Comparison of Structural Analysis of Thin-Walled Structures Accomplished by Isogeometric Analysis and the Finite Element Method.

Isogeometric analysis (IGA) represents a relatively new method of problem-solving in engineering practice. A huge advantage of this method over the finite element method (FEM), is the reduction of the simulation execution time. Non-uniform rational B-splines (NURBS) allow the use of higher-order basis functions, thus increasing the accuracy of the solution. This paper deals with the comparison of structural analysis of thin-walled structural elements using isogeometric analysis and the finite element method. The investigated objects are modelled using a single patch in MATLAB. The basic functions are created from NURBS, which were previously used in the creation of an accurate geometric model. The paper contains a comparison of the results obtained by the above-mentioned methods. All computations are performed in the elastic domain.

Modal Analysis Using Digital Image Correlation Technique.

The present paper discusses a new approach for the experimental determination of modal parameters (resonant frequencies, modal shapes and damping coefficients) based on measured displacement values, using the non-contact optical method of digital image correlation (DIC). The output is a newly developed application module that, based on a three-dimensional displacement matrix from the experimental measurement results, can construct a frequency response function (FRF) for the purpose of experimental and operational modal analysis. From this frequency response function, the modal parameters of interest are able to be determined. The application module has been designed for practical use in Scilab 6.1.0, and its code interfaces directly with the ISTRA4D high-speed camera software. The module was built on measurements of a steel plate excited by an impact hammer to simulate experimental modal analysis. Verification of the correctness of the computational algorithm or the obtained modal parameters of the excited sheet metal plate was performed by simulation in the numerical software Abaqus, whose modal shapes and resonant frequencies showed high agreement with the results of the newly developed application.

Stability Loss Analysis for Thin-Walled Shells with Elliptical Cross-Sectional Area.

The aim of the scientific contribution is to point out the possibility of applicability of cylindrical shells with a constant elliptical cross-sectional shape for stability loss analysis. The solution to the problem consists of two approaches. The first approach is the experimental measurement of critical force levels, where the work also describes the method of production of the sample and jigs that cause the desired elliptical shape. The second approach is solving the problem in the use of numerical methods-the finite strip method together with the finite element method.

Sizing and Topology Optimization of Trusses Using Genetic Algorithm.

Genetic algorithms are a robust method for a solution of wide variety optimization problems. It explores a big space of design variables in order to find the best solution. From the point of view of a user, the algorithm requires the encoding of design variables into the form of strings and the procedure of optimization uses them for optimization. Here, for the structural engineer, it is crucial to find the form of objective function including the constraints of the task and also to avoid critical states during the solution of structural responses. This paper presents the use of genetic algorithm for solving truss structures. The use of genetic algorithm approach is shown on three cases of truss structures.

Influence of Different Strain Hardening Models on the Behavior of Materials in the Elastic-Plastic Regime under Cyclic Loading.

In exact analyses of bodies in the elastic-plastic regime, the behavior of the material above critical stress values plays a key role. In addition, under cyclic stress, important phenomena to be taken into account are the various types of hardening and the design of the material or structure. In this process, it is important to define several groups of characteristics. These include, for instance, the initial area of plasticity or load which defines the interface between elastic and plastic deformation area. The characteristics also include the relevant law of plastic deformation which specifies the velocity direction of plastic deformation during plastic deformation. In the hardening condition, it is also important to determine the position, size and shape of the subsequent loading area. The elasto-plastic theory was used for the analysis of special compliant mechanisms that are applied for positioning of extremely precise members of the Compact Linear Collider (CLIC), e.g., cryomagnets, laser equipment, etc. Different types of deformation hardening were used to simulate the behavior of particular structural elements in the elastic-plastic regime. Obtained values of stresses and deformations may be used in further practical applications or as default values in other strain hardening model simulations.

Experimental and Numerical Analysis of 60-Year-Old Sluice Gate Affected by Long-Term Operation.

Technological units of water reservoirs and power plants include sluice gates which are designed to completely seal the inflow or outflow of water in supply or discharge channels. This article describes the issue of technical assessment of a sluice gate made in the 1950s. Such structures are characterised by states of significant corrosive wear, permanent deformations of contact and sealing surfaces as well as increased levels of residual stresses. In such cases, it is difficult to determine service life using only numerical modelling methods, mainly due to problematic definition of material properties and boundary conditions. Therefore, for safety assessment, it is necessary to verify these facts experimentally. This article presents the procedure for assessing safe operation of the sluice gate on which places with permanent deformation and a broken part of the guide wheel flange were identified. By means of numerical modelling, we identified critical stress values at the locations of reinforcing elements, which were modified, and the stress values were reduced by about 15%. The results of numerical modelling were verified at select locations by experimental measurements during operation using strain gauges. The maximum values of operational normal stresses in the assessed places reached about 27 MPa. Based on the comparison of obtained results and taking into account values of residual stresses reaching up to 190 MPa made by shielded metal arc welding, it can be stated that, for safe operation of the sluice gate, it is necessary to follow the proposed procedure during its lowering and to modify the reinforcing elements structurally.

