The activities of this paper were focused on an in-situ fabrication process for producing two self-healing systems containing dicyclopentadiene and 5-ethylidene-2-norbornene monomers encapsulated in a urea-formaldehyde shell and integration methods applied in the epoxy matrix to analyse and compare the influences of their integration into the neat epoxy matrix. The self-healing systems were first synthesized according to a literature review, and subsequently, an optimization process was conducted for the fabrication process. Neat epoxy specimens were fabricated as reference specimens and subjected to flexural tests. Several integration methods for incorporating the self-healing systems into the epoxy resin were investigated. The optimal method presenting the best dispersion of the healing system was achieved by reducing the viscosity of the epoxy matrix with 10 vol % acetone solution, the addition of a microcapsule in the matrix, and homogenization at 60 °C at 100 rpm. Thermal analysis was performed in order to observe the mass loss obtained with an increasing temperature and phase changes for both poly-urea-formaldehyde (PUF)-dicyclopentadiene (DCPD) and melamine-urea-formaldehyde (MUF)-5-ethylidene-2-norbornene (ENB) systems. The thermogravimetric analysis performed for the PUF-DCPD system indicates a total loss of mass in the range of 30–500 °C of 72.604% and for the MUF-ENB system, indicates a total mass loss in the range of 30–500 °C of 74.093%. Three-point bending tests showed higher mechanical properties for PUF-DCPD (80%) than MUF-ENB (40%) compared to the neat epoxy systems. Numerical simulations were performed to obtain a better understanding of the microcapsule behavior when embedded in an epoxy matrix.

The present investigation was conducted on the low-speed impact response of quasi-isotropic [±45/0/90°]xs hybrid composite through laboratory level experimental tests. The purpose was to understand the behaviour that the different stacking sequences of hybrid glass/carbon fibre composites has on the ability of the material to sustain loads during low-speed impact events without developing critical structural failure in the material and improving the impact energy absorption properties, which is a relevant matter in aerospace and automotive industries. Drop-weight impact tests were carried out on two different laminates, with different stacking sequences, each of which were 16 symmetric inter-ply hybrid laminates named GC [+45G/−45C/0G/90C]4s and, respectively, G-C [+45G/−45G/0G/90G/+45C/−45C/0C/90C]2s, where G stands for glass fibre and C for carbon fibre. Both were comprised of epoxy matrix reinforced carbon/E-glass fibre woven fabric composites. The outcome of changing the hybrid stacking sequence, on the impact performances, was discussed. The damage morphologies and local failure mechanisms were analysed using visual inspection and a high-resolution laser scanner. Under 33 J impact energy, both tested hybrid composites exhibited approximately 10 kN peak load. Nevertheless, one key parameter, the time to peak load, significantly changed; the damage initiation threshold for GC samples occurred immediately before 6 kN, whereas for G-C samples this threshold appeared much earlier. This type of behaviour was partly connected to the delay in the propagation of delamination and fibre breakage, which was influenced by the high elastic energy absorption of the carbon fibres when compared with the glass fibres. The absorbed energy was higher for GC configuration, whereas a higher DI was observed for samples G-C indicating that a high percentage of the total energy was dissipated through the propagation of in-plane and out-of-plane fibre/matrix cracks. No perforation was observed on either configuration; nevertheless, the damage area significantly changed both in size and appearance from one configuration to another.

This paper presents the static mechanical behavior and the dynamic thermomechanical properties of four market-available reinforced and non-reinforced thermoplastics and photopolymer materials used as precursors in different additive manufacturing technologies. This article proposes a characterization approach to further address development of aeronautic secondary structures via 3D-printed composite materials replacing conventional manufactured carbon fiber reinforced polymer (CFRP) composites. Different 3D printing materials, technologies, printing directions, and parameters were investigated. Experimental results showed that carbon-reinforced ONYX_R material exhibits a transition point at 114 °C, a 600 MPa tensile strength, and an average tensile strain of 2.5%, comparable with conventional CFRP composites manufactured via autoclave, making it a suitable candidate for replacing CFRP composites, in the aim of taking advantage of 3D printing technologies. ONYX material exhibits higher stiffness than Acrylonitrile-Butadiene-Styrene Copolymer (ABS), or conventional Nylon 6/6 polyamide, the flexural modulus being 2.5 GPa; nevertheless, the 27 °C determined transition temperature limits its stability at higher temperature. Daylight High Tensile (further called HTS) resin exhibits a tensile strength and strain increase when shifting the printing direction from transversal to longitudinal, while no effect was observed in HighTemp DL400 resin (further called HTP).

A 100 W fibre laser source was used to minimize the surface roughness of 3D-printed Onyx parts. Furthermore, this study aimed to determine the mechanism of surface finishing, the influence of the laser process parameters (laser power, pulse frequency, and laser scanning path) on the surface morphology, and the influence of the scanning path on the dimensional accuracy of the investigated Onyx 3D-printed specimens. A significant reduction in surface roughness of 91.15% was achieved on the S3 Onyx 3D-printed specimen following laser surface polishing treatment using a 50 W laser power and a frequency of 50 kHz. The laser scanning path had little influence on the surface roughness, but had a major impact on the geometrical deviation of the treated sample.

The paper presents the study on the static mechanical properties of PLA (Polylactic Acid) produced with entry-level additive technologies using three printing directions. During the experimental work were tested a total of 15 �dog bone� ASTM D638-14 standard specimens made from additively manufactured polymer (PLA) through FDM (Fused Deposition Modelling) technique, where the material and rectilinear pattern infill geometry and infill percentage of 100% were constant and the printing orientation was varied. Usually technical data sheets that are delivered by filament materials producers include the most satisfactory data which are valid for only one specific printing direction. The printing direction is deliberately selected, in such way that the best material characteristics are achieved. In addition to this matter, as the additive manufacturing market grew significantly in the past couple of years, the filament production market showed a consequential growth. The aftermath of this expansion had a direct impact towards the quality and costs of the filaments used for 3D printing, in order to satisfy both the low-end and high-end users. Therefore, in this frame, the present research provides entry-level additively manufactured PLA performances showing significant changes depending on the different printing directions and determine the build orientation influence on the mechanical properties, in the aim of providing aid for both mechanical designer and product manufacturer at the stage of the printed product mechanical properties.