Volume 23, Issue 6 (November 1995)
Prediction of Creep Properties of Chipboard Used in Stressed-Skin Panels
In recent years, attention has been focused on structural stressed-skin panels (SSP) with lumber webs and sheet metal and chipboard panels as tension and compression flanges, respectively, at Chalmers University of Technology. In this study, certain mechanical and rheological properties of structural chipboard were evaluated as part of a larger project designed to predict the long-term behavior of SSP under load.
Several series of tests were conducted to evaluate short- and longterm properties of urea-formaldehyde bonded chipboard. From eight full-sized panels, specimens were fabricated for use in the determination of chipboard properties. Compressive strength and modulus of elasticity (MOE) were evaluated from short-term tests. Average strength and MOE values of 20 and 4100 MPa, respectively, were obtained. The MOE value was virtually identical to that obtained from initial deformations found in subsequent creep tests, indicating that test method had no effect on MOE.
Three series of long-term (11 000-h) tests were conducted under constant and similar environmental conditions and different combinations of stress-level and loading history. The results suggested that the magnitude of relative creep was greatest under lower stress levels. Two empirical models, a Clouser function without exponential terms and a power function with exponential terms (Clouser, W. S. “Creep of small wood beams under constant bending load,” FPL Report 2150, U.S. Forest Products Laboratory. Madison, WI., 1959), were evaluated as predictors of the deformation-time relationship for chipboard under constant loading. The power function with the exponential terms proved to be superior at extrapolating short-term results to long-term predictions. However, both functions were relatively similar with respect to describing the long-term behavior of chipboard as the compression flange in a SSP. Measuring methods and definition of the initial deformation were found to be important, especially for predicting the relative creep factor for chipboards under different load histories.