Development of Miniature Specimen Test Techniques

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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