Impulse transfer during sand impact with a cellular structure

Compressible cellular metal sandwich structures made from a 3D assembly of square cross section 6061 T6 aluminum alloy tubes, and face sheets of the same alloy have been attached to a vertical pendulum and impacted by synthetic wet sand with an incident velocity of ~300 ms-1. The transmitted impulse of samples with thick (relatively rigid) and thin face sheets are compared to that transferred by an incompressible solid aluminum test block of the same dimensions. A discrete particle-based simulation method was used to simulate the experiments and to investigate the soil particle – structure interaction with the cellular structures. The simulated results agreed very well with experimental data; both showed that the impulse transferred to cellular structures with a 22 MPa core strength was 10-15% less than that transferred to a solid block of similar dimensions. However, the simulations reveal that the some of this apparent mitigation resulted from a subtle sand interaction between the bottom of the test structure and the sand box used for the tests. When this sand box effect was eliminated in the simulations, a small impulse reduction from cellular structures with thin face sheets was still observed. However, this was found to be a result of dynamic deflection of the edges of the front face sheet. When this effect was eliminated (by the use of a rigid front face), the simulations showed a small (5%) impulse reduction occurred for cellular structure whose compressive strength was much less than the pressure applied by the sand. This was a consequence of rapid core compression which increased the travel distance (and lateral spreading) of late arriving sand during the sand loading process. Weak cellular structures, that suffered significant crushing, also reduce the impulse transfer rate.We are grateful to both the U.S. Office of Naval Research (ONR grant number N00014-07-1-0764) managed by Dr. David Shifler and DARPA (DARPA grant number W91CRB-11-1-0005), managed by Dr. Judah Goldwasser for co-supporting this study.This is the accepted manuscript. The final version is available from Elsevier at http://www.sciencedirect.com/science/article/pii/S0734743X15000123

Impulse transfer during sand impact with a cellular structure

Compressible cellular metal sandwich structures made from a 3D assembly of square cross section 6061 T6 aluminum alloy tubes, and face sheets of the same alloy have been attached to a vertical pendulum and impacted by synthetic wet sand with an incident velocity of ∼300 ms-1. The transmitted impulse of samples with thick (relatively rigid) and thin face sheets are compared to that transferred by an incompressible solid aluminum test block of the same dimensions. A discrete particle-based simulation method was used to simulate the experiments and to investigate the soil particle - structure interaction with the cellular structures. The simulated results agreed very well with experimental data; both showed that the impulse transferred to cellular structures with a 22 MPa core strength was 10-15% less than that transferred to a solid block of similar dimensions. However, the simulations reveal that the some of this apparent mitigation resulted from a subtle sand interaction between the bottom of the test structure and the sand box used for the tests. When this sand box effect was eliminated in the simulations, a small impulse reduction from cellular structures with thin face sheets was still observed. However, this was found to be a result of dynamic deflection of the edges of the front face sheet. When this effect was eliminated (by the use of a rigid front face), the simulations showed a small (5%) impulse reduction occurred for cellular structure whose compressive strength was much less than the pressure applied by the sand. This was a consequence of rapid core compression which increased the travel distance (and lateral spreading) of late arriving sand during the sand loading process. Weak cellular structures, that suffered significant crushing, also reduce the impulse transfer rate

Impulse transfer during sand impact with a solid block

A vertical pendulum apparatus has been used to experimentally investigate the impulse and pressure applied by the impact of wet synthetic sand upon the flat surface of a back supported solid aluminum test block. The transferred impulse and maximum pressure applied to the sample were both found to decrease with increasing standoff distance between the bottom of the sand layer and the impact face of the solid block. A particle based simulation method was used to model the sand's acceleration by the explosive and its impact with the test structure. This method was found to successfully predict both the impulse and pressure transferred during the tests. Analysis of the experimentally validated simulations indicates that the momentum transmitted to the test structure is approximately equal to the free field momentum of the incoming sand, consistent with the idea that the sand stagnates against a planar surface upon impact. The decrease in transferred impulse with increasing standoff distance arises from a small reduction in sand particle velocity due to momentum transfer to air particles, and an increase in lateral spreading of the sand particles as the standoff distance increased. This spreading results in a smaller fraction of the sand particles impacting the (finite) area of the test sample impact face

Dynamic compression of square tube cellular structures

Aluminum cellular structures have been fabricated by combining a two-dimensional [0°/90°]2 arrangement of square Al 6061-T6 alloy tubes with orthogonal tubes inserted in the out-of-plane direction. By varying the tube wall thickness, the resulting three-dimensional cellular structures had relative densities between 11 and 43%. The dynamic compressive response of the three-dimensional cellular structure, and the two-dimensional [0°/90°]2 array and out-of-plane tubes from which they were constructed, have been investigated using a combination of instrumented Kolsky bar impact experiments, high-speed video imaging, and finite element analysis. We find the compression rate has no effect upon the strength for compression strain rates up to 2000 s-1, despite a transition to higher-order buckling modes at high strain rates. The study confirms that a synergistic interaction between the colinear aligned and out-of-plane tubes, observed during quasistatic loading, extends to the dynamic regime. Finite element simulations, using a rate-dependent, piecewise linear strain hardening model with a von Mises yield surface and an equivalent plastic strain failure criterion, successfully predicted the buckling response of the structures, and confirmed the absence of strain-rate hardening in the three-dimensional cellular structure. The simulations also reveal that the ratio of the impact to back-face stress increased with strain rate and relative density, a result with significant implications for shock-load mitigation applications of these structures. © 2014 Mathematical Sciences Publishers

Tubular aluminum cellular structures: Fabrication and mechanical response

We explore a novel cellular topology structure based upon assemblies of square cross section tubes oriented in a cross-ply 2D and orthogonal 3D arrangements that can be tailored to support different combinations of through thickness and in-plane loads. A simple dip brazing approach is used to fabricate these structures from assemblies of extruded 6061-T6 aluminum alloy tubes and the through thickness compression of a variety of structures is investigated experimentally and with finite element modeling. We find that the 3D orthogonal structures have an approximately linear dependence of modulus upon relative density. However the strength has a power law dependence upon density with an exponent of approximately 5/3. These cellular structures exhibit almost ideal plastic energy absorption at pressures that can be selected by adjustment of the vertical and in-plane tube wall thicknesses. A finite element model with a nonlinear hardening constitutive law is used to explore the buckling modes of the structure, and to investigate the relationship between cell topology, relative density, tube wall material properties and the cellular structures resistance to compression