A comparison of experimental compressive axial loading testing with a numerical simulation of topologically optimized cervical implants made by selective laser melting.

In recent years, the number of cervical interventions has increased. The stress shielding effect is a serious complication in cervical spine interventions. Topological optimization is based on finite element method structural analysis and numerical simulations. The generated design of cervical implants is made from Ti6Al4V powder by selective laser melting while the optimized cage is numerically tested for compressive axial loading and the results are compared with experimental measurement. Additive manufacturing technologies and new software possibilities in the field of structural analysis, which use the finite element method tools, help to execute implant topological optimization that is useful for clinical practice. The inner structures of the implant would be impossible to make by conventional manufacturing technologies. The resulting implant design, after modification, must fulfill strict application criteria for the area of cervical spine with respect to its material and biomechanical properties. The aim of this work was to alter the mechanical properties of the cervical intervertebral cage to address the clinical concern of the stress shielding effect by topological optimization. A methodology of cervical implant compressive axial loading numerical simulation was created, and subsequent experimental testing was done to obtain real material properties after a selective laser melting process. The weight of the optimized implant was reduced by 28.92 %. Results of the experimental testing and numerical simulation of topologically optimized design showed 10-times lower stiffness compared to the solid cage design, and the real yield strength of the optimized structure is 843.8 MPa based on experimental results.

Application of holographic interferometry in the analysis of stress states in a crack root area.

This paper deals with the visualization and analysis of interaction of a cutting wedge disintegrator with plastic at low loads. For exploratory research, a contactless optical holographic interferometry method was used, allowing a comprehensive picture of the stress state when opening microcracks. An experimental model was set up for the purposes of the research. The structure of the model as well as its geometric parameters had to comply with the applied optical method. The method of holographic interferometry enabled us to record even the initial stages of the crack. Pictures of holographic interferograms allowed us to observe stress fields on the cutting wedge as well as on the loaded body in the form of interference fringes. In order to record the interferograms, we used the method of two exposures so that we gained double-exposure interferograms, which represent the state of the object during the second exposure. The first exposure was caused by superposition of object-related and reference wave after the object was subjected to a load; the second exposure occurred after the load was removed. We used quantitative analysis to determine stress intensity coefficients from holographic interferograms as followed by the calculation of stresses with respect to axes $x$x and $y$y. The analysis was done for loading forces 1.57 N and 3.14 N. As the load applied to the cutting material was increasing, the density of interference fringes was increasing, too.

Submucosal calcifying fibrous tumor of the stomach: A case report.

The calcifying fibrous tumor is a rare benign fibrous tumor which occurs in subcutaneous or deep soft tissues in children and young adults, but also is frequently seen in pleural and intraabdominal locations in older people. Gastric involvement has been only sporadically reported in the literature. We present here our experience with this unusual lesion discovered in a 68-year-old woman. Clinically, the tumor was described as a pendulating, submucosally located mass, in the body of the stomach on a lesser curvature. The calcifying fibrous tumor is a histologically distinct lesion composed of dense hyalinized collagen fibers, inconspicuous scattered fibroblasts, a varying amount of psammoma bodies or dystrophic calcifications and foci of lymphoplasmacytic infiltration. In this report we will focus on a brief review and differential diagnosis of this tumor and other more common or not widely known gastric spindle cell lesions.

Nonfunctioning pituitary adenomas: association of Ki-67 and HMGA-1 labeling indices with residual tumor growth.

BACKGROUND: The postoperative biological behavior of nonfunctioning pituitary adenomas (NFPAs) is variable. Some residual NFPAs are stable long-term, others grow, and some recur despite complete removal. The usual histological markers of tumor aggressiveness are often similar between recurring, regrowing, and stable tumors, and therefore are not reliable as prognostic parameters. In this study, the clinical utility of proliferation indices (labeling index, Li) based on immunohistochemistry targeted at antigens Ki-67 and High-mobility group A1 (HMGA-1) for prediction of NFPA prognosis was investigated. METHODS: Fifty patients with NFPAs were investigated. In each patient, Ki-67 and HMGA-1 Li were evaluated. Based on postoperative magnetic resonance images, patients were classified as tumor-free (18 patients), or harboring a residual tumor (32 patients). The latter group was further subdivided into groups with stable tumor remnants (11 patients) or progressive tumor remnants (21 patients). RESULTS: The median follow-up period was 8 years. No significant relationship between HMGA-1 Li and residual tumor growth was found. Growing residual tumors showed a trend towards higher Ki-67 Li compared with stable ones (p = 0.104). All tumor remnants with Ki-67 Li above 2.2% were growing. The relationship between residual tumor growth and Ki-67 Li exceeding the cutoff value of 2.2% was significant (p = 0.01 in univariate, p = 0.044 in multivariate analysis). CONCLUSIONS: The prognostic significance of the HMGA-1 antigen was not confirmed. In contrast, the Ki-67 Li provides useful and valuable information for the postoperative management of NFPAs. In residual adenomas with a Ki-67 Li above 2.2%, regrowth should be expected, and these tumors may require shorter intervals of follow-up magnetic resonance imaging (MRI) and/or early adjuvant therapy. Future larger studies are needed to confirm the results of this study.