Scholarly article on topic 'Investigation of Sample-size Influence on Tensile Test Results at Different Strain Rates'

Investigation of Sample-size Influence on Tensile Test Results at Different Strain Rates Academic research paper on "Materials engineering"

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Procedia Engineering
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{"micro tensile test" / M-TT / "strain rate" / DIC / S355 / S235 / 1.4301}

Abstract of research paper on Materials engineering, author of scientific article — Martin Rund, Radek Procházka, Pavel Konopík, Jan Džugan, Hugo Folgar

Abstract The paper deals with the determination of dynamic mechanical properties of steel sheets by the miniature testing technique. Several materials were investigated, namely S235, S355 and 1.4301. The newly developed micro-tensile test (M-TT) allows local mechanical properties measurement which can be successfully used for example for weld heterogeneity determination. Furthermore, due to small size samples (tensile sample gauge section dimension of 0,5x1,5x3mm), much higher strain rates can be attained using relatively slow loading velocities in comparison to standard dynamic tests resulting in high results consistency thanks to significant reduction of the oscillations which are typical for common high strain rate tests. The results obtained from M-TT and standard tensile tests were compared.

Academic research paper on topic "Investigation of Sample-size Influence on Tensile Test Results at Different Strain Rates"

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Procedía Engineering 114 (2015) 410-415

Procedía Engineering

www.elsevier.com/locate/procedia

1st International Conference on Structural Integrity

Investigation of sample-size influence on tensile test results at

different strain rates

Martin Runda, Radek Prochazkab, Pavel Konopikc, Jan Dzugand, Hugo Folgard

? Prumyslova 995, Dobrany 33441, Czech Republic

Abstract

The paper deals with the determination of dynamic mechanical properties of steel sheets by the miniature testing technique. Several materials were investigated, namely S235, S355 and 1.4301. The newly developed micro-tensile test (M-TT) allows local mechanical properties measurement which can be successfully used for example for weld heterogeneity determination. Furthermore, due to small size samples (tensile sample gauge section dimension of 0,5x1,5x3 mm), much higher strain rates can be attained using relatively slow loading velocities in comparison to standard dynamic tests resulting in high results consistency thanks to significant reduction of the oscillations which are typical for common high strain rate tests. The results obtained from M-TT and standard tensile tests were compared.

© 2015The Authors.PublishedbyElsevierLtd. This isan open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

Peer-review under responsibility of INEGI - Institute of Science and Innovation in Mechanical and Industrial Engineering Keywords: micro tensile test; M-TT; strain rate; DIC, S355, S235, 1.4301

* Corresponding author. Tel.: 00420 377 197 356; fax: 00420 377 197 310. E-mail address: martin.rund@comtesfht.cz

1877-7058 © 2015 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/4.0/).

Peer-review under responsibility of INEGI - Institute of Science and Innovation in Mechanical and Industrial Engineering doi:10.1016/j.proeng.2015.08.086

1. Introduction

The paper deals with the determination of dynamic mechanical properties of steel sheets by the miniature testing technique in comparison with standard samples. Especially in terms of constructors there is need of determination of local mechanical properties. If the structure is dynamically loaded then the tests should also correspond to this loading. Standard testing procedures require a large materials volume for properties assessment making it impossible to evaluate local properties. At the same time requirements for the high speed loading leads to need of appropriate test machine, which are usually very costly.

Developed micro-tensile test (M-TT) not only allows local properties measurement with the use of small size samples (tensile sample gauge section dimension 0,5x1,5x3mm), but also enable determination of local dynamic tensile properties. The samples can be obtained from the same material volume as small punch test, but maintaining the same loading conditions as a standard samples, do not need any kind of correlations, enabling direct standard test parameters determination. The M-TT thanks to minute specimen size allow to attain high strain rates with the use of relatively slow loading velocities in comparison to standard dynamic tests resulting in high results consistency thanks to significant oscillations reduction typical for high strain rate tests [1].The testing procedure was verified for several materials exhibiting a wide range of strength levels and very good agreement was found confirming the procedure applicability Fig. 1.

1400 1200 1000

400 200 0

0 1 0 20 30 40 50

Strain extensometer in %

Fig. 1 Comparison of records obtained with the use of M-TT and standard tensile samples for various metallic materials.

