Development of Miniature Specimen Test Techniques

TECHNOLOGY-13
Development of Miniature Specimen Test Techniques
EXECUTIVE SUMMARY
Miniature specimen test techniques enable the characterization of mechanical properties using extremely small volume of
the material. Though they have their origin in materials development programmes for the nuclear industry, the miniature
specimen test techniques find very promising applications in the field of remaining life assessment, failure analysis,
properties of weldments, coatings etc. At IGCAR, research and development work on miniature specimen test techniques
includes development of high precision loading equipment, test fixtures, experimental design and implementation,
standardization of experimental procedures, data acquisition and analysis techniques. The applications of these
techniques for evaluating the properties of weldments, irradiated materials, aged materials and coatings have been
successfully demonstrated at IGCAR.
OUTLINE
The miniature specimen test techniques developed at IGCAR
are mainly Shear Punch (ShP) and Ball-Indentation (BI)
tests.
In shear punch testing, a cylindrical punch with a flat end is
forced to punch a hole in a clamped small disc specimen. The
load-displacement plot obtained during the punch test is
similar to that of a conventional tensile test and properties
obtained by analyzing the test curve are correlated with the
corresponding conventional tensile properties. Figure 1
shows the shear punch test fixture designed and developed
at IGCAR.
The Ball-Indentation (BI) test technique is another miniature
specimen technique that requires a very small volume of test
material and can be adopted for in-situ testing on real
structures. In ball-indentation test, the indenter is driven at
a constant speed into the test material and progressive
multiple loadings/partial unloading is performed at the
same test location. The applied indentation loads and
associated penetration depths acquired during the BI test
are used to calculate stress-strain values based on well
established mathematical relationships. Figure 2 shows the
indigenously developed test system for carrying out ballindentation
tests.
These miniature specimen test techniques have been put to
use for various applications. In one of the applications, the
shear punch technique has been used for evaluating the
changes in mechanical properties of irradiated SS316. By
using small specimens for testing irradiated materials, it
makes it possible to take them out of the hot cells and
conduct mechanical testing in comparatively simple and
controlled enclosures with reduced radiological hazards
(Fig. 3). A tensile-shear punch property correlation obtained
using various cold rolled SS316 serves as the base-line data.
Small specimens of 8 mm diameter extracted from the
irradiated SS316 at different locations corresponding to
different neutron fluence (dpa) were shear punch tested and
conventional tensile property changes were estimated using
the derived correlations.
In another application, both shear-punch and ballindentation
tests have been used to determine the
degradation in mechanical properties of modified 9Cr-1Mo
steel on aging at 650ºC. For this, shear punch and ballindentation
test methodologies were first benchmarked with
the conventional tensile testing and validated using various
heat treated microstructures of modified 9Cr-1Mo steel. The
changes in tensile properties of the modified 9Cr-1Mo after
various times of ageing at 650ºC have been obtained using
Shear punch and Ball-Indentation tests (Fig. 4). This study
demonstrates the ability of both shear punch and ballindentation
tests to effectively map the progressive
degradation of modified 9Cr-1Mo on ageing at elevated
temperatures. The present study can be extended to assess
the service-induced degradation of the actual component
through these small specimen test techniques.
Specimen
Punch
Fig. 1 : Shear Punch Test
Fixture
(a)
Fig. 2 : Indigenously developed
table top ball-indentation
test system
Fig. 3 : Shear punch testing of irradiated SS316 specimens (a) test
setup (b) test plots at various fluence
Fig. 4 : The ball-indentation test curves of mod 9Cr-1Mo steel on
aging at 650ºC for different
(b)
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TECHNOLOGY-13
ADDITIONAL INFORMATION
The concept of specimen miniaturization has its origin in the nuclear industry, where because of expensive and limited
irradiation space, specimen sizes were miniaturized for experimental irradiation programmes in reactors. The miniature
specimen test techniques can be broadly classified into the following categories: (a) Tests, which are based on
miniaturization or scaling down of conventional specimen size (like miniature tensile, fatigue, impact and fracture
toughness tests) (b) Tests that are based on novel techniques using disk sized specimens of approximately 0.3 mm
thickness (like disk bend tests, shear punch / small punch tests). (c) Techniques based on ball-indentation (BI) testing.
