Accession Number : AD0810239

Title :   EFFECT OF STRESS RATIO ON FATIGUE CRACK GROWTH AND MODE OF FRACTURE IN 2024-T4 AND 7075-T6 ALUMINUM ALLOYS IN THE LOW-CYCLE RANGE.

Descriptive Note : Summary rept. Jan 63-Jan 65,

Corporate Author : AIR FORCE MATERIALS LAB WRIGHT-PATTERSON AFB OHIO

Personal Author(s) : Wang,D. Y.

Report Date : DEC 1966

Pagination or Media Count : 25

Abstract : Investigation was conducted at minimum-to-maximum stress ratios R of -2.5, -2.0, -1.5, -1.0, 0.0 and 0.6. It was observed that the fracture mechanism of low-cycle fatigue involves Stage II fatigue crack growth which is characterized by the development of ductile striations and two essential types of secondary fracture surfaces. It is shown that ductile fatigue crack growth, which involves two types of ductile striations, is governed by the local irreversible plastic deformation at the crack tip in both alloys. The most important mode of secondary crack growth in 2024-T4 aluminum is the brittle fracture or decohension of the constituent particles ahead of the crack tip and the subsequent growth of the interior cracks toward the main crack. The size of the plastic zone at the crack tip appears to determine the degree of influence of this mode of fracture on overall crack growth rate. In 7075-T6, the dominant mode of secondary crack growth is the quasi-cleavage fracture of the matrix. The frequency of occurrence of these two modes of secondary crack growth increases with the stress ratio, i.e., with increasing mean stress. The change in the modes of crack growth as we change from low to high mean stress contributes to the relative differences in fatigue strength of the two materials at various stress ratios.

Descriptors :   (*ALUMINUM ALLOYS, *CRACK PROPAGATION), FATIGUE(MECHANICS), FRACTOGRAPHY, FRACTURE(MECHANICS), STRESSES

Subject Categories : Metallurgy and Metallography
      Mechanics

Distribution Statement : APPROVED FOR PUBLIC RELEASE