EOCKY MOUNTAIN MINE TIMBEBS. 
33 
MANNER OF FAILURE. 
The prop failures were quite uniform. Compression wrinkles 
occurred usually through one or more knots and, if the prop had a 
slight bend in it, the wrinkles were on the concave side at the point 
of greatest eccentricity. If this bend were at all prominent, tension 
often occurred and in several instances the prop separated in two 
parts as in the brittle cap failure. The influence of checks on the 
form of failures was quite noticeable in the dry props, the shortening 
of the props giving the check, if spiral, the appearance of " unwinding" 
as in the strands of a rope. Figure 6 shows this effect to some extent. 
Fig. 7.— Method used in testing mine caps in bending. 
CAPS. 
The failures of the green caps were quite uniform in character 
among the various species. Near or at the maximum load, com- 
pression took place on the upper surface between the loading points. 
The load fell off slowly, and if continued, failure by tension ultimately 
took place. The green Alpine fir caps, however, had a larger propor- 
tion of tension than compression failures, and the Douglas fir had 
approximately the same number of each. 
The dry caps, as far as could be detected by the eye, generally failed 
in tension near the center. Occasionally compression wrinkles oc- 
curred, but the failures were more often sudden, and some indicated 
brittle .material. There was no material difference in the failure of 
the different species. In the dry material the tension splinters often 
ended at a check, but it was not apparent that the type of failure or 
