Characterization of 3D-printed laminate composites with enhanced mechanical properties

Student thesis: Doctoral Thesis

Abstract

Additive manufacturing (AM) also known as 3D printing/Rapid prototyping is a robust method of manufacturing parts with significant improvement in mechanical properties with continuous carbon fibre reinforcement. 3D printed parts have greater accuracy and precision with no material waste as compared to other traditional manufacturing techniques. The properties of the composite materials depend on the constituent matrix and fibre orientation as well as the volume of the fibre and the matrix. In this project, mechanical characterization of 3D printed polymer composites was carried out using tensile, flexural, fatigue and impact testing along with other key issues such as weak interlayer bonding, voids between the layers and low volume fraction of carbon fibre in the composite parts. Fibre volume fraction was modified by varying the quantity of fibre layer as well as by pressing the large printed plate manufactured using Mark forged Two 3D printer using Fused deposition modelling (FDM). Materials used were continuous carbon fibre with Nylon.

In this study in-plane mechanical properties of continuous carbon fibre reinforced thermoplastic polyamide composite was evaluated and compared against predicted values from classical laminated-plate theory. Strength, stiffness, and Poisson’s ratio of the composite specimens were measured using tensile testing both in longitudinal and transverse direction and the shear properties were also measured. It was determined that the tensile strength and modulus of elasticity values were significantly improved to 603.43 MPa and 85 GPa respectively as compared to 31 MPa and 0.94 GPa for unreinforced nylon specimens. Furthermore, cross-sectional micrographs of specimens were analysed to observe the microstructure and fracture mechanism of the 3D printed composite. Experimentally determined values were used to predict the behaviour of the materials in different orientation using classical laminated-plate theory on the commercially available LAP (Laminated Analysis programme) software.

The results showed that the strength and stiffness of hot-pressed samples increased with the increase in fibre volume content (fraction). Hot pressed samples exhibited the increase in tensile strength by about 27 % and elastic modulus by 11 % because of increasing the fibre volume fraction from 29 % to 35%. Synergetic effect of both short and continuous carbon fibre was also studied, and it was observed that the tensile properties were higher for the samples reinforced with short and continuous fibre than only continuous fibre polymer composites. Effects of voids on 3D printed continuous carbon fibre-reinforced polymer composites were quantified. A microstructure study of the 3D printed polymer composites was carried out using scanning electron microscope (SEM). Fibre volume fraction was calculated using acid digestion technique to determine the amount of fibre contents before and after hot pressing (compaction). From Micro-Computed Tomography (µCT) it was validated that hot pressing reduced the void content which in return increased the strength and modulus.

Additively manufactured composite specimens exhibit anisotropic properties meaning that the elastic response changes with respect to orientation (printing direction). Through thickness tensile strength of 3D polymer composites were determined by printing of continuous carbon fibre reinforced thermoplastic polyamide based composite samples. Surface behaviour was studied using Scanning electron microscopy (SEM) to see the voids and the distribution of the fibres in the samples in the through thickness direction. In the through thickness direction, rectangular specimens have higher tensile strength and elastic modulus as compared to circular specimen due to the lower amount of fibre content. Fatigue analysis of AM samples were also carried out at different stress level and the number of cycles recorded as well as the fracture type. Lap shear testing of the adhesively bonded samples was also carried.
Date of AwardSep 2022
Original languageEnglish
SponsorsSEUPB
SupervisorAlistair McIlhagger (Supervisor) & Edward Archer (Supervisor)

Keywords

  • Polymer composites
  • 3D printing

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