Scholarly article on topic 'Use of Miniature Tensile Specimen for Measurement of Mechanical Properties'

Use of Miniature Tensile Specimen for Measurement of Mechanical Properties Academic research paper on "Materials engineering"

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Abstract of research paper on Materials engineering, author of scientific article — Kundan Kumar, Arun Pooleery, K. Madhusoodanan, R.N. Singh, J.K. Chakravartty, et al.

Abstract This paper gives a comparative study of methods used for evaluation of mechanical properties of materials using miniature and sub-size tensile test specimens. Aim of this paper is to establish the potential of miniature tensile tests which can be useful for life estimation of any in-service-equipment and for development of new materials. Both these applications intend to use very small amount of material for evaluation of the mechanical properties. Apart from various types of novel techniques developed worldwide, the evaluation of mechanical properties from a miniature tensile test has a greater advantage as it is a direct method of measurement of mechanical properties. The paper also discusses various challenges in fabrication of miniature tensile test specimens, testing methodologies and acceptance of the test results. A comparison with the test results from conventional size specimen has been done for establishing the suitability of the miniature test specimen. For establishing the geometrical design of the miniature test specimen and its behaviour over application of tensile load, the tests have been corroborated with a finite element based analysis.

Academic research paper on topic "Use of Miniature Tensile Specimen for Measurement of Mechanical Properties"

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Procedía Engineering 86 (2014) 899 - 909

Procedía Engineering

www.elsevier.com/locate/proeedia

1st International Conference on Structural Integrity, ICONS-2014

Use of Miniature Tensile Specimen for Measurement of

Mechanical Properties

Kundan Kumara*, Arun Pooleerya, K. Madhusoodanana, R N Singhb, J K Chakravarttyb,

B K Duttac and R.K. Sinhad

aReactor Engineering Division, bMechanical Metallurgy Division, cReactor Safety Division Bhabha Atomic Research centre, Mumbai-400085, India dAtomic Energy Commission, DAE, Anushakti Bhavan, Mumbai-400001, India *E-mail ID: kundan@barc.gov.in

Abstract

This paper gives a comparative study of methods used for evaluation of mechanical properties of materials using miniature and sub-size tensile test specimens. Aim of this paper is to establish the potential of miniature tensile tests which can be useful for life estimation of any in-service-equipment and for development of new materials. Both these applications intend to use very small amount of material for evaluation of the mechanical properties. Apart from various types of novel techniques developed worldwide, the evaluation of mechanical properties from a miniature tensile test has a greater advantage as it is a direct method of measurement of mechanical properties. The paper also discusses various challenges in fabrication of miniature tensile test specimens, testing methodologies and acceptance of the test results. A comparison with the test results from conventional size specimen has been done for establishing the suitability of the miniature test specimen. For establishing the geometrical design of the miniature test specimen and its behaviour over application of tensile load, the tests have been corroborated with a finite element based analysis.

© 2014 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.Org/licenses/by-nc-nd/3.0/).

Peer-review under responsibility of the Indira Gandhi Centre for Atomic Research

Keywords:

1. Introduction

Efforts for evaluation of mechanical properties from small or miniature specimen has been a subject of interest among researchers for quite long time. The main driving force for development of miniature test specimens is development of new material. The other reason is the property evaluation of any in-service component by scooping out small volume of material. The limited material available due to scooped out volume from any inservice component restricts the shape and size of the test specimens. The basic philosophy adopted in various test

1877-7058 © 2014 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/3.0/).

Peer-review under responsibility of the Indira Gandhi Centre for Atomic Research doi:10.1016/j.proeng.2014.11.112

standards for conventional tensile test specimens specify dimensions and their ratios; however for miniature specimens such standardisation is not available. This has resulted in large and different designs of the miniature specimens [1-6]. The miniaturisation of specimen cause the so-called 'scaling effect', which leads to different material behaviour in the microscale compared to the mesoscale and macroscale [7]. In mechanics, this effect is limited to the strength dependence on cross-sectional area, however in general it is much wider and may relate not only to specimen size and geometry, but also, to other factors, such as micro-structural constraints, viz. grain size in the cross-section, anisotropy due to microstructure and crystallographic texture, micro-structural and chemical inhomogeneity etc., surface effect and residual stress. These papers [1-6] also describe the strength and ductility characteristics dependence on cross-sectional area of test specimen, specimen fabrication methods, surface conditions, gage length etc.

