Investigation of self-heating and damage progression in woven carbon fibre composite materials, following the fibres direction, under static and cyclic loading

Infrared thermography is commonly used as a non-destructive testing technique for damage monitoring of composite materials under mechanical loadings. Self-heating tests consist in monitoring the stabilized heating of a material submitted to cyclic loading for increasing values of stress level. It appears that the load threshold from which the thermal behaviour changes can be related to the fatigue limit of the tested material. In this paper, this stress threshold is compared to the heating of a woven thermoplastic composite material submitted to a monotonic tensile test. Indeed, during a quasi-static tensile test, the material temperature cools down, due to thermoelastic effect, before warming up again, due to both viscous effects and first damage evolutions. The comparison, which is made for the warp direction only, is also based on microscopic optical scanning of the specimen edge and passive acoustic monitoring. It is shown that thermal changes detected in the composite samples are associated with damage occurring under both static and cyclic loading, for similar stress levels. This result indicates that the static tests make it possible to estimate a damage threshold, therefore a potential fatigue limit, even faster than with self-heating tests, which opens very promising prospects as for the determination of the fatigue limit of woven composite materials reinforced by carbon fibre yarns.

Novel Experimentation for the Validation of Mechanistic Models to Describe Cold Dwell Sensitivity in Titanium Alloys

Previous mechanistic models, proposed to explain the process of damage accumulation and stress redistribution between strong and weak regions inherent within the microstructure of α/β and near α titanium alloys, are validated through a matrix of experiments employing a non-standard variant of the alloy Ti 685. The grain size of the model material was deliberately processed to offer grains up to 20 mm in diameter, to facilitate constitutive measurements within individual grains. A range of experiments were performed under static and cyclic loading, with the fatigue cycle conducted under either strain or load control. Data will be reported to demonstrate significant variations in elastic and plastic properties between grains and emphasise the role of time dependent strain accumulation. Implications for the “dwell sensitive fatigue” or “cold creep” response of conventional titanium alloys will be discussed.

Full-scale testing of adhesively bonded timber-concrete composite beams

This paper presents new findings on adhesively bonded timber-concrete composites with prefabricated concrete parts. Hereby, timber and concrete are bonded solely with adhesive and no metallic connectors have been used. Because the achievement of a continuous bond proved to be a critical point in past studies, special attention is given to that issue. The application procedure of the adhesives is investigated in small-scale bond samples and the manufacturing process in full-scale composite beams with a span of 8 m and a comparatively new polymer mortar is used as adhesive as well as a common epoxy resin. Both adhesives proved to be suitable, although polymer mortars showed strong advantages in terms of applicability and bridging of gaps in comparison to the less viscous epoxy adhesive. The full-scale beams are tested under quasi-static and cyclic loading. The failure occurred more as a bending failure of the timber or compression failure of the concrete. A full bond could be achieved at all full-scale beams. Moreover, an analytical and a finite element model for the calculation of composite beams are presented and validated. It could be seen, that both deformation behavior and failure load are in good accordance with the test results.