Wyniki wyszukiwania - BazTech

1

Numerical study of residual thermal stresses in MMC

Nehari T.

Mechanics and Mechanical Engineering

|

2018

|

Vol. 22, nr 4

1099--1109

EN

In this paper, numerical study analysis of residual thermal stresses in aluminum matrix reinforced with silicon carbide particles with double-crack has been carried out. is studied in order to determine the thermo-mechanical behavior under the effect of different temperature gradients during cooling. For a more realistic simulation of the microstructure of these materials subjected to different loadings, a representative volume element may be used. In this paper, three different types of crack width a = 5 μm, 10 μm, 15 μm, has been carried. The thermal residual stresses are calculated by considering a wide range of cracks of different penetrations proximity to particle of 0.1, 0.2 and 0.5 μm. regarding the distribution of the stresses along the plane of the crack and in vicinity of the particle, results show that the penetration of the crack in the matrix causes an asymmetry. The inter-distance between crack and particle plays an important role regarding the generation of residual stresses. The lower the inter-distance, the higher the internal stresses of normal residual stresses of σzz.

2

The effect of the interface profile on the residual stresses formed in thermally sprayed NiAl coatings on Al2O3 substrate

Golański D., Chmielewski T., Sheng Z.

Przegląd Spawalnictwa

|

2014

|

R. 86, nr 8

41--47

EN

The physical preparation of a surface material to be coated by the thermal spray processing plays very important role as it affects the bonding force between the coating and substrate. When the coating material has different thermal and physical properties than the substrate the thermal residual stresses are generated in a two material system which may reach the level that promotes the coating delamination or cracking. Another factor influencing the stress state in sprayed coatings may lay in the interface profile between the coating and substrate. We may try to affect this factor by analyzing the profile and texture of the substrate surface to be coated. This way we may influence the residual stress built in a deposited coatings. The aim of this paper is to model and analyze the effect of the substrate surface profile on the residual stresses generated during cooling of thermally sprayed metallic coatings onto the ceramic substrates. The numerical computer simulation has been performed to calculate and compare the residual stress state developed in thermally sprayed metallic/ceramic system (NiAl/Al2O3). The main factor analyzed was the variation in the shape of the interface profile between the coating and substrate. The obtained results have been discussed.

PL

Przygotowanie fizyczne podłoża materiału do natryskiwania termicznego odgrywa bardzo ważną role ponieważ wpływa na stopień powiązania między utworzoną na podłożu powłoką. Gdy materiał powłokowy ma różne właściwości cieplne i fizyczne w stosunku do podłoża to w takim układzie powstaje określony stan naprężeń własnych termicznych, który w zależności od wielkości i rozkładu może prowadzić do pęknięć w powłoce lub jej delaminacji od podłoża. Istotnym czynnikiem wpływającym na stan naprężeń własnych w natryskanej powłoce może być kształt linii połączenia między powłoką a podłożem. Można mieć na niego wpływ poprzez analizę kształtu czy tekstury podłoża przygotowanego do natrysku. W ten sposób możemy wpływać na stan naprężeń w natryskiwanych powłokach. Celem pracy było zbudowanie modelu obliczeniowego oraz analiza wpływu profilu linii połączenia powłoki z podłożem na rozkład naprężeń własnych termicznych powstających w fazie chłodzenia natryskanej na podłoże powłoki. Przeprowadzono obliczenia numeryczne dla układu powłoki NiAl natryskanej na podłoże ceramiki Al2O3. Głównym analizowanym czynnikiem był charakter profilu granicy połączenia między powłoką a podłożem. Wyniki obliczeń naprężeń własnych dla sześciu różnych profili granicy połączenia poddano analizie porównawczej oraz dyskusji otrzymanych wyników.

3

Kompozyty spiekane Cr-Al2O3 z dodatkiem renu: wytwarzanie, właściwości, modelowanie, zastosowania

Basista M., Pietrzak K., Węglewski W., Chmielewski M.

Rudy i Metale Nieżelazne

|

2013

|

R. 58, nr 10

556--563

PL

Celem pracy było zbadanie wpływu dodatku renu na właściwości termomechaniczne i użytkowe kompozytów Cr-Al2O3 wytwarzanych metodą spiekania pod ciśnieniem w prasie HP oraz metodą Spark Plasma Sintering (SPS). Uzyskano kompozyty o gęstości przekraczającej 98 % gęstości teoretycznej. Właściwości mechaniczne (m.in. moduł Younga, wytrzymałość na zginanie, twardość, odporność na pękanie, granica plastyczności) oraz odporność na utlenianie wytworzonych materiałów są obiecujące. Zbudowano model numeryczny do obliczeń wielkości naprężeń resztkowych obecnych w materiałach faz kompozytu po procesie spiekania oraz modułów sprężystości. Wykorzystano w tym celu obrazy rzeczywistej mikrostruktury kompozytu otrzymane z tomografii komputerowej. Uzyskano dobrą zgodność wyników modelu z wynikami pomiarów naprężeń metodą XRD. Przedstawiono ponadto porównanie wyników obliczeń numerycznych i pomiarów modułu Younga przy zastosowaniu różnych metod.

