碳纤维复合材料假脚工艺参数对其冲击后疲劳性能的影响

基于三维逐渐损伤理论和有限元法,对碳纤维复合材料假脚的冲击及冲击后疲劳破坏过程进行分析,研究了不同的复合材料体系、几何尺寸、纤维铺设方式等工艺参数对碳纤维假脚的冲击损伤及疲劳性能的影响规律。结果表明,在冲击载荷作用下,碳纤维复合材料假脚的损伤模式主要为基体开裂、纤维压缩和分层;复合材料体系的横向和法向拉伸强度以及剪切强度等参数越小,假脚的冲击损伤面积越大,所能承受的疲劳循环次数越低;随着后龙骨厚度的增加,基体开裂损伤面积越来越大,分层损伤面积略有减小,而纤维压缩损伤几乎没有变化。尽管随着后龙骨厚度的增加,假脚的疲劳循环次数逐渐增大,但是相对于厚度的增加量,疲劳循环次数的增加量相对较小;不同铺层参数对碳纤维复合材料假脚的冲击损伤模式几乎没有影响。适度增加0°铺层的含量,可有效提高碳纤维复合材料假脚的疲劳性能。     

碳纤维复合材料假脚冲击后的疲劳试验

针对一种新型结构的碳纤维复合材料假脚,开展了自由落体冲击试验及冲击后疲劳寿命试验,分析了不同铺层参数、不同冲击能量等因素对其冲击损伤及疲劳寿命的影响规律。结果表明,不同铺层参数对碳纤维复合材料假脚U形结构件的冲击损伤具有显著影响,且随着0°铺层含量的降低,试件的冲击损伤面积增加,外观损伤越来越严重;碳纤维复合材料结构件的疲劳性能对冲击能量比较敏感,随着冲击能量的增加,碳纤维结构件的冲击损伤面积明显增大,其疲劳寿命逐渐降低。在冲击能量从4J增加到10J的情况下,碳纤维复合材料假脚的疲劳寿命大幅度降低了66.8%;尽管随着冲击能量的增加,试件的疲劳寿命逐渐降低,但二者之间并不符合线性关系,即冲击能量存在一门槛值,当冲击能量超过该门槛值后,其对碳纤维结构件疲劳寿命的影响将减弱;仅当碳纤维复合材料假脚的后龙骨厚度为2.7mm,且冲击能量≤4J的情况下,其疲劳寿命才可较好地满足相关安全标准的要求。     

薄面板复合材料蜂窝夹层结构冲击试验

为研究薄面板复合材料蜂窝夹层结构在冲击载荷下的接触力响应和损伤情况,用两种不同质量的冲头对不同面板厚度的复合材料夹层结构进行了多种能量的落锤式冲击试验,并对冲击后的试验件进行了损伤测量。结果表明：冲击能量相对较低时,最大接触力较小,随着冲击能量的增加,最大接触力在增大过程中会出现门槛值,即达到某一值后不再上升。低能量下,冲击损伤表现为面板凹坑和冲击点周围的少量分层,随着冲击能量变大,面板逐渐出现纤维断裂进而被穿透。面板未穿透时,冲头会反弹,接触力-时间曲线的下降段没有台阶,分层区域直径约为冲头直径的1.2倍;面板穿透时,冲头不反弹,接触力-时间曲线下降段出现台阶,分层区域直径约为冲头直径的1.8倍。当最大接触力达到门槛值后,相同冲击能量下,冲头质量越大,冲击持续时间越长,凹坑越深。     

碳纤维复合材料层合板低速冲击损伤应力分析

目的 为掌握碳纤维复合材料板在低速冲击载荷作用下的损伤规律,延缓失效破坏,对其冲击损伤的应力状态进行研究。方法 基于ABAQUS平台,建立碳纤维复合材料层合板低速冲击有限元模型,采用Hashin失效准则和VUMAT用户子程序,对碳纤维复合材料层合板的冲击过程进行数值模拟,同时考虑层合板层内与层间失效,以此来研究低速冲击条件下复合材料的损伤机理,分析冲击损伤过程中的应力变化趋势,讨论应力的分布状态。重点研究铺层角度及铺层距离冲头远近对应力的影响。结果 不同角度铺层的应力传播轨迹均沿着纤维方向和垂直于纤维方向同时扩展,应力均先增加至极限值而后迅速下降;铺层角度越大,板料的承载能力越弱,0°铺层的极限应力为1 432 MPa,而90°铺层的极限应力降至1 206 MPa;离冲头越远的铺层应力越小,达到峰值的时间更早且率先下降,说明远离冲头的铺层更早发生失效。结论 揭示了碳纤维层合板在低速冲击载荷作用下的应力状态及其对损伤的影响规律,能够为复合材料层合板零件设计提供参考。     

