Materials engineered with an internal architecture in order to achieve unusual properties, so-called mechanical metamaterials, are a promising candidate in the ongoing quest for lightweight impact mitigation. During impact events, these materials are subject to high strain rates, and the forces occurring due to the deceleration of the impactor are transmitted in a non-uniform way. The prevailing research in the field of impact mitigation focuses largely on the global effects of architected materials, with less attention being paid to the internal mechanisms of these structured materials. While there have been recent studies on the distribution of forces throughout an impact event, less research is devoted to the transmission of forces and the distribution of energy dissipation. The objective of this study is to examine the transition from static deformation patterns to dynamic phenomena for different types and sizes of microstructure, and to understand both the force transmission through the patch and the energetic distributions in different strain rate regimes. To enable this investigation discretized — geometrically as well as materially — nonlinear Timoshenko-Ehrenfest beams are used in implicit and explicit finite element schemes. The transmitted force levels and energy dissipation are investigated for two auxetic architectures (one for each mechanism resulting in a negative Poisson's ratio) and one non-auxetic architecture. The dynamic force levels transmitted to the back face exhibit an initial peak of a similar magnitude for all investigated strain rates and stabilize to the static stress plateau for each architecture. While the global amount of potential energy remains largely unchanged for all investigated rates, the amount of dissipation and kinetic energy demonstrates a non-linear increase from static deformation to slow and high rate deformation. The phenomena observed in different architectures are highlighted, and the differences are explained and related back to the configurations of the lattices. Notably, the prevalent notion in literature asserting the superiority of negative Poisson's ratio materials for impact mitigation applications is not replicated in this study.

A yield function in the stress resultant space of geometrically exact beams based on the elastoplastic cross-sectional warping problem has been proposed by Herrnböck et al. (Comput Mech, 67(3):723–742, 2021). This plasticity framework has been extended with a hardening tensor to model the kinematic hardening effects in Herrnböck et al. (Comput Mech, 71(1):1–24, 2022). While this framework provides scaling for the yield surface in ideal plasticity, scaling in hardening plasticity has not yet been explored. This paper focuses on the numeric modelling of hardening beams and beam assemblies at different geometric scales. Discretization effects from the introduction of plasticity into the geometrically exact beam model are demonstrated. Furthermore, the effects of scaling are explored, and a method to mitigate undesirable effects in order to achieve a size-agnostic formulation is proposed. Consistent geometric scaling is demonstrated for two alternative scaling approaches of the yield function.

Lightweight materials used for impact mitigation must be able to resist impact and absorb the maximum amount of energy from the impactor. Auxetic materials have the potential to achieve high resistance by drawing material into the impact zone and providing higher indentation and shear resistance. However, these materials must be artificially designed, and the large deformation dynamic effects of the created structures must be taken into consideration when deciding on a protection concept. Despite their promise, little attention has been given to understanding the working mechanisms of high-rate and finite deformation effects of architected auxetic lattice structures. This study compares the static and dynamic elastic properties of different auxetic structures with a honeycomb structure, a typical non-auxetic lattice, at equivalent mass and stiffness levels. In this study, we limit the investigation to elastic material behavior and do not consider contact between the beams of the lattices. It is demonstrated that the equivalent static and dynamic properties of individual lattices at an undeformed state are insufficient to explain the variations observed in impact situations. In particular, the initial Poisson's ratio does not determine the ability of a structure to resist impact. To gain a thorough comprehension of the overall behavior of these structures during localized, high rate compression, the evolution of the elastic tangent properties under compression and shear deformation was monitored, leading to a more profound understanding. Observations made in one configuration of stiffness and mass are replicated and analyzed in related configurations.

Adobe is a traditional masonry made of sundried earthen bricks and mud mortar. Despite a millennial history of buildings of architectural value, adobe still connotes a so called ‘not engineered’ construction type. Namely, the material and structural properties of adobe are still not entirely addressed, resulting in an equally uncertain normative framework for adobe buildings design. However, over the last ten years, a large research program has been conducted in the Netherlands to qualify the material and structural properties of this sustainable building technology. In this paper, a critical analysis of the current normative body for the material characterization of adobe is addressed. Guidelines, prescriptions and requirements related to test methods, materials selection and properties contained in the available building codes for adobe around the world are assessed. A critical normative review is performed using the most recent literature produced on adobe, with particular regards to the results of experimental tests and numerical simulations performed by the authors. On the basis of these findings, some issues have been identified in relation to the knowledge currently condensed in the norms for adobe. A series of programmatic guidelines is aimed at orienting future research on adobe as well as fostering the process of updating its current normative body.