The present work is inspired from the aim of evaluation of mechanical properties of in-service pressure vessels, by way of scooping out small volume of material using a scooping device, Fig. 1 [8]. The scooped material is of boat shaped and has dimensions as 40mm long, 30mm wide and 3mm maximum thickness. The geometries of various test specimens, viz. miniature tensile, fatigue, Charpy and small punch test, which can be made out from the boat sample are shown in Fig. 2 [9].

Fig. 1: Scooping device, scooped region and scooped sample

Fig. 2: Test Specimen layout in scooped sample

Fig. 3: Dimensional details of (a) Conventional Round Specimen (Type-I), (b) Sub-size Flat Specimen (Type-II) and (c) Miniature Tensile (Type-III) Test Specimens and their photographs

This paper is limited to the evaluation of mechanical properties using the miniature tensile test specimen. In order to ensure the usefulness of miniature test techniques it is important to have comparable results of miniature tests and conventional tests with respect to the mechanical properties, viz. UTS, YS and elongation. Two different types of tensile test specimens; namely type-I (conventional round) specimen, type-II (sub-size flat) specimen have been tested and compared with the results of type-III (miniature specimen). The dimensions of all the test specimens are given in Fig. 3. The geometry of the miniature tensile test specimen has also been evaluated using finite element analysis for giving results comparable with conventional tests.

2. Issues Related to Miniature Tensile Tests

Micro-scale uniaxial tests are not yet standardized except for the specific requirements for testing foil materials given in ASTM E345-93 but have the potential to provide uniaxial stress strain data that is representative to macro behaviour. There are many metrological issues, such as specimen geometry, specimen preparation methods, microstructural changes, alignment, ductility, resolution of load cells & strain-measuring devices etc. that can influence the test results. The various issues with respect to development of Miniature Test techniques can be listed as follows [10]:

a) Conformity of geometry of the test specimen with standards: The test specimen may not represent the bulk material due to various methods applied on it for its fabrication, such as EDM, grinding, polishing etc. The damage caused due to specimen preparation methods may be significant in volume-wise so that the result can be affected. Specimen geometry, preparation and surface finish are important parameters to be precisely standardized. Micro-structural features, such as grain shape, phase, orientation, texture, precipitate distribution, and their size relative to the dimensions of the test specimen can have significant effect on the flow stress and ductility.

b) Strain measurement: The miniature-size of test specimen does not allow the use of standard extensometers for strain measurement data. The displacement data obtained from crosshead travel of the machine may give large scatter in data. These days different techniques are being employed for measurement of displacement in miniature testing. LVDT, electrical resistance based transducer, capacitance gauges, line scan cameras, laser interferometry, digital speckle pattern interferometry, digital image correlation (DIC), photoelastic stress analysis and thermoelastic stress analysis are such techniques having their own advantages and disadvantages [10].

c) Validation of test procedures: Even in case of conventional test specimens, the ductility varies due to change in shape and sizes of test specimens. In order to have comparing ductility, the test specimens must be geometrically similar, like L/D ratio for round specimens and L/VA ratio for flat specimens [11]. It is likely that similar relation will be required for miniature test specimens as well. Also, there is a need to address the scattering in data by addressing the proper methods for specimen preparation and limiting the variation in various metrological and metallurgical parameters of the test specimens.

3. Materials and Experimental Procedures

Three different materials, listed in Table-1, were used for carrying out the specimen preparation and testing in the present investigation. Same block and heat of material were used for fabrication of type-I, type-II and type-III test specimens for each material. The tests have been carried out at different test laboratories in order to minimise any result-bias for any particular machine.

Table-1: Materials used for experiments

Material C Cr Mo Mn Ni S P Si Cu Co N V

20MnNiMo 55 0.18 0.078 0.49 1.24 0.58 0.007 0.014 0.23 0.067 0.0069 0.0068 --

CrMoV 0.17 2.51 0.63 0.38 0.8 0.009 0.015 0.28 0.054 0.013 0.065 0.1 7

SS304 LN 0.036 18.6 -- 0.264 8.3 0.001 0.074 0.346 1.178 0.112 -- --

3.1 Testing Fixtures

The miniature specimen having very limited size and grip section, required development of special testing fixture, Fig. 4. Two types of test fixtures were developed are for pinned specimen and for shoulder supported specimens.

Fig. 4: Test Fixtures for Miniature Tensile Test Specimen

3.2 Ductility Measurement for Different-Sized Specimens

In tensile tests, apart from UTS and YS, elongation is also an important measurement which needs to be correlated with conventional test specimen results. As the gage length of miniature specimen is only 5mm, it did not allow mounting of any mechanical extensometer, so earlier the extension measurements were done using the cross-head travel. In general, the measurement of elongation using the cross-head travel gives lower value of extensometer due to the effective smaller length of necking region. In order to minimise the difference in the elongation properties of different gage length specimen, it is important that they are geometrically similar.

