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  • Evaluation of local plastic deformation by detecting heat generation in orthotropic material

    H. Sakamoto, J. Shi & M. Yamamoto Department of Mechanical Engineering and Materials Science, Kumamoto University, Japan

    Abstract

    Ensuring that the heat generation by plastic deformation corresponded to the increasing of its plastic strain energy, the behaviors of the heat distribution of SUS304 which had anisotropy under deformation and fracture was examined by using a thermal image camera with two dimensional array sensors. Furthermore FEM elasto-plastic analysis coupled with heat generation and transient heat conditions was performed. It was shown that the analytical results showed good agreement to the experimental ones, and we could evaluate the anisotropic influence of the material in the plastic deformation and the fracture process as a thermal image distribution macroscopically.

    1 Introduction

    The failure of mechanical members largely depends on the size and the development of plastic deformation from the stress concentration parts. Therefore, it is very important to make clear the local plastic behaviors under deformation. The authors paid attention to heat generation under plastic deformation and the thermography method was used for the detection of the surface temperature under deformation [1]-[4]. This device attracts attention as an effective technique for evaluating a physical phenomenon recently because thermal energy which was radiated from the object can be easily detected [5]-[7]. In this paper, the deformation and fracture behaviors of stainless steel SUS304, which had anisotropy, was measured as continuous thermal images. The specimen surface around crack tip was observed by CCD camera and the COD were measured simultaneously. Furthermore, FEM elasto-plastic deformation

    Damage and Fracture Mechanics VIII, C. A. Brebbia & A. Varvani-Farahani (Editors) © 2004 WIT Press, www.witpress.com, ISBN 1-85312-707-8

  • and fracture analysis and coupled with heat generation and transient heat conduction was performed.

    2 Experiments

    2.1 Thermal image detecting system

    Figure 1 shows the outline of the thermal image detecting system. The specification of infrared thermal video system (TVS-8200) is shown in Table 1. This device consists of infrared camera and image processing unit. The camera has two dimensional array infrared sensors (horizontal 320 x vertical 240) and the 256 colors or gradient thermal image can be continuously obtained every 1/60 second and recorded in the frame memory. These recorded thermal images were transferred to personal computer and image data processing was carried out These measurements of surface heat distribution were executed under light shield condition in order to avoid turbulence from the surrounding.

    Figure 1: Thermal image detection system (TVS8200).

    Table 1: Specification of TVS-8200.

    Range of measurement temperature -40~1200℃ Resolution of temperature 0.4% Number of scanning frames 60 frames/sec Display elements 76800pixels Detector InSb(H 160xV120 elemens) Detecting wave rang 4~4.6 μm

    TVS-8200

    TC TransmitterThermocouple

    Specimen

    Temperature (by Thermocouple)

    Temperature (by Thermography)

    Camera Processor

    Data Logger

    Shield

    Comparison

    VTR

    Damage and Fracture Mechanics VIII, C. A. Brebbia & A. Varvani-Farahani (Editors) © 2004 WIT Press, www.witpress.com, ISBN 1-85312-707-8

    186 Damage and Fracture Mechanics VIII

  • 2.2 Material and specimen

    The material used is stainless steel, SUS304 in JIS. The test specimens were cut out from rolling direction and transverse to rolling direction of the plate in order to examine the effect of anisotropy. The stress-strain relations and the mechanical properties obtained by tensile test at 25mm/min are shown in Fig.2 and table 2. From this figure, it is found that the yielding stress of the rolling direction’s specimen was higher than that of transverse to rolling direction, but work hardening rate of the former was smaller that that of the latter. The geometry and dimensions of the thermal evaluating specimen under deformation and fracture was shown in Fig.3. The center pre-crack was made by electrospark machine with 0.3mm diameter as stress concentration parts. After machining, the strain relief anneal was done in these specimen. The strain gage was pasted at the place shown in the detail A of Fig.3 as monitoring the plastic strain at crack front.

    Table 2: Mechanical properties.

    Young modulus (GPa)

    Yield strength (MPa)

    Roll dir. 191 247

    Trans. Dir. 191 219

    Figure 2: Stress-strain curves.

