LOW-ANGLE FAULTING 07) 
FACTORS WHICH LOWER ANGLE OF FAULTING 
A. EFFECT OF NORMAL COMPONENT OF STRESS 
While the tangential or shearing stress (Pi=P sin 6 cos 6) 
reaches its greatest intensity along planes inclined 45° to the line 
of application of the force P, it is also true, as has just been shown, 
that the intensity of the stress at right angles to this (Pu =P sin? 6) 
is likewise great. This normal stress obviously acts as a frictional 
resistance to shearing by the tangential stress. The value of this 
frictional resistance depends on the shearing strength of the material 
when not in compression.’ As to the potency of this factor, 
Church states that the presence of compressive stress normal 
to the 45° plane is sufficient to strengthen the material for shear- 
ing in that plane, causing separation to occur along a plane where 
the compressive stress is considerably less.? 
Let us see how this plane will be inclined. The intensity of 
the normal stress is expressed by | 
PP sin2 6: 
Its intensity increases as @ increases, and diminishes as 6 
decreases (see Fig. 6). Hence the lower the angle of the fracture 
plane, the less will be the frictional resistance due to normal com- 
pressive stress. 
The intensity of tangential or shearing stress (Pi=P sin 6 cos 8) 
is greatest when 9=45°, and diminishes as 6 becomes less. Shear- 
ing can occur only when this exceeds the shearing strength of 
the material. Here is a limiting factor. 
Comparing 
Pn=P sin? 6 
and 
Pi=P sin @ cos @, 
it is seen that as 6 becomes less P sin?6 diminishes in value more 
rapidly than P sin @ cos @ or, in other words, as the angle @ is 
lowered from 45°, the intensity of normal stress is reduced more 
rapidly than the intensity of the tangential stress. Therefore, 
=W. C. Unwin, op. cit., p. 419. 
21. P. Church, Mechanics of Engineering (New York, 1913), p. 220. 
