Issue 48

Y. Sun et alii, Frattura ed Integrità Strutturale, 48 (2019) 648-665; DOI: 10.3221/IGF-ESIS.48.62 651 film thickness increase, the fracture energy will also increase, and it needs a larger tensile stress to break the film. Furthermore, the fracture morphology depends notably on the film thickness, the substrate elastic and the loading conditions (such as temperature, mechanical stretching and loading velocity). M ORPHOLOGIES AND EVOLUTION OF LOCALIZED WRINKLES IN METAL FILM / ELASTIC SUBSTRATE n 1998, Bowden et al . [14] first reported the microscopic wrinkling in a metal film/elastic substrate, and they observed that the metal film deposited on the flat substrate spontaneously form disordered wrinkles. By prefabricating some regularly arranged patterns on the substrate, they found that the film would form highly ordered wrinkles. Theoretical analysis showed that the patterns on the substrate changed the local compressive stress state and thus altered the local wrinkling morphology. Since then, various global and localized wrinkling patterns have been fabricated in different films/substrates by using some effective stress tuning technologies. The global wrinkles with well-defined wavelengths such as herringbone, sinusoidal and labyrinth patterns can be readily fabricated in various film/substrate materials including the metal films/elastic substrates [2,14,24,28-30], the nonmetal stiff films/elastic substrates [34,95,98-100] and the polymer films/elastic substrates [33,41-43,101]. The localized wrinkles such as the radial patterns can be easily formed in the floating polymer films [16,20,22,46,50,51,102] and the surface of modified elastomer [36]. Furthermore, some researchers have used numerical simulations to report the spiral and radial wrinkling patterns in stiff films/elastic substrates [19,52]. However, the localized wrinkling patterns in metal films deposited on the elastic substrates are rarely observed in experiments. It should be emphasized here that these localized wrinkling patterns do not include the buckle-driven delamination and the folds, creases and ridges induced by the post-buckling evolution of wrinkles. Recent studies show that the wrinkles in the metal films deposited on the elastic substrates can be localized near some surface defect sites. The distribution of the wrinkles is closely related to the formation mechanism of the defects, and the wrinkle morphology is dependent on the defect morphology, the direction of the compressive stress, the film thickness and the elasticity of the substrate. Here, we introduce some localized wrinkle patterns in the metal films deposited on the elastic substrates. Vandeparre et al . [17] reported some localized wrinkle patterns in Ti film deposited on elastic PS substrate with diffusion of toluene solvent. They found that the wrinkle morphology is closely related to the geometry of the diffusion front. When the solvent diffuses from the polymer edge, a linear diffusion front is yielded, which results in the formation of parallel wrinkles, as shown in Fig. 1A(a). If the solvent diffusion starts from a point-like hole defect at a random site in the film, the wrinkles will develop at the defect site and then expand along radial direction, as shown in Fig. 1A(b). The growth of the wrinkles can be stopped by neighboring patterns, and then these wrinkle patterns will join together, which results in the formation of homogeneous global wrinkle patterns, as shown in Fig. 1B. By changing the rheological behavior of the polymer substrate, they also generated complex hierarchical wrinkled patterns. The experimental result and method present new directions in achieving desired localized wrinkle patterns in metal film deposited on the elastic substrates. Since then, the localized wrinkling of the metal film has begun to attract much attention of some researchers, and some localized wrinkle patterns have been generated in different metal film/substrate structures by introducing regular defects in the metal film or substrate. Figure 1 : (A) Gradually growing parallel wrinkles (a) and radial wrinkles (b). (B) Successive optical images (a-c) of two radial wrinkles fusing and final image (d) after fusion of several radial wrinkles. The scale bar corresponds to 20 μm. Images are from ref. [17]. Rand et al . [1] first reported that the surface wrinkles with long-range alignment can be achieved by introducing a simple fracture process in a stiff film. Their experiment results show that the wrinkles are perpendicular to the fracture line and are constrained between the neighboring fracture lines, which implies that the compressive stress near the fracture edge was uniaxial and its direction was parallel to the fracture line. The experimental method provides a possibility to form localized wrinkles by introducing regularly arranged fracture patterns in the metal film or the substrate. Then by effectively controlling I