ROCKY MOUNTAIN MINE TIMBERS. 



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. 



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 



