含孔复合材料层合板静拉伸三维逐渐损伤分析

针对面内静拉伸纤维增强复合材料含中孔层合板，发展了参数化三维逐渐损伤模型. 该模型 可以模拟含中孔层合板损伤起始、发展及最终结构破坏整个过程，并能较好地预测含中孔层 合板的破坏模式和破坏强度. 采用所发展的模型和有限元三维逐渐损伤分析技术即应力分 析、失效判定准则及损伤过程中材料性能退化等，对其他文献所提供的9种不同类型含中孔层合板进行了损伤扩展分析及强度预测，同时对层合板的损伤基本机理、类型及其相互关联作用进行了探讨，计算结果与文献实验结果非常吻合.     

A DAMAGE MECHANICS MODEL FOR FATIGUE LIFE PREDICTION OF FIBER REINFORCED POLYMER COMPOSITE LAMINA

A damage mechanics fatigue life prediction model for the fiber reinforced polymer lamina is established. The stiffness matrix of the lamina is derived by elastic constants of fiber and matrix. Two independent damage degrees of fiber and matrix are introduced to establish constitutive relations with damage. The damage driving forces and damage evolution equations for fiber and matrix are derived respectively. Fatigue tests on 0°and 90°unidirectional laminates are conducted respectively to identify parameters in damage evolution equations of fiber and matrix. The failure criterion of the lamina is presented. Finally, the life prediction model for lamina is proposed.     

An experimental and analytical treatment of matrix cracking in cross-ply laminates

The development of damage in cross-ply Hercules AS4/3502 graphite/epoxy laminates has been investigated. Specific endeavors were to identify the mechanisms for initiation and growth of matrix cracks and to determine the effect of matrix cracking on the stiffness loss in cross-ply laminates. Two types of matrix cracks were identified. These include both straight and curved cracks. The experimental study of matrix crack damage revealed that the curved cracks formed after the straight cracks and followed a repeatable pattern of location and orientation relative to the straight cracks. Therefore, it was postulated that the growth mechanism for curved cracks is driven by the stress state resulting from the formation of the straight cracks. This phenomenon was analytically investigated by a finite-element model of straight cracks in a cross-ply laminate. The finite-element results provide supporting evidence for the postulated growth mechanism. The experimental study also revealed that the number of curved cracks increased with the number of consecutive 90-deg plies. Finally, experimental results show as much as 10-percent degradation in axial stiffness due to matrix cracking in cross-ply graphite/epoxy laminates.     

Prediction of progressive failure in multidirectional composite laminated panels

A mechanism-based progressive failure analyses (PFA) approach is developed for fiber reinforced composite laminates. Each ply of the laminate is modeled as a nonlinear elastic degrading lamina in a state of plane stress according to Schapery theory (ST). In this theory, each lamina degrades as characterized through laboratory scale experiments. In the fiber direction, elastic behavior prevails, however, in the present work, the phenomenon of fiber microbuckling, which is responsible for the sudden degradation of the axial lamina properties under compression, is explicitly accounted for by allowing the fiber rotation at a material point to be a variable in the problem. The latter is motivated by experimental and numerical simulations that show that local fiber rotations in conjunction with a continuously degrading matrix are responsible for the onset of fiber microbuckling leading to kink banding. These features are built into a user defined material subroutine that is implemented through the commercial finite element (FE) software ABAQUS in conjunction with classical lamination theory (CLT) that considers a laminate as a collection of perfectly bonded lamina (Herakovich, C.T., 1998. Mechanics of Fibrous Composites. Wiley, New York). The present model, thus, disbands the notion of a fixed compressive strength, and instead uses the mechanics of the failure process to provide the in situ compression strength of a material point in a lamina, the latter being dictated strongly by the current local stress state, the current state of the lamina transverse material properties and the local fiber rotation. The inputs to the present work are laboratory scale, coupon level test data that provide information on the lamina transverse property degradation (i.e. appropriate, measured, strain–stress relations of the lamina transverse properties), the elastic lamina orthotropic properties, the ultimate tensile strength of the lamina in the fiber direction, the stacking sequence of the laminate and the geometry of the structural panel. The validity of the approach advocated is demonstrated through numerical simulations of the response of two composite structural panels that are loaded to complete failure. A flat, 24-ply unstiffened panel with a cutout subjected to in-plane shear loading, and a double notched 70-ply unstiffened stitched panel subjected to axial compression are selected for study. The predictions of the simulations are compared against experimental data. Good agreement between the present PFA and the experimental data are reported.     

