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Elektronový mikroskop SEM In our miniproject “Fracture: Initiation, Propagation, Consequences and Modeling” the tensile test, computer simulations, observation.

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Presentation on theme: "Elektronový mikroskop SEM In our miniproject “Fracture: Initiation, Propagation, Consequences and Modeling” the tensile test, computer simulations, observation."— Presentation transcript:

1 Elektronový mikroskop SEM In our miniproject “Fracture: Initiation, Propagation, Consequences and Modeling” the tensile test, computer simulations, observation and 3D reconstruction of the fracture surface were carried out. Our project took place at the Department of Materials of the Faculty of Nuclear Sciences and Physical Engineering, CTU in Prague. We had opportunity to see and carry out basic mechanical tests typical for the steel testing. Yield stress, ultimate stress, ductility and Young’s modulus were estimated from the stress-strain curves and compared to the values prescribed for the used steel grade. Tensile test specimen geometry was modeled using Solid Edge software and the virtual tensile test was carried out using the finite element method using software Abaqus. Fracture surfaces were observed by means of optical and scanning electron microscopy. It was stated, that the fracture propagated by coalescence of ductile dimples which we observed under high magnification. 3D fracture surface was reconstructed from the images obtained by light microscopy. Fracture was found to be about 1 mm deep. Conclusions 3D Surface Reconstruction Light Microscopy For the observation of the fracture surface by means of light microscopy we used the metallographic microscope NEOPHOT 32 equipped by OLYMPUS OPTICA DP50-CU digital camera. Fracture surface was divided into several areas. For each area sequence of images with high magnification and different plane of focus was taken. Resulting number of images was about Obtained images were used for the reconstruction of 3D surface. Fracture surface (magnification 30x) Fracture surface (magnification 100x) Scanning Electron Microscopy Fracture of broken specimen was observed by the Scanning Electron Microscope (SEM) JEOL JSM 840A. Aim of this study was to determine the fracture mechanism. Signs of extensive macroscopic as well as microscopic plastic deformation were observed. Under high magnification fracture features known as ductile dimples were observed. Overall view of fracture surface (magnification 50x)Ductile dimples (magnification x) Tensile Test Simulation Tensile test simulation was carried out to get some insight into the stress and strain conditions in the specimen and compare results for different geometry (flat specimen and flat notched specimen) and material model (elastic and elastic-plastic). Geometry of specimens model was created according to the real tested specimens. This model was virtually loaded in the Abaqus software using Finite Element Method procedures. Material properties based on the real tensile test were assigned to the model and mesh of elements was generated. The model was virtually loaded by prescribed displacement. Obtained results are illustrated by modeled stress distribution in the specimens. Material model for elastic-plastic material was based on the real tensile test. Up to the yield stress 280 MPa material remains elastic and then the plasticity occurs (see fig. on the right). In the real test necking and fracture occur after reaching the ultimate stress level. These effects were not considered in the proposed numerical model. Stress component S 11 (in loading direction) - plastic, no notchStress component S 11 (in loading direction) - plastic, notched As can be seen from the images for unnotched specimen, highest and homogenous stress state is localized in the narrow part of the specimen. This stress homogenity is reason why the tensile specimens are prepared in this shape. Otherwise the stress peak (and fracture) would occur under the jaws of the loading machine outside the observed area. In case of notched specimens the stress concentration was observed at the root of the notches. It is very probable, that the fracture would initiate at these point. Stress component S 11 (in loading direction) - elastic, no notchStress component S 11 (in loading direction) - elastic, notched Tensile Test Specimens for the tensile test were manufactured from the steel plate (grade ČSN ) in the workshop. Tensile test gives us some idea about mechanical behavior of the material under uniaxial tensile loading. From the loading curve can be determined yield stress and ultimate stress which can material withstand. Specimen stretched during tensile test. This behavior was documented by taking subsequent photographs with period of 6 seconds. The specimen surface was covered with white elastic paint and small black dots were prepared in order to see local deformation of the steel material. Another tensile test was carried out on round tensile specimen manufactured from steel ČSN The purpose of this test was to obtain the broken specimen which we wanted to use for the 3D reconstruction of the fracture surface. Detail view of 3D fracture surface reconstruction Merged focused images of the fracture surface Last but definitely not least we would like to thank to the “Cesta k vědě”, Gymnázium Karla Sladkovského and Gymnáziu Jaroslava Seiferta staff. Financial support from European Social Fund and Hlavní město Praha is gratefully acknowledged. Our thanks also go to the Department of Materials FNSPE for the support and namely to the Ing. Vladimír Pospíšil for the amount of time invested in the project and to our supervisors Ing. Zuzana Sekerešová and Ing. Radek Mušálek for their time and effort. Acknowledgementí Sequence of the images low depth of field Fully focused image 3D reconstructed surface Notches – Stress concentration Broken round tensile specimens Fracture: Initiation, Propagation, Consequences and Modeling Marek Kovář*, Jiří Švancara*, Tomáš Peták* Supervisor: Zuzana Sekerešová**, Radek Mušálek** Abstract Our research group accomplished in frame of the program Cesta k Vědě at the Department of Materials of Faculty of Nuclear Sciences and Physical Engineering (CTU in Prague) miniproject “Fracture: Initiation, Propagation, Consequences and Modeling”. Tensile tests of two different steels (grades ČSN and ČSN ) were carried out on flat and round tensile specimens. Fracture surfaces of broken specimens were studied using light and scanning electron microscope. Images obtained by light microscopy were processed by IF-analysis (Image Focus Analysis). Result of this analysis is the 3D surface of one chosen fracture surface. Stress conditions in the specimens during the tensile test were simulated by means of Finite Element Method using Solid Edge and Abaqus/CAE software. *High school Karla Sladkovského, Sladkovského náměstí 8, Praha 3, , **Faculty of Nuclear Sciences and Physical Engineering, Trojanova 13, Praha 2, Details are quantify by measure → energy of image Laplacian


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