To obtain mechanical tensile properties of materials it is customary to equip the specimen directly with a device to measure strain and Young’s modulus correctly and only within the measuring length defined by the standards. Whereas a variety of tools such as extensometers, strain gauges and optical systems are available for specimens on coupon level, no market-ready tools to measure strains of single fibres during single fibre tensile tests are available. Although there is a standard for single fibre testing, the procedures described there are only capable of measuring strains of the whole testing setup rather than the strain of the fibre. Without a direct strain measurement on the specimen, the compliance of the test rig itself influences the determination of the Young’s modulus. This work aims to fill this gap by establishing an enhanced method to measure strains directly on the tested fibre and thus provide accurate values for Young’s modulus. It is demonstrated that by applying and then optically tracking fluorescing polymeric beads on single glass fibres, Young’s modulus is determined directly and with high repeatability, without a need to measure at different measuring lengths or compensating for the system compliance. Employing this method to glass fibres, a Young’s modulus of approximately 82.5 GPa was determined, which is in the range of values obtained by applying a conventional procedure. This enhanced measuring technology achieves high accuracy and repeatability while reducing scatter of the data. It was demonstrated that the fluorescing beads do not affect the fibre properties.

The term downcycling is often used anecdotally to describe imperfections in recycling. However, it is rarely defined. Here, we identify six meanings of the term downcycling as used in scientific articles and reports. These encompass the material quality of reprocessed materials, target applications, product value, alloying element losses, material systems, and additional primary production. In a proposal for harmonized and more specific terminology, we define downcycling as the phenomenon of quality reduction of materials reprocessed from waste relative to their original quality. We further identify that the reduced quality can express itself thermodynamically, functionally, and economically, covering all perspectives on downcycling. Dilution, contamination, reduced demand for recycled materials, and design-related issues can cause those downcycling effects. We anticipate that this more precise terminology can help quantify downcycling, keep materials in the loop longer, use materials more often and at higher quality, and therefore assist in reducing material-related environmental impacts.

No abstract

The Young's modulus of single carbon fibres, virgin and recycled, is investigated in this study. Single fibre measurements are enhanced in two fields: firstly, the cross-sectional area is calculated by taking individual measurements with a laser diffraction sensor and applying a sinusoidal fit to account for irregular fibre shapes. Secondly, strains during the loading cycle are measured directly by applying fluorescing tracking markers on the fibre and using one-dimensional digital image correlation. Several markers are applied on each side of the fibre to allow for a statistically verified evaluation. To achieve high-resolution images, a step-wise load cycle is applied. The load is increased in steps of 250 MPa until final failure. It is demonstrated that direct strain measurement and enhanced determination of the Young's modulus are possible for carbon fibres. It was further shown that the recycling of the carbon fibre by pyrolysis reduced the Young's modulus by approximately 10% compared to virgin fibres.

Laser diffraction is a commonly used tool to measure the fibre diameter of carbon fibres prior to mechanical testing. However, non-circularities of carbon fibres need to be considered in order to minimise measuring errors. As the work at hand demonstrates, using a single measurement of the fibre diameter may cause deviations as high as 30% from a computationally determined value. It appears that the error can be minimised by acquiring a data set of several apparent diameters as a function of the angle around the fibre axis. Based on this data, the cross-sectional area can be calculated as a circle with an averaged diameter or as an ellipse by applying an elliptical fitting procedure.

The mechanical properties of plastic-based additively manufactured specimens have been widely discussed. However, there is still no standard that can be used to determine properties such as the interfacial strength of adjacent tracks and also to exclude the influence of varying manufacturing conditions. In this paper, a proposal is made to determine the interfacial strength using specimens with only one track within a layer. For this purpose, so-called single-wall specimens of polylactide were characterised under tensile load and the interfacial area between the adjacent layers was determined using three methods. It turned out that the determination of the interfacial area via the fracture surface is the most accurate method for determining the interfacial strength. The measured interfacial strengths were compared with the bulk material strength and it was found that the bulk material strength can be achieved under optimal conditions in the FFF process. It was also observed that with increasing nozzle temperature, the simultaneous printing of specimens influences the interfacial strength. To conclude, this method allows to measure the interfacial strength without superimposing the influence of voids. However, for example, the interfacial strength within a layer cannot be determined.

The increasing demand for composites leads to a growing amount of end-of-life materialand production waste. The latter consists of a large fraction of unimpregnated fibre waste which is notsufficiently reprocessed using conventional textile processing procedures as they are either too expensiveor their mechanical performance is too low. Using pieces of dry non-crimp fabrics (patches) ina Bulk Moulding Compound process (BMC) displays a straightforward approach of fabric recycling.Adding fillers to the mixture not only offers the opportunity to modify mechanical and electrical propertiesas well as the costs but also a chance for a more holistic approach of dry fibre recycling, whenconventional fillers like chalk are replaced by ground recycled carbon fibres. In this way, all kindof dry fibre wastes can be reused in one process: Larger offcuts are chopped to smaller rectangularpatches whereas waste fractions of small offcuts are processed to carbon fibre powder as filler andprocessed together with resin to produce BMC materials. Mechanical investigations reveal that thepresented approach shows higher specific properties than the conventional filler without compromisingthe process and material quality.

No abstract