The effect of cooling rate on the interlaminar bond strength of continuous fibre reinforced thermoplastic composites
Topic(s) : Manufacturing
Co-authors :
Jiakuan ZHOU (BELGIUM), Frederik DESPLENTERE (BELGIUM), Jan IVENS (BELGIUM)Abstract :
Interlaminar bond strength development is crucial in the processing of continuous fibre reinforced thermoplastic composites. It gradually builds with the interdiffusion and entanglement of polymer chains, thus deemed to be dominated by the polymer matrix traditionally. In this research, a systematic analysis of the effect of cooling rate on the interlaminar bond strength was conducted from the perspective of material properties and mechanics, respectively.
Compression-moulded glass fibre reinforced polypropylene (GF/PP) [0°]4 laminates were made with different cooling rates, and the bond strength was characterised by wedge peel tests. Results showed the bond strength was not affected by the processing temperature with a 10-minute holding time. However, there was a significant bond strength increase (~100%) for fast-cooled GF/PP compared with slowly-cooled counterparts.
A natural explanation for this, from the perspective of material properties, is the different crystallisation behaviour under different cooling. XRD and DSC analysis demonstrated that fast cooling resulted in a lower degree of crystallinity, finer spherulites and smaller spherulitic size, and the formation of β-PP. All those commonly contributed to a more ductile matrix.
However, the delaminated surfaces still showed differences for different cooling, though they all ended up with matrix failure close to the glass fibre surfaces. An underlying but more direct explanation may start from the perspective of microstructures and mechanics, considering the mode I crack in wedge peel tests. To capture the (near) fibre/matrix morphology, chemical etching was performed to preferentially remove the amorphous PP and expose the crystalline spherulites, revealing the fibre-nucleation effect and different lamella growing tendencies under different cooling rates. Consequently, different interfacial mechanical properties and crack propagation paths were triggered by different cooling rates. Naturally, the mode I crack propagated along the weaker one between the fibre/matrix interface or grain boundary. Fast cooling resulted in an interlocked grain boundary as the crack path, indicating a higher fibre/matrix adhesion in combination with more amorphous PP at the glass fibre surface. While slow cooling tends to form a transcrystalline structure and accordingly flat and smooth grain boundary as the crack path, and worse fibre/matrix adhesion. This was further corroborated by nanoindentation on regions near glass fibre and resin pocket regions respectively, and by single-fibre pull-out tests on samples made with different cooling rates.
In summary, this research illustrates the differences in the fibre/matrix morphology and adhesion induced by different cooling rates, and emphasises the corresponding consequences on the bond strength. In this way, it builds the relationship between interlaminar shear strength and fibre/matrix interfacial shear strength, which was not clearly demonstrated in the past.
Compression-moulded glass fibre reinforced polypropylene (GF/PP) [0°]4 laminates were made with different cooling rates, and the bond strength was characterised by wedge peel tests. Results showed the bond strength was not affected by the processing temperature with a 10-minute holding time. However, there was a significant bond strength increase (~100%) for fast-cooled GF/PP compared with slowly-cooled counterparts.
A natural explanation for this, from the perspective of material properties, is the different crystallisation behaviour under different cooling. XRD and DSC analysis demonstrated that fast cooling resulted in a lower degree of crystallinity, finer spherulites and smaller spherulitic size, and the formation of β-PP. All those commonly contributed to a more ductile matrix.
However, the delaminated surfaces still showed differences for different cooling, though they all ended up with matrix failure close to the glass fibre surfaces. An underlying but more direct explanation may start from the perspective of microstructures and mechanics, considering the mode I crack in wedge peel tests. To capture the (near) fibre/matrix morphology, chemical etching was performed to preferentially remove the amorphous PP and expose the crystalline spherulites, revealing the fibre-nucleation effect and different lamella growing tendencies under different cooling rates. Consequently, different interfacial mechanical properties and crack propagation paths were triggered by different cooling rates. Naturally, the mode I crack propagated along the weaker one between the fibre/matrix interface or grain boundary. Fast cooling resulted in an interlocked grain boundary as the crack path, indicating a higher fibre/matrix adhesion in combination with more amorphous PP at the glass fibre surface. While slow cooling tends to form a transcrystalline structure and accordingly flat and smooth grain boundary as the crack path, and worse fibre/matrix adhesion. This was further corroborated by nanoindentation on regions near glass fibre and resin pocket regions respectively, and by single-fibre pull-out tests on samples made with different cooling rates.
In summary, this research illustrates the differences in the fibre/matrix morphology and adhesion induced by different cooling rates, and emphasises the corresponding consequences on the bond strength. In this way, it builds the relationship between interlaminar shear strength and fibre/matrix interfacial shear strength, which was not clearly demonstrated in the past.