Creep of medium and high strength aluminum alloys at normal temperature in moist atmospheres

Abstract

Direct data have been obtained for the creep of high- and medium-strength aluminum alloys in the stress range of O.6–1.2 of the nominal yield strength σ0.2 in laboratory air and in aqueous NaCl solution at room temperature. On this basis using known theories approximating functions have been determined for the creep curves. Stress σ0.2 serves as a natural boundary for the macroelastic and macroplastic regions in the first of which creep is only transient, and in the second there are transient, quasisteady-state, and accelerated stages. Extrapolated estimates of creep strain in the macroelastic region from data measured in the macroplastic region are not physically competent. However, a tendency towards an increase in ductility with an increase in time to failure at stresses greater than σo.2 makes it possible to estimate by extrapolation the time for onset of the accelerated creep stage with low test stresses from measured values at greater stresses in the macroplastic region. Fractographic and strain indices revealed the harmful effect of moist atmospheres on the deformation and failure resistance of alloys with prolonged loading.