碳纤维增强环氧树脂复合材料金字塔点阵夹芯假脚结构在竖向载荷下的力学性能

对碳纤维增强树脂复合材料金字塔点阵夹芯假脚结构在竖向载荷下的力学性能进行研究。制备了三种不同相对密度的假脚,并进行了竖向载荷压缩试验。结果表明,相对密度对结构力学性能的影响显著,载荷-位移曲线呈非线性,峰值载荷和刚度值随相对密度的增加而增大,三种相对密度的破坏模式均为节点的失效和面板的皱曲,结构具有一定的能量吸收能力。建立了金字塔点阵夹芯假脚结构的理论强度预报模型,给出了结构在竖向载荷作用下的挠度响应,获得了四种失效模式和临界破坏载荷。对比了理论计算与试验的峰值载荷、破坏模式和挠度,得到较好的一致性。给出假脚结构参数(面板厚度、杆件角度和杆件直径)对破坏模式和破坏临界载荷的影响,并绘制了结构失效机制图。      

T300/QY8911十字叠层复合材料冲击损伤与破坏的实验研究

对T300碳纤维增强QY8911双马来酰亚胺树脂十字叠层复合材料的冲击损伤与破坏特性进行了实验研究。采用落锤(自由落体)冲击试验方法,测定了三类铺层结构板材的冲击破坏强度和冲击损伤能量门槛值;将冲击强度与破坏模式和静态三点弯曲情况做了对比。用宏、细观方法检测了冲击损伤特性。对低于门槛值的冲击动力学行为给出了分析模型。     

三维正交机织复合材料低周弯曲疲劳力学性能有限元模拟

为了深入理解三维正交机织复合材料（3DOWC）疲劳性能,改进材料抗疲劳设计,结合三维正交机织复合材料试样经纱方向准静态三点弯曲及60%应力水平下的三点弯曲疲劳实验与ABAQUS有限元软件,构建了全尺寸三维实体模型,研究了三维正交机织复合材料在低周循环载荷下的弯曲疲劳性能,经分析得到循环加载下模型应力分布情况和疲劳损伤形态。结果表明：经纱为材料最重要的承载部件,中间加载区域为材料应力集中区,损伤主要位于应力集中区的Z纱通道处的经纱上,随着循环增加,逐渐在中心加载区域的上部和下部形成三角形损伤区域,该研究在复合材料设计与优化中具有指导意义。     

复合材料层合板冲击后压-压疲劳寿命预测方法

针对冲击后复合材料层合板, 发展了含冲击初始损伤层合板的压-压疲劳寿命预测方法。该方法基于无损单向板的力学性能和疲劳特性, 对不同铺层参数、 不同几何尺寸以及不同冲击条件下层合板的疲劳寿命进行预测。为消除人为假设冲击损伤造成的误差, 对层合板在冲击载荷及冲击后疲劳载荷作用下的破坏进行全程分析, 即把冲击后层合板的实际损伤状态直接作为疲劳分析的初始状态。同时基于逐渐损伤思想, 推导了含冲击初始损伤层合板的应力分析过程, 建立了相应的三维逐渐累积损伤模型, 开发了参数化的复合材料层合结构冲击及冲击后疲劳破坏模拟程序, 为复合材料层合结构的抗冲击设计及其疲劳损伤扩展行为研究提供了较好的技术平台。      

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.     

泡沫铝夹层结构复合材料低速冲击性能

以泡沫铝为夹芯材料,玄武岩纤维(BF)和超高分子量聚乙烯纤维(UHMWPE)复合材料为面板,制备夹层结构复合材料。研究纤维类型、铺层结构和芯材厚度对泡沫铝夹层结构复合材料冲击性能和损伤模式的影响规律,并与铝蜂窝夹层结构复合材料性能进行对比分析。结果表明:BF/泡沫铝夹层结构比UHMWPE/泡沫铝夹层结构具有更大的冲击破坏载荷,但冲击位移和吸收能量较小。BF和UHMWPE两种纤维的分层混杂设计比叠加混杂具有更高的冲击破坏载荷和吸收能量。随着泡沫铝厚度的增加,夹层结构复合材料的冲击破坏载荷降低,破坏吸收能量增大。泡沫铝夹层结构比铝蜂窝夹层结构具有更高的冲击破坏载荷,但冲击破坏吸收能量较小;泡沫铝芯材以冲击部位的碎裂为主要失效形式,铝蜂窝芯材整体压缩破坏明显。     