EN

Chromium matrix composites reinforced with alumina ceramic particles exhibit good resistance to high temperatures are thermal shocks. They have enhanced mechanical strength in elevated temperatures, high hardness, oxidation resistance and wear resistance. These exceptional properties make them good candidates for structural applications in automotive, aerospace and energy sectors, such as elements of combustion engines, coatings in aeroengines exhaust systems, or furnace linings. The objective of the present paper is to investigate the effect of rhenium addition on the thermomechanical and service properties of chromium-alumina composites manufactured by powder metallurgy methods. A working hypothesis was made that rhenium, owing to its excellent thermomechanical properties, would enhance the properties of the chromium matrix and, thus, improve the overall performance of the composite. The Cr/Al2O3/Re composites were processed by hot pressing (HP) and by spark plasma sintering (SPS) techniques. Different sizes of chromium powders were used, the addition of rhenium was 2 vol % and 5 vol %. The sintering process was conducted at 1400-1450 °C under pressure of 30-35 MPa in inert gas atmosphere (argon). The density of the sintered composites exceeded 98 % of the theoretical density. The mechanical properties (Young’s modulus, bending strength, hardness, plastic limit) are promising. For example, the compressive strength of the composite was twice as much as that of the sintered pure chromium. The oxidation and corrosion resistance of the composites were also examined and good results were reported. A numerical FE model was developed for the prediction of thermal residual stresses generated in the phase materials after cooling. The model uses micro-CT images of the real material microstructure as the input data. A good agreement of the simulation results and the measurements by X-ray diffraction method was achieved. Young’s modulus of the obtained materials was measured by different methods (mechanical, resonance and ultrasonic) and compared with the developed micro-CT based numerical model. The obtained Cr/Al2O3/Re composites are now being tested as demonstrators of some structural elements in automotive and energy applications.

4

Modelowanie naprężeń własnych w kompozytach MMC z wykorzystaniem metody homogenizacji i techniki cyfrowej obróbki obrazu

Golański D.

Kompozyty

|

2002

|

R. 2, nr 5

354-358

PL

Przedstawiono możliwości modelowania naprężeń wewnętrznych w kompozytach MMC, wykorzystując metodę homogenizacji. Opisano schemat postępowania podczas modelowania naprężeń własnych w strukturze kompozytu WC-stellit. Zastosowana technika modelowania intensywnie wykorzystuje technikę cyfrowej obróbki obrazu oraz metodę elementów skończonych, dzięki czemu umożliwia analizę naprężeń w rzeczywistych strukturach materiałów wielofazowych.

EN

The homogenization method has been utilized with the help of digital image-based technique to analyze the local stress field in the unit cells of metal matrix composite surface layers. The matrix was a nickel-based stellite superailoy with tungsten carbide particulate inclusions. The theory of homogenization concerns statistically homogeneous or periodic composite media of domain Omega epsilon and the representative volume element (RVE) occupying a microscopic region V with characteristic length e (epsilon). Identifying the size of the RVE with e, we introduce two different scales: first is a macroscopic scale denoted by x, in the domain Omega e at which the heterogeneities are invisible and the other one is an microscopic one denoted by y = x/e which enlarges the RVE region by e such that V= eY. Thus, the superscripts introduced in variables indicate their orders as well as the dependency on both x and/or y (Fig. 1). According to the principle of minimum total potential energy for equilibrium, the displacement ue is the solution of the variational problem defined in the domain Omega e (eqn.l). With the help of two-scale asymptotic expansion method (4), the theory asserts that if the selected RVE is periodic and infinitesimally small, the actual displacement, ue, tends to the homogenized one, uo, which is the solution of the macroscopic equations whose coefficients have been homogenized (eqn. 3). Once the macroscopic displacement u° and Delta T are obtained in the macroscopic region, these values are localized to give the micromechanical response of the unit cell. Therefore, the microscopic stress is defined by eqn. (7). Digital image-based (DIB) technique is used to catch and manipulate the image of composite microstructures (unit cells) so that they could be analyzed by the homogenization method. Also, this technique is very helpful for preparing the images of composite unit cells to some other additional processing like generation of 3D structures or changing volume fraction of inclusions. The main procedure of preparation the unit cell of a composite microstructure can be divided into the following four major stages: capture and sampling, selecting and thresholding, exporting, stacking (Fig. 2). The WC-stellite surface layers shown in Figure 3a has been used for calculation of microscopic residual stresses. After the microstructure of the WC-stellite composite has been scanned into computer memory and converted into digital unit cell (Fig. 3b) the homogenization method was used to obtain the effective composite properties dependent on the properties of its constituents and their volume fraction. This was done for the selected unit cell taken from the microstructure of composite (Fig. 3b). Next, the global structure was defined to compose the substrate material covered with a composite surface layer having uniform homogenized properties (Fig. 4). Then, the standard finite element method was applied to solve for the displacement and stress field in the macroscopic scale (global structure). The last step utilizes calculated global displacement field taken for the selected unit cell from the composite's global structure to compute the microscale (local) stress distribution for composite microstructure represented by the selected unit cell. The results of calculations are shown in Figure 5 where principal sigma11 and effective Mises sigma H stress distribution in the analyzed WC-stellite composite unit cell are presented. The applied techniques presented in this paper can be seem to be the adequate tool to compare the local stress state in the metal matrix composites with hard inclusions, as well as to obtain the homogenized composite properties. We shall emphasize, that this kind of analysis is very unique as it allows the real microstructures of composite materials to be analyzed.