APPLICATION OF MEASUREMENT NETS FOR DEFORMATION EVALUATION AT BENDING

Transcription

1 Journal for Technology of Plasticity, Vol. 41 (2016), Number 1 APPLICATION OF MEASUREMENT NETS FOR DEFORMATION EVALUATION AT BENDING Emil Schwarzer *, Milan Dvořák Brno University of Technology, Faculty of Mechanical Engineering, Institute of Manufacturing Technology, Brno, Czech Republic ABSTRACT The article points out various ways of applying measurement nets on the steel sheet specimens. Advantages and disadvantages of the main deposition procedures and its practical use are shortly described. Also the shape of measurement nets pattern is discussed. In this paper the measurement nets were applied for determination of plastic deformation in the bending zone of sheet specimen. In that purpose on selected sheet specimens with different coating layers net pattern in shape of circles were etched by electrochemical method. Bending is performed using a special bend jig. After bending the bent samples were measured by an optical device. Measurements included the thickness of the bent sheet samples and the dimensions of the individual circles after deformation. Based on measurement results the logarithmic deformation φ 1, φ 2 and, φ 3 are then calculated. Key words: Specific nets, electrochemical methods, etching, deformation. 1. INTRODUCTION Measurement nets deposited onto a surface of specimens are common used for the evaluation of the strain state of formed components. The methods consist of measuring dimensions of applied net pattern before and after deformation. There are two main groups of coating methods for deposition of deformation measurement net on steel sheet. a) Methods, when damage does occur on the surface These methods damage the surface layer of material on testing specimens and the risk of distortion during the forming process and measuring data procedure occurs. Scribing by needle (Fig. 1) Punching (stamping) * Corresponding author s emil.schwarzer@seznam.cz

2 58 Fig. 1 Scribing needle surface b) Methods, when damage does not occur on the surface These methods do not damage the surface of the material, however there is a considerable risk of damaging the nets by the molding process (that is called wipe nets). Some of these methods with short description will be listed below. Printing by a stamp or method of metal dies Silk-screen printing The net obtained by above methods is very susceptible to wiping and do not have wide practical use. Photographic method The method is very similar to procedure of making photographs. The disadvantage is low steadiness of the net (easy to wiping). Photographic etching method In this method steel sheet is coated with photographic emulsion sensitive to UV radiation. The layer must have a thickness of only several micrometers, thicker would not go expose. Placing the grid prevents exposing the whole surface (Fig. 2). Subsequent exposure will cause etching of the surface material at locations exposure. The size of etching is dependent on the time of exposure. The method is time consuming. Fig. 2 Photographic etching method

3 59 Electrochemical method The method uses the electrolytic etching. Advantage is the speed of the method (in range of few seconds), simplicity, it does not arise any distortion of the sheet, cost efficiency and resistance during molding. The disadvantages are related to the high requirements in terms of surface condition. This method assumes perfect degreasing, the surface should not have defects, and cannot be corroded. Principle of the method is shown in Fig. 3. Electrode - roller Direction of rolling electrode Felt Raster Plate - sample Fig. 3 - Scheme of electrochemical method The time required for etching depends on the sheet material and the applied stress. The depth of etching is proportional to the application time. Applied net is permanent, wear resistant, and, the procedure does not cause any damage. However, this method can only be used for conductive materials. Necessary equipment for electrochemical etching is commercially available [2]. Photochemical etching nets Photochemical etching belongs to the precise methods for deposition of the measurement nets. The procedure includes the following steps: cleaning the metal surface, covering the surface layer with the emulsion resistant to UV rays, UV lighting, etching, surface cleaning. At normal temperature the steel is etched to a depth 2 to 4 μm. If the procedure is too long than etchant barrier layer deposited in the previous steps could interfere, which would result in deterioration of the edges of particular lines. The main disadvantage of this method is huge laboriousness. The advantages include quality and sharp edges of the net lines and the ability to reproduce a very dense net. Creation of the nets by laser In this case the required pattern of the net is applied to surface of the sample by laser treatment. When using this method the removal of material can be controlled. This method is characterized by high accuracy of the applied net but, depending on the sample size, the procedure may be very time consuming. Moreover, this method of creating nets is very expensive and it is not possible to obtain colored nets. It leads to another limitation since the net could be only of dark color it is impossible to obtain sufficient contrast between a net and a dark-colored surface of the sheet without disturbing the upper material layer. In such cases, we consider the method of screen