The ductility measurement consists of two components, the uniform extension up to necking and the localised extension once necking begins. The extent of uniform extension depends upon the metallurgical condition of material and the effects of specimen size and shape on the development of the neck. Fig. 5 shows the variation of local elongation along with the gage length of a prominently necked tensile specimen [11].

It is seen that shorter the gage length the greater the influence of localised deformation at the neck on the total elongation of the gage length. The total extension is given as

Final Length -Initial Length =uniform extension + local extension at necking

i.e. Lf - Lq= euL0 + a (1)

Gage Length -►

Fig. 5: Ductility results dependence on Gage length

The tensile elongation is given as

ef=( Lf - L0)/L0=eu+ a/Lq (2)

A number of attempts have been made to rationalise the strain distribution in tensile test and based on which it has been concluded that geometrically similar specimens develop geometrically similar necked region. According to Barba's law, a= pVA 0, [11], where A0 is the initial area of cross-section and p is coefficient, the elongation becomes

ef =eu+ PVA,/ Lq (3)

It is generally recognised that in order to compare elongation measurements of different-sized specimens, the specimens must be geometrically similar. The equation shows that for measurement of ductility properties of different-sized specimen the ratio LqWa 0 for sheet specimen and L0/D0 for round specimen must be maintained. Table-2 gives the dimensional relationship followed by different countries vis-a-vis followed in the present work.

Table-2: Dimensional relationships of tensile specimens used in different countries

Type of ASTM ASTM Great Britain Great Britain Germany Present report

Specimen A370 E8M (before 1962) (Current)

Round (L</Do) 4 5 3.54 5 10 5.0 (Type-I)

Sheet (L</Va0) 2.8-14.1 3.2-14.1 4 5.65 11.3 5.5 (Type-II & III)

3.3 Use of Video Extensometer

As the miniature test specimen size does not allow mounting any mechanical extensometer on its gauge section, it has driven the need for using non-contact type extensometer. Various types of such extensometers have been used by many researchers [10], like laser based, DCM, video extensometer etc. In present case video extensometer, Fig. 6, having single camera, Fig. 6(b), with fixed focus has been used for measurement of gauge section extensions. The video extensometer works on the principle of pattern recognition technique. The test specimen is marked with contrast patterns prior to loading in the machine. The camera tracks the marked targets on the test specimen, Fig. 6(c), and works as an extensometer which gives the displacement values during the test.

grasar

» Target I

Target 3 tr • '1 ^ Target 4 \

(f dL « Target 2

É Ii $

(a) Testing Machine

(b) Camera

(c) Test Specimen

Fig. 6: Video Extensometer

4. Results and Discussions

The tests were conducted at room temperature for three materials with a strain rate of 1x10-3 per sec. Fig 7 & 8 give the results of the various tests conducted on the specimens. For 20MnMoNi55, the test of conventional, Type-I specimen, gives the minimum UTS and YS values as 622 MPa and 485 MPa respectively. In comparison to these values the UTS values given by miniature test specimens (type-III) are in the range of 2%-4.5% less. The YS values given by type-III test specimens are in the range of 0.2%-6% less in comparison to type-I specimens. The uniform elongation given by type-III test specimens are in the range of 7.7%-9.8% in comparison to the range of 8.1%-8.2% for type-I specimens. The total elongation of type-III test specimens are 18.2%-20.9% in comparison to 20.3%-21.5% of the type-I specimens. For CrMoV steels there are significant differences in mechanical properties in two perpendicular directions, marked as B1 and C1 as suffixes in the nomenclature in the table-3. Out of total six specimen of type-I tested, three each from block B1 and block C1, the UTS values are 603-605 MPa for block B1, and 615-618 MPa for block C1. The corresponding YS values for B1 and C1 blocks for type-I specimens are 456459 MPa and 471-477 MPa. The elongation values of these specimens are, however, in the same range, 26.5%-28.9% irrespective of their orientation. Block B1 was used for fabrication of miniature specimens and their test results give UTS values as 582-606MPa and YS values as 448-460MPa, which are comparable to their type-I values. The uniform elongation values for type-I and type-III are in agreement however, the total elongation values (22%-23.8%), are less in comparison to type-I specimens' elongation values (~27%). This may be due to the reason the thickness/width ratio 0.445 was taken in this case, whereas specified value of 0.3. For the third material SS-304LN, the minimum UTS and YS values for type-III specimens are 540MPa and 225MPa in comparison to 571MPa and 237MPa for type-I specimens. The higher YS values may be attributed to any residual stresses in the type-III specimen during the fabrication and polishing stage. This needs further tests and analysis. The uniform elongation and total elongation of type-I and type-III specimens are comparable. Overall it can be concluded that the results obtained by miniature test specimens are comparable with those obtained by conventional test specimens within a certain error band. With the precise control of surface finish of test specimen, dimensional deviations, alignment of test fixtures, better strain measurement, the error band can be minimised and potentiality of miniature test specimen can be enhanced.

