Accession Number : AD0486769

Title :   MECHANISMS OF FATIGUE IN 7075-T6 ALUMINUM.

Descriptive Note : Technical rept. 1 Apr 65-31 Mar 66,

Corporate Author : MIDWEST RESEARCH INST KANSAS CITY MO

Personal Author(s) : Grosskreutz, J. C. ; Shaw, G. G.

Report Date : MAY 1966

Pagination or Media Count : 28

Abstract : The mechanisms were studied by means of optical and electron microscopy. Cracks were observed to nucleate at inclusions on the notch surface as a result of highly localized plastic deformation. At least 65% of total life is expended before a crack greater than about 20 microns long can be detected. The final 20% of life is spent in propagating a dominant crack across the gauge section of the sample. The fracture surfaces so produced were studied by electron microscope fractography. The dominant mechanism of crack propagation is striation formation. Striation spacings and surface crack lengths were used to calculate crack growth rates. No consistent power-law relationship between this rate and the crack tip stress intensity factor could be found. Dislocation densities of about 2 x 10 to the 10th power lines/sq cm were observed near the crack tip by transmission electron microscopy. Comparison of the second-phase precipitate structure at the crack tip with the structure of uncycled material reveals evidence of overaging in the fatigued material.

Descriptors :   (*ALUMINUM ALLOYS, FATIGUE(MECHANICS)), FRACTOGRAPHY, CRYSTAL DEFECTS, FRACTURE(MECHANICS), NUCLEATION, MICROSTRUCTURE.

Subject Categories : Properties of Metals and Alloys

Distribution Statement : APPROVED FOR PUBLIC RELEASE