A New Approach to the Determination of Fiber Saturation Point from Mechanical Tests

Estimation of fiber saturation point by mechanical testing involves plotting modulus of rupture or modulus of elasticity values against moisture content. As moisture content increases from zero percent, the modulus first decreases and then becomes constant. The moisture content at which the modulus first becomes constant (the intersection point) can be taken as an estimate of fiber saturation point. Another estimate can be obtained from a plot of percent shrinkage against moisture content where the line is extrapolated to the moisture content corresponding to zero percent shrinkage. Fifteen domestic and tropical species were tested for shrinkage, MOR, and MOE in the green condition and at equilibrium moisture contents established under relative humidities of 90, 80, 60, and 25 percent. Logarithmic MOR and MOE results were used to determine intersection points by least squares regression and these points were averaged (A) for each species. Stress-free shrinkage intersection points (B) for each species were obtained by plotting percent tangential shrinkage against moisture content and reducing the lines for permanent set that are induced by early drying stresses. A regression estimate (C) of mechanical intersection points was obtained by correlating (A) and (B). The best estimate of mechanical intersection point was judged to be the combined average of the regression estimate (C) and the experimental average (A). To determine air-dry to green strength ratios, MOR and MOE values at 12 percent moisture were interpolated from the test data. Correlation of the best estimate of mechanical intersection point with air-dry to green MOR ratios gave a correlation coefficient of 0.9332. A correlation using MOE instead of MOR gave a coefficient of 0.8036.

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