复合材料内多处横向冲击损伤的光纤图像检测1)

基于埋设在复合材料层板中的多方位多模光纤网络的特点,提出了检测层板内部发生多处横向冲击损伤的重构算法.根据光纤损伤图像检测系统获得的图像信号,可实时、定量、直观重构并显示出层板内部各处损伤的位置和各处的损伤程度.在复合材料多点冲击损伤检测试验中,屏幕显示的损伤图示与对半透明试件同时做反向照明检测的结果相一致,证实了重构结果的正确性.     

复合材料结构内埋光纤网络测试其内部损伤试验研究

本文介绍了在复合材料层板内埋入光纤网络，用于测试层板冲击损伤的试验方法和结果。试验表明该法探测损伤是有效的，具有广阔的发展前途。     

含光纤复合材料内部损伤的图像显示

本文研究了含光纤的功能型复合材料内部损伤的图像显示方法和测试系统。通过数字化处理检测网络输出的图像，实现复合材料结构损伤的定量检测和直观显示。冲击损伤检测实验证明了该摄像－图像处理光纤损伤测试系统的有效性。     

缝合复合材料层板低速冲击损伤研究

通过三维动力学有限元法,以空间杆单元模拟缝线的增强作用,建立了缝合复合材料层板在横向低速冲击载荷作用下的渐近损伤分析模型。该模型考虑了缝合层板受低速冲击时的纤维断裂、基体开裂及分层等五种典型损伤形式,采用基于应变描述的Hashin失效准则和Yeh分层失效准则,并将其嵌入ABAQUS/Explicit用户子程序以实现相应损伤类型的判断及其材料性能退化。针对相同铺层的缝合与未缝合层板,模拟了低速冲击作用下的冲击响应和渐进损伤过程,数值结果与试验吻合较好,证明了该方法的合理有效性。同时探讨了冲击速度、缝合密度等对缝合层板冲击响应和损伤的影响。结果表明：有、无缝合层板的低速冲击响应具有类似的变化规律,缝合能够有效减小冲击损伤面积。随着缝合密度和缝线强度的增大,缝合对冲击损伤的抑制作用更为明显。     

复合材料低速冲击损伤的数值模拟研究

为了研究复合材料层合板在低速冲击载荷的作用下层合板内部子层和层间损伤的演化,利用三维动态有限元模拟计算层板低速冲击的过程.在所建立的有限元模型中,分类考虑了不同的损伤模式.并采用薄壳单元模拟分析层合板子层的基体和纤维损伤.对于层合板的层间损伤,采用节点约束失效方法来预测分层损伤.通过仿真结果和实验数据的对比表明,模拟预测的损伤形状和损伤面积与实验结果基本一致.     

材料强度对层板抗弹性能和损伤特性的影响

根据纤维增强复合材料宏细观结构特征,基于ABAQUS软件平台,建立了层合板高速冲击损伤三维有限元分析模型。该模型在复合材料层间引入界面单元模拟层间分层,采用三维粘弹性本构,结合Hashin失效准则模拟单层板面内损伤.利用该模型,深入研究了复合材料层板的抗弹性能和损伤特性,数值分析结果与实验结果吻合良好,证明了该方法的合理有效性。通过数值分析,详细探讨了材料强度参数对层板抗弹性能和损伤特性的影响规律,获得了一些有价值的结论。     

含冲击损伤复合材料层板及加筋壁板剩余强度研究

应用含刚度折减的椭圆形弹性核模拟了层板的损伤问题,研究了复合材料层板及加筋壁板冲击后的剩余强度问题。利用含椭圆核各向异性杂交应力有限单元对损伤层板进行了应力分析,采用基于特征曲线概念的点应力判据预测了含损伤层板、加筋壁板的剩余强度;基于Abaqus用户子程序uel实现了该方法在工程中的应用,并讨论各种参数对剩余强度的影响。研究结果表明此方法是有效的。     

复合材料层合板动态断裂韧性与损伤扩展的实验研究

研究了复合材料层合板在冲击载荷和不同温度条件下的力学行为及破坏机理。在不同温度环境下 ,利用MTS材料试验机 ,采用中等应变速率 (1/S) ,对玻璃布一环氧层板实施了冲击拉伸断裂实验 ,测试了该材料在不同温、不同应变速率下的断裂韧性值。分析了玻璃布 环氧层板材料的破坏过程及内部再伤情况     

复合材料层板的抗贯穿机理与模拟研究

为了研究树脂基纤维增强复合材料层板的抗侵彻贯穿机理和动态力学行为与抗侵彻毁伤的关系, 通过球形破片模拟弹贯穿实验表征了复合材料层板抗高速侵彻的吸能特性;通过高速摄影技术分析了层板 贯穿过程的瞬态变形失效特点;采用CT扫描成像及SEM 电镜分析等手段研究了复合材料层板的抗贯穿破 坏耗能模式。实验结果显示,高速冲击下层板抗贯穿吸能与入射速度成正比;高速侵彻过程是复合材料层板 高应变率变形的动态过程,高应变率动态力学行为对复合层板抗贯穿吸能特性影响显著;冲击波在层板中的 传播特性决定了不同破坏模式阶段的划分以及损伤区域的范围。基于复合层板高速贯穿下的动力学瞬态分 析,建立了复合层板抗高速侵彻吸能的两阶段动态破坏模型,模型计算值与实验值符合良好。研究结果表明, 应变率效应与惯性效应在复合材料层板抗侵彻性能分析中是不可忽视的2个关键因素。     

