A detection of deformation mechanisms using infrared thermography and acoustic emission

Transcription

1 Applied Mechanics and Materials Online: ISSN: , Vol. 474, pp doi: / Trans Tech Publications, Switzerland A detection of deformation mechanisms using infrared thermography and acoustic emission VLADIMIR DEKYS 1, a, PETER KOPAS 1,b, MILAN SAPIETA 1,c And ONDREJ STEVKA 1,d 1 Department of Applied Mechanics, Faculty of Mechanical Engineering, University of Zilina, Univerzitna 1, Zilina, Slovakia a vladimir.dekys@fstroj.uniza.sk, b peter.kopas@fstroj.uniza.sk, c milan.sapieta@fstroj.uniza.sk, d ondrej.stevka@fstroj.uniza.sk Keywords: deformation, Lüder s band, infrared thermography, acoustics emission. Abstract. Change the deformation mechanism in metallic materials may be detectable using infrared thermography and acoustic emission. For samples of mild steel is the creation of Lüder s bands presented in the infrared spectrum. Follow-number increase of these bands is also presented. In the system of acoustic emission is for this deformation mechanism described signal, which allows detection of the deformation process of these technologies. Introduction Tensile test is widely used in the analysis of mechanical or material properties of objects. When the test is usually registered tension force exerted by the machine and deformation of the specimen. Those parameters are measured for example using a force transducer, contact extensometer or by displacement crosshead (when slippage is eliminated in the grips of the machine). Measurements are commonly used, their justification and are significant in terms of determining conventional parameters. If we want to analyze, for example, the distribution of deformation inhomogeneities on the specimen surface, we can use other tools that do not provide a summary value, but local, such as determining the deformation field using a non-contact optical 3D deformation measuring system ARAMIS HS (based on digital image correlation). Fig. 1 shows the deformation field of flat steel samples just before rupture. The classical contact extensometers provide average strain between contact points. Deformation field can also be determined by means of infrared thermography. Define values for strain, respectively stress is subject to thermal stress analysis (TSA). Basic relationships between temperature and the search values (based on infrared radiation incident on the detector) can be determined from the relationship, [1]:, (1) where is the density of specimen, is the heat capacity at constant pressure, is the absolute temperature, is time, is the thermal conductivity of the material,,, and are the spatial coordinates, is the energy conversion rate per unit volume by the thermoelastic effect, is the energy conversion rate per unit volume by the inelastic effect, and is the heat rate from heat source. The first member on the right side of the relationship (1) corresponds to the phenomenon of heat conduction. All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of Trans Tech Publications, (ID: , Pennsylvania State University, University Park, USA-12/05/16,20:47:01)

2 316 Novel Trends in Production Devices and Systems Fig. 1 The inhomogeneous strain field of flat steel samples before rupture, ARAMIS HS. If we are in the analysis of the deformation field with Lüders bands, then it is appropriate to identify an area of transition between plastic and elastic region. Lüders band is characterized by an initially high yield stress followed immediately by a sudden drop in stress. With continued straining the stress stays nearly constant for several percent of strain before normal strain hardening behavior begins. If we want to use extensometers, then try to remove the visual field of camera the contact points of extensometers. If we use optical methods, and auxiliary lights are often required for adequate illumination of the specimen. However, these lights can cause heating of the sample and be a source of spurious radiation in the field of cameras. Several authors [1-7] address the detection, analysis of the simulation Lüders bands. Our approach is based on signal processing of acoustic emission signals to detect and analysis the area in which results these bands. This multi-parametric approach - monitoring more than strictly necessary number of parameters in the experiment, we will want in the future be used for detailed analysis of experiments. We expected that in this area there is a change in emission energy detected in the frequency range up to 1 MHz by means of acoustic emission. The experiment, steel specimen The measurements were performed on a test machine Zwick The test specimen was steel sheet STN (S235JRG2), Fig. 2, thickness 1mm. The specimen was sprayed with spray having known values of emissivity 0,96. The specimen (metal strip) was pulled in the grip of test machine, the relationship between force and time (constant speed of crosshead) is in the Fig. 3. By using of acoustic emission PAC (Physical Acoustic Corporation) PCI 2 measurements system was realized the measure. We used 2 acoustic emission sensors, D9241A (low frequency range, 20kHz-60kHz, LF) and WD (high frequency range, 100kHz-900kHz, HF). The measurements were processed in the SW system AEwin (all PAC).

