Determination of directionality of nonequibiaxial residual stress by nanoindentation testing using a modified Berkovich
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ARTICLE Determination of directionality of nonequibiaxial residual stress by nanoindentation testing using a modified Berkovich indenter Jong-hyoung Kim and Huiwen Xub) Department of Materials Science and Engineering, Seoul National University, Seoul 08826, Korea
Min-Jae Choi Nuclear Materials Division, KAERI (Korea Atomic Energy Research Institute), Daejeon 34057, Korea
Eunju Heo Center for Multi-scale Testing and Assessment at Combined Environment, Seoul National University, Seoul 08826, Korea
Young-Cheon Kima) Materials Research Center for Energy and Clean Technology, School of Materials Science and Engineering, Andong National University, 1375 Gyeongdong-ro, Andong, Gyeongbuk, 36729, Korea
Dongil Kwon Department of Materials Science and Engineering, Seoul National University, Seoul 08826, Korea (Received 26 May 2018; accepted 21 August 2018)
We suggest a new method to evaluate stress directionality, the ratio of principal stresses, using nanoindentation by introducing a modified Berkovich indenter that is extended in one direction from the Berkovich indenter. In a nonequibiaxial stress state, the indentation load-depth curves are shifted differently as the extended axis of the indenter is placed in accordance with each principal direction. The indentation load-difference is proportional to each principal stress and the slopes are defined by the normal and parallel conversion factors whose ratio is constant at 0.58. The suggested method was verified by indentation tests using five nonequibiaxial stressed specimens. The evaluated stress directionality results show agreement with the applied reference values within 620%. Furthermore, we calculated the conversion factor ratios for other modified Berkovich indenters extended to different degrees through finite element analysis and confirmed that the conversion factor ratio was inversely proportional to the extension of the modified Berkovich indenter.
I. INTRODUCTION
At nanoscales, residual stresses occur in thin films, thin rolled plates, and the like. These residual stresses lead to such undesired behaviors as film delamination, buckling, and cracking of the component, which cause problems in applications and also affect such mechanical properties as fatigue characteristics.1,2 Indentation testing can evaluate the residual stresses quantitatively and easily, without damaging the object, and is applicable to both crystalline and amorphous materials3,4; hence many studies have been carried out on evaluating residual stress using indentation. The influence of applied stress on the mechanical properties such as hardness and elastic modulus obtained by nanoindentation was first studied through experiments by Tsui et al.5 and using finite element analysis by Bolshakov et al.6 They investigated the indentation a)
Address all correspondence to this author. e-mail: kimyc@anu.ac.kr b) This author contributed equally to this work. DOI: 10.1557/jmr.2018.329 J. Mater. Res., 2018
curves and real contact areas and found that, though the indentation curves are dependent on stress, the
