Role of plasticity-induced crack closure in fatigue crack growth

The premature contact of crack surfaces attributable to the near-tip plastic deformations under cyclic loading, which is commonly referred to as plasticity induced crack closure (PICC), has long been focused as supposedly controlling factor of fatigue crack growth (FCG). Nevertheless, when the plane-strain near-tip constraint is approached, PICC lacks of straightforward evidence, so that its significance in FCG, and even the very existence, remain debatable. To add insights into this matter, large-deformation elastoplastic simulations of plane-strain crack under constant amplitude load cycling at different load ranges and ratios, as well as with an overload, have been performed. Modeling visualizes the Laird-Smith conceptual mechanism of FCG by plastic blunting and re-sharpening. Simulation reproduces the experimental trends of FCG concerning the roles of stress intensity factor range and overload, but PICC has never been detected. Near-tip deformation patterns discard the filling-in a crack with material stretched out of the crack plane in the wake behind the tip as supposed PICC origin. Despite the absence of closure, load-deformation curves appear bent, which raises doubts about the trustworthiness of closure assessment from the compliance variation. This demonstrates ambiguities of PICC as a supposedly intrinsic factor of FCG and, by implication, favors the stresses and strains in front of the crack tip as genuine fatigue drivers

Plastic zone evolution near a crack tip and its role in environmentally assisted cracking

This paper analyzes the effects of crack tip plastic strains and compressive residual stresses, createdby fatigue pre-cracking, on environmentally assisted cracking of pearlitic steel subjected to localized anodicdissolution and hydrogen assisted fracture. In both situations, cyclic crack tip plasticity improves the behavior ofthe steel. In the respective cases, the effects are supposed to be due to accelerated local anodic dissolution ofthe cyclic plastic zone (producing chemical crack blunting) or to the delay of hydrogen entry into the metalcaused by residual compressive stresses, thus increasing the fracture load in aggressive environment

Fatigue cracking in high-strength cold-drawn pearlitic steel wires for anchorage in rocks

AbstractThis paper analyzes the fatigue crack path in two pearlitic steels with very different microstructural arrangement: a hot rolled pearlitic steel bar and a commercial high-strength cold drawn prestressing steel wire frequently used for anchorage in rocks. In both materials, fatigue cracks are mostly transcollonial and tend to fracture pearlitic lamellae, so that many different micro-phenomena appear such as non-uniform crack opening displacement, micro-discontinuities, branchings, bifurcations and frequent local deflections, all of them creating a sort of microstructural roughness with regard to the fatigue crack path which is different in hot rolled bar and in the cold drawn wire

Desarrollo de embriones somáticos de alcornoque en biorreactores de inmersión temporal.

El sistema de inmersión temporal (SIT) se basa en el empleo de medio líquido y el establecimiento de ciclos de aporte y retirada del medio. Esto permite aprovechar las ventajas del medio líquido (favorece la absorción de nutrientes, el crecimiento del material vegetal y la dilución de los metabolitos excretados), al tiempo que reduce sus inconvenientes (suprime el contacto continuo de los explantos con el medio de cultivo, evitando los problemas de asfixia y vitrificación). Por otra parte el SIT facilita el control y el estudio de la nutrición en el cultivo in vitro y puede contribuir al desarrollo óptimo de los explantos. En este trabajo presentamos la puesta a punto de un SIT aplicado a la obtención de embriones somáticos de albaricoque

Influencia de los factores de riesgo epidemiológico en el abandono del tratamiento de personas afectadas por la tuberculosis pulmonar, Lima Este

En el Perú la Estrategia Sanitaria Nacional de Prevención y Control de la Tuberculosis (ESNPCT) no refleja efectividad en los resultados, teniendo, por el contrario un incremento de los casos MDR y XMDR, que tienen como causa principal a la creciente tasa de casos de abandono al tratamiento anti tuberculosis pulmonar. Esta investigación se hizo con el propósito de determinar los factores de riesgo epidemiológico que influyen en las personas que abandonan el tratamiento de tuberculosis pulmonar dentro de la jurisdicción de la DISA IV de Lima Este, la misma que contiene la mayor prevalencia de casos de TBC a nivel nacional. La investigación fue básica, retrospectiva, teniendo como técnica al análisis documentario. La muestra fueron 456 casos seleccionados de 1034 reportes. El análisis de datos consistió en la identificación de los factores más relevantes que influyen en la condición de abandono al tratamiento, para ello en primer lugar se empleó la regresión logística multinomial. Con el resultado de este análisis se procedió a determinar el nivel de influencia en el abandono, usando para ello la prueba de independencia de Chi Cuadrado (α= 0,05) y el estadístico eta (η), finalmente se calculó el riesgo relativo de cada factor. En términos generales: de los factores socioeconómicos significativos dos (2) fueron considerablemente influyentes: el indicador grado de instrucción (p=0,000<0,05) y en el indicador Si estuvo enfermo hace 3 años (p=0,000<0,05) que aportó en un 61,47% de influencia con un factor de riesgo alto (3,621); De los factores de salud familiar el indicador hacinamiento fue significativo (p=0,015<0,05) aportando un 2,75% de influencia y el indicador Delincuencia (p=0,000<0,05) que aporta un 12,84% de influencia siendo un factor de riesgo (1,575), finalmente de los factores limitantes los indicadores Intolerancia a los medicamentos (p=0,000<0,05) aportando con un 14,89% y siendo un factor de riesgo alto (7,690), los motivos económicos (p=0,000<0,05) aportó con 34,64% siendo un factor de riesgo (2,988) y las complicaciones con otra enfermedad (p=0,000<0,05) aportando con un 16% y siendo un factor de riesgo (1,714)

