Technology Advancements and New Industrial Applications in Welding, 523-526, (1998) Development of a Measurement System for Combined Temperature and Strain Measurements During Welding H.A. Bruck, H.W. Schreier, M.A. Sutton, and Y.J. Chao Department of Mechanical Engineering University of South Carolina Columbia, SC 29208 Abstract Numerical analyses such as thermomechanical Finite Element Methods (FEM) are being used to model welding processes. The effect of material behavior and processing conditions on code predictions need to be directly verified before the codes can be used reliably. The parameters of greatest importance in welding are the temperature fields generated near the weld zone and the subsequent deformations induced by the changes in these fields and the presence of a material inhomogeneity, i.e. the weldment. A new technique called Video Imaging and Thermal Analysis (VITA) has been developed for measuring temperature and 3-D deformation fields during welding processes. VITA consists of a high-speed infrared thermal imaging camera and a stereoscopic video imaging system. The principles of VITA are presented along with verification of the technique’s accuracy using strain gage and thermocouple measurements during Gas Metal Arc Welding (GMAW) of a steel plate. Introduction Numerical models, such as thermomechanical Finite Element Methods (FEM), are being developed to understand welding processes [1,2]. In implementing these analyses, many assumptions are made concerning material properties and processing conditions, including decoupling the mechanical and thermal portion of the analysis [3]. Therefore, the predictions from these analyses must be verified experimentally before they can be reliably used to model welding processes. This paper describes a novel technique known as the Video Imaging and Thermal Analysis (VITA) that can be used for verifying welding models. The VITA technique uses high-speed thermal imaging and stereoscopic video imaging to measure temperature and 3-D deformation fields on the surface of specimens during the manufacturing process. VITA strain and temperature measurements were made during GMAW of thin steel plates for comparison with strain gage and thermocouple measurements in order to verify the accuracy of the VITA technique. Video Imaging and Thermal Analysis (VITA) The VITA technique has been developed to measure deformation and temperature fields during welding processes. This technique involves the use of a stereoscopic vision system for monitoring the 3-D deformations and a high-speed infrared (IR) thermal imaging system for temperature measurements. The stereoscopic imaging approach uses two video cameras to obtain 2-dimensional images of a specimen surface from two different perspectives [4]. By placing a random speckle pattern on the surface of the specimen, these images can then be compared mathematically using a correlation algorithm to determine the profile of the specimen surface, a process known as digital image correlation. Comparing images obtained before and after the surface has been deformed results in 3-D deformation measurements (Figure 1). Figure 1. 3-D deformation measurements using stereoscopic video imaging system The stereoscopic video imaging uses a grid calibration technique to establish parameters describing the video cameras and their relationship to space [6]. By extending this technique to include the IR camera, it is possible to accurately determine the surface temperature and location of any given point on a 3- D surface. This unique combination of video and IR imaging technology provides both 3-D deformation and temperature measurements simultaneously. Data can be obtained at the framing rate of the video cameras, thus dynamic information can be acquired during a thermomechanical event such as a welding process. In order to convert IR measurements to temperature, the specimen surface was calibrated using thermocouples. The calibration process consisted of placing the specimen on a hot plate that could uniformly reach temperatures above 500 o C. IR measurements were then made at various temperatures that were determined using the thermocouple, and a calibration curve was constructed (Figure 2). A function was fit to the data with the form, σ = A*(T-T o ) 4 +B [1] deformed cam 1 deformed cam 2 undeformed cam 2 undeformed cam 1