Chapter 16 Internal Heat Generation in Tension Tests of AISI 316 Using Full-Field Temperature and Strain Measurements Jarrod L. Smith, Veli-Tapani Kuokkala, Jeremy D. Seidt, and Amos Gilat Abstract Full-field temperature and strain measurements were recorded during tension tests of AISI 316 on a hydraulic load frame at a strain rate of 1 s 1. The temperature increase was measured on one side of the specimen using a high speed IR camera while the deformation was measured on the opposite side with a visible camera, each at a frame rate of 500 FPS. Uniform deformation of the specimen was observed up to strains of 0.25 until necking occurred and localization strains reached up to 0.75 at failure. The maximum temperature as measured by the IR camera was 260 ıC before failure. The fraction of plastic work converted to heat (ß) was calculated over the entire gage length of the specimen using the local measurements of stress, strain, and temperature and varied between 0.6 and 0.9 throughout the test. Keywords Infrared Thermography • Digital Image Correlation • Plastic Work • Thermomechanical • Stored Energy 16.1 Introduction Inelastic deformation behavior of all metals is known to be affected by strain rate and temperature. The effects of strain rate and temperature are coupled because a significant portion of the energy required for deformation is also converted to heat during the process which raises the temperature of the material. At low strain rates, plastic deformation takes place in virtually isothermal conditions because as the heat is generated in the material, there is sufficient time for the heat to dissipate into the surroundings. At higher strain rates the temperature in the material reaches higher temperatures because the event occurs in period of time that is too short for the heat to transfer from the material. To observe this behavior, material specimens are normally tested in simple tension tests utilizing a load frame for quasi-static tests and Split Hopkinson Bar for higher strain rate tests. The force on the specimen can be measured by utilizing a load cell for quasi-static tests and by analyzing the elastic waves in the Split Hopkinson Bar. The strains on the surface of the specimen can be determined utilizing Digital Image Coorelation (DIC). DIC is implemented by painting the specimen surface with a black and white speckle pattern and capturing the deformation of the surface with high speed cameras. The images are then processed with software that determines the displacements of the speckles during deformation and then calculates the strains based off of the displacements. Due to recent advancements in Infrared Thermography cameras it is also possible to measure the temperature on the surface of the specimen at high speeds. Simultaneous full-field measurement of the strain and temperature has been completed by many researchers at low strain rates. For example, Saai et al. [1] studied the thermomechanical behavior of Al bi-crystals with tensile tests at the strain rate of ca. 10 2 s 1 making use of simultaneous DIC and IR measurements. In their experiments, the infrared and visible cameras were observing the same specimen area, which was first coated with black paint and then sprayed with white paint to form a random pattern for DIC. They estimated that the emissivity of the paint(s) was 0.96, i.e., very close to that of a black body. The IR images were recorded during the tensile tests at 20 frames per second (fps) with a resolution of 320 240 pixels. Oliferuk et al. [2] used infrared thermography and visible imaging to determine the energy storage rate in the area of strain localization in an austenitic stainless steel similar to AISI 304 L. In their tests, the mean value of strain rate was 6.6 10 1 s 1 and the imaging frame rate of both visible and IR cameras was 538 fps. For local strain determinations, graphite dot markers were painted on one surface of the specimen, while the opposite surface on the IR camera side was covered with soot with estimated emissivity of 0.95. J.L. Smith ( ) • J.D. Seidt • A. Gilat Department of Mechanical Engineering, The Ohio State University, Scott Laboratory, 201 W 19th Ave, Columbus, OH, 43210, USA e-mail: smith.6575@osu.edu V.-T. Kuokkala Department of Materials Science, Tampere University of Technology, P.O.B 589, 33101, Tampere, Finland © The Society for Experimental Mechanics, Inc. 2018 L. Lamberti et al. (eds.), Advancement of Optical Methods in Experimental Mechanics, Volume 3, Conference Proceedings of the Society for Experimental Mechanics Series, DOI 10.1007/978-3-319-63028-1_16 97
RkJQdWJsaXNoZXIy MTMzNzEzMQ==