Hygro-Elastic Deformation of High Pressure Laminates. A Macroscopic Analysis. Theoretical Modelling and Measurements

Doctoral thesis, 1998

Deformation due to environmental changes in moisture arises in many engineering problems of practical interest. The purpose of this work is to develop a theoretical model for the determination of moisture induced deformations in thin plates made of High Pressure Laminates. For the determination of moisture induced deformations, a set of material properties is required. Measuring these properties is also a part of this work. The theoretical models for moisture transfer and elastic deformation are based on Fick's law and the Kirchhoff plate theory, respectively. High Pressure Laminates are classified as a composite material and the behaviour of composites can be analysed in different classes of theoretical problems. Here, the High Pressure Laminate is treated, on the macroscopic scale of observation, as a homogeneous, elastic and orthotropic material. A basic result of the thesis is a simple procedure to account for the effect of hygro-expansion. The moisture moment integral, which essentially is proportional to moisture content profile and thickness of plate, gives a basic time-dependent function I(t) which determines the effect of any uneven moisture distribution over the cross-section of the plate. The moisture-induced moments are proportional to I(t) and they only appear as an additional term in the boundary conditions of the plate. Solutions for simply supported plates are presented for different rectangular shape and, for different stiffness and hygro-expansion coefficients in the two orthotropic directions. Two PC-programmes, MHPL and MecHPL, based on the theoretical models are developed. MHPL deals with determining the moisture content of the material and MecHPL deals with calculating the moisture induced deformations in the plate. These programmes have been validated by using existing programmes and experimental tests. The results of the validations indicate that the models and PC-programmes are correctly formulated.