Low-velocity impact response of a composite structural battery

Recently, ambitious targets have been set out in various branches of transportation to restrict pollutant emissions and to combat environmental degradation. In this frame, with specific reference to the aeronautic field, new designs incorporating electric or hybrid-electric propulsion systems on board aircraft face the problem of penalizing battery characteristics, in terms of energy density and power density, taking an untenable toll on the inert weight of the aircraft. An interesting technology, with the potential to overtake such limitations, is constituted by structural batteries. These are multi-functional structural components, capable of both replacing stress-supporting parts (typically made from metal alloys or carbon-fibers) and storing electrical energy. In the literature, structural batteries can be classified into two categories: multifunctional structures (or decoupled systems) and multifunctional materials (or coupled systems). In the first case different materials within the structural battery perform a single function (either energy storage or load bearing), however the overall composite is multifunctional, whereas in the latter all materials adopt multiple functions (i.e. energy storage and load bearing). Although higher mass savings are predicted for high degrees of structural integration, current research efforts show that structural batteries with low degree of multifunctionality exhibit better overall performances. The purpose of this work is to describe the process used to manufacture a structural battery. Basing on the state of the art, to maximize the mechanical properties of the final product, carbon fiber with twill architecture is used to get both the structural battery anode and the cathode. The separator between the electrodes is made by glass fiber. The matrix is chosen to get both a high adhesion with carbon fibers and a compaction cycle compatible with the battery materials maximum allowable temperature and pressure. Concerning the binders, carbon-coated copper foils are used to avoid galvanic corrosion. The damage tolerance of the obtained structural battery is evaluated through low-velocity impact tests, performed with a customized drop tower equipped with a movable frame that allows measurements with different falling heights. A supporting plate is used to fix the laminate on the ground and a MEMS accelerometer (X16-1D) is placed on the specimen, recording the local acceleration. Moreover, a sensorized hemispherical dart is used to capture the load time history into the impacting zone. The dimensions and geometric specifications of all the equipment is compliant with ASTM D7136 standard. Drop tests results are then compared to numerical ones, obtained by simulating a hemispherical dart impacting a thin composite laminate. Thus, a finite element analysis (FEA) is performed using the explicit finite element code LS-DYNA.