤发生、扩展及最终失效。      

吸湿后碳纤维复合材料正交层板拉伸疲劳性能

湿环境下复合材料疲劳性能会严重退化,影响结构的使用安全,在确定复合材料结构寿命时须考虑湿环境的影响.对常温下湿态和干态下碳纤维复合材料正交层合板的拉伸疲劳性能进行实验,研究饱和吸湿对正交层合板拉伸疲劳性能的影响,获得了两种环境下层合板的S-N曲线.在此基础上,建立有限元模型,并对吸湿后正交层合板的疲劳寿命和损伤演化情况进行预测,计算结果与实验结构吻合较好,证明了模型的有效性.结果表明,饱和吸湿对正交层合板的拉伸疲劳性能影响很大.吸湿后正交层合板的拉伸疲劳寿命明显低于干态情况,而且S-N曲线的斜率稍低,层合板的纤维损伤起始与扩展情况与干态情况也有较大差别.     

碳纤维/环氧树脂复合材料缠绕接头拉伸失效机制

通过试验及数值模拟对碳纤维/环氧树脂复合材料缠绕接头轴向拉伸失效机制进行研究。基于ABAQUS有限元软件,通过连续介质损伤模型及内聚区模型,分别对碳纤维/环氧树脂复合材料缠绕接头各部件及界面进行模拟,编写用户自定义材料子程序(UMAT),建立复合材料的渐进损伤模型,最终得到碳纤维/环氧树脂复合材料缠绕接头的应力分布和载荷-位移曲线,并与试验结果对比确定结构的失效机制。结果表明:有限元分析所得碳纤维/环氧树脂复合材料缠绕接头损伤部位及失效模式与试验吻合,失效载荷与试验值相差较小,证明仿真分析方法的有效性。通过对比失效模式发现,拉伸载荷作用下,链环是主承力部件,其弧形端部是应力集中处,纤维断裂即从此处开始发生并向外扩展,导致链环断裂及整体结构破坏。      

纤维增强复合材料机械连接接头疲劳研究进展

综述了纤维增强复合材料机械连接接头疲劳研究进程、影响因素及破坏方式,同时对目前公开发表的几种预测复合材料接头疲劳寿命方法进行了介绍与分析.最后,对今后研究动向进行了展望.     

复合材料连续损伤力学模型在螺栓接头渐进失效预测中的应用

提出考虑层合板面内(纤维和基体失效)和层间失效的复合材料连续损伤力学模型,对螺栓接头的渐进失效行为进行预测。基于Tsai-Wu强度准则,发展可以判定复合材料面内和层间失效的强度准则。采用幂指数衰减材料退化模型模拟复合材料的损伤扩展过程。建立连续损伤力学模型用以研究0°铺层比例和螺栓直径对复合材料螺栓接头挤压性能的影响,预测结果与实验结果吻合。结果表明:0°铺层比例过高,接头发生剪切破坏,降低连接结构承载能力;增大螺栓直径,层合板损伤受到抑制,可提高复合材料螺栓接头的挤压强度。      

基于渐进损伤模型的复合材料斜接接头的拉伸强度

本文首先以斜接角度、搭接长度以及搭接厚度为参数分别建立了复合材料单边斜接接头和双边斜接接头在拉伸载荷下的参数化有限元模型,采用abaqus软件中UMAT子程序功能编写了材料损伤本构模型,并利用cohesive单元模拟胶接界面损伤。通过渐进损伤分析,模拟出斜接接头的三种破坏模式,最终得到了各参数变化对斜接接头拉伸强度的影响,其中母板最终破坏模式对应最大拉伸强度,并对单边斜接接头和双边斜接接头结果进行了对比分析,得出双边斜接接头强度大于单边斜接接头强度的结论。     

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。      

复合材料蜂窝夹层结构T型接头拉伸性能研究

通过试验测量了复合材料蜂窝夹层结构T型整体接头的拉伸性能,得到其拉伸强度与破坏模式。建立了接头结构有限元模型,利用分类损伤判据、失效准则与刚度退化准则对结构的损伤情况进行模拟,研究了接头的拉伸破坏行为。有限元分析结果与试验结果吻合良好。研究结果表明,结构的薄弱点位于腹板内靠近蒙皮的位置。蜂窝在此处发生面外拉伸破坏,从而导致结构的最终破坏。腹板上的拉伸载荷主要通过过渡区填料传递给蒙皮,腹板与蒙皮间的搭接段对载荷传递的贡献较小。参数研究表明,对于复合材料蜂窝夹层结构T型接头,搭接段长度对结构的强度几乎没有影响,而增大蒙皮蜂窝的高度或采用低模量蜂窝可以提高结构强度。     

Spectrum fatigue of composite bolted joints

The new Swedish fighter JAS39 Gripen has a large number of primary structures made of composites. On those structures a large number of bolted joints are used which during the aircraft's service life will be subjected to spectrum fatigue loading. Consequently it is important to study the spectrum fatigue life of bolted joints. Specimens with a double-lap configuration and six bolts have been fatigue loaded at the load ratios R=−0.2 and R=−5. Specimens were also fatigue loaded with a vertical fin spectrum which had different amounts of elimination of load cycles. A linear damage rule, Miner's rule, was used to predict the spectrum fatigue life. The experimental results show that the shortest fatigue life occurs for specimens loaded at R=−1 followed by specimens loaded at R=−0.2. The longest fatigue life occurred for specimens loaded at R=−5. It was found that 50% elimination of load cycles in the spectrum can be used without affecting the fatigue life. The Miner's rule predictions appeared to overestimate the spectrum fatigue life. From bolt failure it was found that the first bolt row transfers the largest amount of load in the specimens.     

