Back Face Deflection Research Articles

Blast Mitigation in a Sandwich Composite Using Graded Core and Polyurea Interlayer

The dynamic behavior of two types of sandwich composites made of E-Glass Vinyl-Ester (EVE) facesheets and Corecell™ A-series foam with a polyurea interlayer was studied using a shock tube apparatus. The materials, as well as the core layer arrangements, were identical, with the only difference arising in the location of the polyurea interlayer. The foam core itself was layered with monotonically increasing wave impedance of the core layers, with the lowest wave impedance facing the shock loading. For configuration 1, the polyurea interlayer was placed behind the front facesheet, in front of the foam core, while in configuration 2 it was placed behind the foam core, in front of the back facesheet. A high-speed side-view camera, along with a high-speed back-view 3-D Digital Image Correlation (DIC) system, was utilized to capture the real time deformation process as well as mechanisms of failure. Post mortem analysis was also carried out to evaluate the overall blast performance of these two configurations. The results indicated that applying polyurea behind the foam core and in front of the back facesheet will reduce the back face deflection, particle velocity, and in-plane strain, thus improving the overall blast performance and maintaining structural integrity.

Response of metallic pyramidal lattice core sandwich panels to high intensity impulsive loading in air

Small scale explosive loading of sandwich panels with low relative density pyramidal lattice cores has been used to study the large scale bending and fracture response of a model sandwich panel system in which the core has little stretch resistance. The panels were made from a ductile stainless steel and the practical consequence of reducing the sandwich panel face sheet thickness to induce a recently predicted beneficial fluid–structure interaction (FSI) effect was investigated. The panel responses are compared to those of monolithic solid plates of equivalent areal density. The impulse imparted to the panels was varied from 1.5 to 7.6 kPa s by changing the standoff distance between the center of a spherical explosive charge and the front face of the panels. A decoupled finite element model has been used to computationally investigate the dynamic response of the panels. It predicts panel deformations well and is used to identify the deformation time sequence and the face sheet and core failure mechanisms. The study shows that efforts to use thin face sheets to exploit FSI benefits are constrained by dynamic fracture of the front face and that this failure mode is in part a consequence of the high strength of the inertially stabilized trusses. Even though the pyramidal lattice core offers little in-plane stretch resistance, and the FSI effect is negligible during loading by air, the sandwich panels are found to suffer slightly smaller back face deflections and transmit smaller vertical component forces to the supports compared to equivalent monolithic plates.

New Interference Approach for Ballistic Impact into Stacked Flexible Composite Body Armor

In this study a new interference model is developed to investigate the ballistic response of a hybrid, two-layered, flexible body armor. Because the stacking order of layers, which have distinctly different mechanical properties, has a large effect on the V 50 limit velocity for penetration, we apply the analysis to a two-layer system consisting of fabrics of aramid (Kevlar29®) fiber and of ultrahigh molecular weight and drawn polyethylene (Dyneema@) fibers. Using right-circular cylindrical projectiles, previous experimental results of Cunniff (An Analysis of the System Effects in Woven Fabrics Under Ballistic Impact, Textile Research Journal, Vol. 62,1992, pp. 495―509) [using a similar fiber, Spectra@ instead of Dyneema®] showed nearly a factor of 2 difference in the V 50 velocity in the two possible stacking orders, that is, Spectra@ versus Kevlar29® as the strike face. This new model extends our previous fundamental work (A New Membrane Model for the Ballistic Impact Response and V50 Performance of Multiply Fibrous System, International Journal of Solids and Structures, Vol. 40, 2003, pp. 6723―6765) by addressing interference factors in terms of instantaneous material inflow to the impact cone using a convolution over current local strain around the projectile edge during its deceleration. Our previous multilayer model (Modeling of System Effects in Ballistic Impact into Multi-Layered Fibrous Structures for Soft Body Armors, International Jourual of Fracture, Vol. 135, 2005, pp. 219―251) ignored the interference aspect though a subsequent work (Effects of Layer Stacking Order on the V50 Velocity of Two-Layered Hybrid Armor System, Journal of Mechanics of Materials and Structures, Vol. 3, 2008, pp. 627-639) approximated its effects but without instantaneous response of material inflow to local tensile strain. The current model is much more complete and produces velocity, strain, and deformation histories either to perforation or to halting the projectile. In the new model, V 50 velocity differences with stacking order and with layer bonding versus no bonding are even larger. However, backface deflections turn out to be minimally affected. Calculations are very fast and postprocessing figures can be obtained in less than a minute using MATLAB with a laptop (2 MHz Core 2 Duo CPU 2 Gb RAM).

