Finite element modelling and analysis of crack shape evolution in mode-I fatigue Middle Cracked Tension specimens

A numerical procedure was employed to study the shape evolution of fatigue cracks in Middle Cracked Tension specimens. This iterative procedure consists of a 3D finite element analysis to obtain the displacement field in the cracked body, calculation of stress intensity factors along crack front and definition of local crack advances considering the Paris law. Numerical predictions were compared with experimental crack shapes with a good agreement. The evolution of crack shape was analysed for different propagation conditions considering robust dependent parameters. Two main propagation stages were identified: an initial transient stage highly dependent on initial crack shape and a stable stage where the crack follows preferred paths. Mathematical models were proposed for transient and stable stages consisting of exponential and polynomial functions, respectively. The transition between both stages was defined considering two criteria: the rate of shape variation and the distance to stable shape. Finally, the crack shape change was linked with the distribution of stress intensity factor along crack front.http://www.sciencedirect.com/science/article/B6V2R-4RGFD3P-1/1/e08212c92d2476b00aceb521ee4e521

A parameter for quantitative analysis of plasticity induced crack closure

Numerical models have been successfully developed to predict plasticity induced crack closure (PICC). However, despite the large research effort a full understanding of the links between physical parameters, residual plastic wake and PICC has not been achieved yet. The plastic extension of material behind crack tip, Δyp, obtained by the integration of vertical plastic deformation perpendicularly to crack flank, is proposed here to quantify the residual plastic field. The values of Δyp and PICC were obtained numerically in a M(T) specimen using the finite element method. An excellent correlation was found between PICC and Δyp which indicates that this parameter controls the phenomenon, and can be used to quantify the effect of physical parameters. An empirical model was developed to predict PICC assuming that the residual plastic field is a set of vertical plastic wedges, that the linear superposition principle applies and that the influence of a particular wedge exponentially decreases with distance to crack tip. The model was applied successfully to predict PICC for different residual plastic fields which provided an additional validation of Δyp as the parameter controlling PICC

Three-dimensional numerical simulation of Vickers indentation tests

The development of depth sensing indentation equipment has allowed easy and reliable determination of two of the most popular measured mechanical properties of materials: the hardness and the Young's modulus. However, some difficulties emerge in the experimental procedure to calculate accurate values of these properties. This is related to, for example, the tip geometrical imperfections of the diamond pyramidal indenter and the definition of the contact area at the maximum load. Being so, numerical simulation of ultramicrohardness tests can be a helpful tool for better understanding of the influence of these parameters on procedures for determining the hardness and the Young's modulus. For this purpose, specific finite element simulation software, HAFILM, was developed to simulate the ultramicrohardness tests. Different mesh refinements were tested because of the dependence between the values of the mechanical properties and the size of the finite element mesh. Another parameter studied in this work is the value of the friction coefficient between the indenter and the sample in the numerical simulation. In order to obtain numerical results close to reality, a common geometry and size of the imperfection of the tip of Vickers indenter was taken into account for the numerical description of the indenter.http://www.sciencedirect.com/science/article/B6VJS-4FXNRM9-2/1/fc471d8e5a726ca5ef37aaf842f333c

Complications of endomyocardial biopsy in heart transplant patients: a retrospective study of 2117 consecutive procedures

BACKGROUND: Endomyocardial biopsy (EMB) remains the gold standard for the diagnosis of graft rejection after heart transplantation (HT). Our purpose was to evaluate the rate of complications of this invasive procedure. METHODS: This was a retrospective study of 175 patients, who were transplanted between November 2003 and October 2010 and survived more than 1 month after surgery. We evaluated the number of inconclusive EMB and described the incidence, nature, and subsequent management of several complications associated with this procedure. RESULTS: Over a period of approximately 7 years, we performed 2217 EMB yielding 4972 specimens, namely, an average of 2.3 fragments per procedure. The majority of EMBs (95.3%) were performed by the femoral approach. Only 12 EMB (0.57%) were inconclusive. The overall complication rate was 0.71%. During puncture, one patient experienced a vasovagal reaction and another one, a femoral artery false aneurysm. During the biopsy, there was one case of cardiac perforation with tamponade, two cases of supraventricular tachycardia, and three atrioventricular conduction abnormalities. In 19 patients, histological analysis revealed chordal tissue, but only two patients developed mild tricuspid regurgitation. We observed five cases of coronary artery fistulae. The clinical outcomes were favorable in all cases. CONCLUSION: EMB proved to be a suitable, safe method to monitor rejection after HT

