Simulación de la rotura de barras de acero perlítico sometidas a tracción mediante un modelo de fisura cohesiva. Validación experimental

Las probetas cilíndricas fabricadas con materiales metálicos de elevada ductilidad, como el aluminio o el cobre, sometidas a tracción suelen presentar una rotura comúnmente denominada rotura en copa y cono, debido a su geometría. Este tipo de rotura se reproduce numéricamente con éxito mediante el modelo de Gurson-Tvergaard- Needleman, cuya formulación matemática se basa en el fenómeno físico de nucleación, crecimiento y coalescencia de microhuecos. A diferencia de dichos materiales, las barras de acero perlítico, material con una ductilidad apreciable, presentan un frente de rotura plano que no puede simularse correctamente con los modelos antes mencionados, apareciendo una región interior de daño que, en principio, también puede atribuirse a un fenómeno de nucleación y crecimiento de microhuecos, mientras que en el exterior aparece una zona cuya micrografía permite asociar su rotura a un mecanismo de clivaje. En trabajos anteriores los autores han presentado un elemento de intercara cohesivo dependiente de la triaxialidad de tensiones que, incorporado a un código de elementos finitos, permite reproducir de forma razonable el daño que se desarrolla en la región interior mencionada. En este trabajo se presentan los resultados de una campaña experimental que permite validar el modelo desarrollado. Para ello, se ensayan probetas de diferentes diámetros y se comparan los resultados con los obtenidos numéricamente, empleando tres bases extensométricas diferentes en cada uno de los diámetros. Los resultados numéricos se ajustan razonablemente bien a los obtenidos experimentalmente.The cylindrical specimens made of high-ductility metallic materials, such as aluminium and copper, usually fail showing a fracture surface commonly known as cup-cone fracture because of its shape. This type of fracture is successfully reproduced using the Gurson-Tvergaard-Needleman model, which is based on the physical process of nucleation, growth and coalescence of microvoids. Unlike these materials, pearlitic steel bars, which are considerably ductile, show a flat fracture surface that cannot be correctly reproduced with the aforementioned models. In this flat fracture surface, a dark region can be observed in the centre of the specimen, which is the result of a process of nucleation and growth of microvoids, while in the rest of the fracture surface a different region can be identified, which a micrographic study reveals to be the result of a process of cleavage. In previous works, the authors presented a triaxiality-dependent cohesive interface element that, implemented in a finite element code, can reproduce in a reasonably accurate manner the damage that takes place in the dark region mentioned before. The results of an experimental campaign designed to validate the model are presented in this paper. For it, different diameter specimens are tested and these results are compared to those obtained with the numerical models, using three different initial lengths for the strain. Numerical results agree reasonably well with those obtained experimentally.

Análisis del amortiguamiento en vidrio estructural bajo cargas de impacto

Los últimos avances tecnológicos y las tendencias arquitectónicas actuales han impulsado notablemente el uso de una gran variedad de productos de vidrio en la construcción que, en función de sus propias características, permiten diseñar y calcular elementos estructurales de este material bajo condiciones de seguridad. Este trabajo presenta la evaluación y el análisis de las propiedades de amortiguamiento de placas rectangulares de vidrio laminado de 1938 mm x 876 mm con distinto espesor, según el número de capas intermedias de PVB dispuestas. Mediante simulación numérica por elementos finitos y su posterior comprobación experimental, utilizando análisis modal, se determinaron las frecuencias naturales y el amortiguamiento de las placas de vidrio, tanto en condiciones de contorno libre, como en condiciones operacionales del equipo de ensayos de impacto utilizado en el programa experimental, siguiendo las especificaciones de la norma europea UNE-EN 12600:2003.

