Automated Ball Indentation®

ABI® Overview

The ABI® test is based on progressive indentation with intermediate partial unloadings until the required maximum depth (maximum strain) is reached, and then the indenter is fully unloaded. The indentation load-depth data are collected continuously during the test by a 16-bit data acquisition system. The nonlinear spherical geometry of the tungsten carbide indenter allows increasing strain as the indentation penetration depth is increased. Hence, the incremental values of load and plastic depth are converted to incremental values of true-stress and true-plastic-strain values according to established elasticity and plasticity theories.

The force required to indent the material to increased depth values is measured with a force transducer such as a load cell. The current stress at any time is a function of the current indentation force. Periodic partial unloadings during the test are used to determine the elastic strain. The elastic strain is subtracted from the total strain to give the plastic strain. The incremental values of the ABI® -measured true-stress and true-plastic-strain are calculated from the indentation force-depth data (based on elasticity and plasticity theories) and plotted to form a true-stress versus true-plastic-strain curve of the material. The ABI® -derived yield strength is determined from the force-depth data. The strain-hardening exponent, strength coefficient, Lüders strain, uniform ductility, and ultimate strength, are also determined from the ABI® test. Additionally, the ABI® test can be performed without intermediate partial unloadings (i.e., in a single cycle of continuous loading up to the desired maximum indentation depth/strain followed by complete unloading). This approach is preferred for high temperature or high strain rate testing to avoid indentation creep and nonlinear unloading slopes, respectively. The single cycle ABI® test produces a curve of true-stress versus true-strain (i.e., total true strain since the elastic strain component cannot be subtracted due to the elimination of partial unloadings).

The entire test is fully automated (computer-controlled) where the spherical indenter is driven into the test surface at a desired speed which controls the strain rate of the ABI® test, and the indentation force versus penetration depth are continuously collected (using a 16-bit resolution data acquisition system or better) during the entire test.

For laboratory specimens, the test samples can be cooled or heated to the desired ABI® test temperature using an environmental chamber to bring both test sample and indenter to the desired test temperature while the force and displacement transducers are kept outside the chamber. When the depth sensor is positioned outside the environmental chamber the compliance of the testing machine shall be considered. A temperature-resistant LVDT or a clip gage can be used inside the environmental chamber. Testing at higher temperatures can be performed provided that the test surface is not severely oxidized (e.g., by utilizing an inert gas or a vacuum chamber). The test sample and the indenter shall maintain test temperature within ±2.0°C (±4°F) before conducting and during the entire ABI® test.

The stress-strain curve measured with the ABI® test has been demonstrated to correlate with the stress-strain curve measured in a tension test. The localized ABI® test is nondestructive and can be used in-situ to measure the stress-strain properties of a material sample or of a component part in service. Therefore, it can be used to measure stress-strain properties where insufficient material is available to use in a destructive tension test. The ABI® test leaves a shallow spherical depression on the test surface with no sharp edges (hence, no crack initiation sites). Furthermore, it leaves a favorable compressive residual stress at the test site (similar to shot peening but on a slightly larger scale). The ABI® test is also useful in testing small volumes of welds and irregularly shaped heat-affected-zones (HAZs).

The ABI® test is particularly useful where a life extension evaluation is planned for a component and adequate materials property data are not available. Also, it can be used to measure properties for materials that may have service damage that has caused a change in tensile properties during service life (e.g. neutron embrittlement of nuclear pressure vessels). Another important application is the determination of yield strength of ferritic steel components, such as oil and gas pipelines, when no documentation exists for the original and/or repair material and when a deterministic fitness-for-service evaluation is required for safe operation at current or higher (up-rated) pressures.

The ABI® test is a macroscopic/bulk technique that measures the properties on a small volume of material. This capability is valuable in mapping out property gradients in welds and HAZs. The minimum diameter of the indenter must be large enough such that the spherical indentation, produced at the smallest practical depth/strain, covers at least three grains of the metallic sample. This requirement is the same for the minimum thickness of a tensile specimen in order to measure macroscopic/bulk properties. The ABI® technique can be used to measure the stress-strain properties of a material that may have a sharp gradient of mechanical properties. This, for example, exists in a weldment where the base metal and the weld metal have different strength and ductility and the HAZ may have a very sharp gradient of properties. Here the ABI® test can measure the flow properties (true-stress versus true-plastic-strain curve) of a small volume of material and can measure the strength profile along a line traversing from one base metal through the HAZ, the weld metal and continuing through the other base metal.

Although the ABI® test is nondestructive, the strain-hardening exponent (n) determined from the test is a function of the uniform plastic strain of many metallic materials with a power-law true-stress versus true-plastic-strain curve (e.g. nuclear pressure vessels and carbon steel materials).

Although there is no necking (similar to that occurring at maximum force in a tension test), the uniform ductility and ultimate tensile strength are determined from the plot of true-stress versus engineering strain.

The value of Lüders strain (an important property for evaluating steel sheet metals in automotive industry) is calculated from the ABI® -measured yield strength, strain-hardening exponent, and strength coefficient.