Material: 20MnMoNi55

-1. Type-I - 2. Type-I -3. Type-II -4. Type-I -5. Type-II 6. Type-II -7. Type-II

(d) Material: 20MnMoNiS5

• • A

600- ♦ ^ .

■ Type-I UTS ♦

• Type-I VS

A Type-II UTS

550- Y Type-II YS

♦ Type-Ill UTS

500- * Type-Ill YS

• ▼ *

■ *

▼ ★

450- *

Strain, %

Specimen No.

10 12 14 16 18 20 22 24 26 2B 30 32

■ Type-

• Type-A Type-▼ Type-O Type-o Type-

♦ Type* Type-

I B1 Block JTS

I B1 Block YS IC1 Block UTS IC1 Block YS

II B1 Block UTS

II B1 Block YS

III B1 Block JTS III B1 Block YS

Material: CrMoV Steel

Strain, %

Specimen No.

Fig. 7: The engineering stress - strain plot for (a) 20MnMoNi55 steel, (b) Cr-Mo-V steel and (c) SS304LN obtained using Type I, Type II and Type III specimens. The respective variation in yield strength (YS) and ultimate tensile strength (UTS) is shown in (d), (e) and (f).

Uniform Elogn, Type-I Total Elogn, Type-I Uniform Elogn, Type-I I Total Elogn, Type-ll Uniform Elogn, Type-Ill Total Elogn, Type-Ill

Material:2QMnMoNi55

16 g ro

■ Uniform Elogn-Type-l-B1

• Total Elogn-Type-l-B1

A Uniform Elogn-Type-l-C1

▼ Total Elogn-Type-l-C1

♦ Uniform Elogn-Type-ll-B1 < Total Elogn-Typa-ll-B1

• Uniform Elogn-Type-lll-B1

★ Total Elogn-Type-lll-B1

Maten'al:Cr-Mo-V Steel

♦ •

Specimen No.

Specimen No.

Material: SS3Q4LN

1 s°"

D> . | 40"

| 30-"c ■ 3 2010-

Unifonn Elogn, Type-I Total Elogn, Type-I Uniform Elogn, Type-ll Total Elogn, Type-ll Uniform Elogn, Type-Ill Total Elogn, Type-Ill

50 | CO D> 40 O HI

Specimen No.

Fig. 8: Variation in uniform and total elongation for (a) 20MnMoNi55 steel, (b) Cr-Mo-V steel and (c) SS304LN obtained using Type I, Type II and Type III specimens.

5. Finite Element Modeling (FEM)

5.1 Geometrical Modeling

The finite element modelling computations were conducted using a commercial finite element modeling software. The 2-D finite element model of quarter shape of the miniature test specimen was used due to geometric symmetricity, as shown in Fig. 9. The test specimen is modelled with four noded quadrilateral plane stress elements because the thickness of the body or domain is small relative to its lateral (in-plane) dimensions. Mapped meshing was adopted for the gauge length while other sections were free meshed with emphasis given to the number of elements in the gauge section and shoulder sections of the specimen. Geometric nonlinearity was also modeled on top of material non linearity. In order to represent the experimental setup, the loading pin was simulated for providing load to the specimen. The loading pin was treated as 2-D rigid body with friction coefficient of zero. Due to symmetricity only one pin is modeled and the motion in X-direction was arrested. However, it was allowed to experience displacement in the Y direction as in the experimental situation. Contact elements were defined between the loading pin and the shoulder contact line of the specimen model. A concentrated force was applied to the reference point of the pin in the Y direction. The load is applied through the amplitude field in a linear fashion i.e. at time zero, the load value is zero and at time t=1 the load is maximum. All these prescribed conditions are applied in the respective reference node of load pins.

