The acoustic emission (AE) of rock refers to the phenomenon of radiation of acoustic (elastic) waves due to the expansion of original fissures and defects and the initiation, evolution, expansion and fracturing of induced microcracks when a rock undergoes loading [1
]. AE during the loading process of rock is irreversible and reflects the deformation and failure process of rock indirectly. The AE parameters can describe the degree of damage and failure of the rock. At present, the AE method is widely used to evaluate the degree of rock damage and unveil the highest stress level in rock history. Many scholars have revealed the acoustic emission characteristics of rock during loading and established the relationship between stress, strain and acoustic emission characteristics. The influence of the loading rate on AE through the uniaxial and triaxial compression acoustic emission of the rock has been investigated [5
]. The rock damage and failure modes are assessed by acoustic emission parameters [10
]. Therefore, the relationship between the rock deformation and failure process and AE parameters is studied in the test to explore the evolution of AE under different stress paths and experimental environment. This favors in-depth understanding of the evolution characteristics of rock failure and is significant in revealing the rock instability and engineering application of AE.
AE of the rock under loading is irreversible, i.e., it will not occur noticeably until the loading stress magnitude exceeds that of the maximum stress in the loading history. This feature is termed the Kaiser effect, which represents the memory of the rock to the loading history (maximum stress or strain) [13
]. Many defects, such as cracks, pores and particles, exist in the rock. When the rock is submitted to loading, the closure of original fractures and the sliding of particles yield the friction-type AE with low frequency and energy. The microfracturing of rock stems from two failure modes, i.e., tension and shearing. Generation of the new microfracture produces rupture-type AE with high frequency and energy [20
]. The rupture-type AE can memorize the effect of the history maximum stress on the rock, and the friction-type AE is an important factor that disturbs the identification point of the Kaiser effect. Currently, the uniaxial loading mode is usually adopted to determine the stress point of the Kaiser effect. However, in the single loading test (uniaxial compression test or uniaxial tension test) process, the stress point of the Kaiser effect is covered by the considerable AE activities due to the friction and slippage between the surfaces of the microstructures of the rock [21
]. To accurately acquire the stress point of the Kaiser effect, Yoshikawa and Mogi [22
] used the uniaxial double cyclic loading test, where the cyclic peak stress is prescribed to be higher than the previous estimated maximum stress. When the dilatancy stress (cyclic peak stress) is 30–80% of the uniaxial compressive strength (UCS), the Kaiser effect appears. Later, many scholars conducted the cyclic loading and unloading and acoustic emission characteristic test of the rock successively and found that: (1) The Kaiser effect is relevant to the level of the applied stress. When the dilatation of the rock occurs due to the applied stress, the Kaiser effect disappears. Usually the dilatancy stress (crack initiation stress) does not exceed 70–80% of the uniaxial compressive strength (σc
]. When the first loading stress level is less than 30% of σc
in the compaction stage of stress-strain curve of the rock, the Kaiser effect can be easily misjudged due to the acoustic emission generated from closure of original fissures and sliding friction between particles in the rock. When the loading stress level is high, the value of the stress that the rock memorized previously is changed due to the high stress, for which the Kaiser effect in the subsequent reloading is not obvious [26
]. Therefore, to obtain clear Kaiser effect, the appropriate stress range is determined. (2) To inhibit or eliminate the influence of the friction-type acoustic emissions on the identification of the stress point of the Kaiser effect, a certain period of time is maintained after the load is applied to the level of the designed loading stress. The influence of short delay duration on Kaiser effect is small. As time passes (long delay time), the Kaiser effect of some rocks will disappear gradually [23
]. (3) The loading rate and lithology can affect the evolution of Felicity rate and occurrence of the Kaiser effect. However, at present, the factors influencing the acoustic emission characteristics and Kaiser effect are relatively simple. Comprehensive understanding of the influences of the stress level, loading rate and lithology on the acoustic emission characteristics is absent, and the existence of Kaiser effect of rock is further explored in this paper [28
AE originates from the evolution of internal cracks in rocks, being consistent with the damage process of the rock under loading. Therefore, the evolution of initiation, expansion and penetration of internal cracks in rocks can be determined through the variation of AE parameters. The Kaiser effect reflects the memory of rock to irreversible damage under load. The stress level, loading rate and lithology also influence the Kaiser effect of the rock. For the rock of the same type, different stress levels cause different degrees of damage to the rock specimens. Therefore, different AE phenomena occur during testing, and different AE characteristics appear. The difference of the AE characteristics generated in different kinds of rocks in the same stress level is large. At the same time, the loading rate also affects the rock damage degree and causes different AE characteristics. To reveal the influences of stress level, loading rate and lithology on the acoustic emission characteristics, we employ the MTS 816 rock mechanics testing system and the DS5-A acoustic emission testing and analysis system to monitor the mechanical properties and AE characteristics of red sandstone, marble and granite subjected to uniaxial cyclic loading and unloading. The evolution of Felicity ratio is analyzed, and the existence of Kaiser effect is explored. The study provides the foundation for the future application of the Kaiser effect and improvement of the accuracy of the acoustic emission data interpretation.