Investigation of mechanical anisotropy and interlayer bonding in thermoplastic composites produced by additive manufacturing

Anastasiia Khudiakova

Research output: ThesisDoctoral Thesis

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Abstract

Additive manufacturing (AM) methods are a promising alternative to conventional manufacturing processes in the production of carbon fibre (CF) reinforced thermoplastic composites. These composites are lightweight and show outstanding mechanical performance and are therefore of great interest to many industries, such as aerospace, automotive, sports and medical. A major drawback of AM constitutes the layer-by-layer manufacturing approach, inevitably leading to direction-dependent properties of the parts and the accumulation of defects between the layers, which undermines their load-bearing capacity. This crucial topic demands more in-depth research, which is why the present thesis is devoted to the characterisation of mechanical anisotropy and interlayer bonding of parts produced by AM. The topic of mechanical anisotropy was addressed by the systematic analysis of short CF reinforced polyamide 1212 tensile bars produced by selective laser sintering (SLS), which revealed the presence of a preferable fibre orientation. X-ray tomography analysis showed that more than half of the fibres were oriented along the travel direction of the roller distributing the powder. This effect led to highly direction-dependent mechanical properties of the parts that consequently exhibited the highest tensile strength and tensile modulus upon loading along the fibre direction. Further, the anisotropic behaviour of neat and short CF reinforced polylactic acid (PLA) specimens produced by fused filament fabrication (FFF) was characterised by means of tensile testing and digital image correlation (DIC). Specimens were printed with 0°, 45° and 90° raster angles. For both materials, the complete sets of engineering constants could be successfully acquired under the assumption of transverse isotropy. In addition, multi-material parts were produced by direct printing of neat PLA on CF-PLA. Interlayer bonding between the materials was characterised by fracture mechanics tests, namely mode I double cantilever beam (DCB) and cracked round bar tests. The results obtained confirmed that the interlayer bonding of PLA/CF-PLA was at least as strong as that of CF-PLA/CF-PLA. Fracture mechanics tests, namely quasi-static and fatigue mode I DCB tests, were also applied to characterise the effect of manufacturing parameters on the interlayer bonding of unidirectional (UD) CF reinforced polyphenylene sulphide laminates produced by automated tape placement with in-situ consolidation (ATPisc). Regardless of the manufacturing parameters used, all laminates exhibited a complex delamination behaviour with multiple cracking. Different approaches for data reduction and data representation were employed for the quantitative assessment of the laminate quality. In order to make the mode I DCB test less labour-intensive and less dependent on visual crack length measurements, DIC was applied. In the framework of this approach, two data reduction methods were developed to determine the crack length based on either the high strain concentration at the crack tip or the crack tip opening displacement. The methods developed yielded data consistent with those of the top surface analysis method known from literature. Ultimately, it was shown that DIC is a suitable alternative to conventional optical measuring tools, enabling the automated tracking of crack propagation. Besides fracture toughness, the interlayer bonding was characterised in terms of shear strength. For this purpose, compression shear tests (CST) were performed on UD CF/PA6 and steel-CF/PA6 hybrid samples, produced using ATPisc. As a result, CST was revealed to be a simple and straightforward test method that requires only a small sample size and can therefore be recommended for a first qualitative assessment of the interlayer bonding in AM parts.
Translated title of the contributionUntersuchung der mechanischen Anisotropie und Zwischenschichthaftung in thermoplastischen Verbundwerkstoffen hergestellt durch additive Fertigung
Original languageEnglish
QualificationDr.mont.
Awarding Institution
  • Montanuniversität
Supervisors/Advisors
  • Godec, Damir, Co-Supervisor (external)
  • Lomov, Stepan, Assessor A (external), External person
  • Pinter, Gerald Gerhard, Supervisor (internal)
  • Blackman, Bamber, Assessor B (external), External person
Publication statusPublished - 2021

Bibliographical note

embargoed until null

Keywords

  • thermoplastics
  • carbon fibre reinforced composites
  • additive manufacturing
  • mechanical anisotropy
  • interlayer bonding
  • fracture mechanics
  • mode I DCB test
  • interlayer shear strength
  • digital image correlation
  • delamination
  • crack tracking

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