2.5D机织复合材料经向和纬向振动疲劳行为对比

2.5D机织复合材料抗分层、耐冲击,在航空发动机结构上具有巨大的应用前景。本文对一种2.5D机织碳纤维增强双马树脂基复合材料经向和纬向试件,开展了不同名义应力水平下的一阶弯曲共振疲劳试验。试验结果表明：经向试件的振动疲劳性能优于纬向试件,随着应力水平的提高,经向和纬向试件的寿命明显缩短,而固有频率下降百分比增加,试件内部的损伤严重程度和损伤扩展速度都随之提高。2.5D机织复合材料经向和纬向试件在共振疲劳试验过程中的主要失效模式是纱线与基体之间脱粘造成的结构完整性丧失,从而导致试件的刚度持续下降。试件内部损伤的三维电子计算机断层扫描(Computerized tomography,CT)重构图像表明,损伤散布于试件工作段区域,应力水平越高,2.5D机织复合材料经向和纬向试件内部损伤范围越大,损伤程度越高,而且纬向试件内部损伤状态比经向试件严重。利用双对数线性寿命模型,对经向和纬向试件在不同名义应力水平下的共振疲劳试验数据进行拟合,得到2.5D机织复合材料经向和纬向试件共振疲劳应力-寿命(Stress-life,S-N)曲线的数学模型,得到的S-N曲线可用于预测2.5D机织复合材料的寿命。     

三维机织碳/碳复合材料双轴压缩载荷下的力学行为

基于三维机织碳/碳复合材料的细观结构特征, 设计平板十字形试样, 在材料双轴力学性能试验机上开展了复合材料单轴、 双轴加载压缩试验, 对比分析了三维机织碳/碳复合材料在双轴压缩载荷下的力学行为。研究表明: 三维机织碳/碳复合材料的压缩行为表现为非线性、 脆性断裂; 双轴载荷作用下非线性特征更为显著, 压缩模量随应力的增加而增大, 强度与模量相较于单轴有较大幅度增加, 双轴压缩载荷作用下材料的强化效应显著; 试样破坏位置并未出现在试样中心区, 而是发生在试样的加载端部或十字形试样的加载分枝根部, 主要表现为基体开裂、 纤维断裂和层间脱粘, 碳布及其层间界面剪切强度的强弱直接影响材料的压缩强度。     

三维角联锁机织铝基复合材料面内拉伸力学行为与失效机制

针对真空压力浸渗法制备的三维角联锁机织铝基复合材料,采用细观力学有限元模拟与试验结合的方法研究了其面内拉伸变形损伤与断裂力学行为。结果表明:复合材料拉伸应力-应变曲线的计算与试验结果吻合较好,经(纬)向拉伸初始弹性模量、极限强度和断裂应变的计算误差分别为3.96%(1.11%)、1.40%(6.86%)和?5.49%(3.73%);经向拉伸载荷作用下,经纱界面及其邻近基体合金先后发生损伤,随拉伸应变增加损伤累积和交互作用依次引发界面、基体和纬纱失效,变形后期经纱的断裂最终导致复合材料经向拉伸失效;纬向拉伸变形前期,经纱界面和经纬纱之间薄弱的基体合金相继产生损伤和失效现象,经纱在变形中期即出现横向破坏,起主要承载作用的纬纱轴向断裂是纬向拉伸的主要失效机制,由于三维角联锁机织体中纬纱体分远低于经纱,复合材料纬向拉伸模量和强度分别仅为经向的81.8%和56.5%。      

碳纤维三维角联锁机织复合材料弯曲作用下力阻响应

电阻法在碳纤维复合材料结构健康监测(SHM)中具有巨大应用前景。本文研究了碳纤维三维角联锁机织复合材料经向和纬向试件在弯曲作用下力-电阻响应,探究电阻变化与复合材料结构损伤的相关性。试验结果表明：经向和纬向试件在弯曲作用下电阻变化与试件主要承载纱线损伤情况具有相关性。准静态三点弯曲加载下,试件电阻变化可以反映试件承载能力变化：在最大载荷点之前,试件电阻基本不变;主要承载纱线发生断裂损伤时,电阻增加。弯曲疲劳加载下,试件电阻变化可以反映试件承载能力退化情况：在弯曲疲劳加载前期,三维角联锁机织复合材料呈现负压阻效应;随着循环次数增加,基体裂纹、界面脱粘等不可逆损伤不断累积,电阻缓慢增大;在弯曲疲劳加载后期,主要承载纱线断裂,电阻显著增加;试件最终疲劳失效时,电阻急剧增加。     

