Effects of moisture and mechanical loading on wooden materials

In a previous paper (Mårtensson 1988 a), results of tests performed on hardboard in tension under cyclic humidity were presented and comparison with a suggested constitutive model was made. This paper presents the result of further work that has been carried out concerning modelling of the behaviour of tempered hardboard subjected to moisture loading combined with mechanical loading. Tests were made both in compression and bending. The tests in compression were of two types, conventional creep tests during moisture cycling and quasi-relaxation tests during moisture cycling. The model was quantified on the basis of the previous tension tests and the compression tests, in both cases creep tests, and was then checked independently against the other types of test.
Interaction between moisture change and mechanical loading was found to be significant during the first moisture cycle but much smaller during subsequent cycles. Experimental and numerical analysis of composite structural elements was also performed, showing that internal stresses of significant magnitude are developed as a result of variations in moisture. Such stresses can be predicted with reasonable accuracy using the proposed model. This can be of great value in design and development of wood products and wooden structural elements.