Mechano sorptive behaviour of notched beams in bending

Short term bending tests with end-notched beams at constant or varying moisture content have shown an apparent contradictory dependency between moisture content and strength. The higher the moisture content the higher the strength. Varying moisture results in particularly significant differences.In order to study this phenomenon, moisture content and deformations perpendicular to grain were monitored at a stepwise changing humidity (55%, 90% RH at 20 degrees C). The normal stresses perpendicular to grain due to restricted swelling were then obtained by assuming Hooke's law applies and by neglecting deformation due to shear. Compression stresses perpendicular to grain in excess of 6 MPa were found in the vicinity of the notch following a period of adsorption. Similarly, small tension stresses of the order 1 MPa were registered in this area when the specimens were at their most dry condition.It thus seems that the unexpected increase in strength due to moisture uptake may in part be explained by the built-up of compression stresses perpendicular to grain caused by the moisture induced restricted swelling. Such compression stresses act as a reinforcement which must first be overcome before a tension perpendicular to grain failure can develop and ultimately lead to failure.