Mapping of local fibre strain in carbon fibre-reinforced polymers using X-ray diffraction
Topic(s) : Special Sessions
Co-authors :
Sina AHMADVASHAGHBASH (BELGIUM), Babak FAZLALI , Matthias BÖNISCH (BELGIUM), Christophe LE BOURLOT (FRANCE), Eric MAIRE , Christian BREITE (BELGIUM), Mahoor MEHDIKHANI (BELGIUM), Yentl SWOLFS (BELGIUM)Abstract :
State-of-the-art fibre break models overpredict the fraction of diffuse fibre break clusters (those not sharing the same break plane) and underpredict the fraction of coplanar ones. There is currently no convincing theoretical explanation for this observation. This supports the idea that we do not yet fully understand how the real load sharing occurs between broken and intact fibres inside a composite. To unravel this question, we need to gain access to in-situ 3D strain measurements inside fibres. This research provides a possible solution for carbon fibres using microscale 2D/3D mapping of strains by X-ray diffraction (XRD), complemented by computed tomography (CT). The conducted experiments produced new insights into local strain distributions around fibre breaks, thereby allowing the validation of the current load sharing models.
Different specimens were designed and manufactured. Firstly, model micro composites were produced, whereby 1, 2 or 3 carbon fibres were embedded in epoxy resin. The specimen thickness was reduced to 300 μm through polishing to reduce the epoxy background in the diffraction signal. Secondly, thin-ply composite specimens with a [90,0]s layup were manufactured. The idea of this specimen type was to assess whether strains can be measured inside a regular composite using the same XRD strain mapping technique coupled with the reference data from the model composite cases. All specimens underwent incremental strain tests using a customised Deben CT500 loading rig at the ID11 beamline, European Synchrotron Radiation Facility (ESRF). Testing of the model composites was complemented by optical observation to identify fibre break locations through the semi-transparent epoxy resin and to then laterally position the field of view for the following scans. Post-break tomography (see figure) was used to adjust the positions for detailed XRD analysis. This procedure was repeated for higher load levels until the failure of the specimen. The displacement increments were chosen to be large enough to provide noticeable differences in strain levels. To validate the XRD measurements, a small quantity of ceramic particles was distributed in the epoxy resin of the model composites. By using digital image correlation on the CT slices, these random markers allowed us to compute the local strain field and compare it to the XRD data.
The experiments aim to measure the strain recovery length in broken fibres and the exerted strain concentration profiles in the intact neighbours. To map strain via XRD, the data analysis consists of different steps focusing on analysing chosen Regions of Interest (ROIs) around fibre breaks. Strain measurements for a selection of ROIs are displayed in subfigure (d). By assuming a zero strain where the fibre is broken, the data analysis indicates a plateauing strain around 500 µm away on each side of the fibre break. This new technique has enormous potential for validating and refining micromechanical models.
Different specimens were designed and manufactured. Firstly, model micro composites were produced, whereby 1, 2 or 3 carbon fibres were embedded in epoxy resin. The specimen thickness was reduced to 300 μm through polishing to reduce the epoxy background in the diffraction signal. Secondly, thin-ply composite specimens with a [90,0]s layup were manufactured. The idea of this specimen type was to assess whether strains can be measured inside a regular composite using the same XRD strain mapping technique coupled with the reference data from the model composite cases. All specimens underwent incremental strain tests using a customised Deben CT500 loading rig at the ID11 beamline, European Synchrotron Radiation Facility (ESRF). Testing of the model composites was complemented by optical observation to identify fibre break locations through the semi-transparent epoxy resin and to then laterally position the field of view for the following scans. Post-break tomography (see figure) was used to adjust the positions for detailed XRD analysis. This procedure was repeated for higher load levels until the failure of the specimen. The displacement increments were chosen to be large enough to provide noticeable differences in strain levels. To validate the XRD measurements, a small quantity of ceramic particles was distributed in the epoxy resin of the model composites. By using digital image correlation on the CT slices, these random markers allowed us to compute the local strain field and compare it to the XRD data.
The experiments aim to measure the strain recovery length in broken fibres and the exerted strain concentration profiles in the intact neighbours. To map strain via XRD, the data analysis consists of different steps focusing on analysing chosen Regions of Interest (ROIs) around fibre breaks. Strain measurements for a selection of ROIs are displayed in subfigure (d). By assuming a zero strain where the fibre is broken, the data analysis indicates a plateauing strain around 500 µm away on each side of the fibre break. This new technique has enormous potential for validating and refining micromechanical models.