Creep in chipboard. 9. The effect of steady-state moisture content, temperature and level of stressing on the components of creep deflection for a range of boards

Simples of five chipboards, one waferboard, one plywood, one fibre building board and redwood timber (Pinus sylvestris) were loaded in 4-point bending under four stress levels: 30, 45, 67.5 and 75% of the short term ultimate, thereby extending earlier work at 60% stress level. Five environmental conditions were used at each stress level: 10 degrees C 62% relative humidity (rh); 20 degrees C 30% rh; 20 degrees C 65% rh; 20 degrees C 90% rh and 30 degrees C 68% rh. Deflections were recorded at least on a daily basis for a period of 6 months, unless failure of the sample occurred earlier. Values for the elastic, viscoelastic and viscous components of creep deflection were calculated for two time periods using the 4-element rheological model developed in an earlier paper: comparisons were made on the basis of the percentage contribution of each component. The relative proportions of the elastic, viscoelastic and viscous components making up total deflection were found to vary considerably with time, temperature, relative humidity, stress level and material. The relationships were complex with strong interactions occurring among environmental condition, stress level and material. Thus, for example, the effect of high temperature, or high relative humidity, in determining the magnitude of the viscous component was much greater at higher levels of stressing: waferboard appeared to be less sensitive to high humidity and solid timber more sensitive to high temperature than the other materials. T-tests further confirmed the complexity of the relationships. It is concluded that much of the variability in creep deflection recorded in Part 8 of this series is primarily due to changes in the percentage contribution of the viscous component