Materials Structure & Micromechanics of Fracture VI

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Authors: Tomáš Matoušek, Petr Ponížil, Marek Galetka
Abstract: Physical properties of polymer foams depend on their macrostructure. In this paper, we study a range of polyuretane foams from the stereological point of view. Images of plane surfaces of studied samples were created by printing them on a sheet of paper and scanning these imprints. Consequent operations included reconstruction of pore section areas in the plane of printing and computation of pore volumes. The distribution of pore section was compared with data from computer tomography (CT) in order compare both methods.
Authors: G.D. Wang, Luen Chow Chan
Abstract: To evaluate the hydroformablility of tubular components in the tube hydroforming (THF) process, the conventional method is to measure the deformed square or circle grids printed on the surface of the tubular parts. However, the reliability of those measured data is affected greatly by the grid size and its measurement method on the curved surface. It is well-known that material hardness varies under different plastic deformation conditions, especially before and after the forming process. And it is more convenient to obtain the Vickers’ hardness values and distribution around the burst area of deformed components. This paper mainly presents an effective and reliable approach to evaluate the hydroformability of tubular components using micro-hardness measurement. At first, the Vickers’ hardness values and distribution around the burst area of the deformed components were obtained. The plastic strain, together with its distribution in such an area could then be derived by the measured micro-hardness through the developed equations. As a result, it was found to be more suitable to evaluate the hydroformability of tubes using this approach instead of the traditional grids measurement.
Authors: Michal Zouhar, Pavel Hutař, Luboš Náhlík, Zdeněk Knésl
Abstract: Failure of layered materials is frequently caused by the existence of interfaces between single layers. In structures loaded by long-term constant stress the damage can be described on micro-scale by material changes due to active creep mechanism and at the macro-level by a corresponding change of the stress and strain field. In the contribution the behavior of a crack propagating through the interface is analyzed. Primary attention is devoted to a crack with its tip at the interface under creep exposition. The step change of material properties at the bi-material interface means that standard fracture mechanics access is not applicable and a modified approach based on generalization of the classical concept has to be used. The approach is illustrated on the damage of a plastic pipe with protective layer.
Authors: Tomáš Profant, Jan Klusák, Michal Kotoul
Abstract: A bi-material notch composed of two orthotropic parts is considered. The stresses and displacements are expressed using the Stroh-Eshelby-Lekhnitskii formalism for plane elasticity. The potential direction of crack initiation is determined from the maximum mean value of the tangential stress or the local minimum of the mean value of the generalized strain energy density factor in both materials [1, 2]. The matched asymptotic procedure is introduced to derive the change of potential energy for the debonding crack and the crack initiated in the determined direction [3].
Authors: Kirill A. Khvostunkov, Alexey G. Shpenev
Abstract: The model of fiber/matrix interface degradation due to abrasive wear of fiber/matrix contact surfaces is presented. This model explains weak interface composites low cycle fatigue effect and cycle to cycle creep rate increase. The authors consider the results of experiments series carried out at the Institute of Solid State Physics located at Chernogolovka town. During this experiments series some weak interface nickel-oxide composite samples were subjected to a cyclic load in the course of a three point bending test and the samples central point set displacement rate was registered. Then there was created a fiber-matrix interface cyclic degradation model. The authors compared their theoretical assumptions to experimental data (for reference kindly see the below calculations) and evaluated low cycle fatigue influence on composite fracture.
Authors: K.G. Anthymidis, A. Balouktsis, Kostas David, A. Trakali
Abstract: The usage of Computer Numerical Controlled Machines has been generalized over the last decades due to the increased demands for the production of mechanical parts with précised dimensions, higher production rates and products with better treated surface quality. It is well known that the duration of life of a cutting tool is one of the most important parameters during the cutting of metal parts, because it affects the cost of the manufacturing process substantially. Therefore, it is important to know accurately, the relation between the duration of life of the cutting tool and the conditions of the machinery such as cutting velocity, feed rate (fz), the depth of cut (radial and axial) etc. The purpose of this research is to conduct a proper number of cutting experiments in milling, measuring the wear of the cutting tools, in order to conclude in a mathematical model the wear cutting tool. This model can be implemented for the prediction of the cutting tool life, which is very important for the determination of the best cutting conditions. From the experimental diagrams we can come to conclusions for the course of the wear of the cutting tool in connection with the velocity of cutting, for the machinability of the materials used etc. The wear of the used cutting tools was determined by means of optical microscopy and stereoscopy.
Authors: Sharon Kao-Walter, M. Walter, A. Dasari, A. Leon
Abstract: The fracture behaviour of laminated materials was studied in this work. The materials used in this work were low-density polyethylene (LDPE) laminated on polyethylene (PET). The thickness of the LDPE was 27 µm and the PET was 100 µm. Experiments were performed by using a 2-leg trousers specimen to analyse the tearing behaviour of the laminate in relation to the delamination. A clear delamination zone was observed during the crack propagation by tearing. Furthermore, a finite element calculation was performed to simulate the behavior around the crack tip during the tearing. A correlation between adhesion and crack propagation was discussed. Finally, the theory of Essential Work of Fracture (EWF) was used for predicting the specific total work of fracture along the tear path across the plastic zones.
Authors: Martin Ševčík, Zdeněk Knésl, Pavel Hutař, Luboš Náhlík
Abstract: In the case of large diameter polyethylene (PE) and polypropylene (PP) pipes the most usual joint is the butt weld. This paper focuses on the influence of a weld bead on the service life of welded pipes from a fracture mechanics perspective. To this aim the stress concentrator created by the connection between the weld bead and the pipe’s free surface is modelled as a notch. Global parameters defining properties of stress and strain field in the notch region are calculated for materials with a prevailing Norton-type creep. It is demonstrated that in the case of creep loading conditions and ductile failure mode the existence of such stress concentrators has no negative influence on the lifetime of pipes.
Authors: Petr Dymáček
Abstract: The contribution is presenting experimental results of conventional creep and small punch testing performed on miniaturized discs prepared from P91 and P92 steels. Some of the test cases were also numerically modeled by Finite Element Method (FEM) with help of Norton's constitutive model and the results were compared with the experiment. Presented work complements the existing empirical investigations on small punch and conventional creep tests relations.
Authors: Petr Řehák, Miroslav Černý, Jaroslav Pokluda
Abstract: Stress-strain responses of five fcc crystals subjected to triaxial loading are investigated by means of pseudopotential density functional method. Particularly, the influence of superimposed transverse biaxial stresses on the maximum uniaxial stresses is evaluated for both tensile and compressive regions. The obtained results revealed that the compressive strengths (maximum compressive stresses) of all studied metals are more sensitive to the superimposed stresses than their tensile strengths. The compressive strengths were found to be increasing linear functions of the compressive transverse stresses.

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