Ambient multiaxial creep: shakedown analysis of structures using stereo digital image correlation

Abstract

A new framework to utilize stereo-vision digital image correlation (DIC) as a means of identifying the multiaxial cyclic shakedown behavior of structures is demonstrated on high strength steel bars. AISI 1144 carbon steel cylindrical bars are subjected to cyclic tension with nonzero mean stress and constant torque under ambient conditions. An elastic analytical solution is used in a post-processing procedure to extract inelastic strains and estimate the accumulated inelastic strain during cycling. DIC results are used to understand the cyclic strain evolution and determine if the accumulation of plastic strain stabilizes (shakedown). The results are used to construct a load interaction diagram (Bree Diagram) that displays the loading combinations resulting in purely elastic, safe shakedown, and undesirable cyclic inelastic behavior. The manifestation of ambient creep, during the multiaxial cyclic loading, is demonstrated through dedicated experiments. The shakedown criteria has been adapted to account for the ambient creep behavior. Depending on target structural lifetimes and allowable total strains, it is found that the design space can be enhanced by allowing shakedown to occur. An illustration is given for which the design space is enlarged 1.25 times the typical yield-limited approach.