Closure Of Tip Research Articles

Study of crack propagation behavior in single crystalline tetragonal zirconia with the phase field method

The tetragonal to monoclinic phase transformation and its toughening effect on the Mode I crack propagation in single crystalline zirconia is investigated by a coupled phase field model. The developed phase field model can simulate the evolution of the tetragonal to monoclinic phase transformation and crack propagation simultaneously. Two tetragonal lattice orientations: 0° and 90°, are studied and compared with the case that the material is untransformable. The numerical results show that the phase transformation always initiates from the crack tip. The phase transformation dramatically reduces the normal stress near the crack tip and slows down the crack growth speed for both orientation angles. A closure of the crack tip may even be observed shortly after the phase transformation, which is caused by the transformation stain and the stress re-distribution at the junction of the monoclinic phase and the crack phase. The lattice orientations have great influences on the final monoclinic microstructure and the transformation toughening effect. This work shows the validity of the phase field method to simulate the microstructure evolution during the crack propagation and it can serve as a robust tool to investigate the transformation toughening behavior of Zirconia ceramics.

Crack-contact problem for an elastic layer with rigid stamps

The plane crack-contact problem for an infinite elastic layer with two symmetric rectangular rigid stamps on its upper and lower surfaces is considered. The elastic layer having an internal crack parallel to its surfaces is subjected to two concentrated loads p on its upper and lower surfaces trough the rigid rectangular stamps and a pair of uniform compressive stress <TEX>$p_0$</TEX> along the crack surface. It is assumed that the contact between the elastic layer and the rigid stamps is frictionless and the effect of the gravity force is neglected. The problem is reduced to a system of singular integral equations in which the derivative of the crack surface displacement and the contact pressures are unknown functions. The system of singular integral equations is solved numerically by making use of an appropriate Gauss-Chebyshev integration formula. Numerical results for stress-intensity factor, critical load factor, <TEX>$\mathcal{Q}_c$</TEX>, causing initial closure of the crack tip, the crack surface displacements and the contact stress distribution are presented and shown graphically for various dimensionless quantities.

Fast intestinal barrier recovery following ischemia/reperfusion damage in the human gut: myosin light chain mediated closure of the villus tip

In a recently developed human model of intestinal ischemia reperfusion (I‐IR) a rapid reversal of intestinal damage by shedding of injured enterocytes and villus tip closure was described. Here we investigated the putative involvement of myosin light chain kinase (MLCK) mediated actin contraction in villus tip closure.During surgery, an isolated part of the jejunum scheduled to be removed was subjected to 30 min ischemia and subsequent reperfusion. Immunohistochemistry was used to study the localization of MLCK, phosphorylated‐MLC (pMLC), filamentous actin and claudin‐3. Protein levels of MLCK, pMLC and claudin‐3 were semi‐quantitatively analyzed using western blot. Intestinal claudin‐3 production was determined by qPCR.After 30 min ischemia subepithelial spaces were noted as retractions of the basal membrane. A 30 min reperfusion period resulted in disintegration of epithelial lining at the villus tip, with epithelial cell damage and loss of claudin‐3. Both MLCK and pMLC increased following ischemia and were distributed around the disrupted villus tips after 30 min of reperfusion (MLCK; 918±430 versus 641±338 INT, P&lt;0.01) (fig 1). After 60 min reperfusion complete barrier recovery was observed.This study shows fast gut barrier recovery following I‐IR damage in the human gut after myosin light chain kinase mediated closure of the villus tip.Figure 1A) Before ischemiaB) After 30 min ischemiaC) After 30 min reperfusionD) After 60 min reperfusion

Population trends of Rooks Corvus frugilegus in Spain and the importance of refuse tips

