Title: Fatigue of Concrete by Constant and Variable Amplitude loading
Author(s): Jan Ove Holmen
Publication: Symposium Paper
Appears on pages(s): 71-110
Keywords: amplitude; deformation; failure; fatigue (materials);
fatigue tests; loads (forces); plain concrete; static tests; -- strains; stresses.
In this investigation, the effect of various load histories on the fatigue behaviour of plain concrete was studied. (1). A total of 462 cylindrical specimens (100x250 mm) were tested statically and dynamically in compression. In addition, a total of 18 cylinders (150x300 mm) plus 132 cubes (100 mm) were tested statically. In addition to fatigue strength, special interest has been paid to the deformation characteristics of the concrete. A total of 140 specimens were tested under constant amplitude loading. Based on the fatigue strength results, empirical ex-pressions between the stress level (S), number of cycles (N) and probability of failure (P), S-N-P relationships, were derived. Using deformation characteristic results, an empirical expression for the total longitudinal strain (&maX) as a function of the cycle ratio (N/NF) and the loading time (t) was derived. This expression can be used to predict the fatigue life from deforma-tions early in the life. A total of 180 specimens were tested under various variable load histories of a given statistical distribution. The effect of both small and large amplitudes on fatigue strength and deforma-tion characteristics was examined. Based on fatigue strength results, the validity and limitations of the Palmgren-Miner hypothesis were examined. The PM hypothe-sis was found to give more or less unsafe predictions of the fatigue life depending on the load histories. Features of the loading histories affecting the accuracy of the hypothesis are discussed and an empirical relationship between loading histogram parameters and the Miner-sum at failure is presented. The empirical expression for the total longitudinal strain de-rived for constant amplitude tests is modified for tests under variable load histories.