复合材料层合板机械连接结构累积损伤模型和挤压性能试验研究

建立了三维累积损伤有限元模型, 采用扩展拉格朗日乘子法对螺栓表面和复合材料层合板孔壁间的接触行为进行了模拟。对复合材料层合板中纤维断裂、基体开裂和纤维2基体剪切3 类基本损伤类型的产生、扩展以及它们之间的关联性进行了研究。计算得到了复合材料层合板单钉连接结构的载荷2位移曲线, 预测了它们的初始挤压破坏载荷。同时, 进行了T300 帘子布/ Q Y8911 双马来酰亚胺树脂层合板单钉单搭螺栓挤压强度试验,验证了不同铺层类型和结构尺寸对复合材料层合板机械连接挤压性能的影响。试验结果和计算结果吻合较好, 证明了该模型和算法的有效性。      

螺栓连接层合板孔边挤压应力三维分布

孔边应力是复合材料连接设计中强度校核的重要依据,装配间隙对其有明显的影响。针对准各向同性铺层层合板单搭接连接结构,首先采用弹性基剪切梁模型推导了螺杆挠度的解析解,给出挤压载荷沿着层合板厚度方向的分布。在此基础上,将能计及装配间隙的二维情况下孔边挤压应力分布的Persson模型推广到三维情况,得到了孔边挤压应力分布及应力集中系数。理论结果通过三维细节有限元方法进行了验证。最后采用理论方法分析了装配间隙、层合板厚度及钉载对应力集中系数的影响。     

结合改进单胞模型的单钉双剪层合板螺栓连接结构挤压性能的多尺度表征分析

为提高螺栓连接层合板结构的可靠性和承载能力,基于ABAQUS软件及用户子程序(USDFLD),结合改进的单胞模型,建立了考虑组分材料失效的多尺度数值模型。利用该模型表征分析了单钉双剪层合板螺栓连接结构的力学性能,研究了铺层形式及几何尺寸对连接结构性能的影响。该模型的预报结果与试验结果吻合较好。结果表明:准各向同性层合板螺栓连接结构的挤压强度高于正交各向异性层合板连接结构的挤压强度,前者的失效模式为挤压失效,后者为剪出失效,该模式导致结构承载能力降低,设计中应避免。层合板边径比大于3时,不同宽径比连接结构的挤压强度趋近稳定值;但相同边径比的连接结构,其挤压强度随宽径比的增大而增大,连接结构设计时应给予考虑。      

不同孔隙率CFRP层合板静态力学性能研究

为了研究孔隙率对织物碳纤维/环氧树脂复合材料层合板静态力学性能的影响规律,分别测量了孔隙率为0.33%至1.50%的CFRP层合板的弯曲强度和层间剪切强度,并进行有限元模拟.在适用于复合材料单向板的改进Hashin失效准则基础上,建立了适用于织物纤维增强复合材料静态力学强度的失效准则.通过引入复合材料基本强度参数预测不同孔隙率CFRP层合板的力学性能,结合刚度突然退化模型,采用ABAQUS软件建立了有限元模型.试验结果表明,随着孔隙率的增加,复合材料层合板的弯曲强度和层间剪切强度均呈下降趋势.有限元模型较为准确地预测了不同孔隙率织物碳纤维/环氧树脂复合材料层合板的弯曲强度和层间剪切强度.     

搭接长度和铺层方式对CFRP复合材料层合板胶接结构连接性能和损伤行为的影响

针对不同搭接长度和铺层方式的碳纤维增强树脂（CFRP）复合材料层合板单搭胶接结构进行了拉伸试验,观察了试件的受力过程和失效形态,获得了载荷-位移曲线;同时基于连续损伤力学模型和三维Hashin失效准则模拟了CFRP复合材料层合板的层内损伤形成和演化,并利用内聚力模型来模拟层间及胶层的失效损伤,对CFRP复合材料层合板单搭胶接结构在拉伸作用下的失效强度和损伤机制进行了预测,通过对比验证了该数值方法的有效性;通过数值试验比较不同搭接长度和铺层方式的单搭胶接结构及双搭胶接结构的连接强度和损伤行为,并提出了一种优化的CFRP复合材料层合板胶接结构。结果表明:CFRP复合材料层合板胶接结构的极限失效载荷随着搭接长度的增大逐渐增加并趋于稳定值,且结构的失效形式逐渐从胶层自身剪切失效过渡到邻近胶层的层合板层间分层失效;CFRP复合材料层合板胶接结构的连接强度和损伤行为随着铺层方式的不同而改变,通过对3种铺层方式的对比和分析,得到性能最好的铺层方式是[0₃/90₃]_2S;在搭接长度为5~20 mm时,通过对搭接长度进行优化,得到单搭胶接结构的最优搭接长度是17 mm,双搭胶接结构的最优搭接长度是19.3 mm,与搭接长度为20 mm相比,单搭胶接结构和双搭胶接结构的连接强度分别提高了13.26%和0.43%。      

