EXPERIMENTAL DETERMINATION OF LIMITING TRANSFORMATION WITH HIGHER STRAIN RATE. Jan Boček a Pavel Doubek, Michaela Kolnerová, Lukáš Kovárník - PDF

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METAL Hradec nad Moravicí EXPERIMENTAL DETERMINATION OF LIMITING TRANSFORMATION WITH HIGHER STRAIN RATE Jan Boček a Pavel Doubek Michaela Kolnerová Lukáš Kovárník a TU of Liberec Hálkova

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METAL Hradec nad Moravicí EXPERIMENTAL DETERMINATION OF LIMITING TRANSFORMATION WITH HIGHER STRAIN RATE Jan Boček a Pavel Doubek Michaela Kolnerová Lukáš Kovárník a TU of Liberec Hálkova Liberec CR Abstract Nowadays is used in the car industry for production of superstructure quite a number of different sorts of sheet metals. Mechanical tests of these materials are proceeds under different conditions than really straining is. When car is runs and when it clashes into the impediment so its superstructure is deformated by high strain rate. Classic tests are carried out by very lower strain rates. There is a big problem in this section of mechanical exams that's why we must practise these exams by high strain rate. Only after we will have true values for objective appreciation of several materials we will be only able to evaluate pertinence of other materials for construct a several parts of superstructure.. INTRODUCTION Problems with detection and generating of forming limit diagrams are already dealing with a number of authors in a lot of publications. Generally are defined limiting states parameters of straining different sorts of element deformation mesh and methods of making deformations meshes. Only few authors present method for measured deformed mesh used in their experiment so far from to make a survey of these methods. Different results of experiment may be achieved also by using of different detection methods and evaluation of forming limit diagrams. 2. PROCESS OF STRAINING [2] One of most using method of graphics visualization of limiting strain materials are forming limit diagrams (FLD). Premise for making as most exact FLD are mastered methods of production testing samples with strain mesh and their result measuring. For straining of these samples by low strain rates is using in Technical University of Liberec Department of Engineering Technology Section of Metal Forming and Plastics modernized hydraulic press CBA 3/63. For straining by high strain rates was evolved special pneumatic machinery that shoots a drawing punch by pressure air into the sample (Fig..). Fig.. High rate machinery METAL Hradec nad Moravicí 3. ABSTRACT OF METHODS BY EVALUATION DEFORMATION MESHES Metering and evaluation deformed meshes of testing samples is the longest and the most laboured part of all testing process. In the start of the measuring are chosen suitable elements on the spot samples. Most often are chosen those elements which closely abutting with beginning fracture but aren't affected by that. For measuring of strain by circular elements (the most often) of deformed mesh building-up on the measuring sample we can for example use one of sequentially described methods. Types and methods for evaluation of measuring values are very different for several methods. We can sequentially use measured values for next statistic evaluation or graphics processing for example for drawing a forming limit diagram. 3..Workshop microscope [2] Measuring of deformation meshes with the help of workshop microscope is one of the most common methods of findings final deformations of testing samples. Principle of measuring consists in findings size of deformed element (with regard to definition of limiting strain). When we use a mesh with circular elements (Fig.2.) we after measure deformation on chosen elements length of primary and secondary axes of created ellipses (primary axis 3-4 secondary -2). Thus determined values was continuously registered and recounted into the required values of strain in major (lengthwise) φ in minor (transverse) direction φ2 and then used to create forming limit diagrams. Fig.2. Workshop microscope and method of measuring deformed element In laboratories of Department of Engineering Technology TU of Liberec is measuring deformed meshes practise by older workshop microscope Zeiss (Fig.2.) which is connect with PC and makes it possible direct digitising and registration of measuring values. These values are records into computer by program Microsoft Excel for findings logarithmic strains. Resulting deformations are imports into computer program Statistica 6. and at the end are created forming limit diagrams (Fig.3.). This computer program allows display not only zones of dots with regression curves (forming limit curves FLC) and regression equations but also tolerance and confidence limits.. Fig.3. Graphic visualization of program Statistica 6. 