HOW FAILURE UNDER STRESS OCCURS IN TIMBER. 429 
fairly well with the crushing strength as got from ordinary longitudinal compression, 
but has no relation to the tensile strength. This is due to the fact that in timber 
the elastic limit is about ‘6 of the breaking load, and also that the ultimate 
compressive strength is approximately ‘6 of the calculated strength factor. There 
is thus no direct connection. But, on the assumption that the limit of elasticity 
varies directly as the breaking load and therefore as the modulus of rupture, the 
- value of the latter is usually considered as a good indication of the compressive 
streneth of the timber. 
In the tests for elasticity we have seen that the modulus varies with the 
position of the section, being greater when resting on the broad side than when on the 
narrow side. We would expect, therefore, that the stress at the point of ultimate 
fracture would show a corresponding difference. But the tests hardly bear this out. 
Some of the results have been grouped in pairs, and comparisons are made between their 
moduli of elasticity and of rupture under similar and dissimilar conditions, but these 
ratios are not constant. H, and E, are Young’s modulus with broad side horizontal and 
vertical respectively, and f, and f, are the calculated extreme fibre stresses obtained 
under similar conditions. 
AsH. 
: is 
Test 31 ! Broad side tangential Ey _ ) Ey _ ‘915 En = -97 fete 1:15 
” 12 ” ” ” Ey Ky Ky Sv 
: i } : i is = 67 9s 65 ” = "745 ” =1 
” 15 ” ” ” 
sy, TOL tangential Je Sale 
i ” ” =: — 1°38 JH _ 15 
4 = radial Doras ate Sa : 
12) tangential F 
eS ” ” =1:07 = JV—] 
me low 5, radial i 28 Jy 
Boxwoop. 
Test 4 | Broad side tangential Ey Eu Uae 
— Se —S => » 65 —_- = 1 09 
lO \ Re » radial Ey g Ey 8 Sv 
“5 or * 5 tangential = (925 = “885 esices 
a 5 », radial 
A high Young’s modulus showing a high degree of stiffness is quite compatible with 
a comparatively low ultimate strength, as the presence of any flaw, though not evident 
in the elastic stage, will be shown by a low breaking load. Thus beams which show the 
