Issue 48

O. A. Mocian et alii, Frattura ed Integrità Strutturale, 48 (2019) 230-241; DOI: 10.3221/IGF-ESIS.48.24 231 several severe impacts arising from operational events like collision, flying debris, tool drop or bird strike. Among these, low velocity impacts can severely reduce the strength of the whole structure due to indentation and localized internal skin and/or core damage [11, 12], without any perception on the impacted side, [13-15]. As mentioned by Feng and Aymerich [15] the low velocity impact can develop various damage patterns in sandwich composites as resulting from different failure mechanisms as: matrix cracking, fiber fracture, face-core debonding through delaminations, and core crushing. All of them interact through complicated synergies when increasing the impact energy. Many research results can be found in the aspects of quasi static, dynamic and energy absorption characteristics. In the studies of Steeves et al. [16], Jiang et al. [17] and Vitale et al. [18], the collapse of a sandwich beam under three-point bending was analyzed through several types of failure events: core shear, face micro buckling, facesheet indentation and core crushing, depending on the geometry and density of the foam core. The dominant regimes of each collapse mode were marked out by drawing failure maps. The predictions were found close to the experimental results. Sokolinsky et al. [19] used linear higher-order sandwich panel theory (HSAPT) to describe the mechanical response of sandwich beams with aluminum facesheets and PVC foam core. They showed that the linear HSAPT can be efficiently used to estimate vertical displacements of soft-core sandwich beams up to high load levels with great accuracy. Zhang et al. [20] studied the flexural performance of composite sandwich beams with different GFRP ribs in four-point bending tests and concluded that beams failed due to skin compressive failure when positioned edgewise and in core shear failure when positioned flatwise. Styles et al. [21] investigated the effect of core thickness on the failure behavior of aluminum foam core sandwich structures under flexural loading using the 3D digital image correlation method. A full field strain analysis for each core thickness was conducted and different failure mechanisms were found. Regarding the dynamic impact investigations of sandwich structures, attention has gradually shifted from experimental testing to analytical analysis and numerical simulation due to the high cost of testing and the inability to accurately capture the damage states, especially in the case of low velocity impact where damage is nearly imperceptible. Extensive studies have been recently made on sandwich structures with composites as facesheets. In [22] analytic and semi-analytic solutions have been obtained for the quasi-static case in studying the problem of damage progression and interaction in a sandwich beam with laminated facesheets and a homogeneous core. It was shown that core/facesheet interactions generate energy barriers to the propagation of delaminations in the facesheet; this explains the experimental findings of low velocity impact tests showing that in some cases multiple delamination damage in the facesheet typically remains localized near the striker. A procedure to numerically simulate the low-velocity impact tests and residual flexural strength tests of sandwich structures was developed by the VUMAT subroutine in ABAQUS/Explicit, [23]. The damage morphology of both facesheets and the corrugated core was clearly identified. The numerical and experimental load-displacement characteristics for all cases were compared and there was a reasonably good agreement between them, except for the prolonged load plateau stage when the predicted load overestimated the experimental result, probably because the interface debonding was not considered in the numerical simulation. Funari et al. [24] developed a much more refined numerical technique by incorporating in the model moving mesh cohesive modelling, crack initiation and nucleation at core/skin interfaces. Arbitrary Lagrangian–Eulerian (ALE) interface elements were used for the sandwich structure combined with a cohesive fracture approach where debonding phenomena may occur in presence of multiple delaminations. It is therefore realized a reduction of the computational costs, required to predict crack onset and progressive evolution of debonding phenomena. Cohesive models for sandwich core/skin interfaces were calibrated by means of comparisons with numerical and experimental data for mode I and mode II configurations. They also studied, [25], the influence of inertial effects on debonding phenomena and crack propagation in different core typologies of sandwich structures based on fracture parameters determined experimentally on commercially available foams. Park et al. [26] investigated the force-time history curves of impacted sandwich structures and concluded that the damage resistance appears to be dependent on both the facesheet materials and core thickness: the lower the stiffness becomes, the greater the core thickness affects the impact resistance. Crupi et al. [27] conducted a theoretical and experimental analysis for the impact response of glass fiber reinforced aluminum honeycomb sandwiches. They used 3D computed tomography to investigate the failure mode and internal damage and demonstrated that the use of glass-epoxy reinforcement on aluminum honeycomb sandwiches enhanced the energy absorption and load carrying capacities. Zhu et al. [28] studied through theoretical, experimental and numerical methods the damage and failure mode maps of composite sandwich panels subjected to quasi-static indentation and low velocity impact. The effect of facesheet thickness was found to be vital contribution to the failure mode and its corresponding ultimate load, while core density affected only the failure mode. Dawood et al. [29] demonstrated through experimental and numerical means that the punching resistance of GFRP sandwich panels with balsa wood cores can be increased by locally stiffening the panels near the location of the concentrated load. The low velocity impact response in metallic facesheets such as titanium, stainless steel and aluminum is different from composites as metals are ductile under localized deformation. Xie et al. [30] studied the low velocity impact response of