三维正交机织复合材料的动态压缩性能

本文作者利用分离式Hopkinson压杆装置对玻璃纤维三维正交机织复合材料进行了高应变率下面外、面内方向的压缩试验,并在万能试验机下进行了相应的准静态压缩。获得3 个高应变率及准静态下的应力-应变曲线,观察了试样的破坏形貌。结果表明：玻纤三维正交机织复合材料是应变率敏感材料,最大应力、压缩模量随着应变率的增大而增大。三维正交结构使复合材料体现出各向异性：面外的最大应力、失效应变比面内大;面内的压缩模量大于面外,且压缩模量对于应变率的变化比面外方向敏感;经纬向相比,纬向的最大应力大于经向。     

复杂面内应力状态下平面编织高铝纤维增强氧化铝基复合材料强度及疲劳寿命预测方法

针对平面编织氧化铝基复合材料提出了一种复杂面内应力状态下的强度准则和疲劳寿命预测方法。通过拉伸、压缩及纯剪切试验,分别获得了材料的静强度指标。考虑材料拉、压性能的差异和面内拉-剪联合作用对材料强度的影响机制,提出了修正的Hoffman强度理论。采用该强度理论预测得到的偏轴拉伸强度与试验结果基本一致,偏差不超过10%。开展了偏轴角θ=0°、15°、30°、45°,应力比R=0.1,频率f=10 Hz的拉伸疲劳试验,试验结果表明随着偏轴角的增加,相同轴向拉伸载荷下的疲劳寿命逐渐降低。由于面内剪切应力分量的作用,疲劳失效由纤维主导逐渐过渡到纤维和基体共同主导的模式。基于单轴疲劳寿命曲线,采用Broutman-Sahu剩余强度模型表征剩余强度随疲劳循环次数的变化规律,结合剩余强度演化模型和修正的Hoffman强度理论,提出了一种面内复杂载荷条件下的疲劳寿命预测模型,并引入疲劳剪切损伤影响因子表征拉-剪应力联合作用对材料疲劳行为的影响。采用本文提出的疲劳寿命预测模型,预测不同偏轴角拉伸疲劳寿命,预测结果与试验结果基本一致,偏差在1倍寿命范围内。比较结果表明在给定应力比、温度和疲劳载荷频率条件下,该疲劳寿命预测模型可以用来预测平面编织氧化铝基复合材料拉-剪复杂面内载荷条件下疲劳寿命。      

2.5维机织复合材料强度准则

2.5维机织复合材料已有较为广泛的应用,目前对该类复合材料强度理论的研究还相对较少。根据2.5维机织复合材料拉伸破坏的细观机理,基于单向复合材料的三维Hoffman准则,建立了2.5维机织结构复合材料拉伸破坏准则,通过对2.5维机织复合材料3种结构24个试件进行拉伸试验,与计算预测结果的对比表明了本文建立的强度准则的合理性。研究表明,纤维拉伸断裂是2.5维机织复合材料拉伸破坏的主要原因;相比基于最大应力准则、Hashin准则建立的强度准则,基于Hoffman准则建立的强度准则综合考虑了纤维在外载荷作用下各应力分量对纤维断裂破坏的影响,其预测结果与实际试验结果更为接近;在其他条件不变情况下,随纱线取向角增大,纱线拉伸断裂应力呈非线性降低。随纱线纤维体积含量增加,纱线拉伸断裂应力成线性增加。     

高强度钢30Cr3SiNiMoVA的疲劳特性研究

为了获得超高强度钢30Cr3SiNiMoVA的疲劳设计数据,对该材料进行了疲劳性能实验。分别对该材料的抛光和未抛光试样进行轴向拉压疲劳实验。用升降试验法测定材料的条件疲劳极限,用成组实验法在四级应力水平下测试其疲劳寿命。结合升降法所得的条件疲劳极限值和成组法实验数据,采用双参数最小二乘法拟合S-N曲线,运用概率方法求得P-S-N曲线。通过对两种试样的S-N曲线分析可知:未抛光试样的疲劳寿命要低于抛光试样的疲劳寿命,并且疲劳载荷级越低,两类试样之间的疲劳寿命差越大,抛光试样的疲劳极限大约是未抛光试样的1.4倍。相关实验现象表明30Cr3SiNiMoVA的疲劳寿命对试样表面粗糙状态非常敏感,建议该类材料在应用过程中要采取有效措施改善构件的表面状态。文中实验数据可为该材料的工程应用提供参考。     