层合板连接结构挤压强度测试方法的研究

利用单钉单剪层合板连接结构挤压性能测试方法和单钉双剪层合板连接结构挤压性能测试方法 (ASTM D 5961A/B),对T700/5429开孔层合板螺栓连接结构的挤压性能进行试验研究,分析不同测试方法对开孔层合板连接结构挤压性能的影响。研究表明:两种试验方法中层合板连接构件均在连接孔处出现有效的挤压破坏形式,但单剪连接试验的稳定性差,影响因素较多,试验数据的离散性较大,且测得的连接结构的极限挤压强度和偏移挤压强度均低于双剪试验的结果。因此,双剪试验方法更适合用于复合材料层合板连接结构设计中挤压许用值的测量。     

湿热环境对碳纤维增强树脂基复合材料力学性能的影响及其损伤机理

采用加速吸湿法研究经3种湿热环境(湿度为85%RH,温度分别为25,70,85℃)处理后CFRP层合板的吸湿特性,对吸湿前后的碳纤维增强树脂基复合材料(CFRP)层合板分别进行拉伸、压缩、剪切实验,研究其力学性能变化规律,利用扫描电镜和红外光谱分析湿热环境中CFRP层板的损伤机理,最后采用最小二乘法拟合提出湿热环境下CFRP层合板力学性能的预测公式。结果表明:CFRP层合板的吸湿初期特性符合Fick定律;相同湿度下环境温度越高,CFRP的吸湿速率和平衡吸湿率越大,达到吸湿平衡所需时的间越长;3种湿热环境处理后的CFRP层板的90°拉伸和剪切力学性能下降最明显;经湿热环境处理后水分子通过氢键与环氧树脂发生缔合,但CFRP层合板中的各组分未发生化学结构变化;拟合建立的不同湿热条件下力学性能衰退公式与实验结果基本一致。     

干涉对复合材料层合板连接系统的极限挤压强度影响

本文对复合材料层合板单面螺纹抽钉紧固件干涉配合连接结构和高锁螺栓间隙配合连接结构的静挤压强度进行了试验研究.分别考虑了不同干涉量配合和间隙配合对紧固件连接结构极限挤压强度的影响.在静拉伸试验中,采用了卡口引伸计来实时监测孔的变形量.根据ASTM D 5961挤压试验方法标准,得到极限挤压强度及偏移量为2%的2%偏移挤压...     

不同湿热条件下碳纤维/环氧复合材料湿热性能实验研究

对比研究了环氧5228A树脂及碳纤维/环氧5228A树脂复合材料层合板在3种湿热环境(水煮、70℃水浸,70℃85%相对湿度)下的湿热性能,考察了湿热条件对复合材料层间剪切性能的影响规律,并从吸湿特性、物理化学特性、树脂力学性能、湿应力等方面分析了不同湿热环境下复合材料性能衰减的机制。研究表明,碳纤维/高温固化环氧树脂复合材料层间剪切性能主要是由吸湿率决定,相同吸湿率不同湿热条件下性能的下降幅度基本相同;3种湿热条件下该树脂及其复合材料未发生化学反应、微裂纹等不可逆变化,复合材料层合板湿热老化机制主要是吸入水分后基体增塑和树脂、纤维湿应变不一致导致的湿应力对复合材料性能的负面作用。     

复合材料层合板单钉双剪连接挤压强度的一种工程估算方法

将上限理论应用到复合材料层合板单钉双剪连接挤压强度分析中, 把连接结构的位移场划分为动态区域(层合板)和静态区域(紧固件), 并认为失效发生在位移可动场和不动场之间的钉孔边受挤压部分。由于受挤压孔孔边各层应力状态不一样, 受挤压孔边各层的失效区域和失效模式也各不相同。从宏观上研究复合材料层合板单钉连接孔边的失效区域和失效模式, 结合上限理论提出了一种估算复合材料单钉连接挤压强度的工程算法。通过与试验结果对比, 发现该方法能较好地预测出复合材料单钉双剪连接挤压强度。     

TC4钛合金构件电磁铆接极限间隙量研究

维修中拆卸原铆钉将导致钉孔间隙明显增大,进而直接影响维修后铆接构件性能.为了实现大间隙量下钛合金铆接构件的有效维修,本文通过试验对TC4钛合金构件电磁铆接极限间隙量进行了研究,并评估了铆接构件拉剪性能.结果表明:电磁铆接通过绝热剪切成形的方式,能有效实现大间隙量下钛合金构件的干涉铆接;随着钉孔间隙量的增加,镦头直径和高...     

