28 research outputs found
A modified energyâbased fatigue parameter for short fiber reinforced polymers: Performance analysis with varying thicknesses, load ratios, and fiber orientations
The durability of Short Fibre Reinforced Polymers (SFRPs) is affected by several variables. Accurate unifying fatigue parameters are thus essential for an efficient characterization campaign. This research investigated the fatigue behaviour of a Polyphtalamide PA6T/6I reinforced with 50% of glass ïŹbres. Two thicknesses (1.6 mm and 3 mm), two orientations from the injection moulding direction (0° and 90°) and three load rations (-0.5, 0.1, 0.5) were investigated. A new fatigue parameter, called Alternating Energy Density (AED), was presented; its ability to correlate the fatigue tests results was compared to other known fatigue parameters â cyclic mean strain rate and cyclic creep energy density. All the fatigue parameters were found to be independent of specimen thickness. Furthermore, the prediction of the fatigue lifetime based on AED showed greater accuracy with respect to the other known investigated methods. AED could thus be used for accurate and efficient lifetime prediction of SFRPs
3D Printed Short Carbon Fibres Reinforced Polyamide: Tensile and Compressive Characterisation and Multiscale Failure Analysis
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 specimensâ 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
Design and Optimisation of a 3D Printed Composite Prosthetic Foot: a Finite Element Feasibility Study
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
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 carâs 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
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-SMCâs 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
Helicobacter pylori primary and secondary genotypic resistance to clarithromycin and levofloxacin detection in stools: A 4-year scenario in Southern Italy
Antibiotic resistance has become an emerging problem for treating Helicobacter pylori (H. pylori) infection. Clarithromycin and levofloxacin are two key antibiotics used for its eradication. Therefore, we reviewed our experience with genotypic resistance analysis in stools to both clarithromycin and levofloxacin in the last four years to evaluate time trends, both in naive and failure patients. Patients collected a fecal sample using the THD fecal test device. Real-time polymerase chain reaction was performed to detect point mutations conferring resistance to clarithromycin (A2142C, A2142G, and A2143G in 23S rRNA) and levofloxacin (substitutions at amino acid position 87 and 91 of gyrA). One hundred and thirty-five naive patients were recruited between 2017-2020. Clarithromycin resistance was detected in 37 (27.4%). The time trend did not show any significant variation from 2017 to 2020 (p = 0.33). Primary levofloxacin resistance was found in 26 subjects (19.2%), and we observed a dramatic increase in rates from 2017 (10%) to 2018 (3.3%), 2019 (20%), and 2020 (37.8%). Ninety-one patients with at least one eradication failure were recruited. Secondary resistance to clarithromycin and levofloxacin was found in 59 (64.8%) and 45 patients (59.3%), respectively. In conclusion, our geographic area has a high risk of resistance to clarithromycin. There is also a progressive spreading of levofloxacin-resistant strains
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 quasiâstatic 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