Obtención de la tensión de fractura del vidrio en ensayos de anillos concéntricos utilizando MEF

Las discrepancias entre las funciones de distribución acumulada de la tensión de fractura del vidrio es una de las dificultades encontradas para incorporar la filosofía probabilista para el diseño del vidrio. Se utilizan tres ensayos para la caracterización de la tensión de fractura del vidrio: flexión de viga en cuatro puntos, ensayo con dobles anillos concéntricos con pequeña superficie y con gran superficie. La comparación directa de las funciones de distribución presenta discrepancias claras relacionadas con el estado de tensión -uni o biaxial- y con el comportamiento de la estructura -grandes o pequeños desplazamientos. Con un procedimiento iterativo, se ajustan los resultados de los ensayos (carga - presión- probabilidad de rotura) a través de un modelo de elementos finitos que permite estimar las aéreas efectivas de cada espécimen. Los parámetros finales para la tensión de rotura mejoran la estimación de la probabilidad de la carga de rotura de cada probeta con errores inferiores al 10%.

Estudio de la fisuración en muestras de hormigón debido a la corrosión de las armaduras

La corrosión del acero en estructuras de hormigón armado puede causar fisuración debido a la expansión volumétrica del óxido respecto al acero inicial. Para estudiar esta fisuración, se realizaron ensayos de corrosión acelerada en primas de hormigón con un tubo liso de acero como armadura. Después de corrosión, se estudió el patrón de fisuración resultante en secciones transversales de los prismas y se realizó un análisis cuantitativo para comparar el patrón entre muestras. Complementariamente, los ensayos se simularon utilizando un modelo que reproduce la expansión del óxido y que trabaja con elementos finitos con fisura embebida adaptable para reproducir la fractura del hormigón según el modelo de fisura cohesiva estándar. Los resultados numéricos coincidieron con los experimentales, aunque en secciones con tubo se observó mayor sensibilidad a los parámetros del óxido que en secciones con barra maciza, que se investigará en el futuro para obtener información sobre el óxido

Caracterización probabilística de vidrio mediante diferentes tipos de ensayos

El vidrio no puede ser tratado como un material estructural convencional desde el punto de vista de la resistencia mecánica. Su naturaleza, como material frágil, junto con la inevitable presencia de microfisuras en su superficie y las consecuencias de accidentes por posibles fallos, exigen métodos rigurosos que garanticen un cálculo seguro de los elementos estructurales de vidrio, cuya resistencia a rotura depende en gran medida del tamaño del elemento y del tipo de carga a la que está sometido. Por lo tanto, su cálculo debe basarse en conceptos probabilísticos y en criterios de mecánica de la fractura, en sustitución de un cálculo convencional de vidrio según tablas deducidas de programas experimentales y posterior aplicación del concepto de tensiones admisibles. Con el fin de analizar y comparar las características mecánicas de vidrios templados, termoendurecidos y recocidos, se realizó un amplio programa experimental de ensayos de flexión a cuatro puntos y de anillos concéntricos de pequeña superficie, seguido de un ajuste de los resultados mediante una función de distribución triparamétrica de Weibull. Glass cannot be handled as a conventional structural material from the point of view of the mechanical strength. Its nature as brittle material, together with the inevitable presence of micro-cracks on its surface and the consequences of eventual failures, demand rigorous methods to achieve a safe design for glass elements, whose stress resistance is very much dependent on the integrity of its surface, element size and loading pattern. Thus, its design must rely on probabilistic concepts and fracture mechanics criteria, substitutive of the conventional glass design based on charts derived from experimental programs and subsequent application of the admissible stress concept. In order to analyze and compare the strength characteristics of tempered, heat-strengthened and annealed glass, a large experimental programme based on four-point bending and coaxial double ring tests was performed and the results were fitted using a three-parameter Weibull cumulative distribution function.

