A study by the lattice discrete element method for exploring the fractal nature of scale effects

Nowadays, there are many applications in the field of Engineering related to quasi-brittle materials such as ceramics, natural stones, and concrete, among others. When damage is produced, two phenomena can take place: the damage produced governs the collapse process when working with this type of material, and its random nature rules the nonlinear behavior up to the collapse. The interaction among clouds of micro-cracks generates the localization process that implies transforming a continuum domain into a discontinue one. This process also governs the size effect, that is, the changes of the global parameters as the strength and characteristic strain and energies when the size of the structure changes. Some aspects of the scaling law based on the fractal concepts proposed by Prof Carpinteri are analyzed in this work. On the other hand, the Discrete Method is an interesting option to be used in the simulation collapse process of quasi-brittle materials. This method can allow failures with relative ease. Moreover, it can also help to relax the continuum hypothesis. In the present work, a version of the Discrete Element Method is used to simulate the mechanical behavior of different size specimens until collapse by analyzing the size effect represented by this method. This work presents two sets of examples. Its results allow the researchers to see the connection between the numerical results regarding the size effect and the theoretical law based on the fractal dimension of the parameter studied. Two main aspects appear as a result of the analysis presented here. Understand better some aspects of the size effect using the numerical tool and show that the Lattice Discrete Element Method has enough robustness to be applied in the nonlinear analysis of structures built by quasi-brittle materials

Damage process in glass fiber reinforced polymer specimens using acoustic emission technique with low frequency acquisition

The damage process in fiber-reinforced polymers (FRP) has been the subject of continuous studies in recent decades using different approaches. In particular, the acoustic emission technique has been proved to be a powerful tool in the monitoring of structures of this type of material due to the large acoustic activity captured when this material is loaded to rupture. The present work explores several indexes that were calculated from the experimental recording of acoustic emission signals, and their efficiency in describing the failure process in structures. One of these indexes, called c value, is originally proposed, and its sensibility it is compared with other classical parameters that are usually employed in the Acoustic Emission analysis. A 3-point bending test has been performed on a glass fiber reinforced polymer plate. Our own methodology was proposed to identify AE signals in semi-automatic manner. This methodology also allowed a faster analysis of the global parameters during the process data of the AE test. The global parameter evolution obtained from the acoustic emission data during the damage process could be considered as precursors of the more meaningful event and as an aid to understand in which way the structure is going to the collapse

Crack path estimation in the shot-earth 772 by a discrete element method

In the present paper, the fracture behaviour of the shot-earth 772, in terms of crack path, is numerically investigated. To such an aim, a version of the lattice discrete element method is used to numerically simulate fracture toughness testing performed, on the shot-earth 772, by employing the Modified Two-Parameter model. A comparison between the numerical crack path and the experimental ones is performed, highlighting as the lattice discrete element method is able to capture the kinked crack shape, typical of quasi-brittle material

Applications of lattice method in the simulation of crack path in heterogeneous materials

The simulation of critical and subcritical crack propagation in heterogeneous materials is not a simpleproblem in computational mechanics. These topics can be studied with different theoretical tools. In the crackpropagation problem it is necessary to lead on the interface between the continuum and the discontinuity, andthis region has different characteristics when we change the scale level point of view. In this context, this workapplies a version of the lattice discrete element method (LDEM) in the study of such matters. This approach letsus to discretize the continuum with a regular tridimensional truss where the elements have an equivalent stiffnessconsistent with the material one wishes to model. The masses are lumped in the nodes and an uni-axial bilinearrelation, inspired in the Hilleborg constitutive law, is assumed for the elements. The random characteristics of thematerial are introduced in the model considering the material toughness as a random field with defined statisticalproperties. It is important to highlight that the energy balance consistence is maintained during all the process.The spatial discretization lets us arrive to a motion equation that can be solved using an explicit scheme ofintegration on time. Two examples are shown in the present paper; one of them illustrates the possibilities of thismethod in simulating critical crack propagation in a solid mechanics problem: a simple geometry of grade material.In the second example, a simulation of subcritical crack growth is presented, when a pre-fissured quasi-brittlebody is submitted to cyclic loading. In this second example, a strategy to measure crack advance in the model isproposed. Finally, obtained results and the performance of the model are discussed

Analysis of Acoustic Emission Activity during Progressive Failure in Heterogeneous Materials: Experimental and Numerical Investigation

This work focuses on an experimental and numerical investigation into monitoring damage in a cube-shaped concrete specimen under compression. Experimental monitoring uses acoustic emission (AE) signals acquired by two independent measurement apparatuses, and the same damage process is numerically simulated with the lattice discrete element method (LDEM). The results from the experiment and simulation are then compared in terms of their failure load, final configurations, and the evolution of global parameters based on AE signals, such as the b-value coefficient and the natural time approach. It is concluded that the results from the AE analysis present a significant sensitivity to the characteristics of the acquisition systems. However, natural time methods are more robust for determining such differences, indicating the same general tendency for all three data sets

Long-Range Correlations and Natural Time Series Analyses from Acoustic Emission Signals

This work focuses on analyzing acoustic emission (AE) signals as a means to predict failure in structures. There are two main approaches that are considered: (i) long-range correlation analysis using both the Hurst (H) and the detrended fluctuation analysis (DFA) exponents, and (ii) natural time domain (NT) analysis. These methodologies are applied to the data that were collected from two application examples: a glass fiber-reinforced polymeric plate and a spaghetti bridge model, where both structures were subjected to increasing loads until collapse. A traditional (AE) signal analysis was also performed to reference the study of the other methods. The results indicate that the proposed methods yield reliable indication of failure in the studied structures

