An corrective elastic analysis of semichemical corrugating medium (SCM) in single wall corrugated fiberboald (FC) under the shearing force W applied along the machine direction (MD) on the surface of kraft liner (KL）was studied by the corrective method of the beam bending stress analysis. And the difference between results of this method and the FEM calculation mactched by this model was investigated. Then folllowing resu]ts are obtained. Normal stress s_x and s_y of SCM in the machine and the thickness direction of FC distribute antisymmetrically for the joint, and the shear stress txy does symmetrically. Maximums of s_x, s_y and t_x_y are near distances L/32, Ｌ/16 and L/8 of MD from the joint on the inner surface, which L is the wavelength of SCM, and external values are on the outer surface at same positions and are negative sign for the inner surface. sx of the joint is small value +/- W/(2Ts), which W and Ts are the appled shear force and the SCM thickness, however s_x of the FEM method is zero. And s_y and t_x_y of the joint are zero.

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Size estimation of plastic deformation zone at the crack tip of paper under fracture toughness testing was attempted not only by means of thermographic observation but also by means of the calculation based on the linear elastic fracture mechanics (LEFM). Stress concentration at the notch tips occurs at the latter stage of the testing. As developing patterns of plastic deformation zone observed by thermography changes from type(i) to type(ii) arid further to type(iii), the calculated zone size decreases. The zone calculation is useful especially in the brittle paper as a method describing the behavior of plastic deformation zone during the testing.

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