Ankle sprains: from diagnosis to management. the physiatric view

Ankle injuries are the most frequently encountered injuries in clinical practice. They are often managed by general practicians, and not only by orthopaedic or physiatric physicians. This injury is usually non-complicated, but some care should be taken to assure an adequate management and to exclude severe lesions. The stability of the ankle is necessary for functional activity of lower extremity, allowing walking and participation in other high demanding activities like running or jumping. There is a constant concern in adopting the best diagnostic and treatment procedures to enhance the recovery and to prevent the chronic joint instability. According to this, there should be proposed comprehensive strategies focusing the rehabilitation view. The ankle is a complex articular structure with contributions from the talocrural, subtalar, and inferior tibiofibular joints. The full understanding of the functional anatomy and biomechanics is the first step for the evaluation of the etiologic factors. The recognition of the mechanism of injury, and the risk factors, should be carefully addressed to make an accurate diagnosis, proper management and to implement prophylactic measures, knowing that the lateral ligamentous complex is the most commonly injured. As always, diagnosis can be made taking an adequate history, performing a thorough physical examination, and when necessary, requesting complementary studies. The priority in initial assessment it's to clear out some severe complications, like fractures, that can mimic or that can be associated with ankle sprains. Although the conventional radiology is suitable for most cases, that has been greatly improved through the institution of the Ottawa Rules, in selected patients the severity of the damage is best evaluated with other imaging resources. Treatment of acute ankle sprains depends on the severity of the injury. Most acute lateral ligament injuries are best treated nonsurgically and will regain satisfactory ankle stability after functional treatment. The conservative approach has been found to be equally effective in treating grade I and II ankle sprains. Some controversy still exists regarding the appropriate treatment of grade III injuries. General belief is that the majority of these patients may also be treated well with conservative management. Surgical reconstruction may be necessary, in cases that develop chronic functional instability, and especially in athletes with high demands on ankle joint stability. The purpose of this article is to review the biomechanics, clinical examination, diagnosis, management and secondary prevention of ankle sprains. We discuss the use and benefit of different modalities and outline a three-phase intervention program of rehabilitation based on recent guidelines

Comparative Analysis of Fatigue Life Predictions in Lateral Notched Round Bars under Multiaxial Loading

The main objective of this study is to predict the multiaxial fatigue life for lateral notched round bars made of DIN 34CrNiMo6 high strength steel. In-phase bending-torsion tests for different stress amplitudes and different ratios of the normal stress to the shear stress were performed. Single torsion and single bending tests were also conducted for different stress amplitudes. In addition, the beach marking technique was used to mark crack fronts on fracture surfaces for the different loading paths studied. Experimental fatigue life was compared with predictions obtained by the Coffin-Manson model. The stress-strain state at the notch root was evaluated using the Theory of Critical Distances (TCD) and the Equivalent Strain Energy Density (ESED). Finally, a comparative analysis of the fatigue life predictions obtained by the two above-mentioned approaches was carried out. The former gave good results for higher lives but was too conservative for lower lives. The latter led to good correlations in the range studied here

Stability Properties of Networks with Interacting TCP Flows

The equilibrium distributions of a Markovian model describing the interaction of several classes of permanent connections in a network are analyzed. It has been introduced by Graham and Robert. For this model each of the connections has a self-adaptive behavior in that its transmission rate along its route depends on the level of congestion of the nodes on its route. It has been shown that the invariant distributions are determined by the solutions of a fixed point equation in a finite dimensional space. In this paper, several examples of these fixed point equations are studied. The topologies investigated are rings, trees and a linear network, with various sets of routes through the nodes

Fatigue life predictions in polymer particle composites

This paper presents a study on fatigue life predictions in three polymer particle composites with different volume fractions of filler and different particle sizes. Central hole notched specimens were analysed using a fracture mechanics approach. A solution for the stress intensity factor of corner cracks at a hole was obtained using the finite element method and considering quarter-circular and quarter-elliptical cracks of different sizes. The solution was compared with a literature solution and significant differences were found. Fatigue crack propagation tests were performed at room temperature and constant loading amplitude, for stress ratios R=0 and R=-0.75. Finally, fatigue lives, crack shape evolution and final crack length were predicted assuming an initial crack size and considering that the crack maintains a quarter-elliptical shape. The comparison with experimental fatigue lives indicated the presence of initial defects larger than the silica particles; however, these large sizes can be explained by the residual stresses measured near the hole.http://www.sciencedirect.com/science/article/B6V35-454FDJ5-7/1/4231c9abb6a4b1b364cde431359b1e1