Influencia de un recubrimiento de vinilo en la tensión de fractura del vidrio en ensayos de anillos concéntricos con gran superficie

El comportamiento post-rotura de los vidrios laminados es uno de los temas que están siendo investigados para explicar la capacidad de carga remanente tras la rotura de la primera lámina. En investigaciones previas se ha observado que en el caso de impacto humano en vidrios recocidos se llega a una capacidad hasta 3 veces superior, sin explicación clara del comportamiento estructural del conjunto. Para realizar un acercamiento a la resistencia a la rotura del vidrio laminado se ha planificado una campaña de ensayos de rotura con anillos concéntricos de grandes superficies en vidrio recocido, termoendurecido y templado, con dos series adicionales de vidrio recocido y termoendurecido con una capa de butiral adherida justo después del proceso de fabricación. Para realizar la comparación de las distribuciones de Weibull de las distintas tensiones de rotura se utiliza un proceso iterativo basado en la distribución real de tensiones obtenida con un modelo de elementos finitos ajustado con datos experimentales. Las comparaciones finales muestran un aumento apreciable de la resistencia (45%) en el caso de vidrios recocidos, y menor en el de los termoendurecidos (25%).The post-fracture behavior of the laminated glasses is one of the research topics that are being studied to explain the load capacity after the break of the first sheet. Previous experimental work have shown, that in case of human impact in annealed glasses, the capacity of bearing load it can be up to 3 times higher without clear explanation of the structural behavior of the plate. To make an approximation to the post-fracture resistance, a experimental program to test annealed, heat-tempered and toughened glass plates has been prepared. Two additional series of annealed and heattempered, with a layer of polyvinyl butyral adhered just after the manufacturing process, have also been incorporated. Coaxial Double Ring with large test surface areas Coaxial Double Ring with large test surface areas is the standard that has been followed. To make the comparison of Weibull's distributions of the different fracture stress, an iterative process based on the actual stress distribution obtained with a finite elements model updated with experimental results has been used. Final comparisons show a great stress improvement for the annealed glass plates (45 %), and a minor increment for the heat-tempered (25 %).

Binding of high-risk human papillomavirus E6 oncoproteins to the human homologue of the Drosophila discs large tumor suppressor protein

In the majority of cervical cancers, DNAs of high-risk mucosotpropic human papillomaviruses (HPVs), such as type 16, are maintained so as to express two viral proteins, E6 and E7, suggesting an essential importance to carcinogenesis. The high-risk HPV E6 proteins are known to inactivate p53 tumor suppressor protein but appear to have an additional, molecularly unknown function(s). In this study, we demonstrate that these E6 proteins can bind to the second PDZ domain of the human homologue of the Drosophila discs large tumor suppressor protein (hDLG) through their C-terminal XS/TXV/L (where X represents any amino acid, S/T serine or threonine, and V/L valine or leucine) motif. This finding is similar to the interaction between the adenomatous polyposis coli gene product and hDLG. E6 mutants losing the ability to bind to hDLG are no longer able to induce E6-dependent transformation of rodent cells. These results suggest an intriguing possibility that interaction between the E6 protein and hDLG or other PDZ domain-containing proteins could be an underlying mechanism in the development of HPV-associated cancers.

A Rap guanine nucleotide exchange factor enriched highly in the basal ganglia

Ras proteins, key regulators of growth, differentiation, and malignant transformation, recently have been implicated in synaptic function and region-specific learning and memory functions in the brain. Rap proteins, members of the Ras small G protein superfamily, can inhibit Ras signaling through the Ras/Raf-1/mitogen-activated protein (MAP) kinase pathway or, through B-Raf, can activate MAP kinase. Rap and Ras proteins both can be activated through guanine nucleotide exchange factors (GEFs). Many Ras GEFs, but to date only one Rap GEF, have been identified. We now report the cloning of a brain-enriched gene, CalDAG-GEFI, which has substrate specificity for Rap1A, dual binding domains for calcium (Ca2+) and diacylglycerol (DAG), and enriched expression in brain basal ganglia pathways and their axon-terminal regions. Expression of CalDAG-GEFI activates Rap1A and inhibits Ras-dependent activation of the Erk/MAP kinase cascade in 293T cells. Ca2+ ionophore and phorbol ester strongly and additively enhance this Rap1A activation. By contrast, CalDAG-GEFII, a second CalDAG-GEF family member that we cloned and found identical to RasGRP [Ebinu, J. O., Bottorff, D. A., Chan, E. Y. W., Stang, S. L., Dunn, R. J. & Stone, J. C. (1998) Science 280, 1082–1088], exhibits a different brain expression pattern and fails to activate Rap1A, but activates H-Ras, R-Ras, and the Erk/MAP kinase cascade under Ca2+ and DAG modulation. We propose that CalDAG-GEF proteins have a critical neuronal function in determining the relative activation of Ras and Rap1 signaling induced by Ca2+ and DAG mobilization. The expression of CalDAG-GEFI and CalDAG-GEFII in hematopoietic organs suggests that such control may have broad significance in Ras/Rap regulation of normal and malignant states.