模拟长纤维增强树脂层合板冲击损伤的数值方法

建立了一套模拟树脂层合板低速冲击下损伤过程的方法.该方法采用Mindlin板模型并考虑板的大变形,使用以前提出的一种修正的Newmark时间积分方案进行层板的动态分析,全面考虑了复合材料的脱层破坏、基体开裂和纤维断裂等破坏形式及其相互作用.还建立了一种适合于 Mindlin板的计算应变能释放率的方法;构造了一种连续的罚函数来动态加人弹簧约束以考虑脱层间的接触效应;证明了球板之间的冲击过程是刚性系统,应使用A（α）稳定的数值方法求解;实现了模拟过程中的自适应分析.使用所开发的软件NBHULI完成了长纤维增强碳/环氧复合材料层板在低速冲击下损伤过程的模拟,取得了一些重要的结果.     

Element-failure concepts for dynamic fracture and delamination in low-velocity impact of composites

An element-failure algorithm is proposed and incorporated into a finite element code for simulating dynamic crack propagation and impact damage in laminated composite materials. In this algorithm, when a crack is propagating within a finite element, the element is deemed to have partially failed, but not removed from the computations. Consequently, only a fraction of the stresses that were computed before the crack tip entered the element contribute to the nodal forces of the element. When the crack has propagated through the element, the element is completely failed and therefore can only resist volumetric compression. This treatment of crack propagation in isotropic solids allows fracture paths within individual elements and is able to accommodate crack growth in any arbitrary direction without the need for remeshing. However, this concept is especially powerful when extended to the modeling of damage and delamination in fibre-reinforced composite laminates. This is because the nature of damage in composite laminates is generally diffused, characterized by multiple matrix cracks, fibre pullout, fibre breakage and delaminations. It is usually not possible to define or identify crack tips in the tradition of fracture mechanics. Since parts of a damaged composite structure are often able to partially transmit load despite the presence of some damage, it is advantageous to model the damaged portions with partially failed elements. The damage may be efficiently modeled and tracked using element-failure concepts, with the application of appropriate failure criteria and damage evolution laws. The idea is to embody the effects of damage into the effective nodal forces of the finite element. In this paper, we report the novel use of element-failure concepts in the analysis of low-velocity impact damage of composite laminates. The initiation and propagation of delaminations arising from the impact are predicted and the results show qualitative agreement with experimental observation of the formation of multiple delaminations in impact-damaged specimens. While such delaminations do not permit transmission of tensile stress waves across the cracked surfaces, transmission of compressive stress waves are allowed in the simulation. It is further shown that, when elements are allowed to fail, the dynamic stress wave distributions are altered significantly. In the element-failure algorithm, the issue of interpenetration of delamination surfaces in the model does not arise. This is a significant advantage over the conventional method of explicitly modeling the delamination surfaces and crack front, where generally, much computational time must be spent in employing contact algorithms to ensure physically admissible solutions. Finally, we also demonstrate the simulation of crack propagation of pre-notched specimens of an isotropic material under initial conditions of mode II loading using the element-failure algorithm. The numerical results showed that the cracks propagated at an angle of about 70° with respect to the notches, in agreement with the experimental results of Kalthoff.     

DAMAGE PROGRESSIVE MODEL OF COMPRESSION OF COMPOSITE LAMINATES AFTER LOW VELOCITY IMPACT

IntroductionCompressivepropertiesofcompositelaminatesafterlowvelocityimpactareoneofthemostseriouscircumstanceswhichmustbetakenintoaccountindamagetolerancedesignofcompositestructures[1].Impactdamagegenerallymanifestsintheformsofdelaminations,matrixcracksandfibrefracture[2 ,3].Sofaralotofresearcheshavebeentakeninlowvelocityimpactbehaviorofcompositelaminatesandtheirpost_impactcompressiveproperties.Compressivefailuremechanismsofcompositelaminatesafterlowvelocityimpacthavenotbeenknownclearly .Ther…     

Nonlinear progressive damage model for composite laminates used for low-velocity impact

In order to effectively describe the progressively intralaminar and interlam- inar damage for composite laminates, a three dimensional progressive damage model for composite laminates to be used for low-velocity impact is presented. Being applied to three-dimensional （3D） solid elements and cohesive elements, the nonlinear damage model can be used to analyze the dynamic performance of composite structure and its failure be- havior. For the intralaminar damage, as a function of the energy release rate, the damage model in an exponential function can describe progressive development of the damage. For the interlaminar damage, the damage evolution is described by the framework of the continuum mechanics through cohesive elements. Coding the user subroutine VUMAT of the finite element software ABAQUS/Explicit, the model is applied to an example, i.e., carbon fiber reinforced epoxy composite laminates under low-velocity impact. It is shown that the prediction of damage and deformation agrees well with the experimental results.     

