Programmed Plastic Deformation in Mathematically-Designed Architected Cellular Materials

The ability to control the exhibited plastic deformation behavior of cellular materials under certain loading conditions can be harnessed to design more reliable and structurally efficient damage-tolerant materials for crashworthiness and protective equipment applications. In this work, a mathematically-based design approach is proposed to program the deformation behavior of cellular materials with minimal surface-based topologies and ductile constituent material by employing the concept of functional grading to control the local relative density of unit cells. To demonstrate the applicability of this design tactic, two examples are presented. Rhombic, and double arrow deformation profiles were programmed as the desired deformation patterns. Grayscale images were used to map the relative density distribution of the cellular material. 316L stainless steel metallic samples were fabricated using the powder bed fusion additive manufacturing technique. Results of compressive tests showed that the designed materials followed the desired programmed deformation behavior. Results of mechanical testing also showed that samples with programmed deformation exhibited higher plateau stress and toughness values as compared to their uniform counterparts while no effect on Young’s modulus was observed. Plateau stress values increased by 8.6% and 13.4% and toughness values increased by 5.6% and 11.2% for the graded-rhombic and graded-arrow patterns, respectively. Results of numerical simulations predicted the exact deformation behavior that was programmed in the samples and that were obtained experimentally.

Manufacturing and Characterization of Open-Cell Aluminum Foam Produced via Infiltration of Leachable Space Holder

Open-cell Al-Si foam samples were produced using infiltration casting technique. The metal infiltration process was performed in a specially designed and built setup consisting of a vertical chamber resistance furnace, a pressurization chamber connected to an Argon gas cylinder through a control manifold. To control the relative density of the produced foams, non-compacted and compacted preforms (5 MPa) were prepared from 2 or 4 mm NaCl particles. The compaction was performed using a hydraulic press in the same infiltration chamber. Argon pressure of 3 bars was applied to infiltrate the preforms with the aluminum alloy after melting at 750 °C. The produced aluminum foam specimens show no lack of filling, a high degree of preform replication, and good homogeneity of pore sizes. The preliminary physical and mechanical characterization tests, including relative density, plateau stress, densification strain, and elastic modulus of the foam, are comparable to the values reported in previous investigations, in which more complicated, time-consuming, higher energy, and costly techniques were used. Further investigations on wider ranges of particle sizes, compaction, and infiltration pressures are currently in progress.

Sacrificial Cladding with Brittle Materials for Blast Protection

In the following work, sacrificial claddings filled with different brittle materials were investigated, from concrete foam to granular media. They were subjected to blast loading using an explosive driven shock tube, while a sensor measures the load transmission and a high speed camera records the compression of the core. From a macroscopic point of view, concrete foam and granular media can act efficiently as a crushable core but differs greatly in terms of energy dissipation mechanisms. To compare them, granular media was at first treated as a cellular material, and different key parameters (plateau stress, densification strain) were computed using the energy absorption efficiency methodology. The presented tests results, coupled with observation in literature, allow a better understanding on the crushing process of a granular media. In particular, granular media tend to work as a core even for low intensity load, contrary to more classical crushable core.

Energy absorption characteristics of circular-celled honeycombs under in-plane quasi-static compressive loadings

The energy absorption characteristics of hexagonally packaged circular-celled honeycombs and quadrilater-ally packaged circular-celled honeycombs are obtained under in-plane quasi-static compressive loadings through finite element analysis. The stress–strain curves, deformation modes, energy absorption efficiency, specific plateau stress, normalized energy absorption and energy absorption diagrams are discussed. The cell arrangement patterns influence the shapes of stress–strain curves and deformation modes. The densification strain is in linear relationship with the relative density and the specific plateau stress is proportional to relative density. The hexagonally packaged circular-celled honeycombs have the largest specific plateau stress in the x2 direction for a given relative density. The normalized energy absorption is nearly proportional to the strain before densification and increases with increasing relative density for a given strain in one compression direction. The envelope line in the energy absorption diagram is approximately a straight line tangent to the shoulder points through the origin. The hexagonally packaged circular-celled honeycombs outperform the quadrilaterally packaged circular-celled honeycombs in in-plane energy absorption.