3 Applied Mechanics and Materials Vol Fig. 2 The test specimen, thickness 1mm, STN (S235JRG2). Fig. 3 The relationship of force at the time of the test on the test machine. The circle represents an important area of Lüdersovych bands, in the range second of the test. The measurement results are shown in Fig Fig. 4 LF sensor, number of hits during the test. Fig. 5 HF sensor, number of hits during the test.

4 318 Novel Trends in Production Devices and Systems Fig. 6 LF sensor, number of counts during the test. Fig. 7 HF sensor, number of counts during the test. Fig. 8 LF sensor, number of counts and duration of hits during the test. Fig. 9 HF sensor, number of counts and duration of hits during the test. The emission measurements show that the transition between elastic and plastic region of the material is clearly identifiable with extreme (minimum value) with hits and also counts (with extreme - maximum value). These results apply to the LF sensor. We are not able present similar results for the HF sensor on the basis of the measurements. The parameters for measure were: speed of crosshead 0.7mm/s, gain of pre-amplifier 20dB, threshold 50dB, sampling frequency 2MHz, hit definition time (HDT) = single channel event timeout (SCETO) = 1000 μs, peak definition time (PDT) = rise time-time out (RTTO) = 2000 μs, hit lockout time (HLT) = re-arm time out (RTO) = μs, max duration = 500 ms. The infrared camera FLIR SC7500 with cooled InSb detector 320x256 pixels was used to analyze the occurrence and spread of slip bands. In the Fig are some of results for frame rate 383Hz.

5 Applied Mechanics and Materials Vol Fig. 9 Increased temperature of the specimen due to the growth of deformation around the grips. Fig. 10 Discovering the slip bands, which are connected to the specimen areas with increased temperature. Fig. 11 Lüders bands that formed in areas with optimal conditions for this type of deformation. Fig. 12 Continued growth of the Lüders bands. Fig. 13 Images of the area before the subsequent hardening. Fig 14 Snapshot of the temperature field in stage after fracture specimen. Image is for a moment after 13 second test. Summary It is a variety of approaches to carry out experimental work. One of them may be a multiparameter approach that characterized such monitoring and analysis is not only necessary but a larger number of parameters, when the larger number of parameters is acceptable. When analyzing the Lüders bands using infrared thermography are frequently used methods of detection areas in which these bands occur. Based on measurements made using acoustic emissions, we concluded that the detection of this field can be done already by detecting hits and counts of steel and when presented, setting the system parameters of acoustic emission. Hits and county achieved in this yield phenomenon extreme values (minimum and maximum).

6 320 Novel Trends in Production Devices and Systems A numerical simulation of deformation and stress on the specimen will be a next step. We will base upon the experiences and research works from our Department of applied mechanics, Faculty of mechanical engineering and our co-workers e.g. [8]. Acknowledgement This research project is partially supported by the EU Structural Funds, Operational Programme Research and Development, Project Title: Development of an optimum technology for the analysis of limit states of structural elements in contact, ITMS and by the grant project Slovak VEGA 1/1000/12 References [1] W. Gongyao, F. Qingming, Y. Bing, J. Wenhui, Thermographic studies of temperature evolutions in bulk metallic glasses An overview, Intermetallics 30 (2012) [2] E. Corona, J.A. Shaw, M.A. Iadicola, Buckling of steel bars with Lüders bands, Int. J. Solids Struct., 39 (2002) [3] J. Zhang, Y. Jiang, Lüders bands propagation of 1045 steel under multiaxial stress state, Int. J. Plast., 21 (2005) [4] J. A. Shaw, S. Kyriakides, Initiation and propagation of lacalized deformation in elasto-plastic strip under uniaxial tension, Int. J. Plast., 13 (1998) [5] T. Pottier, F. Toussaint, H. Louche, P. Vacher, Inelastic heat fraction estimation from two successive mechanical and thermal analyses and full-field measurements, Mec. Theor. Appl. (A/Solids), 38 (2013) [6] A. Rusinek, J.R. Klepaczko, Experiments on heat generated during plastic deformation and stored energy for TRIP steels, Mater. Des., 30 (2009) [7] S. Dumoulin, H. Louche, O.S. Hopperstad, T. Borvik, Heat sources, energy storage and dissipation in high-strength steels: Experiments and modeling, Mec. Theor. Appl. (A/Solids) 29 (2010), [8] M. Zmindak, P. Novak P., Special approach for thermal modelling fibre reinforced composites with larger aspect ratio, Communications (Zilina, Slovakia), 14 (2010)

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