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.