###### 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. c 20202016 December 201917 January 2020202016 December 201917 January 202020202016 December 201917 January 20202020202020202  
Cyclic loading often results in a rock failure at a lower stress level than its monotonically determined strength. Crack growth at a stress level much lower than fracture stress is called subcritical crack growth. Subcritical crack growth occurs under cyclic loads, monotonic loads, or their combination. Subcritical crack growth under cyclic loading is usually characterized by the Paris law [4]. For a cyclic load, the rate of crack growth is a function of the range of the stress intensity factor:  
\[\frac{da}{dN}=c(\Delta K)^{m} \tag{1}\]  
where \(da/dN\) is the crack growth rate, \(\Delta K\) is the range of stress intensity factor during the fatigue cycle, and \(C\) and \(m\) are constants that depend on the material, environment and stress ratio. [PERSON] and [PERSON] [5] reported the validity of Paris' law for Westerly granite. They found that \(C\) was dependent on the stress ratio. Under monotonic loads, subcritical crack growth due to stress corrosion by an environmental agent can be expressed by [PERSON] law [6]:  
\[v=A\Big{(}\frac{K}{K_{c}}\Big{)}^{n} \tag{2}\]  
where \(v\) is the crack velocity, \(K\) is the stress intensity factor, \(K_{c}\) is fracture toughness, and \(A\) and \(n\) are subcritical crack growth parameters. Stress corrosion is considered to be the primary mechanism underlying subcritical crack growth in rocks [7]. The present pore water in the crack tip environment can induce crack propagation by facilitating weakening reactions. For silicon dioxide, most commonly found in rocks, the strong silicon-oxygen bonds are replaced with much weaker hydrogen bonds [7]. This phenomenon is called stress corrosion. There have been many studies on crack growth in rocks or brittle rock-like materials under monotonic loading [8; 9; 10; 11; 12; 13; 14; 15]. In monotonic compression tests using flaw-induced specimens, tensile or wing cracks appear first and propagate parallel to the applied compressive loading direction. Secondary or shear cracks appear later and are responsible for specimen failure as a rule. Crack coalescence occurs through secondary cracks or a combination of wing cracks and secondary cracks [8; 9; 10; 11; 12]. However, crack growth under cyclic loading has not been sufficiently investigated. Most experiments have focused on the fatigue properties of intact rock and the influence of cyclic frequency and strain rates [16; 17; 18; 19; 20; 21; 22; 23; 24; 25; 26]. Thus, research related to crack propagation and coalescence under cyclic loading is very rare.