On Stress Intensity Factors of Multiple Cracks at Small Distances in 2-D Problems

In the present work a method is proposed to predict stress intensity factors (SIFs) of strongly interacting cracks at spacings that are substantially smaller than crack lengths. The method is intended for applications where cracks are observed in stack-like/staggered arrangements as in damage patterns of some natural materials with dense lamellar microstructures. The new calculation procedure is inspired by the analytical method of Kachanov (1987) that due to its simplicity has shown to be a powerful tool for analysis of crack interactions. Although in 3-D the accuracy of Kachanov’s method remains good at quite close spacings, in 2-D problems it, however, quickly drops as the distance between cracks decreases, underestimating the effect of crack interactions, especially in ordered staggered arrangements. In this work we introduce new modeling assumptions that are suited for problems where stress fields have high gradients – a typical situation in the case of closely spaced parallel cracks. The accuracy of the method is examined on the example of two stacked cracks. The use of SIFs for estimation of material elastic compliance is also discussed.     

Modeling of a locking mechanism between two rough surfaces under cyclic loading

The work is motivated by experimental studies on energy dissipation due to micro-slip in mechanical joints. It has been observed that the loss of energy undergoes certain evolution under cyclic shear loading. It manifests itself in the form of the gradual decrease approaching a steady state as cycling progresses. This behavior has a repeatable character if contact is re-established and subjected to cycling again. In the present work, a simple multiple-asperity model is developed that suggests a physical hypothesis that when two rough surfaces are brought in contact and subjected to shear loads a certain locking mechanism comes into play at the interface. Such locking occurs due to the tendency of the interface to adapt its contact microstructure to the loading conditions. The adaptation mechanism is described in the present work through the contact inclination angles. The developed model is simple in application and it relates micro-characteristics of the contact to macro-behavior of the system exhibiting itself in energy dissipation.     

Photo-thermo-refractive glass co-doped with Nd as a new laser medium

Photo-thermo-refractive (PTR) glass demonstrates refractive index change after exposure to UV radiation followed by a thermal treatment that enables recording of high efficiency holographic optical elements. This work demonstrates feasibility of function of this material as a complex optical medium which posseses both photosensitive and luminescent properties and paves a way for creation of monolythic solid state lasers where resonator components can be holographically recorded inside of a laser medium. It was found, that incorporating of Nd³⁺ ions in PTR glass does not affect photosensitivity required for hologram recording. It was demonstrated that emission wavelength, spectral width, and cross section of Nd³⁺ luminescence in PTR glass are typical for silicate laser glasses and Nd-doped PTR glass can be considered as a promising laser medium for monolithic solid state lasers.     

Asymmetric effects of ovarian hormones on hemispheric activity: Evidence from dichotic and tachistoscopic tests.

Previous studies have suggested a role of endogenously cycling ovarian hormones in the modulation of perceptual asymmetries. In the present investigation, participants were given perceptual asymmetry tests including verbal and nonverbal tachistoscopic and dichotic listening tasks during the menstrual and midluteal phases of the menstrual cycle. Salivary levels of estrogen and progesterone were also measured. Analyses of performance on the tachistoscopic tests revealed that left visual field, but not right visual field, accuracy was significantly lower for both verbal and nonverbal tasks at the midluteal phase than at the menstrual phase. Right ear performance on the nonverbal dichotic test was significantly reduced at the midluteal phase. Results suggest suppression of right hemisphere processing areas and possible reduction in callosal transfer efficiency at higher levels of ovarian steroids. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

UV, Raman and XRD study of polymorphism of poly(methyl-n-propylsilane)

Raman, UV and XRD studies have been performed to characterize the structures of differently prepared samples of poly(methyl-n-propylsilane). The results demonstrate polymorphism of this polymer between T_c and T_g. At room temperature the polymer can exist in up to four modifications which comprise one amorphous disordered phase and three more ordered modifications, differing in the interchain organization and in the silicon backbone conformations. The latter are considered to be deviant, transoid and all-anti, respectively. The number of the modifications present and relative amount of each strongly depends on the preparation method and thermal history of the sample as well as on the molecular weight.     

Prediction and comparison of textural properties of magnetic copolymer supports for enzyme immobilization

Magnetic polymers supports have proven to be valuable materials for enzyme immobilization, as they allow recovering the catalyst by magnetic separation, precluding the need for costly and time-consuming separation steps. In this study, magnetic copolymer supports were synthesized using styrene (STY) and different crosslinking agents (divinylbenzene, ethylene glycol dimethacrylate, or triethylene glycol dimethacrylate) and initiators (azobisisobutyronitrile or benzoyl peroxide) and used to immobilize Candida antarctica lipase B (CALB). The aim was to obtain biocatalysts with high enzymatic activity and satisfactory morphological properties for use in biotransformation reactions. Two morphological properties known to influence the immobilization yield were taken into consideration, specific surface area, and swelling index. Experimental data were compared to the predictions of a model based on molar balance, method of moments, numerical fractionation, and elementary gel structures. The high correlation (R² = 0.9974) between experimental and predicted values demonstrated the suitability of the model for estimating the textural properties of enzyme supports. CALB was successfully immobilized, showing high hydrolytic activity (500–700 U g⁻¹) and good thermal stability at 50°C. CALB/STY-EGDMA-M was 14 times more stable than free CALB. The results confirm the efficiency of the immobilization method and the suitability of the copolymers for enzyme immobilization.     

The use of recycled low-density polyethylene films from protective prepreg for the development of nanocomposites with bentonite clay

Prepreg is a pre-impregnated composite fiber where a thermoset polymer matrix material is present. Before being used, these materials are kept at low temperatures and have a low-density polyethylene (LDPE) film for your protection. The increase in the use of structural composites based on prepregs causes an increase in the amount of protective LDPE film. This material is usually discarded or incinerated and can cause great damage to the environment. Thus, the present work aimed to study the feasibility of recycling up to 100% of protective LDPE (rLDPE) to develop blends-based nanocomposites with rLDPE/virgin LDPE (60/40, 70/30, 80/20, 90/10 and 100/0) with the addition of 5 wt% of compatibilizer agent (maleic anhydride grafted LDPE, LDPE-g-MA), and 1 wt% of bentonite (BNT) using a co-rotational twin-screw extruder, followed by hot pressing in a hydropneumatic press and die-cutting. Water absorption test, thermal aging resistance, morphological characterization by scanning electron microscopy, mechanical properties by Izod impact strength and tensile tests, and differential scanning calorimetry were performed. By the results, it was possible to verify the viability of 100% of the recycling of rLDPE. The results showed a good distribution of 1 wt% of BNT in the matrix, better mechanical properties when compared with virgin LDPE. Furthermore, the thermal properties, water absorption test, and thermal aging resistance showed no statistical differences between the samples. These results confirm the effectiveness and the environmental gain in the use of the recycled material.