Analysis Of Failure Mechanisms In Platelet-Reinforced Composites

The short-term mechanical strength of platelet-reinforced polymer composites was modeled using classical two-dimensional stress-transfer analysis. The stress field in the platelet and at the platelet/matrix interface was described in the presence of a matrix crack perpendicular to the interface. Modeling takes into account the tensile strength of the platelet, its adhesion to the matrix, and also considers the internal stress state resulting from processing. Platelet rupture and interface delamination were considered to be the two key failure mechanisms, depending on the ratio of platelet strength to interface strength. The transition between the two failure events was predicted to occur at a critical platelet length, the value of which depends on the elastic properties of the platelet and matrix, on the platelet geometry and strength, on the platelet/matrix adhesion, and on the internal stress state. The approach was applied to the case of low volume fraction silicon oxide platelets/poly(ethylene terephthalate) composites, where the size of the platelets was accurately controlled either below or above the predicted critical length. Compression molded composites, with perfect alignment of the platelets, and injection molded composites, were prepared and tested. The toughness of the compression molded composites was found to be accurately predicted by the strength model, with a 100% increase in the case of platelets smaller than the critical length compared to larger platelets. Injection molded composites with platelets larger than the critical length were found to fail without yielding. By contrast, when the platelets were smaller than the critical length, the injection molded composites exhibited excellent ductility. The general agreement obtained between the predicted and observed toughening transition shows the importance of filler size and stress state on the strength of platelet-reinforced composite

Biaxial fragmentation of thin silicon oxide coatings on poly(ethylene terephthalate)

Crack patterns of 53 nm and 103 nm thick silicon oxide coatings on poly(ethylene terephthalate) films are analyzed under equibiaxial stress loading, by means of a bulging cell mounted under an optical microscope with stepwise pressurization of film specimens. The biaxial stress and strain are modeled from classical elastic membrane equations, and an excellent agreement is obtained with a finite element method. In the large pressure range, the derivation of the biaxial strain from force equilibrium considerations are found to reproduce accurately the measured data up to 25% strain. The examination of the fragmentation process of the coating under increasing pressure levels reveals that the crack onset strain of the oxide coating is similar to that measured under uniaxial tension. The fragmentation of the coating under biaxial tension is also characterized by complex dynamic phenomena which image the peculiarities of the stress field, resulting in considerable broadening of the fragment size distribution. The evolution of the average fragment area as a function of biaxial stress in the early stages of the fragmentation process is analyzed using Weibull statistics to describe the coating strengt

U–Pb zircon and monazite geochronology of post-collisional hercynian granitoids from the Central Iberian Zone (Northern Portugal)

In the Central Iberian Zone (CIZ) of the Iberian Massif large volumes of granitoids were emplaced during the post-collisional stage of the Hercynian orogeny (syn- to post-D3, the last ductile deformation phase). Twelve granitic units and a quartz monzodiorite were selected for a U–Pb zircon and monazite geochronological study. They represent successive stages of the D3 event. The Ucanha-Vilar, Lamego, Sameiro and Refoios do Lima plutons are coeval (313±2 Ma, 319±4 Ma, 316±2 Ma and 314±2 Ma, respectively) and belong to the earliest stage. Later on the Braga massif was emplaced, its different units yielding the same age: 309±3 Ma for the Braga granite, 309±1 Ma for the Gonça granite and 311±5 Ma for a related quartz monzodiorite. The Braga massif is subcontemporaneous with the Agrela and Celeirós plutons (307±3.5 Ma and 306±2 Ma, respectively), in agreement with field data. The Briteiros granite is younger (300±1 Ma), followed by the emplacement of the Peneda–Gerês massif (Gerês, Paufito, Illa and Carris granites). The Gerês granite, emplaced at 296±2 Ma, seems to represent a first magmatic pulse immediately followed by the intrusion of the Paufito granite at 290±2.5 Ma. For the Carris granite a minimum emplacement age of 280±5 Ma was obtained. Based on these results the following chronology is proposed: (1) syn-D3 biotite granitoids, 313–319 Ma; (2) late-D3 biotite-dominant granitoids, 306–311 Ma; (3) late- to post-D3 granitoids, ca. 300 Ma; (4) post-D3 granitoids, 290–296 Ma. These chronological data indicate that successive granitic intrusions were emplaced in the CIZ during a short time span of about 30 Ma that corresponds to the latest stages of the Hercynian orogeny. A rapid and drastic change occurred at about 300 Ma, between a compressive ductile tectonic regime (D3, ca. 300–320 Ma) associated to calc-alkaline, monzonitic and aluminopotassic plutonism and a fragile phase of deformation (D4) which controlled the emplacement of the subalkaline ferro-potassic plutonism at 290–296 Ma.Junta Nacional de Investigação Científica e Tecnológica - French Embassy Cooperation Programme and Research-Formation Network no. 38. PRAXIS - project 2r2.1rCTAr391r94