Underwater blast loading of sandwich beams: Regimes of behaviour

Finite element (FE) calculations are used to develop a comprehensive understanding of the dynamic response of sandwich beams subjected to underwater blast loading, including the effects of fluid–structure interaction. Design maps are constructed to show the regimes of behaviour over a broad range of loading intensity, sandwich panel geometry and material strength. Over the entire range of parameters investigated, the time-scale associated with the initial fluid–structure interaction phase up to the instant of first cavitation in the fluid is much smaller than the time-scales associated with the core compression and the bending/stretching responses of the sandwich beam. Consequently, this initial fluid–structure interaction phase decouples from the subsequent phases of response. Four regimes of behaviour exist: the period of sandwich core compression either couples or decouples with the period of the beam bending, and the core either densifies partially or fully. These regimes of behaviour are charted on maps using axes of blast impulse and core strength. The simulations indicate that continued loading by the fluid during the core compression phase and the beam bending/stretching phase cannot be neglected. Consequently, analyses that neglect full fluid–structure interaction during the structural responses provide only estimates of performance metrics such as back face deflection and reaction forces at the supports. The calculations here also indicate that appropriately designed sandwich beams undergo significantly smaller back face deflections and exert smaller support forces than monolithic beams of equal mass. The optimum transverse core strength is determined for minimizing the back face deflection or support reactions at a given blast impulse. Typically, the transverse core strength that minimizes back face deflection is 40% below the value that minimizes the support reaction. Moreover, the optimal core strength depends upon the level of blast impulse, with higher strength cores required for higher intensity blasts.

AN EXPERIMENTAL AND PRELIMINARY ANALYTICAL STUDY ON THE IMPULSE RESPONSE OF METALLIC SANDWICH PANELS

To investigate the structural response of sandwich panels loaded by impulsive loads, a systematic investigation has been conducted, and some experimental results are reported and discussed in this paper. Quantitative results were obtained based on the measurement in the tests by a pendulum with corresponding sensors, and then the deformation/failure patterns of front face and core were classified and analyzed systematically. Finally, an empirical equation is proposed to predict the back face deflection of the panels.

Sandwich panels for blast protection in water: simulations

Abstract Edge supported metallic sandwich panels subject to underwater blast respond in a manner dependent on the relative timescales for water cavitation, core crushing, and structural response in bending. This article examines the response to impulses representative of an (especially relevant) domain: wherein the water cavitates before the core crushes. In some cases, core crushing ceases before the overall structural deformation is complete, leading to what is termed strong-core response. In other situations, core crushing continues beyond the stage at which the dry-face of the sandwich panel ceases to deform, a behavior that is known as weak-core response. It is known that weak-core response provides superior panel performance in terms of three performance metrics: the back-face deflection, the tearing susceptibility of the faces and the loads transmitted to the supports. The relevant regime of behavior is determined by the magnitude of the transmitted impulse from the water, the relative strength of the core, and the aspect ratio of the panel, so that a panel must be designed carefully against a known threat to achieve optimal performance. This paper initiates the development of a map of response for sandwich panels having I-core geometry, delineating strong, weak and other behavior in terms of some of the design parameters mentioned above. The effect of fluid – structure interaction is included in the simulations, and these results are contrasted with those in which fluid – structure interaction is omitted and an initial impulse is instead given to the wet-face of the panel.

Mechanical response of metallic honeycomb sandwich panel structures to high-intensity dynamic loading

Explosive tests were performed in air to study the dynamic mechanical response of square honeycomb core sandwich panels made from a super-austenitic stainless steel alloy. Tests were conducted at three levels of impulse load on the sandwich panels and solid plates with the same areal density. Impulse was varied by changing the charge weight of the explosive at a constant standoff distance. At the lowest intensity load, significant front face bending and progressive cell wall buckling were observed at the center of the panel closest to the explosion source. Cell wall buckling and core densification increased as the impulse increased. An air blast simulation code was used to determine the blast loads at the front surfaces of the test panels, and these were used as inputs to finite element calculations of the dynamic response of the sandwich structure. Very good agreement was observed between the finite element model predictions of the sandwich panel front and back face displacements and the experimental observations. The model also captured many of the phenomenological details of the core deformation behavior. The honeycomb sandwich panels suffered significantly smaller back face deflections than solid plates of identical mass even though their design was far from optimal for such an application.

