Wrinkle Formation and Initial Defect Sensitivity of Steered Tow in Automated Fiber Placement

This paper aims to study the wrinkle formation of a prepreg with initial defect during steering in automated fiber placement (AFP). Wrinkle formation has a detrimental effect on the mechanical properties of the final product, limiting the AFP applications. A theoretical model for wrinkle formation has been developed in which a Pasternak foundation and a Koiter imperfection model are adapted to model viscoelastic characteristics of the prepreg tack and initial defect of the prepreg, respectively. The initial defect is defined as a slight deviation of the tow’s mid-plane from a horizontal shape. The initial defect is generated in the tow by moving the tow through the guidance system, pressure of the roller, and resin tackiness. Galerkin method, along with the finite difference method (FDM), are employed to solve the wrinkle problem equation. The proposed method is able to satisfy the different boundary conditions for the wrinkle problem completely. The numerical results show that increasing the initial defect leads to a decrease in critical load and an increase in critical steering radius. To validate the theoretical model, experimental results are presented and compared with model-predicted results. It is shown that the model is well able to capture the trends and values of wrinkle formation wavelengths obtained from the experiment.

Failure in Two-Dimensional Materials: Defect Sensitivity and Failure Criteria

Two-dimensional (2D) materials have attracted a great deal of attention recently owing to their fascinating structural, mechanical, and electronic properties. The failure phenomena in 2D materials can be diverse and manifested in different forms due to the presence of defects. Here, we review the structural features of seven types of defects, including vacancies, dislocations, Stone-Wales (S-W) defects, chemical functionalization, grain boundary, holes, and cracks in 2D materials, as well as their diverse mechanical failure mechanisms. It is shown that in general, the failure behaviors of 2D materials are highly sensitive to the presence of defects, and their size, shape, and orientation also matter. It is also shown that the failure behaviors originated from these defects can be captured by the maximum bond-stretching criterion, where structural mechanics is suitable to describe the deformation and failure of 2D materials. While for a well-established crack, fracture mechanics-based failure criteria are still valid. It is expected that these findings may also hold for other nanomaterials. This overview presents a useful reference for the defect manipulation and design of 2D materials toward engineering applications.

Design and Simulation of Sensor Based on One Dimensional Photonic Crystal Metallodielectric with Two Defect

In this paper we report characteristics of a 1D metallodielectric photonic crystal with a two defect rod. Simulations of the field propagation for the corresponding Transverse Electric (TE) modes were carried out using Finite Difference Time Domain (FDTD) technique on Maxwell’s Equation. The results show that for certain chosen parameters of the transmittance varies if the refractive index of the defect rod is changed and that approximate linier changes for an refractive index range of 1.3 – 1.5 with a slight increase for a second defect change and a relatively steep decrease for a first defect change. A second defect index change can be used as the first defect sensitivity control The characteristics change due to variation of the defect rod radius is also considered for sensor design optimization