Multiaxial Fatigue Assessment of Friction Stir Welded Tubular Joints of Al 6082-T6

The present paper addresses the problem of designing aluminium friction stir (FS) welded joints against multiaxial fatigue. After developing a bespoke FS welding technology suitable for joining aluminium tubes, some one hundred welded tubular specimens of Al 6082-T6 were tested under pure axial, pure torsional and biaxial tension-torsion loading. The influence was explored of two independent variables, namely the proportional or nonproportional nature of the biaxial loading and the effect of axial and torsional non-zero mean stresses. The experimental results were re-analysed using the Modified Wöhler Curve Method (MWCM), with this bi-parametrical critical plane approach being applied in terms of nominal stresses, notch stresses, and also the Point Method. The validation exercise carried out using these experimental data demonstrated that the MWCM is applicable to prediction of the fatigue lives for these FS welded joints, with its use resulting in life estimates that fall within the uniaxial and torsional calibration scatter bands. The approach proposed in the present paper offers, for the first time, a complete solution to the problem of designing tubular FS welded joints against multiaxial fatigue loading

Extreme Observations and Diversification in Latin American Emerging Equity Markets

In this paper, we focus on the tails of the unconditional distribution of latin American emerging markets stock returns. We explore their implications for portfolio diversification according to the safety first principle, first proposed by Roy (1952). We find that the Latin American emerging markets have significantly fatter tails than industrial markets, especially, the lower tail of the distribution. We consider the implication of the safety first principle for a U.S. investor who creates a diversified portfolio using Latin America stock markets. We find that a U.S. investor gains by adding Latin American equity markets to her purely domestic portfolio. For different parameter specifications, we find a more realistic asset allocation than the one suggested by the literature based on the traditional meanvariance framework.

Gradient elasticity: a transformative stress analysis tool to design notched components against uniaxial/multiaxial high-cycle fatigue

This paper investigates the accuracy of gradient elasticity in estimating high-cycle fatigue strength of notched components subjected to both uniaxial and multiaxial fatigue loading. A novel design methodology is formulated by combining Ru and Aifantis’ gradient elasticity with the Theory of Critical Distances and the Modified Wöhler Curve Method. The keyfeature of this innovative design methodology is that, via the Theory of Critical Distances, gradient elasticity’s length scale parameter is directly estimated from conventional material fatigue properties (i.e., the plain fatigue limit and the threshold value of the stress intensity factor). From a stress analysis point of view, the proposed approach directly post-processes the gradient-enriched stress states determined, at the hot-spots, on the surface of the component under investigation (and independently of the sharpness of the stress concentrator being assessed). The accuracy and reliability of this design method was checked by using a large number of experimental results taken from the literature and generated by testing notched metallic samples under uniaxial as well as under multiaxial fatigue loading. This comprehensive validation exercise demonstrates that the systematic usage of this transformative design approach leads to the same level of accuracy as the one which is obtained by applying the classic Theory of Critical Distances. This result is certainly remarkable since the proposed approach is not only very efficient from a computational point of view, but it also allows high-cycle fatigue damage to be assessed by directly postprocessing gradient-enriched stress states determined on the surface of the component being assessed

Interest Rate Volatility and Contagion in Emerging Markets: Evidence from the 1990s

In this paper we use high frequency interest rate data for a group of Latin American countries to analyze the behavior of volatility through time. We are particularly interested in understanding whether periods of high volatility spillover across countries. Our analysis relies both on univariate and bivariate switching volatility models. Our results indicate that high-volatility episodes are, in general, short-lived, lasting from two to seven weeks. We find some weak evidence of volatility co-movements across countries. Overall, our results are not overly supportive of contagion' stories.

Regime-Switching Stochastic Volatility and Short-Term Interest Rates.

In this paper, we introduce regime-switching in a two-factor stochastic volatility model to explain the behavior of short-term interest rates. The regime-switching stochastic volatility (RSV) process for interest rates is able to capture all possible exogenous shocks that could be either discrete, as occurring from possible changes in the underlying regime, or continuous in the form of `market-news' events. We estimate the model using a Gibbs Sampling based Markov Chain Monte Carlo algorithm that is robust to complex nonlinearities in the likelihood function. We compare the performance of our RSV model with the performance of other GARCH and stochastic volatility two-factor models. We evaluate all models with several in-sample and out-of-sample measures. Overall, our results show a superior performance of the RSV two-factor model.Short-term interest rates, stochastic volatility, regime switching, MCMC methods.

