Non-Destructive Evaluation of Deformation and Fracture Properties of Materials using Stress-strain Microprobe
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Advanced Technology Corporation, 661 Emory Valley Road, Suite A, Oak Ridge, TN 37830
ABSTRACT Tensile deformation and fracture properties of several metallic materials, welds, and their heataffected-zones were determined non-destructively using the Stress-Strain Microprobe (SSM) system. The system is based on automated ball indentation (ABI) technique and involves straincontrolled multiple indentations at the same location on the material surface by a small spherical indenter. The technique permits evaluation of tensile deformation parameters such as yield strength, ultimate tensile strength, strength coefficient, and strain-hardening exponent, and a fracture energy parameter called indentation energy to fracture. ABI tests were conducted on carbon steels, stainless steels, nickel alloys, aluminum alloys, Zircaloys, electronic soldering materials and several nuclear pressure vessel steels (in the unirradiated, neutron irradiated, and irradiated and thermally annealed conditions). For all these test materials and conditions, the ABI-derived results were found to agree with the data from conventional standard test methods. In addition to the laboratory applications of SSM, it can be used as an in-situ testing instrument for non-destructive assessment of deformation and fracture properties of operating structural components. INTRODUCTION
Techniques to characterize deformation and fracture behavior of components are needed to assess the degradation in properties and estimate remaining life of structures in various applications. While destructive tests using material removed from components do provide relevant data, in-situ measurements using non-destructive techniques on real structures are desirable. Another important criterion is that the technique should be capable of yielding these data using minimal amount of material so that gradient properties in welds and their heataffected-zones (HAZ) can be evaluated. One such technique is the Automated Ball Indentation (ABI). While the idea of ball indentation is not new [1], ABI technique is unique because it incorporates fully automated measurement of impression parameters. It does not require measurement of diameter of the indentation using elaborate profilometry, optical interferometry, etc., which renders the traditional methodology unsuitable for on-line monitoring of the mechanical properties of components. Based on this principle, a portable/in-situ stress-strain microprobe system was developed recently by Advanced Technology Corporation [2] to test minimal material, and determine tensile properties (e.g. yield strength, tensile strength, strainhardening exponent, strength coefficient) and fracture property (in terms of a fracture energy parameter called indentation energy to fracture, (IEF)). SSM system and the test methods incorporate known and accepted theoretical and empirical relationships governing material response to multiaxial indentation loading [3]. ABI is a relatively simple, rapid (- few minutes) and non-destructive technique, requires small amoun
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