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

Time-intensity transformation and internal stress in UV-curable hyperbranched acrylates

The photocuring of three different highly functional acrylates—Di-pentaerythritol penta/hexaacrylate (DPHA) and two hyperbranched molecules (HBP), one with a stiff polyester and one with a more flexible polyether structure—was investigated by means of photorheology, photo differential scanning calorimetry, and beam bending. Special attention was paid to the influence of the composition of DPHA/HBP reactive blends and UV intensity on gelation and vitrification and the resulting dynamics of the internal stress. It was found that adding HBPs to DPHA did not influence gelation significantly, but shifted the onset of vitrification to higher conversions and thus caused lower internal stresses in the material. Increasing UV intensity increased both the conversion at vitrification, thus retarding the build-up of internal stresses, and the ultimate conversion, thus increasing the final stress level. The obtained conversion, gelation, and vitrification data were assembled into time-intensity transformation diagrams, thus providing a useful tool for optimizing photocurin

The role of diacylglycerol lipase in constitutive and angiotensin AT(1) receptor-stimulated cannabinoid CB1 receptor activity

The cannabinoid CB1 receptor (CB1R) is,a G protein-coupled receptor, which couples to the G(i/o) family of heterotrimeric G proteins. The receptor displays both basal and agonist-induced signaling and internalization. Although basal activity of CBIRs is attributed to constitutive (agonist-independent) receptor activity, studies in neurons suggested a role of postsynaptic endocannabinoid (eCB) release in the persistent activity of presynaptic CB1Rs. To elucidate the role of eCBs in basal CB1R activity, we have investigated the role of diacylglycerol lipase (DAGL) in this process in Chinese hamster ovary (CHO) cells, which are not targeted specifically with I Agonist-induced G protein activation was determined by detecting dissociation G protein subunits expressed in CHO cells with bioluminescence resonance energy transfer (BRET), after labeling the alpha and beta subunits with Renilla luciferase and enhanced yellow fluorescent protein (EYFP), respectively. Pre-incubation of the cells with tetrahydrolipstatin (THL), a known inhibitor of DAGLs, caused inhibition of the basal activity of CBIR. Moreover, preincubation of CHO and cultured hippocampal neurons with THL increased the number of CBIRs on the cell membrane, which reflects its inhibitory action on CB1R internalization in non-simulated cells. In CHO cells co-expressing CB1R and angiotensin AT(1) receptors, angiotensin II-induced Go protein activation that was blocked by both a CBIR antagonist and THL. These data indicate that cell-derived I mediators have a general role in the basal activity of CB1Rs in non-neural cells and neurons, and that this mechanism can be stimulated by AT(1) receptor activation

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

The effect of processing conditions on the morphology, thermomechanical, dielectric, and piezoelectric properties of P(VDF-TrFE)/BaTiO3 composites

In this study (0-3) P(VDF-TrFE)/BaTiO3 composites containing up to 60vol% of ceramic phase were prepared by solvent casting or compression molding. Their thermomechanical, dielectric, and piezoelectric properties were investigated, and discussed in the light of the properties of the basic components, the processing route and the resulting morphology. The crystalline structure of the P(VDF-TrFE) matrix was found to be highly dependent on the processing route, while the structure of BaTiO3 was not affected by any of the processing steps. The mechanical properties of the solvent cast materials showed a maximum at 30vol% BaTiO3, while they increased monotonically with BaTiO3 content for compression molded materials. This difference was attributed to a higher amount of porosity and inhomogeneities in the solvent cast composites. Permittivity as high as 120 and piezoelectric coefficient d 33 up to 32pC/N were obtained for compression molded composites, and the observed decrease in d 33 with aging time was attributed to the effect of mechanical stress release in the polymer matri

Fearfulness Affects Quail Maternal Care and Subsequent Offspring Development

International audienceOur study investigated relationships between a precocial bird’s fearfulness and maternal care, and the implication of maternal care as a vector for non-genomic transmission of fearfulness to chicks. We compared care given to chicks between two sets of female Japanese quail selected to present either high (LTI) or low fearfulness (STI). Chicks, from a broiler line, were adopted by these females following a sensitization procedure. Chicks’ fearfulness after separation from their mother was assessed by well-established procedures. LTIs took longer to present maternal responses, pecked chicks more during the first days post-hatch, presented impaired maternal vocal behaviour and were globally less active than STI females. Chicks mothered by LTIs presented more fearful reactions than did chicks mothered by STIs, supporting the hypothesis of a non-genetic maternal transmission of fearfulness. We suggest that the longer latencies required by LTIs to become maternal are a consequence of their greater fear of chicks, and that their lower general and vocal activity could be components of a heightened antipredatory strategy. We discuss the transmission of maternal fearfulness to fostered chicks, taking into account the possible implication of several well-known mechanisms underlying maternal effects

Differential β-arrestin2 requirements for constitutive and agonist-induced internalization of the CB1 cannabinoid receptor

CB1 cannabinoid receptor (CB1R) undergoes both constitutive and agonist-induced internalization, but the underlying mechanisms of these processes and the role of beta-arrestins in the regulation of CB1R function are not completely understood. In this study, we followed CB1R internalization using confocal microscopy and bioluminescence resonance energy transfer measurements in HeLa and Neuro-2a cells. We found that upon activation CB1R binds beta-arrestin2 (beta-arr2), but not beta-arrestin1. Furthermore, both the expression of dominant-negative beta-arr2 (beta-arr2-V54D) and siRNA-mediated knock-down of beta-arr2 impaired the agonist-induced internalization of CB1R. In contrast, neither beta-arr2-V54D nor beta-arr2-specific siRNA had a significant effect on the constitutive internalization of CB1R. However, both constitutive and agonist-induced internalization of CB1R were impaired by siRNA-mediated depletion of clathrin heavy chain. We conclude that although clathrin is required for both constitutive and agonist-stimulated internalization of CB1R, beta-arr2 binding is only required for agonist-induced internalization of the receptor suggesting that the molecular mechanisms underlying constitutive and agonist-induced internalization of CB1R are different

Ankyrin G Membrane Partners Drive the Establishment and Maintenance of the Axon Initial Segment

International audienceThe axon initial segment (AIS) is a highly specialized neuronal compartment that plays a key role in neuronal development and excitability. It concentrates multiple membrane proteins such as ion channels and cell adhesion molecules (CAMs) that are recruited to the AIS by the scaffold protein ankyrin G (ankG). The crucial function of ankG in the anchoring of AIS membrane components is well established, but a reciprocal role of membrane partners in ankG targeting and stabilization remained elusive. In rat cultured hippocampal neurons and cortical organotypic slices, we found that shRNA-mediated knockdown of ankG membrane partners (voltage-gated sodium channels (Nav) or neurofascin-186) led to a decrease of ankG concentration and perturbed the AIS formation and maintenance. These effects were rescued by expressing a recombinant AIS-targeted Nav or by a minimal construct containing the ankyrin-binding domain of Nav1.2 and a membrane anchor (mABD). Moreover, overexpressing mABD in mature neurons led to ankG mislocalization. Altogether, these results demonstrate that a tight and precocious association of ankG with its membrane partners is a key step for the establishment and maintenance of the AIS

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