T-stress determination using thermoelastic stress analysis

T-stress and mixed-mode stress intensity factors have been determined experimentally using thermoelastic stress analysis and using a finite element method. Pure mode I, strong mixed-mode I and II, and interacting cracks have been used as the case studies. A new technique has been proposed to identify the crack tip from thermoelastic images. It has also been shown that using three terms of Williams's stress field formulation to determine the T-stress, yields a more accurate solution than using only the first two terms of the expansion

Some experimental observations of crack-tip mechanics with displacement data

Estudio de la mecánica en el vértice de la grieta mediante datos de desplazamiento.In the past two decades, crack-tip mechanics has been increasingly studied with full-field techniques. Within these techniques, Digital Image Correlation (DIC) has been most widely used due to its many advantages, to extract important crack-tip information, including Stress Intensity Factor (SIF), Crack Opening Displacement, J-integral, T-stress, closure level, plastic zone size, etc. However, little information is given in the literature about the experimental setup that provides best estimations for the different parameters. The current work aims at understanding how the experimental conditions used in DIC influence the crack-tip information extracted experimentally. The influence of parameters such as magnification factor, size of the images, position of the images with respect the crack-tip and size of the subset used in the correlation is studied. The influence is studied in terms of SIF and T-stress by using Williams’ model. The concept of determination of the K-dominance zone from experimental data has also explored. In this regard, cyclic loading on a fatigue crack in a compact tension (CT) specimen, made of aluminium 2024-T351 alloy, has been applied and the surface deformation ahead of the crack tip has been examined. The comparison between theoretical and experimental values of KI showed that the effect of subset size on the measured KI is negligible compared to the effect of size of the image.Universidad de Málaga. Campus de Excelencia Internacional Andalucía Tech

Experimental and analytical study of cracks under biaxial fatigue

Most mechanical components experience multi-axial cyclic loading conditions during service. Experimental analysis of fatigue cracks under such conditions is not easy and most works tend to focus more on the simpler but less realistic case of uni-axial loading. Consequently, there are many uncertainties related to the load sequence effect that are now well known and are not normally incorporated into the growth models. The current work presents a new methodology for evaluating overload effect in biaxial fatigue cracks. The methodology includes evaluation of mixed-mode (KI and KII) stress intensity factor and the Crack Opening Displacement for samples with and without overload cycle under biaxial loading. The methodology is tested under a range of crack lengths. All crack-tip information is obtained with a hybrid methodology that combines experimental full-field digital image correlation data and Williams' elastic model describing the crack-tip field.Universidad de Málaga. Campus de Excelencia Internacional Andalucía Tech

Crack Growth Behavior in the Threshold Region for High Cycle Fatigue Loading

This paper describes the results of a research program conducted to improve the understanding of fatigue crack growth rate behavior in the threshold growth rate region and to answer a question on the validity of threshold region test data. The validity question relates to the view held by some experimentalists that using the ASTM load shedding test method does not produce valid threshold test results and material properties. The question involves the fanning behavior observed in threshold region of da/dN plots for some materials in which the low R-ratio data fans out from the high R-ratio data. This fanning behavior or elevation of threshold values in the low R-ratio tests is generally assumed to be caused by an increase in crack closure in the low R-ratio tests. Also, the increase in crack closure is assumed by some experimentalists to result from using the ASTM load shedding test procedure. The belief is that this procedure induces load history effects which cause remote closure from plasticity and/or roughness changes in the surface morphology. However, experimental studies performed by the authors have shown that the increase in crack closure is a result of extensive crack tip bifurcations that can occur in some materials, particularly in aluminum alloys, when the crack tip cyclic yield zone size becomes less than the grain size of the alloy. This behavior is related to the high stacking fault energy (SFE) property of aluminum alloys which results in easier slip characteristics. Therefore, the fanning behavior which occurs in aluminum alloys is a function of intrinsic dislocation property of the alloy, and therefore, the fanned data does represent the true threshold properties of the material. However, for the corrosion sensitive steel alloys tested in laboratory air, the occurrence of fanning results from fretting corrosion at the crack tips, and these results should not be considered to be representative of valid threshold properties because the fanning is eliminated when testing is performed in dry air

