An investigation of irregular crack path effects on fracture mechanics parameters using a grain microstructure meshing technique

Electronic version of an article published as Journal of Multiscale Modeling, Vol. 4, Iss. 1, atricle 1250001, 2012, http://dx.doi.org/10.1142/S1756973712500011 © World Scientific Publishing Company, http://www.worldscientific.com/worldscinet/jmmA sub-grain size finite element modelling approach is presented in this paper to investigate variations in fracture mechanics parameters for irregular crack paths. The results can be used when modelling intergranular and transgranular crack growth where creep and fatigue are the dominant failure mechanisms and their crack paths are irregular. A novel method for sub-grain scale finite element mesh consisting of multiple elements encased in ~50–150 μm-sized grains has been developed and implemented in a compact tension, C(T), mesh structure. The replicated shapes and dimensions were derived from an isotropic metallic grain structure using representative random sized grain shapes repeated in sequence ahead of the crack tip. In this way the effects of crack tip angle ahead of the main crack path can be considered in a more realistic manner. A comprehensive sensitivity analysis has been performed for elastic and elastic-plastic materials using ABAQUS and the stress distributions, the stress intensity factor and the J-integral have been evaluated for irregular crack paths and compared to those of obtained from analytical solutions. To examine the local and macroscopic graph path effects on fracture mechanics parameters, a few extreme cases with various crack-tip angles have been modelled by keeping the macroscopic crack path parallel to the axis of symmetry. The numerical solutions from these granular mesh structures have been found in relatively good agreement with analytical solutions

Optimization Makes Estimation Much More Worse

Mean-variance optimization as a modern portfolio theory is a major model for theoretical purposes, however, in practice portfolio managers don’t have enough interest despite some other ad hoc methods for many reasons such as estimation errors. Recently, the significance of modern portfolio theory has been analyzed that it doesn’t beat the simple naïve 1/N rule not only in many real empirical databases but also in a simulation. By this paper, due to inherent weakness of Sharpe ratio we first express more common use and adjusted measurements such as adjusted expected utility of portfolio under ambiguity aversion to analyze their effects on portfolio optimization after this consideration, because using only sample mean and variance (Sharpe ratio) to evaluate performance value for the portfolio models may be subject to considerable bias. Second, we propose a new model based on the new measurement (adjusting ambiguity Sharpe ratio) to improve portfolio optimization problem. Our result states that by using the new measurement mean- variance optimization beats the naïve rule by applying the adjusted measurement and also the novel model outperforms Markowitz in terms of Sharpe ratio while the interesting is that for adjusting Sharpe ratio inverse result exists. Therefore, our study expresses optimization makes estimation almost worse when we try to use a measurement as an optimization target. Keywords: portfolio selection, optimization, measurement, Sharpe ratio

A Novel Approach to Construct Portfolio that Addresses Variance of Utility Function

Many studies in the area of portfolio selection have done based on trade-off among various moments especially between mean and risk of sample returns. Merton (1980) argued that the instability of portfolio weights and sampling errors are due more to estimate the amount of mean. Indeed, it is difficult to estimate the expected return from time series of realized expected return. By this paper, we first answer to this question how to eliminate the wrong effect of mean sample returns instead of ignoring it from calculating portfolio weights and how would be the relation of other moments after this consideration. Second, the substantial evidence from experiments shows that hide information exposes ambiguity aversion to investor’s behavior, and hence decision making under ambiguity for portfolio choice has led to improving tractability of the main features of asset returns. By considering the volatility of utility preferences as ambiguity then individuals prefer to stabilize their utility preferences to maximize their expected utility. Keywords: utility function, portfolio selection, risk aversion, ambiguity aversio

The Effect of Current Expected Variance of Return on Future Trading Optimal Strategy

Due to the presence of transaction cost, most investors do not keep changing their portfolio to an optimal portfolio over time. This paper adopts a new approach to investigate the linkages between current optimal portfolio variance and the expected future portfolios variances. It is given a closed-form solution for optimal dynamic portfolio selection with trading cost; considering the minimum variance of the utility function as an optimal or selected portfolio by an investor for any period of time based on Gârleanu & Pedersen (2013) framework. Finally, we introduce the multi-period portfolio model based on CRRA preference utility function. Keywords: multiperiod portfolio selection, higher-order moments, CRRA utility function, optimal wealth change

Computational fluid dynamics of coronary arteries with implanted stents: effects of Newtonian and non‐Newtonian blood flows