4 60 printing is better solution because it offers the choice of color net and related maximizing contrast. For galvanized materials laser method, in comparison with chemical etching is more accurate and faster. Track created by laser has half depth and width compared to footprint created by chemical etching. Therefore, the net created by laser gives a more reliable results, while there is less thinning of material than chemical etching. This method replaces chemical etching especially when we need to create a net on the sheet of large area. 2. SHAPES OF MEASUREMENT NETS Some typical patterns of the measurement net are depicted in Fig. 4. Circular patterns are common used in practice since the square net shows many disadvantageous from the viewpoint of subsequent evaluation. Fig. 4 Measurement nets of various shapes 3. EXPERIMENTAL PROCEDURE Practical application of measurement nets for evaluation of material deformation is shown in case of bending steel sheet specimens. Bending process was performed on the bending jig device with graduated scale for bending radius. Steel sheets (S235JRG2 according to EN standard) with nominal size 90x10x1 mm were used in experiment. Samples from the experiment were divided into groups in relation to the applied coating., see Table 1. The measurement net in shape of circles (Fig.5) were etched electrochemically onto the top surface of the specimens. Test samples were bent on a radius R11, R17, R23, R29 a R35 mm. [3], [4], [5]. Measurement is performed on the optical measuring device Scope-Check with accuracy 0.001mm (Fig. 6). We measured the thickness of the bent sheet samples and the dimensions L 1 and L 2 of the ellipses (shape of the initial circle pattern after deformation - Fig.7) [6]. Results for individual samples are shown in Tab. 1.

5 61 Fig. 5 The sample with the net of circular pattern after bending on the bending jig device P - rightside, L - leftside, S - middle Fig. 6 - Measuring optical device Scope-Check with accuracy mm with a clamped sample Fig. 7 - Measurement of thickness T of the sample

6 62 Tab. 1 - Measured thickness of selected samples (TC- top coat, BP -without coat, Z7- base experiment, PL- plasma) TC12 Z7 BP15 BP16 Z51 Z12 T T T T T T T T T T Fig. 8 is a photo of the etched ellipse with dimensions L 1 a L 2 on the bent sample. During deformation the net circle changes to an ellipse, where their axes define the direction and magnitude of maximum and the minimum logarithmic deformation pressed parts [7]. Fig. 8 - Measured length of the sample L 1 and L 2 after bending As seen from the results, the length L 1 are longer than length L 2 which is logical, because the bending direction was in direction L 1. Determination of the size of the logarithmic deformation φ 1, φ 2, φ 3 carried out by formulas (1) to (3). Because the changed sheet thickness during drawing, deformation φ 3 is calculated by the formula (3) [8], [9], [10]. φ 1 ln L 1 (1) d

7 63 φ 2 ln L 2 (2) d φ3 φ1 φ2 (3) Fig. 9 Determination of the size of the logarithmic deformation φ 1, φ 2, φ 3 The bent samples were measured by length L 1 and L 2. In the initial state the diameter of etched circles were measured, see Fig. 9.[11], [12]. The resulting deformation, averages of individual samples are shown in Table 2 to Table 4. L L1 L2 ød φ 1 φ 2 φ 3 sample S P TC12 Tab. 2 Determination of the size of the logarithmic deformation φ 1, φ 2, φ 3 for the sample TC12

8 64 L L1 L2 ød φ 1 φ 2 φ 3 sample S P Z7 Tab. 3 Determination of the size of the logarithmic deformation φ 1, φ 2, φ 3 for the sample Z7 L L1 L2 ød φ 1 φ 2 φ 3 sample S P BP15 Tab. 4 Determination of the size of the logarithmic deformation φ 1, φ 2, φ 3 for the sample BP15

