Damage Control: Thermal Fatigue Assessment

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Assessment Techniques

Historically, two considerations have encouraged the development of formal fatigue analysis methods. The first has been the need to provide designers and engineers methods that are practical, easily implemented, and cost-effective. The second consideration has been the need to reconcile accurate and meaningful analytical approaches.^[1]  Conventionally, three primary fatigue analysis methods have been used to estimate fatigue life; these are the stress-life (S-N) approach, the strain-life (ε-N) approach, and the fracture mechanics (crack growth) approach.^[1]  As discussed herein, specific versions of these three assessment methods are options for analysts in API 579-1/ASME FFS-1, Fitness-For-Service (API 579).^[2]  Modern fatigue assessments typically utilize design-by-analysis (DBA) techniques to linearize and classify elastic stresses^[3] into different components, and they often leverage computational tools such as FEA to resolve cyclic thermal-mechanical stresses at critical locations. Common fatigue cycle definitions and terminology are highlighted in Figure 1. When performing an analysis, cyclic thermal-mechanical stresses and strains computed over a typical (often idealized) operating cycle are extracted as inputs for the fatigue life assessment.

The current (3rd) edition of API 579^[2] introduced a new section (Part 14), devoted solely to evaluating fatigue life. The complete technical basis for Part 14 is provided in Welding Research Council (WRC) Bulletin 550.^[5] Additionally, the fatigue methods in Part 14 of API 579 are summarized in detail in [6-8]. For the sake of brevity, only a short summary of the API 579 fatigue assessment procedures is provided herein. Part 14 is written as a multi-level procedure covering screening, current ASME design Code methods, and advanced methods.^[9] The advanced methods include fatigue assessment of welded joints using the equivalent strain, Master S-N Curve Method (based on the Battelle Structural Stress approach^[10]), and a smooth bar fatigue assessment method that incorporates a multiaxial fatigue criterion with a strain-life equation, coupled with a critical plane approach.^[11-13] Cycle counting methods for both welded joint and smooth bar fatigue methods are also provided, and methods to evaluate fatigue in the subcritical crack-growth regime are also covered.^[14] An overview of the fatigue analysis methods outlined in API 579^[2] is provided below.