An efficient way of extracting creep properties from short-time spherical indentation tests
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Jin Haeng Lee Korea Atomic Energy Research Institute, Daejeon 305-353, Republic of Korea
Hyungyil Leea) Department of Mechanical Engineering, Sogang University, Seoul 121-742, Republic of Korea (Received 9 June 2015; accepted 6 October 2015)
Indentation as a means to extract creep properties has the advantage that it can be applied directly to micro/nano-structures. Many studies on indentation creep reported at least partially poor agreement with creep parameters derived from uniaxial test. One important reason for the incompatibility is the neglect of transient creep. Another one is the choice of equivalent stress and strain measures to relate the different material responses. Applying a material model that accounts for transient creep effects we propose an efficient method for deriving creep properties from shorttime spherical indentation tests. We first determine a subsurface point where the material response is very close to that observed in uniaxial tests. We then map the load–displacement data to the material response, expressed in terms of two dimensionless variables, at this point. Converting the dimensionless variables data to stress, strain, and strain rate data, we finally determine the material’s creep coefficient and exponent.
I. INTRODUCTION
Contributing Editor: George M. Pharr a) Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2015.322
occurs in the steady-state regime, where the strain rate is constant. This assumption is however highly contested, for matching indentation creep data to uniaxial creep data succeeded for some materials but failed for others; some researchers reported good agreement regarding the creep exponent but large deviation regarding the creep coefficient. For instance, indenting indium specimens, Lucas and Oliver8 obtained creep exponents similar to the one from uniaxial creep tests; yet, there was a significant difference between the creep coefficients. Ma et al.9,10 and Choi et al.11 found unsatisfactory agreement for the creep exponent. Wang et al.12 observed that while creep exponents agree well, the strain rate from the indentation test vastly differs from the uniaxial creep test. Fujiwara and Otsuka,13 Liu and Chen,14 Mahmudi et al.15,16 and Marques et al.,17 on the other hand, reported good agreement for solder alloys; Tagaki et al.18 for an Al–Mg alloy; and Geranmayeh19 for cast Mg–6Al–1Zn–0.7Si alloy. Numerical attempts to elucidate the differences between uniaxial creep and indentation creep have been made, though mainly for sharp indentation (e.g., Takagi et al.,18 Chen et al.,20 Stone et al.,21 Su et al.,22 Dean et al.,23 and Cordova and Shen24). Many of these studies were limited to a specific material (type), or neglected the influence from transients. Ma et al.25 related indentation variables to material properties by introducing a parameter that represents the amount of pile-up or sink-in. Takagi and Fujiwara26 suggested two coefficients to convert
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Ó Materials Research Society 2015
As nanomechanical properties are of incr
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