Co-authors​ :

 Sinem ELMAS (TURKEY), Buse ATAC , Cahit Orhun SENOL (TURKEY), Serra TOPAL (TURKEY), Mehmet YILDIZ , Hatice HATICE S. SAS (TURKEY) 

Fiber-reinforced thermoplastic composites are gaining importance in the aerospace industry due to their superior properties compared to thermoset counterparts. Polyether-ketone-ketone (PEKK), a semicrystalline polymer with advantageous characteristics, stands out as an ideal material for aerospace structures. Key features such as rapid processing time and in-situ consolidation enable easy adaptation to various manufacturing methods. However, the aerospace industry faces challenges related to manufacturing and service-induced damages, such as microcracks, and delamination, which can significantly impact durability and performance. Addressing such issues requires an effective repair process to restore the composite's properties. This study explores the potential of annealing, a thermal treatment process conducted above the glass transition temperature (Tg), for healing the thermoplastic matrix and improving mechanical properties. The focus is on understanding the impact of annealing treatment on matrix microcracks, supported by acoustic emission (AE) inspection, in carbon fiber-reinforced PEKK(CF/PEKK) composites produced through the Vacuum-Bag-Only(VBO) process. The laminate layup follows a quasi-isotropic and symmetric [0/45/90/(-45)2/90/45/(0)2/45/90/-45]s. Microcracks induced into the samples are evaluated for load range using AE. The microcracked samples are annealed in an oven for one hour at 180°C, with heating and cooling rates set at 8°C/min. Mechanical characterization and damage analysis are performed through flexural and compression tests, supported by AE inspection, both before and after annealing. The VBO-manufactured laminates exhibit aerospace-grade quality, with a void content of 0.61± 0.18%. Annealing at 180°C for one-hour results in partial recrystallization, as evidenced by Tg shifts observed with DSC analysis. Mechanical characterization reveals a 10% decrease in flexural strength for microcrack-induced samples, accompanied by the formation of transverse cracks. However, annealing leads to a slight increase in flexural strength, indicating matrix recovery. While compressive strength initially decreases by 7.7% due to microcracks, it improves by 2% after annealing. From fracture surfaces, catastrophic failures within the gage region are observed, but annealing alters fracture behavior. AE analysis highlights distinct failure modes under bending and compressive loads, with fiber-related failures dominating under bending and matrix-related failures under compressive loads. The presence of microcracks introduces complexities in both tests, contributing to increased matrix and interface failures. The AE behavior of CF/PEKK composites after annealing is closely linked to changes in the material's microstructure, residual stresses, and interfacial properties. The findings underscore the potential of the annealing process to enhance the mechanical properties of thermoplastic composite laminates and, notably, influence failure behavior.