Effects Of Meso-structure On The Mechanical Behavior And Multi-objective Optimization Design Of Cellular Metals

Cellular metals exhibit low densities, high specific stiffness and strength, high energy-absorbing capabilities and novel thermal properties comparing to fully-dense metals. They have been widely used in many fields for their multi-functionality, such as aircraft, spacecraft, automobile and offshore oil production platforms. The strong connections between mechanical behavior and cell structures of cellular metals are generally acknowledged, but not complete and in-depth. In the present work, the influences of cell structure defects on the macroscopic mechanical properties of 2-D honeycombs are investigated, optimization design of cell structure for open-cell metal foams is developed, and further more, the multi-objective optimization design are carried out for specified metal foam structures.Finite element simulations are performed to study the effect of randomly removing cell walls on the dynamic crushing behaviour of honeycomb structures. The influences of the imperfection and impact velocity on the deformation mode and plateau stress are investigated. Simulation results reveal that both imperfection and impact velocity affect the deformation modes as well as the critical velocities of mode transition. It is found that the deformation mode of imperfect honeycomb is determined by the combined action of two mechanisms, i.e. the deformation localization caused by inertia effect and deformation bands dispersion introduced by random distributed defects. The plateau stress is found to be proportional to the square of the impact velocity when the imperfect honeycombs are deformed at transitional mode or dynamic mode. When the impact velocity is near the critical velocity between transitional mode and dynamic mode, honeycombs with small fraction of imperfection exhibit higher plateau stress, comparing to those of regular honeycombs having the same relative density. However, when the imperfection further increases, the plateau stress decreases obviously near the critical velocity between quasi-static mode and transitional mode.The dynamic crushing behavior of honeycombs with randomly distributed solid inclusions is studied. Simulation results reveal that the deformation mode and critical velocities remain the same as regular honeycombs after introduction of solid inclusions. The plateau stress of honeycombs with solid inclusions is found to be proportional to the square of impact velocity. Comparing to regular honeycombs with same density, the compression strength of honeycombs with solid inclusion is found to decrease significantly under low velocity impact. However, as the impact velocity increases, inertia effects would result in spinous stress protuberances in the stress-strain curves of honeycombs with solid inclusions, moreover, the cell wall crushing of honeycombs with solid inclusions dissipates more energy than shear bands of regular honeycombs, accordingly, plateau stress of honeycomb with solid inclusions can be 10% higher than that of regular honeycomb.A dual-size cellular structure design is proposed and used to improve the mechanical properties of open-cell metal foams fabricated by the infiltration technique. Assuming a spherical shape of cells and idealized face-centered cubic (FCC) arrangement of cells, numerical simulations on the axial compression of open-cell foams with dual-size cellular structure are performed. The results show that the stiffness and strength of metal foam with secondary cells are much higher than those of uniform cell foams. The analysis on the deformation mechanism reveals that cell wall bending is the dominated mechanism in uniform cell foams, however, In dual-size foams, larger proportion of solid materials deforms, lead to an increase on mechanical properties, the combined action of cell wall bending and axial compression also lead to a different flow behavior compared with uniform cell foams. According to the numerical results, optimal dual-size open-cell aluminum foams are then manufactured. Their mechanical properties are tested. The stiffness and yield strength of dual-size foams are 48% and 19% higher than uniform cell foam, respectively. The optimized radius ratio and volume ratio of secondary cells to large cells are acquired. The experimental results fit with numerical prediction qualitatively.The geometric model of dual-size foams is further used for steady heat conduction simulations. The effective thermal conductivity of open-cell foams having various density and cell radius ratios are calculated by FEA method. The fitting functions of yield stress and thermal insulation parameter of dual-size foams are constructed by least square method. The multi-objective optimization design model including three objective functions, i.e. yield stress, thermal insulation parameter and structure weight, is proposed for metal foam plate. Solving the model with restriction method, the optimized density, cell radius ratio and plate thickness are acquired. Finally, the relationship graph of thermal insulation parameter and yield stress of metal foam plate is provided. It reveals that the comprehensive performance of dual-size foam plate is much better than that of uniform cell foam plate.