GENERAL EXPLANATION RELATED TO THE DESCRIPTION
The small specimen test techniques like disk specimen tests (shear punch,
small punch) and ball-indentation tests do not have equivalent
conventional counterparts. These novel test techniques need to be first
validated and benchmarked before being effectively put to use. The
methodology adopted at IGCAR is to first carry out experiments which
involve performing both conventional and small specimen tests on various
simulated heats of widely ranging microstructures and mechanical
properties prepared from the alloy of interest. The data from the small
specimen test technique is analyzed, compared and correlated with the
corresponding conventional test properties. For an unknown condition of
the alloy, using the small specimen test and the correlations derived, the
conventional properties of the bulk material are obtained.
Using this methodology, an excellent linear correlation (Fig. 5) between Fig. 5 : The shear punch–tensile correlation for Mod
conventional tensile and shear punch tests has been demonstrated for a
9Cr-1Mo steel
wide variety of materials such as AISI type 304 stainless steel, AISI type 800
True Stress, MPa
316 stainless steel, 2.25Cr-1Mo & 9Cr-1Mo ferritic steels etc. Similarly the
ball-indentation (BI) tests have been standardized for accurately 600
modified 9Cr-1Mo
evaluating the tensile properties for various materials like AISI 1025
carbon steel, 2.25Cr-1Mo steel, modified 9Cr-1Mo steel and AISI 316 400
Mod 9Cr- Tensile
1Mo
steels. A methodology has been developed for including the usually
Tensile
BI
ignored pile-up effects in calculating the stress- strain data from ballindentation
tests for the ferritic steels. Figure 6 reflects the excellent
BI
200
BI
BI
agreement between the stress- strain curve estimated from ballindentation
tests and the conventional tensile stress-strain curves for 0.00 0.02 0.04 0.06 0.08
True Strain
modified 9Cr-1Mo steel.
Fig. 6 : Excellent agreement between BI and tensile
stress strain curves
BRIEF DESCRIPTION OF THEORETICAL BACKGROUND
Shear Punch testing is basically a blanking operation and the principal mode of deformation is shear occurring in the
narrow clearance zone between the specimen and lower die surface. The relations between the shear and the uniaxial
tensile is linear in nature. However, the shear punch-tensile correlations equations for strength and ductility parameters
are mostly empirical in nature due to the complex stress states in the clearance zone. Indentation tests using spherical
indenters are unique because the geometry of indentation changes with increasing penetration, corresponding to unique
stress-strain value for a particular depth. As a result, spherical indentation test technique is a unique method for
determining the complete stress-strain curve of metals using a greatly reduced volume of test material. Compared with the
conventional hardness tests, this technique offers both strength and ductility parameters from analysis of the measured
indentation load-depth curve.
ACHIEVEMENT
Miniature specimen test techniques developed in-house have been validated and benchmarked with conventional
mechanical tests for various alloy classes. Their applications in number of areas like properties of irradiated materials,
weldments, aged conditions and coating etc have been successfully demonstrated. These techniques will be used for
assessing service-induced degradation of the actual structure through in-situ ball-indentation tests and shear punch
tests with very small volume of scooped out material.
PUBLICATIONS ARISING OUT OF THIS STUDY AND RELATED WORK
1. V.Karthik, K.Laha, P.Parameswaran, K.V.Kasiviswanathan and Baldev Raj, J. Testing and Eval. , 35 ( 2007).
2. V.Karthik, K.V.Kasiviswanathan, A.Vijayaraghavan, K.Laha and Baldev Raj, Trans. IIM (In Press).
Further inquiries:
Shri V.Karthik, Post-Irradiation ExaminationDivision
Metallurgy and Materials Group, IGCAR, e-mail: karthik@igcar.gov.in
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