Moisture Content Changes and Creep of Wood

Beech beams 2 by 2 mm in cross section were supported over a span of 60 mm and loaded at the center of the span. The beams were exposed at controlled temperatures for 24 hours to completely dry conditions or to a variety of relative humidities. The central deflection of the beams was observed with a cathetometer; similar observations were made on beams kept at a fixed humidity to provide information on the creep under steady conditions. Test results showed the increase in deflection due to cycling is very large; the beam loaded to three-eighths of the maximum broke after 14 complete cycles. The deflection before failure was 19 millimeters. Further studies showed there is a fatigue effect associated with moisture content cycling and that small beams can be broken by this means at loads well below the short-term breaking load. The dependence of the magnitude of the additional deflection on the change in moisture content was investigated and it was found that the points for deflection obtained during the change from 53 percent to 93 percent relative humidity lie on the same curves as those obtained when cycling from zero percent relative humidity. The effect of temperature on the additional deflection was studied and it was found that the temperature effect is significant at the 1 percent level for urea, but only at the 10 percent level for relative deflection. Study on behavior of wood in shear showed that moisture cycling produces marked increases of deformation in shear loading as well as in bending. Further study showed no change in Young’s modulus with successive cycling.

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