Influence of osteoporosis on the compressive properties of femoral cancellous bone and its dependence on various density parameters

Data collection of mechanical parameters from compressive tests play a fundamental role in FE modelling of bone tissues or the developing and designing of bone implants, especially referring to osteoporosis or other forms of bone loss. A total of 43 cylindrical samples (Ø8 × 16 mm) were taken from 43 freshly frozen proximal femora using a tenon cutter. All femora underwent BMD measurement and additionally apparent- and relative- and bulk density (ρapp, ρr, ρb) were determined using samples bordering the compressive specimen on the proximal and distal regions. All samples were classified as "normal", "osteopenia" and "osteoporosis" based on the DEXA measurements. Distal apparent density was most suitable for predicting bone strength and BMD. One novel aspect is the examination of the plateau stress as it describes the stress at which the failure of spongious bone progresses. No significant differences in mechanical properties (compressive modulus E; compressive stress σmax and plateau stress σp) were found between osteopenic and osteoporotic bone. The results suggest that already in the case of a known osteopenia, actions should be taken as they are applied in the case of osteoporosis A review of the literature regarding extraction and testing methods illustrates the urgent need for standardized biomechanical compressive material testing.

Quasi-State Compressive Properties of Functionally Graded Aluminum Matrix Syntactic Foams

The uniform aluminum matrix syntactic foams (SFs) were prepared by the stir casting method, with alumina hollow spheres (2-3 mm and 3-4 mm) and expanded glass (2-3 mm) as reinforcements, and ZL111 aluminum alloy as matrix. The functionally graded aluminum matrix syntactic foams (FG-SFs) were obtained by superimposing two uniform aluminum matrix syntactic foams. Quasi-static compression tests were performed. The plateau stress of FG-SFs containing only hollow spheres decreased slightly with increasing volume fraction of SF containing 3-4 mm hollow spheres. The FG-SFs containing 2-3 mm hollow spheres and 2-3 mm expanded glass showed the highest plateau stress. The energy absorption behavior of all samples fluctuated in a small range. The initial position of shear band depended on the volume fraction of uniform aluminum matrix syntactic foams, reinforcement type and size. The cracks always appeared first in the uniform aluminum matrix syntactic foams containing expanded glass.

In-plane dynamic crushing behaviors of joint-based hierarchical honeycombs with different topologies

Introducing the hierarchy into cellular materials has attracted increasing attention in the effort to pursue improved absorbed-energy abilities and impact resistance. In this paper, the dynamic crushing properties and energy absorption capacities of joint-based hierarchical honeycombs with different topologies were explored by means of explicit dynamic finite element (FE) analysis using ANSYS/LS-DYNA. Four types of joint-based hierarchical honeycombs with uniform cell-wall thickness were firstly constructed by substituting each vertex of regular honeycombs with a smaller self-similar cell (hexagon or square). The respective influences of hierarchical parameters and impact velocities on in-plane dynamic deformation modes, mechanical characteristic and energy absorption of joint-based hierarchical honeycombs were discussed. Research results showed that the hierarchy had a far greater influence on the in-plane deformation modes of honeycombs. Compared with regular honeycombs, the dynamic plateau stress and specific energy absorption of joint-based hierarchical honeycombs can be improved if the proper hierarchical parameters were chosen. Adding the joint-based hierarchy into regular honeycombs can enhance the crushing stress efficiency (CSE) of the specimens. In addition, by introducing a non-dimensional dynamic sensitivity index, the dynamic shock enhancement of hierarchical honeycombs was also investigated. These researches are useful for the multi-objective dynamic optimization design and controllable properties of cellular materials.

In-plane Dynamic Crushing of a Novel Bio-inspired Re-entrant Honeycomb With Negative Poisson's Ratio

In order to seek higher crashworthiness and energy absorption capacity, based on biological inspiration, a novel bio-inspired re-entrant honeycomb (BRH) structure with negative Poisson's ratio is designed by selecting lotus leaf vein as biological prototype. The numerical simulation model is established by the nonlinear dynamics software ABAQUS and further compared with the available reference results to verify the feasibility. The dynamic compression behavior and energy absorption capacity of two types of BRH (BRH-Ⅰ and BRH-Ⅱ) are firstly compared with conventional re-entrant honeycomb (RH). The simulation results show that BRH have better mechanical properties and energy absorption characteristics. Then, the crushing behavior of BRH-Ⅱ under different impact velocities are systematically studied. Three typical deformation modes of BRH-Ⅱ are observed through the analysis of deformation profile. The quasi-static plateau stress is closely related to the cellular structure. Based on one-dimensional shock theory, the empirical equations of dynamic plateau stress for BRH-Ⅱ with different relative densities are given by using least-square fitting. In addition, the effects of impact velocity and relative density on plateau stress and energy absorption behavior are also studied. The results show that the energy absorption capacity of BRH-Ⅱ is increased nearly six times compared with RH at the same impact velocity.