Shear Stresses Resulting from Swelling in Laminated Assemblies

The technique developed here makes it possible to study swelling or shrinkage forces which are restrained in shear. It could be used in studying the behavior of various types of laminated or cross-banded products. The shear stress near the center was found to be essentially zero during uniform sorption in normal wood. The shear stress on the outer area was slightly greater for the 1-1/2-inch thick laminate than for the 3/4-inch thick laminate. With a total change of about 4 percent moisture content, it was found that the shear stresses developed were higher when the change occurred over a 48-hour period than when it occurred over a 16- to 20-hour period. In compression wood, transverse shear stresses caused by uniform sorption are greater in the center area of the glue line than at the outer edge. Two types of laminates were used. The normal wood samples were old-growth Douglas-fir from the coast region with a rate of growth of 30 rings per inch and specific gravity of 0.47. Douglas-fir compression wood with a rate of growth of 13 rings per inch and specific gravity of 0.48 was obtained from northern Idaho. The block pairs that made up a test laminate were cut to 1-inch lengths and 4 and 7 inches in width and in thicknesses from 3/4 to 1-1/2 inches. The wood blocks were bonded to aluminum strips with epoxy resin of a rigid mixture. Two types of SR-4 strain gages were used. The wood and aluminum laminations remained in the conditioning room at all times until they were transferred to a specially built humidity chamber installed in a floor model testing machine. As test proceeded, and swelling occurred, the shear stresses which were transmitted to the aluminum center strip resulted in a reduction of the preload.

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