International Journal of Advances in Mechanical and Civil Engineering, ISSN: 2394-2827 Volume-4, Issue-2, Aprl.-2017 http://iraj.in INVESTIGATING THE RELATIONSHIP BETWEEN HARDNESS AND YIELD STRENGTH OF SS400 STEEL USING NANOINDENTATION 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 temperature 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 materials in the bridge and building construction due The hardness of the material 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 steels. 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 strength of materials [1]-[3]. Furthermore, the indentation technique has several advantages, such as the short testing time and the low where hc is the contact depth. cost. In early works, the relationship between hardness and B. Determination of the Yield Strength The true stress - strain curve of structural steel can be yield strength has been constructed by combining the 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 y is a yield strain, st is the strain at the investigated based on the open literatures for the starting-point of strain hardening, is a plastic indentation technique [6]. Although that method is property defined as the ratio of st and y, E is acceptable, the inconsistent input data of the yield elastic modulus, and n is strain hardening exponent. strength and hardness values are a limitation of the The yield strength of structural steel can be study because the hardness-yield strength correlation determined from the indentation data by using a is constructed based on the data collected from seven reverse algorithm, which consists of following different studies [6]. The yield strength now can be dimensionless 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. Investigating the Relationship Between Hardness and Yield Strength of Ss400 Steel Using Nanoindentation 31 International Journal of Advances in Mechanical and Civil Engineering, ISSN: 2394-2827 Volume-4, Issue-2, Aprl.-2017 http://iraj.in where aijk and bijk are coefficients, and C is the assembled from the hardness and the yield strength 2 values at the different strain rates. loading curvature calculated as C Pm/ h m . III. EXPERIMENTS In this study, the relationship between hardness and yield strength is found to be described by an exponent A. Specimen Preparation equation as follows: The specimen for the indentation test is a flat plate with a size of 20 mm x 12 mm x 8 mm. Fig. 1 shows the samples of SS400 steel after being mounted and where a and b are material dependent constants, polished. 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 Stiffness Measurement Fig. 3. The relationship between hardness and yield strength. (CSM) technique by controlling a loading strain rate. Berkovich indenter made of industrial diamond with It can be recognized that (6) well describes the an elastic modulus of 1140 GPa and Poison’s ratio of hardness-yield strength relationship with an R-square 0.07 is imployed. The maximum displacement is of 0.85. The correlation between hardness and yield 2000 nm, which is applied to all points during strength constructed based on the indentation results indentation tests. is reliable and acceptable. I. IV. RESULTS AND DISCUSSIONS CONCLUSIONS A. Load-Displacement curves In this study, the nanoindentation test is performed on Fig. 2 presents the load-displacement curves of structural steel with different strain rate. The structural steel. The results show that the maximum relationship between the hardness and the yield load increases when the strain rate increases. From strength was investigated based on assembling the the indentation data that are extracted from the load- values of hardness and yield strength at different displacement curves, several material properties can strain rates. The hardness-yield strength relationship be calculated, such as the hardness and the yield can be described by an exponent equation. strength. 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 of yield strength from hardness measurements,” Metall Trans, Fig. 2. The load-displacement curves of nanohardness test at vol. 2, pp. 1979–83, July 1971. different loading strain rates. [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. [6] Busby JT, Hash MC, Was GS, “The relationship between Hardness hardness and yield stress in irradiated austenitic and ferritic Fig. 3 shows the correlation between hardness and steels,” J Nucl Mater, vol. 336, pp. 267–278, Sep. 2004. yield strength. The input data of this correlation are Investigating the Relationship Between Hardness and Yield Strength of Ss400 Steel Using Nanoindentation 32 .