In softwood material, the coupling between mechanical loading and hydric state is known as the mechanosorptive effect. However, the coupling with viscoelastic effect remains unclear so far, especially when the loading is controlled by strain. In this context, the present paper is focused on the process of creation and recovery of 'hygrolock' behaviour, i.e. a stress locking effect which occurs in a phase of drying under load. For this purpose, several relaxation tests were first performed on small-scale silver fir specimens in order to express the relaxation function in terms of the ambient humidity. Then, two mechanosorptive tests were carried out in order to induce hygrolock stresses in the same sample loaded in sustained strain condition, and subjected to cyclically varying humidity. Based on the assumption of stress partition, the analysis of the test results clearly shows the existence of a hygrolock stress. From these experimental evidences, a law is finally proposed to describe the evolution of the hygrolock stress in terms of the hydric state of the softwood material.Electronic supplementary material The online version of this article (
ABSTRACT. An incremental formulation suitable for modelling of materials with damaging viscoelastic behaviours is proposed in this work. A constitutive law based on linear viscoelasticity coupled with strain dependent damage is developed. The viscoelastic model is represented by a generalized Maxwell's chain. It is governed by a set of internal stress variables attached to the branches of the Maxwell's chain. The damage evolution is governed by values gained by a pseudo strain. The coupled law is turned into an incremental form suitable for the numerical analysis of damaging time dependent structures. Taking advantage of the incremental form, the coupled damaging viscoelastic law is implemented as a step-by-step procedure. The calculation procedure consists of a damaging elastic step followed by a number of damaging viscoelastic steps. The damage variable is adjusted at each step, according to the value gained by the pseudo strain. Exemplary calculations are worked out for two cases of uniaxial and biaxial variable or cyclic loadings. The results show the efficiency of the incremental model. It is worth noticing that the time increment used for the calculations is not necessarily small. As a consequence, precise analysis of damaging time dependent structures can be performed for low calculation cost.
ABSTRACT. Stress states caused in wood by drying periods are often the source of considerable structural disorders, when this material is used as structural material. The origin of these stresses is generally due to the viscoelastic behaviour of the wood combined with the dimensional variations (shrinkage) related to the moisture content. In order to better understand this phenomenon, a slice of green wood is submitted to natural drying in stable environmental conditions. The wood slice is placed on an electronic balance so as so to measure the moisture content variation during the drying period. Simultaneously, the displacements caused in the slice by the drying are captured by a video camera. An incremental relaxation model based on the generalized Maxwell's chain is used to analyze the evolution of the stresses induced by the drying process within the wood slice. Numerical results show the development of tensile stresses in the material. Analyzing these results leads to the conclusion that the stresses are due to the orthotropic behaviour of the wood material combined with anisotropic drying shrinkage. A tensile stress concentration is evidenced in a zone where a crack was finally observed during the test.
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