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INVESTIGATING THE RELATIONSHIP BETWEEN HARDNESS AND STRENGTH OF SS400 USING

1NGOC-VINH NGUYEN, 2VIET-HUNG TRUONG, 3THAI-HOAN PHAM, 4SEUNG-EOCK KIM

1,2PhD Student, Dept. Of Civil and Environmental Engineering, Sejong University, Seoul, Korea. 3PhD, Dept. Of Civil Engineering, Vinh University, 182 Le Duan, Vinh City, Viet Nam. 4Professor, Dept. Of Civil and Environmental Engineering, Sejong University, 98 Gunja-dong, Gwangjin-gu, Seoul, 143- 747, Korea. E-mail: [email protected], [email protected], [email protected], [email protected]

Abstract - In this study, Nano Hardness test was carried out at room with different strain rates from 0.002 s-1 to 0.2 s-1,. A reverse algorithm was used to determine the yield strength from the load-displacement curve of the indentation test. From the obtained yield strength and hardness at different strain rates, the hardness-yield strength relationship was constructed.

Keywords - Hardness; Nanoindentation; Reverse algorithm; Yield strength.

I. INTRODUCTION II. METHODS

Structural steel is one of the most widely used A. Determination of the Hardness in the bridge and building construction due The hardness of the can be extracted from to several advantages, such as the low cost and easy the load-penetration depth curve of indentation, as machinability and weldability. The hardness (H) and follows [1]: the yield strength (y) are the important material properties of structural . In order to determine the hardness and yield strength, several tests must be where Pm is a maximum applied load, Ac is the carried out, such as the tensile, compressive, and projected contact area. In the indentation technique, indentation tests, in which the indentation test is a the project area is calculated by using the following good choice since this test allows extracting both the equation [2] hardness and yield [1]-[3]. Furthermore, the indentation technique has several advantages, such as the short testing time and the low cost. where hc is the contact depth.

In early works, the relationship between hardness and B. Determination of the Yield Strength yield strength has been constructed by combining the The true - strain curve of structural steel can be expressed as follows [3]: results from tensile and indentation tests [4], [5]. In these studies, the yield strength was obtained from the tensile test, while the hardness was obtained from indentation test. However, determining the H/y ratio by using these both tensile and indentation tests is a high cost and time-consuming task. The relationship between hardness and yield strength was also where ɛy is a yield strain, ɛst is the strain at the investigated based on the open literatures for the starting-point of strain ,  is a indentation technique [6]. Although that method is property defined as the ratio of ɛst and ɛy, E is elastic acceptable, the inconsistent input data of the yield modulus, and n is strain hardening exponent. The strength and hardness values are a limitation of the yield strength of structural steel can be determined study because the hardness-yield strength correlation from the indentation data by using a reverse is constructed based on the data collected from seven algorithm, which consists of following dimensionless different studies [6]. The yield strength now can be equations [3] calculated by using a reverse algorithm for indentation technique [3].

The aim of this study is to investigate the relationship between the hardness and the yield strength by using indentation technique.

where aijk and bijk are coefficients, and C is the 2 loading curvature calculated as C Pm/ h m .

Proceedings of 26th Research World International Conference, Bangkok, Thailand, 5th-6th January 2017, ISBN: 978-93-86291-83-7 12 Investigating the Relationship Between Hardness and Yield Strength of Ss400 Steel Using Nanoindentation III. EXPERIMENTS assembled from the hardness and the yield strength values at the different strain rates. A. Specimen Preparation In this study, the relationship between hardness and The specimen for the indentation test is a flat plate yield strength is found to be described by an exponent with a size of 20 mm x 12 mm x 8 mm. Fig. 1 shows equation as follows: the samples of SS400 steel after being mounted and polished.

where a and b are material dependent constants, which are 71.12 and 0.00089, respectively.

Fig. 1. Specimen of structural steel after polishing.

B. Nano Hardness test Nanohardness tests are carried out at room temperature using Continuous Measurement (CSM) technique by controlling a loading strain rate. Berkovich indenter made of industrial diamond with an of 1140 GPa and Poison’s ratio of Fig. 3. The relationship between hardness and yield strength. 0.07 is imployed. The maximum displacement is 2000 nm, which is applied to all points during It can be recognized that (6) well describes the indentation tests. hardness-yield strength relationship with an R-square of 0.85. The correlation between hardness and yield IV. RESULTS AND DISCUSSIONS strength constructed based on the indentation results is reliable and acceptable. A. Load-Displacement curves I. Fig. 2 presents the load-displacement curves of CONCLUSIONS structural steel. The results show that the maximum load increases when the strain rate increases. From In this study, the nanoindentation test is performed on the indentation data that are extracted from the load- structural steel with different strain rate. The displacement curves, several material properties can relationship between the hardness and the yield be calculated, such as the hardness and the yield strength was investigated based on assembling the strength. values of hardness and yield strength at different strain rates. The hardness-yield strength relationship can be described by an exponent equation.

REFERENCES

[1] Oliver WC, Pharr GM, “An improved technique for determining hardness and elastic-modulus using load and displacement sensing indentation experiments,” J Mater Res, vol. 7, pp. 1564–1583, June 1992. [2] Dao M, Chollacoop N, Van Vliet KJ, Venkatesh TA, Suresh S, “Computational modeling of the forward and reverse problems in instrumented sharp indentation,” Acta Mater, vol. 49, pp. 3899–3918, Aug. 2001. [3] Pham T-H, Kim J-J, Kim S-E, “Estimating constitutive equation of structural steel using indentation,” Int J Mech Sci, vol. 90, pp. 151-161, April 2014. [4] Cahoon JR, Broughton WH, Kutzak AR, “The determination Fig. 2. The load-displacement curves of nanohardness test at of yield strength from hardness measurements,” Metall Trans, different loading strain rates. vol. 2, pp. 1979–83, July 1971. [5] Currey JD, Brear K, “Hardness, Young’s modulus and yield stress in mammalian mineralized tissues,” J Mater Sci Mater, B. The Relationship between Yield Strength and vol. 1, pp. 14–20, June 1990. Hardness [6] Busby JT, Hash MC, Was GS, “The relationship between Fig. 3 shows the correlation between hardness and hardness and yield stress in irradiated austenitic and ferritic yield strength. The input data of this correlation are steels,” J Nucl Mater, vol. 336, pp. 267–278, Sep. 2004.

Proceedings of 26th Research World International Conference, Bangkok, Thailand, 5th-6th January 2017, ISBN: 978-93-86291-83-7 13