13th International Conference on Fracture June 16–21, 2013, Beijing, China -1- Wrinkling and Delamination of Thin Films on Compliant Substrates Haixia Mei and Rui Huang* Department of Aerospace Engineering and Engineering Mechanics, University of Texas, Austin, TX 78712, USA * Corresponding author: ruihuang@mail.utexas.edu Abstract When the surface of a soft material is coated with a stiff thin film, compression induced buckling instability of the thin film results in surface wrinkles, and wrinkling of the thin film may lead to fracture and delamination. In this work, cohesive interface elements are employed in finite element analysis to simulate progressive wrinkling and wrinkle-induced delamination of an elastic thin film on a compliant substrate. The critical strain for initiation of wrinkle-induced delamination compares closely with an analytical formula based on a strength criterion. Subsequent growth of the delamination crack depends on the interface toughness. Co-evolution of wrinkling and buckle-delamination is simulated using a sufficiently large model. The interaction between the two buckling modes is elucidated. Keywords: Thin films, Wrinkling, Delamination 1. Introduction Stiff thin films on compliant substrates are used in a wide range of technological applications, including flexible electronics [1], thin film metrology [2], and micro/nano-fabrication [3]. Similar material structures are abundant in nature [4]. The mechanical interaction between a stiff film and a compliant substrate leads to a rich variety of phenomena that either limit or inspire practical applications of the hybrid system integrating hard and soft materials. In particular, recent interests in micro/nano-scale thin film materials have exploited mechanical instability (buckling and wrinkling) as an enabling mechanism for novel applications. Mechanics of surface wrinkling has been studied extensively over the last decade [5-17]. While most of these studies have assumed perfect bonding between the film and the substrate, it has been occasionally pointed out that wrinkling may cause interfacial delamination [5, 18, 19]. On the other hand, interfacial delamination is a necessary condition for buckling of thin films attached to relatively stiff substrates [20-22]. Simultaneous buckling and delamination has also been observed in compressed thin films on compliant substrates [23-25]. Some experiments have shown both surface wrinkling and buckle-delamination co-existing in the same film/substrate system [18]. Figure 1. (a) wrinkling with no delamination; (b) Wrinkling and delamination. In a previous work [18], we proposed a quantitative criterion for selection of the initial buckling mode by comparing the critical conditions for surface wrinkling and buckle delamination. The favored buckling mode at the onset of instability depends on the elastic mismatch between the film and the substrate as well as on the size of pre-existing interfacial delamination. More recently, by analytical and finite element methods, we presented a study on concomitant wrinkling and buckle-delamination for an elastic thin film on a highly compliant substrate [26]. In comparison with a nonlinear finite element analysis, an approximate formula was suggested to estimate the normal traction at the interface and to predict initiation of wrinkle-induced delamination. In this (b) (a)
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