Accession Number:

ADA210567

Title:

Fatigue Crack Initiation Mechanics of Metal Aircraft Structures

Descriptive Note:

Final rept. 1 Aug 1986-10 Oct 1987

Corporate Author:

ROCKWELL INTERNATIONAL CORP THOUSAND OAKS CA SCIENCE CENTER

Personal Author(s):

Report Date:

1988-08-01

Pagination or Media Count:

93.0

Abstract:

The purpose of this research was to advance the understanding of deformation process that control the local stresses and strains at the grain size level in order to formulate a physically based model of crack initiation under spectrum loads. Experimental techniques of high spatial resolution strain measurements developed at the Science Center are used to investigate the mechanical behavior of individual constrained grains in Al 2219-T851. Local mechanical properties and yield criteria are deduced from measurements of the local-stressstrain hysteresis loop width. Two stages of flow are identified, each involving different yield locus behavior. Mathematical models of the local grain stiffness and the constrained plastic strain range are presented, and a phenomenological description of slip band nucleation and hardening are stated as functions of the local plastic strain range. These are then incorporated into a crack growth equation modified for plastic deformation to calculate the cycles required for a microcrack to reach the first subsurface grain boundary when propagating in a region of localized plastic flow. This provides a convenient engineering definition of crack initiation because it avoids the anomalous accelerated crack growth often found within the grain of initiation.

Subject Categories:

  • Properties of Metals and Alloys

Distribution Statement:

APPROVED FOR PUBLIC RELEASE