Anthropogenic food from refuse tips can affect population dynamics in birds, especially gulls, but the evidence is mostly circumstantial. We combine analyses of long‐term population data and natural experiments to show a positive effect of refuse tips on the growth of the Spanish breeding population of Rooks Corvus frugilegus. In this isolated population of around 2000 breeding pairs, monitored since 1976, birds in colonies less than 10 km from tips fed largely on refuse, particularly during periods of lowest natural food availability. Three lines of evidence support the hypothesis that the supply of refuse influenced breeding numbers, suggesting that this population is limited by food: 1) between 1976 and 2003, the two population nuclei that had access to tips increased 2.1 and 3.7 times more than that without a tip nearby; 2) annual colony growth between 1996 and 2003 was strongly correlated with the availability of tips when other potentially important variables were taken into account; 3) the number of breeding pairs in refuse‐foraging colonies declined rapidly after the closure of the local tip and recovered only when a supply of refuse was restored. The effect of tips on colony growth was stronger when the availability of natural foraging habitat around the colonies was low, suggesting that anthropogenic food acts as a buffer against shortage of natural food. Artificial food supplementation may be an effective tool to increase the breeding population of target species, especially those facing a reduction of their foraging habitats. The potential effects on bird species of Directive 1999/31/CE, which is enforcing a massive closure of tips in Europe, are discussed.

Postoperative complications after surgery for Typoid Ileal performation in adults in Kumasi

The surgical complications of typhoid ileitis such as ileal perforations (TIP) continue to cause many deaths especially in countries with poor sanitation and limited health care facilities. To assess and highlight the complications associated with typhoid ileal perforation A prospective study of the postoperative complications after closure of TIP was conducted between Jan. 2002 and Dec. 2005 at the Komfo Anokye Teaching Hospital (KATIH.) A total of 248 patients aged between 16 and 54 years with a mean age of 24.9 (8.3) and a median of 23.5 years were operated upon for TIP. There were 180 males (72.6%) and 68 females (27.4%) with a male to female ratio of 4: 1. The diagnosis of TIP was based on clinical, radiological and operative findings. The over all complication rate was 49.3%. The most common postoperative complication was wound infection (52.4 %). The most serious were persistent peritonitis (34.7%) and enterocutaneous fistula (10.0%) with a mortality of 33.3 % and 22.2 % respectively. The overall mortality was 10.9%. These complications significantly increased the duration of hospital stay of survivors by several days (p<0.01) Very serious complications occur after surgery for TIP in adults. These complications may contribute to the high mortality from this disease.

Shielding and amplification of a penny-shape crack due to the presence of dislocations

The interaction between a penny-shape crack and a dislocation in crystalline materials is investigated within the framework of dislocation dynamics. The long-range and singular stress field resulting from the crack is determined by modeling the crack as continuous distribution of dislocation loops. This distribution is determined by satisfying the traction boundary condition at the crack face, resulting into a singular integral equation of the first kind that is solved numerically. This crack model is integrated with the dislocation dynamics simulation technique to yield the stress field of the combine system of crack and different types of dislocations situated at different positions in a three dimensional space. The integrated system is then used to investigate the dislocation behavior and its influence on the crack opening displacement and the characteristic of the stress field near the crack tip. It is shown that, depending on the relative position of the dislocation and its character, the dislocation may result in reduction in the stress amplitude at the crack tip and in some cases in closure of the crack tip. These analyses yield shielding and amplification zones near the crack providing an insight of the dislocation influence on the crack. The full dislocation dynamic analysis reveals the nature of the crack dislocation interaction and the manner in which the dislocation morphology changes as it is attracted to the crack surfaces, as well as the changes it causes to the crack profile.

Analytic Description of the Fatigue Crack-Growth Rate in Metals for Different Load Ratios

We propose an analytic description of the fatigue crack growth rate in metals based on experimental data taking into account the regularities of changes in the kinetic diagrams of fatigue fracture depending on different load ratios R , R * and the phenomenon of closure of the crack tip. We establish an empirical relationship between the effective ΔK eff and nominal ΔK Rranges of the stress intensity factors for the load ratio R (for the load ratio R *, the phenomenon of fatigue crack-tip closure is absent) valid within the limits of existence of fatigue crack-tip closure: from the critical threshold effective stress intensity factor range ΔK {fcl , th , R} to the critical effective stress intensity factor range ΔK {fcl , R}.

