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Damage tolerance of high-performance composites
Citation Link: https://doi.org/10.15480/882.4276
Publikationstyp
Doctoral Thesis
Date Issued
2022
Sprache
English
Author(s)
Advisor
Referee
Title Granting Institution
Technische Universität Hamburg
Place of Title Granting Institution
Hamburg
Examination Date
2022-01-28
TORE-DOI
First published in
Number in series
41
Citation
Technische Universität Hamburg (2022)
High-performance composite laminates are structural materials that combine low density with high stiffness and strength. High safety and reliability requirements for structural parts require the consideration and detailed knowledge about damage tolerance of materials and structures.
The multi-scale nature of composites results in the occurrence of different failure modes and a complex failure behaviour where matrix failure at the micro-level influences the failure process at all higher observation levels. Consequently, the occurring matrix damage has a decisive influence on the damage tolerance. The research hypothesis of this thesis is:
Understanding and adapting the matrix's damage behaviour is fundamental to improving the damage tolerance of high-performance composites.
To adapt the matrix's damage behaviour, carbon-fibre reinforced plastics (CFRP) with a few-layer graphene (FLG) modified matrix with layer thicknesses varying from ultra-thin-ply (28 μm / 30 g/m2) to thick-ply (220 μm / 240 g/m2) were investigated. The FLG modification introduces additional damage modes into the matrix and significantly increases the composites mode I and mode II inter-laminar energy release rates (ERR). The modification increases the damage initiation stress of the notched tensile specimen and the usable design space and damage tolerance. For ultra-thin-ply laminates, the notched strength is increased due to crack blunting because the FLGs initiate distributed damage on the microscale in highly stressed areas. The damage resulting from low-velocity impact (LVI) decreases with the FLG modification for all layer thickness, but only the residual compressive strength of thick-layer laminates improves. Additionally, the FLG modification leads to a considerable improvement of the compressive strength.
As another approach to influence the matrix damage, bio-inspired laminates with a helicoidal layup with a pitch angle of 2.07° were realised. Due to the small angle between the ply, the laminates suppress the formation of delamination damage and only exhibit subcritical matrix cracking before final failure. As a result, the compressive residual strength after LVI is significantly increased compared over 45°-quasi-isotropic (QI) layup, despite a lower proportion of 0}{\degree} fibres in the load direction. Due to the low portion of load oriented fibres, the unnotched tensile strength is lower than 45°-QI layups. However, the formation of helicoidal matrix damage leads to crack blunting, and as a result, almost no notch sensitivity occurs, and similar notched strengths as 45°-QI layups are achieved, despite the low portion of load oriented fibres.
Additionally, this thesis investigated the influence of temperature on LVI and the residual strength to further understand the influencing factors of damage tolerance. Because the matrix's mechanical performance decreases with increasing environmental temperature, the composite's damage behaviour is temperature-sensitive. LVIs with impaction energies between 8 J and 25 J and temperature ranging from -20 °C to 80 °C were investigated. A change in temperature leads to a substantial change in damage behaviour. With increasing temperature, the delamination area reduces and fibre-failure occurs on the impacted side, which reduces the residual tensile strength considerably. The compressive residual strength was determined at 20 °C and 80 °C. The results point out that an elevated temperature during compressive loading has a more decisive influence than the impaction energy. To further understand the influence of temperature on the impact damage and the residual strength, temperature- dependent material properties were determined and implemented into a continuum damage model (CDM) to describe CFRP's progressive failure under temperature influence. LVI simulations validated that the material model could predict the damage resulting from different temperatures and impact energies and following residual compressive strength. The results reveal that not an increase of the inter-laminar ERR cause a reduction in delamination size but the change in intralaminar damage and overall structural response.
The multi-scale nature of composites results in the occurrence of different failure modes and a complex failure behaviour where matrix failure at the micro-level influences the failure process at all higher observation levels. Consequently, the occurring matrix damage has a decisive influence on the damage tolerance. The research hypothesis of this thesis is:
Understanding and adapting the matrix's damage behaviour is fundamental to improving the damage tolerance of high-performance composites.
To adapt the matrix's damage behaviour, carbon-fibre reinforced plastics (CFRP) with a few-layer graphene (FLG) modified matrix with layer thicknesses varying from ultra-thin-ply (28 μm / 30 g/m2) to thick-ply (220 μm / 240 g/m2) were investigated. The FLG modification introduces additional damage modes into the matrix and significantly increases the composites mode I and mode II inter-laminar energy release rates (ERR). The modification increases the damage initiation stress of the notched tensile specimen and the usable design space and damage tolerance. For ultra-thin-ply laminates, the notched strength is increased due to crack blunting because the FLGs initiate distributed damage on the microscale in highly stressed areas. The damage resulting from low-velocity impact (LVI) decreases with the FLG modification for all layer thickness, but only the residual compressive strength of thick-layer laminates improves. Additionally, the FLG modification leads to a considerable improvement of the compressive strength.
As another approach to influence the matrix damage, bio-inspired laminates with a helicoidal layup with a pitch angle of 2.07° were realised. Due to the small angle between the ply, the laminates suppress the formation of delamination damage and only exhibit subcritical matrix cracking before final failure. As a result, the compressive residual strength after LVI is significantly increased compared over 45°-quasi-isotropic (QI) layup, despite a lower proportion of 0}{\degree} fibres in the load direction. Due to the low portion of load oriented fibres, the unnotched tensile strength is lower than 45°-QI layups. However, the formation of helicoidal matrix damage leads to crack blunting, and as a result, almost no notch sensitivity occurs, and similar notched strengths as 45°-QI layups are achieved, despite the low portion of load oriented fibres.
Additionally, this thesis investigated the influence of temperature on LVI and the residual strength to further understand the influencing factors of damage tolerance. Because the matrix's mechanical performance decreases with increasing environmental temperature, the composite's damage behaviour is temperature-sensitive. LVIs with impaction energies between 8 J and 25 J and temperature ranging from -20 °C to 80 °C were investigated. A change in temperature leads to a substantial change in damage behaviour. With increasing temperature, the delamination area reduces and fibre-failure occurs on the impacted side, which reduces the residual tensile strength considerably. The compressive residual strength was determined at 20 °C and 80 °C. The results point out that an elevated temperature during compressive loading has a more decisive influence than the impaction energy. To further understand the influence of temperature on the impact damage and the residual strength, temperature- dependent material properties were determined and implemented into a continuum damage model (CDM) to describe CFRP's progressive failure under temperature influence. LVI simulations validated that the material model could predict the damage resulting from different temperatures and impact energies and following residual compressive strength. The results reveal that not an increase of the inter-laminar ERR cause a reduction in delamination size but the change in intralaminar damage and overall structural response.
Subjects
Damage Tolerance
Thin-Ply
Graphene
Low-Velocity Impact
DDC Class
600: Technik
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