Estudio de la propagación de ondas guiadas sobre un riel ferroviario

La propagación de ondas elásticas en estructuras donde por lo menos una de las dimensiones es mucho mayor que las otras dos, las llamadas ondas guiadas, es un tema actualmente vinculado atécnicas de Ensayos No Destructivos. Varios sistemas basados en este tipo de ondas permiten definir regiones con daño en forma rápida solo con una entrada que será emisora receptora, esto tiene gran ventaja en el estudio de estructuras de difícil acceso. Otra de las características de los métodos de detección de daño basados en la propagación de este tipo de ondas es que permite también escanear con rapidez grandes regiones de inspección. Existen muchas estructuras de gran interés en ingeniería, en las cuales se puede aplicar técnicas de inspección basadas en ondas guiadas, tubos, cables, rieles, recipientes a presión, entre otros. El estudio de este tipo de ondas fue iniciado por Lamba comienzos del siglo XX, el mismo propuso una solución analítica para representar la propagación espacial-temporal de este tipode ondas para el caso de una placa. La propagación de ondas guiadas en estas estructuras tienen la característica de ser muy sensible a la forma de la dimensión finita de la estructura en análisis. El estudio de estas ondas es fundamental en el cálculo de las llamadas curvas de dispersión que serán específicaspara cada geometría estudiada. Estas curvas de dispersión permiten describir, dada una excitaciónaplicada sobre la estructura en análisis, en qué ondas modales dicha excitación se va a descomponer.Publicado en: Mecánica Computacional vol. XXXV no.33Facultad de Ingenierí

Idoneidad de la prescripción antibiótica en atención primaria en la Comunidad Autónoma Vasca

ObjetivoEstudiar la variabilidad e idoneidad de la prescripción antibiótica en atención primaria en la Comunidad Autónoma Vasca (CAV).DiseñoEstudio descriptivo, prospectivo.EmplazamientoCentros de atención primaria de la red reformada y no reformada de la CAV.Mediciones y resultados principalesSe estudiaron 3.182 infecciones obtenidas de consultas sobre enfermedad infecciosa mediante muestreo estratificado por área sanitaria durante febrero, marzo y abril de 1998. La idoneidad de la prescripción de antibióticos (ATB) se comparó con unos patrones elaborados tras una extensa revisión bibliográfica. Un 68,6% de las consultas son infecciones que no requieren ATB (catarro de vías altas [IRA], bronquitis aguda, gripe y gastroenteritis). Se utilizan ATB en un 28,5% de estos casos (IC, 26,6-30,5). Destaca la bronquitis aguda, con un 90,7% (IC, 87,3-93,4). La prescripción innecesaria supone un 39,7% de la prescripción total antibiótica (IC, 37,2-42,1%), suponiendo un gasto anual innecesario de 1.155 millones de pesetas.En el 31,4% de las infecciones que requieren tratamiento con ATB, se opta por uno no adecuado en un 23,7% (IC, 21-26,7%).En total, la idoneidad del tratamiento global fue del 72,2% (IC, 70,6-73,8). El hecho de trabajar en la red reformada se asoció a una mejor prescripción en todos los casos. Los pediatras prescriben mejor en el caso de las infecciones no susceptibles de tratamiento con ATB.ConclusionesLa prescripción de ATB no se realiza conforme a la evidencia científica disponible. Los tratamientos incorrectos tienen lugar sobre todo en procesos benignos y autolimitados. Es necesaria una política local de uso de ATB que incluya colaboración multidisciplinaria y formación continuada efectiva.ObjectiveTo study the variability and suitability of antibiotic prescription in primary care in the Basque Country.DesignProspective, descriptive study.SettingBasque Country health care centres both within and not in the reformed network.Measurements and main results3182 infections from consultations for infectious disease were studied through sampling stratified by health area during February, March and April 1998. The appropriateness of antibiotics (ATB) prescription was compared with some standards after an extensive bibliographic review. 68.6% of consultations were infections not needing ATB (infections of respiratory airways, acute bronchitis, flu and gastro-enteritis). ATB were used in 28.5% of these cases (CI: 26.6-30.5), especially in acute bronchitis (90.7%) (CI: 87.3-93.4). Unnecessary prescription supposes 39.7% of total antibiotic prescription (CI: 37.2-42.1%), which means unnecessary annual expenditure of 1155 million pesetas. In 31.4% of the infections that require ATB treatment, in 23.7% inappropriate treatment was chosen (CI: 21-26.7%). Overall appropriateness of treatment was 72.2% (CI: 70.6-73.8). Working in the reformed system was linked to better prescription in all cases. Paediatricians prescribed better in the case of infections not susceptible to ATB treatment.ConclusionsATB prescription is not consistent with the available scientific evidence. Incorrect treatments occur especially in benign and self-limiting processes. A local policy of ATB use that includes multi-disciplinary collaboration and effective ongoing training is necessary

Purification and functional characterisation of rhiminopeptidase A, a novel aminopeptidase from the venom of Bitis gabonica rhinoceros

This study describes the discovery and characterisation of a novel aminopeptidase A from the venom of B. g. rhinoceros and highlights its potential biological importance. Similar to mammalian aminopeptidases, rhiminopeptidase A might be capable of playing roles in altering the blood pressure and brain function of victims. Furthermore, it could have additional effects on the biological functions of other host proteins by cleaving their N-terminal amino acids. This study points towards the importance of complete analysis of individual components of snake venom in order to develop effective therapies for snake bites