Zpět na hlavní stránku Semináře ÚT
| Úterý 10. 4. 2018, 13:00, posluchárna B | |
Behaviour of brittle materials under dynamic loading. |
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| Jaroslav Buchar a Jan Trnka, Ústav termomechaniky AV ČR, v. v. i. | |
| The lecture deals with some achievement on description of brittle materials behavior at highstrain-rate loadings such as: air blast loading or percussive drilling of rocks, ballistic impact against ceramic armour or transparent windshields, plastic explosives used to damage or destroy concrete structures, soft or hard impacts against concrete structures and many others in civil and military applications. The most popular dynamic testing techniques used for this which are based on the use of split Hopkinson pressure bar methodologies and/or plate impact testing methods are briefly described. The influence of the strain rate on the material strength is discussed. Some constitutive equations are presented. Some of them are used in the numerical simulation of some ballistic loading of ceramics. |
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| Středa 28. 3. 2018, 11:00, posluchárna B | |
Finite Fracture Mechanics and its Applications to Composite Materials |
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| Vladislav Mantič, Department of Continuum Mechanics and Structural Analysis, School of Engineering, University of Seville, Spain | |
| The basic assumption of Finite Fracture Mechanics (FFM) is to allow crack growth by (possibly) finite steps, in opposite to the hypothesis of crack growth by infinitesimal steps adopted in classical Linear Elastic Fracture Mechanics (LEFM). The coupled (stress and energy) criterion of FFM introduced by D. Leguillon (2002) requires that both stress and energy conditions are simultaneously fulfilled for such a finite crack advance. A quite general formulation of the coupled criterion of FFM leading to an optimization problem is introduced. Several examples of applications of this coupled criterion to the prediction of damage initiation in form of cracks at micro- and meso-scale in composites are presented. | |
| Středa 28. 3. 2018, 14:00, posluchárna B | |
Brief introduction to optimization and topology optimization |
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| Dr. Paulo Salvador Britto Nigro, Software Developer and Researcher of Virtual.PYXIS optimization, São Paulo, Brazil | |
The lecture will address the following topics in Topology Optimization (OT):
Dr Paulo Salvador Britto Nigro is a Software Developer skilled in Numerical Simulation applied to |
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| Středa 28. 3. 2018, 10:00, posluchárna B | |
Micromechanics of Martensitic Laminates |
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| Doc. Ing. Hanuš SEINER, Ph.D., Institute of Thermomechanics, Czech Academy of Sciences / Visiting Fulbright Scholar at the University of Minnesota, Minneapolis, USA | |
| The talk will summarize the main theoretical aspects of mechanics of geometrically ordered microstructures appearing in single crystals of shape memory alloys, called martensitic laminates. It will be shown that the formation of the laminates can be explained based on the concept of non-linear elasticity and energy-minimizing sequences. The applicability of this theoretical framework will be illustrated on two technologically important examples: i) branched laminates at the phase interfaces; ii) highly mobile laminate-laminate interfaces in the ferromagnetic shape memory alloys. For both cases, explicit constructions of energy upper-bounds will be shown, and the implications of the theoretical findings for designing of new alloys with advanced functionalities will be discussed. The development of these upper-bounds and the exploration of their properties are the main subjects of the current speaker’s research at the University of Minnesota, done in collaboration with prof. R.D. James and his research group. |
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| Středa 7. 3. 2018, 10:00, posluchárna B | |
Catch the yield surface, experimentally, theoretically, and computationally |
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| Dr. Li-Wei Liu, Department of Civil Engineering, National Taiwan University, Taipei 10617, Taiwan / Institute of Thermomechanics of CAS, v. v. i., Prague |
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| The yield surface of a material is the boundary of the elastic region where every stress point inside the region result from the elastic response of the material. The experimental evidence shows that the yield surface changes position, size, shape, and orientation during the material undergoing the plastic loading which results in the permanent deformation. Based on the experimental observation, the modelling of the yield surface evolution is a key point to completely simulate the plastic behavior of the material. Most experiments of yield surface detection were conducted in the two-dimensional space (axial-torsional or bi-axial). Due to the complete stress space is six dimensional, detecting the yield surface in the space whose dimension is more than two can collect more detail of the yield surface evolution. For the experiment of yield surface detection, the determination of yield point underpins the accuracy of the geometry of the yield surface. Nowadays, test machines used for the experiments of yield surface detection are usually servo-controlled hydraulic system, hence the scatter of data should be taking into account in the determination of yield point. To this end, an automated yield stress determination based on the Weibull distribution is introduced. After conducting the experiment in the axial-torsional-hoop stress space, yield points are obtained according to the yield-stress determination and