In-Situ Measurements of Mechanical Properties Using Novel Automated Ball Indentation System

Determination of the integrity of any metallic structure is required either to ensure that failure will not occur during the service life of the components (particularly following any weld repair) or to evaluate the lifetime extension of the structure. A portable/in-situ stress-strain microprobe system was developed to evaluate nondestructively in situ the integrity of metallic components (including base metal, welds, and heat-affected zones (HAZs)) The microprobe system utilizes an innovative automated ball indentation (ABI) technique to determine several key mechanical properties (yield strength, true-stress/true-plastic-strain curve, strain-hardening exponent, Luders strain, elastic modulus, and an estimate of the local fracture toughness). This paper presents ABI test results from several metallic samples. The microprobe system was used successfully to nondestructively test in-situ a circumferentially welded Type 347 stainless steel pipe. Four V-blocks were used to mount the testing head of the microprobe system, allowing a 360° inspection of property gradients in the weld and its HAZ. The ABI test is based on strain-controlled multiple indentations (at the same penetration location) of a polished surface by a spherical indenter (0.25 to 1.57-mm diameter). The microprobe system and test methods (1) are based on well demonstrated and accepted physical and mathematical relationships which govern metal behavior under multiaxial indentation loading. A summary of the ABI test technique is presented here, and more details are given elsewhere (1-14). The microprobe system currently utilizes an electromechanically driven indenter, high-resolution penetration transducer and load cell, a personal computer (PC), a 16-bit data acquisition/control unit, and copyrighted ABI software. Automation of the test, where a 486 PC and a test controller were used in innovative ways to control the test (including a real-time graphic and digital display of load-depth test data) as well as to analyze test data (including tabulated summary and macro-generated plots), make it simple, rapid (less than 10 min for a complete ABI test), accurate, economical, and highly reproducible. Results of ABI tests (at several strain rates) on various base metals, welds, and HAZs at different metallurgical conditions are presented and discussed in this paper.