2976 Chapter 24 



MODULUS OF RUPTURE 



Structural flakeboards must support, without danger of failure in bending, 

 loads anticipated in service and those imposed during construction. Modulus of 

 rupture (MOR) is the measure of ultimate strength in bending. Section 24-2 

 noted that the MOR of construction exterior-grade plywood in the United States 

 is about 8,000 psi along the grain of face veneers and 4,000 psi across the grain. 

 For 2-MW waferboard and 2-MF flakeboard, the National Particleboard Associ- 

 ation (1980) requires MOR of at least 2,500 and 3,000 psi, respectively (see 

 table 24-2). Ultimate bending-load specifications for APA RATED STURDI- 

 FLOOR panels are given in table 24-3 and related discussion, and those for APA 

 RATED SHEATHING in table 24-5 and related discussion. 



McNatt (1973) found that tensile strength of particleboards parallel to the 

 surface is closely correlated with MOR (R^ = 0.90) as follows (at 9-percent 

 moisture content): 



Tensile strength, psi = -221 + 0.523 (MOR, psi) (24-1) 



Factors and procedures that increase modulus of elasticity, also increase 

 MOR, as summarized in the following paragraphs. 



Flake orientation. — Ultimate tensile strength of flakeboards is closely relat- 

 ed to orientation of flakes within the board (fig. 24-29 right); e.g., when flakes 

 in a board are all aligned at 45 degrees to the axis of applied tension load, tensile 

 strength of the board is only about one-tenth that of a board with flake grain 

 perfectly aligned with tension load direction. 



Price (1978) found that panels with aligned flakes had an MOR of about 5,574 

 psi along the major axis and 3,169 along the minor axis, whereas panels with 

 random flakes averaged about 4,701 psi (table 24-12). 



Price (1978) also applied concentrated and distributed loads to these panels. 

 He found that the 5/8-inch panels — both with aligned and random flakes — were 

 more than adequate for APA RATED STURDI-FLOOR, and that the 1/2-inch 

 panels of both designs were more than adequate for APA RATED SHEATHING 

 panels, as specified in tables 24-3, 24-4, and 24-5. He found that for 200- and 

 300-pound concentrated loads applied IVi inches from the edge on 1- and 3-inch 

 disks, respectively, oriented and random panels had similar deflection values. 

 Failure loads applied on the disks, however, were higher for the random panels 

 than for the oriented panels, and the random panels retained more strength after a 

 3-day water spray than the oriented panels. In general, the oriented panels 

 deflected less than random panels in tests of distributed load over a 24-inch span. 

 For a 16-inch span, dry-tested 5/8-inch random panels deflected less than orient- 

 ed panels. 



Wood density and species. — In flakeboards described by table 24- 1 1 , Hse 

 (1975c) found that MOR ranged from 3,914 psi for white oak boards at 44.5 lb/ 

 cu ft panel density to 10,080 psi for sweetbay boards at 49.5 Ib/cu ft. MOR 

 increased with panel density; at constant density, values were highest for sweet- 

 bay, followed in decreasing order by red maple, black tupelo, sweetgum, white 

 ash, southern red oak, hickory, post oak, and white oak. To achieve a MOR of 

 4,500 psi with randomly placed flakes, Hse found that flakeboard density varied 



