The article reports on the influence of annealing PVDF in an autoclave process on the PVDF phase composition. DSC, FTIR and XRD measurements serve to observe the phase changes in an already stretched, polarised and β-phase rich film. Annealing was conducted between 90 and 185 ∘C to cover a broad range of curing processes in an autoclave. The β-phase is found to be stable up to near the melting range at 170 ∘C. At 175 ∘C, the non-piezoelectric α-phase dominates and the piezoelectric γ- and γ′-phases appear. The γ-phase grows at elevated temperatures and replaces the β-phase. This observation stresses the importance of developing new methods to reactivate the polarisation after annealing, in particular for the integration of PVDF as a sensor in laminated structures, such as CFRP.
Disbond arrest features combined with a structural health monitoring system for permanent bondline surveillance have the potential to significantly increase the safety of adhesive bonds in composite structures. A core requirement is that the integration of such features is achieved without causing weakening of the bondline. We present the design of a smart inlay equipped with a micro strain sensor-system fabricated on a polyvinyliden fluorid (PVDF) foil material. This material has proven disbond arrest functionality, but has not before been used as a substrate in lithographic micro sensor fabrication. Only with special pretreatment can it meet the requirements of thin film sensor elements regarding surface roughness and adhesion. Moreover, the sensor integration into composite material using a standard manufacturing procedure reveals that the smart inlays endure this process even though subjected to high temperatures, curing reactions and plasma treatment. Most critical is the substrate melting during curing when sensory function is preserved with a covering caul plate that stabilizes the fragile measuring grids. The smart inlays are tested by static mechanical loading, showing that they can be stretched far beyond critical elongations of composites before failure. The health monitoring function is verified by testing the specimens with integrated sensors in a cantilever bending setup. The results prove the feasibility of micro sensors detecting strain gradients on a disbond arresting substrate to form a so-called multifunctional bondline.
In comparison to bolted joints, structural bonds are the desirable joining method for light-weight composite structures. To achieve a broad implementation of this technology in safety critical structures, the issues of structural bonds due to their complex and often unpredictable failure mechanisms have to be overcome. The proposed multifunctional bondline approach aims at solving this by adding two safety mechanisms to structural bondlines. These are a design feature for limiting damages to a certain size and a structural health monitoring system for damage detection. The key question is whether or not the implementation of both safety features without deteriorating the strength in comparison to a healthy conventional bondline is possible. In previous studies on the hybrid bondline, a design feature for damage limitations in bondlines by means of disbond stopping features was already developed. Thus, the approach to evolve the hybrid bondline to a multifunctional one is followed. A thorough analysis of the shear stress and tensile strain distribution within the hybrid bondline demonstrates the feasibility to access the status of the bondline by monitoring either of these quantities. Moreover, the results indicate that it is sufficient to place sensors within the disbond stopping feature only and not throughout the entire bondline. Based on these findings, the three main working principles of the multifunctional are stated. Finally, two initial concepts for a novel multifunctional disbond arrest feature are derived for testing the fundamental hypothesis that the integration of micro sensors into the disbond stopping feature only enables the crack arrest and the health monitoring functions, while reaching the mechanical strength of a conventional healthy epoxy bondline. This work therefore provides the fundamentals for future investigations in the scope of the multifunctional bondline.
Collective absorption, so far determined by numerical simulations, is explained in physical terms for cold and warm plasma. After deducing a few general relations for flat targets, interface phase mixing and nonadiabatic electron acceleration in the skin layer are identified as the main physical processes leading to irreversibility, that is, to absorption.
Use of silane coupling agents enables the significant improvement of the performance of thermoplastic composites. Water-borne silane coupling agents may offer a more environmentally friendly alternative. Compositions of nylon 66 with silane and water-borne silane treated mineral fillers: clay, talc, mica and wollastonite were prepared and comparatively evaluated. Filler dispersion, mechanical properties, and thermoplastic matrix-mineral substrate interface were studied. I t may be gathered from the results of the study that water-borne silanes by themselves or in combination with some additives exhibit equal or better performance than regular silanes.
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