Extending the range of constant strain rate nanoindentation testing

Benoit Merle,George M Pharr,Wesley H Higgins

doi:10.1557/jmr.2019.408

Benoit Merle, George M Pharr + Show 1 more

Open Access

https://doi.org/10.1557/jmr.2019.408

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Abstract

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.

Highlights

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
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 obtained by standard nanoindentation testing
The basic principle underlying the new method is simple: if the reduced elastic modulus of the material is known, the basic equations used to extract hardness from nanoindentation load–displacement data obtained under constant strain rate loading conditions can be used to continuously measure the hardness without continuous stiffness measurement technique (CSM) measurements of the contact stiffness

Summary

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. A significant part of that research makes use of impact testing, which produces ballistic strain rates ($1000 sÀ1) at the time the indenter makes initial contact with the sample [12, 13] These strain rates are not sustained throughout the indentation process, and as a result, indentation deformation and plasticity take place under varying strain rate conditions. This means that the apparent measured hardness represents a combination of strength experienced at different strain rates, making interpretation of experimental data rather complex

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