Permanent Strains from Cyclic Variable‐Amplitude Loadings

Abstract

Cyclic triaxial tests were performed on a well‐graded railroad ballast with a maximum particle size of 38 mm (1.5 in.). Samples were compacted in a split mold to reproduceable density states and subjected to repeated applications of compressive axial stresses. The cumulative permanent axial strains that developed, for constant‐amplitude loadings, were found to be dependent upon the permanent strain after one cycle of loading, the loading magnitude, and the number of applied loading cycles. Staged tests were conducted in which a sequence of variable loads was applied to the same sample. For these variableamplitude tests, the final permanent strains were independent of the order of the applied loadings. A superposition method is presented to characterize the mixed loading conditions. The results for the variable‐amplitude tests indicated that the maximum load controlled the permanent strain development, with negligible additional strains developing after returning to a lower repeated stress level. Partial unloading cycles were also investigated. These partial unloading cycles caused very little additional cumulative inelastic strains.