ABOUT EQUIVALENT ELLIPSE CONCEPT APPLIED FOR ENDURANCE LIMIT DETERMINATION OF POROUS MATERIALS

摘要

The paper is a continuation of the analysis presented at the PM94 in Paris. The endurance limit of elasto-plastic porous materials containing pores of different shape and size was theoretically determined. Using analysis of the range of plastic zones formed near the inclusion and their early cracking and based on the threshold value of K below which crack propagation stops, the stable stress level was determined. In the present paper equivalent ellipse concept was used, and a stress distribution near the real pore was replaced by stress field in the tip of certain ellipse. Analysis of pore shape in tested material reveals that about 80% of the pores can be replaced by the ellipse with axis ratio a/b between 0.5 and 2. Shapes with a/b ratio less than 0.5 are not active in the fracture process and those with a/b more than 2 are producing smaller values of stress intensity factor. Surprisingly it was found that circular shape of the pores is most dangerous which explains why so many earlier investigations using circular pore shape model are so well supported by the experiment.