PhD Oral Exam - Hamid Hamidi, 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.
Most of the existing works on fatigue of long fiber composite laminates approach the problem from the material point of view. These works have also been mainly on thin laminates. This is because the laminates (particularly unidirectional laminates) are considered to be homogeneous and uniform. This may or may not be true because of the heterogeneous nature of the composite (consisting of fibers and matrix and interface), and the significant variability which gives rise to random spatial variation in the properties. A composite laminate is actually both a structure and a material. For thin laminates, the structural aspect is usually scanned over and only the material aspect is focused on. This gives rise to the expectation that the laminate should behave as a material (with good homogeneity and with little variability). This has resulted in failure of many failure criteria.
The importance of the structural aspect is more evident for the case of thick laminates, subjected to flexural loading while being constrained by bolts. To study the fatigue behavior of such materials (and structures), both the structural aspects and the material aspects must be taken into consideration. This is the subject of study of the present research. A combined material and structural approach as the application of coupon level material properties (material level) into the 3D finite element model (structural level) is introduced and applied. By this it examines the behavior of thick unidirectional glass/epoxy laminates subjected to flexural loads while being constrained by bolts. Both theoretical and experimental studies are carried out to validate each other. The theoretical part consists first of structural analysis which provides the locations of potential failure. This is followed by the application of fatigue failure criteria at these locations. Good correspondence is seen between the experimental and the theoretical results.
The agreement between the results of experiments with those of developed fatigue progressive damage modeling (FPDM) shows that the combined material and structural approach is suitable to study the fatigue behavior of thick composite laminates subjected to bolt loads.