Characteristics of Crack Growth in Rock-Like Materials under Monotonic and Cyclic Loading Conditions

Tae Young Ko,Sean Seungwon Lee

doi:10.3390/app10020719

Abstract

Experiments with gypsum as a model rock material were conducted to investigate the characteristics of crack growth under monotonic and cyclic loading. The specimens had two pre-existing flaws that were placed at different inclination angle, spacing and continuity. Tensile or wing cracks and secondary or shear cracks were observed in both the monotonic and cyclic tests. Wing cracks or tensile cracks initiated at (or near) the tips of flaws and grew parallel to the loading direction. Secondary or shear cracks occurred after initiation of the wing crack and culminated in a final failure. Secondary cracks started at the tips of flaws and propagated in the colinear direction of flaws or perpendicular to loading. Six types of coalescence were observed. Both the monotonic and cyclic tests showed almost identical coalescence types. Coalescence occurred due to the internal shear cracks in specimens containing colinear flaws, while it occurred through combinations of internal shear cracks, internal wing cracks and tension cracks in specimens with non-colinear flaws. Fatigue cracks occurred in tests under cyclic loads. Finally, the subcritical crack growth parameters under monotonic and cyclic loading were determined. Although there were variations in the parameters, the parameter “n” showed similar values.

Highlights

Rocks and underground structures often subjected to cyclic loading due to natural earthquakes, vibrations by car or train, and drill-and-blast by humans
Rocks may exhibit a pattern of failure which differs from a pattern of monotonic loading failure
Under uniaxial cyclic loading, a fracture occurs along an inclined plane which develops over the entire height of the specimen

Summary

Introduction

Rocks and underground structures often subjected to cyclic loading due to natural earthquakes, vibrations by car or train, and drill-and-blast by humans. Ground structures such as man-made masonry arch bridges need to be assessed for stability due to earthquakes or cyclic loads [1]. Royer-Carfagni and Salvatore [2] showed the difference of failure patterns in marble under monotonic and cyclic loading. Monotonic and cyclic loading may cause a different pattern of fracture propagation and coalescence in rocks. Confining stress affects the stress–strain relation, and it modifies the material behavior under monotonic and cyclic loading [3]

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