Association of INAD with NORPA is essential for controlled activation and deactivation of Drosophila phototransduction in vivo

Visual transduction in Drosophila is a G protein-coupled phospholipase C-mediated process that leads to depolarization via activation of the transient receptor potential (TRP) calcium channel. Inactivation-no-afterpotential D (INAD) is an adaptor protein containing PDZ domains known to interact with TRP. Immunoprecipitation studies indicate that INAD also binds to eye-specific protein kinase C and the phospholipase C, no-receptor-potential A (NORPA). By overlay assay and site-directed mutagenesis we have defined the essential elements of the NORPA–INAD association and identified three critical residues in the C-terminal tail of NORPA that are required for the interaction. These residues, Phe-Cys-Ala, constitute a novel binding motif distinct from the sequences recognized by the PDZ domain in INAD. To evaluate the functional significance of the INAD–NORPA association in vivo, we generated transgenic flies expressing a modified NORPA, NORPAC1094S, that lacks the INAD interaction. The transgenic animals display a unique electroretinogram phenotype characterized by slow activation and prolonged deactivation. Double mutant analysis suggests a possible inaccessibility of eye-specific protein kinase C to NORPAC1094S, undermining the observed defective deactivation, and that delayed activation may similarly result from NORPAC1094S being unable to localize in close proximity to the TRP channel. We conclude that INAD acts as a scaffold protein that facilitates NORPA–TRP interactions required for gating of the TRP channel in photoreceptor cells.

Protein Interactions with the Glucose Transporter Binding Protein GLUT1CBP That Provide a Link between GLUT1 and the Cytoskeleton

Subcellular targeting and the activity of facilitative glucose transporters are likely to be regulated by interactions with cellular proteins. This report describes the identification and characterization of a protein, GLUT1 C-terminal binding protein (GLUT1CBP), that binds via a PDZ domain to the C terminus of GLUT1. The interaction requires the C-terminal four amino acids of GLUT1 and is isoform specific because GLUT1CBP does not interact with the C terminus of GLUT3 or GLUT4. Most rat tissues examined contain both GLUT1CBP and GLUT1 mRNA, whereas only small intestine lacked detectable GLUT1CBP protein. GLUT1CBP is also expressed in primary cultures of neurons and astrocytes, as well as in Chinese hamster ovary, 3T3-L1, Madin–Darby canine kidney, Caco-2, and pheochromocytoma-12 cell lines. GLUT1CBP is able to bind to native GLUT1 extracted from cell membranes, self-associate, or interact with the cytoskeletal proteins myosin VI, α-actinin-1, and the kinesin superfamily protein KIF-1B. The presence of a PDZ domain places GLUT1CBP among a growing family of structural and regulatory proteins, many of which are localized to areas of membrane specialization. This and its ability to interact with GLUT1 and cytoskeletal proteins implicate GLUT1CBP in cellular mechanisms for targeting GLUT1 to specific subcellular sites either by tethering the transporter to cytoskeletal motor proteins or by anchoring the transporter to the actin cytoskeleton.