A Multi-Parameter Approach for Calculating Crack Instability

An accurate fracture control analysis of spacecraft pressure systems, boosters, rocket hardware and other critical low-cycle fatigue cases where the fracture toughness highly impacts cycles to failure requires accurate knowledge of the material fracture toughness. However, applicability of the measured fracture toughness values using standard specimens and transferability of the values to crack instability analysis of the realistically complex structures is refutable. The commonly used single parameter Linear Elastic Fracture Mechanics (LEFM) approach which relies on the key assumption that the fracture toughness is a material property would result in inaccurate crack instability predictions. In the past years extensive studies have been conducted to improve the single parameter (K-controlled) LEFM by introducing parameters accounting for the geometry or in-plane constraint effects]. Despite the importance of the thickness (out-of-plane constraint) effects in fracture control problems, the literature is mainly limited to some empirical equations for scaling the fracture toughness data] and only few theoretically based developments can be found. In aerospace hardware where the structure might have only one life cycle and weight reduction is crucial, reducing the design margin of safety by decreasing the uncertainty involved in fracture toughness evaluations would result in lighter hardware. In such conditions LEFM would not suffice and an elastic-plastic analysis would be vital. Multi-parameter elastic plastic crack tip field quantifying developments combined with statistical methods] have been shown to have the potential to be used as a powerful tool for tackling such problems. However, these approaches have not been comprehensively scrutinized using experimental tests. Therefore, in this paper a multi-parameter elastic-plastic approach has been used to study the crack instability problem and the transferability issue by considering the effects of geometrical constraints as well as the thickness. The feasibility of the approach has been examined using a wide range of specimen geometries and thicknesses manufactured from 7075-T7351 aluminum alloy

Penbactam for Helicobacter pylori eradication: A randomised comparison of quadruple and triple treatment schedules in an Iranian population

Background & study aims: Selection of the best drug regimens for eradication of Helicobacter pylori infection especially in patients at risk of peptic ulcer relapses and the development of complications is challenging. This study assessed and compared the efficacy of the two common PPI based triple therapies to a quadruple therapy including PPI, metronidazole, amoxicillin and a bismuth compound in Iranian population. Patients & Methods: Three hundred and thirty patients with peptic ulcer and H. pylori infection were included in the study. Patients were randomly assigned to one of the three treatment protocols all given twice daily: (a) A 14-day quadruple therapy (OMAB group) comprising omeprazole 20. mg, metronicazole 500. mg, amoxicillin 1. g, and bismuth subcitrate 240. mg; (b) A 14-day triple regimen (OCP group) comprising omeprazole 20. mg plus clarithromycine 500. mg and penbactam 750. mg and (c) A 14-day triple regimen (OCA group) comprising omeprazole 20. mg plus clarithromycine 500. mg and amoxicillin 1. g. Cure was defined as a negative urea breath test at least six weeks after treatment. Results: The per-protocol eradication rates achieved with both OCP regimen (87.0%) and OCA treatment (90.8%) were significantly higher than the OMAB treatment protocol (56.0%); however, no significant difference emerged in eradication rates between the two triple treatment schedules. No significant differences between the groups were found in most side-effects. Conclusion: Two-week quadruple therapy showed a lower eradication rate compared to common triple treatment schedules when used as first-line eradication treatment for H. pylori infection in Iranian population. © 2012 Arab Journal of Gastroenterology

Cryopreservation of Ram Semen in Extenders Containing Soybean Lecithin as Cryoprotectant and Hyaluronic Acid as Antioxidant

