Effect of Repeated Loading and Salt-Water Immersion on Flexural Properties of Laminated White Oak

This paper reports the results of a Forest Products Lab. study sponsored by the Bureau of Ships to learn the combined effect of moisture and repeated loadings on glued laminated structures similar to those used in naval vessels. Nine six-lamination, 1-3/4 inch by 3-3/4 inch by 43 inch, white oak specimens were assigned, side-matched, to each of the following five test groups: 1) dry control — tested without water exposure or loading; basis for groups 2, 3, 4; 2) repeated load, wet exposure–3 month salt water immersion followed by loading at half the ultimate strength of the control group 4 for 3, 6, or 9 million cycles; 3) repeated load, dry exposure–repeated loadings in 75?F., 65 percent relative humidity (RH) atmosphere at 50 percent stress of ultimate strength of control group 1; 4) wet control–3 month salt water immersion. 5) cyclic exposure and repeated load–half the specimens were tested without water or repeated load exposure, and the rest were subjected to 3 cycles (3 month immersion in salt water followed by 1 month drying in 75?F., 65 percent RH atmosphere) followed by loading at stress equal to 50 percent of dry controls, for 3, 6, or 9 million cycles. Test results indicated that immersion alone yields considerable strength and stiffness reduction, while immersion followed by repeated loadings yields increased bending strength. Dry repeated loading does not affect bending strength, though stiffness is increased somewhat. Some group 5 specimens failed under repeated loading, likely caused by higher moisture content, hence decreased MOR, of test specimens over the control group. Despite the limited number of tests and mismatching of specimens, it was observed that 1) glue bond is not affected by combined moisture and repeated load, since failures occurred in the wood itself; 2) bending loads up to 50 percent of the ultimate strength may be applied to a wet or dry member up to 9 million times without appreciably affecting bending strength and stiffness.

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