Fatigue resistance and residual strength of riveted joints in FML

Static and fatigue tests have been carried out on 190-mm-wide fibre metal laminate (FML) riveted lap joints. The specimens were made with a Glare 3-3/2-0.3 material, i.e. a laminate composed of three layers of 2024-T3 aluminium alloy, thickness 0.3 mm and two double layers (0/90) of pre-preg FM-94-27%-S2 glass fibre. Seven configurations were tested which differed in the number of rivet rows (two or three), in the rolling direction of metal layers (perpendicular or parallel to the load direction), in the rivet type (solid or special, such as Hi-Lok or Lock-Bolt), in the rivet material for solid rivets (7050-T73 or 2017-T3), in the rivet diameter (4.8 or 5.5 mm) and in the presence of interlaminar doublers in the overlap area (titanium, aluminium, glass fibre). An additional difference was in the pre-formed rivet head: solid rivets had countersunk head, while Hi-Loks and Lock-Bolts had protruding head. The fatigue tests demonstrated the efficiency of a selective local reinforcement in the overlap area; in some cases, the fatigue resistance was so high that fatigue cracks nucleated in the laminates, rather than in the overlap area, as commonly expected.     

Instantaneous mechanical fastening of quasi-isotropic CFRP laminates by a self-piercing rivet

A modified self-piercing rivet (SPR) has been proposed to mechanically fasten CFRP laminates. The modified SPR consists of a rivet body and two flat washers. The two flat washers were used to suppress delamination in the CFRP laminates at the point of piercing. The advantages of the modified SPR for fastening CFRP laminates are instantaneous process time and low cost. Any pretreatments such as surface treatments or hole drilling are not required. In this study, the viability of the modified SPR for a quasi-isotropic CFRP laminate was investigated by tensile and fatigue tests on the single lap joints. The experimental results showed that the tensile strength of a modified SPR joint was slightly higher than a bolted joint. In tension–tension fatigue tests, a fatigue limit at N_f = 10⁷ cycles was about 50% of the tensile joint strength. Experimental results showed that the modified SPR was one of the promising fasteners for future mass-production CFRP automobiles.     

Near surface mounted CFRP laminates for shear strengthening of concrete beams

A Near Surface Mounted (NSM) strengthening technique was developed to increase the shear resistance of concrete beams. The NSM technique is based on fixing, by epoxy adhesive, Carbon Fiber Reinforced Polymer (CFRP) laminates into pre-cut slits opened in the concrete cover of lateral surfaces of the beams. To assess the efficacy of this technique, an experimental program of four-point bending tests was carried out with reinforced concrete beams failing in shear. Each of the four tested series was composed of five beams: without any shear reinforcement; reinforced with steel stirrups; strengthened with strips of wet lay-up CFRP sheets, applied according to the externally bonded reinforcement (EBR) technique; and two beams strengthened with NSM precured laminates of CFRP, one of them with laminates positioned at 90° and the other with laminates positioned at 45° in relation to the beam axis. Influences of the laminate inclination, beam depth and longitudinal tensile steel reinforcement ratio on the efficacy of the strengthening techniques were analyzed. Amongst the CFRP strengthening techniques, the NSM with laminates at 45° was the most effective, not only in terms of increasing beam shear resistance but also in assuring larger deformation capacity at beam failure. The NSM was also faster and easier to apply than the EBR technique. The performance of the ACI and fib analytical formulations for the EBR shear strengthening was appraised. In general, the contribution of the CFRP systems predicted by the analytical formulations was slightly larger than the values registered experimentally. Performance of the formulation by Nanni et al. for NSM strengthening technique was also appraised. Using bond stress and CFRP effective strain values obtained in pullout bending tests with NSM CFRP laminate system, the formulation by Nanni et al. predicted a contribution of this CFRP system for the beam shear resistance of 72% the experimentally recorded values.     

Evaluation of impact damage mechanism of multi-axial stitched CFRP laminate

This study focuses on multi-axial stitched fabric, which is a thick, high performance reinforcement for large-scale composite structures. The effects of impact damage on multi-axial stitched CFRP laminates molded by vacuum-assisted resin transfer molding (VARTM) method were evaluated. Impact damage within material was evaluated by ultrasonic scanning device and optical cross-sectional observations. Probed images obtained by both non-destructive and destructive methods were compared, and internal damage distributions of multi-axial stitched CFRP laminates were clarified. In addition, residual compressive strength and fatigue property of impact-damaged CFRP laminates were evaluated by in situ damage growth monitoring using the thermo-elastic stress analyzer (TESA). Three-dimensional damage distribution of impacted CFRP laminate was obtained from ultrasonic C-scan images and cross-sectional photographs. Damage progress behavior was observed on a destructive and non-destructive basis by post-impact fatigue (PIF) test.     

