Strain-localisation phenomena in polymer-based composites at fibre/matrix level visualised by nanoscale digital image correlation and finite element modelling
Topic(s) : Experimental techniques
Co-authors :
Elise VAN VLIERBERGHE (BELGIUM), Sarah GAYOT , Nathan KLAVZER , Christian BREITE (BELGIUM), Thomas PARDOEN (BELGIUM), Yentl SWOLFS (BELGIUM)Abstract :
Existing multiscale models for fibre-reinforced polymers (FRP) [1] currently lack experimentally validated physics-based microscale models as input parameters [2]. These computational models typically include matrix models identified and validated at the macroscale on neat polymer specimens. However, the literature points out that these matrix models may not consider the influence of confining the polymer in between fibres, thus neglecting the size effect on the matrix response, a local increase in strength [3], and the in-situ effect, the presence of an interphase [4]. Additionally, multiscale models typically include fibre models with constant properties over the cross-section, which do not account for the skin-core structure of carbon fibres [5].
A recently developed nanoscale digital image correlation (nano-DIC) technique [6] was used to determine the mechanical behaviour of FRPs at the fibre/matrix level and improve the predictive capability of current computational models. Two indium-speckled unidirectional FRPs, namely a carbon/epoxy and a glass/PMMA system, were subjected to transverse compression within a scanning electron microscope. The speckle pattern displacements were translated into corresponding radial and shear strain maps at the fibre/matrix level (Fig. 1a-b). At the fibre/matrix interface, a distinct negative strain gradient in the matrix was captured and related to an interphase due to the in-situ effect. Submicrometre shear bands were also noticed in the matrix strip between two fibres, regardless of the spatial configuration of those fibres. A skin-core structure was captured in the radial strain maps of the carbon fibres. The skin of the carbon fibres, which corresponds to 15-30 % of the radius, was identified through an increase in radial strain towards the interface – a feature absent in the case of the glass fibres.
The nano-DIC strain maps were compared with a finite element (FE) model based on matrix properties validated at the macroscale and homogenous properties over the fibre cross-section (Fig. 1c). The 3D FE model, with recreated fibre configurations, included cooling down after curing, followed by transverse compression using the displacements obtained by nano-DIC. Thermal residual stresses arise due to the mismatch of thermal expansion coefficients between the fibre and matrix materials. This addition to the model leads to strain concentrations localised around the fibres, which are also observed in nano-DIC. The comparison of the FE model and nano-DIC results revealed inconsistencies within the computational model. In particular, no radial strain increase towards the interface was observed for the carbon fibres. This effect could only be obtained by adding a skin within the fibre with a radially increasing Young’s modulus.
The nano-DIC results and the inconsistencies in the model prove the importance of reliable physics-based micromechanical models for accurate designs of composite structures.
A recently developed nanoscale digital image correlation (nano-DIC) technique [6] was used to determine the mechanical behaviour of FRPs at the fibre/matrix level and improve the predictive capability of current computational models. Two indium-speckled unidirectional FRPs, namely a carbon/epoxy and a glass/PMMA system, were subjected to transverse compression within a scanning electron microscope. The speckle pattern displacements were translated into corresponding radial and shear strain maps at the fibre/matrix level (Fig. 1a-b). At the fibre/matrix interface, a distinct negative strain gradient in the matrix was captured and related to an interphase due to the in-situ effect. Submicrometre shear bands were also noticed in the matrix strip between two fibres, regardless of the spatial configuration of those fibres. A skin-core structure was captured in the radial strain maps of the carbon fibres. The skin of the carbon fibres, which corresponds to 15-30 % of the radius, was identified through an increase in radial strain towards the interface – a feature absent in the case of the glass fibres.
The nano-DIC strain maps were compared with a finite element (FE) model based on matrix properties validated at the macroscale and homogenous properties over the fibre cross-section (Fig. 1c). The 3D FE model, with recreated fibre configurations, included cooling down after curing, followed by transverse compression using the displacements obtained by nano-DIC. Thermal residual stresses arise due to the mismatch of thermal expansion coefficients between the fibre and matrix materials. This addition to the model leads to strain concentrations localised around the fibres, which are also observed in nano-DIC. The comparison of the FE model and nano-DIC results revealed inconsistencies within the computational model. In particular, no radial strain increase towards the interface was observed for the carbon fibres. This effect could only be obtained by adding a skin within the fibre with a radially increasing Young’s modulus.
The nano-DIC results and the inconsistencies in the model prove the importance of reliable physics-based micromechanical models for accurate designs of composite structures.