2d Frame Analysis V2 Crack -
An examination of cracked crystals reveals an important trend that cracks consistently branch out of Y-shaped (triple) junctions or from the edge of monolayer crystals (e.g., see Fig. 3d). This observation suggests the existence of specific crack nucleation sites and preferential crystallographic directions over which cracks spread within the plane of monolayer crystals. Given that the origin of the strain is composition conversion, we think that unsaturated bonds (at the edges) or defect sites (within the interior domains of the film) serve as local sites at which the incorporation of S atoms into the lattice of MoSe2 is initiated. A similar contribution of defect sites in the formation of cracks and Y-shaped branching behavior has been recorded in TMD26 films subjected to biaxial strain.
2d Frame Analysis V2 Crack -
To quantitatively study the conversion-induced strain in converted MoS2 crystals, we carry out PL spectroscopy, in which probing the optical bandgap provides direct information about the type (tensile/compressive) and the strength of the strain.28,29,30,31 For this purpose, we use a partially cracked MoS2 crystal that enables a direct comparison between the emission spectra of cracked and continuous regions (Fig. 4a, b) within a single film. Mapping the emission energy at the maximum PL intensity (Fig. 4c) indicates that compared to cracked regions, the continuous (i.e., crack free) regions emit light at lower energies. We also observe a significant drop in the intensity of the light emission in continuous regions (Fig. 4d). These two distinctions can be concurrently observed in the representative PL spectra of cracked and continuous films as displayed in Fig. 4e. Since the PL spectra of cracked regions imitate the standard emission spectrum of a pristine CVD-grown MoS2 film, we denote the continuous regions as the strained parts. Thus, the reduction of the optical bandgap and the drop in the emission intensity are taken as indications of a biaxial tensile strain that acts on continuous regions.31,32
Composition modulation synthesis of ternary alloys of atomically thin transition metal dichalcogenides gives rise to intrinsic biaxial strain. A team led by Ali Adibi at Georgia Institute of Technology reported the onset of a substantial biaxial strain in monolayer MoS2xSe2(1-x) that is intrinsically linked to the two-step composition modulation synthesis used to grow the ternary alloy. As the S atoms replace the Se atoms of the starting MoSe2 host crystal, the resulting alloy forms a stretched lattice and develops a large biaxial tensile strain. Morphological and spectroscopic characterisations suggest that such strain results in the onset of fracture in the crystal, and further relaxes via formation of cracks within the crystal domains. Theoretical modelling indicates that pre-existing cracks give a substantial contribution in weakening the strength of the synthesized van der Waals alloy.
Fracture is a process relevant to many kinds of applications of materials. Competition between two modes, namely, the crystal plane cleavage and dislocation nucleation at the crack tip, is the main factor governing the ductile or brittle behaviour of crystals1,2. Since the establishment of the well-known Griffith model3, it has been questioned whether linear elastic fracture mechanics hold true in the crack tip down to all length scales and, specifically, whether it holds true for recently developed two-dimensional (2D) materials4,5 is debatable.
It is expected that the defect-free nature and high strength in 2D4,5 may provide a higher chance to achieve the onset of plasticity, but on the other hand, most molecular dynamics simulations show that the fracture of graphene as well as MoS2 is brittle15,16. To understand the atomistic features when fracture occurs in 2D materials, a direct observation of the cracking process at the crack tip is necessary from the viewpoint of basic physics and practical use. Previous work by in situ TEM demonstrated the capability of dynamical atomic-scale observations on the sublimation and cracking process of few layer graphene17 or monolayer graphene17. Moreover, environment-assisted cracking in MoS2 also needs to be investigated and compared with pure cracking that is of particular importance in real-world applications. The environmental susceptibility of MoS2 has drawn much attention18 that would have significant influence on their physical properties19.
TMD materials have been known to be susceptible to environment18,27. In the following we will discuss about the environment-assisted cracking of monolayer MoS2, the so-called stress corrosion crack (SCC). For SCC, the KIC usually becomes smaller because of a chemical reaction assisted by release of the decohesion energy. Meanwhile, the yield stress also decreases. The as-grown monolayer MoS2 sample on a sapphire substrate was exposed to ultraviolet (UV) light with controlled humidity inside the chamber (see Methods)27. Highly oxidative chemical groups, such as O3 and oxygen-related radicals, can be created under UV that can quickly react with MoS2. This method was first applied to visualize the grain boundaries of 2D materials28.
