Advanced Infusible Thermoplastic Resins for Lightweight Aerospace Composites

Abstract

This Ph.D. dissertation is a novel study on the development of advanced liquid thermoplastic-based composites for aerospace applications. Elium® is a new commercially available poly methyl methacrylate (PMMA)-based liquid thermoplastic resin that goes under radical polymerization at room temperature. This research addresses the detailed characterization of the Elium® 188 O resin through several techniques such as differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), Fourier Transform Infrared Spectroscopy (FTIR), and rheological analysis. The characterization is followed by a progressive processability study by incorporating smart graphene-coated glass fabric sensors to monitor the resin flow behavior, dielectric properties, pressure variations within the mold, race-tracking, and cure/post-cure behavior in comparison to the conventional epoxy resin. These smart sensors were successfully utilized to effectively monitor the entire resin infusion process from vacuum compaction to cure/post-cure of both thermosetting and thermoplastic resins.

Comprehensive mechanical performance of the thermoplastic composites incorporating commonly used glass fabrics (GFs) and carbon fibers (CFs) reinforcements used in aerospace applications was conducted in terms of Dynamic Mechanical Analysis (DMA), flexural and tensile tests. The microstructures of these composites were thoroughly analyzed using X-ray Computed Tomography (XCT). Whereas the fractured surfaces were analyzed through a detailed Scanning Electron Microscopy (SEM) analysis. The mechanical tests study showed promising results in terms of comparable strengths of the thermoplastic composites to the conventional thermosetting counterparts. Furthermore, complex infusible aerostructures including carbon fiber/Elium® honeycombs and their sandwich structures were successfully manufactured and tested.

Laminates with artificial cracks were manufactured via a vacuum-assisted resin transfer molding (VARTM) process and were subsequently healed through interdiffusion at different temperatures, pressures, and contact times in a heated press. The healed specimens were then tested using double cantilever beam (DCB) tests in Mode I configuration exhibiting fracture toughness as high as 0.74 N/mm, comparable to those reported for the adhesively bonded samples. Replicating the impact-induced damage in the form of delamination and healing them at the optimum parameters also led to 96% flexural strength recovery.

The formability of Elium® composites was investigated using five different composite variants by conducting 90° V-bending tests. The analysis of variance (ANOVA) study revealed a dominant contribution of holding time (around 50%) followed by forming temperature (around 30-40%).The XCT images confirmed the reduction of voids and porosity at higher forming temperatures with a holding time. In general, near-net shapes were achieved for all fabric types at 210°C and a holding time of 2 min with minimum defects. The springback effect for all fabric types was also validated through analytical modeling.

Date of Award	12 Dec 2023
Original language	American English
Supervisor	Rehan Umer (Supervisor)