Abstract: The strain field around a mode Ⅰ crack tip with plane stress deformation forFe-0.5%Si material specimen was measured by SEM,micro grid method and digital imageprocessing. Finite element method(FEM) for elastic-plastic deformation was used to cal-culate the experimental object. Comparison of experimental and FEM results showed thatFEM based on the updated Lagrangian formulation was in good accordance with experi-mental measurement.But some factors,e.g.,plastic dilatancy,should be included as cracktip was nea...