Lightweight phenolic resin-impregnated aramid paper honeycombs, commercially known as Nomex ® honeycombs, are promising cores for sandwich structures in aerospace applications due to their high ratios of stiffness and strength to density. The out-of-plane compressive properties of the Nomex honeycombs have been widely investigated under quasi-static and low strain rates (up to 300 s -1 ). There is a need to understand the behaviour of this structure under higher strain rate compression. This will widen the applicability of these structures to more areas such as debris impact and other impacts which induce high strain rates. This paper reports the out-of-plane compressive responses of Nomex honeycombs subject to quasi-static loading and high strain rate dynamic loading up to 1500 s -1 . The work involves experimental measurements and numerical modelling and validation. The compressive responses of the honeycombs were measured using a sensitive magnesium alloy Kolsky bar setup with front and back face impacts. The failure modes of the Nomex honeycombs were identified to be different under quasi-static and dynamic compressions. Under quasi-static compression, the honeycombs failed with local phenolic resin fracture after the elastic buckling of the honeycomb walls. For the dynamic compression, the honeycombs failed with the stubbing of cell walls at the ends of specimens. A finite element (FE) numerical model was devised and validated with the experimental data. The FE model considered the strain rate effect of phenolic resin material. The model predictions were in good agreement with the experimental measurements and facilitated interpreting the out-of-plane compressive response of the Nomex honeycombs. It was shown that there was a linear compressive strength enhancement up to 30% from quasi-static to strain rate of 1500 s -1 . The strength enhancement was governed by two mechanisms: the strain rate effect of the phenolic resin and inertial stabilization of the honeycomb unit cell walls, where 61%-74% of the enhancement was contributed by the inertial stabilization of the unit cell walls. In addition, it was shown that the impact method and initial imperfections had negligible effect on the compressive response of the Nomex honeycombs.
A triaxial compression test was carried out to investigate the effect of ultra high molecular weight polyethylene (UHMWPE) on concrete's behavior, in which four kinds of fiber volume fraction (0%, 0.3%, 0.5%, 1.0%) and five kinds of confining pressure (0MPa, 3MPa, 6MPa, 12MPa, 18MPa) were designed. The experiment result shows that both confining pressure and fiber volume fraction have influences on the stress-strain curves of the fiber reinforced concrete where the confining pressure's improvement will increase peak stress and mixing fiber will increase limit strain. According to that, a formula is given to describe the relationship between the strength of UHMWPE fiber reinforced concrete, confining pressure and fiber volume fraction. Then by using compression toughness index method, the relationship between the toughness of UHMWPE fiber reinforced concrete, confining pressure and fiber volume fraction is also revealed. When the confining pressure is 18 MPa and the fiber volume fraction is beyond 0.5%, the toughness improvement is the most remarkable.
The expansion tube is normally used in impact-resistant components, owing to the stable expansion force, and high specific energy-absorption capacity. This paper reports the deformation mechanism and energy absorption characteristics of expansion tube subject to impact velocity ranging from 0.083 m/s to 48.84 m/s. The effects of cone piston semi angle, tube wall thickness, and impact velocity on expansion tube performance were studied via experimental measurements and numerical modeling. The mechanical responses of the expansion tube were measured using a universal gas gun setup. The energy absorption capacity of the expansion tube absorber was identified to be different under quasi-static and dynamic compressions. The dynamic expansion force is lower than the quasi-static expansion force, which is about 62.2%-76.6% of the static expansion force, but the deformation mechanisms of the tube under quasi-static loading and dynamic impact are same. A finite element numerical model was built and validated with the experimental data. The finite element predictions were in good agreement with the experimental measurements. It was shown that the decrease in the friction coefficient is the main reason for the dynamic expansion force lower than the quasi-static expansion force. The influence of cone piston semi angle and tube wall thickness are significant on the energy absorption capacity of the tube. The dynamic expansion response does not change significantly when the impact velocity is less than 50 m/s. Under dynamic impact, the change of energy absorption efficiency is negligible, and the plastic deformation energy is about 64%-71.5% of the total kinetic energy of striker. INDEX TERMS Dynamic impact, energy absorption characteristics, expansion response, expansion tubes, friction coefficient.
This novel hybrid fibre composites combining stiff composites with soft composites are developed to improve the ballistic impact resistance of composite beams while maintaining good quasi-static loading bearing capacity. The ballistic impact performance of the hybrid beams have been investigated experimentally at a projectile velocity range of 11 0 50 ms 300 ms v , including ballistic limits, failure modes, energy absorption capacity and the interaction between stiff and soft composite parts. For each type of monolithic beams, i.e. stiff, soft and hybrid monolithic beams, three categories of failure modes have been identified: minor damage with rebound of projectile at the low impact velocities, fracture of beam at the medium impact velocities and perforation of beam at the high impact velocities.The critical velocity of hybrid monolithic beam was similar to that of the soft monolithic beam under the same failure mode, and higher than that of the stiff monolithic beam. For the sandwich beams with stiff, soft and hybrid face sheets, the failure modes were similar to those of the monolithic beams. Among the monolithic beams, the hybrid and soft monolithic beams exhibited better energy absorption capacity than the stiff monolithic beams. As for the sandwich beams, the hybrid-face sandwich beams absorbed more kinetic energy of projectile 2 than the soft-face sandwich beams at higher projectile velocity. The advantages of the stiff/soft hybrid construction include: (i) at lower impact velocity, the soft composite part survived with negligible damage under impact; (ii) due to the buffer effect of the soft part at the front face, stress distribution within the stiff part of the hybrid monolithic beams is more uniform than that of the stiff monolithic beams.
Bio-inspired self-similar hierarchical honeycombs are multifunctional cellular topologies used for resisting various loadings. However, the crushing behavior under large plastic deformation is still unknown. This paper investigates the in-plane compressive response of selective laser melting (SLM) fabricated hierarchical honeycombs. The effects of hierarchical order, relative density as well as constituent material are evaluated. The results show that at small deformation, the AlSi10Mg alloy hierarchical honeycombs show great advantages over the elastic modulus and compressive strength than 316L steel hierarchical honeycombs. As the relative density and hierarchical order increase, the failure mechanism of AlSi10Mg alloy honeycombs gradually changes from a bending-dominated mode to a fracture-dominated mode; whereas all the 316L steel honeycombs fail due to the distortion of original unit cells. At large deformation, the AlSi10Mg alloy honeycombs behave with brittle responses, while the 316L steel honeycombs exhibit ductile responses, showing a negative Poisson’s ratio behavior and gradient deformation of hierarchical unit cells. The addition of unit cell refinements improves the elastic modulus of AlSi10Mg alloy honeycombs and advances the densification of 316L steel honeycombs. In addition, the effect of constituent material on the compressive response of hierarchical honeycombs has been discussed. This study facilitates the development and future potential application of multifunctional ultra-light sandwich structures.
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