Vincent Picandet

Influence of compactness and hemp hurd characteristics on the mechanical properties of lime and hemp concrete

ABSTRACT Research on concrete made with hemp is part of a sustainable development policy in the building field. The very low apparent density of the hemp hurds confers to lime and hemp concrete lightness and low thermal conductivity, however compared to usual construction materials, it has low mechanical strength. The presence of fibres in the hemp, the optimization of the granular to binder ratio, and the compaction during the casting process represent parameters which could significantly increase the compressive strength and change the mechanical behaviour of the material. A brief study of these parameters is presented and discussed in this paper.

Effect of compaction on mechanical and thermal properties of hemp concrete

ABSTRACT Some preliminary studies, dealing with the process optimisation of pre-cast building elements made of Lime and Hemp Concrete (LHC), have shown that compression during casting lead to significant improvements: better mechanical characteristics and facing. However, this compaction leads to an increase of the weight to volume ratio and to a decrease in porous volume. Thus, the amount of entrapped air inside material, which contributes to decrease the thermal conductivity, is lower. Our data actually show a slight increase in thermal conductivity when compactness increases. The goal of this study is to compare the effect of compaction during casting on both mechanical and thermal characteristics of hardened specimens in order to evaluate the relevance of such a process.

From discrete to nonlocal continuum damage mechanics: Analysis of a lattice system in bending using a continualized approach:

It is shown herein that the bending problem of a discrete damage system, also called microstructured damage system or lattice damage system, can be rigorously handled by a nonlocal continuum damage mechanics approach. It has been already shown that Eringen’s nonlocal elasticity was able to capture the scale effects induced by the discreteness of a microstructured system. This paper generalizes such results for inelastic materials and first presents some results for engineering problems modelled within continuum damage mechanics. The microstructured model is composed of rigid periodic elements connected by rotational elastic damage springs (discrete damage mechanics). Such a discrete damage system can be associated with the finite difference formulation of a continuum damage mechanics problem, i.e. the Euler–Bernoulli damage beam problem. Starting from the discrete equations of this structural problem, a continualization method leads to the formulation of an Eringen’s type nonlocal model with full coupling between nonlocal elasticity and nonlocal continuum damage mechanics. Indeed, the nonlocality appears in this continualized approach both in the constitutive law and in the damage loading function. A comparison of the discrete and the continuous problems for the cantilever shows the efficiency of the new micromechanics-based nonlocal continuum damage modelling for capturing scale effects. The length scale of the nonlocal continuum damage mechanics model is rigorously calibrated from the size of the cell of the discrete repetitive damage system. The new micromechanics-based nonlocal damage mechanics model is also analysed with respect to available nonlocal damage mechanics models.

On Nonlocal Computation of Eigenfrequencies of Beams Using Finite Difference and Finite Element Methods

In this paper, we show that two numerical methods, specifically the finite difference method and the finite element method applied to continuous beam dynamics problems, can be asymptotically investigated by some kind of enriched continuum approach (gradient elasticity or nonlocal elasticity). The analysis is restricted to the vibrations of elastic beams, and more specifically the computation of the natural frequencies for each numerical method. The analogy between the finite numerical approaches and the equivalent enriched continuum is demonstrated, using a continualization procedure, which converts the discrete numerical problem into a continuous one. It is shown that the finite element problem can be transformed into a system of finite difference equations. The convergence rate of the finite numerical approaches is quantified by an equivalent Rayleighs quotient. We also present some exact analytical solutions for a first-order finite difference method, a higher-order finite difference method or a cubic Hermitian finite element, valid for arbitrary number of nodes or segments. The comparison between the exact numerical solution and the approximated nonlocal approaches shows the efficiency of the continualization methodology. These analogies between enriched continuum and finite numerical schemes provide a new attractive framework for potential applications of enriched continua in computational mechanics.

Static and dynamic behaviors of microstructured membranes within nonlocal mechanics

AbstractIn this paper, the static and dynamic behaviors of a finite microstructured rectangular membrane were studied. The microstructured membrane model comprised an equal number of elastic spring...

