Analysis of bending creep of wood under natural environmental conditions by a Fourier transformation procedure

Bending creep tests of glued-laminated wood beams of four sizes, ranging fr om 1.1 cm(2) to 45.6 cm2 in cross section, were done under natural environm ental conditions for about one year. The bending creep tests were conducted by a two-point loading method, and stresses at the surface of the beams we re 6 Mpa. Deflection and mass of the beam, and environmental conditions wer e measured hourly using an electronic data recorder. The effects of specime n size on creep behavior under non-steady state moisture conditions were an alyzed using a Fourier transformation procedure. Creep components of typical periods were resolved by applying an inverse Fo urier transformation to an amplitude spectrum of time trend creep curves. T he creep components of daily and seasonal changes were distinguished within the amplitude spectrum. Well-known mechano-sorptive creep effects were observed in the creep compon ents of the daily and the seasonal changes resulting from the environmental conditions. For daily moisture changes the size effect was large for the small specimen s and decreased very: much with increasing specimen size, because the moist ure movement occurs only in the surface part of the specimens. For the seas onal, long-period moisture changes, the specimen size effect was less for a ll specimen sizes, because the moisture can penetrate deeply enough into th e specimens during long exposure to the environmental conditions. The resul ts show that the size effects are remarkably large for short-period creep o f small specimens with cross sections below 5 cm(2).