PhD Oral Exam - Mohammadhossein Ghayour, 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

Automated Fiber Placement (AFP) is a new robot technology that has the advantage of the rapid manufacturing process and reduce material waste. However, using the fiber tows instead of composite prepreg sheets, new defects are introduced into the composite structures. These defects can cause geometrical discontinuities and local material inhomogeneities. Although the effect of these defects on the in-plane mechanical performance of composite structures can be found in the literature, there is still a lack of knowledge in studying the out-of-plane and impact response of the composite laminates.

The main objective of this research study is to investigate the effect of induced tow-gaps on the Low-Velocity Impact (LVI) response and Compression After Impact (CAI) response of the thin composite plates. For this purpose, defective quasi-isotropic Carbon/Epoxy composite plates have been manufactured with the AFP technique. Two millimeters of tow-gaps are embedded in all composite plies to provide an average of 8% of pre-cured volumetric gaps in all specimens.

The test specimens are subjected to impact loads with impact energies of 5 J, 10 J, and 15 J, and impact responses and projected delamination of the defective plates are compared with the baseline sample. Delamination areas are measured using the Ultrasonic C-scan technique. CAI tests are carried out to evaluate the residual compressive strengths of the defective impacted samples. Furthermore, Digital Image Calibration (DIC) technique is used during the CAI tests to measure the in-plane strains of the specimen under compressive loading. Effect of curing process on the gap formation evaluates by measuring the Induced Gap Shrinkage Factor (IGSF) using microscopic observation. Three-point bending tests are carried out on both short and standard beams to measure the interlaminar shear strength and flexural stiffness/strength of the composite beams with tow-gaps.

Furthermore, Induced Defect Layer Method (IDLM), a robust meso-macro model, is developed for damage analysis of the defective laminates. This method uses a geometrical parameter to incorporate induced gaps in the elastic, inelastic, and softening behavior of the material. The main advantage of the proposed method is that a novel homogenization technique is used to include tow-gaps in composite damage analysis of defective laminates. In addition, IDLM evaluates the mechanical response of the defective laminates with acceptable precision and less computational time compared to the current numerical models.

Results indicate that tow-gaps can reduce the impact resistance of the composite plates by about 17% for low levels of impact energy. It is also shown that the interaction of induced gaps and impact damage at the compressive residual strength reduction of impacted thin composite plates is significant and cannot be neglected. Numerical results show that the IDLM is a robust damage method for damage analysis of the composite laminates with manufactured gaps.