2. Experimental material

For this tests several material were used, namely S355JR, S235JR and 1.4301 (X5CrNi18-10). These materials are very typical representatives of structural steels and are very widespread. S355JR and S235JR are perlitic-feritic steels with defined minimal yield stress and absorbed energy (27J at 20°C). Steel 1.4301 (X5CrNi 18-10) is referred to as resistant to corrosion in the environment of conventional type.

3. Tensile test

Tests execution was done at room temperature at different loading velocities resulting in initial strain rates ranging from 0.001; 0,1; 1; 10s-1. The servo-hydraulic system MTS BIONIX (load capacity 25kN and velocity of the actuator 1000mm/s) (see Fig. 2a)) was used for standard samples. M-TT samples were measured primarily at servo-electric test machine with load capacity of 5kN (see Fig. 2b)). This machine was used up to strain rates 0.1s-1. For stain rates 1 and 10s-1 the MTS BIONX was used. Dimension of the samples is pictured in Fig. 3a,b.

Fig. 2a) Servohydraulic test machine MTS BIONIX; b) Servo-electric test machine 5kN

Fig. 3a) Dimensions of M-TT; b) Dimensions of standard sample

3.1 Digital Image Correlation (DIC)

Traditionally used mechanical extensometers attached to the sample can successfully measure longitudinal and transversal strains; however, their use at high strain rate testing is almost impossible. Firstly, there is often insufficient amount of data acquired from the extensometer during the dynamic event, and secondly, dynamic tests could be destructive to the extensometer itself. Especially for high strain rate testing, optical methods proved to be more suitable [2]. For the optical measurement there is possible to use a various of systems. Very good know and often used is for example laser extensometer or video extensometer. These systems are more convenient for standard samples. Because the accuracy of strain measurement is crucial in the case of M-TT, it was found [3, 4] that the best results can be attained with the use of Digital Image Correlation (DIC) system. The principle of the Digital Image Correlation (DIC) method has been known since 1970s [5]. It is based on the recognition of change in the sequence of images.

For the need of this work DIC system Aramis was used. For this system is necessary application of the random pattern on the specimen prior the testing. The test itself is recorded by one (2D in-plane deflection measurement) or two (3D) cameras. Under the load, the specimen is deformed and so is the applied pattern. Comparing the images, changes in the pattern are registered and displacements and strains are calculated. Systems based on this method

enable 3D strains measurements of either testing samples or real components. Accuracy of this system is up to 0,01% strain [6]. Example of strain measurement with DIC system is pictured on Fig. 4.

■ ■ ■ ■

|pr ■ - tap

H ■ m

Fig. 4 Example of strain measurement at different stages of tensile test for M-TT.

Tests were performed in constant actuator velocity and the strain rate was calculated from the initial gauge length and initial velocity. After tests specimen dimensions were measured and also cross section reduction and elongation was evaluated.

An example of the records obtained from the material S355JR are shown in Fig. 5. For greater clarity all data were processed and average values of Ultimate strength (UTS) and Yield Strength (YS) in dependency of strain rate are summarized in Tab. 1. Graphical representation could be found in Fig. 6 to Fig. 8. The reduction in ductility can be shown analysing the stress ratio between the ultimate tensile strength and the yield stress (UTS/YS). Dependency of the strain rate and the ration of UTS and YS is plotted in Fig. 9.

500 iff 400

8 300 R 200 100 0

X\ \ 1 n V-, ^ \

-S355_001_3 — MTT_S355_0,001_2

-S355_01_5 —MTT_S355_0,1_ 4

—Standard S355 1 7 — -MTT S355 1 5

| -Standard_S355_10_11 — MTT_S355_10_8

0 5 10 15

Engineering Strain (%)

Fig. 5 Comparison of results obtained from standard samples and M-TT, material S355JR

Tab. 1 Tensile test results

é YS UTS Material é YS UTS Material é YS UTS

s-1 MPa MPa s-1 MPa MPa s-1 MPa MPa

0,001 209,7 318,6 0,001 378,6 552,6 0,001 295,6 728,5

Standard S235 0,1 257,5 340,5 Standard S355 0,1 411,8 572,6 Standard 1.4301 0,1 350,1 672,4

1 313,1 358,4 1 455,3 589,5 1 362,2 685,3

10 359,9 393,5 10 468,7 619,5 10 421,2 714,8

0,001 222,2 327,6 0,001 383,0 546,1 0,001 319,0 733,7

MTT S235 0,1 242,4 348,3 MTT S355 0,1 416,5 555,7 MTT 1.4301 0,1 356,0 710,4

1 324,2 368,6 1 437,1 574,3 1 379,0 712,3

10 365,0 389,6 10 477,2 641,9 10 445,0 740,6

S235JR

450 400 350 300 ' 250 2001 150 100 50

—MTT_YS —o—Standard YS —MTT_UTS —b—Standard_UTS

0,1 logs (s-1)