碳纳米管/碳纤维增强复合材料层合板低速冲击响应和破坏的数值模拟

碳纳米管/碳纤维增强复合材料（carbon nanotube/carbon fibre reinforced plastic,CNT/CFRP）是一种多尺度复合材料,比传统CFRP有更好的综合性能和更广阔的应用前景。对CNT/CFRP在低速冲击下的响应和破坏进行了数值模拟研究。首先,基于先前的研究通过引入基体增韧因子、残余强度因子并改进损伤耦合方程,建立了新的FRP动态渐进损伤模型;然后,利用新建立的本构模型并结合黏结层损伤模型,对4种碳纳米管含量的增韧碳纤维增强树脂基复合材料层合板在5个能量下的冲击实验进行了数值模拟;最后,将模拟结果与文献中的相关实验结果进行了比较,并讨论了冲击速度的影响。结果表明:新建立的FRP本构模型能够预测CNT/CFRP层合板在低速冲击载荷作用下的响应、破坏过程和分层形貌,模拟得到的载荷-位移曲线和破坏形貌与实验吻合较好;冲击速度会影响CNT/CFRP层合板拉伸和压缩破坏的比例,相同的冲击能量下,更大的冲击速度会造成更多的拉伸破坏。     

Effects of Environmental Degradation on Flexural Failure Strength of Fiber Reinforced Composites

Fiber-reinforced composite laminates are often used in harsh environments that may affect their long-term durability as well as residual strength. In general, environmental degradation is observed as matrix cracking and erosion that leads to deterioration of matrix-dominated properties. In this work, cross-ply laminates of carbon fiber reinforced epoxy were subjected to environmental degradation using controlled ultraviolet radiation (UV) and moisture condensation and the post-exposure mechanical properties were evaluated through elastic modulus and failure strength measurements. Additionally, both degraded and undegraded were subjected to cyclic fatigue loading to investigate possible synergistic effects between environmental degradation and mechanical fatigue. Experimental results show that the degradation results in reduced failure strength. Greater effects of degradation are observed when the materials are tested under flexural as opposed to uniaxial loading. Based on strength measurements and scanning electron microscopy, we identified various damage modes resulting from exposure to UV radiation and moisture condensation, and cyclic loading. The principal mechanisms that lead to reduction in mechanical properties are the loss of fiber confinement due to matrix erosion, due to UV radiation and moisture condensation, and weakened/cracked ply interfaces due to mechanical fatigue. An empirical relationship was established to quantify the specific influence of different damage mechanisms and to clarify the effects of various degradation conditions.     

Behavior of graphite/epoxy plates with holes under biaxial loading

An experimental investigation was conducted to study the behavior under biaxial tensile loading of quasiisotropic graphite/epoxy plates with circular holes and to determine the influence of hole diameter on failure. The specimens were 40 cm×40 cm (16 in.×16 in.) laminates of [0/±45/90] _s layup. Four hole diameters, 2.54 cm (1.00 in.), 1.91 cm (0.75 in.), 1.27 cm (0.50 in.) and 0.64 cm (0.25 in.), were investigated. Deformations and strains were measured using strain gages and birefringent coatings. Equal biaxial loading was introduced by means fo four whiffle-tree grip linkages and controlled with a servohyraulic system. Initially, the circumferential strain is uniform around the boundary of the hole. Subsequently, with increasing load, regions of high strain concentration with nonlinear response develop at eight characteristic locations 22.5 deg off the fiber axes. Failure in the form of cracking and delamination initiates at these points. Maximum strains at failure on the hole boundary reach values up to twice the ultimate strain of the unnotched laminate. The effect of hole diameter on strength was described satisfactorily using an average biaxial-stress criterion. Good correlation was also obtained with theoretical predictions based on a tensor-polynomial failure criterion for the lamina and a progressive degradation model.     

