Piezoresistive poly(vinylidene fluoride) (PVDF) nanocomposites are very intriguing for strain sensor applications in structural health monitoring (SHM) systems. In general, high piezoresistive sensitivity combined with broad measurable strain ranges are greatly favored in those sensors. Here, a facile strategy, i.e. constructing strain susceptible conductive networks using hybrid filler systems consisting of carbon nanotubes (CNTs, 0.5-1 wt %) and carbon black (CB, 0.5-4 wt %), was introduced to tune both electrical conductivity and piezoresistive sensitivity of melt mixed PVDF nanocomposites. At the same filler content CNTs, due to their larger aspect ratio, contribute more to electrical conductivity improvements of nanocomposites than CB, while contacts between CB particles are more sensitive to tensile strain. With retained ductility of PVDF, tunable electrical conductivity and ΔR/R0-strain sensitivity can be achieved by combining the advantages of CNTs and CB by adjusting the conductive network structure. Conductivity improvement is more remarkable if the mass ratio of CNTs to CB (mCNTs/mCB), varied between 1:1 and 1:4, is higher in hybrid filler compositions. Lower mCNTs/mCB ratios result in higher ΔR/R0 values in PVDF nanocomposites whether they have the same content of total filler or similar/the same initial electrical resistivity. At 10% tensile strain, the highest ΔR/R0 of 0.65 was obtained for the nanocomposite filled with 0.5 wt % CNTs and 0.5 wt % CB, while that for the counterpart containing 1 wt % CNTs is 0.35 at the same strain. The concept of using hybrid fillers provides a low-cost and effective way to fabricate piezoresistive polymer nanocomposites toward SHM applications.

Carbon nanotube dispersions have been immersedin a maleated polypropylene film former system andapplied to insulating glass filament yarns compatible to polypropylene matrix. The percolation threshold can be reached below 0.5 wt% carbon nanotubecontent. In order to achieve a homogeneous multifunctional interphase, the average sizing thickness must be above 450 nm tailored during a coating process by varying the solid content of the sizing. The temperature and the velocity of the coating process have a great impact on the volume resistivity of the multifunctional interphase. Electro-mechanical behavior as sensors in unidirectional composites containing 50 vol% glass fiber reinforcement is monitored during tensile and compression loading. The largest electrical sensitivity is achieved for tensile loading in the axis of fibers.

Online commingled yarns were spun with three different polymeric matrices, namely polypropylene (PP), polyamide (PA) and polylactic acid (PLA) and glass fibers. Tailored sizings were applied for the three matrices and the resulting mechanical performance of unidirectional composites was evaluated and compared. Significant improvements in the fiber/matrix bonding were achieved by employed sizing chemistry in order to achieve multifunctional interphases. The pure silane coupling agents provide the best performance for all matrices investigated. However, an additional film former has to be added in order to achieve fiber processing. Film formers compatible to the matrices investigated were adapted. The consolidation behavior during isothermal molding was investigated for polypropylene matrix. Different fiber volume contents could be realized and the resulting mechanical properties were tested.

The electrical contacting of an embedded sensoryarn in a unidirectional glass fiber polypropylene (GF/PP) composite realizedby laser processing was investigated. It was shown that compared to thecircular drilled hole the same electrical contacting quality can be achieved atan increased mechanical performance level.

In this comparative study two sensor yarns, such as carbon filament yarns and carbon nanotube coated glass filament yarns are analyzed regarding their electrical property changes during textile manufacturing into multi-layer weft knitted structures as well as during compression molding into a composite plate due to the different electro-mechanical behavior, various sensitivities, and changing ways of acting as an in-situ sensor: yarns act either as a conductive fiber itself (carbon) or as an electrical conductive sizing (CNT-coated glass). Both sensor yarns under investigation have to be considered differently with regard to changes in the electrical conductivity due to mechanical loading during textile processing and possible healing of cracks during compression molding.