1. Introduction
High temperature thermal systems such as power plant steam piping work and gas turbines1, chemical plant pressure vessels2-3, aero-engine combustion structures4, etc., may fail due to accumulated damage caused by long-term creep, fatigue, CF under elevated temperature. In engineering applications, it is always inevitably required to precisely evaluate the material properties, residual life of in-service components, which has led to the development of non-destructive or “quasi” non-destructive technique. On the basis of these demands, various small-sized, miniature specimen techniques have been developed5-8 and are being increasingly used to assist in condition minoring and life management programs9-11. In order to obtain comparable constitutive behavior as SSFS specimen, there are three critical issues on the development of miniaturized testing: i) the standardization for high temperature LCF miniature specimens for sub-sized thin structures, e.g., coating-substrate systems12-13 or narrow heat-affected zones (HAZs)2,14, ii) improvements in LCF testing apparatus leading to high accuracy test rigs, and iii) robust mechanics-based theoretical methods for LCF data transferability and correlation.
Data from small specimen creep testing has a direct input into remaining life evaluation and has become increasingly attractive for power plant application11,15. Such data can also be used to generate creep constitutive laws for welded materials14,16,17. The main small specimen and testing types that are used to obtain high temperature tensile and creep properties include the conventional sub-sized uniaxial specimens18 and several specialized miniature specimen test types including: the impression creep test19-21, the small punch tensile, creep and fracture tests22-26, the small ring creep test27-29, the small tensile two bar creep test30, and the more recent miniature thin-plate tensile test12,13,31. One of the unique advantages that small punch creep test has is the very small thickness (~ 0.5 mm), however, there is no universally accepted conversion techniques available for data interpretation32,33. Recently, small punch related constitutive model was developed to predict the uniaxial tensile stress-strain response of materials based on an energy principle, which was capable to accurately convert the load-displacement data into stress-strain description34. In addition, instrumented indentation test (IIT) was applied in remaining life assessment, a good agreement was obtained between the standard specimens and the numerical prediction35. Impression creep testing requires a small amount of material to only produce steady-state creep data after a transient primary stage, which is attractive in the localized deformation and damage behavior36-37. The two-bar specimen can produce the full stage creep curves, but it is difficult to make it very thin to ensure that the two bars fail at the same time30. Inverse approaches33,38-39have been adopted in miniaturized beam/thin-plate bending specimen/small punch creep tests to obtain creep properties, based on analytical solutions and numerical modelling.
Up to date, most of the research focus on the miniaturized creep tests, however, there has been a significant missing gap in small specimen LCF test as many issues are still waiting for the technical solutions40-42. For example, almost all the testing approaches, such as testing rigs and gripping fixtures, are specially designed for the standard specimens. Moreover, the miniature specimens are apt to suffer from torsional damage during high temperature cyclic testing. In the recent years, cylindrical small-sized specimens had been successfully applied on LCF tests by Dzugan et al .,41,42, which were not compared and validated by the standard specimen tests. Small punch high cycle fatigue tests had been conducted on Ti-6Al-4V43,44, however, it was very difficult to derive stress-strain hysteresis response. An alternative miniature fatigue test method, using a thin-plate specimen, could provide a much more reliable fatigue data. For example, LCF tests were carried out by Nozaki et al., and Nogami et al. ,45,46 using round bar and thin-plate specimens, and little difference was found in LCF behavior. Thermal-mechanical fatigue tests were performed on Ni-based superalloy, and the results showed good agreement of the fatigue lives between miniature and conventional specimen tests47. Although the miniature LCF testing mentioned above has paved the way for evaluating the cyclic properties and is capable to represent cyclic plasticity behavior of SSFS specimen using a relatively long enough gauge length45-47, more extensive investigations on LCF and CF with smaller-sized specimens at higher temperature are still scare before standardization of the testing methods.
The current work proposes a new experimental and numerical framework for non-standard MTP specimen, which aims to duplicate the LCF behavior of SSFS specimen at elevated temperature. The organization of the paper is as follows: the experimental methodology is presented in Section 2, consisting of material, specimen design, experimental set-up, and comparisons of uniaxial tensile responses between SSFS and MTP specimens. A high temperature unified visco-plasticity (UVP) modelling framework is introduced in Section 3 including data interpretation. The comparisons of high temperature LCF testing results between MTP and SSFS testing are presented in Section 4, followed by detailed discussions and future technical improvements in Section 5.