The impulsive response of sandwich beams: Analytical and numerical investigation of regimes of behaviour

An analytical model is developed to classify the impulsive response of sandwich beams based on the relative time-scales of core compression and the bending/stretching response of the sandwich beam. It is shown that an overlap in time scales leads to a coupled response and to the possibility of an enhanced shock resistance. Four regimes of behaviour are defined: decoupled responses with the sandwich core densifying partially or completely, and coupled responses with partial or full core densification. These regimes are marked on maps with axes chosen from the sandwich beam transverse core strength, the sandwich beam aspect ratio and the level of blast impulse. In addition to predicting the time-scales involved in the response of the sandwich beam, the analytical model is used to estimate the back face deflection, the degree of core compression and the magnitude of the support reactions. The predictions of the analytical model are compared with finite element (FE) simulations of impulsively loaded sandwich beams comprising an anisotropic foam core and elastic, ideally plastic face-sheets. The analytical and numerical predictions are in good agreement up to the end of core compression. However, the analytical model under-predicts the peak back face deflection and over-predicts the support reactions, especially for sandwich beams with high strength cores. The FE calculations are employed to construct design charts to select the optimum transverse core strength that either minimises the back face deflections or support reactions for a given sandwich beam aspect ratio or blast impulse. Typically, the value of the transverse core strength that minimises the back face deflection also minimises the support reactions. However, the optimal core strength depends on the level of blast impulse, with higher strength cores required for greater blasts.

The Underwater Blast Resistance of Metallic Sandwich Beams With Prismatic Lattice Cores

The finite element method is used to evaluate the underwater blast resistance of monolithic beams and sandwich beams containing prismatic lattice cores (Y-frame and corrugated core) and an ideal foam core. Calculations are performed on both free-standing and end-clamped beams, and fluid-structure interaction effects are accounted for. It is found that the degree of core compression in the free-standing sandwich beam is sensitive to core strength, yet the transmitted impulse is only mildly sensitive to the type of sandwich core. Clamped sandwich beams significantly outperform clamped monolithic beams of equal mass, particularly for stubby beams. The Fleck and Deshpande analytical model for the blast response of sandwich beams is critically assessed by determining the significance of cross-coupling between the three stages of response: in stage I the front face is accelerated by the fluid up to the point of first cavitation, stage II involves compression of the core until the front and back faces have an equal velocity, and in stage III the sandwich beam arrests by a combination of beam bending and stretching. The sensitivity of the response to the relative magnitude of these time scales is assessed by appropriately chosen numerical simulations. Coupling between stages I and II increases the level of transmitted impulse by the fluid by 20–30% for a wide range of core strengths, for both the free-standing and clamped beams. Consequently, the back face deflection of the clamped sandwich beam exceeds that of the fully decoupled model. For stubby beams with a Y-frame and corrugated core, strong coupling exists between the core compression phase (stage II) and the beam bending/stretching phase (stage III); this coupling is beneficial as it results in a reduced deflection of the back (distal) face. In contrast, the phases of core compression (stage II) and beam bending/stretching (stage III) are decoupled for slender beams. The significance of the relative time scales for the three stages of response of the clamped beams are summarized on a performance map that takes as axes the ratios of the time scales.

The Response of Metallic Sandwich Panels to Water Blast

Metallic sandwich panels subject to underwater blast respond in a manner dependent on the relative time scales for core crushing and water cavitation. This article examines the response at impulses representative of an (especially relevant) domain: wherein the water cavitates before the core crushes. Three core topologies (square honeycomb, I-core, and corrugated) have been used to address fundamental issues affecting panel design. Their ranking is based on three performance metrics: the back-face deflection, the tearing susceptibility of the faces, and the loads transmitted to the supports. The results are interpreted by comparing with analytic solutions based on a three-stage response model. In stage I, the wet face acquires its maximum velocity with some water attached. In stage II, the core crushes and all of the constituents (wet and dry face and core) converge onto a common velocity. In stage III, the panel deflects and deforms, dissipating its kinetic energy by plastic bending, stretching, shearing, and indentation. The results provide insight about three aspects of the response. (i) Two inherently different regimes have been elucidated, designated strong (STC) and soft (SOC), differentiated by a stage II/III time scale parameter. The best overall performance has been found for soft-core designs. (ii) The foregoing analytic models are found to underestimate the kinetic energy and, consequently, exaggerate the performance benefits. The discrepancy has been resolved by a more complete model for the fluid/structure interaction. (iii) The kinetic energy acquired at the end of the second stage accounts fully for the plastic dissipation occurring in the third stage, indicating that the additional momentum acquired after the end of the second stage does not affect panel performance.