PhD.Dissertation:Ceren Yıldırım

MULTIFACETED CHARACTERIZATION AND OPTIMIZATION OF AEROSPACE GRADE THERMOPLASTIC COMPOSITES JOINTS: ADVANCES IN SURFACE TREATMENTS, MECHANICAL PERFORMANCE, AND ENVIRONMENTAL RESILIENCE

Ceren Yıldırım
Manufacturing Engineering, PhD Thesis Dissertation, 2024

Thesis Jury

Prof. Mehmet Yıldız (Thesis Advisor), Prof. Burç Mısırlıoğlu, Assoc. Prof. Hatice Sinem Şaş Çaycı, Prof. Cengiz Kaya, Assoc. Prof. Bertan Beylergil

Date & Time: July 11st, 2024 – 09:00 AM

Place: FENS L027

https://sabanciuniv.zoom.us/j/2164839010

Keywords: Thermoplastic Composites, Adhesively Bonded Joints, Surface Treatments, Structural Health Monitoring, Failure Analysis

Abstract

This thesis is composed of four interconnected and complementary investigations on novel manufacturing processes to increase the performance of carbon fiber-reinforced polyether-ketone-ketone (CF/PEKK) composite laminates and adhesively bonded CF/PEKK joints for aerospace composite structures. As such, in-situ and autoclave consolidated laminates are manufactured by automated fiber placement (AFP) and then extensively characterized through utilizing mechanical and thermomechanical methods along with multi-instrumental structural health monitoring (SHM) techniques. Subsequently, these laminates are joined adhesively under four different surface preparation conditions, namely, non-treated (NT), peel-ply (PP), mechanical abrasion (MA), and atmospheric plasma activation (APA).  

The first study physically, thermally, and mechanically characterizes autoclave-consolidated CF/PEKK laminates and examines their damage behavior under tensile loading conditions. Using a multi-instrument SHM approach, including acoustic emission (AE), digital image correlation (DIC), and infrared thermography (IRT), the study provides insights into failure mechanisms and damage progression in high-performance thermoplastic composites (TPCs). The obtained results can shed light on identifying the appropriate damage severity index for TPCs, which is a rather valuable parameter towards estimating the remaining useful life of CF/PEKK composite structures.

After developing a comprehensive understanding of physical, thermal, and mechanical properties as well as the failure mechanism of CF/PEKK as an adherend, the second study demonstrates the effectiveness of APA on adherend surfaces and the mechanical behavior of adhesively bonded joints compared to NT-, PP-, and MA-treated surfaces. The mechanical assessments of single lap-joints (SLJs) under tensile and flexural loadings reveal that APA-treated joints exhibit superior load-carrying capacities, with a notable increase in shear and flexural strength. Post-fractured surface analyses and in-situ AE monitoring confirm enhanced interface interactions along with promotion to the cohesive failure in APA-treated joints.

The third study assesses the impact of surface treatments on Mode-I and Mode-II fracture toughness of adhesively bonded CF/PEKK joints, showing significant enhancements with APA treatment. The findings indicate substantial improvements in fracture toughness values for APA-treated samples, with AE analyses correlating with slower delamination and occurrence of cohesive failure.

These studies on adhesively bonded joints are further extended by investigating the influence of environmental conditions (i.e., high temperature, low temperature, and cycling hygrothermal conditions) on the mechanical and thermomechanical performances, durability, and reliability of adhesively bonded CF/PEKK joints. Thus, it becomes possible to mimic the service conditions of aerospace grade composite structures. The results indicate that environmental conditions dramatically alter fracture toughness and dynamic-mechanical behaviors, thus highlighting the importance of considering environmental factors in aerospace structural design.