Translaminar fracture behavior of hybrid woven-ply peek thermoplastic laminates under isothermal and kerosene flame exposure

The growing demand to use thermoplastic matrix composites in aeronautic has been confronted with the need to understand the fracture mechanisms under different service conditions. Thus, the present work aims at studying the influence of different thermal stress conditions on the fracture behavior of PolyEther Ether Ketone-reinforced carbon/glass fiber hybrid laminates (CG/PEEK), as a function of heating temperature and fire exposure time. Compact Tensile (CT) specimens are exposed to isothermal (from 350°C to 550°C in a high temperature furnace) and critical service conditions (kerosene flame exposure characterized by a heat flux of 116 kW/m 2 and a temperature of 1150°C) exposure conditions. Monotonic tensile tests are then conducted to assess the mode I translaminar fracture toughness (FT) at room temperature. Crack propagation is monitored during mechanical loading using a Digital Image Correlation (DIC) device combined with a binarization algorithm. Then, the G-R curves have been obtained from the compliance method. Under isothermal conditions, the residual mechanical properties degrade as temperature increases, particularly once thermal decomposition is about to start, due to the formation of porosities and extensive delamination. Under flame exposure, the microscopic and tomographic observations reveal thermally- and mechanically-induced damages with a heterogeneous distribution due to temperature gradients within the plies of laminates. Depending on the pyrolysis degree of each ply, the load bearing capabilities of the plies gradually deteriorate from the exposed to the opposed side. The critical FT values shows a decreasing trend (as a function of fire exposure time) with increasing ply number of charred regions through the thickness.