Fatigue tensile behavior of carbon/epoxy composite reinforced with non-crimp 3D orthogonal woven fabric

An experimental study of the in-plane tension-tension fatigue behavior of the carbon fiber/epoxy matrix composite reinforced with non-crimp 3D orthogonal woven fabric is presented. The results include pre-fatigue quasi-static test data, fatigue life diagrams, fatigue damage progression, and post-fatigue quasi-static test data for the warp- and fill-directional loading cases. It is revealed that the maximum cycle stress corresponding to at least 3 million cycles of fatigue life without failure, is in the range of 412-450 MPa for both loading directions. This stress range is well above the static damage initiation threshold and significantly above the first static damage threshold (determined by the onset of low energy acoustic emission). The second static damage threshold, determined by the onset of high energy acoustic emission and related to the appearance of local debonds and intensive transverse matrix cracking falls within this range. The established correlation between a 3000,000 cycle fatigue stress limit on one side and the second static damage threshold stress on the other is of a high practical importance, because it will significantly reduce the amount of future fatigue tests required for this class of composites. Surprisingly, for equal maximum cycle stress level, the fatigue life under fill-directional loading appears about three times shorter than that under warp-directional loading. The 100,000 cycle, 500,000 cycle and 1000,000 cycle fatigue loading with 450 MPa maximum cycle stress has resulted in so high variations of post-fatigue static modulus, strength and ultimate strain, that no consistent and statistically meaningful trends could have been established; further extensive experimental studies are required to reliably quantify this effect.     

基于迟滞耗散能的纤维增强陶瓷基复合材料疲劳寿命预测方法

纤维增强陶瓷基复合材料(CMCs)在疲劳载荷作用下,纤维相对基体在界面脱粘区往复滑移导致其出现疲劳迟滞现象,迟滞回线包围的面积,即迟滞耗散能,可用于监测纤维增强CMCs疲劳损伤演化过程。提出了一种基于迟滞耗散能的纤维增强CMCs疲劳寿命预测方法及考虑纤维失效的迟滞回线模型,建立了迟滞耗散能、基于迟滞耗散能的损伤参数、应力-应变迟滞回线与疲劳损伤机制(多基体开裂、纤维/基体界面脱粘、界面磨损与纤维失效)之间的关系。分析了疲劳峰值应力、疲劳应力比与纤维体积分数对纤维增强CMCs疲劳寿命S-N曲线、迟滞耗散能和基于迟滞耗散能的损伤参数随循环次数变化的影响。疲劳寿命随疲劳峰值应力增加而减小,随纤维体积含量增加而增加;迟滞耗散能随疲劳峰值应力增加而增加,随应力比和纤维体积分数增加而减小;基于迟滞耗散能的损伤参数随纤维体积分数增加而减小。      

Synergistic Effects of Temperature,Oxidation and Multicracking Modes on Damage Evolution and Life Prediction of 2D Woven Ceramic-Matrix Composites under Tension-Tension Fatigue Loading

In this paper, the synergistic effects of temperature, oxidation and multicracking modes on damage evolution and life prediction in 2D woven ceramic-matrix composites (CMCs) have been investigated. The damage parameter of fatigue hysteresis dissipated energy and the interface shear stress were used to monitor the damage evolution inside of CMCs. Under cyclic fatigue loading, the fibers broken fraction was determined by combining the interface/fiber oxidation model, interface wear model and fibers statistical failure model at elevated temperature, based on the assumption that the fiber strength is subjected to two-parameter Weibull distribution and the load carried by broken and intact fibers satisfy the Global Load Sharing (GLS) criterion. When the broken fibers fraction approaches to the critical value, the composite fatigue fractures. The evolution of fatigue hysteresis dissipated energy, the interface shear stress and broken fibers fraction versus cycle number, and the fatigue life S–N curves of SiC/SiC at 1000, 1200 and 1300 °C in air and steam condition have been predicted. The synergistic effects of temperature, oxidation, fatigue peak stress, and multicracking modes on the evolution of interface shear stress and fatigue hysteresis dissipated energy versus cycle numbers curves have been analyzed.     

