This study experimentally investigates the fatigue mechanical response of synthetic intermittent jointed rocks subject to random cyclic compression with different loading amplitudes and durations. Our results report the influence of the two loading parameters on the mechanical behavior of synthetic jointed rocks, involving the strength and deformation characteristics, the fatigue damage evolution and the progressive failure behavior. Both the fatigue strength and the deformation modulus of jointed samples decrease with increasing loading duration, while they exhibit opposite variation with increasing loading amplitude. The irreversible plastic strain exists in each hysteresis loop observed in stress-strain curves of jointed samples, which develops in a two-stage manner with increasing cycle number. Under higher loading amplitude or duration, the synthetic jointed rock is characterized by higher cumulative damage. Five crack coalescence patterns are observed in the present study based on the interaction between two adjacent joints, and the progressive failure process of tested jointed sample is captured via a high resolution industrial camera.

1 Introduction

Rock engineering structures are likely to be subjected to seismic loading during their service lives. Structural damage induced by earthquakes has been reported in numerous recent seismic sequences, such as Nepal (2015), Tohoku (Japan, 2011), Christchurch (New Zealand, 2010) and Wenchuan (China, 2008) (Wang & Zhang 2013). Since the earthquake loading is random and cycle in essence, it is thus crucial to characterize the mechanical behavior of rocks under random cyclic loading, especially for the intermittent jointed rocks that widely exist in various engineering structures.

Considerable study has concentrated on the mechanical properties of rock materials subjected to regular cyclic loading via laboratory experiments. The strength and deformation characteristics are the primary results in these cyclic loading tests. Early researches revealed the hysteresis of the cyclic stress-strain curves and the accumulation of the deformation of intact rocks (Burdine 1963; Attewell & Farmer 1973). Scholars further reported that both the fatigue strength and deformation modulus of intact rocks decrease with increasing cycle number (Fuenkajorn & Phueakphum 2010; Ma et al. 2013); they also concluded that the fatigue mechanical properties of intact rocks were significantly affected by the cyclic loading parameters. Moreover, the fatigue failure mechanism of intact rocks has been revealed via a series of regular cyclic loading tests (Cerfontaine & Collin 2017; Liu et al. 2018). In contrast to the intact rocks, the researches on intermittent jointed rocks mostly focused on their static mechanical properties (Singh et al. 2002; Prudencio et al. 2007). Since jointed rocks are highly sensitive to cyclic loading and their mechanical behavior quite differs from that of intact rocks, the fatigue response of jointed rocks to regular cyclic loading was also investigated (Brown & Hudson 1974; Prost 1988). Li et al. (2001) indicated that the preexisting intermittent joints significantly affect the fatigue strength and deformation behavior of rock materials under regular cyclic triangular loading. Liu et al. (2017, 2018) further investigated the influence of joint geometric configurations and cyclic loading parameters on the fatigue response of intermittent jointed rocks subject to regular cyclic sinusoidal loading.

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