Sean Ryan

Metal Fatigue in Gas Pipelines

         Sean Ryan

Hello everyone. So, this time around, I’ll be discussing a certain article with you all. The article is Destruction Patterns of X70 Steel Sample, Possessing Cracks of Corrosion -Mechanical Origin, under Cyclic Loading. While initially a daunting title, this article can be broken down quite a bit easier. The main purpose of the article is to test the steel from a gas pipe that exploded and determine the properties of the material that might have been the cause.

Over the last few years there have been many cases where both oil pipes or gas pipe have failed and exploded. These gas pipe explosions often end up killing people due to their violent expansion. Making sure that the pipelines are secure, and the material can contain the pressure that is produced by the gases is essential in keeping the environment and the people who work with the pipes safe.

Olaf, A (2007) AB on the Gas Pipe [Online Image]

The first major thing to note is the difference between mechanical fatigue, and corrosion fatigue. Mechanical fatigue is when the actions of a material weaken it. For instance, when you bend a paperclip it gets weaker each time until it snaps. Corrosion fatigue is then the mechanical action damages the material when it is in a corrosive environment. This article mainly focuses on the latter. The amount of time that metal takes to fail is measured in loading cycles, or the amount of times the material has been in use and then stops being in use. A simple way to think about this is an airplane, whose cycle consists of a takeoff and landing.

 The paper then goes into the experiment that was performed. They used what is called a fatigue machine, which puts stress onto a material in the same ways that it would experience while in use. They used this machine to cause cracks, then the cracks were measured as well as the thermodynamic stability. While another dauntingly large phrase, this just means the ability of the material to no give off nor absorb heat. At first, measurements were made every 3 cycles, but then moved to every 10 cycles as the experiment progressed. The observations were done under a microscope, while the thermodynamic stability was tested with an electrode. The sample that they used ended up cracking along the corrosion crack that already existed in the material.

 Over the course of the experiment, they discovered that when the damage due to corrosion fatigue on metal gets worse, the ability of the sample to avoid absorbing or giving off heat is worse. The thermodynamic stability was tested by the electrode mentioned earlier, and gave data called the electrode potential. This value has a direct relationship with thermodynamic stability, so when electron potential is lower, so is the stability. They realized that the relaxation of metals, which occurs when the sample has the load removed from it and the metal is no longer deformed, did not account for the lower stability.

Using the data points they collected, they ended up using a nonlinear regression model to predict changes. That is, they used a program to match an equation that wasn’t a straight line to the data points they had collected in order to be able to predict the thermodynamic stability of the sample outside of their tests. They found that as more and more loading cycles occurred, that damage to the material in the form of the crack length depended less on the number of cycles that happened, and more on the potential of the material on each cycle. This would mean that if the thermodynamic stability is lower, the material if more susceptible to breaking.

They discovered that the larger the deformation became, the more that the potential of the material stabilized. This was attributed to the relaxation of the metal after the loading cycle. Another thing that was concluded was that the more loading cycles that occurred, the lower the thermodynamic stability in a corrosive medium, which then leads to more fatigue damage taking place. To sum up the tests in this article, they wanted to know what caused the failure in the original pipe, and whether it was simply because the pipe was old and had been in use for a long time, or whether there were other factors at play. They determined that, while the length of operation did have an effect on the damage  done to the pipe, the main contributor was that the metal became more likely to absorb and give off heat during operation, which caused it to contract and expand during operation, as well as to slightly change its properties. This, all happening in a corrosive medium, lead to the propagation of any microscopic cracks that may exist in the metal.

Though this article may be a bit much to take in at one time, the points made in this blog hopefully aim to make it more digestible by the most amount of people. Thank you for listening, and I hope this helped to shed some light on some more topics.

Nasibullina, O. A. 1. ksu33@bk. r., & Gareev, A. G. 1. ag. gareev@mail. r. (2019). Destruction Patterns of X70 Steel Sample, Possessing Cracks of Corrosion-Mechanical Origin, under Cyclic Loading. Materials Science Forum, 946, 20–24. https://doi.org/10.4028/www.scientific.net/MSF.946.20