Inspired by the experimental observation that the maximum peeling force of elastic films on rigid substrates does not always emerge at the steady-state peeling stage, but sometimes at the initial one, a theoretical model is established in this paper, in which not only the effect of the film's bending stiffness on the peeling force is considered, but also the whole peeling process, from the initiation of debonding to the steady-state stage, is characterized. Typical peeling force-displacement curves and deformed profiles of the film reappear for the whole peeling process. For the case of a film with relatively large bending stiffness, the maximum peeling force is found arising at the initial peeling stage and the larger the stiffness of the film, the larger the maximum peeling force is. With the peeling distance increasing, the peeling force is reduced from the maximum to a constant at the steady-state stage. For the case of a film with relatively small stiffness, the peeling force increases monotonically at the initial stage and then achieves a constant as the maximum at the steady-state stage. Furthermore, the peeling forces in the steady-state stage are compared with those of the classical Kendall model. All the theoretical predictions agree well with the existing experimental and numerical observations, from which the maximum peeling force can be predicted precisely no matter what the stiffness of the film is. The results in this paper should be very helpful in the design and assessment of the film-substrate interface.

An improved shear-lag model is developed in this paper to study the effects of interface roughness on the mechanical properties of unidirectional fiber-reinforced polymer composites with a staggered structure, in which the roughness is incorporated by establishing equilibrium equations for the fiber platelets with varying thickness along its axial direction. The stress transfer and effective Young’s modulus of composites are mainly investigated due to the influence of fiber’s surface roughness. Since the polymer matrix can be chosen as thermoplastic or thermosetting materials, a uniformly interfacial shear stress distribution due to the frictional transfer along fiber/matrix interfaces and a non-uniformly one due to the elastic transfer are analyzed, respectively. It is found that when the surface roughness becomes larger, fibers in the former will carry more tensile loads, while the tensile loads keep almost invariant in fibers and the shear stress reduces in matrix in the latter. Moreover, the effective Young’s modulus of composites will be enhanced due to increasing fiber’s surface roughness. However, the enhancing effect will gradually reduce with an increasing aspect ratio of fibers. The results should be very useful for the design of novel fiber-reinforced polymer composites, especially for those that needed interfacial modifications in order to improve the interfacial adhesion, for example, carbon-fiber reinforced polymer composites.