PhD Oral Exam - Yasser Sami Elsherbini, 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.
Composite materials are widely used in many applications owing to their advantages over conventional ones. Among the different available manufacturing techniques of composites, automated fiber placement (AFP) attracted the attention of many industries due to its speed of material deposition and repeatability in manufacturing. Unfortunately, the occurrence of gaps between material strips during AFP manufacturing process is unavoidable. Even though there have been many studies that focused on the effect of these gaps on the static properties of the AFP laminates, to our knowledge, no work has been performed to test their effect on fatigue behavior.
In this dissertation, the effect of the induced gaps on fatigue performance of carbon/epoxy AFP laminates was investigated both experimentally and numerically. In the experimental part, fatigue tests were conducted on both reference, free from defects laminates, and defected laminates. Then, the fatigue performance of both types was compared and analyzed to obtain the effect of gaps. For better understanding the fatigue behavior of laminates containing gaps, many parameters were taken into consideration such as laminate stacking sequence, gap shape, gap orientation and number of gaps. Based on analyzing the results of fatigue testing of different stacking sequences, a few design recommendations are provided that can enhance the performance of the defected laminates and alleviate the effect of gaps.
In addition, infrared thermography was used as a non-destructive technique for in-situ detection of damage during fatigue loading. In order to examine the nature of the inherent damage within the laminate due to gaps, sectioning and inspection of specimens using scanning electron microscopy (SEM) were performed. The extensive fatigue experiments revealed the existence of a threshold stress value below which the effect of gaps on fatigue performance diminishes. The main drawbacks in obtaining this threshold values using the traditional long fatigue testing method were the large number of specimens and long-time for the fatigue tests.
Consequently, infrared thermography and Risitano method were applied on AFP laminates containing gaps to provide a quick method for obtaining the threshold values. This method has a great potential in saving time and material required for performing traditional fatigue tests to develop stress/life curves. The obtained results of threshold values were in good agreement with the results obtained from the conventional method.
In the numerical part, a fatigue progressive damage model (FPDM) was developed using Ansys Parametric Design Language (APDL) and applied to the case of laminates containing gaps. The progressive damage model presented in this work is an integration of fatigue life model, failure criterion, sudden and gradual degradation of strength/stiffness. The predicted results from the model were compared to the experimental results for different stacking sequences. The model showed a good agreement with the experimental results for the case of unidirectional laminates. For the case of cross-ply laminates more work should be done for better prediction of results due to the complex nature of damage for off-axis laminates. Nevertheless, the model can be helpful in saving time and material in the preliminary design steps to have an idea about the damage behavior and the performance of the designed part.