5.2 Material Modeling

In the elastic analysis of simulation, the material properties of the columns were defined by elastic modulus and Poisson's ratio. In the nonlinear analysis stage, material nonlinearity or plasticity was included in the FEM using a mathematical model known as the associated plasticity flow rule with von-Mises yield criterion. In the finite element simulation of miniature specimen, the plastic properties are defined together with the isotropic hardening rule. It means that the yield surface size changes uniformly in all directions such that the yield stress increases in all stress directions as plastic straining occurs. In the incremental plasticity model true stresses (t) and true plastic strains (tp) of a conventional test specimen were specified. This is because, during the tensile test, the cross-section area of the sample decreases due to elongation, while each subsequent increase of sample length takes place over an already elongated sample length. The true stress-true strain defines the flow curve appropriately as they represent the basic flow characteristics of the material. The incremental plasticity model requires the true stress-strain curve from the point corresponding to the last value of the linear range of the engineering stress-strain curve to the ultimate point of the stress-strain curve. The material properties used for the finite element simulation were determined through standard uniaxial tensile test and are shown in Table-3.

Table-3: Material Properties Used In FE Simulation

Material: 20MnMoNi55 Young's modulus (GPa): 194 Poisson's ratio 0.29

True stress(MPa) True plastic strain

401 0.0021

455 0.0027

479 0.0044

503 0.0097

534 0.0177

583 0.0322

620 0.0476

5.3 Finite Element Results

A concentrated load of 300 N (for two pins load would be 600N) is applied on the reference node of the pin in a linearly varying manner i.e. at time t=0, the load was zero and at time t=1, the load applied is (300N) with the use of amplitude option in FEM software. For each time step both the load taken by the specimen and the corresponding elongation in the specimen is stored in the software. Fig. 10 shows the load-time diagram obtained by the FE simulation of specimen and experimental results, which are in good agreement.

Fig. 9: Geometrical Modelling and Results of FE analysis

Fig. 10: Comparison of true stress - true strain curves obtained experimentally and from FE analysis 6. Conclusion

Tensile properties were determined for three different material using Type I, Type II and Type III test specimens and their results were found to be in good agreement. UTS values given by type-III specimens are in the range of 2%-4.5% less in comparison to type-I specimens for all three materials. The YS values given by type-III test specimens are in the range of 0.2%-6% less in comparison to type-I specimens for 20MnMoNi55 and CrMoV steel, however these values are approximately 7-12% higher in case of SS304LN material. The higher YS values may be attributed to any residual stresses in the type-III specimen during the fabrication and polishing stage. This needs further tests and analysis. The uniform elongation and total elongation values of type-III specimen are more or less in agreement to the type-I specimen for 20MnMoNi55 and SS304LN, however these values could not be in agreement as different thickness of specimen was used for CrMoV steel. The true stress - true strain curves obtained experimentally and through FEM analysis of miniature specimen were also in good agreement. These results suggest the usefulness of miniature specimen technology and for tapping this potential there is a need for a coherent international effort for standardizing the technique and to adopt these as standard practices for the mechanical property evaluation.

7. Acknowledgements

Some part of the present work has been done at National Metallurgical Laboratory, Jamshedpur under

BRNS project. Authors are thankful to both BRNS and NML for their respective works.

References

1. Panayotou N F et al, 'Design and use of Nonstandard Tensile Specimens for Irradiated Materials Testing' SPT-888, P201-219

2. Rosinski Stan T et al, 'Application of Sub-size specimens in Nuclear Plant Life Extension' ASTM STP 1204, pp 405-416

3. Yuanchao Xu et al, 'Application of the miniature specimen technique to material irradiation tests and surveillance for reactor components' International Journal of Pressure Vessels and Piping 77 (2000), pp 715721

4. Yutaka Kohno et al, 'Specimen size effects on the tensile properties of JPCA and JFMS' Journal of Nuclear Materials, pp 283-287 (2000) 1014-1017

5. Recommendation of miniaturized techniques for mechanical testing of fusion materials in an intense neutron source, P. Jung et al, Journal of Nuclear Materials 232 (1996) 186-205

6. Miniature Tensile Test Specimens for Fusion Reactor Irradiation Studies, R.L. Klueh, Nuclear Engineering and Design/Fusion 2 (1985) 407-416

7. Sergueeva A V et al, 'Gage length and Sample size effect on measured properties during tensile testing', A526 (2009), Mat.Sc and Engg A

8. Kumar K et al, 'Development of Boat Sampling Technique' Report No. BARC/2002/I/013

9. Kumar K et al, 'Miniature Specimen Technique as an NDT Tool for Estimation of Service Life of Operating Pressure Equipment' Proceeding of the International Conference & Exhibition on Pressure Vessels and Piping, "OPE 2006 - CHENNAI", 7-9, February 2006, Chennai, India

10. Lord J D et al, 'Aspects of strain and strength measurement in miniaturised testing for engineering metals and ceramics' Materials Science and Technology, pp 127-148 (2010) Vol 26

11. Dieter G. E. 'Fundamentals of Mechanical Metallurgy'