三向正交纤维增强铝基复合材料经向拉伸渐进损伤及其断裂力学行为

用真空压力浸渗法制备了新型三向正交碳纤维增强铝基(CF/Al)复合材料,根据其内部纱线截面形状和机织结构特征建立了考虑界面作用的细观力学有限元模型,并将数值模拟与实验相结合研究了复合材料在经向拉伸载荷作用下的渐进损伤与断裂力学行为。结果表明,铝基复合材料拉伸弹性模量、极限强度与断裂应变的实验结果,分别为120.7 GPa、771.75 MPa和0.83%。数值模拟的计算误差分别为-3.21%、1.75%和-9.63%,宏观应力-应变曲线的计算结果与实验曲线吻合得较好。在经向拉伸载荷作用下复合材料的基体合金与Z向纱之间的界面先发生失效,随着拉伸应变量的增大纱线交织处基体合金的损伤逐渐累积并先后发生Z纱和纬纱的局部开裂失效,在拉伸变形后期基体合金的失效和经纱断裂最终使复合材料失去承载能力。铝基复合材料的拉伸断口呈现出经纱轴向断裂以及纬纱和Z向纱横向开裂的形貌,起主要承载作用的经纱其轴向断口较为平齐且纤维拔出长度较短,复合材料经向拉伸时表现出一定的脆性断裂特征。     

2.5D机织复合材料的渐进损伤与失效模拟

为了研究典型2.5D机织复合材料的压缩性能,开展了复合材料单胞结构的经向和纬向压缩实验,并通过对材料编织结构的细观表征,建立了细观尺度的单胞有限元模型来模拟压缩载荷下单胞内部的变形及渐进失效过程。结果表明,2.5D机织复合材料在受压时表现出明显的非线性力学响应,材料沿经向的压缩模量和强度均高于纬向;经向压缩时材料的主要破坏模式有经纱的横向开裂、纤维束间的界面分层破坏、纬纱的压溃及基体的开裂,纬向压缩时出现的主要破坏模式是纬纱的压溃、纬纱纤维束的断裂及基体开裂;通过对比试验与有限元结果,认为所建立的细观有限元模型能够准确预测材料单胞在压缩载荷下的应力-应变响应,并且能够模拟编织结构中的损伤起始和演化过程。      

Experimental determination of the mode I delamination fracture and fatigue properties of thin 3D woven composites

The mode I delamination fracture toughness and fatigue strength of thin-section three-dimensional (3D) woven composite materials is experimentally determined. The non-crimp 3D orthogonally woven carbon–epoxy composites were thin (2 mm) and consequently their through-thickness z-binder yarns were inclined at a very steep angle (about 70°) from the orthogonal direction. The steep z-binder angle has a marked effect on the delamination toughening and fatigue strengthening mechanisms. Experimental testing revealed that the fracture toughness and fatigue resistance increased progressively with the volume content of z-binders. However, the steep angle caused the z-binder yarns bridging the delamination crack to deform and fail in shear and through-thickness tension, rather than in-plane tension which usually occurs in thick 3D woven composites. Mode I pull-off tests on a single woven z-binder yarn embedded within the composite revealed that the crack bridging traction load, strain energy absorption and failure mechanism were strongly affected by the steep angle.     

Tensile Properties and Failure Mechanism of a New 3D Nonorthogonal Woven Composite Material

Tensile properties and failure mechanism of a newly developed three-dimensional (3D) woven composite material named 3D nonorthogonal woven composite are investigated in this paper. The microstructure of the composite is studied and the tensile properties are obtained by quasi-static tensile tests. The failure mechanism of specimen is discussed based on observation of the fracture surfaces via electron microscope. It is found that the specimens always split along the oblique yarns and produce typical v-shaped fracture surfaces. The representative volume cell (RVC) is established based on the microstructure. A finite element analysis is conducted with periodical boundary conditions. The finite element simulation results agree well with the experimental data. By analyzing deformation and stress distribution under different loading conditions, it is demonstrated that finite element model based on RVC is valid in predicting tensile properties of 3D nonorthogonal woven composites. Stress distribution shows that the oblique yarns and warp yarns oriented along the x direction carry primary load under x tension and that warp yarns bear primary load under y tension.     

Tension-tension fatigue behavior of GFRP woven laminates at low temperatures

This paper describes an experimental study on the fatigue damage behavior of GFRP woven laminates in terms of stiffness degradation and residual strength under cyclic loading at low temperatures. Uniaxial, load-controlled, tension-tension fatigue tests were conducted at room and low temperatures. The applied stress versus cycles to failure (S-N) relationships and fatigue limits were obtained for the GFRP woven laminates and the microcrack evolution due to fatigue loading was characterized using optical microscopy. Temperatures were also measured using a thermocouple embedded in the center of the specimens.     

