Accession Number:

ADA162663

Title:

Investigation into the Fatigue Crack Initiation Process in Metals.

Descriptive Note:

Final technical rept. 3 Aug 83-2 Sep 85,

Corporate Author:

ROCKWELL INTERNATIONAL THOUSAND OAKS CA SCIENCE CENTER

Report Date:

1985-12-01

Pagination or Media Count:

90.0

Abstract:

The foundation for a physically derived model of fatigue crack initiation in metals under spectrum loading is described. the work focuses on the microplastic deformation properties of a surface subject to stresses under the bulk elastic limit. A theoretical description of the highly localized plastic strains in individual surface grains, known to be pertinent to aluminum alloys, is developed with the support of high spacial resolution, high sensitivity strain measurements. These are made within individual grains of a Al 2219-T851 model material by a scanning electron microscopy technique. An important finding is that fatigue causes the local flow stresses in individual large surface grains to fall dramatically to less than 50 of their prefatigue values. the local stress-strain behavior is then controlled by the external load spectra, but is greatly influenced by the reaction stresses within a microplastic grain generated when its plasticity is constrained by the essentially elastic surface. Because of the constraint, the local stresses in a grain differ markedly from the applied. The theoretical models developed allow these stresses to be calculated and local mechanical properties of the surface including flow stress and plastic moduli to be estimated from experimental data. An especially interesting anti-clockwise motion of the external stress-local strain hysteresis loop near grain boundaries is observed, and is explained by a simple dual-domain deformation model. It results from an inhomogeneity in mechanical properties with a higher flow stress within the grain near the boundary. Opportunities for the use of such deformation theories to predict crack initiation lifetime are discussed.

Subject Categories:

  • Mechanics

Distribution Statement:

APPROVED FOR PUBLIC RELEASE