NADP-Dependent Isocitrate Dehydrogenase from Arabidopsis Roots Contributes in the Mechanism of Defence against the Nitro-Oxidative Stress Induced by Salinity

NADPH regeneration appears to be essential in the mechanism of plant defence against oxidative stress. Plants contain several NADPH-generating dehydrogenases including isocitrate dehydrogenase (NADP-ICDH), glucose-6-phosphate dehydrogenase (G6PDH), 6-phosphogluconate dehydrogenase (6PGDH), and malic enzyme (ME). In Arabidopsis seedlings grown under salinity conditions (100 mM NaCl) the analysis of physiological parameters, antioxidant enzymes (catalase and superoxide dismutase) and content of superoxide radical (O2  ∙−), nitric oxide (NO), and peroxynitrite (ONOO−) indicates a process of nitro-oxidative stress induced by NaCl. Among the analysed NADPH-generating dehydrogenases under salinity conditions, the NADP-ICDH showed the maximum activity mainly attributable to the root NADP-ICDH. Thus, these data provide new insights on the relevance of the NADP-ICDH which could be considered as a second barrier in the mechanism of response against the nitro-oxidative stress generated by salinity

Estimation of interfacial fracture toughness based on progressive edge delamination of a thin transparent coating on a polymer substrate

Evaluation of interfacial toughness of sub-micron-thickness layers deposited on a ductile substrate is a challenging task which has motivated different experimental approaches Fragmentation testing was used in the present study as a means of interface characterization of a silicon-nitride-coated polyimide substrate. During the test, after an initial rapid segmentation-cracking phase, the coating fragments developed edge delaminations which propagated in a stable manner with further increase in the applied strain The debonding process was modelled by the finite element method Incorporating a cohesive zone at the front of the interfacial crack The edge cracks were found to be dominated by mode II loading. By fitting the predicted delamination evolution to the experimental data for coating fragments of differing geometry, the mode II critical energy release rate was estimated at 30 J m(-2) (C) 2010 Acta Materialia Inc Published by Elsevier Ltd All rights reserve

Prediction of the adhesive fillet size for skin to honeycomb core bonding in ultra-light sandwich structures

The formation of resin fillet between honeycomb core cell walls and skin in light sandwich structures was studied to gain a better understanding of the bonding process. A method was developed for tailoring the amount of adhesive between 8 and 80 g/m2. The size of the adhesive menisci and the contact angles between the adhesive and the skin and the core materials were measured. A model was developed to predict the size of the menisci, based on the surface energy of skin and honeycomb materials. When adhesive films were used for bonding, up to 50% of the adhesive did not form the menisci whereas 100 % did when the newly-developed adhesive deposition method was used, which allowed better bonding with lower weight

Tailoring the interfacial interactions in ferroelectric fluorinated polymer/ceramic nanocomposites

In this work composites of PVDF-TrFE containing 60 vol% untreated and surface modified BaTiO3 were produced by solvent casting with two procedures. Their morphology and structure were characterized by scanning electron microscope, X-ray diffraction and differential scanning calorimetry. The effect of the processing conditions and of the surface modification of BaTiO3 on the viscoelastic, dielectric and piezoelectric properties was investigated. The surface modification of BaTiO3 allowed obtaining composite films with low porosity and good filler dispersion, and hence higher storage modulus and lower loss tangent, in a wider processing window. Furthermore it reduced the dielectric losses at low frequency and modified the decay kinetic of the d33 piezoelectric coefficient with respect to composites made with untreated particles