Effect of closure of the tip of a fatigue crack on the asymmetry of loading cycles and on the thresholds of crack resistance

On the basis of the experimental data, it is shown that, in the presence of closure of the tip of a fatigue crack, the imaginary influence of the positive load ratio R on the kinetics of crack growth and on the thresholds of crack resistance expressed via the threshold range of stress intensity factors ΔK th,R is actually the quantitative representation of changes in the characteristic of crack closure ΔK cl op provided that we use the nominal range of stress intensity factors ΔK.

THE SIGNIFICANCE OF CRACK TIP DEFORMATION FOR SHORT AND LONG FATIGUE CRACKS

Abstract— The behaviour of physical short mode I cracks under constant amplitude cyclic loading was investigated both numerically and experimentally. A dynamic two‐dimensional elastic‐plastic finite element technique was utilised to simulate cyclic crack tip plastic deformation. Different idealisations were investigated. Both stationary and artificially advanced long and short cracks were analysed. A parameter which characterises the plastically deformed crack tip zone, the strain field generated within that zone and the opening and closure of the crack tip were considered. The growth of physically short mode I cracks under constant amplitude fully reversed fatigue loading was investigated experimentally using conventional cast steel EN‐9 specimens. Based on a numerical analysis, a crack tip deformation parameter was devised to correlate fatigue crack propagation rates.

Life-History Strategy of the Herring Gull: Changes in Survival and Fecundity in a Population Subjected to Various Feeding Conditions

The present study compares breeding parameters and adult survival rate in a herring gull colony before and after the closing of a large refuse tip where breeders used to find most of their food. During the first study period (1983-88) food of human origin was abundant and virtually always available. During the second period (1989-90) such food was scarce. The annual adult survival rate was time-dependent. It varied from 0.826 (SE = 0.031) in 1987-88 to 0.975 (SE = 0.022) in 1985-86. The average survival rate calculated for 1983-90 was 0.881 (SE = 0.014). There was no significant difference in adult survival between males and females. The closure of the refuse tip was not followed by a significant decrease in adult survival rate. After the refuse tip was closed, mean clutch size and mean production of young per breeding pair decreased by 6.7% and 49.1%, respectively. Mean adult body weight decreased by 4.6% for males and by 4.7% for females. The proportion of non-breeders among former breeders and the proportion of 3- and 4-year-old individuals among ringed birds did not change after closure of the tip. The results are discussed in terms of the life-history theory, which predicts that in long-lived species a decrease in food supply should affect fecundity before affecting adult survival.

Interpretation of hydraulic fracturing shut-in curves for tectonic stress measurements

The closure process after shut-in of the cracks which are induced by hyraulic fracturing on the surface of a wellbore is analyzed to discuss the physical meaning of shut-in curves. The analysis is based on the linear theory of elasticity and also on linear fracture mechanics. It is shown that the pressure decay characteristics of shut-in curves are primarily governed by the leak-off rate, the equipment compliance and the partial crack closure. It is also shown that the downhole fluid pressure at the onset of the closure of the crack tip is very close to the compressive tectonic stress normal to the crack plane. This pressure can be detected as the point of maximum curvature on each shut-in curve. Finally, the methods which have been proposed so far for the determination of instantaneous shut-in pressures are also applied and their applicability and reliability are discussed.

Influence of room temperature aging on the threshold condition for a short fatigue crack.

The effects of room temperature aging treatment are discussed on the critical condition for the onset of growth of non-propagating cracks on 0.15% C low carbon steel, with special emphasis on the length of the critical non-propagating crack and on the crack opening displacement at the crack tip. It is found from the experimental analysis that the aging treatment of a fatigue precracked specimen for 30 days at room temperature introduced the closure of a crack tip of the pre-crack and the reduction of crack opening displacement at the wake of crack, together with an improvement in crack growth resistance of the microstructure. This may cause an increase in the endurance limit of the specimen, through the enhancement of effective stress for the onset of growth of the critical non-propagating crack.