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This paper regards the determination of the SIF in plate composed of twomaterials with crack starting from free edge and growing in direction perpendicular tothe interface. The RGB photoelastic method was used for the following capabilities: fullfield investigation, rapidity, appreciable precision. The result shows that the influenceof the interface is limited within a little area around interface then a greater part ofcrack path is good fitted by the homogeneous Irwin-Westergard formulation. Thegeneral form of the relation between SIF and a-dimensional crack length presents amaximum value of KI for a/h1 dependent to the geometric h1/h2 ratio. The next goal is tofind an analytical relation that describe correctly the stress field at crack tip in a zonenear the interface.