P. 934-940A soybean lecithin‐based extender supplemented with hyaluronic acid (HA) was assayed for effectiveness to improve the quality of frozen–thawed ram semen. HA has not been tested yet in an extender containing soybean lecithin for freezing ram semen. Thus, the aim of this study was to analyse the effects of soybean lecithin at 1% or 1.5% along with HA at 0, 0.5 and 1 mg ml‐1 in a Tris‐based extender on the motion characteristics, membrane integrity (HOST), viability, GSH peroxidase (GSH‐PX) activity, lipid peroxidation and acrosomal status after freezing–thawing. Semen was collected from four Mehraban rams during the breeding season and frozen in the six lecithin×HA extenders. The extender containing 1.5% lecithin supplemented with no HA yielded higher total motility (52.5%±1.6), viability (55.8%±1.6) and membrane integrity (44.5%±1.7), but the effects of the lecithin concentration did not reach signification. Linearity‐related parameters, ALH, BCF, lipid peroxidation, GSH‐PX activity, morphology and acrosomal status were not affected by the extender composition. In general, adding HA significantly decreased sperm velocity (1 mg ml‐1 HA), total motility (only with 1.5% lecithin), viability (1 mg ml‐1 HA for 1% lecithin; both concentrations for 1.5% lecithin) and membrane integrity. In conclusion, adding HA to the freezing extender supplemented with soybean lecithin failed to improve quality‐related variables in ram semen. Increasing the lecithin content could have a positive effect, but further studies are needed.S

Organizational Pathology in Staff Department of Shiraz University of Medical Sciences based on Three-branch Model

Introduction: Pathology of organizations is one of the most key actions that make the university officials aware of the strengths, weaknesses, opportunities and challenges of an organization and allows these officials to take the right actions accordingly. Therefore, this study examined the pathology of human resources in staff departments of Shiraz University of Medical Sciences. Methods: This was a cross-sectional study. Two hundred employees participated in this study. Random sampling was used and the data were collected by an organizational pathology questionnaire based on the three branch model. Data analyzed by SPSS software. Results: The results showed that behavioral factors (mean = 2.73) and contextual factors (mean= 3.34) had the highest and the lowest impacts on human resources malfunction in Shiraz University of Medical Sciences. Among subsets of structural factors, the payment system (average rank of 2.5, mean=2.21±0.72 and p-value ≤0.001), behavioral factors, and job security (mean rank = 2.03, mean=2.28±0.93 and p-value ≤0.001) and among subsets of contextual factors, customer-orientation (mean rank= 1.31, mean=3.07±0.70 and p-value 0.19) had the most impact on organizational pathology in this university. Conclusion: Considering that the behavioral factors had the greatest impact to the damages, it seems that the university has to adopt programs and policies in terms of creating appropriate organizational culture, increasing motivation and job satisfaction, adopting effective leadership, providing opportunities for training and development of employees and improving job security for the staff. Indeed, it is worth mentioning that solving some of these issues is beyond the powers of the executive organs, so solving these types of problems at macro-organizational level in the country is of vital importance

Some experimental observations of crack-tip mechanics with displacement data

In the past two decades, crack-tip mechanics has been increasingly studied with full-field techniques. Within these techniques, Digital Image Correlation (DIC) has been most widely used due to its many advantages, to extract important crack-tip information, including Stress Intensity Factor (SIF), Crack Opening Displacement, J-integral, T-stress, closure level, plastic zone size, etc. However, little information is given in the literature about the experimental setup that provides best estimations for the different parameters.The current work aims at understanding how the experimental conditions used in DIC influence the crack-tipinformation extracted experimentally. The influence of parameters such as magnification factor, size of theimages, position of the images with respect the crack-tip and size of the subset used in the correlation is studied. The influence is studied in terms of SIF and T-stress by using Williams’ model. The concept of  determination of  the K-dominance zone from experimental data has also explored. In this regard, cyclic loading on a fatigue crack in a compact tension (CT) specimen, made of aluminium 2024-T351 alloy, has been applied and the surface deformation ahead of the crack tip has been examined. The comparison between theoretical and experimental values of KI showed that the effect of subset size on the measured KI is negligible compared to the effect of size of the image