A comparison of geometry effect on tensile testing of wood strands

Abstract

There is currently no ASTM standard for testing the tensile mechanical properties of strands used in wood-strand–based composites. In this study we compared the ultimate tensile strength (UTS) parallel to grain and tensile modulus of elasticity (MOE) for southern pine (Pinus spp.) wood strands from an oriented strand board plant in which one treatment consisted of rectangle-shaped specimens and the other treatment consisted of samples milled into a tapered (‘‘dog-bone’’) shape. For bone-shaped samples, the measurements observed were 16 and 27 percent higher for MOE and UTS, respectively, than for the rectangular samples, and this was attributed to the generally accepted fact that dog-bone–shaped geometry yields measurements that are closer to true population parameters. Variation in mechanical properties was not statistically different for the two test methods. This study quantified that tensile testing of the rectangular strands will underestimate the true strength and MOE of the southern pine material. Because both methods resulted in similar levels of variability in test results, in-plant testing, using the traditional rectangular sample, may be acceptable for quality control as long as there is a recognition that the UTS and MOE values will be substantially more underestimated than those of smaller, dog-bone–shaped samples. As such, for future standards development, consideration should be given to the geometry of the strand when determining mechanical properties. Given the large amount of studies that use rectangular strands, there may be a need for a methodology to relate test results for rectangular and bone-shaped specimens.