The interaction between matrix cracks and interface delamination is a major failure mechanism in composite laminates and has been a subject of active research in recent years. Although coupled failure behaviour of composite laminates between matrix cracks and delamination has been observed in experiments, accurate modelling of this phenomenon for application in composite structures remains challenging. In this work, a three-dimensional separable cohesive element (SCE) is proposed to enable the modelling of interaction between matrix cracking and interfacial delamination in laminated fibre-reinforced composite materials. It is demonstrated that traditional cohesive elements are incapable of modelling the coupled failure mechanisms accurately if partitioning is not allowed. The SCE may be partitioned according to the configuration and geometry of matrix cracks in adjacent plies, thus maintaining appropriate connection between plies. Physically, the original interface is split and new interfaces are formed to bond the homologous cracked solids during fracturing process. The stress concentration induced by matrix cracks and the load transfer from cracked solid elements to interface cohesive element are effectively modelled. The proposed SCE is applied to model progressive failure in composite laminates and the results are found to agree with experiments.

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To realize an efficient translation in a narrow space, a surface milli-walker based on linear piezoelectric actuation is proposed, fabricated, and tested in this paper. Operating at the second-order out-of-plane flexural vibration mode, the proposed surface walker is able to realize a linear movement on demand. The surface walker has a weight of 0.135 g with package size of 6 × 2 × 2 mm. Experimental results show that the maximum velocity and drag force of the walker can reach up to 200 mm/s and 10 mN, respectively. The velocity of the walker can be smoothly adjusted over a range of 0.15–200 mm/s. In addition, its startup time is as quick as 6 ms. The milli-walker holds great promises for numerous potential applications in precise instruments.@en

The cohesive element (CE) has been widely used to model delamination in laminated composites. The penalty stiffness, interfacial strength and fracture toughness are three CE parameters that often determine the accuracy of the numerical analysis by finite elements (FEs). The fracture toughness may be experimentally determined through standard fracture tests. The interfacial strength is difficult to obtain experimentally but also plays a crucial role in predictive numerical modelling. The penalty stiffness is a numerical parameter whose range of values may influence computational results. Recently, the CE has been deployed to model the initiation and propagation of delamination in composite structures where no pre-existing cracks or delamination has been assumed. The parameters are often calibrated, but comprehensive studies on the influence of CE parameters and their selection criteria remain limited in the literature. In the present paper, the effects of CE parameters on delamination modelling are systematically examined. The sensitivity of the overall progressive damage process and failure load to the assumed CE strengths and stiffness is studied based on selected examples of fracture and coupon-level tests. With varying CE strengths employed in the FE analyses, two distinct kinds of failure behaviour, namely the strength-sensitive and strength-insensitive delamination processes, have been observed, depending on the geometry and ply layups of composite materials. On the other hand, the penalty stiffness of CE mainly affects the computational efficiency and accuracy of delamination modelling. The failure mechanisms and physics behind are investigated to get better understanding on the numerical effects of CE parameters, and thus provide the selection guidelines in practical analyses.@en

The cohesive element has been widely employed to model delamination and matrix cracking in composite materials. However, an extremely fine mesh along the potential crack path is required to achieve accurate predictions of stresses within the cohesive zone. A sufficient number of cohesive elements must be present within the cohesive zone ahead of the crack tip, resulting in very high computational cost and time for application to practical composite structures. To mitigate this problem, an adaptive floating node method (A-FNM) with potential to reduce model size and computational effort is proposed. An element with adaptive partitioning capabilities is designed such that it can be formulated as a master element, a refined element and a coarsened element, depending on the damage state in the progressive damage process. A relatively coarse overall mesh may be used initially, and by transforming the element configurations adaptively, the local refinement and coarsening schemes are applied in the analysis. The localized stress gradient ahead of the crack front within the refinement zone is captured by the refined elements. The refinement and coarsening operations are performed at the elemental level with fixed nodal connectivity, so that global successive remeshing in adaptive mesh refinement (AMR) techniques is avoided; this is the key difference between AMR and A-FNM. It is demonstrated that, without loss of accuracy, the present method simplifies the modelling procedure and reduces computational cost.

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A three-dimensional separable cohesive element (SCE) is proposed to enable the modelling of interaction between matrix cracking and interfacial delamination in laminated fibre-reinforced composite materials. It is demonstrated that traditional cohesive elements are incapable of modelling the coupled failure mechanisms accurately if partitioning is not allowed. The SCE may be partitioned according to the configuration and geometry of matrix cracks in adjacent plies, thus maintaining appropriate connection between plies. Physically, the original interface is split and new interfaces are formed to bond the homologous cracked solids during fracturing process. The stress concentration induced by matrix cracks and the load transfer from cracked solid elements to interface cohesive element are effectively modelled. A comprehensive set of cases of multiple matrix crack configurations from plies of different fiber angles is considered. The proposed SCE is applied to model progressive failure in composite laminates and the results are found to agree with experiments.

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