Revisiting the Dexterity of Birnbaum's Explanation of Hydrogen-Induced Hardening or Softening in Macroscopic Stress–Strain Tensile Tests

Abstract Experimental measurements of the stress–strain curve from uniaxial tension tests in hydrogen yielded both macroscopic hardening and softening results despite the fact that hydrogen increases the mobility of dislocations according to the hydrogen-enhanced localized plasticity (HELP) mechanism for embrittlement. In his seminal paper, Howard Birnbaum (Scripta Metallurgica et Materialia, 31(2), pp. 149–153, 1994) using dislocation plasticity explained the contradictory outcomes of the uniaxial tension test on the basis of the two fundamental principles of HELP, hydrogen-enhanced dislocation mobility, and hydrogen-induced plastic flow localization; principles whose effects have been experimentally observed in almost all metals and alloys. By way of example, Birnbaum showed that the flow stress increases in hydrogen if the increased dislocation mobility cannot balance the shear localization hardening effect. In this work, we revisit the dexterity of Birnbaum's theory from a continuum plasticity standpoint and develop a framework to calculate macroscopic flow stress versus plastic strain curves in the presence of plastic flow localization in hydrogen. We incorporate the hydrogen effect on the constitutive response of the material through its effect on the activation enthalpy and volume characterizing the motion of dislocations—parameters that can be measured experimentally. Then, on the basis of Birnbaum's theory, we demonstrate that the competing and combined hydrogen effects on enhanced localization and enhanced dislocation mobility can result in either macroscopic hardening or softening response depending on their individual intensity and magnitude.