PhD Thesis Defence: Mr. Rishi Raj (25/02/25)
Thesis title:
Advanced thermoplastic laminates with tailored interface for aerostructures
Faculty advisor(s):
Prof. Subodh Kumar (Co-Supervisor - Prof. Suryasarathi Bose)
When?
25th February, 2025 (Tuesday), 02:30 PM (India Standard Time)
Where
KPA Auditorium, Materials Engineering
Abstract
Carbon fiber reinforced polymer (CFRP) laminates are currently used in commercial aircrafts replacing their metallic counterparts. Epoxy is used as matrix in CFRPs in the primary structures of aircrafts such as fuselage, wings and tail parts, due to its superior properties. However, there are many disadvantages in CF/Epoxy laminates such as, slow production rate due to curing, lack of recyclability, brittleness, and susceptibility to catastrophic failure. Additionally, epoxy requires rivets for joining, have a limited pot life, and pose challenges in damage repair. Their sensitivity to high temperatures also restricts their use in applications near engine components. To overcome these problems, high temperature thermoplastics such as PEEK can be used instead of epoxy as matrix. It does not require curing and the presence of phenyl groups in the main backbone chain provides high strength, thermal stability and rigidity to the polymer, and it can be used at high temperatures (150-200oC) for prolonged time without losing structural integrity. However, the surface energy of CF and PEEK are very different. The lack of functional groups and smooth surface of CF increases the difficulty in wetting of PEEK over CF. The poor interface between PEEK and CF will give rise to poor mechanical properties such as= interlaminar shear strength (ILSS), Flexural strength (FS), fracture toughness etc. This work endeavors to achieve good mechanical properties as well as functional properties such as, deicing and electromagnetic shielding, in CF/PEEK laminates.
Chapter 1 is introduction giving a holistic view of thermoplastic based laminates in aerostructures. Current challenges in CF/Epoxy laminates and state-of-the-art literature, which provides approaches to overcome those challenges. It also discusses the outlook and perspective of the literature review, and the objectives of the thesis.
Chapter 2 contains information about the materials experimental techniques and characterization methods used in the thesis.
Chapter 3 aims to optimize the processing parameters, pressure, temperature and residence time, in compression molding to produce CF= /PEEK laminates. The results show that 410oC temperature, 5+10+15+20 MPa pressure cycle and 30 minutes at each pressure produced the best laminate with 66 MPa ILSS and 658 MPa FS, which are much superior to the commercial CF/Epoxy laminates currently being used. The deicing time was 120 s in electrothermal heating at 2V to melt frozen droplets of 10 μL water and EMI shielding effectiveness was 47.5 dB.
Chapter 4 aims to improve the deicing and EMI shielding performance by incorporating stainless steel (SS) mesh at the top and bottom of the CF/PEEK laminate. To improve the bonding with PEEK matrix SS mesh was coated with Polyetherimide (PEI). ILSS, FS and storage modulus showed an improvement of 9%, 16% and 44% after the incorporation of PEI coated SS mesh. The deicing time was reduced from 120 s to 45 s. EMI shielding effectiveness also improved from 47.5 dB to 60 dB. The dominant mechanism of EMI shielding changed from absorption to reflection.
Chapter 5 aims to improve Mode I fracture toughness (GIC) and ILSS by electrospraying graphene oxide (GO) and then growing ZnO nanorods by SILAR technique on CF surface. GIC and ILSS showed an increase of 19% and 13% for ZnO+GO-CF/PEEK laminates. The SEM fractographs showed the interfacial debonding was suppressed and the major mode of failure changed to matrix deformation. A minor increment in EMI shielding effectiveness was also observed from 47.5 dB to 49 dB.
Chapter 6 explores the shockwave damage of the neat CF/PEEK laminates and ZnO+GO-CF/PEEK laminates. The damaged laminates were then healed using appropriate temperature and pressure to recover the loss in mechanical properties. The neat CF/PEEK laminate lost 35% and 66% ILSS at 367 bar and 545 bar damaging pressure, whereas these losses were 29% and 37% for ZnO+GO-CF/PEEK laminate. After the healing treatment, the neat CF/PEEK laminate recovered 100% and 74% of original ILSS value at 367 bar and 545 bar respectively, whereas these values were 82% and 75% for ZnO+GO-CF/PEEK laminate. Thus, the interfacial strengthening by ZnO+GO reduced the damage. However, the recovery is similar because the broken ZnO or GO nanoparticles could not be repaired by healing treatment.
In summary, this thesis contributes to enhance the current knowledge on improving the important structural and functional properties of CF/PEEK laminates, which is highly desirable as an aerostructure material in the current scenario. It also explores the damage aspects and recovering the properties after damage.