A localization analysis of a non-uniform damage lattice in presence of strength gradient

The failure of a non-uniform axial damage chain under uniform tension is studied both with discrete damage mechanics (DDM) and continuum damage mechanics (CDM). It is shown that a micomechanics-based nonlocal CDM model may be built from a DDM formulation, that may include material heterogeneities. DDM is based on a microstructured model consisting in multiples elastic-damage springs, whose elastic yield threshold is variable and depends on the position along the chain. We aim to develop a nonlocal CDM model as a relevant continuous formulation of the lattice DDM system. To do this, we rely upon a continualisation procedure applied to the difference formulation of the lattice problem, which gives us a nonlocal propagating damage model. The boundary conditions of the nonlocal CDM problem are equivalent to a finite length damage cohesive law. Analytical and numerical results show a strong proximity of the discrete and enriched continuous approaches for this heterogeneous bar problem, as well as the effectiveness of the nonlocal damage model to capture the softening localization phenomenon in heterogeneous quasi-brittle fields.

Particle Size Distribution

In this chapter, a state of the art of Particle Size Distribution (PSD) measurement of bio-based aggregates and characterization methods is presented. Shiv particles coming from the stem of plants cultivated either for their fibers (hemp, flax, etc.) or for their seeds (oleaginous flax, sunflower, etc.) are very different from the mineral aggregates typically used in concretes. Owing to the structure of the stem of the plant they are made from, such aggregates are generally malleable, elongated and highly porous with a low apparent density. Irregular shape are generally observed, especially in case of shiv coming from fiber plant due to the shredding action of the decortication process. Such ground bio-mass lead usually to uni-modal size distribution that can be efficiently characterized using basic distribution models with two parameters. Starting from the standardized tools and techniques developed for mineral aggregates, other technics using image processing are investigated and discussed in the global perspectives of the effect of the PSD on the properties of the in-service building material.

Bulk Density and Compressibility

Hemp is made of highly deformable particles. Depending on the water content, on the particle size distribution, and on many other material parameters such as initial porosity and retting of the processed stalks, the mechanical behaviour of shiv in bulk can change significantly. In a compaction process, the mass per volume of the raw material increases with the applied stress and some creep or relaxation effects occur as observed in wood based materials. Hence, the mechanical properties of the bulk impact the packaging of the raw material as the shiv density inside the final mix and the in-service properties of the composite material. In this way, the bulk compressibility is primarily useful to manage the building processes, from transport of the raw material, to mixing and casting.

Scale effects in the static response of a one-dimensional quasi-brittle damage lattice

In this paper, we investigate the failure of a discrete elastic-damage axial system using both a discrete and an equivalent continuum approach. The microstructured damage chain consists of a one-dimensional damage lattice with direct nearest–neighbour interactions, which is composed of a series of periodic elastic-damage springs (axial lattice system treated in terms of discrete damage mechanics). We show that the damage lattice equations are equivalent to the centred finite difference formulation of a continuum damage mechanics (CDM) evolution problem. Such a discrete damage system reveals some scale effects on both the structural strength and stiffness. The nonlocal CDM in the hardening branch and cohesive damage model in the softening branch considered here are built using a continualisation procedure applied to the nonlinear difference equations of the lattice system. With this procedure, the difference equations to be solved are approximated by higher order differential equations. Using a rational asymptotic method, the continualised model appears to be equivalent to a nonlocal CDM model in the damage propagation zone. A finite length cohesive model is obtained in the softening range. A comparison of the discrete and the continuous problems for damage chains brings out the effectiveness of the new micromechanics-based nonlocal and cohesive continuum damage model, especially for capturing scale effects.

Nonlocal Continuum Damage Mechanics Approach of a Discrete Axial Chain under Non-Uniform Axial Load

The failure of a discrete elastic-damage axial system is investigated using both a discrete and anequivalent continuum approach. The discrete damage mechanics (DDM) approach is based on amicrostructured model composed of a series of periodic elastic-damage springs (axial DDM latticesystem). Such a damage discrete system can be associated with the finite difference formulation of aContinuum Damage Mechanics (CDM) evolution problem.The nonlocal CDM models considered in this paper are mainly built from a continualizationprocedure applied to centered finite difference schemes. A comparison of the discrete and thecontinuous problems for the chains shows the effectiveness of the new micromechanics-basednonlocal Continuum Damage modeling, especially for capturing scale effects.