Monitoring the damage evolution of flexural fatigue in unidirectional carbon/carbon composites by electrical resistance change method

This paper reported simultaneous monitoring damage evolution of flexural fatigue in unidirectional carbon-fiber-reinforced carbon composites (C/C composites) by electrical resistance change (ERC) methods. The degree of irregularity in electrical resistance changes increased with stress levels increasing. The shapes of electrical resistance change rate–fatigue cycle curves can reflect stress levels and damage types of tested samples: sawtooth shapes reflected delamination at a higher stress level; and “peak” shapes reflected inner damages in one fiber bundle at the fatigue limit stress level. In addition, the similarity of initial electrical resistance–fatigue life curve and S–N curve was observed clearly. In summary, ERC methods can monitor the damage evolution and qualitatively estimate the fatigue life of unidirectional C/C composites.     

Low velocity impact and compression after impact characterization of woven carbon/vinylester at dry and water saturated conditions

The low velocity impact response and compression after impact strength of dry and water saturated plain weave carbon/vinylester composites have been determined. The composites employed T700 carbon fibers and vinylester 510A and 8084 resins. Quasi-static impact tests were conducted on dry C/VE510A and C/VE8084 to estimate the threshold impact force required to initiate damage in the composites. Falling-weight impact tests were conducted on the composites over a range of impact energies from 6.7 to 47 J. Destructive inspection of damaged panels revealed damage in the form of matrix cracks as well as delamination between fiber bundles. The quasi-static estimation of the threshold impact force was in reasonable agreement with that measured in the impact test. To examine structural degradation due to impact loading, impacted panels were tested in compression (CAI). The CAI strength decreased with increasing impact energy. Absorbed moisture caused further reductions of the CAI strength.     

Micromechanics modeling of fatigue failure mechanisms in a hybrid polymer matrix composite

Initiation of fatigue damage for a hybrid polymer matrix composite material was studied via 3-Dimensional viscoelastic representative volume element modeling in order to gain further understanding. It was found that carbon fiber reinforced composites perform better in fatigue loading, in comparison to glass fiber reinforced composites, due to the fact that the state of stress within the matrix material was considerably lower for carbon fiber reinforced composites eliminating (or at least prolonging) fatigue damage initiation. The effect of polymer aging was also evaluated through thermal aging of neat resin specimens. Short-term viscoelastic material properties of unaged and aged neat resin specimens were measured using Dynamic Mechanical Analysis. With increasing aging time a corresponding increase in storage modulus was found. Increases in the storage modulus of the epoxy matrix subsequently resulted in a higher state of predicted stress within the matrix material from representative volume element analyses. Various parameters common to unidirectional composites were numerically investigated and found to have varying levels of impact on the prediction of the initiation of fatigue damage.     

Influences of thermal cycling and low-energy impact on the fatigue behavior of carbon/PEEK laminates

Effects of low-energy impact and cyclic thermal loading on fatigue behavior of carbon fiber reinforced polyetheretherketone (carbon/PEEK) laminates have been examined. The fatigue behavior of the virginal composites, low-energy impacted composites, and low-energy-impacted and thermally exposed composites were investigated. Cyclic thermal loading was performed in the temperature range between 60 and −60°C. Residual tensile strength was measured to aid in understanding the influence of low-energy impact on the retention of tensile strength. Fatigue testing involved a stress ratio of 0.1, with a frequency of 3 Hz. The Weibull distribution function was used to evaluate the ultimate tensile strength and fatigue life. S–N curves were plotted and the influence of thermal cycling and the low-energy impact on the fatigue sensitivity of the carbon/PEEK laminates was investigated. Stiffness variation during fatigue testing was monitored and differences in stiffness reduction for three test conditions were compared. C-scan was used to investigate the damage zone under different low-energy impacts and to understand damage propagation during fatigue testing. Moreover, scanning electron microscopy (SEM) was used to examine the fracture morphologies of carbon/PEEK composites in both tensile failure and fatigue failure conditions.     

Fatigue Life Prediction of Carbon Fiber-Reinforced Ceramic-Matrix Composites at Room and Elevated Temperatures. Part I: Experimental Analysis

This paper presents an experimental analysis on the fatigue behavior in C/SiC ceramic-matrix composites (CMCs) with different fiber preforms, i.e., unidirectional, cross-ply and 2.5D woven, at room and elevated temperatures in air atmosphere. The experimental fatigue life S???N curves of C/SiC composites corresponding to different stress levels and test conditions have been obtained. The damage evolution processes under fatigue loading have been analyzed using fatigue hysteresis modulus and fatigue hysteresis loss energy. By comparing the experimental fatigue hysteresis loss energy with theoretical computational values, the interface shear stress corresponding to different peak stress, fiber preforms and test conditions have been estimated. It was found that the degradation of interface shear stress and fibres strength caused by oxidation markedly decreases the fatigue life of C/SiC composites at elevated temperature.