Identification of Regulators for Ypt1 GTPase Nucleotide Cycling

Small GTPases of the Ypt/Rab family are involved in the regulation of vesicular transport. Cycling between the GDP- and GTP-bound forms and the accessory proteins that regulate this cycling are thought to be crucial for Ypt/Rab function. Guanine nucleotide exchange factors (GEFs) stimulate both GDP loss and GTP uptake, and GTPase-activating proteins (GAPs) stimulate GTP hydrolysis. Little is known about GEFs and GAPs for Ypt/Rab proteins. In this article we report the identification and initial characterization of two factors that regulate nucleotide cycling by Ypt1p, which is essential for the first two steps of the yeast secretory pathway. The Ypt1p-GEF stimulates GDP release and GTP uptake at least 10-fold and is specific for Ypt1p. Partially purified Ypt1p-GEF can rescue the inhibition caused by the dominant-negative Ypt1p-D124N mutant of in vitro endoplasmic reticulum-to-Golgi transport. This mutant probably blocks transport by inhibiting the GEF, suggesting that we have identified the physiological GEF for Ypt1p. The Ypt1p-GAP stimulates GTP hydrolysis by Ypt1p up to 54-fold, has a higher affinity for the GTP-bound form of Ypt1p than for the GDP-bound form, and is specific to a subgroup of exocytic Ypt proteins. The Ypt1p-GAP activity is not affected by deletion of two genes that encode known Ypt GAPs, GYP7 and GYP1, nor is it influenced by mutations in SEC18, SEC17, or SEC22, genes whose products are involved in vesicle fusion. The GEF and GAP activities for Ypt1p localize to particulate cellular fractions. However, contrary to the predictions of current models, the GEF activity localizes to the fraction that functions as the acceptor in an endoplasmic reticulum-to-Golgi transport assay, whereas the GAP activity cofractionates with markers for the donor. On the basis of our current and previous results, we propose a new model for the role of Ypt/Rab nucleotide cycling and the factors that regulate this process.

GIPC and GAIP Form a Complex with TrkA: A Putative Link between G Protein and Receptor Tyrosine Kinase Pathways

NGF initiates the majority of its neurotrophic effects by promoting the activation of the tyrosine kinase receptor TrkA. Here we describe a novel interaction between TrkA and GIPC, a PDZ domain protein. GIPC binds to the juxtamembrane region of TrkA through its PDZ domain. The PDZ domain of GIPC also interacts with GAIP, an RGS (regulators of G protein signaling) protein. GIPC and GAIP are components of a G protein-coupled signaling complex thought to be involved in vesicular trafficking. In transfected HEK 293T cells GIPC, GAIP, and TrkA form a coprecipitable protein complex. Both TrkA and GAIP bind to the PDZ domain of GIPC, but their binding sites within the PDZ domain are different. The association of endogenous GIPC with the TrkA receptor was confirmed by coimmunoprecipitation in PC12 (615) cells stably expressing TrkA. By immunofluorescence GIPC colocalizes with phosphorylated TrkA receptors in retrograde transport vesicles located in the neurites and cell bodies of differentiated PC12 (615) cells. These results suggest that GIPC, like other PDZ domain proteins, serves to cluster transmembrane receptors with signaling molecules. When GIPC is overexpressed in PC12 (615) cells, NGF-induced phosphorylation of mitogen-activated protein (MAP) kinase (Erk1/2) decreases; however, there is no effect on phosphorylation of Akt, phospholipase C-γ1, or Shc. The association of TrkA receptors with GIPC and GAIP plus the inhibition of MAP kinase by GIPC suggests that GIPC may provide a link between TrkA and G protein signaling pathways.