Finite Element Analysis of Delamination inWoven Composites under Quasi-Static Indentation

Delamination initiation and propagation in plain woven laminates and 3D orthogonal woven composites during short beam shear (SBS) test were analyzed using finite element (FE) analyses. Two kinds of 3D woven composites, containing single z-yarns and double z-yarns, were considered. The FE models were guided by experimental observations from SBS tests for the same material systems. A series of mechanisms including creation and evolution of matrix cracks and delaminations were modeled discretely. The force-displacement curves obtained from the FE simulations were compared with those from experiments. Further parametric studies were conducted to investigate the effects of z-yarns and interlaminar fracture toughness on delamination in woven composites. The results from the FE simulations revealed that z-yarns in 3D woven composites can play a major role in impeding propagation of interlaminar cracks. On the other hand 2D plain woven laminates without any z-reinforcement demonstrated higher interlaminar fracture toughness due to undulation in yarns. 3D woven composites with double yarns showed better damage tolerance than single yarn 3D woven composites and their behavior was very similar to composite laminates with high interlaminar fracture toughness.     

2.5D机织复合材料压缩性能实验与数值模拟

为了研究2.5D机织复合材料的压缩损伤和失效机制,验证双尺度渐进损伤有限元数值模拟方法的有效性,对这类复合材料分别沿经纱方向和纬纱方向进行了准静态压缩实验,获得了其相应的应力-应变曲线,并测定了材料的初始弹性模量和极限强度。在此基础上,利用双尺度渐进损伤有限元数值方法模拟分析了材料的压缩应力-应变响应和损伤演化行为,取得了与实验吻合较好的模拟结果。结果表明:2.5D机织复合材料在纬向压缩下的主要失效模式是纬纱的轴向压溃与断裂,可获得相对较高的压缩强度;但在经向压缩下,经纱因弯曲会承受附加弯矩作用,从而对周围基体造成挤压,故在经纱轴向断裂之前容易出现经纱之间基体的压溃和纱线之间的分层开裂,使强度降低,不利于发挥纤维的承载优势。     

Influence of inclusion size on S-N curve characteristics of high-strength steels in the giga-cycle fatigue regime

Fatigue fracture of high-strength steels often occurs from small defect on the surface of a material or from non-metallic inclusion in the subsurface zone of a material. Under rotating bending loading, the S-N curve of high-strength steels consists of two curves corresponding to surface defect-induced fracture and internal inclusion-induced fracture. The surface defect-induced fracture occurs at high stress amplitude levels and low cycles. However, the subsurface inclusion-induced fracture occurs at low stress amplitude levels and high-cycle region of more than 10⁶ cycles (giga-cycle fatigue life). There is a definite stress range in the S-N curve obtained from the rotating bending, where the crack initiation site changes from surface to subsurface, giving a stepwise S-N curve or a duplex S-N curve. On the other hand, under cyclic axial loading, the S-N curve of high-strength steels displays a continuous decline and surface defect-induced or internal inclusion-induced fracture occur in the whole range of amplitudes. In this paper, influence factors on S-N curve characteristics of high-strength steels, including size of inclusions and the stress gradient of bending fatigue, were investigated for rotating bending and cyclic axial loading in the giga-cycle fatigue regime. Then, based on the estimated subsurface crack growth rate from the S-N data, effect of inclusion size on the dispersion of fatigue life was explained, and it was clarified that the shape of S-N curve for subsurface inclusion-induced fracture depends on the inclusion size.     

Estimate Interface Shear Stress of Woven Ceramic Matrix Composites from Hysteresis Loops

An approach to estimate the fiber/matrix interface shear stress of woven ceramic matrix composites during fatigue loading has been developed in this paper. Based on the analysis of the microstructure, the woven ceramic matrix composites were divided into four elements of 0^o warp yarns, 90^o weft yarns, matrix outside of the yarns and the open porosity. When matrix cracking and fiber/matrix interface debonding occur upon first loading to the peak stress, it is assumed that fiber slipping relative to matrix in the interface debonded region of the 0^o warp yarns is the mainly reason for the occurrence of the hysteresis loops of woven ceramic matrix composiets during unloading and subsequent reloading. The unloading interface reverse slip length and reloading interface new slip length are determined by the interface slip mechanisms. The hysteresis loops of three different cases have been derived. The hysteresis loss energy for the strain energy lost per volume during corresponding cycle is formulated in terms of the fiber/matrix interface shear stress. By comparing the experimental hysteresis loss energy with the computational values, the fiber/matrix interface shear stress of woven ceramic matrix composites corresponding to different cycles can then be derived. The theoretical results have been compared with experimental data of two different woven ceramic composites.