基于振动疲劳试验的复合材料螺栓连接预紧力松弛特性

在强迫弯曲振动试验的基础上,建立了基于模态参数(共振频率和阻尼比)表征螺栓连接结构动态性能的分析方法和试验测试手段;通过施加不同初始预紧力和激振频率,探究碳纤维/环氧复合材料螺栓连接预紧力松弛的时变行为及其影响因素。结果表明:在10h振动疲劳过程中,螺栓初始预紧力越小,激振频率越大,连接件预紧力松弛程度越大;振动疲劳损伤会导致连接结构刚度衰退、阻尼增加;复合材料螺栓连接松弛受到材料黏弹性以及界面摩擦的共同影响,其中约50%的松弛是由复合材料黏弹性效应引起的。      

CFRP/titanium hybrid material for improving composite bolted joints

The structural joining remains an essential challenge for the development of composite aerospace structures: every structural interconnection means a disturbance of an optimized structure resulting in an increase in overall structural weight. The lightweight potential of advanced, high-performance fiber composite materials is affected more strongly by mechanical fastening techniques than by conventional metallic materials due to the low shear and bearing capabilities of CFRP materials. Local embedding of thin titanium layers into the composite laminate in the coupling region results in a considerable improvement in structural efficiency of bolted and riveted joints in CFRP structures. This improvement is not only obvious in the increase in shear and bearing capabilities, but also in the resulting possibilities for a design no longer burdenend by local material thickening, excentricities and additionally excited local bending stresses.

This report displays experimental results demonstrating the advantageous influence of titanium hybridization on specific characteristics of CFRP-materials, thus proving the mechanical potential of CFRP/titanium hybrid materials when used as an advanced reinforcement technique for highly loaded composite joints.     

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.     

Bearing failure of composite bolted joints with non-uniform bolt-to-washer clearance

This paper investigates the effect of the relative positions of the bolt and the washer on bearing failure behavior of bolted composite joints with various lateral constraints. Two extreme diametral fit positions, with a positive or negative bolt hole-to-washer clearance, were considered. Experimental results show that the relative location can affect the initial failure loads but not the ultimate failure loads. The measured failure loads were also used to validate an existing analytical model.     

On the prediction of bolted single-lap composite joints

A new set of failure criteria to predict composite failure in single-lap bolted-joints is proposed. The present failure criteria are an extension of Chang–Lessard criteria considering a three-dimensional stress field and including out-of-plane failure modes. The advantage with respect to other three-dimensional failure criteria is the consideration of non-linear shear stress–strain relationship. The failure criteria were implemented in a finite element model and validated through comparison with experiments in literature. Stresses were calculated by a non-linear finite element model developed in ABAQUS/Standard which considers material and geometric nonlinearities. A progressive damage model was implemented in a USDFLD subroutine. The model predicted the effect of secondary bending and tightening torque showing an excellent agreement with experimental results. Moreover, results were compared with those reported in literature using Hashin failure criteria. In addition, a parametric study was carried out to analyse the influence of friction coefficient and tightening torque.     

The static and fatigue strength of bolted joints in composites with hygrothermal cycling

The effects of hygrothermal cycling upon the performance of a bolted composite joint was examined. Bolt torque relaxed as the number of environmental cycles increased. Comparison with analytical results suggested that the bolt torque “zigzag” behavior probably results from the natural sensitivity of bolted composite joints to the existing ambient temperature and moisture. The washer effect and specimen surface finishes were investigated to study the friction effect on joint bearing performance. Fatigue tests of specimens exposed to hygrothermal cycling exhibited greater hole elongation than specimens not exposed. High preload does improve static failure strength and the fatigue life of specimens under room conditions. Tests run on IM6/3501-6 material specimens with hygrothermal cycling show reasonable declines in fatigue life. The moisture weight gains of composites under hygrothermal cycling exposure were measured and compared to numerical results; good correlation was obtained. As a result, bolt failure occurred in some fatigue tests and is most frequently associated with large thickness-to-diameter ratio. The peak-to-peak stress was defined to study the effect of the R-ratio on bolts for the bolt failure problem.     

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。     

Improving bearing performance of composite bolted joints using z-pins

The effect of z-pins on the bearing properties and damage tolerance of composite bolted joints is experimentally studied in this paper. The region around bolt-holes in carbon/epoxy laminates was reinforced in the through-thickness direction with different volume contents and sizes of fibrous z-pins. Bearing test results show that the z-pins improved the bearing stiffness (by 7.5–9.6%), ultimate load (7.7–12.8%), failure strength (7.4–9.8%), and elastic strain energy absorption to bearing failure (8.5–16.3%) of the composite joints. The bearing properties increased at a quasi-linear rate with the z-pin content, but were not dependent on pin diameter. Stiffness is improved by z-pins increasing the through-thickness tensile modulus around the bolt-hole of the joint. Post-mortem microstructural examination of the failed joint specimens revealed that z-pins improve the bearing strength by reducing cracking near the bolt-hole via an interlaminar bridging toughening mechanism that involves debonding and frictional sliding of pins within the damaged region. The elastic strain energy to failure is increased by the through-thickness stiffening and toughening provided by the z-pins. This study proves that the reinforcement of bolt-holes with z-pins increases the bearing properties without the weight penalty incurred with the traditional strengthening method of thickening the laminate around holes.