Refine
Year of publication
- 2020 (22) (remove)
Document Type
- Article (22) (remove)
Language
- English (22)
Has Fulltext
- no (22)
Effect of voids in a heat-flux dependent theory for thermoelastic bodies with dipolar structure
(2020)
The moisture absorption behavior of flax fiber-reinforced epoxy composites is deliberated to be a serious issue. This property restricts their usage as outdoor engineering structures. Therefore, this study provides an investigation of moisture in flax fibers on the performance of the flax/epoxy composite materials based on their shear responses. The ±45° aligned flax fibers exposed to different relative humidities (RH) and the vacuum infusion process was used to manufacture the composite specimens. The optimum shear strength (40.25 ± 0.75 MPa) was found for the composites manufactured with 35% RH-conditioned flax fibers, but the shear modulus was reduced consistently with increasing RH values. Although shear strength was increased because of fiber swelling with increased moisture absorption rate until 35% RH environments with good microstructures, nonetheless, strength and modulus both started to decrease after this range. A very poor microstructure has been affirmed by the SEM images of the composite samples conditioned at 90% RH environments.
In our present paper, we approach the mixed problem with initial and boundary conditions, in the context of thermoelasticity without energy dissipation of bodies with a dipolar structure. Our first result is a reciprocal relation for the mixed problem which is reformulated by including the initial data into the field equations. Then, we deduce a generalization of Gurtin’s variational principle, which covers our generalized theory for bodies with a dipolar structure. It is important to emphasize that both results are obtained in a very general context, namely that of anisotropic and inhomogeneous environments, having a center of symmetry at each point.
The objective of this study is to investigate the influence of veneering technique (hand-layering vs. milling) on the fracture resistance of bi-layer implant-supported zirconia-based hybrid-abutment crowns. Mandibular molar copings were anatomically designed and milled. Copings were then veneered by hand-layering (HL) (n = 20) and milling using the Cad-On technique (LD) (n = 20). Crowns were cemented to zirconia hybrid-abutments. Ten samples of each group acted as a control while the remaining ten samples were subjected to fatigue in a chewing simulator. Crowns were loaded between 50 and 100 N for 1.2 million cycles under simultaneous temperature fluctuation between 5 and 55 °C. Crowns were then subjected to static load a to fracture test. Data were statistically analysed using the one-way ANOVA. Randomly selected crowns from each group were observed under scanning electron microscopy to view fractured surfaces. Study results indicate that during fatigue, LD crowns had a 100% survival rate; while HL crowns had a 50% failure rate. Fracture resistance of LD crowns was statistically significantly higher than that of HL crowns at the baseline and after fatigue (p ≤ 0.05). However, fatigue did not cause a statistically significant reduction in fracture resistance in both LD and HL groups (p > 0.05). Copings fractured in the LD crowns only and the fracture path was different in both LD and HL groups. According to the results, it was concluded that milled veneer implant-supported hybrid-abutment crowns exhibit significantly higher fracture resistance, and better withstand clinical masticatory loads in the posterior region compared to the hand-layered technique. Also, fatigue application and artificial aging caused no significant strength reduction in both techniques. Clinical significance: Different veneering techniques and materials (hand-layering or milling) act differently to clinical forces and environment and may be prone to early chipping during service. Therefore, practitioners are urged to consider the appropriate veneering protocol for posterior implant-supported hybrid-abutment restorations.
Our study is dedicated to a composite, which, in fact, is a mixture of two thermoelastic micropolar bodies. We formulate the mixed initial boundary value problem in this context and define the domain of influence for given data. For any solution of the mixed problem we associate a measure and prove a second-order differential inequality for it. Based on the maximum principle for the heat equation and on the second-order differential inequality, we establish an estimate which proves that the thermal and the mechanical effects, at large distance from the domain of influence, are dominated by an exponential decay.
The aim of the current work was to illustrate the effect of the fibre area correction factor on the results of modelling natural fibre-reinforced composites. A mesoscopic approach is adopted to represent the stochastic heterogeneity of the composite, i.e. a meso-structural numerical model was prototyped using the finite element method including quasi-unidirectional discrete fibre elements embedded in a matrix. The model was verified by the experimental results from previous work on jute fibres but is extendable to every natural fibre with cross-sectional non-uniformity. A correction factor was suggested to fine-tune both the analytical and numerical models. Moreover, a model updating technique for considering the size-effect of fibres is introduced and its implementation was automated by means of FORTRAN subroutines and Python scripts. It was shown that correcting and updating the fibre strength is critical to obtain accurate macroscopic response of the composite when discrete modelling of fibres is intended. Based on the current study, it is found that consideration of the effect of flaws on the strength of natural fibres and inclusion of the fibre area correction factor are crucial to obtain realistic results.
