Tensile Properties of Loblolly Pine Growth Zones
For the determination of tensile properties of the earlywood or latewood zone of loblolly pine, modified standard ASTM tensile specimens were machined so that the critical sections at mid-length of specimens (2 1/2 inches length of uniform cross section) consisted entirely of either earlywood or latewood. Specimens were tested to failure in both the green and air-dry condition.
Specific stress and specific stiffness of latewood in the air-dry condition are more than 50% and 63% higher, respectively, than corresponding values for earlywood. More than one-third of the tensile strength of latewood is attributed to factors other than density. Moisture affects the specific strength of latewood more than the specific strength of early-wood. The moisture effect on specific stiffness is approximately the same for both earlywood and latewood. The property affected most by moisture is the specific stress at proportional limit.
Same growth zones were tested by microtome sections of approximately 100 μm in thickness. Maximum tensile properties of any growth zone obtained from microtome sections were approximately one-half of the values obtained for the same zone from modified standard tensile specimens.
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