Development of failure models and criteria for laminated composites

复合材料作为结构部件的潜力目前受到了阻碍，因为我们缺乏理解和预测故障引发和传播的能力。这一领域的发展可以带来更快、更经济的设计，以及更轻、更高效的结构。该项目将通过(I)每种失效模式的实验表征、(Ii)失效机制的分析解释和(Iii)用于失效传播的数值模拟工具的开发，来提高由单向铺层组成的层合复合材料的失效预测能力。对每种失效模式的实验研究将特别详细，侧重于导致每种失效模式的事件序列及其相互作用的定性和定量描述。这些事件最初发生在微观机械尺度上，然后逐渐增长，导致结构故障。这方面的一个例子是纤维压缩扭结。在失效复合材料中观察到的扭结带是一系列事件的结果，包括基体开裂、纤维弯曲失效和最终纤维微屈曲，并且取决于纤维错位和剪切中的基质非线性行为。在这些事件中，例如，基质裂纹本身就是基质微裂纹生长和融合的结果。实验调查的目的是提供关于这些过程的广泛、详细、明确的信息。它将需要设计试验台，并将利用密集的仪器(如声发射、摄影测量和应变计)以及光学和扫描电子显微镜。实验结果将成为开发分析模型的基础，该模型描述了导致故障的物理事件序列及其相互作用。这些分析模型将形成对每种失效模式的物理、力学解释。它们可以理解为每种失效模式的物理理论，将观察到的事件转化为涉及材料(例如，弹性、强度、韧性)和几何特性(例如，纤维直径、典型纤维错位大小和分布、典型基质微裂纹大小和分布)的数学表达式。这些模型的结果将是一组方程，这些方程将被表示为失效准则，直接用于设计。为了对复杂结构进行准确的分析，必须考虑数值模型。为此，将开发一个包括以前的失效准则的先进的数值失效模型，用于商业有限元软件。为了准确地模拟失效扩展和避免虚假的网格依赖，数值模型将基于一种有待开发的适合于层合复合材料各种失效问题的弥散裂纹方法。这些都与复合材料可能表现出的多种失效模式以及每种失效模式如何影响材料响应有关。例如，基质破裂将导致断裂面上的牵引力矢量的剪切分量降为零，如果为正，则法向分量也降为零；计算模型应该能够准确地再现这一点，并正确地考虑过程的断裂能量。最后，数值模型将根据项目期间获得的实验数据以及文献中发表的数据进行验证。

The potential of composite materials as structural parts is currently hindered by our lack of ability to both understand and predict failure initiation and propagation. Developments in this area can lead to faster and more economical design, as well as lighter and more efficient structures. This project will contribute to the capability of predicting failure of laminated composites consisting of unidirectional plies, by focusing on (i) experimental characterisation of each failure mode, (ii) analytical interpretation of the failure mechanisms and (iii) development of numerical simulation tools for failure propagation.The experimental investigation into each failure mode will be particularly detailed, focusing on the qualitative and quantitative description of the sequence of events leading to each failure mode and their interaction. These events take place initially at a micromechanical scale, and then progressively grow to cause structural failure. An example of this is fibre compressive kinking. The kink bands observed in failed composites are the outcome of a sequence of events which include matrix cracking, fibre-bending failure and eventually fibre micro-buckling, and are dependent on fibre misalignments and matrix nonlinear behaviour in shear. Within these events, matrix cracking for instance, is itself the result of the growth and coalescence of matrix micro-cracks. The experimental investigation aims at producing extensive, detailed, univocal information on these processes. It will require design of test rigs, and will make use of intensive instrumentation (e.g. acoustic emission, photogrammetry and strain gauges) as well as optical and scanning-electron microscopy.The experimental findings will be the basis for the development of analytical models describing the sequence of physical events leading to failure and their interaction. These analytical models will form a physical, mechanist, interpretation of each failure mode. They can be understood as a physical theory for each failure mode, translating the observed events into mathematical expressions involving material (e.g. elastic, strength, toughness) and geometric (e.g. fibre diameter, typical fibre misalignments magnitude and distribution, typical matrix micro-cracks size and distribution) properties. The outcome of these models will be a set of equations, which will be expressed as failure criteria, for direct use in design.For the accurate analysis of complex structures, numerical models have to be considered. For this reason, an advanced numerical failure model including the previous failure criteria will be developed, to be used within commercial finite elements software. In order to accurately model failure propagation and avoid spurious mesh dependency, the numerical model will be based on a to-be-developed smeared-crack methodology appropriate for the variety of issues of failure in laminated composites. These have to do with the multiple failure modes composites can exhibit and how each failure mode affects the material response. For instance, matrix cracking will result in the shear components of the traction vector on the fracture plane being reduced to zero, as well as the normal component if positive; the computational model should be able to reproduce this accurately, as well as correctly accounting for the fracture energy of the process. Finally, the numerical model will be validated against experimental data obtained for this effect during the project as well as published in the literature.