Extending the range of constant strain rate nanoindentation testing
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Extending the range of constant strain rate nanoindentation testing Benoit Merle1,a)
, Wesley H. Higgins2, George M. Pharr2,b)
1
Materials Science & Engineering, Institute I, Friedrich-Alexander-Universität Erlangen-Nürnberg (FAU), Erlangen D-91058, Germany; and Department of Materials Science & Engineering, Texas A&M University, College Station, Texas 77843, USA 2 Department of Materials Science & Engineering, Texas A&M University, College Station, Texas 77843, USA a) Address all correspondence to this author. e-mail: [email protected] b) This author was an editor of this journal during the review and decision stage. For the JMR policy on review and publication of manuscripts authored by editors, please refer to http://www.mrs.org/editor-manuscripts/. Received: 18 September 2019; accepted: 17 December 2019
Constant strain rate nanoindentation hardness measurements at high sustained strain rates cannot be made in conventional nanoindentation testing systems using the commonly employed continuous stiffness measurement technique (CSM) because of the “plasticity error” recently reported by Merle et al. [Acta Mater. 134, 167 (2017)]. To circumvent this problem, here we explore an alternative testing and analysis procedure based on quasi-static loading and an independent knowledge of the Young’s modulus, which is easily obtained by standard nanoindentation testing. In theory, the method applies to any indentation strain rate, but in practice, an upper limit on the rate arises from hardware limitations in the testing system. The new methodology is developed and applied to measurements made with an iMicro nanoindenter (KLA, Inc.), in which strain rates up to 100 s−1 were successfully achieved. The origins of the hardware limitations are documented and discussed.
Introduction Developing methods to measure the small-scale mechanical behavior of materials at high strain rates would be highly beneficial to a broad range of technical applications. These include providing improved mechanical properties as input for crash simulations, the development of new types of armor and impact-resistant materials, and enhancing the properties of cutting tools and materials used under extreme machining conditions such as forming, cutting, and piercing. Small-scale, high-rate testing would also allow for the selective characterization of coatings and individual material phases, which are too small for the reference macroscopic methods (split Hopkinson pressure bar [1, 2, 3], Taylor cylinder impact [4], and pressure-shear plate impact [5]). From a scientific point of view, small-scale measurements could also be used to experimentally validate atomistic simulations since they are inherently restricted to small volumes and high strain rates [6, 7, 8]. Although most nanoindentation testing to date has been performed at strain rates below about 0.1 s1, a great deal of research activity has recently focused on extending the
technique to high strain rates [9, 10, 11, 12, 13, 14, 15, 16, 17]. A significant part of that research makes use of im
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