Damage Control: Thermal Fatigue Detection

Editor’s Note:   This is the first edition of a new regular column offering practical insights into various damage mechanisms affecting equipment in the O&G, petrochemical, chemical, power generation, and related industries. Each article will cover either the detection, assessment, or mitigation of a particular damage mechanism, and will incorporate observations from industry experts in materials and corrosion, fitness-for-service, inspection, and mechanical integrity.

Readers are encouraged to send us suggestions for future topics, comments on the current article, and raise issues of concern. All submissions will be reviewed and used to pick topics and guide the direction of this column. We will treat all submissions as strictly confidential. Only Inspectioneering and the author will know the names and identities of those who submit. Please send your inputs to the author at damagecontrol@inspectioneering.com.

Introduction

This article reflects the first in a series on damage mechanisms that will appear in this recurring Inspectioneering column entitled “Damage Control.” Each damage mechanism discussed will be delineated into a set of three successive articles, each covering different aspects of a specific damage mechanism. The first article in each set will cover the detection of damage, including typical damage locations, morphology, common inspection techniques, and documented historical failures. The second article will highlight how the type of damage can be characterized and evaluated using engineering calculations, fitness-for-service methods, or advanced analysis. The third article will address what steps can be taken to mitigate or remediate the damage on affected equipment, whether relating to design/fabrication recommendations or process/operating practices. Lastly, the goal of each set of articles will be to offer practical guidance for, and real examples of, in-service degradation or failures attributed to the damage mechanism being covered.

The inaugural topic discussed in this column is thermal fatigue, a specific form of fatigue driven by varying metal temperature gradients and ensuing differential thermal expansion. In general, fatigue is a complex metallurgical process that is cycle-dependent and in which failure of a component occurs due to repeated or cyclic loading, which creates cyclic stresses. Furthermore, cyclic stresses can result from mechanical loading (applied loading or vibration) or thermal loading (metal temperature gradients). The application of these cyclic stresses produces slip lines in the crystals of a metal that develop into small cracks. These small cracks then propagate, coalesce, and result in an eventual fracture, which usually involves little-to-no gross plastic deformation.

Consequently, at one time, fatigue failures were mistakenly thought to show that the metal had lost its ductility.^[1] To this end, fatigue failure involves progressive, localized, permanent structural change due to fluctuating stresses and strains.^[2] To accurately estimate the fatigue life (or cycles to failure) in a component, the fatigue driving force and resistance parameters need to be well understood. As such, geometry, material properties, and accurate characterization of the applied loading (fatigue driving force) are required. Furthermore, fatigue test data that describes material behavior as a function of stress or strain magnitude and the number of accumulated cycles (fatigue resistance parameters) is also required to carry out meaningful remaining life calculations.^[3]

Brief Historical Overview