Deformation of Cross-Ply Composite Laminates with Cracked Ceramic Matrix

The deformation of cross-ply ceramic-matrix composite laminates under biaxial loading is studied theoretically. Experimentally, microscopic damage evolution in cross-ply ceramic-matrix composite laminates has not been well characterized yet, mainly due to the difficulties involved in testing of anisotropic plates under biaxial loading. It is assumed that the initial damage mechanism observed in ceramic-matrix composite laminates is the formation of tunneling macrocracks both in the 90°- and 0°-plies. The paper addresses the stiffness-deterioration behavior of cross-ply ceramic-matrix laminates with transverse and longitudinal macrocracks. The analysis is based on the equivalent-constraint model of a damaged laminate. Numerical results for SiC/CAS cross-ply laminates of various lay-ups are presented and discussed__________Translated from Prikladnaya Mekhanika, Vol. 41, No. 1, pp. 37–46, January 2005.     

Designing for tolerance to impact damage at fastener holes in graphite/epoxy laminates under compression

This paper illustrates how fracture mechanics tools may be used to estimate the residual strength of impact damaged fastener holes in composite laminates. Finite element analysis is carried out in order to study the effect of the delamination damage. It is shown that as the size of the damage is increased, a stage is reached after which a significant further increase in damage does not result in a significant decrease in residual compressive strength.     

Characterisation of pure and mixed mode fracture in composite laminates

The extensive use of advanced fibre composite materials for aircraft construction has necessitated the development of a damage tolerance methodology for aircraft components. Such a methodology would facilitate the design of more efficient and reliable composite structures and their maintenance in service. Therefore, it is necessary to characterize and understand the complex failure modes of fibre composites, including the influence of temperature, moisture and various defects arising from manufacture or service conditions.This paper briefly discusses the present status of some approaches to the experimental characterization of pure and mixed-mode fracture of composite laminates.     

基于连续丝束剪切技术的变角度复合材料层合板的热屈曲分析

连续丝束剪切(Continuous Tow Shearing, CTS)铺放技术是一种新的变角度层合板制作技术，这种新技术能显著减少丝束铺放过程中产生的丝束重叠和间隙等缺陷，然而，采用CTS技术铺设时，层合板的厚度将随着纤维角度的变化而变化．本文基于一阶剪切变形理论并应用Chebyshev-Ritz法对这种变厚度的变角度复合材料层合板的热屈曲问题进行了研究．假设纤维方向角沿板的长度方向按照线性变化，获得了均匀温度变化时变厚度层合板的临界热屈曲荷载．通过与现有文献的比较验证了本文方法的正确性，并进一步讨论了纤维铺设技术、纤维方向角的变化以及边界条件的不同对变角度复合材料层合板的临界屈曲温度的影响．研究结果表明，在体积相同的情况下，采用CTS铺设较传统的自动丝束铺放(AFP)可以进一步提升变角度层合板的临界屈曲温度．本文的研究结果可为变角度复合材料的设计提供一定的参考．     

On the description of anisotropic damage in composite laminates

A general anisotropic damage theory of cracked laminates is formulated here. The deformation of composite laminates is composed of matrix elastic strains, pseudo-elastic damage strains due to cracking and permanent damage strains due to interlaminar slip. The surface of damage initiation is constructed according to the concept of linear elastic fracture mechanics for the virgin material. After the initial damage, a pesudo-elastic damage can be used to describe the damage behaviour if interlaminar slip is negligible. Damage evolution, load induced anisotropy and interlaminar intralaminar interaction for composite laminates are examined; the latter can perturb the normality structure of damage strain rate. Explicit expressions are given for pseudo-elastic (or secant) moduli of the damaging composite laminates, under a non-interacting assumption imposed on the cracks between different families.     

Non-linear forced periodic oscillations of laminates with curved fibres by the shooting method

Large-amplitude forced vibration, before damage onset, of variable stiffness composite laminated plates with curvilinear fibres are studied. The fibre paths considered change linearly in relation to one Cartesian coordinate. The plates are rectangular and with clamped edges. The displacement field is modelled by a third order shear deformation theory and the equations of motion, in the time domain, are obtained using a p-version finite element method. The in-plane inertia is neglected, still taking into consideration the in-plane displacements, and the model is statically condensed. The condensed model is transformed to modal coordinates in order to have a reduced model with a smaller number of degrees-of-freedom. A shooting method using fifth-order Runge–Kutta method, as well as adaptive stepsize control, is used to find periodic solutions of the equations of motion. Frequency-response curves of composite laminates with different curvilinear fibre angles and various thicknesses are plotted and compared. Tsai–Wu criterion is employed in order to predict the damage onset. When it is detected that damaged started, the continuation method is interrupted and no further points of the response curve are computed. The reason behind this interruption is that the model does not include the effects of damage. Examples of bifurcations are presented and studied in detail, using projections of trajectories in a phase plane and Fourier spectra. The time histories and frequency spectra of steady-state stresses are plotted for VSCL plates with different fibre angles. The steady-state stresses are also displayed for bifurcated branches of the solutions.