The Mechanics Behind Equipment Fatigue in the Oil & Gas Industry

This post is based on an article from the September/October 2008 issue of Inspectioneering Journal by Richard Green at Accurate Metallurgical Services. You can find the original article here.

Fatigue is a big problem for equipment in the oil & gas industry. It is estimated that fatigue failure mechanisms are responsible for over 90% of mechanical failures in the field. Because of this it is incredibly important to detect fatigue damage before it becomes significant. Significant fatigue damage can be defined as either the lowest possible amount of fatigue detectable by NDT inspection techniques, or as when the residual strength of a part is decreased to below the minimum required strength of the part. Detecting significant cumulative damage is the goal of fatigue testing.

A part’s fatigue properties can be improved by increasing its size and weight. To look at an example: in the aerospace field, weight is a premium. Airframes are built with lightweight materials such as aluminum, titanium, and graphite composites. This is done so that doubling or even tripling the size of the component does not contribute a significant increase in the weight of the component. Given these high safety margins, parts can tolerate a certain level of cracking without failing.

On the other hand, because of the hostile environment inside the engine, engine parts are made of nickel, iron and cobalt alloys. These materials have much higher densities than the materials used in the airframe, so increasing the size of engine parts will will have a much larger effect on the overall weight. Such weight gains are unacceptable in engine design so increasing component size isn't really a viable option when working with jet engine parts. Because of these tighter safety margins, 

In component testing, the location of the crack initiation site can be as important as the fatigue life. For example, a dental implant can be designed so that the crack initiates in the post area rather than the implant area. This is because cracks that initiate in the post can be replaced relatively easily but repairing cracks that initiate in the implant area, which is located in the jawbone, often requires extensive surgery.

Fatigue testing in a laboratory replicates the mechanical effects needed for fatigue in a controlled environment. When the goal of the testing program is material characterization, force and strain are typically chosen as the methods for applying cyclic stress. The testing of assemblies is often performed using displacement of load control. Force control involves straining a specimen until a force detection device, known as a load cell, senses the desired force and feeds back a signal to the control mechanism. Strain control involves straining a specimen until the specimen has elongated to a desired distance.

Force control allows tests to be performed at high frequencies. Most load cells are capable of load changes at frequencies in excess of 100 Hz. Care must be used when performing testing at higher frequencies. High frequencies can generate heat by the mechanism of internal friction, and this adiabatic heating of the test material can lead to erroneous data. High frequency testing permits the accumulation of a high quantity of cycles prior to crack initiation. Curves such as fatigue threshold curves are generated using force control mode. The location of the load cell facilitates the exposure of specimens to hostile environments such as high temperatures and corrosive mediums without damage to the load frame.

With an improved understanding of fatigue mechanics, engineers can better predict the life expectancy of the part. With laboratory fatigue data reliable maintenance schedules can be planned, instead of basing maintenance schedules on “best guesses.”

Inspection service doors can be placed close to areas where fatigue cracks are likely to develop. Fatigue mechanics answers many “what if” questions. For example, if a fire were to occur on a plane, is the structural integrity compromised? Fatigue mechanics also reduces costs by justifying the extension of component inspection intervals or even eliminating some of the inspections all together.