The meaning and general phenomenology of cyclic fatigue are briefly reviewed, with a particular emphasis on the practical and conceptual differences between man made and geological materials testing. After a synthetic review of previous studies, the experimental apparatus and methodology are described. The results of tests on a crystalline limestone are exposed and discussed, and the behaviour under cyclic fatigue is related to hysteresis loop feature changes under increasing loads; differences between oligocyclic and policyclic behaviour are described. In the concluding part, the problem of the representation of fatigue behaviour is dealt with. A representation in terms of survival rates at different loads is proposed.
Having withstood a certain load when applied once does not warrant the material will withstand the same load when applied many times. A particular phenomenon, termed "cyclic fatigue" occurs when the material is repeatedly stressed, even at stress levels noticeably lower than the ultimate strength. Cyclic fatigue behaviour has been long since recognized as an important factor in metals and other man made materials, and an enormous amount of experiences has been carried out on this subject, especially in the fields of vehicle, aeronautical and power generation engineering, whilst in the field of rock materials only a dozen or so of them has been studied (see table 1). The problem of defining the fatigue behaviour of rocks, however, is not devoided of practical interest.
With reference to cyclic fatigue due to compression stresses, the subject of this report, literature data collected for 12 rocks are collectively plotted in the "normalized" S/N graph of fig. 1 (S = maximum stress in the loading cycle, expressed as a percentage of the ultimate strength, N = number of cycles at failure).
It can be seen in fig. 1 that a wide dispersion of experimental points does not conceal completely a S/N correlation, possibly of the exponential type, having an assymptote (a practical assymptote, obviously, since the number of cycles cannot actually be pushed to infinity) at a stress level representing a fraction of the ultimate strength, that could be termed, at large, the "fatigue limit" of the rock material: a stress level that can be withstood no matter how large the number of cycles is. The wide dispersion does not arise only from having collectively plotted data from different rock types: even when tests are carried out on a single rock type a considerable dispersion of results, by far wider than the dispersion observed in tests on metals and other man made materials, occurs. We should not forget that, when dealing with a geologic material, we credit the term "material" of a different meaning from the meaning the same term has when referring to an artificial construction material; the latter, indeed, encompasses the features of its own mechanical behaviour in its technical definition, having been withstand well defined designed to stress level.
