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Nicolas Krumenacker and Pascal Hubert specimens cut per sample beam, five specimens are reserved for dimensional analysis and subsequent mechanical testing, and an additional specimen is reserved to conduct a representative corner laminate quality assessment (micro-CT). 2.3 Corner consolidation and porosity assessment Corner specimens destined for mechanical testing are scanned at high resolution (> 1500 dpi) on a standard Canon home-office scanner; all specimens of a given sample are scanned together. A custom Matlab code was developed to process the scanned images, recognise and segment the various corners in each image, and effectively detect the convex and concave edges of each corner for subsequent analysis. The output is a mean (or average) thickness profile for each sample as a function of the concave, tool-side edge, which is designated as the reference edge for convex corners. In addition, the code computes the dimensions necessary for the analysis of curved beam strength (CBS) mechanical testing data as well as the mean percent corner thickness deviation for each sample. Besides obvious practical reasons, the true benefits of this dimensional analysis tool are i) to remove human error from thickness measurements—particularly in the corner, and ii) to generate a complete thickness profiles in the corner regions as opposed to relying on micrometer point measurements. A select few specimens are additionally analyzed via high-resolution x-ray micro-computed tomography (micro-CT) using a Skyscan 1172 High Resolution micro-CT unit in order to complement the above thickness deviation analysis and examine the local laminate quality in terms of local macro- porosity at corners. The selected specimens are trimmed (25 mm flange length and 18 mm width) using a lab-scale Struers precision lab-bench saw fitted with a composite-specific diamond precision cut-off blade. The trimmed section are then x-rayed over a 180° span at 0.28° intervals with the following radiographic parameters: no filter, voltage source of 40 kV, intensity of 250 μA intensity, image size of 4000 x 2096 pixels, and pixel resolution of 4.96 μm. The x-ray images are reconstructed in 3D using equipment-specific software for post-analysis and viewing. It should be noted that it is impractical to visualize micro-voids and to calculate accurate void contents given current equipment and protocol limitations. Nevertheless, pertinent qualitative insights may still be drawn for estimating the extent of macro-porosity. 2.4 Curved beam strength testing ASTM D6415 mechanical testing standard is selected to assess the effect of processing deficiencies on the interlaminar tensile behaviour of the corner specimens. This standard is one of the few recognized standards to be devised specifically for the mechanical testing of corner laminates. One of the clear advantages is that the corners are loaded under four-point bending with no end-loading. A pure bending moment is generated in the corner that results in a simpler stress state with no interlaminar shear. In addition, the pure bending moment induces an interlaminar tensile stress that is nearly twice that induced by an equivalent end-load [23]. A second advantage is the ease and speed of testing. The general property generated by this test and used in this work is the curved beam strength (CBS), which is “the moment per unit width (M/w) applied to the curved test section that causes a sharp decrease in applied load or delamination(s) to form” [24]. Mechanical testing is conducted on a MTS Insight electromechanical testing frame equipped with a 5 kN load cell. Corner specimens are placed in the standard CBS fixture build by Wyoming Test Fixtures Inc. specifically for ASTM D6415, as illustrated in Figure 2B. A loading rate of 1 mm/min is applied (twice the prescribed rate) in order to ensure specimen failure within 10 min. The curved beam strength is then calculated for each specimen per the standard at the last data point preceding the first load drop. The mean CBS is finally calculated for each sample. To complement the mechanical testing, one failed specimen is selected per sample that exhibits a representative overall failure mode. Each selected specimen is photographed at higher magnification using a Dino-Lite Edge portable digital microscope to reveal the extent of delamination. A small vice is used to slightly bend the open the specimen angle in order to render the delamination(s) more visible.PDF Image | VACUUM-BAG-ONLY COMPLEX-SHAPE PREPREG LAMINATE
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