AN EXPERIMENTAL-STUDY ON THE OPENING AND CLOSURE BEHAVIOR OF FATIGUE SURFACE, CORNER AND THROUGH-THICKNESS CRACKS AT NOTCHES

The opening and closure behaviour of surface, corner and through-thick ness cracks in thin notched plates of FeE460 (K-t=2.5) and Al5086 (K-t =3.4) was experimentally studied. The cracks were initiated and examin ed under uniaxial fully reversed constant amplitude and two-step loadi ng. Crack opening load values were measured during crack growth in not ch sections with a nonuniform stress distribution using small strain g auges glued to the specimen surface, very close to the crack tip. The results represent a comprehensive set of experimental data for crack o pening load values in dependence on crack lengths a, c and load level including the influence of overloads and covering all types of cracks. The results indicate uniform relationships between crack opening load levels and all crack types. Crack opening and closure occur at nearly the same strain level, which depends on the applied load level. The c rack opening load values measured at large notched specimens differ fr om those measured at similar smaller specimens because of the differen t local stress gradients.