Accession Number : AD0638904

Title :   OBSERVATIONS ON MICRO-MECHANISMS OF FATIGUE CRACK PROPAGATION IN 2024 ALUMINUM.

Descriptive Note : Memo. rept.

Corporate Author : NAVAL RESEARCH LAB WASHINGTON D C

Personal Author(s) : Meyn,D. A.

Report Date : JUN 1966

Pagination or Media Count : 42

Abstract : The propagation of fatigue cracks in 2024-T3 aluminum in air was observed to occur by two different mechanisms depending upon the crack tip stress amplitude. Propagation at low amplitudes developed straitions of extreme flatness usually separated by fissures. A study of matched areas from mating fracture surfaces in the electron microscope showed that the flat and fissured sections of each striation on one fracture surface mated respectively with the flat and fissured sections on the other fracture surface. It was concluded that low amplitudes encouraged propagation by an environmentally aided mechanism, as suggested by P. J. E. Forsyth, and his basic model was adopted. Propagation at high amplitudes produced striations showing evidence of extensive local plastic deformation: isolated tear dimples were common, slip lines were well developed on the striations, and the striations were of a ridge-and-valley contour. Study of matched areas of mating fracture surfaces indicated that ridges and valleys matched ridges and valleys, respectively. A model such as proposed by McEvily, Boettner and Johnston for plastics, involving ductile tearing and compressive resharpening at the crack tip, was adopted to explain the observations made. (Author)

Descriptors :   (*ALUMINUM ALLOYS, FRACTURE(MECHANICS)), (*FRACTOGRAPHY, ALUMINUM ALLOYS), FATIGUE(MECHANICS), MICROSTRUCTURE

Subject Categories : Properties of Metals and Alloys

Distribution Statement : APPROVED FOR PUBLIC RELEASE