Development of stretchable sensors for enhanced shear characterization of woven fabric composites - UBC Library Open Collections
Shear is known to be the dominant mode of deformation in woven fabric composites when forming complex, doubly-curved parts. Picture frame (PF) testing has been widely employed in the literature to quantify the mechanical properties of woven fabrics under shearing, albite a number of uncertainty factors present during this test set-up. In the present study, common sources of such uncertainties (imperfections) are first classified and their effects on characterization data are studied, through both analytical and numerical approaches. Namely, the mechanical response of fabrics under PF testing is recognized to be highly sensitive to the imperfections stemmed from either misplacing the fabric into the fixture, or deviation of fixture from an ideal shearing frame. In addition, to prevent the fabric/fixture contacts, corner cut-offs are usually introduced to the fabric by experimenter during PF testing, exacerbating the shear angle mismatches between the sample center and fixture frame. Upon the above general, theoretical assessment of PF test imperfections, their adverse effects on shear characterization of fabrics have been experimentally demonstrated through novel sensors integrated within the yarns of a typical polypropylene/glass plain weave, capturing the local tensile deformation of the material during the shear tests. Finally, a novel frameless picture frame (FPF) shear testing approach was introduced for mitigating of imperfections as seen in the conventional PF test. In the new set-up, the frame is inscribed on the sample itself by locally consolidating the fabric at regions corresponding to the clamping areas in the conventional PF. A high controllability was realized over the sample during preparation and installation phases of the new test set-up. Furthermore, high shear strain deformation behavior as well as normalized force-shear angle response of the fabric was assessed during the FPF testing and was proven to be more consistent than the PF test, by means of eliminating sample size effects and significantly improving the data repeatability under cyclic loads. The embedded PDMS/CCF sensor into the fabric further confirmed the performance of the new test by monitoring local yarn strain level; it was significantly lower in a needle integrated version of FPF test when compared to the PF test.
