PSI - Issue 36

V.V. Kharchenko et al. / Procedia Structural Integrity 36 (2022) 59–65 V. V. Kharchenko et al. / Structural Integrity Procedia 00 (2021) 000 – 000

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The use of hardness testers with another method of determination, which allows one to perform testing in an express mode due to the mobility and versatility, however, is accompanied by a sharp decrease in repeatability and inaccuracy in the hardness measurement and, consequently, the error of determination of mechanical characteristics. Unfortunately, in Ukraine, there are no standards such as ASTM A956-02, ASTM A1038-10, ISO 14577-1, which could be used for portable instruments based on other operation modes (dynamic, ultrasonic, impedance type). 3. Instrumented indentation The instrumented indentation method is more accurate and informative. Recently, it has been widely used in determining a variety of mechanical characteristics. In determining Young's modulus, Poisson's ratio, strengthening index, yield and strength limits, relative elongations, as demonstrated by Huber and Tsakmakis (1998), Jiang et al. (2009), Beghini et al. (2011), Nagaraju et al. (2017), Bolzon (2018), Peng et al. (2018), Kravchuk (2019), Katok (2020), one typically uses the single-cycle and multi-cycle indentation of the material surface under stress loading at a constant indenter displacement rate. In the creep determination, as shown by Arai (2017) and Lu et al. (2019), the identification of the specimen floor with a constant load within a time interval is performed. The instrumented indentation method is also used to investigate the aging processes by Mathew et al. (1999) and Das et al. (2010), determination of the strain curve in uniaxial tension by Beghini et al. (2006) and Fu et al. (2015)], investigation of residual stresses by Swadener et al. (2001) and Peng et al. (2018), determination of the mechanical characteristics under low-cycle loading by Nguyen et al. (2019), crack resistance characteristics by Murty et al. (1998) and Ghosh et al. (2010), etc. both at the macro, micro, and nano levels. A typical indentation diagram is illustrated in Fig. 2.

a

b

2 n F d А d D   =    

Fig. 2. Standard indentation diagram (a); (b) indentation diagram with coordinates « stress in the imprint F/d 2 – strain d/D».

The main international normative document governing the instrumented indentation method is ISO 14577-1 (2015). This standard regulates the determination of the Martens hardness (HM, HM s , HM diff ), indentation hardness HIT, Young's modulus E IT , creep characteristics C IT , and relaxation R IT during indentation, as well as elastic W elast and plastic W plast components of indentation (first part). The proposed set of physical and mechanical properties of materials has not found wide application both in industry and in scientific institutions. Therefore, the development of methods is in progress to determine the characteristics of mechanical properties from the results of tests. Unfortunately, most techniques are valid for certain classes of materials, which provide a wide scope for their improvement. Standard ISO 14577-1 (2015) covers all ranges of indentation (macro, micro, and nano levels) and a wide range of indenters: orthogonal pyramid-shaped diamond indenter with a square base (Vickers pyramid); triangular diamond pyramid (such as a modified Berkovich pyramid); carbide ball; diamond cone indenter with a spherical tip. For further development of the standard for evaluating strength properties, the International Organization for Standardization published a technical report ISO/TR 29381 (2008). It was developed regarding the first and second parts of the previous 2002 version of ISO 14577-1 (2015). It proposes three ways of determination: type of stresses and strains, inverse finite element analysis, and a neural network approach. Unfortunately, standard ISO/TR 29381 (2008) provides schematics and calculation structures without defining all key parameters unambiguously. Additional literature and the results of our investigation should be used for their definition.