Modelling damage and residual indentation from composites impact

损伤容限是复合层压板的主要问题。对于低速撞击，外部损伤可能看不见，或者几乎看不见，而内部损伤可能已经发生。为了评估这一点，开发了冲击后压缩强度（CAI）测试，这是通过实验量化复合材料层合板在低速冲击损伤后强度损失的行业标准。从本质上讲，这个测试影响的是不同水平的低能量板。选用0.5mm压痕板，在边缘压缩下加载至失效。这种破坏荷载给出了碰撞后的残余强度，它必须超过最大结构设计荷载；因此，一个结构被证明可以安全飞行，几乎看不到损坏。许多研究者已经对CAI试验进行了研究，以数值预测冲击过程中的层内和层间损伤。包括IFB在内的一些研究人员在将冲击损伤建模与冲击后压缩强度分析相结合方面也取得了有限的成功。尽管做出了这些努力，但迄今为止仍忽视了一个重要方面；即根据冲击能准确预测局部压痕。这不是一个微不足道的问题；它将需要三维建模和本构律的损伤，塑性和松弛后的负荷去除；此外，与内部损伤的准确预测强耦合将是必不可少的。这就是本研究计划的目的。将开展广泛的材料测试计划，从中将开发新的本构模型，以表征局部冲击破碎和局部冲击复合材料的时间依赖松弛卸载。这项工作将推进由于低速撞击而导致的表面和内部损伤之间相互作用的一般科学知识，并将是迈向CAI测试程序的全虚拟模拟方法的重要一步。虽然研究将集中在CAI测试配置上，但该主题与许多其他影响相对较小的应用相关；例如冰雹对风力涡轮机叶片的冲击和异物对复合材料层压板的一般冲击。

Damage tolerance is a major concern with composite laminates. For low velocity impact external damage may be invisible, or barely visible, whereas significant internal damage may have occurred. In order to assess this, the Compression strength After Impact (CAI) test has been developed and is the industry standard to experimentally quantify loss in strength of a composite laminate after damage from low velocity impact. Essentially, this test impacts plates with various levels of low energy. The plate with a 0.5mm indent is selected, and loaded under edgewise compression to failure. This failure load gives the residual strength after impact which must exceed the maximum structural design load; thus a structure is proven safe to fly with barely visible damage.Numerous researchers have investigated the CAI test to numerically predict intra- and inter-ply damage during impact. A few researchers, including IFB, have also had limited success to couple impact damage modelling with post-impact compression strength analysis. Despite these efforts one important aspect has been overlooked to date; namely, to predict local indentation accurately with respect to impact energies. This is not a trivial problem; it will require 3D modelling and constitutive laws for damage, plasticity and relaxation after load removal; also, a strong coupling with accurate prediction of internal damage will be essential. This is the aim of this research proposal. An extensive materials test program will be undertaken from which new constitutive models will be developed to characterise the local impact crushing and time dependent relaxation unloading of locally impacted composites. The work would advance general scientific knowledge of interaction between surface and internal damage due to low velocity impact and would be an essential step toward a full virtual simulation methodology for the CAI test procedure. Although research will focus on the CAI test configuration the topic is relevant to many other applications involving relatively low impact; for example hailstone impact with wind turbine blades and general impact of foreign bodies against composite laminates.