Predicting the fatigue life and introducing structural health monitoring in cord reinforced rubber composites
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Fatigue behaviour of cord-rubber composites is of great interest as components based on these materials are often subjected to cyclic loading during in-service conditions. Unfortunately, there appears a lack of scientific literature addressing this topic compared to that of fibre reinforced plastics (FRPs). First conventional wedged grips were evaluated as a means to measure the tensile fatigue behaviour of single carbon cord reinforced hydrogenated nitrile butadiene rubber (CC-HNBR) composite samples. However, in these tests, failure was dominated by shear induced interfacial failure mechanisms (debonding), rather than cord fracture. Consequently, an alternative fatigue test for measuring the cord-dominated failure of cord-rubber composites was developed using bollards to introduce the load without creating premature failure near the gripping area. Later, frequency effect and R ratio dependence were investigated based on a comprehensive characterisation of non-relaxing tensile fatigue of the CC-HNBR composites. Frequency has negligible effects on the fatigue life within testing regimes (2, 5, 10 and 20 Hz) due to a very limited self-heating of the rubber matrix as detected using the thermal imaging. Higher R ratios tend to increase the fatigue life due to possible crack tip blunting as a consequence of strain induced crystallisation (SIC) of the HNBR matrix. Life predictions of those model composites were made by various constant life diagram (CLD) models. Attempts were further made by modifying Harris’s CLD and their applicability were examined. It showed that predictions made by the modified Harris’s CLD and piecewise linear CLD were in a good agreement with experimental data at different R ratios. II A novel experimental set-up was described that replicated in a simplified way the real-pulley situation encountered under typical service conditions was used to investigate the effect of the bending curvature, frequency and R ratio effects on the life-time of the CC-HNBR composite subject to coupled tension and bending conditions. Cord-dominated fracture occurred close to the point where the specimen just left the pulley as detected using thermal imaging due to the combined effects of bending and maximum tension at this site. There was a reduction in fatigue life as a result of a tighter bending radius of the pulley which created a higher level of bending strain whilst similar effects of R ratio and frequency were observed to those in tensile fatigue. Finally, efforts were made to develop smart self-sensing glass cord rubber composites with integrated damage detection capabilities. A simple swelling and infusion method was developed to incorporate carbon nanotubes (CNTs) into the existing elastomeric adhesive coating of glass cords. Conductive CNT-infused glass cords with good self-sensing functions were achieved without affecting the bonding provided by the coating with rubber matrix. The effectiveness of using these smart cords as interfacial damage sensors in cord-rubber composites was demonstrated under static and cyclic loading. It showed the possibility to identify both reversible deformation and irreversible interfacial damage. The simplicity of the proposed swelling and infusion methodology provides great potential for large-scale industrial production or modification of CNT functionalised elastomeric products such as cord-rubber composite.
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