B-spline approximation methods for digital image reconstruction in strain measurement

A non-invasive digital imaging procedure has been successfully developed to quantify the strain history during electric resistance welded (ERW) pipe forming. A curve fitting method was used to reconstruct a continuous and smooth pipe profile from digital image data. Three B-spline approximation methods (one-dimensional, two-dimensional, and trigonometric B-spline) were evaluated using the same set of digital image data, and validated against the strain gauge measurements. Trigonometric B-spline provides superior curve fitting and strain prediction. Using the trigonometric B-spline approximation method, a 3D strain history contour plot was generated to optimize the mill setup and identify the strain reversal.