In concluding, we note that for a long time researches have been interested in determining the exact atomic structure at the plastic crack tip zone6 that is a singular field according to classical fracture theory3. Observation at the crack tip is the most direct approach but has been limited by both materials and instruments. Apart from providing an atomistic picture of the crack tip, here we have observed the emission and dynamics of dislocations in MoS2 sample with 1% sulfur vacancy concentration. In addition, the 2D mode of cracking in MoS2 has, to our knowledge, not been reported before and has broader conceptual significance. We observed the edge dislocation emission that is different from the more frequently observed screw dislocation emissions in 3D cracks33. In 2D cracking, there is easier fluctuation in 3D space, and hence the extreme singular strain at the crack tip can be relaxed in part by 3D fluctuations. There is a higher atomic diffusion rate at the surface, and hence there could be easier dislocation climb in 2D. Such processes may account for the higher ductility/plasticity in 2D materials when compared with their 3D bulk counterparts. The plastic zone of MoS2, which is a few nanometers in size, may be enlarged or shrunk by temperature control or defect/dopant engineering for various purposes.
How to cite this article: Ly, T. H. et al. Dynamical observations on the crack tip zone and stress corrosion of two-dimensional MoS2. Nat. Commun. 8, 14116 doi: 10.1038/ncomms14116 (2017).
T.H.L. contributed to this work in the experimental planning, experimental measurements, data analysis and manuscript preparation. J.Z. conceived the idea and contributed to this work in the experimental planning, experimental measurements, data analysis and manuscript preparation. M.O.C. did the TEM measurements. L.-J.L. provided the MoS2 grown on sapphire. Y.H.L. contributed to the experimental planning, data analysis and manuscript preparation.
DIC methods have also been used for assessment of concrete crack development. Helm in  shown how to use DIC for assessment of specimens with multiple growing cracks. Similarly, Rui et al. in  presented DIC-based measurement system of crack generation and evolution during static testing of concrete sleepers. In  Gehri et al. shown a study on an automated crack detection and measurement based on DIC. Finally, DIC techniques allow for measurements and calculations of strains localization and the width of the fracture process zones on the surface of notched concrete beams [16,17].
Crack assessment using DIC methods is very precise but also require huge computational resources and is very time-consuming. As a result, it is mainly used off-line for assessment after the tests. It is also possible to apply other non-destructive monitoring techniques such as Acoustic Emission (AE)  or microwave sensors . On the other hand, in recent years, convolutional neural networks (CNN) have been developed and applied for online automatic detection of concrete cracks and structural damage. See for example, recent state-of-the-art reviews [5,6]. In  Cha et al. described an autonomous system for structural visual inspection using region-based deep learning for detecting multiple damage types. In  a system for real-time crack assessment with wall-climbing unmanned aerial system is presented. Roberts et al. in  shown a system for low-cost pavement condition health monitoring and analysis. In  Deng et al. presented a region-based CNN with deformable modules for visually classifying concrete crack. Finally, an application of CNN for detection of flaws in concrete using ultrasonic tomography is described in .
After training, the model was tested by detecting cracks in new images. The model produces a bounding boxes on the images along with a percentage of how accurate this bounding box is based on the trained model. This value provides users with an assessment of how good the detection of cracks is. Figure 8 shows tested images containing cracks. It can be noted that while the crack on the left image was properly detected and localized as one crack with certainty 99% (inside the green bounding box) and also not properly divided into two cracks: inside the blue bounding box (with certainty 96%) and inside the red bounding box (with certainty 81%).
Example of two tested images: the crack on the left image properly detected and localized as one crack (with certainty 100%), the crack on the right image simultaneously properly detected as one crack (with certainty 99%) and not properly detected as two cracks (with certainty 96% and 81%, respectively).
The CivEng Vision system, developed at Cracow University of Technology (CUT), was used for acquisition, storing and processing the images [38,39]. The images were then processed using DIC method for computing deformation fields and crack width visible on the surface of the beam. Finally, the images were processed by trained R-CNN model to automatically detect and localize the cracks on the surface. Figure 10 shows four images of the side surface of the beam, taken using the CivEng Vision system, during the three-point bending test.