Cyclic Stress in a drill string???

[wertyu1980]

I need help with the concept of cyclic stress.

Suppose a drill string is drilling a hole. The string is made up of number of connections along it's length. Now while drilling, the drilling conditions are difficult. Say the drill bit is having trouble drilling through the formation. This will induce stress in the string. The string is vibrating in a lateral direction.

Now, due to stress/vibration, one of the connections is fractured...meaning string breaks off. Now, every connection is torqued to a certain value. Due to the drilling condition & rotating action of the string, it is torqued further to a point it breaks.

How would you define this fracture/parting of the drill string? Was it due to cyclic bending stresses, torsional loading, or fatigue???

I would really appreciate if you can explain these terms.
Thank you.

 

[DrillerNic]

It depends upon the mechanisom of the break: you'd have to look at the fracture surfaces to make an assessment of the actual fracture mechanism.

I've twisted off drill pipe (the pin breaks in the box of the connection) which is usually a result of torsional loading (in my case it certainly was- some numpty on the beach made the crossover at the top of the BHA weaker than the rest of the drill string), and I've had to keep a close watch on the pipe for fatigue due to drilling wells with very high build angles (rotating a drill string round a 14deg/ 100ft build section is just like bending the pipe backwards and forwards).