    Simultaneously, the heat distributions on the specimen’s surface were measured and recorded by the thermography. The thermal images obtained were converted into the distribution of temperature rise on specimen surface by a thermal image processing. The distribution of temperature rise per unit time was obtained as the differential thermal image of current one and referent one shown in Fig.5.

    0 0.02 0.04 0.06 0.08 0.1 0.12 0.14 0

    100

    200

    300

    400

    500

    Strain (ε)

    St re

    ss (M

    Pa )

    Roll dir. Trans. dir.

    T.Speed 25mm/min

    Damage and Fracture Mechanics VIII, C. A. Brebbia & A. Varvani-Farahani (Editors) © 2004 WIT Press, www.witpress.com, ISBN 1-85312-707-8

    Damage and Fracture Mechanics VIII 187

  • Figure 3: Geometry and dimensions of specimen.

    (a) Clip gage and CCD unit (b) Temperature measuring

    positions by thermocouple

    Figure 4: Appearance of the deformation measurement unit and the measuring points.

    C rack Tip

    4

    4 4

    4

    M easuring Point

    1ch

    2ch

    3ch

    9.0 4.5

    B

    Electrospark machined crack with wire of 0.3mm diameter

    detail A

    3.0

    Thermocouple

    Strain gage

    detail B

    A

    40 C

    li p

    ga ge

    le ng

    th

    45

    75

    15 0

    Damage and Fracture Mechanics VIII, C. A. Brebbia & A. Varvani-Farahani (Editors) © 2004 WIT Press, www.witpress.com, ISBN 1-85312-707-8

    188 Damage and Fracture Mechanics VIII

  • Figure 5: Thermal image processing.

    3 FE Simulation of the heat generation and conduction under the plastic deformation and fracture

    In order to examine the relation between the distribution of the temperature rise measured by TVS and the behavior of the plastic deformation and fracture, the elasto-plastic FE deformation analysis coupled with heat generation and transient heat conduction was performed by displacement control method. The thermoelastic effect was considered in elastic region. It was assumed that plastic strain energy was perfectly converted into heat. The Crank-Nicolson’s time integration method was adopted in heat conduction analysis. The plane stress condition was assumed and the strain hardening is expressed by Swift’s equation. In the fracture process, the nod’s constrain at crack tip was released according to crack propagating rate obtained by the experiment. Analytical constants in elasto-plasticity were calculated from the stress-strain relation in this Fig.2.

    4 Results and discussion

    Figures 6 and 7 show the temperature rise and the behavior of COD on the specimen’s surface measured by the thermocouples and the thermography at the points shown in Fig.4 (b), respectively. The arrow in Fig.6 shows crack propagation beginning point. The temperature rise curves of the thermocouples correspond well to ones of thermography. Paying attention to the temperature- rise at each position, the temperatures are going up in order of ch.1, ch.2, ch.4, and ch.3 and rise suddenly when the crack propagation starts. The broken line in these figures shows the simulation result of ch.1 by FE analysis.

    Damage and Fracture Mechanics VIII, C. A. Brebbia & A. Varvani-Farahani (Editors) © 2004 WIT Press, www.witpress.com, ISBN 1-85312-707-8

    Damage and Fracture Mechanics VIII 189

  • (a) Roll dir. (b) Trans. dir.

    Figure 6: Temperature rise on specimen’s surface measured by thermocouples.

    (a) Roll dir. (b) Trans. dir. Figure 7: Temperature rise on specimen’s surface measured by thermography.

    The behaviors of crack propagation observed by CCD camera and the change in the surface temperature rise according to the crack propagation are shown in Figure 8 and 9 in rolling direction and transverse to rolling direction, respectively. The indicated time in these figures shows the elapsed time from the crack propagation beginning.

    0 5 10 15 20 25 30 35 40 45 -2

    0

    2

    4

    6

    8

    10

    12

    14

    Time (s)

    Te m

    pe ra

    tu re

    ri se

    (℃ ) T.G 1chT.G 2ch

    T.G 3ch T.G 4ch

    ROLL T.Speed 25mm/min

    0 5 10 15 20 25 30 35 40 45 -2

    0

    2

    4

    6

    8

    10

    12

    14

    Time (s)

    Te m

    pe ra

    tu re

    ri se

    (