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Derivation of plastic stress–strain relationship from ball indentations: Examination of strain definition and pileup effect

Published online by Cambridge University Press:  31 January 2011

Jeong-Hoon Ahn  and
Dongil Kwon
Jeong-Hoon Ahn
Affiliation:
School of Materials Science and Engineering, Seoul National University, Seoul 151–742, Korea
Dongil Kwon
Affiliation:
School of Materials Science and Engineering, Seoul National University, Seoul 151–742, Korea
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Abstract

The ball indentation technique has the potential to be an excellent substitute for a standard tensile test, especially in the case of small specimens or property-gradient materials such as welds. In our study, the true stress–true strain relationships of steels with different work-hardening exponents (0.1–0.3) were derived from ball indentations. Four kinds of strain definitions in indentation were attempted: 0.2sinγ, 0.4hc/a, ln[2/(1 + cosγ)], and 0.1tanγ. Here, γ is the contact angle between the indenter and the specimen, hc is the contact depth, and a is the contact radius. Through comparison with the standard data measured by uniaxial tensile testing, the best strain definition was determined to be 0.1tanγ. This new definition of strain, in which tanγ means the shear strain at contact edge, reflected effectively the work-hardening characteristics. In addition, the effects of pileup or sink-in were considered in determining the real contact between the indenter and the specimen from the indentation load–depth curve. The work-hardening exponent was found to be a main factor affecting the pileup/sink-in phenomena of various steels. These phenomena influenced markedly the absolute values of strain and stress in indentation by making the simple traditional relationship Pm/σR ≈ ≈ 3 valid for the fully plastic regime.

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Articles
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Journal of Materials Research , Volume 16 , Issue 11 , November 2001 , pp. 3170 - 3178
Copyright
Copyright © Materials Research Society 2001

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