Date of Award

Spring 2004

Document Type


Degree Name

Doctor of Philosophy (PhD)


Materials and Infrastructure Systems

First Advisor

Ray Sterling


Fatigue is the most common cause of mini-HDD drill rod failure. It can occur at stress levels far below the normal operating stress in most drill stem components. Fatigue failures occur because the drill rod, after it has been forced into a curved path, undergoes a cyclic bending stress oscillating from tension to compression in concert with the other stress components caused by torque, thrust, or pullback.

When a rod breaks underground, there is considerable extra cost caused by the delay in “fishing” out the broken rod as well as the cost of replacing the rod. On the other hand, it is expensive to replace drill strings before the end of their useful life as the drill rod is often the most expensive consumable used in the mini-HDD process.

In order to analyze the fatigue life of mini-HDD rod, a fatigue test machine was built in the laboratory of the Trenchless Technology Center. Twelve fatigue tests were conducted in this laboratory on the drill pipes donated by Drilltube, Inc. and Texas Pup, Inc. The pipes donated by Texas Pup were used as the preliminary tests to gain experience in the testing methodology. The pipes donated by Drilltube, Inc. were used to construct an S-N curve for this type of drill pipe. Those two kinds of pipes did show a difference in fatigue life behavior.

During the five preliminary tests, the strong effect of make up torque and joint lubrication was observed. In the later seven tests, six failures occurred. Five of these failures occurred at the joint area, and one failure occurred in the body area. This is consistent with the reports of field failure: the joint area is the weakest area in the whole pipe. From the test results of the second series of seven tests, an S-N curve for that type of pipe was made. The fatigue limit for this pipe (based on a simple bending stress approximation) is less than 22 ksi. Since the “Atlas of Fatigue Curves” indicates a value in the range of 55 ksi to 65 ksi for similar steels, mini-HDD drill rod fails at considerably less than the normally accepted values. The reason for this was assumed to be due to stress concentration in the joint area.

Since the stress in the joint area is very complex, numerical analysis was used to simulate the stress concentration. An ANSYS model was constructed and run for bending, tension, and thrust conditions.

After applying the stress concentration factor observed within the joint area (approximately 3.0), the fatigue limit did fall within the expected range.

In addition to the experimental and simulation results, the research investigated the theoretical stress condition in different phases of the drilling process compared to the normal stress assumptions used in practice. The S-N curve results were also used to provide an indication of the cost of drill rod fatigue using some illustrative assumptions about curved drilling and drill rod usage.