With advances in interpolation and noise filtering, a pure strain calculation can be derived using the fundamental definition of the strain tensor and a shell model with only a few assumptions. Deformation data acquired on the inner surface of the pipe is used to calculate the positional knots that define the inner shell surface from which the displacement vectors of the median plane can be calculated. Using a shell model around the median plane, the strains can be calculated on the inner and outer surfaces based purely on the displacement of the knot positions. To validate this procedure, an FEM is built using a nonlinear stress-strain response for the steel to model its elasto-plastic behavior with associated kinematic hardening behavior. The model uses a second order shell element with plasticity, large deformation, and large strain capabilities.

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