For structures consisting of a thin film bonded to a compliant substrate, wrinkling of the thin film is commonly observed as a result of mechanical instability. Although this surface undulation may be an undesirable feature, the development of new functional devices has begun to take advantage of wrinkled surfaces. The wrinkled structure also serves to improve mechanical resilience of flexible devices by suppressing crack formation upon stretching and bending. If the substrate has a reduced thickness, buckling of the entire structure may also occur. It is important to develop numerical design tools for predicting both wrinkle and buckle formations. In this paper we report a comprehensive finite element-based study utilizing embedded imperfections to directly simulate instabilities. The technique overcomes current computational challenges. The temporal evolution of the wrinkling features including wavelength and amplitude, as well as the critical strains to trigger the surface undulation and overall structural buckling, can all be predicted in a straightforward manner. The effects of model dimensions, substrate thickness, boundary condition, and composite film layers are systematically analyzed. In addition to the separate wrinkling and buckling instabilities developed under their respective geometric conditions, we illustrate that concurrent wrinkling and buckling can actually occur and be directly simulated. The correlation between specimen geometry and instability modes, as well as how the deformation increment size can influence the simulation result, are also discussed.

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Many sensor technologies commonly used for structural health monitoring (SHM) still suffer from high energy demand, large form factors and weight, and indirect damage detection. These constraints limit the application of these sensor systems for SHM of various space, aeronautical, mechanical and civil structures. In this study, a self-sensing thin film fabricated from poly(3-hexylthiophene) (P3HT) and multi-walled carbon nanotubes (MWNTs) is proposed for strain sensing. Unique to this nanocomposite is its ability to generate a photocurrent in response to light illumination, and it will be shown that the magnitude of the generated photocurrent scales with the level of applied strain. First, two types of P3HT-based sample sets with and without MWNTs have been prepared by spin coating. Second, current–voltage measurements have been used for characterizing electrode response due to applied strains. Lastly, strain sensing characterization tests have been conducted, where thin film generated photocurrent has been measured during applied cyclic tensile strains. The nanocomposites’ strain sensitivity, linearity and sensing range have been analyzed and compared. The results show that these P3HT-based thin films can be used for photocurrent-based strain sensing. These films do not require an external power source, have small form factors and can directly measure structural strains.

Surface buckling (wrinkling) driven by mechanical instability is commonly observed in thin-film structures with a compliant substrate. The resulting undulation, while sometimes undesirable, has been increasingly exploited to enhance mechanical and/or functional performances of many thin film devices. In this study a practical finite element modeling approach is introduced to simulate wrinkle formation in thin films atop a compliant substrate. The proposed technique is robust and easy to implement, and it overcomes typical challenges in computationally modeling the buckling instability. Using a two-dimensional geometry under the plane strain or generalized plane strain conditions, with randomly distributed imperfections bearing different material properties at the film/substrate interface, we demonstrate the model's capability in triggering surface instability during direct compression and out-of-plane tensile loading. With sufficient mesh refinement, the predicted wrinkling wavelength, amplitude, and critical strain to activate wrinkle formation are shown to be close to analytical solutions. The effect of imperfection distribution is systematically studied, and a valid range of imperfection spacing is identified. The present numerical approach can be applied to predicting buckling instability in the design and analysis of thin film/compliant substrate systems over a wide range of material and geometric conditions. Directions for future studies are also discussed.

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