Fig. 6 YS and UTS dependence on various strain rates, S235JR

700 600 500 400 300 200 100 0

S355JR

—MTT YS —o—Standard_YS —MTT_UTS —b—Standard_UTS

0,001 0,01 0,1

logs (s-1)

Fig. 7 YS and UTS dependence on various strain rates, S355JR

1.4301

800 700 600 500 400 300 200 100

—MTT_YS —o—Standard_YS —MTT_UTS —s—Standa rd_UTS

0,01 0,1

logé (s-1)

Fig. 8 YS and UTS dependence on various strain rates, 1.4301

3,0 2,5

^ 2,0 <

o •e 2

1,0 0,5 0,0

MTT_S235 MTT_S355 MTT_1.4301

-B— Standard_S235 -O—Standard_S355 -o— Standard_1.4301

0,001 0,01 0,1

logé (s-1) Fig. 9 Ratio UTS/YS vs. strain rate

4. Conclusion

Within this work, tensile tests using standard and M-TT specimens were performed. Tests were carried out at room temperature with strain rate 0.001-10s-1. The investigated materials were two perlitic-feritic structure materials S355JR and S235JR. Third investigated material was austenitic steel 1.4301. On the basis of results shown in Tab. 1 it is clearly visible, that there is no significant difference between standard and M-TT specimen. Difference in the results does not exceed 8%. Subsequently should be noted, that using the M-TT samples not only saves costs for

samples production but also allow dynamic measurement on standard machines. Actuator velocity for standard samples was 150 mm/s and for M-TT 27 mm/s.

Concerning the dynamic testing for the evaluation of strain rate sensitivity, there is clearly shown trend of strain rate sensitivity increase with strain at steels S355JR and S235JR. According to our expectation, austenitic steel did not show significant strain rate sensitivity for UTS but only in YS. Similar results were described in e.g. [7], where the increase of UTS begins with higher strain rates (about 102s-1). It can also be seen that the increase of strain rate leads to a decrease of area reduction, in other words a decrease of ductility. The reduction in ductility can be shown analysing the stress ratio between the ultimate tensile strength and the yield stress (UTS/YS).

Acknowledgements

This study was created by project Development of West-Bohemian Centre of Materials and Metallurgy No.: LO1412, financed by the MEYS of the Czech Republic.

References

[1] Konopik, P., Dzugan, J., Prochazka, R.: Determination of fracture toughness and tensile properties of structural steels by small punch test and micro-tensile test, Metal 2013, 15-17.5. 2013, Brno, Czech Republic.

[2] Konopik, P., Dzugan, J., Rund, M.: Dynamic Tensile And Micro-Tensile Testing Using DIC Method, Metal 2014, May 21st - 23rd 2014, Brno, Czech Republic, ISBN 978-80-87294-52-9.

[3] Konopik, P., Dzugan, J., Prochazka, R.: Evaluation of local mechanical properties of steel weld by miniature testing technique, Materials Science & Technology 2013, October 27-31, 2013, Montreal, Quebec, Canada, ISBN 978-0-87339-762-9, pp. 2404-2411

[4] Dzugan, J., Prochazka, R., and Konopik, P., "Micro-Tensile Test Technique Development and Application to Mechanical Property Determination," Small Specimen Test Techniques, 6th Volume, STP 1576, Mikhail A. Sokolov and Enrico Lucon, Eds., pp. 12-29, doi:10.1520/STP157620140022, ASTM International, West Conshohocken, PA 2014.

[5] Sutton, m. Digital Image Correlation: Principles Develpments and Applications for Paramter Estimation, University of South Carolina. http://www.gdr2519.cnrs.fr/ecole2011/ET_2011/M_Sutton.pdf.

[6] GOM: ARAMIS System. [online] http://www.gom.com/3d-software/aramis-software.html.

[7] Cadoni, E., Fenu, l., Forni, D., Strain rate behaviour in tension of austenitic stainless steel used for reinforcing bars, Construction and Building Materials, Volume 35, October 2012, Pages 399-407, ISSN 0950-0618, http://dx.doi.org/10.1016/j.conbuildmat.2012.04.081.