Biaxial Compression Datigue and the Discontinuity of Concrete

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Title: Biaxial Compression Datigue and the Discontinuity of Concrete

Author(s): Eric C. M. Su and Thomas T. C. Hsu

Publication: Materials Journal

Volume: 85

Issue: 3

Appears on pages(s): 178-188

Keywords: biaxial loads; compression; compressive strength; concretes;cyclic loads; fatigue (materials); fatigue strength; fatigue tests; strength; stresses; General

Date: 5/1/1988

Abstract:
A biaxial compression fatigue machine was built to test plain concrete plates of 6 x 6 x 1.5 in. (15.2 x 15.2 x 3.8 cm). Brush-type loading platens were used to eliminate the end friction. The test program covered four principal biaxial-compression stress ratios and a range of load cycles from 1 to 2,000,000. Within this range of cycles the S-N (fatigue stress versus number of cycles) curve was found not to be a straight line as commonly assumed. This S-N curve can be closely represented by a modified Richard and Abbott's expression. The observed S-N curves of concrete can each be idealized by two straight lines that have significantly different slopes. The slope for low-cycle fatigue is several times greater than that for high-cycle fatigue. These two straight lines form a distinctive kink at the intersection. This point of intersection represents the discontinuity stress and can be used as its definition. Compared to the authors' tests, the S-N curves obtained in the literature are found to be not conservative because of the end friction exerted by the solid loading platens on the test specimens. The fatigue strength of concrete under biaxial compression is greater than that under uniaxial compression for any given number of load cycles. Observation of failure modes and the measurements of deformations also indicate that concrete possesses similar failure patterns under static and fatigue loadings.