In this study, the impact of a steel spherical projectile on an alumina ceramic is considered. New experimental and numerical results are presented and analysed. Numerical results are obtained using the Finite Element Method and the upgraded viscosity regularized Johnson-Holmquist-2 constitutive model to describe the ceramic material behaviour. First, a short investigation is done using 2d Finite Element simulations to establish a proper numerical framework. Second, the numerical framework is extended to 3d and experimental results are used to validate the framework and the ceramic material model. This shows that all relevant ceramic failure mechanisms are captured correctly and the framework and model can be used to simulate sphere impact on ceramic material. Third, the simulations are used to analyse the failure processes in the ceramic material in more detail. Here the focus lies in obtaining information which can currently not be retrieved from experiments. Timing and interaction of propagating conical and radial cracks are investigated and corroborate with the typical failure mechanisms observed in sphere impact on ceramic material.

A local damage model has been recently developed for the numerical simulation of the static behaviour of adobe bricks. Mesh insensitivity of the local model was obtained by generalizing the damage delay concept based on a Dirichlet boundary condition decomposition integrated in an implicit solver. The regularization properties of the model were proven before only in statics. In this study, mesh independence is demonstrated in dynamics analysing the problem of a cantilever bar uniaxially loaded at high deformation rates. Furthermore, the physical background of the delay formulation is interpreted regarding the main failure processes in compression exhibited by quasi brittle materials used in masonry. Two limitations of the model in correctly simulating the dynamic behaviour of masonry bricks have been observed. Corrections to the original damage delay formulation are proposed in this study. These enhance the capability of the model to address also distributed failure of traditional geo-materials and the inherent rate dependence also at high strain rate regimes. The improvements are demonstrated in this paper by means of numerical simulations of both theoretical tests and practical applications. These consist of experimental tests in compression recently performed by the authors at different strain rates, from statics to high velocity impact tests.

A local damage model is proposed for the numerical assessment of the static performance of Adobe masonry components. The model was applied to simulate the experimental behaviour of sundried soil bricks and mud mortar tested in uniaxial compression and bending. Numerical simulations of the model are made mesh objective by means of a rate dependent regularization algorithm in statics. This is achieved using a generalization of the damage delay concept based on a decomposition of the Dirichlet boundary condition. It allows non-dimensionality of model parameters mathematically needed to prevent loss of ellipticity of the equilibrium equations of the model. The entire regularization algorithm is integrated within an implicit Newton-Raphson solver.

This study presents the numerical analyses conducted to investigate the impact behavior of different porous concretes, which have also been cast and tested experimentally. For a realistic representation of the real porous concretes containing arbitrary shaped air pores, a mesh generation code was developed in which the aggregates in the mixtures were directly extracted through computed tomography. In the code, mineralogically different aggregates in porous concretes with gravel could also be individually defined. In the explicit finite element analyses conducted, porous concrete was considered as a four-phase material, consisting of aggregates, interfacial transition zones (ITZ), bulk cement paste and air. The pore size distribution and the fragmentation behavior of the concretes were also numerically analyzed. Among the parameters that have been investigated both numerically and experimentally, aggregate grading, which determines the porosity and pore size distribution of the material, was found to have a dominant effect on the strength as well as the fragmentation properties of porous concretes. Although the amount of ITZ is higher in mixtures containing finer aggregates, those mixtures had higher impact strengths compared to coarser aggregate ones again owing to their much finer pore structures.

The deformation-to-fracture evolution of a flexible polymer material under high-strain-rate compressive loading conducted by a split Hopkinson pressure bar (SHPB) setup was investigated. Representative tests were carried out at different strain rate levels, followed by the characterization of dynamic damage after each test. Craze and crack patterns on the end surface of the specimen were carefully analyzed. The failure patterns appear along the radial and circumferential directions. The sequence of their formation with increasing strain/stress level was revealed. The mechanisms resulting in the craze and crack patterns were analyzed. The heterogeneous stress distribution in the specimen and the resultant damage morphologies were demonstrated. This research not only shows the deformation-to-fracture evolution of a flexible polymer material under SHPB loading, but also provides a better clarification of the localized stress distribution in the tested material via SHPB technique.

Adobe is an ancient building technology made of sun dried bricks joined together by mud mortar. This paper deals with the physical and mechanical characterization of three different typologies of adobe bricks and one typology of mud mortar produced in Europe. They differed in terms of internal soil element proportions and amount of organic content. Physical tests consisted of granulometry, moisture content and density tests. The mechanical characterization consisted of uniaxial compressive tests and three point bending tests. Tests were performed according to modern material standards. The main mechanical properties both in tension and compression were determined at different curing conditions. The outcome provided in this study offers a general overview on the assessment of the mechanical performance of adobe in relation to the properties and interactions of its soil constituents. In fact, the comparison between components with the same soil mineralogical family and production process made it possible to assess both at a qualitative and quantitative level the effect of the physical properties of the mixture (such as fiber and clay percentages or moisture content) on the mechanical parameters of the resulting bricks and mortar. This paper proposes new predictive formulations of the most relevant material parameters in strength and deformation, such as compressive strength, deformation at peak stress and ultimate displacement for both adobe bricks and mortar. They quantify the influence that water content, clay percentage and fiber reinforcement produce on the mechanical performance of the tested adobe components. This was made possible by means of multivariate statistical analyses on the mechanical parameters derived from all the tested samples.