Franc Kosel

The differential ant-stigmergy algorithm: an experimental evaluation and a real-world application

This paper describes the so-called Differential Ant-Stigmergy Algorithm (DASA), which is an extension of the Ant-Colony Optimization for a continuous domain. An experimental evaluation of the DASA on a benchmark suite from CEC 2005 is presented. The DASA is compared with a number of evolutionary optimization algorithms, including the covariance matrix adaptation evolutionary strategy, the differential evolution, the real-coded memetic algorithm, and the continuous estimation of distribution algorithm. The DASA is also compared to some other ant methods for continuous optimization. The experimental results demonstrate the promising performance of the new approach. Besides this experimental work, the DASA was applied to a real-world problem, where the efficiency of the radial impeller of a vacuum cleaner was optimized. As a result the aerodynamic power was increased by twenty per cent.

A new approach to monitoring thermal fatigue cracks in die casting moulds

Abstract Die casting moulds are exposed to high cyclic temperatures, mechanical loads and severe chemical conditions. Thermal and mechanical loads cause high local stresses and consequently surface cracks. This paper introduces a new approach to establishing thermal fatigue cracks in die casting moulds by measuring the resulting defect-fins on aluminium alloy castings in the actual die casting process. The investigation showed that cracks occurred sooner and were bigger if the mould material had a lower hardness. The maximum depth of the defect-fin observed on the casting due to thermal fatigue crack on the mould with the lowest hardness (42 HRc) at 10000 cycles was 0.37 mm, the maximum width of the defect-fin was 0.76 mm and maximum length was 9.6 mm. The observed cracks were bigger closer to the entrance of melt flow, especially at the places with stress concentrators, due to higher melt temperature and higher melt flow.

Numerical simulation of rotational symmetric tube bulging with inside pressure and axial compression

The process of rotational symmetric tube bulging with inside pressure and axial compression enables the standard tubes to be formed into different rotational symmetric hollow parts in such a way that their central part is expanded into a desired shape while the ends remain unchanged. The superposition of axial compression contributes to a more favorable forming stress state, which is reflected in larger forming limits and smaller wall thinning in the widened area. The problems characterizing the process are a limited range of compression stability and difficulties met when establishing and optimizing the technological parameters of the process whose course cannot be defined in an analytical way. Based on a physical model of the forming process a numerical model was built. Using ABAQUS code the model was simulated over the entire stress/forming region. The comparison of the computer simulated forming process with the experimentally obtained results showed that the model was highly accurate. Finally, the paper studies the influences of particular parameters on the stability of the process, showing on a practical example how it is possible to achieve tube bulging without wall thinning.

Intelligent location of simultaneously active acoustic emission sources: Part I

Part I describes an intelligent acoustic emission locator, while Part II discusses blind source separation, time delay estimation and location of two continuous acoustic emission sources. Acoustic emission (AE) analysis is used for characterization and location of developing defects in materials. AE sources often generate a mixture of various statistically independent signals. A difficult problem of AE analysis is separation and characterization of signal components when the signals from various sources and the mode of mixing are unknown. Recently, blind source separation (BSS) by independent component analysis (ICA) has been used to solve these problems. The purpose of this paper is to demonstrate the applicability of ICA to locate two independent simultaneously active acoustic emission sources on an aluminum band specimen. The method is promising for non-destructive testing of aircraft frame structures by acoustic emission analysis.

Biaxial Constrained Recovery in Shape Memory Alloy Rings

In this article biaxial constrained recovery in a thick-walled shape memory alloy (SMA) ring with a rectangular cross-section is modeled using the theory of generalized plasticity, which is developed by Jacob Lubliner and Ferdinando Auricchio. As a mechanical obstacle that delays free recovery in a SMA ring, a steel ring is used. The result of constrained recovery is the generation of high stresses in both the rings. All equations are written in a closed form in terms of infinite series. Theoretical results are compared with experimental findings and good agreement is found when SMA rings are in the domain of recoverable strains.

Elastoplastic buckling of circular annular plates under uniform in-plane loading

Abstract This paper deals with the elastoplastic buckling of a circular annular plate, with various axially symmetric boundary conditions and uniform axially symmetric in-plane radial loads on the inner and outer edge. The analysis is based on the standard linear buckling equations and the material behaviour is modelled by the small strain J 2 flow and deformation theories of plasticity where an elastic linear hardening rheological model of the material is considered. The solutions are obtained using the equilibrium approach where the governing differential equation is solved by the finite difference method which leads to the determination of eigenvalues of a homogeneous system of linear equations. Elastoplastic buckling loads for axially symmetric and asymmetric buckling shape modes with m waves in the circumferential direction are calculated and compared for both theories of plasticity. For one case, an experiment was performed and the results were compared with theoretical predictions.

Approximative formula for post-buckling analysis of non-linearly elastic columns with superellipsoidal cross-sections

Approximative formulas for post-buckling analysis of non-linearly elastic columns made of Ludwick material are developed for free-clamp, hinge—hinge, and clamp—clamp supports. The columns have a superellipsoidal cross-section. Comparison between analytically obtained and numerical solutions showed good agreement. Additionally, post-buckling configurations for all three types of columns and materials are given in diagrams, from where the influence of material constants on the shape of the deflection curve can be examined.

A novel experimental setup for multiparameter aeroelastic wind tunnel tests

A comprehensive experimental approach to the problem of determining static stability boundary (divergence) or dynamic stability boundary (flutter) of a given multiparameter aeroelastic system in a low speed wind tunnel is presented. The experimental setup with the corresponding measurement algorithm is based on the well-known theoretical model of an elastically supported typical wing section with two degrees of freedom. The design of the experimental system makes it possible to vary structural parameters within the determined parameter space. An automated measurement algorithm for determination of static or dynamic stability boundary by monitoring the aeroelastic system response to an initial impulse in the time domain is developed. The multiparameter experimental results of the case study using a NACA 0012 airfoil are compared to the analytical solutions of the theoretical model. The comparison shows that the experimental setup represents a reliable platform for aeroelastic wind tunnel test purposes. In addition, series of experiments were conducted to investigate the effect of airfoil thickness distribution on dynamic and static stability boundary. The results are compared to those of the NACA 0012 airfoil.

ON STATIC STABILITY OF NONLINEARLY ELASTIC EULER'S COLUMNS OBEYING THE MODIFIED LUDWICK'S LAW

It is illustrated in this paper that a nonlinearly elastic column, depending upon the values of different material parameters involved, exhibits several stability characteristics and types of buckling which are generally observed separately in distinctively different structural systems. By introducing finite disturbances it is shown that the column may buckle well before the bifurcation buckling load is reached. The proposed approach can be useful in engineering practice since it can be utilized to study the stability of uniaxial structural elements made from rubber or any other material which obeys the modified Ludwicks constitutive model.

Mesh deformation based on artificial neural networks

In the article a new mesh deformation algorithm based on artificial neural networks is introduced. This method is a point-to-point method, meaning that it does not use connectivity information for calculation of the mesh deformation. Two already known point-to-point methods, based on interpolation techniques, are also presented. In contrast to the two known interpolation methods, the new method does not require a summation over all boundary nodes for one displacement calculation. The consequence of this fact is a shorter computational time of mesh deformation, which is proven by different deformation tests. The quality of the deformed meshes with all three deformation methods was also compared. Finally, the generated and the deformed three-dimensional meshes were used in the computational fluid dynamics numerical analysis of a Francis water turbine. A comparison of the analysis results was made to prove the applicability of the new method in every day computation.