JEC COMPOSITES MAGAZINE - Issue #113 - June 2017 - 65

X-ray computed tomography for assessment of mechanical properties and monitoring damage

Fig. 3: Sample analysis using the
helical µCT detection mode [12]

Mapping volume strain distribution in an adjacent flow weld
line: combining helical µCT and full-scale FEM
The second contribution of this work was to study the influence of fibre
arrangement on strain localization in the vicinity of a weld line. A second configuration of injection-moulded structural samples containing
two neighbouring injection gates was used. This approach was based
on ex-situ μCT acquisitions and full-scale modelling. In this context,
we report the use of helical cone-beam µCT along the weld line that
was used to cover a 20 millimetre-long domain at a spatial resolution
of 3 microns during ten hours (Figure 3). Then, µCT 3D images were
used as an input for full-scale FE modelling to assess the heterogeneous
3D strain fields. Regular structured meshing from scaled images was
defined by cuboids of 8 nodes with structural displacements for each
node in the X, Y and Z directions. The model size reached 105 million degrees of freedom. Periodic boundary conditions were applied
to assess nodal mechanical fields. Finite element runs were performed
on a 24-core workstation with 192 GB of RAM. The main hypothesis
assumed was the continuity of the traction and displacement across the
perfect interface between the fibres and the matrix. Numerical strain
fields were compared to experimental fields obtained by digital image
correlation (DIC) during uniaxial tensile tests.
The combined qualitative analysis of microstructure features in the
weld line and 3D full-scale computation revealed the presence of a high
volume fraction of out-of-plane oriented fibres in the weld line before
the first contact point. Complementarily, these zones were coincident
with locations of high strain levels at the macro-scale revealed by DIC
during in-plane tensile tests. Superior results were achieved by full-scale
modelling since DIC based on optical imaging is limited to address microscopic heterogeneity represented by traces of fibre orientation [12].
Quantitative analysis of damage kinetics
based on in-situ tests
SEM-based analyses
In this context, a "stair-step" plate equipped with a large injection gate
was used. The acquisition protocol was applied to different samples
cut at different orientations compared to the main flow direction.
Qualitative and quantitative analyses of local damage in a field of heterogeneous fibre distributions were performed. First, discontinuous
in-situ tensile tests coupled with scanning electron microscopy (SEM)
were conducted. The (2D) surface observations were used to assess
the damage scenario of the studied material as well as the characteristic lengths of each damage mechanism. A home-designed experimental procedure was successfully used to monitor surface changes in

Fig. 4: Quantitative evaluation of surface damage based on SEM in-situ discontinuous tensile tests [3]

a SEM chamber using two detection modes based on back-scattered
electrons and secondary electrons. Combining these two acquisition
modes made it possible to extend the use of 2D SEM scans from classical qualitative observations to the quantification of damage evolution
(Figure 4) [3].
Due to the surface aspect of SEM analyses, the range of investigation
was limited. Complementary to surface observations, the assessment
of damage kinetics in SFRPs should be based on 3D observations considering the following aspects:
* Injection-moulded SFRPs have a 3D multi-layered structure. Thus,
damage investigation should potentially include the contribution of
the total sample thickness.
* Damage evolution laws should provide information about the initiation of new damage sites and expansion of existing cavities. Since
cavities have 3D morphologies with different degrees of complexity (at
fibre tips, interfacial domains, matrix cavities), the number of separate
damage sites detected on the surface cannot provide a reliable insight
about 3D aspects of damage.
Based on these considerations and the characteristic damage aspects
identified based on SEM micrographs, there was a need to use volumetric characterization techniques such as X-ray µCT.
No113 June 2017

/ jec composites magazine

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