where ri: = number of cycles at the stress level 



Nj = life at the stress level 



By fixing the allowable sum of the cycle ratios, as above, the duration 

 of pipe usage may be determined. The fatigue properties of the materials 

 must be known in some detail to establish the values of Nj. Experiments 

 which will yield such data are common. Careful record keeping during the 

 life of the system will serve as an aid to the operators in determining the 

 probability of weaknesses in the pipe or joints. Another possibility is the 

 system used by the crew of the Glomar Challenger. Each time the ship puts 

 into port the pipe is inspected with an internal sonde and any pipe and/or 

 joints with suspected defects are given a more thorough inspection before 

 being put back into use. 



Life of the Pipe. It is possible to analyze the life of a member if 

 nothing is known about the material except the ultimate tensile stress. The 

 results will, of course, be approximate, but they can be used as a safe basis 

 for design. 



It will be assumed that the stress variation, ±Aa, due to the dynamic 

 load is 20% of the static stress, a^^. The part is to have an infinite life. The 

 material is V-1 50 having an ultimate tensile stress of 1 50,000 psi. It is 

 desired to find the allowable mean stress, Og^, due to static loading. 



If no fatigue test data on the endurance limit are available, it can be 

 assumed that one-third the ultimate stress is the endurance limit. In this 

 case 50,000 psi is the endurance limit in completely reversed loading. 



On the basis of the above, Aa = 0.20 Og^. From the assumption on 

 the endurance limit 



A0„ = 



3 



A (J *^3V 



Therefore = 0.60 — • — 



123 



