We address multi-walled carbon nanotubes (MWCNTs) for structural health monitoring in adhesive bonds, such as in building structures. MWCNT-loaded composites are employed to sense strain changes under tension load using an AC impedance measurement setup. Different weight percentages of 1, 1.5, 2 and 3 wt % MWCNTs are added to the base epoxy resin using different dispersion times, i.e., 5, 10, and 15 min. The equivalent parallel resistance of the specimens is first measured by applying an alternating voltage at different frequencies. To determine the mechanical as well as sensory properties, the specimens are then subjected to a tensile test with concurrent impedance measurement at a fixed pre-chosen frequency. Using alternating voltage, a higher sensitivity of the impedance reading can be achieved. Employing these sensors in buildings and combining the readings of a network of such devices can significantly improve the buildings’ safety. Additionally, networks of such sensors can be used to identify necessary maintenance actions and locations.

The material properties of 3D printed continuous fiber composites have been studied many times in the last years. However, only a minimal number of samples were used to determine the properties in each of the reported studies. Moreover, reported results can hardly be compared due to different sample geometries. Consequently, the variability of the mechanical properties (from one sample to the other) is a crucial parameter that has not been well quantified yet. In the present work, the flexural properties of 3D printed continuous carbon fiber/nylon composite specimens were experimentally quantified, using batches of 15 test specimens. In order to account for the possible influence of the quality of the prepreg filaments on the observed variability, three different filament rolls were used to manufacture the different batches. Also, two configurations were tested, with a fiber direction parallel (longitudinal) or perpendicular (transverse) to the main axis of the specimens. The results show moderate to high variabilities of the flexural modulus, flexural strength and maximum strain. The coefficient of variation was more than twice as high in the transverse case as in the longitudinal case.

A controlled laminate consolidation is one of the most essential approaches in the production of fiber-reinforced thermoplastics components. With the use of specific consolidation models, almost the entire strength potential of the material can be exploited. However, a controlled thermo-mechanical in situ consolidation (TMIC) strategy in the fused filament fabricated (FFF) process of continuous fiber-reinforced polymer composites (CFRPC) has not been considered so far and leads to deconsolidation defects in the 3D-printed material. These defects in terms of micro and macro volumetric flaws in the joining zone indicate a poor process parameter selection and inadequate thermo-mechanical consolidation. These imperfections lead to a reduction in the fiber volume content and a significant deterioration in the mechanical properties. In this work, a self-developed test rig is presented, which is able to influence and monitor the consolidation during the additive manufacturing (AM) process with a TMIC unit in a controlled manner. To evaluate the test rig, the mechanical construction and the implemented sensors were tested for full functionality. Subsequently, test specimens were fabricated for mechanical characterization using three-point bending (3PB) tests and microstructural analysis. Based on these results, the influence of TMIC, with its dependent process parameters (consolidation force, temperature, printing speed), is presented. A perspective on the future development of controlled consolidation in AM of CFRPC is shown.

Additive manufacturing (AM) of continuous fiber reinforced composites (CFC) is becoming increasingly important in the field of lightweight construction. The major advantage lies particular in the automated manufacturing process and in the production of complex geometries. Most work was done with traditional fusion filament fabrication (FFF) and the lack of separate consolidation units. AM CFCs produced in this manner have shown increased consolidation-related volumetric flaws -i.e. deconsolidation defects decreasing mechanical performance. The presence of deconsolidation defects, normally indicates either poor process parameters selection or inadequate in situ consolidation. Recently, there were several efforts in modifying the FFF of CFCs to minimize deconsolidation defects through in situ thermo-mechanical pressing. However, there are only limited fundamental knowledge on the in situ thermo-mechanical consolidation of FFF-CFCs. The present paper analyses the stated problem and proposes ways to decrease deconsolidation in FFF CF-PA6 laminates. For this purpose, we used a self-developed FFF 3D Printer coupled with a thermo-mechanical pressing unit. The influence of extrusion-, consolidation temperature, printing speed and in situ consolidation pressure on laminate microstructure and flexural strength was investigated. A comparison with FFF laminates printed in a common 3D printer was performed. The controlled 3D printing consolidation process leads to a homogeneous distribution of fibers in the matrix material and to an improvement of the material parameters of the reinforcing composite.