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。     

A periodic unit-cell simulation of fiber arrangement dependence on the transverse tensile failure in unidirectional carbon fiber reinforced composites

The effect of fiber arrangement on transverse tensile failure in unidirectional carbon fiber reinforced composites with a strong fiber-matrix interface was studied using a unit-cell model that includes a continuum damage mechanics model. The simulated results indicated that tensile strength is lower when neighboring fibers are arrayed parallel to the loading direction than with other fiber arrangements. A shear band occurs between neighboring fibers, and the damage in the matrix propagates around the shear band when the interfacial normal stress (INS) is sufficiently high. Moreover, based on the observation of Hobbiebrunken et al., we reproduced the damage process in actual composites with a nonuniform fiber arrangement. The simulated results clarified that the region where neighboring fibers are arrayed parallel to the loading direction becomes the origin of the transverse failure in the composites. The cracking sites observed in the simulation are consistent with experimental results. Therefore, the matrix damage in the region where the fiber is arrayed parallel to the loading direction is a key factor in understanding transverse failure in unidirectional carbon fiber reinforced composites with a strong fiber/matrix interface.     

A theoretical model of matrix cracking in composite laminates under thermomechanical loading

A theoretical approach is presented for analyzing the ply cracking in general symmetric laminates subjected to any combination of in-plane mechanical loading and uniform temperature changes. The equivalent constraint model proposed by the authors in a previous work is used to account for the cracking interaction between laminae in the laminates. By using a superposition scheme and the stress field solutions the energy release rate for a ply cracking is explicitly expressed as a function of stiffness reduction parameters of the laminates. The ratio of mode I to mode II is introduced for construction of the fracture criterion. The effects of the laminate parameters and the crack spacing on the energy release rate and the mode mixity are illustrated. Finally, the model is used to predict the thermomechanical load for the first-ply-cracking. Project supported by the National Natural Science Foundation of China (No. 19972076) and the Germen Research Foundation (DFG).     

Analysis of elastic crack bridging in ceramic matrix composites

A micromechanics analytical model based on the consistent shear lag theory is developed for predicting the failure modes in fiber reinforced unidirectional stiff matrix composites. The model accounts for a relatively large matrix stiffness and hence its load carrying capacity. The fiber and matrix stresses are established as functions of the applied stress, crack geometry, and the microstructural properties of the constituents. From the predicted stresses, the mode of failure is established based on a point stress failure criterion. The role of the microstructural parameters of the constituents on the failure modes such as self-similar continuous cracking, crack bridging and debonding parallel to the fibers is assessed.     

A novel technique for time-resolved detection and tracking of interfacial and matrix fracture in layered materials

A novel experimental technique is developed for time-resolved detection and tracking of damage in the forms of delamination and matrix cracking in layered materials such as composite laminates. The technique is non-contact in nature and uses dual or quadruple laser interferometers for high temporal resolution. Simultaneous measurements of differential displacement and velocity at individual locations are obtained to analyze the initiation and progression of interfacial fracture and/or matrix cracking/delamination in a polymer matrix composite laminate system reinforced by graphite fibers. The measurements at multiple locations allow the speeds at which interfacial crack front (mode-I) or matrix cracking/delamination front (mode-II dominated) propagates to be determined. Experiments carried out use three-point bend configurations. Impact loading is achieved using a modified Kolsky bar apparatus with a complete set of diagnostics for load, deformation, deformation rate, and input energy measurement. This technique is used to characterize the full process of damage initiation and growth. The experiments also focused on the quantification of the speed at which delamination or damage propagates under primarily mode-I and mode-II conditions. The results show that the speed of delamination (mode-I) or the speed of matrix cracking/delamination (primarily mode-II) increases linearly with impact velocity. Furthermore, speeds of matrix failure/delamination under primarily mode-II conditions are much higher than the speeds of mode-I crack induced delamination under mode-I conditions.     

