Andreia Dias

Use of the hole-drilling method for measuring residual stresses in highly stressed shot-peened surfaces

The same shot-peening treatment was applied to five steels with different mechanical properties. The induced residual stress profiles were analyzed using X-ray diffraction and incremental hole drilling (IHD). The results of both techniques showed that IHD can still be successfully used for measuring shot-peening residual stresses, even if these exceed the yield strength of the bulk material. Expected errors due to the plasticity effect are reduced by the strain hardening of the surface. For an assessment of the reliability of IHD data, strain-hardening variation was quantified by microhardness measurements to estimate the yield strength of the plastified layer. All the main calculation methods for IHD were applied. The results were compared and discussed with respect to the characteristics of each method.

Residual stress analysis near a cold expanded hole in a textured Alclad sheet using X-ray diffraction

In this paper we present the methods of determination and the stress obtained at the periphery of a cold expanded hole in a 2024-T3 Alclad aluminum alloy sheet. The measurements in the aluminum clad were performed by the sin2Ψ method, taking experimental precautions to deal with the texture effects. In the core aluminum a special method had to be implemented to determine the stress values in a direction not accessible to the X-ray diffraction. The strains were measured in sample orientations selected according to the texture characteristics and stress factorsFij were used to calculate the stress tensor. TheFij values were determined assuming a quasi-isotropic material behavior, after concluding that the stress results were not significantly affected by factors calculated for textured material. The residual stress profile, both in the clad and in the sheet, shows a nearly axisymmetric stress state. Compressive stresses were observed near the periphery of the hole, with values that are higher on the exit than on the entrance face. Residual stresses were also higher in the hoop direction than in the radial direction. They decreased with the radial distance to the hole and affected the previous stress state over a distance of 6 mm. The plastic deformation induced by the cold expansion is well evidenced by the FWHM values, which in the affected zone decrease with increasing distance from the hole edge.

Multifactor analysis on the effect of collagen concentration, cross-linking and fiber/pore orientation on chemical, microstructural, mechanical and biological properties of collagen type I scaffolds

This work evaluates the effect of processing variables on some physicochemical and mechanical properties of multi- and unidirectional laminar collagen type I scaffolds. The processing variables considered in this study included microstructure orientation (uni- and multidirectional fiber/pore controlled by freeze-drying methodology), cross-linking (chemical - using genipin and glutaraldehyde, and physical - using a dehydrothermal method), and collagen concentration (2, 5 and 8mg/ml). The biocompatibility of the scaffolds obtained in each of the evaluated manufacturing processes was also assessed. Despite previous research on collagen-based platforms, the effects that these processing variables have on the properties of collagen scaffolds are still not completely understood. Unidirectional scaffolds presented higher resistance to failure under stress than multidirectional ones. The cross-linking degree was found to decrease when the concentration of collagen increased whilst using chemical cross-linkers, and to increase with the concentration of collagen for the dehydrothermal cross-linked scaffolds. Pore orientation indexes of both unidirectional and multidirectional scaffolds were not influenced by collagen concentration. Cross-linked scaffolds were more hydrophobic than non-cross-linked ones, and presented water vapor permeability adequate for use in low-to-moderate exuding wounds. Pore size ranges were compatible with cell in-growth, independently of the employed cross-linking and freezing methodologies. Moreover, scaffolds cross-linked with glutaraldehyde presented higher in-growth of primary oral mucosa fibroblasts than those cross-linked with genipin or with the dehydrothermal treatment. This multi-factor analysis is expected to contribute to the design of collagen type I platforms, which are usable on several potential soft tissue-engineering applications.

CHAPTER 7:Recent Trends and Perspectives for the Extraction of Natural Products

In this chapter we present and discuss the most recent trends and perspectives on the extraction of natural products. This was made considering: (i) the natural‐origin extracts/target compounds that are currently being more studied and extracted (as well as in their potential applications); (ii) the most recently extracted vegetable raw materials that can be explored as sources for the envisaged target compounds; (iii) the current and most promising strategies regarding the extraction methodologies that are expected to be further developed and employed on the extraction of natural products; and (iv) the novel/alternative solvents, solvent mixtures and extraction additives that could be beneficially used in the above referred extraction methodologies. To accomplish this task, we performed a literature search (covering the period between 2000 and 2011) and used several specific search descriptors in order to verify the above referred trends in the extraction of natural products. Results showed that, in recent years, there has been a clear renewed/increased interest in the extraction of natural products and in their advantageous applications, namely as food supplements, phytomedicines, nutraceuticals, cosmetics and natural pesticides. The most used natural‐origin raw materials to obtain these natural products were terrestrial plant‐origin raw materials. In addition, there was a greater interest in the extraction from plant residues/wastes, from marine organisms and from microorganisms. In terms of the most employed natural products extraction methods, supercritical fluid extraction methods seemed to keep the interest of researchers, while other methods, such as ultrasound, microwave, pressurized‐liquid, sorptive and combined/hyphenated extraction techniques, have gained more attention in recent years. Finally, a recent interest was also observed in using novel extraction solvents (or solvent mixtures) that may improve process safety and sustainability, and/or that may improve extraction yields, selectivities and stabilities of the target compounds. Moreover, it is expected that the observed trends will be maintained in the near future as they were mostly motivated by recent consumer demands and by safety, environmental and regulatory issues.