Structural Flakeboards and Composites 3039 



Modulus of rupture. — Average MOR in stress parallel to the grain of face 

 plys was least — 4,495 psi — for panels with single 1/24-inch veneers on each 

 face over an oriented core. It was greatest — 15,843 psi — for panels with two 1/ 

 16-inch veneers cross-laminated on each face over an oriented core. 



With values for all core construction pooled, MOR differed significantly with 

 face veneer thickness. As noted in footnote^ of table 24-27, horizontal shear 

 failures in the specimens with 1/10-inch veneers precluded accurate determina- 

 tion of their MOR, but MOR increased as veneer thickness increased from 1/24- 

 to 1/16-inch. The tabulation of MOE vs. veneer thickness — which could be 

 accurately determined before horizontal shear failures occurred — is probably 

 also a good indicator of variation of MOR (stressed parallel) with veneer 

 thickness. 



When stress was perpendicular to the grain of the outermost face veneer, 

 MOR decreased as face veneer thickness increased. 



Interactions of core construction with face construction and number of face 

 plys affected MOR, as follows: 



Face and MOR 



core construction Stressed parallel Stressed perpendicular 



- Psi 



Single veneer 



Random 9,153 2,646 



Oriented 5,846 5,277 



Three-layer 7,167 5,122 



Two veneers, cross-laminated 



Random 8,056 — 



Oriented 13,161 — 



Three-layer 7,207 — 



MOR of panels with two veneers cross-laminated on each face over oriented 

 cores averaged significantly higher than MOR in panels with single face veneers 

 over oriented cores. In panels with random cores, however, MOR was slightly 

 greater for those with single veneers on each face. Little difference was detected 

 between one-ply and two-ply faces when the core was fabricated in three layers. 



Of all panels with one-ply veneer overlays, those with random cores yielded 

 highest MOR (i.e., 9,153 psi) when stressed parallel to the grain of the face 

 veneer, but had lowest MOR (2,646 psi) when stressed perpendicular to the 

 grain. 



Summary. — Table 24-29 summarizes Hse's (1976) results in terms of the 

 construction giving the best performance in each property tested. Two veneers, 

 cross laminated on each face over a core of oriented flakes, yielded strongest 

 panels. Nevertheless, panels with single-ply faces over random cores appeared 

 more than adequate for most structural applications; MOR was 9,153 psi when 

 averaged over the three veneer thicknesses tested, and MOE was 1 ,73 1 ,000 psi. 

 Economy in use of veneer also favors single-ply faces. 



For panels with single-ply faces, a random core is preferable to oriented or 

 three-layer cores in most properties, even though such construction is less stable 

 across the grain direction of the face veneer. Both strength and dimensional 



