Proposal of an estimation method of moisture gradient in lumbers based on the eigen-strain methodology

Abstract

Lumbers must be dried to stabilize their shape, to strengthen, and to prevent corrosion and discoloration before using. Since dehydration usually occurs from outer surface, moisture content in inner layers becomes higher than that of outside. Therefore, higher moisture gradient which may cause unexpected deformation and cracks in materials can be seen after drying. Today, there are some non-destructive techniques to measure moisture content but it is difficult to measure moisture gradient in the thickness direction without sectioning. This study aims to propose an estimation method of moisture slope for whole log based on the eigen-strain methodology in which three-dimensional residual stress distributions can be evaluated from released strains on surface. In the proposed method, shrinkage by dehydration is expressed by eigen-stains which are estimated by an inverse analysis from released strains by strain gauges in processing. When released strains due to moisture gradient are obtained in lumber processing of necessity, the process of this method can be regarded as nondestructive, essentially. In order to prove the effectiveness of this method, numerical simulations were carried out for a log with moisture gradient by using FEM (Finite Element Method). In this analysis, a split liner for prevent splitting was conducted to a log to measure released strains on surface. Moisture gradient could be estimated accurately from released strains when moisture gradient in the thickness direction was relatively steep. However, estimation accuracy became poorer when slope of moisture gradient was relatively moderate. In order to improve the estimation accuracy, a log was split in sequential processes and a formulation was attempted by consolidation those sequential histories. And, estimation accuracy could be improved successfully by adopting the sequential manner.