Fatigue behaviour of open-hole composites containing bis-maleimide polymer as self-healing agent

• Κύριος συγγραφέας: Χαντζή, Μαριάννα-Γεωργία
• Άλλοι συγγραφείς: Chantzi, Marianna-Georgia
• Γλώσσα: English
• Έκδοση: 2022
• Θέματα: Composites; Self-healing; Open-hole; Electrospinning; Cohesive zone model; Σύνθετα; Αυτοΐαση; Ανοιχτή οπή; Ηλεκτροϊνοποίηση; Μοντέλα συνεκτικής ζώνης

In the current diploma thesis, bis-maleimide (BMI) self-healing (SH) resin based on Diels-Alder (DA) reaction mechanism was integrated into high performance carbon fiber reinforced plastics (CFRPs) by using two variants of electrospinning process technique: (i) the solution electrospinning (SEP) and (ii) the melt electro-writing process (MEP). After modification, all CFRPs were exposed to quasi-static tensile tests, fatigue tests under tensile-tensile loading conditions, while their self-healing capability was further assessed. Based on these, reference and BMI modified CFRPs containing (open-hole, OH) or no hole (unnotched) having [45°/-45°/0°/90°]2S stacking sequence were fabricated for the needs of the current study. In the case of modified CFRPs, 1 wt% of graphene nano-platelets (GNPs) were also incorporated into SHA resin to further modify the CFRPs. The SHA was incorporated locally on the center of each CFRP and surrounding the hole (in the case of OH CFRPs). Unnotched CFRPs were tested only under quasi-static tensile tests, while OH ones were tested under both quasi-static and fatigue conditions. According to quasi static experimental results, it was shown that the incorporation of the BMI SHA led to decrease in strength and stiffness for the unnotched specimens, while for the OH specimens it led to decrease in stiffness and slightly increase in strength. The BMI-GNP modified by SEP specimens (both unnotched and OH) showed the best tensile properties. Fatigue tests showed similar behavior for all tested material types: as the cycles progressed the apparent modulus of the specimens decreased and the hysteresis loops moved towards higher strains. The incorporation of the BMI SHA led to increase in fatigue life of most of the modified specimens even by 50%. The application of the healing cycles showed partial restoration of the damage as observed in c-scan plots. The modified CFRPs (BMI and BMI-GNP) by MEP, exhibited the best behavior under fatigue tension-tension loading. Finite element models (FEM) for the simulation of quasi-static tensile tests of modified OH CFRPs were developed utilizing cohesive contact interactions to account for interlaminar delaminations. All analytical models withstood a higher load compared to the experimental results but were capable of simulating the damage initiation and evolution that was observed in the experiments. Τhe models were deemed satisfactory and helped draw conclusions about the damage evolution in OH modified specimens under tensile loads. In the end of the thesis, suggestions are made for further development of the experimental procedure and the numerical simulations.