Fatigue Characterisation and Monitoring in 3D Printed Short Fibres Reinforced Polyamide

The use of fused filament fabrication technology as a competitor of the conventional manufacturing processes for end-use parts production is of a great interest. However, a deep knowledge of the mechanical properties and failure behavior of printed parts is required to safely design final usable components. The present work aims to characterize the mechanical properties of a 3D printed short carbon fiber reinforced polyamide and to investigate the relationship between its unique morphology and its failure behavior. Tensile tests were conducted on rectangular specimens printed with an alternating sequence of layers with 0°/90° and ±45° raster orientation. Fracture surfaces of failed specimens were analyzed by Scanning Electron Microscopy (SEM). Possible failure mechanisms were then inferred by surface morphology analysis. Moreover, several specimens’ geometries were designed and printed to characterize the in-plane and out-of-plane material compressive properties. Considerations about the longitudinal strength of the material were eventually drawn

Administration of a Synbiotic to Free-Living Elderly and Evaluation of Serum Cytokines. A Pilot Study

Ten free-living elderly were administered with a synbiotic [fermented milk containing Lactobacillus rhamnosus Gorbach and Goldin (LGG)] and oligofructose as a prebiotic for one month. Serum cytokines were evaluated before (T0) and after (T1) synbiotic administration. At T0, values of Interleukin (IL)-12, IL-6, IL-10, IL-1 and Tumor Necrosis Factor (TNF)- were lower than normal controls, with the exception of IL-8, thus confirming previous results on the impairment of both innate and adaptive responses in elderly. At T1, the synbiotic was able to significantly increase, depressed values of IL-1, IL-6 and IL-8 with a trend to a modest increase for the restant cytokines. In conclusion, the synbiotic used in this study seems to be very beneficial to elderly for its capacity to maintain the immune homeostasis, even if an increase in dosage and prolongation of administration time are required for a better modulation of the aged adaptive immune response

Morphology-induced fatigue crack arresting in carbon fibre sheet moulding compounds

Carbon Fibre Sheet Moulding Compounds (CF-SMCs) are tow-based composite materials. Interrupted fatigue tests, combined with computed tomography, were performed here to investigate the damage mechanisms in high in-mould flow CF-SMC. The tow-based microstructure created obstacles for fatigue damage propagation, increasing the CF-SMC’s resistance against cyclic loading. Failure is shown to nucleate inside the tows, but inter-tow crack propagation tends to be hindered by the presence of the other tows. Tows oriented perpendicularly to the initial fatigue crack stop the crack itself, showing an intrinsic crack arrest mechanism. Additionally, pre-existing manufacturing cracks or voids do not propagate at all. As a result, flatter slopes of the SN diagrams were observed for CF-SMC than for other carbon or glass fibre composites with short, long and even continuous fibres

Experimental characterization of a Polymer Metal Hybrid (PMH) automotive structure under quasi-static, creep, and impact loading

A feasibility study on a short fibre reinforced Polymer Metal Hybrid (PMH) solution of a car’s suspension control arm has been conducted through a simplified demonstrator, representative of the most critical portion of this component. It was injection moulded in two versions: an all composite one and a PMH version, in which the short fibre reinforced composite was over-moulded on to an aluminium insert. The demonstrator underwent quasi- static, creep and impact tests to simulate most of the loading conditions experienced by a suspension arm during its lifetime. The mechanical behaviours of the two demonstrator versions were compared to highlight the differences introduced by the proposed novel PMH solution. In particular, the ductile metal insert ensured the compliance of the PMH demonstrators with the automotive specific safety requirement of avoiding the complete separation at failure, which was successfully obtained in all testing conditions

Fatigue crack growth analysis in composite bonded joints by back face distributed strain sensing and comparison with X-ray microtomography

Fatigue crack growth in composite, single lap bonded joints is analysed by back face distributed strain sensing, using optical backscatter reflectometry. Crack growth is inferred from the back-face strain profile measured with high spatial resolution along optical fibres bonded on the joints. The indications about the position of the crack front are compared with micro computed tomography scans conducted on specimens at suitable interruptions of the fatigue tests. By the proposed technique, crack initiation can be clearly detected, and measurements of crack growth are in good agreement with observations, with decreasing error with increasing crack length

Estimating crack tip position in adhesively bonded joints subjected to mode II quasi‐static loading

This study aimed at estimating crack tip position in adhesive bonded joints under mode II quasi-static loading using experimental and numerical approaches. Experimental techniques were utilized and compared, including optical backscatter reflectometry, visual testing, and a novel strategy based on digital image correlation. Additionally, a finite element analysis was employed to identify the numerical crack tip position and the extent of damage within the bondline. This analysis revealed that a significant portion of the crack propagation region in the adhesive is occupied by the fracture process zone. Moreover, optical backscatter reflectometry shows the potential to detect this process zone within the adhesive that the other methods may not detect. This capability is particularly beneficial for detecting damage at early stages.OBR and DIC were used for crack initiation and propagation in ENF joint under mode II.OBR back-face strain demonstrated its ability to detect potential early-stage damage.Trapezoidal TSL outperformed bi-linear TSL in FE crack propagation modeling.A significant portion of crack propagation region was occupied by FPZ in adhesive

Effects of a Hypocaloric Diet on Obesity Biomarkers: Prevention of Low-Grade Inflammation since Childhood

Body mass index (BMI), serum cytokines and serum obesity markers were evaluated in 33 obese children before, during and after a hypocaloric diet. The cytometric bead array "human inflammatory kit" was used for the evaluation of serum interleukin (IL)-1 beta, IL-6, IL-10 and tumor necrosis factor-alpha. On the other hand, the following obesity biomarkers were evaluated by means of a flowcytomix-human obesity 9 plex kit: Soluble Isoform of CD40 Ligand; Soluble Intercellular Adhesion Molecule-1; Leptin; Monocyte Chemoattractant Protein 1; Myeloperoxidase; Osteoprotegerin; Resistin and Soluble TNF-receptors. Actually, throughout the study modifications of BMI were negligible and, therefore, serum cytokines and obesity markers did not show any significant changes in comparison with baseline values. On the other hand, at the different time points considered the majority of obesity markers were higher than normal controls, thus indicating a low grade inflammation in childhood obesity. Therefore, attempts at reducing this inflammatory status in children which predisposes to the metabolic syndrome outcome are discussed

Material characterization and fatigue data correlation of short fibre composites: effects of thickness, load ratios and fibre orientation at elevated temperatures

In design engineering of short fiber reinforced components a given material can be used for two or more parts of the same system having different wall thicknesses. The thickness, together with numerous parameters, has an impact on the cyclic behavior of short fiber reinforced thermoplastics under fatigue loading. This research analyses and compares the fiber orientation by µCT as well as the quasi-static and cyclic behavior of two sets of specimens with 1.6 and 3 mm wall thicknesses made of PA6T/6I GF50. Fatigue criteria based on cyclic mean strain rate or on cyclic creep energy density are evaluated to assess the most accurate and independent of the thickness