Arbitrage and convergence: Evidence from Mexican ADRs

This paper investigates the convergence between the prices of ADRs and Mexican traded shares using a sample of 21 dually listed shares. Since both markets have similar trading hours, standard arbitrage considerations should make persistent deviation from price parity rare. We use a STAR model, where the dynamics of convergence to price parity are influenced by the size of the deviation from price parity. Based on different tests, we select the ESTAR model. Deviations from price parity tend to die out quickly; for 14 out of 21 pairs it takes less than two days for the deviations from price parity to be reduced by half. The average half-life of a shock to price parity is 3.1 business days, while the median half-life is 1.1 business days. By allowing a non-linear adjustment process, the average half-life is reduced by more than 50% when compared to the standard linear arbitrage model. We find that several liquidity indicators are positively correlated to the speed of convergence to price parity.ADRs, nonlinear convergence, arbitrage, ESTAR

Volatility Dependence and Contagion in Emerging Equity Markets

In this paper we use weekly stock market data for a group of Latin American countries to analyze the behavior of volatility through time. We are particularly interested in understanding whether periods of high volatility are correlated across countries. The analysis uses both on univariate and bivariate switching volatility models. Our results do not rely on the correlation coefficients, but on the co-dependence of volatility regimes. The results indicate that high-volatility episodes are, in general, short-lived, lasting from two to twelve weeks. We find strong evidence of volatility co-movements across countries, especially among the Mercosur countries.

Microstructural length scale parameters to model the high-cycle fatigue behaviour of notched plain concrete

The present paper investigates the importance and relevance of using microstructural length scale parameters in estimating the high-cycle fatigue strength of notched plain concrete. In particular, the accuracy and reliability of the Theory of Critical Distances and Gradient Elasticity are checked against a number of experimental results generated by testing, under cyclic bending, square section beams of plain concrete containing stress concentrators of different sharpness. The common feature of these two modelling approaches is that the required effective stress is calculated by using a length scale which depends on the microstructural material morphology. The performed validation exercise demonstrates that microstructural length scale parameters are successful in modelling the behaviour of notched plain concrete in the high-cycle fatigue regime

The linear-elastic Theory of Critical Distances to estimate high-cycle fatigue strength of notched metallic materials at elevated temperatures

This paper investigates the accuracy of the linear-elastic Theory of Critical Distances (TCD) in estimating high-cycle fatigue streng th of notched metallic materials experienc- ing elevated temperatures during in-service operations. The TCD postulates that the fatigue damage extent can be estimated by directly post-processing the entire linear-elastic stress field acting on the material in the vicinity of the crack initiation locations. The key feature of this theory is that the high-cycle fatigue assessment is based on a scale length parameter that is assumed to be a material property. The accuracy of this design method was checked against a number of experimental results generated, under axial loading, by testing, at 250 °C, notched specimens of carbon steel C45. To further investigate the reliability of the TCD, its accuracy was also checked via several data taken from the literature, these experimental results being generated by testing notched samples of Inconel 718 at 500 °C as well as notched specimens of directionally solidified superalloy DZ125 at 850 °C. This validation exercise allowed us to prove that the linear-elastic TCD is successful in estimating high-cycle fatigue strength of notched metallic materials exposed to elevated temperature, resulting in estimates falling within an error interval of ±20%. Such a high level of accuracy suggests that, in situations of practical interest, reliable high-cycle fatigue assessment can be performed without the need for taking into account those non-linearities characterising the mechanical behaviour of metallic materials at high temperature, the used critical distance being still a material property whose value does not depend on the sharpness of the notch being designed

A Multiaxial Stress-Based Critical Distance Methodology To Estimate Fretting Fatigue Life

This work presents a methodology for fretting fatigue life estimation based on the evaluation of a multiaxial fatigue parameter at a critical distance below the contact surface. The fatigue parameter is defined using the Modified Wöhler Curve Method together with a measure of shear stress amplitude based on the Maximum Rectangular Hull concept. To apply the approach in the medium-cycle fatigue regime, the critical distance is assumed to depend on the fatigue life. Available fretting fatigue experiments conducted on a cylinder-on-flat contact configuration made of Al-4%Cu alloy were used to evaluate the methodology. Most of the fatigue life estimates were within factor-of-two boundaries