Rheological and mechanical characterization of poly(lactic acid)/polypropylene polymer blends

Poly(lactic acid) (PLA) was melt blended with polypropylene (PP) with the aim of replacing commodity polymers in future applications. Since cost of PLA is quite high, it is not economically feasible to use it alone for day to day use as a packaging material without blending. This paper reports the preparation of poly(lactic acid)/polypropylene polymer blends (PLA/PP) using a laboratory scale single screw extruder. Rheological and mechanical properties of the prepared blends were determined. The rheological experiments were carried out on a capillary rheometer, the effect of shear rate, temperature and PLA content on the flow activation energy and true viscosity of the blends were described. Mechanical properties of the blends were investigated on dog bone-shaped samples obtained by injection molding; tensile tests were performed using Testometric M350-10KN. The effect of PLA content on Young’s modulus, strain at break and stress at break of the blends were described. The rheological results showed that the true viscosity of the blends is between that of the pure polymers, whereas the flow activation energy of the blends is less than that of the pure polymers. The mechanical results showed incompatibility between PLA and PP in the blend.     

Numerical analysis of the stress intensity factors for repaired cracks from a notch with bonded composite semicircular patch

In this paper, we investigated the crack growth behaviour of cracked thin aluminium plate repaired with bonded composite patch. The finite element method is used to study the performance of the bonded composite reinforcement or repair for reducing the stress concentration at a semicircular lateral notch and repairing cracks emanating from this kind of notch. The effects of the adhesive properties and the patch size on the stress intensity factor variation at the crack tip in mode I were highlighted. The obtained results show that the stress concentration factor at the semicircular notch root and the stress intensity factor of a crack emanating from notch are reduced with the increase of the diameter and the number of the semicircular patch. The maximal reduction of stress intensity factor is about 42% and 54%, respectively, for single and double patch. However, the gain in the patch thickness increases with the increase of the crack length and it decreases when the patch thickness increases. The adhesive properties must be optimised in order to increase the performance of the patch repair or reinforcement.     

Analysis of the plastic zone size ahead of repaired cracks with bonded composite patch of metallic aircraft structures

In this study, the three-dimensional and non-linear finite element method is used to estimate the performance of the bonded composite repair of metallic aircraft structures by analyzing the plastic zone size ahead of repaired cracks. Several calculations have been realized to extract the plasticized elements around the crack tip of repaired crack. The obtained results show that the presence of the composite patch reduces considerably the size of the plastic zone ahead of the crack. The effects of the adhesive properties and the patch thickness on the plastic zone size ahead of repaired cracks were analyzed.     

Analysis of the effect of load direction on the stress distribution in dental implant

In this work, the finite element method is used to compute the distribution of stresses in dental prosthesis. The stress analysis is particularly focused at the interface bone–implant in different positions: distal zone, medial zone and proximal zone of these components. The effects of the intensity and the direction of loading on the stress variation were highlighted.     

Finite element analysis of the behaviour of microvoids in the cement mantle of cemented hip stem: Static and dynamic analysis

In this study, the three-dimensional finite element (FE) method is used to analyse the stress distribution around microcavities in the cement mantle of total hip arthoplasty (THA). Static and dynamic loading were analysed. The effect of the position of the microdefect on the stress distribution is also highlighted. The obtained results show that microcavity located in the proximal zone of the prosthesis is subject to higher stress field. The static loading generates higher stresses than dynamic one if the microcavity is located in the proximal and distal zones of the prosthesis. The inverse case is observed when the microcavity is located in the medial zone.     

Three-dimensional numerical simulation of free convection and entropy generation in a cubic cavity containing a heat source

The present article provides a three-dimensional numerical investigation of thermal convection and entropy generation. The lattice Boltzmann method, coupled with the finite difference approach, is applied to perform numerical simulations. The validation of these numerical approaches for thermal convection simulation and entropy calculation is performed by comparing our numerical results with those in the published literature for the case of benchmark problems. The physical geometry studied in this paper concerns a hot obstacle having the shape of a plus sign (+) placed in the center of a cubic enclosure. This cube is filled with air of a Prandtl number of 0.71 and characterized by two cold vertical walls. The heat exchange between the fluid and the hot body is studied as a function of the Rayleigh number (${10}^3\le \mathit{Ra}\le {10}^7$). The performed simulations show that the heat transfer rate can be increased by about 429% by switching from $\mathit{Ra}={10}^3$ to ${10}^7$. The entropy generation due to fluid friction, heat transfer, and total entropy are also calculated and discussed. For an irreversibility coefficient $\mathit{\phi }={10}^{-4}$, the analysis of the results showed that for low values of the Rayleigh number ($\mathit{Ra}={10}^3$), the entropy production due to temperature gradients predominates over that produced by viscous effects. In the cases of $\mathit{Ra}={10}^4$ and ${10}^5$, entropy generation is due to both fluid friction and heat transfer. However, when the Rayleigh number becomes large ($\mathit{Ra}{\ge 10}^6$), entropy generation due to viscosity predominates over entropy production related to heat exchange. These results have important implications for the optimization and design of heat transfer systems in various industrial applications.     

Resistive-switching in yttria-stabilized hafnia ceramics

Yttria-stabilized hafnia ceramics are high-temperature oxide ion conductors that lose a small amount of oxygen, both at high temperatures and on the application of a small dc bias. At zero applied bias, a small amount of p-type conductivity is present. This increases with low bias and is attributed to reactions initiated at the positive electrode/ceramic interface. With a further increase in bias, n-type conductivity is initiated at the negative electrode/ceramic interface. After a short time-lapse, the overall conductivity increases rapidly by 1.5–3 orders of magnitude and is reversible, with hysteresis, on subsequent removal of the bias. Switching has been observed over the range 457–531°C and is sensitive to both temperature and oxygen partial pressure in the surrounding atmosphere. This is the first example of low field, resistive switching in bulk hafnia ceramics, in contrast to most examples of resistive switching which are observed in nanometre-thick devices using similarly applied voltages.