In this work, some inaccuracies and limitations of prior indentation theories, which are based on experimental observations and the deformation theory of plasticity, are investigated. Effects of major material properties on the indentation load-deflection curve are examined via finite element (FE) analyses based on incremental plasticity theory. It is confirmed that subindenter deformation and stress–strain distribution from deformation plasticity theory are quite dissimilar to those obtained from incremental plasticity theory. We suggest an optimal data acquisition location, where the strain gradient is the least and the effect of friction is negligible. A new numerical approach to indentation techniques is then proposed by examining the FE solutions at the optimal point. Numerical regressions of obtained data exhibit that the strain-hardening exponent and yield strain are the two key parameters which govern the subindenter deformation characteristics. The new indentation theory successfully provides a stress–strain curve and material properties with an average error of less than 3%.

A numerical approach to spherical indentation techniques for material property evaluation

A new method for acquiring true stress–strain curves over a large range of strains using a tensile test and finite element method

A study on robust indentation techniques to evaluate elastic–plastic properties of metals

Nanocrystalline (nc) iron was produced by mechanical attrition and compacted into near fully dense samples. Isothermal annealing at 800 K resulted in grain sizes between 15 and 24 nm. A newly available Automated Ball Indentation system was used to study the compressive mechanical properties of the samples. The ABI method proved useful in examining the mechanical properties of nc iron on a more quantitative level than previously possible by conventional hardness testing methods. Stress–strain curves were obtained which indicated a compressive behavior similar to that of perfectly plastic materials: low strain hardening at high flow stresses around 3 GPa and a low room-temperature strain-rate sensitivity. The flow stresses were independent of the grain size in the range of the present study. The deformation pile-up around the indentations seems to have formed inhomogeneously, exhibiting intense plastic deformation in localized shear bands.

Compressive mechanical behavior of nanocrystalline Fe investigated with an automated ball indentation technique

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.

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

A field indentation microprobe (FIM) apparatus was developed (and patented) to evaluate, nondestructively in situ, the integrity of metallic structures. This study investigated the applicability of using a new automated ball indentation (ABI) test, which is a major part of the FIM, to measure the flow properties of metallic materials including those exhibiting Lueders or inhomogeneous strains (carbon steels, titanium alloys, aluminum alloys, etc.) and to estimate their fracture toughness. The ABI test is based on multiple indentations (at the same penetration location) of a polished metallic surface by a spherical indenter. Automation of the test, where a computer and test controller were used in innovative ways to control the test as well as to analyze test data, made it simple, rapid, accurate, economical, and reproducible. Results of ABI tests on different metals, welds, and irradiated materials are presented and discussed in this paper. Excellent agreement was obtained between ABI-derived data and those from standard ASTM uniaxial tensile and fracture toughness tests. 17 refs., 20 figs., 2 tabs.

Use of Automated Ball Indentation Testing to Measure Flow Properties and Estimate Fracture Toughness in Metallic Materials

A computer-controlled system consisting of a servohydraulic test machine, an in-house designed test controller, and a desktop computer has been developed for performing automated fracture toughness and fatigue crack growth testing both in the laboratory and in hot cells for remote testing of irradiated specimens. Both unloading compliance and dc-potential drop can be used to monitor crack growth. The test controller includes a dc-current supply programmer, a function generator for driving the servohydraulic test machine to required test outputs, five measurement channels (each consisting of low-pass filter, track/hold amplifier, and 16-bit analog-to-digital converter), and digital logic for various control and data multiplexing functions. The test controller connects to the computer via a 16-bit wide photo-isolated bidirectional bus. The computer, a Hewlett-Packard series 200/300, inputs specimen and test parameters from the operator, configures the test controller, stores test data from the test controller in memory, does preliminary analysis during the test, and records sensor calibrations, specimen and test parameters, and test data on flexible diskette for later recall and analysis with measured initial and final crack length information. During the test, the operator can change test parameters as necessary. 24 refs., 6 figs.

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A computer-controlled automated test system for fatigue and fracture testing

The objective of this investigation is an estimation of the crack-arrest toughness, particularly of irradiated materials, from voltage versus time output of an instrumented setup during a test on a Charpy V-notch (CVN) specimen. This voltage versus time trace (which can be converted to force versus displacement) displays events during fracture of the specimen. Various stages of the fracture process can be identified on the trace, including an arrest point indicating arrest of brittle fracture. The force at arrest, F{sub a}, versus test temperature, T, relationship is examined to explore possible relationships to other experimental measures of crack-arrest toughness such as the drop-weight nil-ductility temperature (NDT), or crack-arrest toughness, K{sub a}. For a wide range of weld and plate materials, the temperature at which F{sub a} = 2.45 kN correlates with NDT with a standard deviation, sigma, of about 11 K. Excluding the so-called low upper-shelf energy (USE) welds from the analysis resulted in F{sub a} = 4.12 kN and {sigma} = 6.6 K. The estimates of the correlation of the temperature for F{sub a} = 7.4 kN with the temperature at 100-MPa{radical}m level for a mean American Society of Mechanical Engineers (ASME) type K{sub Ia} curve through crack-arrest toughnessmore » values show that prediction of conservative values of K{sub a} are possible.« less

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DL Thomas

Papers

Use of Automated Ball Indentation Testing to Measure Flow Properties and Estimate Fracture Toughness in Metallic Materials

A computer-controlled automated test system for fatigue and fracture testing

Use of Forces from Instrumented Charpy V-Notch Testing to Determine Crack-Arrest Toughness