Dynamic compression of square honeycomb structures during underwater impulsive loading

Abstract

Significant reductions in the fluid structure interaction regulated transfer of impulse occur when sandwich panels with thin (light) front faces are impulsively loaded in water. A combined experimental and computational simulation approach has been used to investigate this phenomenon during the compression of honeycomb core sandwich panels. Square cell honeycomb panels with a core relative density of 5% have been fabricated from 304 stainless steel. Back supported panels have been dynamically loaded in through thickness compression using an explosive sheet to create a plane wave impulse in water. As the impulse was increased, the ratio of transmitted to incident momentum decreased from the Taylor limit of 2, for impulses that only elastically deformed the core, to a value of 1.5, when the peak incident pressure caused inelastic core crushing. This reduction in transmitted impulse was slightly less than that previously observed in similar experiments with a lower strength pyramidal lattice core and, in both cases, was well above the ratio of 0.35 predicted for an unsupported front face. Core collapse was found to occur by plastic buckling under both quasistatic and dynamic conditions. The buckling occurred first at the stationary side of the core, and, in the dynamic case, was initiated by reflection of a plastic wave at the (rigid) back face sheet-web interface. The transmitted stress through the back face sheet during impulse loading depended upon the velocity of the front face, which was determined by the face sheet thickness, the magnitude of the impulse, and the core strength. When the impulse was sufficient to cause web buckling, the dynamic core strength increased with front face velocity. It rose from about 2 times the quasistatic value at a front face initial velocity of 35 m/s to almost 3 times the quasistatic value for an initial front face velocity of 104 m/s. The simulations indicate that this core hardening arises from inertial stabilization of the webs, which delays the onset of their buckling. The simulations also indicate that the peak pressure transmitted to a support structure from the water can be controlled by varying the core relative density. Pressure mitigation factors of more than an order of magnitude appear feasible using low relative density cores. The study reveals that for light front face sandwich panels the core strength has a large effect upon impulse transfer and the loading history applied to support structures.

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

“…Theoretical assessments indicate this beneficial effect arises from two phenomena: a reduction in the impulse acquired by the sandwich panel front face as a result of a fluid-structure interaction (FSI) effect [3,4,[8][9][10] and the higher flexural stiffness and strength of the sandwich. It has also been experimentally shown that the forces transmitted to supports when rigid back supported sandwich panels are impulsively loaded in water are significantly less than equivalent solid plates [11][12][13][14]. These reductions arise from a combination of beneficial FSI effects (which reduce the transmitted impulse) and a low core crushing stress.…”

“…After the beneficial FSI effect had been confirmed in water [2][3][4][5][6][7][8], numerous sandwich panel concepts for impulse and pressure mitigation have been explored [7][8][9][10][11][12][13][14][15][16][17].…”

“…The through thickness (vertical) forces transmitted to supports are governed by the dynamic strength of the core which in turn depends upon the core relative density, the inertial strengthening of its structural members, and the strength (modified by strain rate hardening) of the material used for its construction [13,17,20]. As the core relative density is decreased, the quasi-static strength of cores made from trusses begins to exceed that of the honeycomb webs because of their higher buckling resistance [21,22].…”

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“…this case, the transmitted pressure is controlled by the dynamic crush strength of the core, σ cD . The required minimum thickness for the cellular core, h min , can be estimated from [Ashby et al 2000] and [Wadley et al 2007b] as…”

Elastic and transport properties of the tailorable multifunctional hierarchical honeycombs

“…Vaziri and collaborators focused on metallic sandwich panels with different kinds of cellular cores such as hexagonal honeycombs [18], square honeycombs [19][20][21][22][23], open-cell rhombic dodecahedron cellular structures [24] and pyramidal truss cores [25][26][27][28], and explored some of their multifunctional applications, such as energy absorptions [27], sustaining shock loadings [19][20][21] and underwater impulsive loadings [22][23].…”