PSI - Issue 57

Giovanni M. Teixeira et al. / Procedia Structural Integrity 57 (2024) 670–691 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

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1. Introduction

The three main alternatives to the mechanism based D TMF method are Taira’s LCF Method [1], Sehitoglu TMF Damage Approach [2] and the Strain Range Partitioning (SRP) [3]. Taira’s phenome - nological method was first introduced in 1973 and is the simplest among D TMF , Sehitoglu and SRP. Fatigue damage is assumed to be related to the plastic strain range (powered) by a damage factor (  ) and a temperature independent material constant C [4]. ( ) ⋅ Δ ⋅ = (1) S ehitoglu’s damage-based model is the one that requires the largest number of parameters. Damage is evaluated as a sum of three damage mechanisms: 1 = 1 + 1 + 1 (2) Where is the mechanical fatigue contribution, whereas and represent the influence of oxidation and creep. Unlike D TMF the creep contribution is an additive term, rather than a multiplicative factor. The SRP Method, developed by Manson et al. in 1971, is based on the partition of the inelastic strain range into time-independent plastic and time-dependent creep strain ranges, taking the following form: 1 = 1 + 1 + 1 + 1 (3) Where N – Number of cycles to failure PP – plasticity in tension and plasticity in compression The major challenge in accounting for thermo-mechanical fatigue at the design stage is related to testing and model calibration. The tests are expensive and time consuming. Material data is scarcely available in the literature and frequently (when they are available) not suitable for the fatigue method (e.g., D TMF ) one wants to use. As the various TMF fatigue phenomena (oxidation, thermal, mechanical and creep fatigue) are interrelated, defining their exact influence on the crack nucleation and propagation is not that straightforward and similarly the task of resolving the parameters for all equations involved (calibration). Although the phenomena of oxidation and creep may happen in the absence of alternating loads, in the context of the D TMF approach they are computed only if the crack tip opening displacement range ( CTOD  ) is non-zero, i.e., they are considered only in the presence of mechanical fatigue (unlike Sehitoglu and SRP). 1.1. Rate Independent Elasto-Plasticity The theoretical background for the modern plasticity theories date back to the 19 th Century and the experiments performed by the French engineer Henri Tresca, starting in 1864. Since then, many models have been developed with the aim of describing the cyclic visco elasto-plastic response of various engineering materials under mechanical and thermo-mechanical loading. Thermo-mechanical fatigue ( TMF ) analysis requires a deep understanding of the material behavior ( hardening, softening, creep, relaxation ) at high temperature, including the mechanisms of nucleation and propagation of microcracks where multiple phenomena ( creep, oxidation, hydrogen embrittlement ) may concur to produce a mechanical failure. One of the simplest material models that includes the Bauschinger effect was proposed by Prager in 1955. It is CC – creep in tension and creep in compression PC – plasticity in tension and creep in compression CP – creep in tension and plasticity in compression