Composite material Moisture Mechanics - Project description

Goal of project

The goal is computational tools by which global deformation properties of a wood fibre composite material can be analysed and the deformations of a product predicted. This over-all goal is divided into two steps:

 1. A numerically implemented model of the microstructure of wood fibre composite materials, enabling analysis of the global deformations of a (small) piece of the composite material when exposed to homogeneous states of climate (moisture and temperature) and mechanical load.

2. A numerically implemented continuum mechanics model of the deformation properties of a composite material, the numerical values of the model being obtained by the microstructure model.

Content and approach of research work

In analogy with the goal, the work will be divided into two steps: a licentiate project dealing with the microstructure modelling and a subsequent PhD project dealing with the continuum modelling. In the microstructure modelling the material will be assumed to be composed of a network of fibres embedded in matrix. For the numerical calculations, a laminated composite network model can be proposed: the layers are built up by fibre networks and the matrix material, respectively, interacting at nodal points in a spatially discretized model of an assigned structure. Geometrically the network may be modelled as a framework of curved beams interacting at their points of contact. For cellulose fibre materials without any matrix component this kind of numerical model is being developed at the present. First constant and then time-dependent conditions are considered. The constitutive equations for the constituents can probably be assumed to be of the form

              D e = a D s t + b D u + c s D u + d D Q

where e indicates the strain, a, b , c and d parameters, s stress, u moisture and Q temperature. Tests needed for calibration and verification will be carried out in co-operation with industry. The second step comprises the development of a suitable continuum mechanics model for the composite material and a homogenization technique by which parameters in the continuum model can be determined from numerical results obtained by the composite network model.

Implementation of the continuum model in a finite element code enables calculation of deformations of
composite material products that are in non-homogenous and varying states of moisture and stress. The model will verified by tests of the shape stability of panel products when exposed to gradients and variations in moisture and temperature. The proposed kind of modelling is new. The research on network mechanics modelling of fibre materials without any matrix ongoing at the division of Structural Mechanics, LTH, is internationally in the front of research. Network mechanics modelling involves heavy numerical calculations and has gained growing interest due to the development of high capacity computers and in recent years also from growing interest for network modelling research among physicists.

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