9 65 4. RESULTS AND FINAL REMARKS In the current article determination of deformations φ 1, φ 2, and, φ 3 by measurement net in the bent area of sheet metal with coating is described. In all cases deformation φ 1 has the positive values and deformation φ 2 negative values. As far as φ 3 is concerned negative values dominate for samples TC12 and Z7. But for sample B15 positive values of deformations occur. Test samples were bent in special jig radiuses R11, R17, R23, R29 and R35. For bending operation special jig developed at the Technical University in Brno, Institute of Manufacturing Technology was used. Measuring was performed by optical device Scope-Check with an accuracy of mm. Arithmetic means for sample TC12 were calculated: for φ 1 = 0.053, φ 2 = , and, for φ 3 = For sample Z7: for φ 1 = 0.060, φ 2 = , and, φ 3 = For sample BP15: for φ 1 = 0.025, φ 2 = , and, φ 3 = Standard deviation was obtained for the test sample TC12: for φ 1 = 0.007, φ 2 =0.010, and, φ 3 = For sample Z7: for φ 1 = 0.007, φ 2 = 0.013, and, φ 3 = 0,016 and for sample BP15: for φ 1 = 0.011, φ 2 = 0.011, and, φ 3 = Acknowledgments The article was published by the support of the Faculty of Mechanical Engineering: BUT FME-S from 2012 and VAV Acknowledgements for Brno University of Technology who financed this technical article. REFERENCES [1] Tatíček F Methods of study of materials under different speed deformation, during pressing,.: Doctoral thesis. Technical University, Institute of Manufacturing Technology, Czech Technical University in Prague, [2] ČSN EN 13144: Metallic and Other Inorganic Coatings: Method for Quantitative Measurement of Adhesion for Tensile Test, Czech Standards Institute, (2003) pp [3] Dvořák M., et al.: Technology II, Academic Publishing CERM, Ltd., Brno, (2001) pp [4] Patent EP , US 6, 525,481: Method of Making a Physically and Chemically active Environment by Means of a Plasma jet and the Related Plasma Jet. Brno: Masaryk University, (2005). 6 pp. [5] Hušek M., Dvořák M.: Test of adherence multifunctional coating on the sheet using a graduated bending jig. Engineering Technology, Vol. XV, No. 2, (2010) pp [6] Blanks T.: Metals Handbook: Mechanical testing, American Society for Metals, (1985) pp [7] Čada R.: Surface formability of metallic materials, Ostrava-Poruba: Technical University of Ostrava, (1998) pp [8] Kraus V.: Surface modification, Plzeň: University of West Bohemia, (2000) pp [9] Kreibich V.: Theory and Technology of Surface Treatment, Publishing House of CVUT, Prague,(1996) pp

10 66 [10] Krejčík V.: Surface Treatment of Metals II, Publishing House of Technical Literature, Prague (1988). [11] Hermann F., Schiller M.: Testing of paints and protective coatings, Pardubice- Green suburb: SYNPO a.s. (2007). [12] Samek R.: Analysis of the limit state of plasticity and technological formability. Brno: University of Defense in Brno, (1988) 230 pp.

11 67 PRIMENA MERNIH MREŽA ZA ODREDJIVANJE DEFORMACIJA KOD SAVIJANJA Emil Schwarzer *, Milan Dvořák Brno University of Technology, Faculty of Mechanical Engineering, Institute of Manufacturing Technology, Brno, Czech Republic REZIME U radu je prikazano određivanje deformacija u procesu savijanja čeličnih uzoraka korišćenjem mernih mreža. U prvom delu rada analizirane su prednosti i nedostaci mernih mreža kao i njihova moguća geometrija. U ovom radu primenjena je mreža sa krugovima. Savijanje čeličnih uzoraka vršeno je pomoću specijalnog alata. Merenje geometrije deformisane mreže nakon deformacije vršeno je pomoću posebnog optičkog uređaja Scope-Check sa tačnošću od 0,001mm. Savijanje je vršeno na sledeće radijuse: R11, R17, R23, R29 a R35 mm. Pored promene geometrije u ravni vršeno je i merenje promene debljine lima, na taj način određene su sve tri deformacije. Metoda je primenjena za jedan broj različitih čeličnih materijala. Rezultati su analizirani i statistički obrađeni. Ključne reči: merne mreže, elektrohemijska metoda, deformacije