This study introduces and compares computational fluid dynamics of Newtonian and non-Newtonian blood flows in coronary arteries, with and without considering stents. Three blood flow models, including Newtonian, Carreau, and non-Newtonian power-law models, were simulated to investigate their effect, and the solution algorithm includes drawing the geometry, creating the desired mesh, and then simulating Newtonian and non-Newtonian blood flow different models and comparing them with each other, is presented in the article. A Newtonian fluid model has been commonly used in the simulation of blood flow, whereas blood has non-Newtonian properties due to the nature of a solution containing suspended particles. The goal of this research is to investigate the differences between the models built with Newtonian and non-Newtonian fluid assumptions. In addition, a stent was designed and the effect of the stent on blood flow parameters was investigated for all three flow models, including Newtonian, Carreau, and non-Newtonian power-law models. Stents are medical devices that can be placed in arteries to open up blood flow in a blocked vessel. Stents can affect the wall shear stress. Knowing the slight deformation of the shear stress makes the importance of stent implantation and also helps to optimize the design of the intravascular stent, which can affect the occlusion of the vessels. The distribution of the velocity, pressure, and wall shear stress in all blood flow models with and without considering the effect of stents have been investigated and finally compared. Therefore, in general, the innovation of this article is to find the effect of implanted stents on blood flow parameters with different blood flow models, both Newtonian and non-Newtonian. A comparison of Newtonian and non-Newtonian flows showed that in the case of the Carreau non-Newtonian model, the wall shear stress was higher. In addition, in the results of the geometric model with a stent effect compared to the geometric model without a stent effect, it is evident that there was a higher velocity and wall shear stress

Validation of the persian version of spiritual well-being questionnaires

Background: Spiritual well-being is an important issue in health sciences, hence the need for validated instruments to assess this aspect of health in the Iranian population. The aim of the current study was to determine the validity of the Persian versions of 2 most common measures of spiritual health (Spiritual Well-Being Questionnaire SWBQ or Spiritual Health and Life-Orientation Measure SHALOM and Spiritual Well-Being Scale SWBS). Methods: This was a cross-sectional study via a convenience sampling method in Iran University of Medical Sciences with 170 participants aged above 18 years comprising students, teachers, and administrative staff and managers. The study was conducted from September 7, 2014 to September 20, 2015 in Tehran. Four questionnaires, namely the SWBQ, SWBS, General Health Questionnaire (GHQ-12), and Oxford Happiness Questionnaire (OHQ), were used. Statistical analysis was done using SPSS 18 and LISREL (version 8.2). Cronbach�s alpha, intra-class correlation coefficient, Pearson correlation, and confirmatory factor analysis were employed to assess the validity and reliability of the questionnaires. Results: Cronbach�s alpha for the SWBQ and the SWBS was greater than 0.85. The repeatability of both questionnaires was between 0.88 and 0.98. The Pearson correlation for the SWBQ and the SWBS ranged from 0.33 to 0.53; and all the correlations were significant. The respondents who indicated a higher spiritual well-being also reported better general health and happiness. Conclusion: The Persian versions of the SWBS and the SWBQ have good reliability, repeatability, and validity to assess spiritual health in the Iranian population. © 2018, Shiraz University of Medical Sciences. All rights reserved

Evaluation of coronary stents: a review of types, materials, processing techniques, design, and problems

In the world, one of the leading causes of death is coronary artery disease (CAD). There are several ways to treat this disease, and stenting is currently the most appropriate way in many cases. Nowadays, the use of stents has rapidly increased, and they have been introduced in various models, with different geometries and materials. To select the most appropriate stent required, it is necessary to have an analysis of the mechanical behavior of various types of stents. The purpose of this article is to provide a complete overview of advanced research in the field of stents and to discuss and conclude important studies on different topics in the field of stents. In this review, we introduce the types of coronary stents, materials, stent processing technique, stent design, classification of stents based on the mechanism of expansion, and problems and complications of stents. In this article, by reviewing the biomechanical studies conducted in this field and collecting and classifying their results, a useful set of information has been presented to continue research in the direction of designing and manufacturing more efficient stents, although the clinical-engineering field still needs to continue research to optimize the design and construction. The optimum design of stents in the future is possible by simulation and using numerical methods and adequate knowledge of stent and artery biomechanics

Validation of the persian version of spiritual well-being questionnaires

Background: Spiritual well-being is an important issue in health sciences, hence the need for validated instruments to assess this aspect of health in the Iranian population. The aim of the current study was to determine the validity of the Persian versions of 2 most common measures of spiritual health (Spiritual Well-Being Questionnaire SWBQ or Spiritual Health and Life-Orientation Measure SHALOM and Spiritual Well-Being Scale SWBS). Methods: This was a cross-sectional study via a convenience sampling method in Iran University of Medical Sciences with 170 participants aged above 18 years comprising students, teachers, and administrative staff and managers. The study was conducted from September 7, 2014 to September 20, 2015 in Tehran. Four questionnaires, namely the SWBQ, SWBS, General Health Questionnaire (GHQ-12), and Oxford Happiness Questionnaire (OHQ), were used. Statistical analysis was done using SPSS 18 and LISREL (version 8.2). Cronbach�s alpha, intra-class correlation coefficient, Pearson correlation, and confirmatory factor analysis were employed to assess the validity and reliability of the questionnaires. Results: Cronbach�s alpha for the SWBQ and the SWBS was greater than 0.85. The repeatability of both questionnaires was between 0.88 and 0.98. The Pearson correlation for the SWBQ and the SWBS ranged from 0.33 to 0.53; and all the correlations were significant. The respondents who indicated a higher spiritual well-being also reported better general health and happiness. Conclusion: The Persian versions of the SWBS and the SWBQ have good reliability, repeatability, and validity to assess spiritual health in the Iranian population. © 2018, Shiraz University of Medical Sciences. All rights reserved