ABSTRACT

Microcracks progressively occur, propagate, coalescence and finally lead to failure when it is subjected to stress. When subjected to loads over a long period, failure may occur below the material strength. It can be considered as a time-dependent behavior of materials. During this behavior, acoustic emissions (AE) are normally generated and using these signals we can evaluate the stability of materials. In this study, we conducted both on static and dynamic long-term strength tests. In the static test, we adopted a subcritical crack growth test for Mode I and Mode II. In case of the dynamic test, cyclic four point bending test and cyclic shear test were used. From the static test, we estimated the characteristics of delayed failure and long-term strength of granite and from the dynamic test, we estimated the fatigue life of concrete and got an S-N curve. We also evaluated the static and dynamic long-term stability using cumulative acoustic emissions curve.

1. INTRODUCTION

Acoustic emission (AE) is one of elastic waves which is generated when new cracks or cracks propagate in material (Ishida et al., 2017). It occurs when the applied stress exceeds a certain threshold value, it can be related to the long-term strength or fatigue limit. Particularly brittle materials have small displacement or strain before failure, but even in this case, AE tends to occur continuously.

The long-term stability is important in terms of long-term utilization of rock structures such as radioactive waste disposal, CO2 storage, and underground storage of compressed air, etc. The long-term stability can be classified into delayed failure due to static creep and fatigue failure due to dynamic cyclic loading. The related limited researches have been conducted by some researchers due to the time limitation and experimental difficulties (Wilkins, 1980, Backers, 2006, Kim and Kemeny, 2008, Rinnie, 2008, Ko, 2008, Nara et al., 2010, Park and Jeon, 2006).

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