ON THE DAMAGE TOLERANCE OF C/C-SIC COMPOSITE HOT STRUCTURES

Thanks to their excellent structural performance at high temperature, the Ceramic‐Matrix Composites (CMCs) are good candidates for light‐weight structures in high temperature applications, such as applications to re-entry vehicles. Beyond the well-known high temperature resistance, the CMCs do also show good damage tolerance properties, making them suitable for applications to large primary structures. Unfortunately, the production at the industrial level is currently at an embryonic stage due to the criticalities in the involved manufacturing processes which do not allow to produce defects free components. During the design phase, the presence of these manufacturing related defects are taken into account by introducing knock-down factors reducing the mechanical properties. However, this approach can be highly conservative and can strongly limit the intrinsic advantages of using such materials. Moreover, even if suitable non-destructive techniques (such as tomography) can be adopted to identify the manufacturing defects, still uncertainties exist about these defects evolution and, above all, about their influence on the structural performance (in term of stiffness and strength) of real components. The adoption of suitable material and fracture mechanics numerical models, could help to correctly predict the stiffness and strength characteristics of CMCs at coupon and subcomponent level. Which such advanced numerical models the manufacturing defects and their influence on the structural performance could be taken into account leading to a strong reduction of the currently adopted knockdown factors. The present work aims to investigate the damage tolerance of a C/C-SiC hot structure acting as aerodynamic control surface of a reusable re-entry vehicle. These vehicles are exposed to severe environmental conditions when re-entering Earth atmosphere. Indeed, the highest loaded areas such as nose, leading edges and control surfaces can experience temperatures up to 1650°C. Since the structural integrity of a re-entry vehicle needs to be guaranteed during all the mission phases and for multiple missions, a damage tolerant reliable design is required. Starting from the validation at coupon level of the material and fracture mechanics numerical models, by means of experimental data available in literature, a full parametric Finite Element model has been developed. The proposed parametric model is able to consider any delamination in any planar and thickness location all along the body flap domain. In order to reduce the computational costs, the entire body flap was modelled by means of layered shell elements, while layered solid elements have been used in the delaminated area. Hence, to connect the global coarse model and the local refined model, a global-local approach has been implemented. Finally, a sensitivity analysis has been performed finalised to assess the influence of location (in plane and trough the thickness) and dimension (radius) of a circular delamination on the damage tolerance of the investigated structure.