2 METAL Hradec nad Moravicí For lucidity I introduce basic parameters of workshop microscope Zeiss in the sequent table (Table.). Table. Chosen values of workshop microscope Workshop microscope Measuring range Accuracy Magnification Shift XYZ Angle Shift XY Angle Maximum weight of sample Weight of machinery Zeiss R5U/6 3x4x5 mm 36 o mm o Ocular 63x Objective 6x Total 8x c. 5 kg c. 3 kg 3.2. System ARGUS [2345] Measuring machinery ARGUS is contact less optical system for testing 3D sheets strains during forming process (Fig.4.) using for example by Škoda Auto a.s. company. Fig.4. Measuring system ARGUS with detail of camera On the measuring samples were created deformation meshes that were deformated together with these steel samples. Size of meshes dots (Fig.5.) is standardly between and 6 mm. This grid is after forming scanning by camera with CCD chip. From pictures were by using of image processing computed 3D coordinates of grid dots. With the help of volume conservation law is computed distribution of major and minor strain then is determined reduction of material thickness and sequentially program creates a forming limit diagram (FLD). The findings are visualization by 3D models (Fig.6.) cuttings and output listings. Fig.5. Detail of deformation mesh 3 METAL Hradec nad Moravicí Advantages of this system are mobility (system may be toted in personal car) flexibility (easy modification by view size and system discriminability) large measuring range (objects from size of mm 2 to some few m 2 ) strain measuring range (from 5% up to some few %) high accuracy and number of measured values (dots at the object surface) as well as transparent analyse of measuring findings - graphic visualization. Fig.6. Sensing specimen and relevant 3D model Outputs from measuring may be for example in form of values of 3D translations 3D strains (Mises Tresca) and finally values of primary and secondary strains (Major Minor strain). How was noted previously in introduction we also may carried out cuttings and determined modifications of material thickness (Fig.7.). Fig.7. Other examples of visualization system ARGUS For clearness I in table finally introduce (Table 2.) some few basic parameters of different types of system Argus. Table 2. Chosen values of measuring system ARGUS ARGUS - type 8M 3M 2M 4M 2M Resolution of camera 24x768 28x24 6x2 Sensing flat mm 2 to m 2 Number of dots by type to 3 Deformation range 5 % to 3 % Maximum accuracy to 2 % 248x x284 Sensor size Weight of machinery 9x7x2 mm 7 kg 4 METAL Hradec nad Moravicí 3.3. CCD camera [26] Strain sensing of forming sample by digital camera is carrying out in actual time. When is guaranteed camera's synchronization with deformed measuring sample's surface then is possible to destine from back analyse of individual samples accurate moment of fissure inception and to that appropriate limit sample strain. Scheme of this process is featured in figure Fig.8. ϕ 7 6 v = 28 mm.s - v = 273 mm.s ϕ 2 Fig.8. Measuring with CCD camera Experiments with digital camera equipped by CCD chip were preceded also in Technical University of Liberec Department of Engineering Technology Section of Metal Forming and Plastics. In special modified drawing tool was create mechanical synchronisation shift between forming of sample surface and CCD camera for guaranteed constant focal distance of sensing (Fig.9.). After experiment camera was connected with PC and obtained values were transferred through high-speed interface Fire Wire with minimum damage quality of findings notation. Measuring of chosen elements was carrying out on the PC display by the aid of special simple dedicated program. This program runs on the principle of calibration known size in our instance it's size of the strain mesh element. Accuracy of measuring values is given by quality of camera sensing component so size of output figure in. Pixel is basic unit of size for PC and it's able to determine quality of image too. More meaning that image is superior and measured values are more accurate. Into the future is planned procuring all machinery by special automatic software that will be able to measuring both distances and also processing input image in really time. Fig.9. Attachment scheme of CCD camera 5 METAL Hradec nad Moravicí 3.4. Magnifier and ruler [2] This evaluation method is one of the oldest methods and now isn't practically using. Measuring principle consists in apposition of ruler or graph paper on deformed sample and destined sizes of chosen element by the aid of enlarging magnifier (Fig..). From description is evident that this process was very laboured and relatively inexact. Fig.. Magnifier and ruler 4. Conclusion In previous chapter was in more detail defined some few basic processes of findings limit values of deformed samples. In technical experience is using quite a number of another methods however their measuring procedure is analogous to some of here described. Final strain values and forming limit diagrams compiled from them are known for single methods therefore there is hard work to be able to compare findings of each other methods. For samples which will always be created from one testing material and loaded up to defined limit state will be used number of evaluation methods. These measurements will be practise on quite a high number of common materials using for car superstructure. Science findings should be already known within short time and they should therefore specify our view on problem as whole. Intercom paring of obtained data measured under the same technological conditions but different evaluation methods is our main purpose. This paper has been prepared under the terms of solution of research program MSM REFERENCES [] BOČEK J. Vliv rychlosti deformace na velikost mezního přetvoření Diplomová práce Liberec: TU v Liberci s [2] BOČEK J. Vliv rychlosti přetvoření na mezní deformace pevnostních plechů Teze disertační práce Liberec: TU v Liberci s [3] Msol: ARGUS [6.2.26] [4] Gom: Optical Measuring Techniques ARGUS [6.2.26] [5] Mcae Systems: ARGUS [6.2.26] [6] KOVÁRNÍK L. aj. Moderní digitální technika při analýze deformace plechových výlisků. In Sborník z konference Medzinárodná vedecká konferencia Forming 24 Bratislava: Slovenská technická univerzita v Bratislave 24 s ISBN
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