【作者】 庄兴民；

【导师】 晏雄；

【作者基本信息】 东华大学 ， 纺织工程， 2005， 博士

【摘要】 超高分子量聚乙烯(UHMWPE)纤维具有优异的综合性能。由于其原料相对廉价和来源广泛,废料可回收再利用及具有上述特点的缘故,使其作为复合材料增强体在复合材料领域占有重要的地位。本研究采用正交试验方案,获得优化的UHMWPE/PE层合板复合工艺。在UHMWPE/PE复合材料结构设计时,纤维含量是决定复合材料拉伸强度和剪切强度的重要因素,可以根据需要设计。复合工艺条件中,温度的影响最大,温度升高有利于提高界面强度,但会对纤维强度造成影响,时间和压力的影响相对较小,因此可选用低温长时间的工艺条件,使复合材料达到良好的综合性能。 为了优化设计UHMWPE/PE复合材料层合板和准确评价其性能,必须深入地研究该类材料的损伤破坏机理,进而揭示其损伤扩展规律、预测因损伤的存在对材料刚度、强度等宏观性能的影响。在求解复合材料力学问题时,通常模拟用数值模型可分为以下三种类型:(1)通过使用平面应变元素,来模拟层合板复合材料的任意横截面,但由于复合材料层合板的力学行为总是具有复杂的三维变形,所以使用平面应变元素的二维模拟是不够的。(2)有人应用经典层合理论,把层合板复合材料简化假定为单一等效刚度板,并通过使用壳单元进行模拟,而此方法不能独立地揭示层内破裂和层间分离的分布及其特征等。(3)通过使用三维实体单元模拟层合板复合材料。很显然,为了得到有效的数值计算结果,网格必须很细,并要求材料具有一定的周期性,计算量相当庞大。随着加载过程,材料内部可能会出现基体开裂、纤维和基体界面的脱粘、纤维断裂、层间分层等复杂的损伤模式,当采用有限元数值方法模拟这一损伤破坏过程时,针对微结构的任何改变,都必须重新划分单元进行迭代计算,直至结构演化趋于稳定,然后,再增加载荷,进行同样的模拟步骤。这种仿真过程要求计算机容量极大,计算机工作时间很长。所以,目前对真实的多层复合材料层合板的破坏过程进行仿真计算,一般是花更多还原

