Dynamic Cyclic Tests Research Articles

A review of certain aspects of the Bauschinger effect in metals

Certain aspects of the Bauschinger effect (B.E.) in metals (in particular two-phase systems) are reviewed. The B.E. is usually determined from simple forward and reverse loading tests, and is manifested by the reverse flow curve exhibiting a reduced elastic limit, a well-rounded appearance of the initial plastic portion and permanent softening vis à vis the forward hardening curve. Even under such simple loading systems a reliable a priori estimate of the reverse flow behaviour is still lacking. A number of macroscopic and microscopic models which have been proposed for reproducing the forward and reverse flow behaviour are discussed. The macroscopic models are phenomenological in nature and base on the continuum theory of plasticity. The microscopic approach aims to identify the hardening mechanisms and the magnitude of the internal stresses created within the deforming material; the B.E. is a natural consequence of the unrelaxed internal stresses that are developed. Some of the strengths and weaknesses of the models are discussed in the light of experimental data available from quasi-static forward and reverse loading tests on a variety of materials. Recourse is also made to data from some dynamic cyclic loading tests. In two-phase systems the volume fraction of the second phases appears to be one parameter that can be correlated (at least qualitatively) with certain reverse flow features. A definitive account of the role of second-phase constituents is still lacking. In conclusion the implications of the B.E. in plate fabrication processes which involve reverse strain cycles, such as some pipemaking processes, are discussed.

Influence of Degree of Shear Stress Reversal on the Liquefaction Potential of Saturated Sand

A series of dynamic cyclic shear tests under undrained conditions were conducted to determine the conditions to induce liquefaction of saturated sands, using a ring torsion apparatus to apply cyclic shear stresses under nearly plane strain conditions. In addition to completely reversed cyclic tests, partially reversed and unreversed cyclic tests were performed by superposing static initial shear stress on dynamic shear stress pulses. Most specimens were of a relative density of approximately 40 per cent, and the frequency of the cyclic shear stress ranged from 1Hz to 12 Hz. Sudden failure of the specimens which was clearly recognized in the completely reversed and partially reversed tests was not observed in the unreversed tests. Irrespective of the degree of stress reversal, liquefaction failure became imminent when the ratio of the peak shear stress to the vertical effective stress reached a certain critical value. The degree of stress reversal had considerable effect on the conditions to cause initial liquefaction, but had no significant effect on the conditions to cause complete liquefaction. For completely reversed tests, the conditions to induce liquefaction was nearly independent of the frequency of the cyclic shear stress from 1Hz to 12Hz.