Electromagnetic emission (EME) testing and acoustic emission (AE) testing are applied to investigate the failure of a brittle, dielectric material under mechanical load. A setup for three point flexure tests comprising simultaneous monitoring of EME and AE was used to induce fracture of epoxy resin specimens. The influences of the orientation and the distance of the crack surface on the detectable EME signals are the subjects of investigation. As EME sensor a capacitive sensor was used. Tests with an artificial test source are carried out to characterize the system response of the sensor, the attached amplifier and acquisition cards as well as the included bandpass filters. We propose an EME source based on the surface charge density modelled at the position of the fracture plane. Results of finite element method modelling of the EME source are compared to experimental results and show very good agreement. The experimental results show a clear directional character of the emitted electromagnetic field and a strong dependence of the detected signals amplitude on sourcesensor distance. A significant influence of the measurement chain on the detected electromagnetic signals bandwidth was found. Furthermore it is shown that the electromagnetic signals consist of three contributions originating from different source mechanisms. These are attributed to the separation and

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.

By using a novel fiber-reinforced cement paste in a 3D-printing process, a material with vastly improved flexural and tensile strength can be created. The extrusion-based process leads to a high degree of fiber orientation. A fiber content of 3 vol. % results in static flexural strengths of above 100 MPa. High-strength fiber-reinforced materials have potential to be used in lightweight construction. Due to the susceptibility of lightweight structural members to oscillating dynamic loads, cyclic flexural and tensile tests with up to 10 million cycles are carried out. To characterize the complex (micro-)cracking behavior of the material, the tests are augmented by external deformation sensors (strain gauges, fiber-optic sensors, photogrammetry) and microstructural analysis (X-ray CT, acoustic emission sensors, coda-wave interferometry). The experimental data will serve as the basis for multiscale-modelling approach which uses representative volume elements created from high-resolution CT scans.

Carbon fiber reinforcement used in concrete has become a remarkable alternative to steel fibers. Admixing short fibers to fresh concrete and processing the material with a 3D printer leads to an orientation of fibers and a material with high uniaxial strength properties, which offers an economic use of fibers. To investigate its mechanical behavior, the material is subjected to flexural and tensional tests, combining several measuring techniques. Numerical analysis complements this research. Computed tomography is used with several post-processing algorithms for separating matrix and fibers. This helps to validate fiber alignment and serves as input data for numerical analysis with representative volume elements concatenating real fiber position and orientation with the three-dimensional stress tensor. Flexural and uniaxial tensional tests are performed combining multiple measuring techniques. Next to conventional displacement and strain measuring methods, sound emission analysis, in terms of quantitative event analysis and amplitude appraisal, and also high-resolution digital image correlation accompany the tests. Due to the electrical conductibility of carbon fibers, the material’s resistivity could be measured during testing. All sensors detect the material’s degradation behavior comparably, showing a strain-hardening effect, which results from multiple, yet locally restricted and distributed, microcracks arising in combination with plastic deformation.