Issue 48

Y. Sun et alii, Frattura ed Integrità Strutturale, 48 (2019) 648-665; DOI: 10.3221/IGF-ESIS.48.62 652 the evolution of the fracture morphology, it is able to precisely tune the morphological evolution of the localized wrinkles. Recently, Wu et al . [32,63] introduced the curved cracks to the elastic substrates and created some localized wrinkle patterns in a thin Al film deposited on a PDMS substrate by thermal evaporation. They studied the controllable mechanisms of the wrinkle patterns. They observed that the wrinkles unevenly distribute on the convex side of the crack near the irregular cracks, such as shown in Fig. 2A. The ordered wrinkle patterns can be fabricated by patterning the substrate with regular cracks, such as the radial wrinkles induced by concentric circle cracks, as shown in Fig. 2B. Based on theoretical analysis, they found that the compressive stress is always concentrate on the convex side of the curved crack, causing the wrinkles to be localized on the convex side of the crack. On the concave side of the curved crack, however, the compressive stress is released, which results in a suppression effect to the wrinkling. The curvature radius of the curved crack has a significant effect on the distribution of the wrinkles. The smaller curvature radius is more significantly to suppress the wrinkling on the concave side of the curved crack, which can be validated from Fig. 2B. It is clear from Fig. 2B (a) that the radial wrinkles develop from the convex side of c 1 and then extend to the concave side of c 2 and disappear. When curvature radius of the curved crack becomes larger, the curvature of the crack cannot afford to suppress the wrinkling at the concave side, thus the wrinkles shown in Fig. 2B (b) run through c 2 and c 3 . The experimental results provide a new insight into controlling the localized wrinkling in metal film/elastic substrate, and the experimental technique can be developed to effectively control the distribution and orientation of the winkling by introducing the regularly arranged patterns into the substrate before film deposition. Figure 2 : (A) Wrinkle patterns near irregular cracks. (B) Optical image of the radial wrinkles induced by concentric circle cracks c 1 , c 2 and c 3 . Images are from ref. [63]. Many studies show that the deposition of the metal film can slightly increase the temperature of the elastic substrate due to the heat radiation from sputtering source and the bombardment of the metal particles with high energy. During deposition, the substrate is expanded and the metal film is placed under a tensile stress. When the stress reaches a critical value, cracks start to form in the metal film. After deposition, the substrate contracts, and the film is placed under a compressive stress. When the stress is beyond a critical value, disordered wrinkles will appear in the areas away from the cracks. Near the cracks, the metal film undergoes a plastic deformation, and the compressive stress is concentrated in deformation zone, which results in the formation of locally ordered wrinkles. Therefore, by effectively controlling the deposition temperature to produce the fracture patterns, it is able to form the localized wrinkles near the cracks. Recently, Yu et al . [2] studied the formation and evolution of the wrinkled stripes localized by straight crack in a Fe film/PDMS substrate with thermal treatment. The wrinkled stripe is composed of many parallel straight wrinkles normal to the crack plane, and these wrinkles always form on both sides of the new crack, as shown in Fig. 3. They found that the morphologies of the wrinkles and cracks are obviously dependent on the film thickness and the elasticity of the substrate. As the film thickness increases, the crack spacing s decreases and eventually reaches a stable value, the crack number N increases, the old crack width w is almost unchanged while the new crack width increases. Meanwhile, the wrinkle length L decreases, while the wrinkle wavelength  and amplitude A increase significantly, as shown in Fig. 3B. With the increases of the curing time c T of the substrate, the elasticity of the substrate will decrease, which causes the decreases of the crack spacing, the crack width, the