| dc.description.abstract | The present work is aimed at the extraction of material’s yield and creep parameters using
cantilevers by conducting constant deflection rate and constant load tests respectively.
Under the assumption of Euler-Bernoulli’s beam theory, stress and strain components
are considered along beam length only. A stress and strain gradient throughout the
cantilever makes bending rich in information as stress-strain rate-strain at every location
of the beam corresponds to a single uniaxial test. By extracting these stress-strain rate strain
information from multiple locations of the cantilever throughout deformation, high
throughput feature of bending is utilized by extracting all the material yield and creep
parameters from a single cantilever. The position dependent strain is measured using
digital image correlation (DIC) in this work. The estimation of stress in a cantilever
during non-linear deformation (i.e., non-linear dependence of stress on strain and/or strain
rate), however, needs numerical methods which solve for stress distribution throughout
the cantilever using equilibrium equations based on material’s constitutive behavior, beam
geometry, loading and boundary conditions. Such numerical methods are developed to
gain understanding of the stress redistribution which is found to be transient in nature
and finally saturates when permanent (plastic or creep) strain are large (∼3-4%) such
that elastic strain are negligible. The influence of material’s yield and creep parameters
on stress redistribution profile and associated timelines is studied and limitations of the
existing analytical expressions for saturated stress profile are discussed. The numerical
method is also utilized to estimate the effect of limitations on strain accuracy measured
by DIC on the extracted parameters. Therefore, the present work aims at developing
cantilever bending combined with DIC for strain measurement as an alternative testing
technique to extract material’s yield and creep parameters from a single cantilever.
The procedure for extraction of yield parameters like yield strength and strain hardening
exponent is established for a beam of tension-compression symmetric, strain rateinsensitive
material using the concept of the ‘invariant point’. These points are identified
using numerical methods as the unique locations at every cross-section of the cantilever,
one each in tensile and compressive regions, where stress remains almost unchanged during
stress redistribution. The hardening exponent and yield strength are measured from a single
cantilever with better statistics and thereby improved reliability using strain measured
at the invariant points in tensile and compressive regions within an accuracy of 99.5% and
90% respectively for pure copper and aluminium alloys. The procedures for measuring
strain rate sensitivity for a tension-compression symmetric, strain-rate sensitive material
and yield parameters for a tension-compression asymmetric, strain-rate insensitive material
are also proposed and the challenges are highlighted with the understanding that
bending has the potential to measure yield parameters for these latter systems as well in
future.
The timelines associated with stress saturation under creep deformation have been
quantified using numerical methods in terms of a parameter stress saturation time (SST).
Based on the recommendations obtained from SST, loading profile for T22 boiler steel
is redesigned in the form of small loading steps due to which stress gets sufficient time
to relax during loading itself and does not exceed yield strength during redistribution.
Thus, creep parameters can be extracted at loads as high as yield strength, which is not
possible otherwise. This makes testing faster and thereby efficient because creep rates are
higher at high load and steady state is achieved faster. In contrast, a high SST at low
loads has been identified to explain the misinterpretation of experimental data in terms
of mechanism shift at low loads for P91 steel.
The numerical method is further developed to include primary creep response at loads
above yield which holds relevance to room-temperature creep response of an hcp system,
i.e., titanium alloy Ti-6Al. In case of Ti-6Al, it is found from uniaxial creep tests conducted above yield that prior plastic deformation does not affect creep behaviour which
implies that plasticity affects only the initial stress distribution. The invariant points
which remain invariant to stress redistribution even under the combined effect of creep
and plastic deformation are identified based on the numerical methods. The strain at
these locations in tension as well as in compression, measured using DIC, are utilized to
extract equivalent primary creep response for Ti-6Al using a single cantilever. Therefore,
the present work aims at establishing bending as the testing technique to measure yield
and creep parameters for a range of materials (FCC, BCC, HCP) and testing conditions
(RT-600oC) utilizing minimum sample volume with reduced testing and better statistics. | en_US |