A Millennial Myosin Census

The past decade has seen a remarkable explosion in our knowledge of the size and diversity of the myosin superfamily. Since these actin-based motors are candidates to provide the molecular basis for many cellular movements, it is essential that motility researchers be aware of the complete set of myosins in a given organism. The availability of cDNA and/or draft genomic sequences from humans, Drosophila melanogaster, Caenorhabditis elegans, Arabidopsis thaliana, Saccharomyces cerevisiae, Schizosaccharomyces pombe, and Dictyostelium discoideum has allowed us to tentatively define and compare the sets of myosin genes in these organisms. This analysis has also led to the identification of several putative myosin genes that may be of general interest. In humans, for example, we find a total of 40 known or predicted myosin genes including two new myosins-I, three new class II (conventional) myosins, a second member of the class III/ninaC myosins, a gene similar to the class XV deafness myosin, and a novel myosin sharing at most 33% identity with other members of the superfamily. These myosins are in addition to the recently discovered class XVI myosin with N-terminal ankyrin repeats and two human genes with similarity to the class XVIII PDZ-myosin from mouse. We briefly describe these newly recognized myosins and extend our previous phylogenetic analysis of the myosin superfamily to include a comparison of the complete or nearly complete inventories of myosin genes from several experimentally important organisms.

Controlling small guanine–nucleotide-exchange factor function through cytoplasmic RNA intramers

ADP-ribosylation factor (ARF) GTPases and their regulatory proteins have been implicated in the control of diverse biological functions. Two main classes of positive regulatory elements for ARF have been discovered so far: the large Sec7/Gea and the small cytohesin/ARNO families, respectively. These proteins harbor guanine–nucleotide-exchange factor (GEF) activity exerted by the common Sec7 domain. The availability of a specific inhibitor, the fungal metabolite brefeldin A, has enabled documentation of the involvement of the large GEFs in vesicle transport. However, because of the lack of such tools, the biological roles of the small GEFs have remained controversial. Here, we have selected a series of RNA aptamers that specifically recognize the Sec7 domain of cytohesin 1. Some aptamers inhibit guanine–nucleotide exchange on ARF1, thereby preventing ARF activation in vitro. Among them, aptamer M69 exhibited unexpected specificity for the small GEFs, because it does not interact with or inhibit the GEF activity of the related Gea2-Sec7 domain, a member of the class of large GEFs. The inhibitory effect demonstrated in vitro clearly is observed as well in vivo, based on the finding that M69 produces similar results as a dominant-negative, GEF-deficient mutant of cytohesin 1: when expressed in the cytoplasm of T-cells, M69 reduces stimulated adhesion to intercellular adhesion molecule-1 and results in a dramatic reorganization of F-actin distribution. These highly specific cellular effects suggest that the ARF-GEF activity of cytohesin 1 plays an important role in cytoskeletal remodeling events of lymphoid cells.

Interaction with mLin-7 Alters the Targeting of Endocytosed Transmembrane Proteins in Mammalian Epithelial Cells

To investigate the targeting mechanism for proteins bound to the mammalian Lin-7 (mLin-7) PDZ domain, we created receptor protein chimeras composed of the carboxyl-terminal amino acids of LET-23 fused to truncated nerve growth factor receptor/P75. mLin-7 bound to the chimera with a wild-type LET-23 carboxyl-terminal tail (P75t-Let23WT), but not a mutant tail (P75t-Let23MUT). In Madin-Darby canine kidney (MDCK) cells, P75t-Let23WT localized to the basolateral plasma membrane domain, whereas P75t-Let23MUT remained apical. Furthermore, mutant mLin-7 constructs acted as dominant interfering proteins and inhibited the basolateral localization of P75t-Let23WT. The mechanisms for this differential localization were examined further, and, initially, we found that P75t-Let23WT and P75t-Let23MUT were delivered equally to the apical and basolateral plasma membrane domains. Although basolateral retention of P75t-Let23WT, but not P75t-Let23MUT, was observed, the greatest difference in receptor localization was seen in the rapid trafficking of P75t-Let23WT to the basolateral plasma membrane domain after endocytosis, whereas P75t-Let23MUT was degraded in lysosomes, indicating that mLin-7 binding can alter the fate of endocytosed proteins. Altogether, these data support a model for basolateral protein targeting in mammalian epithelial cells dependent on protein–protein interactions with mLin-7, and also suggest a dynamic role for mLin-7 in endosomal sorting.