Effect of Head Speed and Span on Modulus of Rupture in Several Types of Commercial Hardboards

Thirteen types of hardboard representing four producers, four sources of raw material, three production processes, two thicknesses (1/8- and 1/4-inch), and two classes (treated and untreated) were included in the sampling plan to investigate the relationship of head speed and span length to modulus of rupture in the static bending test on commercial hardboards. Eight head speeds ranging from 0.05 to 4.00 inches per minute were employed in testing material over 4- and 8-inch spans. As a check on previous work, a supplementary experiment included testing one type of board on 2-, 3-, and 6-inch spans for part of the range of head speeds. Values of moisture content and specific gravity were determined so that the effect of these factors on head speed and span could be eliminated in the statistical analysis of results. Thus the true effects of head speed and span on modulus of rupture could be evaluated. Test results indicated a range of head speeds from 0.50 to 2.00 inches per minute that did not yield significantly different modulus of rupture values. This was valid for any span from 4 to 8 inches. Furthermore, it was determined that after a critical rate of fiber strain is reached, modulus of rupture values fall off sharply. This value is estimated as being between 0.33 and 0.50 inches per inch per minute, with the 1/8-inch-thick material probably having a higher critical value than the-1/4-inch material. The test results suggest the advisability of changing the static bending procedure in the current tentative ASTM standard (D1037-52T) to recommend the use of a 6-inch span and any head speed between 0.50 and 2.00 inches per minute for all commercial hardboards regardless of thickness or class of board.

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