In-situ Scanning Electron Microscopy (SEM) Observation of Shale Fracture and Crack Propagation

To better understand the formation and evolution of hierarchical fracture networks in shale, Zhendong Cui et al. [1] employed an in-situ scanning electron microscopy (SEM) observation system using a JEOL JSM-5410LV SEM (JEOL Ltd., Akishima, Japan) to investigate the development and progression of damage and crack growth in shale under tensile loading. For SEM observation, one surface of each sample was polished using 1200-grit metallographic sandpaper to facilitate clearer identification of micro- and nanoscale damage and cracks.

Prior to metal sputter coating of the polished surface, the arched specimens were mounted onto a pair of tensile grips using superglue (Figure 1a). The specimen–grip assembly was then placed into the tensile stage of the in-situ SEM observation system within the JSM-5410LV (Figure 1b). After applying a preliminary preload (if necessary), the specimen chamber was sealed and evacuated. Once the notch tip was located and magnified in the SEM, a tensile load was applied at a displacement rate of 1×10⁻⁴ mm/s. The focus was maintained at the notch tip of each specimen to observe the initiation of micro- and nanoscale damage and cracks. Sequential images were captured to document the intermediate stages of damage and crack development at micro- and nanoscale resolution.

Figure 1:Technical details of the specimen setup: (a) the arched specimen adhered to a pair of tensile grips, and (b) the arched specimen mounted in the in-situ tensile stage and SEM observation system (JSM-5410LV). Inset: the arched specimen after tensile fracture.

The arched specimen, with an artificial notch cut along its bending edge, enables effective observation of damage and crack propagation during the brittle fracture of shale. This arched design induces a compressive effect, which reduces tensile stress concentration at the crack tip and partially prevents unstable fracture in the brittle shale. Induced and natural pores and cracks were observed at various scales along the main crack path and fracture surfaces, as shown in Figure 2.

Figure 2:Bidirectional crack coalescence at the microscale: (a) coalescence of adjacent crack wings forming a bidirectional connection, followed by the development of a series of en echelon micro-damage cracks; (b) merger of bidirectionally growing cracks along both sides of an isolated micro rock bridge.

Observations revealed that crack initiation zones developed around the crack tip under tensile stress concentration, where micro- and nanoscale cracks nucleated. Crack growth typically occurred through the sequential formation and coalescence of damage zones, characterized by intermittent echelon microcracks ahead of the crack tip. Mineral anisotropy and localized stress accumulation around the crack tip led to crack kinking, deflection, and branching. Due to elastic recovery and the presence of induced compressive stress, crack propagation was often accompanied by the arrest or closure of previously branched cracks. The branching and interaction of cracks formed a three-dimensional hierarchical network, comprising induced branching cracks with similar paths and natural structures such as nanopores, bedding planes, and microcracks.

[1] Zhendong Cui, Weige Han, In Situ Scanning Electron Microscope (SEM) Observations of Damage and Crack Growth of Shale, Microscopy and Microanalysis, Volume 24, Issue 2, 1 April 2018, Pages 107–115, https://doi.org/10.1017/S1431927618000211.