2988 Chapter 24 



(range of 66 to 120 inches). The 1/2-inch plywood had a more consistent average 

 MDH (64 inches for both edges), but the range was 36 to 78 inches for the 

 outside edge and 12 to 102 inches for the inside edge. 



Among flakeboards, yellow-poplar panels had the greatest and 1/2-inch ori- 

 ented panels the least impact resistance. Results for random pine panels were 

 similar to those for random mixed species panels. The 5/8-inch random flake- 

 board had a higher MDH than similar oriented flakeboard for the wet test but a 

 lower MDH for the dry test. The difference may have occurred because of 

 excessive thickness swell and water absorption by the oriented panels. 



Except for yellow-poplar flakeboards, the impact resistance and deflection of 

 1/2-inch panels in the dry test were similar to those in the wet test. Among 5/8- 

 inch panels, the random panels and plywood had more impact resistance when 

 wet than when dry tested. Oriented panels and plywood deflected more when 

 wet tested than when dry tested. 



After impact failure, all panels supported the 200-pound concentrated load. 

 Except for a few cases, the deflection ratio — final deflection/initial deflection — 

 exceeded 1.50 but was greater than 1.86 only for the wet 5/8-inch oriented 

 mixed species flakeboards. Among 1/2-inch panels, plywood had the lowest 

 ratio for both wet and dry conditions. But the ratio for 5/8-inch random flake- 

 board was less than that for 5/8-inch plywood for the dry test and equal for the 

 wet test. 



The 200-pound concentrated load caused less initial deflection for all types of 

 flakeboards than for plywood when equal thicknesses were compared. After 

 failure, the plywood deflected slightly less than flakeboard of equal thickness 

 except for the dry tested 1/2-inch yellow-poplar flakeboards and wet tested 5/8- 

 inch oriented mixed species flakeboards. 



INTERLAMINAR SHEAR STRENGTH AND STIFFNESS 



Resistance to in-plane shear deformation is a significant mechanical-property 

 requirement for materials in webs of composite beams and structural dia- 

 phragms. No data specific to flakeboards made of southern hardwoods are 

 published, but data from other species give useful information probably applica- 

 ble to southern hardwoods. 



Interlaminar shear strength. — From evaluations of 10 particleboards at 

 about 9-percent moisture content, McNatt (1973) found that internal bond 

 strength is linearly correlated with interlaminar shear strength (fig. 24-41); at 

 internal bond strength of 70 psi, his data indicate interlaminar shear strength of 

 252 psi. 



In-plane shear modulus. — Hunt (1978) compared the change in shear mod- 

 uli of sheathing-grade aspen waferboard with sanded Douglas-fir structural-I- 

 grade Douglas-fir plywood over a range of relative humidities from 36 to 87 

 percent. He found that throughout this range, the flakeboard had in-plane shear 

 moduli about IVi times greater than that of the plywood. No comparable data are 

 published describing flakeboards of southern hardwoods, nor are data available 

 from systematic studies of the effects of weathering on this relationship. 