Numerical experimental analysis of hybrid double lap aluminum-CFRP joints

Due to their reliability and ease of assembly, both the adhesively bonded and the mechanical joints are commonly used in different fields of modern industrial design and manufacturing, to joint composite materials or composites with metals.As it is well known, adhesively bonded joints are characterized by high stiffness and good fatigue life, although delamination phenomena localized near the free edges may limit their use, especially for applications where corrosive environments and/or moisture can lead to premature failure of the bonding. In these cases, a possible alternative is given by the well-known mechanical joints. On the contrary, these last joints (bolted, riveted) require a preliminary drilling of the elements to be joined, that may cause localized material damage and stress concentration, especially for anisotropic laminates characterized by high stress concentration factors and easy drilling damaging, with significant decrease of the load-carrying capacity of the joined elements. In order to exploit the advantages of the bonded joints and those of the mechanical joints, both industrial manufacturing and research activity have been focused recently on the so called hybrid joints, obtained by the superposition of a mechanical joint to a simple adhesively bonded joint.In order to give a contribution to the knowledge of the mechanical behavior of hybrid bonded/riveted joints, in the present work a numerical–experimental study of bonded/riveted double-lap joints between aluminum and carbon fiber reinforced polymer (CFRP) laminates, has been carried out. It has permitted to highlight both the static and the fatigue performance of such joints obtained by using aluminum and steel rivets, as well as to known the particular damage mechanisms related also to the premature localized delamination of the CFRP laminate due to the riveting process.     

Performance of steel beams strengthened with CFRP laminate – Part 2: FE analyses

A new method for repairing and strengthening steel is under development and consists of using CFRP (carbon-fibre-reinforced-polymer) laminates bonded to the steel substrate. Research on this method has been conducted by a few research groups in recent years. The idea is to let the CFRP laminate carry a large part of the stresses and thereby reduce the load on the steel, which may have had its capacity lowered due to deterioration or fatigue. The present paper presents the results of FE analyses of steel beams strengthened with bonded CFRP laminates. The interfacial shear and peeling stresses that appear in the bond line between the steel and CFRP laminate are studied in both the elastic and plastic phase of the steel beam. Comparisons with the results obtained from laboratory tests conducted on steel beams strengthened with bonded CFRP laminates show that the behaviour of the strengthened beams can be captured using FE analyses. The distribution of the shear and peeling stresses near the end of the bond line were obtained from the FE analyses, together with the interfacial stresses that develop near beam mid-span due to the yielding of the steel. These stresses may exceed the capacity of the adhesive and cause debonding in this region.     

Novel test method for accurate characterization of intralaminar fracture toughness in CFRP laminates

A novel initial crack insertion method, “intralaminar film insertion method”, was proposed to investigate the fracture toughness of unidirectional carbon fiber reinforced plastic (CFRP) laminates when the crack propagates inside the ply and not in the interlayer resin-rich area. Here, a release film was inserted inside a single lamina during the resin impregnation process of prepreg manufacturing. Mode I intralaminar fracture toughness tests were carried out for conventional CFRP laminates and interlayer toughened CFRP laminates. For comparison, two conventional methods were used to introduce initial cracks. One is the “interlaminar film method”, where a release film is inserted between two prepreg plies during the lay-up process. The other is the “machined slit method”, where a slit notch is machined in parallel to the layer of CFRP laminates. It was demonstrated that the proposed “intralaminar film method” can correctly evaluate the intralaminar fracture toughness of both conventional CFRP laminate and interlayer toughened CFRP laminate from the initial value to the propagation value. For this range, it was also found that the intralaminar fracture toughness of interlayer toughened CFRP laminate was the same as that of conventional CFRP laminate. Thus, the intralaminar fracture toughness was not influenced by interlayer toughening.     

碳纤维/铝/环氧复合板的初步研究

本文揭示了碳纤维/铝/环氧复合板的显微结构,比重、热胀性能与碳纤维含量的关系,探讨了增强组份的表面处理、碳纤维/铝的叠层结构设计、碳纤维含量、纤维混杂对复合板力学性能的影响。试验表明:碳纤维/铝/环氧复合板具有轻质、低热胀、高强度等特点,当碳纤维增强环氧的含量为55vol%时,它的拉伸,弯曲与剪切强度达到或超过芳纶纤维或玻璃纤维/铝/环氧复合板的相应值。