Full-field measurement of residual strains in cold bent steel plates

With the recent American Association of State Highway and Transportation Officials bridge design code acceptance of cold bent fracture-critical and nonfracture-critical plates and the increasing use of cold bending by the bridge industry, there is a need to better understand and predict residual strains in cold bent steel. However, measuring residual strains is a challenging task since the bending process typically interferes with conventional instrumentation. This paper introduces three-dimensional Digital Image Correlation (DIC) as a new, non-destructive approach to measuring residual strains in cold bent steel that captures full-field strain data with high accuracy. DIC was used to measure residual strains in 18 cold bent 12.7 mm thick ASTM A36 steel plates, bent using a press-brake to a constant radius of 102 mm with varying angles (10, 20, and 30 degrees) and plate widths (76.2 mm and 203 mm). The measured residual strains are compared to analytical predictions and both two- and three-dimensional finite element models. A parametric study, using the validated finite element models, is completed for additional widths (12.7 mm and 140 mm) and thicknesses (6.35 mm and 19.1 mm) to investigate the effect of these parameters on circumferential strains, contact pressure, strain state, and to make modeling recommendations. Results are used to develop a means of predicting peak residual strains based on the final plate dimensions - a useful quality control measure.