Influence of fiber alignment on pseudoductility and microcracking in a cementitious carbon fiber composite material

Abstract

This research examines the effect of fiber alignment on the performance of an exceptionally tough 3D-printable short carbon fiber reinforced cementitious composite material, the flexural strength of which can exceed 100 N/mm2. The material shows pseudoductility caused by strain-hardening and microcracking. An extrusion-based manufacturing process allows accurate control over the spatial alignment of the fibers’ orientation, since extrusion through a tight nozzle leads to nearly unidirectional alignment of the fibers with respect to the directional movement of the nozzle. Specimens were investigated using mechanical tests (flexural and tensile load), augmented by non-destructive methods such as X-ray 3D computed tomography and acoustic emission analysis to gain insight into the microstructure. Additionally, digital image correlation is used to visualize the microcracking process. X-ray CT confirms that about 70% of fibers show less than 10° deviation from the extrusion direction. Systematic variations of the fiber alignment with respect to the direction of tensile load show that carbon fibers enhance the flexural strength of the test specimens as long as their alignment angle does not deviate by more than 20° from the direction of the acting tensile stress. Acoustic emission analysis is capable of evaluating the spatiotemporal degradation behavior during loading and shows consistent results with the microstructural damage observed in CT scans. The strong connection of fiber alignment and flexural strength ties into a change from ductile to brittle failure caused by degradation on a microstructural level, as seen by complementary results acquired from the aforementioned methods of investigation.

Evaluation of the Behavior of Carbon Short Fiber Reinforced Concrete (CSFRC) Based on a Multi-Sensory Experimental Investigation and a Numerical Multiscale Approach

“…The results correspond well to other published data on the alignment of samples containing the thinner 'Tenax-J HT C261′ fiber [10,16], showcasing that the usage of the thicker Kreca-Chop fiber as a model system still leads to accurate results as far as alignment is concerned.…”

“…The results correspond well to other published data on the alignment of samples containing the thinner 'Tenax-J HT C261 fiber [10,16], showcasing that the usage of the thicker KrecaChop fiber as a model system still leads to accurate results as far as alignment is concerned. As can be seen, both the θ-and ϕ-angles scatter tightly around specific angles, showing the effectiveness of the alignment during extrusion.…”

“…Small-sized specimens with dimensions of 60 × 12 × 3 mm were manufactured as described in [16] and subjected to the static three-point bending tests on the universal testing machine Wolpert 10 kN, Ludwigshafen, Germany with a 500 N load cell. The test was conducted in displacement-controlled mode with a constant rate of 0.11 mm/min.…”

“…Similar to the description in [16], the dry concrete components are admixed in a BECKEL bucket mixer (Schwallungen, Germany), water and plasticizer are added and mixing is continued until nearly all agglomerates are dispersed. Finally, 1 vol.-% carbon fibers (PX35, Zoltek Corporation, Bridgeton, MO, USA) are added and the mixing process is continued shortly for 40 s twice, while in between the bucket's walls are cleared from dry fibers.…”

“…The basic CT data originate from miniature specimens in order to attain images capable of resolving the meso scale. Next to fiber detection, this method helps to discover micro matrix cracks and their development during fatigue testing, as shown in [16]. Here, we especially examine miniature specimens under flexural loading combined with sound wave-based non-destructive testing methods giving an insight into the load-bearing behavior.…”