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。     

含圆孔复合材料层板在拉伸载荷下损伤破坏过程的模拟

本文应用有限元法模拟受拉含孔夏合材料层板损伤起始、累积直至破坏的过程。在模拟中引入了作者早些时建立的包含基体开裂层非对称约束影响的损伤本构关系，以及裂纹密度与损伤区应力关系函数等。本文给出损伤累积模拟方法并编制了程序，用该程序可以获得层板加载过程中各层的损伤状态、计算其刚度衰减和应力重分布以及最终破坏载荷。给出了数值模拟算例并与现有研究结果进行了比较。     

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。     

Experimental and analytical comparisons of failure in thermoplastic composite laminates

Failure characteristics of Gr/PEEK were studied, using an experimental investigation and a fully nonlinear ply-by-ply finite-element technique. The stacking sequence of the laminates (with centrally located holes) investigated were: 0, 90, ±45 deg, (0/45/90/−45 deg)_2s and (0±45/90 deg)_2s. The [0 deg] laminate failure was characterized by splitting at the extremities of the hole and along the fibers. The [90 deg] laminates failed in the transverse direction, whereas the [±45 deg] laminates exhibited considerable elongation to failure. In the case of the quasi-isotropic laminates, the failure progression appeared to be due dominantly to matrix cracking followed by fiber failure. Analytical predictions of the failure process showed reasonably good correlation with the experimentally determined data.     

Fiber fracture and strength degradation in unidirectional graphite/epoxy composite materials

An experimental study was conducted to determine the influence of load factor on fiber fracture development and residual strength of fatigue loaded unidirectional graphite/epoxy composite laminated. 8-phy composite laminates with a layer of release cloth imbedded at the middle ply were fatigued at different load levels and were examined for fiber fracture and residual strength at several stages of life based on the average number of cycles to failure (according to S-N data). From the experimental results, it is evident that the number of fiber fractures is nearly constant after the first few percent of the life. It is also suggested that the load level is much more important than the number of cycles of loading in the determination of the state of fiber fracture. This behavior was interrupted at high load levels (S60% S_u where the final fracture was highly affected by the longitudinal matrix splittings. Residual strength is found to be independent of the global fiber fracture density, and to be controlled by local behavior such as matrix cracking, local clustering of fiber fractures, and other local stress concentrations.     

基于树脂层强度预测分层萌生载荷

 轴向拉伸下的层合板在自由边缘附近存在奇异应力场,容易产生分层萌生进而导致结构破坏．而大多数基于材料力学方法的分层萌生研究对强度参数的确定有较强的主观性,缺乏合理解释．本文通过在层间插入一定厚度的树脂层,将分层萌生视为层间树脂层在三维应力状态下的强度破坏,根据有明确试验标准的树脂强度和Mohr 判据判断是否发生分层萌生．采用Pipes-Pagano 的均匀轴向拉伸模型计算应力场,对层间应力与树脂层面外应力进行了对比．取临界长度为4 个单层板厚度对T800/914 层合板的分层萌生进行了预测,结果表明预测值与实验值吻合良好．     

角铺设复合材料层合板内部脱层破坏的研究

在面内剪切外载作用下，角铺设复合材料层板板最终的宏观破坏模式是脱层，然而从细观角度来看，宏观的脱层破坏可以对应不同的细观损伤过程，以破坏面的形貌为例，有些破坏面的形貌为例，有些破坏面主要由裸露的纤维和纤维迹组成，而有些破坏面则主要由矩齿形基体材料组成。不同的过程对应着不同的力学性能、诸如脱层强度、韧性等、本文从细观角度研究了脱层破坏过程，并就铺设角、界面强度、基体开裂强度对该过程的影响进行了讨论。