Mechanical metamaterials at the theoretical limit of isotropic elastic stiffness

Acknowledgements H.N.G.W. is grateful for support for this work by the ONR (grant number N00014-15-1-2933), managed by D. Shifler, and the DARPA MCMA programme (grant number W91CRB-10-1-005), managed by J. Goldwasser.Peer reviewedPostprintPostprintPostprintPostprin

Numerical study on load-bearing capabilities of beam-like lattice structures with three different unit cells

The design and analysis of lattice structures manufactured using Additive Manufacturing (AM) technique is a new approach to create lightweight high-strength components. However, it is difficult for engineers to choose the proper unit cell for a certain function structure and loading case. In this paper, three beam-like lattice structures with triangular prism, square prism and hexagonal prism were designed, manufactured by SLM process using AlSi10Mg and tested. The mechanical performances of lattice structures with equal relative density, equal base area and height, and equal length for all unit cells were conducted by Finite Element Analysis (FEA). It was found that effective Young’s modulus is proportional to relative density, but with different affecting levels. When the lattice structures are designed with the same relative density or the same side lengths, the effective Young’s modulus of lattice structure with triangular prism exhibits the maximum value for both cases. When the lattice structures are designed with the same base areas for all unit cells, the effective Young’s modulus of lattice structures with square prism presents the maximum. FEA results also show that the maximum stress of lattice structures with triangular prisms in each comparison is at the lowest level and the stiffness-to-mass ratio remains at the maximum value, showing the overwhelming advantages in terms of mechanical strength. The excellent agreements between numerical results and experimental tests reveal the validity of FEA methods applied. The results in this work provide an explicit guideline to fabricate beam-like lattice structures with the best tensile and bending capabilities

Matrix cracking of fiber-reinforced ceramic composites in shear

The mechanics of cracking in fiber-reinforced ceramic matrix composites (CMCs) under general loadings remains incomplete. The present paper addresses one outstanding aspect of this problem: the development of matrix cracks in unidirectional plies under shear loading. To this end, we develop a model based on potential energy differences upstream and downstream of a fully bridged steady-state matrix crack. Through a combination of analytical solutions and finite element simulations of the constituent stresses before and after cracking, we identify the dominant stress components that drive crack growth. We show that, when the axial slip lengths are much larger than the fiber diameter and when interfacial slip precedes cracking, the shear stresses in the constituents are largely unaffected by the presence of the crack; the changes that do occur are confined to a 'core' region within a distance of about one fiber diameter from the crack plane. Instead, the driving force for crack growth derives mainly from the axial stresses - tensile in the fibers and compressive in the matrix - that arise upon cracking. These stresses are well-approximated by solutions based on shear-lag analysis. Combining these solutions with the governing equation for crack growth yields an analytical estimate of the critical shear stress for matrix cracking. An analogous approach is used in deriving the critical stresses needed for matrix cracking under arbitrary in-plane loadings. The applicability of these results to cross-ply CMC laminates is briefly discussed

A concept for mitigating head injury under translational blunt impact

© 2015 Taylor & Francis This study assesses a bi-layer composite concept for mitigating the severity of injury due to translational blunt impact of an unprotected head at moderately high speeds. The concept comprises crushable foam and a stiff face-sheet on the impacting face. Approximate analytical models for acceleration–time histories of prototypical impact scenarios are used to guide the design. The key design variables probed experimentally are the crushing strength of the underlying foam and the tile size. The efficacy of the composite systems and the foams alone is ascertained through a series of drop impact tests with an instrumented head-form at a representative impact velocity (6.7 m/s, 15 mph), using three commercial viscoelastic foams, with and without face-sheets. The measurements are analysed in terms of five performance metrics: the peak acceleration, the Gadd severity index (GSI), the head injury criterion (HIC), the skull fracture correlate (SFC) and the head impact power (HIP). The experiments demonstrate that, with the addition of a face-sheet, each of these metrics can be reduced substantially (by as much as a factor of two) relative to those of the foam alone. The benefits derive from spreading of contact forces over a larger area of foam by the face-sheet

Effects of weave architecture on mechanical response of 2D ceramic composites

A meso-scale finite element model is developed to investigate effects of weave architecture on strain and stress evolution in an eight harness-satin SiC/SiCN composite. Fiber tows are modeled explicitly using elastic rebar layers embedded within elastic/plastic effective medium elements. Effects of through-thickness constraint are investigated using several idealized test geometries, ranging from a single (unconstrained) ply to a fully-constrained two-ply lay-up with periodic boundary conditions in the through-thickness direction. A parallel experimental study of surface strain evolution in a representative SiC/SiCN composite is used to assess the model predictions. The results indicate that, because of bending and straightening of wavy tow segments at the locations of tow cross-overs, strain and stress concentrations arise. The effects are exacerbated by reductions in the constraints on bending and straightening caused by matrix damage, especially in surface plies. The implications of the results in the fracture process and on potential mitigation strategies are discussed

Effects of weave architecture on mechanical response of 2D ceramic composites

A meso-scale finite element model is developed to investigate effects of weave architecture on strain and stress evolution in an eight harness-satin SiC/SiCN composite. Fiber tows are modeled explicitly using elastic rebar layers embedded within elastic/plastic effective medium elements. Effects of through-thickness constraint are investigated using several idealized test geometries, ranging from a single (unconstrained) ply to a fully-constrained two-ply lay-up with periodic boundary conditions in the through-thickness direction. A parallel experimental study of surface strain evolution in a representative SiC/SiCN composite is used to assess the model predictions. The results indicate that, because of bending and straightening of wavy tow segments at the locations of tow cross-overs, strain and stress concentrations arise. The effects are exacerbated by reductions in the constraints on bending and straightening caused by matrix damage, especially in surface plies. The implications of the results in the fracture process and on potential mitigation strategies are discussed

A concept for mitigating head injury under translational blunt impact

© 2015 Taylor & Francis This study assesses a bi-layer composite concept for mitigating the severity of injury due to translational blunt impact of an unprotected head at moderately high speeds. The concept comprises crushable foam and a stiff face-sheet on the impacting face. Approximate analytical models for acceleration–time histories of prototypical impact scenarios are used to guide the design. The key design variables probed experimentally are the crushing strength of the underlying foam and the tile size. The efficacy of the composite systems and the foams alone is ascertained through a series of drop impact tests with an instrumented head-form at a representative impact velocity (6.7 m/s, 15 mph), using three commercial viscoelastic foams, with and without face-sheets. The measurements are analysed in terms of five performance metrics: the peak acceleration, the Gadd severity index (GSI), the head injury criterion (HIC), the skull fracture correlate (SFC) and the head impact power (HIP). The experiments demonstrate that, with the addition of a face-sheet, each of these metrics can be reduced substantially (by as much as a factor of two) relative to those of the foam alone. The benefits derive from spreading of contact forces over a larger area of foam by the face-sheet