744 JOURNAL OF FORESTRY 



of this computation assume a test piece has slopes of 1 to 10 and 1 to 15. 

 For computation of absolute slope these ratios are converted into dec- 

 imals and are equivalent to .10 and .067, respectively. Squaring these 

 and extracting the square root, gives .12 which expressed as a ratio 

 equals 1 to 8.3. 



In correlating the data, all the sticks with slopes of grain between 

 certain limits as 1 to 30 and ] to 39.9, 1 to 25 and 1 to 29.9, 1 to 20 and 

 1 to 24.9, etc., were averaged together with respect to strength proper- 

 ties and slopes. All sticks with slopes not greater than 1 to 40 were 

 averaged together and treated as if they were straight-grained. 



The average values of strength property and slope were plotted in 

 diagrams such as figures 1, 2, and 3, in which specific gravity and 

 moisture content were also plotted. These figures were for white ash 

 only but the curves showing the relation of strength properties to 

 slope of grain are not essentially different for the other species. 



Figure 4 presents a comparison of the three species with respect to 

 the influence of slope of grain on a combination of important strength 

 properties. The maximum difference between the three curves does not 

 exceed 3 per cent until a slope in excess of 1 to 12.5 is reached, thus 

 indicating that in the aggregate the three species are practically alike 

 with respect to the relation of slope of grain to strength properties. 



Table 1 shows the percentage by which material with the various 

 slopes of grain falls below straight-grained material in various strength 

 properties. 



Inspection of the figures and table shows that compressive strength 

 is but little affected until quite steep slopes of grain (1 to 10 or greater) 

 is reached. Modulus of elasticity — stiffness — is more affected and be- 

 gins to suffer a really appreciable decrease at a slope of 1 to 15. 

 Modulus of rupture — strength in bending— decreases even more rapidly 

 and has about 10 per cent deficiency at a 1 to 20 slope and nearly 20 

 per cent at 1 to 15. The most pronounced effect is on work to 

 maximum load and maximum drop — both measures of shock-resisting 

 ability — which are considerably deficient even at a slope of 1 to 25 

 and decrease very rapidly as slope of grain increases. 



As a result of these tests, it has been recommended to the War and 

 Navy Departments and to aircraft manufacturers that slopes of grain 

 in excess of 1 to 20 should not be permitted in highly stressed parts. 

 The advisability of restricting the slope of grain to 1 in 20 as now pro- 

 vided in grading rules for select structural southern yellow pine and 

 Nos. 1 and 2 structural Douglas fir is confirmed by the tests. 



