Understanding Delamination Suppression at High Deformation Rates in Through-Thickness Reinforced Laminated Composites

该提案的重点是最先进的纤维增强塑料（FRP）形式的复合材料的冲击性能，通过Z钉扎引入全厚度增强。在过去的20年中，复合材料在主要轻质结构中的应用稳步增长，增加了对新的和先进的复合材料技术的需求。最近的例子包括大型民用飞机，如波音787和空中客车A350，高性能汽车，如迈凯轮650 S，以及民用基础设施，如M6高速公路上的Mount Pleasant桥。FRP由塑料材料的薄层（层片）制成，其中嵌入有高刚度和强度纤维。通过在称为“固化”的过程中施加热和压力，层片在堆叠中结合在一起。所得到的组件是FRP层压板。FRP在几个工程领域中使用增加的主要原因是相对于传统合金的上级面内比刚度和强度以及由于不存在腐蚀而导致的长期环境耐久性。FRP的另一个关键优点是，它们可以通过优化增强纤维在层压板上的取向来适应特定的设计载荷。然而，FRP易于分层，即层片在层压件的厚度上逐渐脱离。这是由于标准FRP层压板在整个厚度方向上没有增强，因此面外机械性能明显低于面内机械性能。根据美国空军的数据，在使用中的FRP构件中，60%的结构失效是由分层引起的。冲击是FRP层压板脱层的主要原因，能量通常在20 J左右，足以在FRP板中产生多个脱层。这种能量水平的代表性场景是2cm直径的石头以110 km/h的速度撞击层压板。在航空航天撞击情景中可能更为严重。例如，涡轮风扇发动机的认证要求风扇叶片能够承受质量为几公斤量级的鸟以超过300 km/h的速度撞击，撞击能量为数千焦耳。在FRP中引入全厚度增强是改善全厚度机械性能和抑制分层的可行策略。Z型钉扎是一种全厚度加固技术，在固化前将短FRP棒插入层压板中。已证明Z-钉扎在准静态、疲劳载荷和低速/低能量冲击载荷下抑制脱层特别有效。尽管如此，很少有人知道的Z-销层压板承受高能量/高速冲击，其影响是由复杂的瞬态现象发生在散装FRP层压板和多层板界面的性能。总的来说，这些现象通常被称为“高应变率”效应。有一些证据表明，Z-钉扎也有利于高速撞击，但这不是决定性的。目前的知识缺乏可以通过过度设计和昂贵的大规模结构测试来规避，但这不是中长期可持续的解决方案。该项目旨在填补上述知识空白，通过将高变形率下的新实验表征与可用于设计和认证耐冲击损伤复合材料结构的新建模技术相结合。开发合适的建模技术对工业开发特别重要，因为它将减少复合结构认证所需的测试量，从而显著降低成本并缩短标记的准备时间

This proposal focuses on the impact performance of state-of-the-art composites in the form of fibre-reinforced plastics (FRPs) with through-thickness reinforcement introduced via Z-pinning. The application of composites in primary lightweight structures has been steadily growing during the last 20 years, increasing the requirement for new and advanced composites technologies. Recent examples include large civil aircraft, such as the Boeing 787 and the Airbus A350, high performance cars, such as the McLaren 650S, and civil infrastructure, such as the Mount Pleasant bridge on the M6 motorway. FRPs are made of thin layers (plies) of plastic material with embedded high stiffness and strength fibres. The plies are bonded together in a stack by applying heat and pressure in a process known as "curing". The resulting assembly is the FRP laminate. The main reasons for the increasing usage of FRPs in several engineering fields are the superior in-plane specific stiffness and strength with respect to traditional alloys and the long-term environmental durability due to the absence of corrosion. Another key advantage of FRPs is that they can be tailored to specific design loads via optimising the orientation of the reinforcing fibres across the laminate stack. FRPs are, however, prone to delamination, i.e. the progressive dis-bond of the plies through the thickness of the laminate. This is due to the fact that standard FRP laminates have no reinforcement in the through-thickness direction, so the out-of-plane mechanical properties are significantly lower than the in-plane ones. According to the US Air Force, delamination can be held responsible for 60% of structural failures in FRP components in service. Impacts are the main cause of delamination in FRP laminates with energies usually in the order of 20J, sufficient to produce multiple delaminations in FRP plates. A representative scenario for such energy level is that of a 2cm diameter stone impacting a laminate at a speed of 110 km/h. In aerospace impact scenarios can be much more severe. For example, the certification of turbofan engines requires the fan blades to be able to withstand an impact with a bird whose mass is in the order of a few kilograms at speed in excess of 300 km/h, with impact energies of thousands of Joules. Introducing through-thickness reinforcement in FRPs is a viable strategy for improving the through-thickness mechanical properties and inhibiting delamination. Z-pinning is a through-thickness reinforcement technique whereby short FRP rods are inserted in the laminate before curing. Z-pinning has been proven to be particularly effective in inhibiting delamination under quasi-static, fatigue loading and low velocity/low energy impact loading. Nonetheless, little is known regarding the performance of Z-pinned laminates withstanding high energy/high speed impacts, whose effects are governed by complex transient phenomena taking place within the bulk FRP laminates and multiple ply interfaces. Overall, these phenomena are commonly denoted as "high strain rate" effects. There is some evidence that Z-pinning is beneficial also for high-speed impacts, but this is not conclusive. The current lack of knowledge may be circumvented with overdesign and expensive large-scale structural testing, but this is not a sustainable solution in a medium to long-term scenario. This project aims to fill the knowledge gap outlined above, by combining novel experimental characterisation at high deformation rates with new modelling techniques that can be used for the design and certification of impact damage tolerant composite structures. The development of suitable modelling techniques is particularly important for industrial exploitation, since it will reduce the amount of testing required for certification of composite structures, with a significant reduction of costs and shorter lead times to mark