Insertion into the labour market of people living in poverty and/or the socially excluded

Iniciar una reflexió sobre la situació de les persones allunyades del mercat laboral a causa de les seves condicions de pobresa i d'exclusió, ens aboca irremeiablement a constatar les dificultats afegides que l'actual context de crisi comporta per a un substancial nombre de persones en edat activa, pero socialment i personalment molt copejades. Els qui, ja per si mateixos, tenen molt difícil accedir a una feina en aquest mercat tan fortament competitiu i precari, veuen disminuir les seves possibilitals reals d'inserció.Iniciar una reflexión sobre la situación de las personas alejadas de mercado laboral en razón de sus condiciones de pobreza y de exclusión, nos aboca irremediablemente a constatar las dificultades añadidas que el actual contexto de crisis comporta para un sustancial número de personas en edad activa, pero social y personalmente muy golpeada. Quienes, ya de por si, tienen muy difícil acceder a un empleo en este mercado tan fuertemente competitivo y precario, están viendo disminuir sus posibilidades reales de inserción.To initiate thought on the situation of people not able to join the labour market through poverty and exclusion , we are irrevocably heading lowards finding out what added difficullies thtl the current economic crisis will bring for a substantiant number of people of working age, but who have received hard knocks in their social and personal lives. People who already filnd il very difficulty to find work in such a strongly competitive and precarious market are watching their real chances of insertion slip away

Fatigue crack propagation in cold drawn steel

[EN]This paper deals with the fatigue crack growth in pearlitic steel wires in the form of hot rolled bar and cold drawn wire. The progress of crack front was analysed by means of the evolution of the aspect ratio with the relative crack depth, and this latter was evaluated during the tests by a compliance method. Results show that cold drawing is beneficial from the point of view of crack growth rate, i.e., cracking is slower in the cold drawn wire (final commercial product) than in the hot rolled bar (base material). In spite of the oriented pearlitic microstructure of the cold drawn steel, fatigue crack propagation develops in mode I, i.e., cracking takes place by maintaining its original plane. A materials science reasoning is proposed to explain this behaviour on the basis of the pearlitic microstructure of the steel and the large geometry changes in the vicinity of the crack tip

Naturaleza y ciudad. Perspectivas para la ordenación de la infraestructura verde en los planes territoriales metropolitanos en España

La infraestructura verde se está asentando en los últimos años como una herramienta clave para la planificación del espacio libre metropolitano desde una perspectiva estratégica y multifuncional. Ante la gran diversidad de aproximaciones teóricas y aplicadas a este concepto que coexisten en la actualidad, el presente trabajo plantea como objetivos abordar una revisión crítica del mismo y explorar sus posibilidades de implementación a través la planificación territorial a escala metropolitana, para lo cual se realiza un análisis comparado de los planes de Barcelona, Bilbao y Sevilla

Growth data from a field trial of Quercus suber plants regenerated from selected trees and their half-sib progenies by somatic embryogenesis

The development of reliable clonal propagation technologies is a requisite for performing Multi-Varietal Forestry (MVF). Somatic embryogenesis is considered the tissue culture based method more suitable for operational breeding of forest trees. Vegetative propagation is very difficult when tissues are taken from mature donors, making clonal propagation of selected trees almost impossible. We have been able to induce somatic embryogenesis in leaves taken from mature oak trees, including cork oak (Quercus suber). This important species of the Mediterranean ecosystem produces cork regularly, conferring to this species a significant economic value. In a previous paper we reported the establishment of a field trial to compare the growth of plants of somatic origin vs zygotic origin, and somatic plants from mature trees vs somatic plants from juvenile seedlings. For that purpose somatic seedlings were regenerated from five selected cork oak trees and from young plants of their half-sib progenies by somatic embryogenesis. They were planted in the field together with acorn-derived plants of the same families. After the first growth period, seedlings of zygotic origin doubled the height of somatic seedlings, showing somatic plants of adult and juvenile origin similar growth. Here we provide data on height and diameter increases after two additional growth periods. In the second one, growth parameters of zygotic seedlings were also significantly higher than those of somatic ones, but there were not significant differences in height increase between seedlings and somatic plants of mature origin. In the third growth period, height and diameter increases of somatic seedlings cloned from the selected trees did not differ from those of zygotic seedlings, which were still higher than data from plants obtained from somatic embryos from the sexual progeny. Therefore, somatic seedlings from mature origin seem not to be influenced by a possible ageing effect, and plants from somatic embryos tend to minimize the initial advantage of plants from acorn