Date of Award


Document Type


Degree Name

Doctor of Philosophy (PhD)


A closed-loop system for constant true strain rate test was modified using a Zenith Z-158 microcomputer connected to an optical sensor and MTS tensile testing machine. This new system has several desirable features: (1) the system is easy to operate; (2) the system works with any round shape of ASTM standard specimen; (3) the system works with a constant true strain rate or any form of strain rate; and (4) the system can be controlled to pause or stop at any pre-selected time or amount of true axial strain. Three types of materials, electrical grade iron, austenitic stainless steel, and ferritic stainless steel, were selected for a constant true strain rate test at room temperature. The results from this study lead to the conclusions that: (1) the true stress-true plastic strain curves of all materials employed in this study obey two forms of the generalized power law; (2) the true plastic strains at maximum load appears to depend on strain rates; (3) the true stresses at maximum load of electrical grade iron and ferritic stainless steel appear to depend on the initial grain size; and (4) the true plastic strain at maximum load of austenitic stainless steel appears to depend on initial grain size. Grain strain studies conducted after constant true strain rate tests show that the distribution of grain strain is not uniform over the entire necking region and is independent of the strain rate and grain size. The results of including the grain strain effect into the constitutive equations show that the true stress at maximum load of electrical grade iron and ferritic stainless steel can be expressed in terms of the linear intercept in the axial direction of the corresponding strain.