The Inelastic Behavior of Wood: A New Concept for Improved Panel Forming Processes

The purpose of this study was to investigate the effect of temperature, moisture content, direction of test (radial and tangential), and strain level on the inelastic behavior of wood in compression perpendicular to the grain, and to relate this behavior to the particleboard process, Yellow-poplar (Liriodendron tulipifera L.) was chosen for the experimental work because it is even-grained and compresses more readily than denser hardwoods. Specimens were tested in compression per pendicular to the grain in the radial and in the tangential directions under each combination of the following conditions. Temperature: 80?, 120?, 160? and 200?F. Moisture Content: Green, 18, 12, and 6 percent. Strain Level: 0.0240 and 0.0360 inches per inch. Rate of Strain: 0.013 inches per minute. Duration of Stress: 10 minutes. The following conclusions, applicable to the inelastic behavior of yellow-poplar in compression perpendicular to the grain, could be drawn from the experimental evidence: Unrecovered strain, stress relaxation, coefficient of relaxation b, and maximum stresss at strain level are significantly affected by moisture content, temperature, strain level and direction of test independently and by various interactions between them. Only the coefficient of relaxation b is not affected by direction of test, indicating that the structural elements in the radial and tangential directions relax at similar average rates. The coefficient of relaxation b is independent of unrecovered strain and stress relaxation, but is inversely dependent on maximum stress at strain level. Differences in the inelastic behavior between the radial and tangential directions can be attributed to the rays in the radial direction. Moisture content, more than any other effect, influences the inelastic behavior in both the radial and tangential directions. Elasticity under compression increases with increasing moisture content. Yellow-poplar does not relax in a linear manner for short relaxation times.

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