Abstract

High demands of composites application in various industries have led to top-quality manufacturing techniques, including automated fiber placement and double diaphragm forming. These advanced technologies are restrained by defects formation during the layup and forming process. Several studies have been conducted to investigate the effect of process parameters and to identify the different forming mechanisms for better understanding defect formation. Among these parameters, the bending property of uncured prepreg is the primary factor that can dominate the most common emerging defect known as wrinkle during composite forming. Perceiving these properties in depth could reduce the costs of the experiment extensively as they could be proper input for simulation software to have better defect prediction. Since composites are classified as viscoelastic materials, the main goal of this thesis is to study the effect of viscoelasticity on the bending stiffness of uncured carbon epoxy prepreg. For this purpose, a stress relaxation test has been implemented using a tensile test machine at room and elevated temperatures. Also, the effect of temperature on the viscoelastic behavior of the sample is discussed. In conformity with the experimental observations, a new nonlinear viscoelastic model is proposed and compared to linear models. It is shown that this proposed model is able to address viscoelastic behavior not only in the prior and after elastic region but also out of the testing time range. Within this thesis, a bending test method for defining the bending behavior of prepreg materials at forming conditions was developed based on a horizontal cantilever beam. Then, both linear and nonlinear aspects have been coupled with constitutive bending equations to represent the bending behavior and its time dependency. At last, a finite element (FEM) is performed in the ABAQUS platform in order to depict the results correlations. Moreover, a simple deformation mechanism has been simulated to represent the effect of deformation rate perfectly.