Hot isostatic pressing can be utilized to reduce the anisotropic mechanical properties of Al–Si–Mg alloys fabricated by laser powder-bed fusion (L-PBF). The implementation of post processing densification processes can open up new fields of application by meeting high quality requirements defined by aircraft and automotive industries. A gas pressure of 75 MPa during hot isostatic pressing lowers the critical cooling rate required to achieve a supersaturated solid solution. Direct aging uses this pressure related effect during heat treatment in modern hot isostatic presses, which offer advanced cooling capabilities, thereby avoiding the necessity of a separate solution annealing step for Al–Si–Mg cast alloys. Hot isostatic pressing, followed by rapid quenching, was applied to both sand cast as well as laser powder-bed fused Al–Si–Mg aluminum alloys. It was shown that the critical cooling rate required to achieve a supersaturated solid solution is significantly higher for additively manufactured, age-hardenable aluminum alloys than it is for comparable sand cast material. The application of hot isostatic pressing can be combined with heat treatment, consisting of solution annealing, quenching and direct aging, in order to achieve both a dense material with a small number of preferred locations for the initiation of fatigue cracks and a high material strength.
The methodology of the approximation and interpretation of thermal desorption spectra (TDS) of hydrogen in some carbon nanostructures and graphite has been developed and applied for such materials.
The methodology is based on a definite approximation by the symmetrical Gaussians of the hydrogen thermal desorption spectra, obtained by using one single heating rate, for carbon materials and nanomaterials, and a definite processing of the Gaussians, in the approximation of the first-order reactions and the second-order ones. It results in determining (with a satisfactory accuracy, for the further physical analysis), from TDS data of one single heating rate, the activation energies and pre-exponential factors of the rate constants of desorption processes corresponding to the main TDS peaks with different temperatures of the maximum desorption rate. The developed methodology contains several successive steps of its implementation, including the use of several “criterions of truth” and the final verification and/or modification of the results, with the help of numerical modeling methods. This technique is not less informative, but much less time-consuming in experimental terms compared to the generally accepted classical Kissinger method, which demands using of several heating rates, and has strict limits of applicability. Furthermore, the methodology allows one to reveal physics and atomic mechanisms of the main desorption processes through thermodynamic analysis of the obtained peak characteristics and comparison with the corresponding independent experimental and theoretical data.
The purpose of such a methodology is to further reveal the weakly studied physics of the main states of hydrogen in carbon materials and nanomaterials, and not the thorough detailed mathematical description of the spectra. For this case, both the large difference and the large spread of the known experimental and theoretical values of the thermodynamic characteristics of the main desorption processes, important for hydrogen storage problems, are also taken into account.
Background:
Glaucoma, a characteristic type of optic nerve degeneration in the posterior pole of the eye, is a common cause of irreversible vision loss and the second leading cause of blindness worldwide. As an optic neuropathy, glaucoma is identified by increasing degeneration of retinal ganglion cells (RGCs), with consequential vision loss. Current treatments only postpone the development of retinal degeneration, and there are as yet no treatments available for this disability. Recent studies have shown that replacing lost or damaged RGCs with healthy RGCs or RGC precursors, supported by appropriately designed bio-material scaffolds, could facilitate the development and enhancement of connections to ganglion cells and optic nerve axons. The consequence may be an improved retinal regeneration. This technique could also offer the possibility for retinal regeneration in treating other forms of optic nerve ailments through RGC replacement.
Methods:
In this brief review, we describe the innovations and recent developments in retinal regenerative medicine such as retinal organoids and gene therapy which are specific to glaucoma treatment and focus on the selection of appropriate bio-engineering principles, biomaterials and cell therapies that are presently employed in this growing research area.
Results:
Identification of optimal sources of cells, improving cell survival, functional integration upon transplantation, and developing techniques to deliver cells into the retinal space without provoking immune responses are the main challenges in retinal cell replacement therapies.
Conclusion:
The restoration of visual function in glaucoma patients by the RGC replacement therapies requires appropriate protocols and biotechnology methods. Tissue-engineered scaffolds, the generation of retinal organoids, and gene therapy may help to overcome some of the challenges in the generation of clinically safe RGCs.
Additive manufacturing offers unique possibilities to produce complex components, which would not be possible or only very difficult to be realizable with conventional manufacturing processes. The potential of additive manufacturing of aluminum alloys has already been realized in prototyping, but still offers potential in the establishment of series applications. For enabling the establishment of additive manufacturing in series applications, comprehensive knowledge about the material behavior is necessary. This study examines the torsional behavior of additive manufactured AlSi10Mg samples and shows the correlation between tensile and torsional strength. Comprehensive torsion tests with strain gauges were performed and significant directional dependencies were shown. The correlation factor in the area of torsional yield strength/tensile yield strength and torsional strength/tensile strength was determined and was compared with the yield/failure conditions according to Tresca and von Mises.