PhD Oral Exam - Hassan Abdullah Alshahrani, Mechanical Engineering

When studying for a doctoral degree (PhD), candidates submit a thesis that provides a critical review of the current state of knowledge of the thesis subject as well as the student’s own contributions to the subject. The distinguishing criterion of doctoral graduate research is a significant and original contribution to knowledge.

Once accepted, the candidate presents the thesis orally. This oral exam is open to the public.

Abstract

Composite materials are increasingly replacing metals for many modern structures used in the aerospace and automotive industry. Textile composites are favored due to their supe¬rior forming capabilities to produce complex shapes. However, the formability of a textile composite is limited by failure modes, such as wrinkling, which remain challenging issues during the forming process. The ability to accurately predict wrinkles and ultimately pre¬vent them during the forming process is highly desirable for process optimization in an industrial environment. To predict those defects, a thorough understanding of the defor¬mation behavior of textile-woven prepregs is required. Out-of-plane bending is one of the deformation mechanisms that govern the appearance of wrinkles during composite forming. This thesis therefore presents an experimental, theoretical, and numerical study on the out-of-plane bending behavior of woven out-of-autoclave prepregs with application in forming simulations.

Within this thesis, a new test method for characterizing the bending behavior of prepreg materials at forming conditions was developed based on a vertical cantilever test associated with a linear actuator and load cell. This test method allowed for sufficient control of the deflection shape, testing rates, and processing temperatures within the range of the thermosetting resin. Investigations for out-of-plane bending behavior and viscoelastic be-havior at the forming process conditions were undertaken using the developed test method.

Through the cantilever beam theory, where the prepreg yarn is composed of two external vis¬coelastic polymer plies with a linear elastic ply, a theoretical model is proposed to estimate the bending stiffness over a range of processing conditions. A new approach for consider¬ing the testing rate and temperature with respect to a reference value is also established. Experimental tests were carried out to estimate the model parameters and to validate the proposed model. The predicted bending stiffness was found to be in a good agreement with experimental values at selected conditions. However, there were slight differences due to the complexity of undulation in the woven fabric structure. Then, a finite element model for the bending behavior of multilayered prepregs was developed by considering the actual bending behavior of the material. The prepreg ply was modeled by incorporating the characterized behavior of intra-ply shear and inter-ply friction. The effect of stacking sequences on out-of-plane bending deformation during the forming process was studied experimentally and numerically. Moreover, the feasibility of using a viscoelastic approach and its application in forming simulations were analyzed.

Finally, a series of real forming experiments using a double diaphragm process were car¬ried out to investigate the formability of textile out-of-autoclave thermoset prepregs over complex geometry for aerospace applications. A one-step procedure was used for both the forming and curing processes using the same experimental setup. A finite element model was developed to simulate the double diaphragm forming process, with consideration for the diaphragm material properties at forming conditions. In addition, important considera-tions, such as local fiber compressive stresses, shear angle distributions, and stacking lay-up sequences, were analyzed to identify the potential causes of wrinkles in the formed parts. The resulting knowledge from the modeling methodology allowed the designers to reliably choose the appropriate stacking sequences and suitable process parameters for complex structures prior to conducting expensive trial and error tests.