designed probing paths. To further capture the global information from these yield points and observe the evolution of yield surface during different pre-loading paths, a convex-closed-cubic polynomial, which is capable of description of the yield surface evolution, including translation, expansion/ contraction, rotation, affine deformation, and distortion in the three dimensional space, is proposed and the corresponding three-stage estimation for parameters of the polynomial is developed. This polynomial enable us to observe the yield surface evolution from the three dimensional point of view and it can also be a candidate of potential yield functions. Furthermore, the computation of elastoplastic models needs more attention to the special mathematical structure of the model containing ordinary differential equations, algebraic equations, and inequalities. Exploring the underlying structure of elastoplastic models shows part of them possesses internal symmetry that is the pseudo-sphere of real pseudo-Euclidean space Rp,q on which the proper orthochronous pseudo-orthogonal group SOo(p,q), a sub group of the Lie group, leaves acts. Based on the internal symmetry, a return-free integration is developed and it keeps the computed stress point on the yield surface automatically and exactly without any extra enforcement during the plastic deformation. | |
| 15. 2. 2018, 14:00, posluchárna B | |
Complementary near field technique for assessment of materials with added value |
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| Dr. Adriana Savin, Head of Nondestructive Testing Department, National Institute of Research and Development for Technical Physics, Iasi, Rumunsko | |
| The National Institute of Research and Development for Technical Physics (NIRDTP) is a part of the national institutes R&D network coordinated by the Ministry of Research and Innovation - National Authority for Scientific Research and Innovation. Institute performs basic and applied research in the field of advanced materials with novel structures and properties, devices (i.e., sensors, transducers, actuators, measuring systems) based on advanced materials, new preparation methods and characterisation techniques, including non-destructive evaluation and magnetometry, electrical and magnetic separation, and devices for applications in engineering, healthcare, and biotechnology.
Nondestructive Testing Department (NDT) performs theoretical and applicative research in the field of electromagnetic testing of cylindrical and plate products including composite materials; calculation of the fields scattered by material discontinuities located at different areas of the multilayered medium by solving the forward problems; theoretical optimization of the operation of different types of sensors. Department also performs ultrasonic testing, development of specific methods for ultrasonic signal processing with FFT, digital filtering, neuro-fuzzy networks, development of the algorithms for defects localization and automaticclassification of flaws. In this lecture, a new possibility of using sensor with metamaterial lens for the nondestructive evaluation of metallic strip gratings and carbon fiber reinforced plastics will be presented. The sensor has enhanced spatial resolution due to the apparition of evanescent waves in the space between strips and between carbon fibers respectively, during the excitation by transversal electromagnetic field polarized along z-axis. The evanescent waves can be manipulated by a lens made from two conical Swiss rolls that act as a field concentrator. The detection has spatial resolution better than λ/2000. |
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| 25. 1. 2018, 14:00 posluchárna B | |
Evolution and Verification of a Kinematic Hypothesis
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| Prof. Herbert A. Mang, Institute for Mechanics of Materials and Structures, Vienna University of Technology | |
| Splitting of the strain energy into its “non-membrane” and membrane percentage provides insight into the load-carrying mechanism of structures, subjected to proportional loading. It may be useful, for example, for sensitivity analysis of the initial post-buckling behavior of beams, arches, plates, and shells, and assemblies of such structures. The task of this work is to determine this percentage without computing insignificant numbers such as the values of the strain energy and its membrane part. It is hypothesized that this percentage is proportional to the acceleration of a fictitious particle, moving along a curve on the unit sphere. The curve is described by the vertex of the normalized “fundamental eigenvector” of the so-called “consistently linearized eigenvalue problem”. The proportionality factor is obtained from the initial condition for the “non-membrane” percentage of the strain energy, hypothesized as twice the initial velocity of the particle. The lower bound of this factor signals the constancy of this percentage with increasing load, whereas the upper bound indicates a monotonic increase or decrease up to its ab initio predictable value at a stability limit or to an unphysical asymptotic limiting value. The proof of the universal validity of the two hypotheses begins with their verification for the special cases of a membrane stress state and pure bending. The assertion that this is a sufficient condition for the universal validity of these hypotheses is subsequently verified for an example with a monotonically increasing “non-membrane” percentage of the strain energy. It is finally confirmed by an indirect proof of their validity for a non-monotonic course of this percentage. A by-product of this work are conditions for extreme values of the stiffness of structures, subjected to proportional loading. | |
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