applications

Welds and Heat-Affected Zones

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. 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.

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.