M. Lamine Boubakar

Nonlinear tensile behaviour of elementary hemp fibres. Part I: Investigation of the possible origins using repeated progressive loading with in situ microscopic observations

Abstract The aim of this study is to achieve a better understanding of the nonlinear tensile behaviour of the elementary hemp fibre. This is of great importance in view of the need to develop an efficient predictive tool for the design of natural fibre reinforced composites. This first paper investigates the possible mechanisms responsible for the nonlinear behaviour, using repeated progressive tensile loading with in situ polarised light microscopy. The persistence of residual strain has been confirmed during testing when the tensile load was released. Only a certain fraction of this residual strain is reversible, and the reversibility is time-dependent. Beyond the yield level, the fibre’s rigidity is not deteriorated, but significantly increased as a function of the number of loading cycles and the level of strain. A new scenario involving a stick–slip mechanism, extension and re-orientation of the microfibrils and shear strain-induced crystallisation of the amorphous cellulose is proposed.

Creep behaviour of single hemp fibres. Part II: Influence of loading level, moisture content and moisture variation

This work investigates the tensile creep behaviour of single hemp fibres under constant and cyclic loading coupled to constant or variable moisture content environment. Results show that the primary creep strain rate of such fibres decreases with the increasing stress, while the secondary creep strain rate increases. Load cycling at an average load higher than constant creep load produces a large additional extra creep strain and an increase of the creep rate. Both primary and secondary creep strain rates increase with the increasing moisture content. More creep is also observed in cyclic humidity conditions than in a constant environment at the high-humidity. In agreement with some observations on synthetic fibres, we showed that this accelerated creep is only observed for high moisture cycling rates. This mechanosorptive effect is consistent with sorption-induced stress-gradient explanations proposed in literature.

Creep behaviour of single hemp fibres. Part I: viscoelastic properties and their scattering under constant climate

The literature on the time-dependent behaviour of single bast fibres such as flax and hemp is extremely poor. The aim of this extensive study is to characterise the long-term behaviour of elementary hemp fibres and to establish suitable constitutive laws. Single hemp fibres are shown to exhibit both instantaneous strain and delayed, time-dependent strain when tensile loaded under constant climate. The creep behaviour appears to be a logarithmic function of time with a high strain rate during the primary creep and a lower and constant one during the secondary creep. A large scattering both in time-dependent properties and behaviour was observed on a batch of 25 single fibres. Three main creep behaviours were observed. Type II is truly linear as a function of the logarithm of time while Type I and Type III are strongly nonlinear and can be described, respectively, by concave and convex functions. A rheological model based on an anisotropic viscoelastic law and on a truncated inverse Gaussian spectrum of viscous mechanisms was shown to successfully describe all the experimentally observed behaviours.

Comportement élastoplastique anisotrope pour l'analyse numérique des coques minces en grandes transformations

ABSTRACT Due to rolling and cristallographie texture development, thin sheets metal for stamping exhibit an initial orthotropic plastic behaviour that have to be incorporated in numerical modelling of sheet forming processes. A mechanical modelling of the behaviour is presented. It is based on a formulation in a rotating frame updatted using the proper rotation tensor. The stress calculation algoritlim associated is based on the prediction-correction method in the rotating frame and explicitly takes into account shell kinematics with mixed transverse shear interpolation. Numerical results on deep drawing problems show the accuracy and the efficiency of the proposed approach.