【Abstract】 Ultra-high molecular weight polyethylene (UHMWPE) fibers have excellent integrated properties. Their materials are relatively inexpensive and source broad. Flotsam can be reclaimed. Moreover, UHMWPE fibers as reinforcing materials have above properties, so they play an important role in region of composites. In this study orthogonal experimental design was utilized, optimum processing conditions of UHMWPE/PE laminates were obtained. Fibers content is one of important factors that affect tensile strength and shear strength in structural design of UHMWPE/PE laminates. Temperature is a most important factor among all processing parameters in this research. Rising temperature is useful for enhancing interfacial strength, but it can affect the strength of UHMWPE fibers. The effects of duration and pressure on are relatively small. So good integrated properties of UHMWPE/PE laminates can be achieved, if low temperature and long duration of processing conditions are used.In order to optimize the designs and evaluate exactly the performances of UHMWPE/PE laminated composites, it is necessary to study the damage mechanisms of these materials, and to indicate the rules of the damage propagation, then to predict the effects of what on the various performances such as stiffness, strength and so on with the damage developing. In the research about the mechanical problems of composite materials, numerical models used in the conventional simulations can be classified into the following three types. Firstly, any cross section of the laminated composite is modeled by using plane-strain elements. However, the mechanical behavior of the laminated composite always has the complex three-dimensional deformations. Therefore two-dimensional simulation using plane-strain elements are not adequate. Secondly, the laminated composite is sometimes assumed to be a single equivalent stiffness plate by means of classical lamination theory, and modeled by using shell elements. This method cannot simulate the in-layer fracture and the interlaminar delamination separately. Thirdly, the laminated composite is modeled by using three-dimensional solid elements. Obviously, in order toobtain the effective results of numerical calculation, the mesh must be very fine and material having certain periodicity is required, so the calculating amount is quite huge. With increasing loading, the multiple damage modes which are interfacial fracture between the fiber and matrix, the matrix cracking and interlaminar delamination etc. in the laminated composites may be occurred. When this damage process is simulated by using finite element numerical method, elements must be remeshed and iteratively calculated to any change of the microstructure, until the structure evolves and tends towards stability. Then, the same simulation step is carried out with further increasing the loading. Such simulating process requires the computer is great in capacity and the computer working time is very long. Accordingly, it cannot usually be afforded to spend that numerically simulating the damage processes of true multi-layer of laminated composites. Based on the quasi-3-demensional model concept, unreasonable aspect associated with fibers area modeling of Nishiwaki’s model was modified, and the numerical model of UHMWPE/PE laminated composites was proposed in this paper. The damage propagation processes of UHMWPE/PE laminated composites subjected to tensile loading were investigated and different fractural phenomena such as the transverse crack and interlaminar delamination etc. in the composite laminates were simulated at the same time by using aforementioned modified numerical model. The damage mechanisms were revealed, the effects of ply stacking angles and sequences on were discussed and the strengths were predicted for those materials. Tensile stress-strain curves of experimental data were consistent with those of the numerical simulating for aforementioned laminated composites. The levels of tensile stresses that correspond to transverse crack, interlaminar delamination and their propagation can be predicted. Though stacking sequences of layers of above UHMWPE/HDPE quasi-isotropic laminates are different, sequences of initial cracking of their single ply are same, namely, first to be 90° layer. The damage initiation and propagation correlate with ply stacking angles and sequences of UHMWPE/HDPE quasi-isotropic laminates.In order to verify further this modified finite element mechanical model, next studies are that correlative experiments will be performed by means of acoustic emission technology etc. The damage mechanisms of self-reinforced polyethylene composites laminates (UHMWPE/HDPE) being subjected to tensile loading were investigated by acoustic emission technique and a scanningelectron microscope technique in this study. The correlations were established between the dominant failure mechanisms and acoustic emission events amplitude for model specimens which exhibited the dominant damage mechanisms. Results revealed that fiber-matrix interfacial debonding, matrix plastic deformation and cracking, fiber pull-out, fiber breakage and interlaminar delamination are associated with acoustic emission events having amplitude range 30 dB to 45 dB (low amplitude events), 30 dB to 60 dB (low amplitude events), 60 dB to 80 dB (middle amplitude events), 80 dB to 97 dB (high amplitude events) and 60 dB to 85 dB (middle amplitude events), respectively. These correlations can be used to monitor the damage growth processes in the UHMWPE/HDPE composite laminates exhibiting multiple modes of damage, to evaluate the structure integrality and predict the life of these materials. Results from this study revealed that the acoustic emission technique is a viable and effective tool for identifying the damage mechanisms in the UHMWPE/HDPE composite materials.Using the correlations established between the types of damage in the UHMWPE/HDPE laminates and the acoustic emission results in terms of the events amplitude, measuring and analyzing amplitudes of acoustic emission signals were performed for different types of UHMWPE/HDPE quasi-isotropic laminates under the tensile loading conditions. Accumulative numbers of acoustic emission events for [0/90/45/-45]s (type A), [0/45/-45/90]s(type B), [45/-45/0/90]s(type C) specimens of UHMWPE/HDPE quasi-isotropic laminates vs tensile stress curves are different each other, corresponding loading levels of their same type of damage occurred are not equal. Results revealed that ply stacking angles and sequences of UHMWPE/HDPE quasi-isotropic laminates affect the damage growth processes of these laminates. The acoustic emission characteristics of damage growth processes and the fracture mechanisms in those laminates were revealed. The validity of the finite element mechanical model established in this study was proved.更多还原