Issue 48

O. A. Mocian et alii, Frattura ed Integrità Strutturale, 48 (2019) 230-241; DOI: 10.3221/IGF-ESIS.48.24 232 titanium honeycomb sandwich structures through experimental and numerical investigations. The identified failure modes were permanent indentation on the impacted facesheet and localized honeycomb core crush under the impact location. Sun et al. [31] studied the effect of skin thickness, core thickness, core height and cell size on indentation characteristics, such as peak forces, failure modes and energy absorption, for sandwich panels with aluminum facesheets and honeycomb core. The principal failure mechanisms identified were shear and tension damage for the facesheets and plastic folding and buckling for the honeycomb core. The parametric studies conducted through numerical analysis revealed that by modifying the cell size changed considerably the failure modes and by increasing the core height decreased the peak forces. Despite the extensive research conducted in low velocity impact of sandwich structures, a considerable amount of research is still needed to reveal their complex damage and failure characteristics. In this study, the damage and failure mechanism and energy absorption properties of foam core sandwich panels with aluminum and GFRP facesheets subjected to low velocity impact are investigated. Drop weight impact tests are carried out to induce damage into the sandwich panels. Different impact velocities are selected as to cause different damage response of the sandwich panels, from barely visible impact damage to complete perforation. Damage characteristics are evaluated by measuring the damaged area or depth of indentation of the impactor and energy absorption is evaluated by using specific parameters as: normalized absorbed energy, specific energy absorption and crush force efficiency. E XPERIMENTAL METHODOLOGY Materials and specimens ll tested sandwich panels consist of two identical facesheets and a foam core. Facesheets are of two major types: aluminum and composite. The aluminum is Al 6082-T6, which is one of the strongest alloys from the 6xxx series, due to the heat-treated and artificially aged processes. The thickness of the aluminum facesheets is 1.5 mm. The composites are made from 8 layers of fiber glass roving of 500 g/m 2 and a general-purpose epoxy system, EPOLAM 2017. Three different types of composite facesheets were obtained by adding different amounts of short glass fibers in the resin matrix: type A - 0 g of short glass fibers / 100 g resin, type B - 6.4 g of short glass fibers / 100 g resin and type C - 9.3 g of short glass fibers / 100 g resin. Thickness of the facesheets slightly varies around 2 mm due to the manual manufacturing process. The foams used as core for the sandwich panels are polyurethane (PUR) Necuron 100 of density 100 kg/m 3 and commercial extruded polystyrene (PS) of density 32 kg/m 3 , both having thicknesses of 12 mm. Facesheets were bonded to the core using an epoxy adhesive, type Araldite AW106 (Huntsman). The panels were cut into square specimens of 140x140 mm. Low velocity impact testing Impact tests were conducted according to ISO 6603-2:2000 [32] and ASTM 7136 [33] standards, using an instrumented impact tower presented in Fig. 1. The INSTRON Ceast 9340 is a gravitationally accelerated impact drop tower that can reach impact velocities up to 4.6 m/s and impact energies of almost 430 J. It is equipped with an instrumented impactor, having a hemispherical head of 20 mm diameter, which can directly measure the impact force using a strain gauge transducer which is mounted inside, on an elastic element. The sandwich plates are placed on an adjustable in height test specimen support with a circular hole of 100 mm diameter (Fig. 1), which eventually allows the striker to fall if the plate is perforated. A clamping ring is pressed over the sandwich plate by a pneumatic system with a maximum force of 3 kN. This force cannot be adjusted during testing. The machine's software allows the user to obtain other impact characteristics such as: absorbed energy, displacement of the impactor, and of course the measured contact force during impact. Therefore force-time and force-displacement plots can be easily obtained. Data acquisition was done with a frequency of 200 kHz for a maximum estimated time of 40 ms which proved as being sufficient to follow all the significant events. Only the first impact was considered for monitoring the impact phenomena and comparisons of the responses of the sandwich panels which were tested. Details of the experimental procedure can be found in previous work [34, 35]. The objective of the impact tests was to cause impact damage to the panels, from barely visible impact damage to complete perforation. Several specimens were tested at impact velocities ranging between 1.5 and 4.5 m/s in order to select the impact velocities that manage to cause various failure mechanisms. Three impact velocities were selected hereby: 3 m/s, 3.5 m/s and 4 m/s and a constant mass of the impactor as 13.15 kg. The corresponding impact energies and drop heights are presented in Tab. 1. The impacted sandwich panels were abbreviated as following: sandwich panel (SP) with composite type (A, B or C) or aluminum (Al) facesheets, foam core type (PUR or PS) and impact velocity. Therefore, as an example, SPA_PUR_3 means A