PhD Oral Exam - Emad Pourahmadi, 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

Despite fabrication difficulties, the utilization of thermoplastic composite laminates is expanding, especially in the aerospace industry, owing to their outstanding characteristics, such as high toughness and recyclability. Compared to established manufacturing procedures, such as hand layup autoclave process, automated manufacturing techniques, such as Automated Fiber Placement (AFP), offer the potential to economize time and costs. An advantage of manufacturing thermoplastic composite laminates using AFP lies in the possibility of in-situ consolidation, thereby eliminating the necessity of any secondary consolidation processes. However, short processing time during the AFP method leads to a significant contrast in the quality of in-situ-consolidated thermoplastic composite laminates in terms of interlaminar bond strength and other material properties when compared to that of their autoclave-reconsolidated counterparts. The present thesis focuses on this aspect and aims to develop an efficient micromechanical computational model based on the finite element method that can predict the interface strength and other material properties, including stiffness and strength, of in-situ-consolidated Carbon/PEEK thermoplastic composite laminate. Two batches of laminate samples are fabricated by AFP with in-situ consolidation. One of the batches is subsequently re-consolidated in an autoclave to serve as a reference for a comparative study (i.e., in-situ consolidated vs. autoclave re-consolidated). The Short-Beam Shear (SBS) test, due to delamination failure mode, is chosen to measure the Interlaminar Shear Strength (ILSS). The interface strength properties caused by AFP in-situ consolidation are computationally determined using the cohesive zone model and the SBS test results. The manufactured samples undergo micrographic study and thermoanalytical Differential Scanning Calorimetry (DSC) testing to gather the essential data for the computational model, including fiber volume fraction, interlaminar resin pocket, void content and degree of crystallinity. Then, realistic two-dimensional Representative Volume Elements (RVEs) are generated at a micro-scale based on the obtained information from micrographic examination and DSC analysis. These 2D RVEs were first used in the finite element simulation to predict the transverse tensile strength, resulting from the AFP in-situ consolidation process, using the Drucker-Prager law along with ductile failure criterion to take into account the plastic deformation of the matrix, as well as crack onset and evolution in the neat PEEK resin. Furthermore, the effective stiffness properties, such as transverse elastic and out-of-plane shear moduli, influenced by AFP in-situ consolidation were predicted by applying periodic boundary conditions and using the homogenization theory. The obtained results reveal that while the AFP in-situ consolidation manufacturing process reduces the transverse stiffness properties of Carbon/PEEK thermoplastic composite laminate 10% to 20%, the transverse tensile strength value may even decrease up to 44%, in comparison with the autoclave treatment. The outcomes of this thesis demonstrate that the mechanical performance of Carbon/PEEK thermoplastic composite laminates is significantly affected by the AFP in-situ consolidation process. The predicted interfacial strength and effective material properties provide essential input parameters for subsequent finite element modeling, analysis, and structural design of thermoplastic composite components produced through the AFP in-situ consolidation process.