Controlling Environmental Fatigue in Aerospace Al Alloys by Multiscale Crack Tip Measurements and Modeling

摘要

The objective of this research is to quantitatively establish the governing crack tip mechanics conditions and damage mechanisms pertinent to environmental crack propagation. The central goals are: (1) to develop accurate predictions of crack tip stresses and plastic strains for incorporation into micromechanical descriptions of crack growth, (2) to validate crack tip mechanics models by high resolution experiments, and (3) to resolve physical characteristics of fatigue crack tip hydrogen damage. Environment has a dominant and deleterious effect on fatigue crack propagation in airframe and engine components. Despite this known fact, a fundamental understanding of environment-enhanced fatigue remains elusive. We have recently examined in detail the characteristics of plasticity and crack path during environmental fatigue cracking in airframe aluminum alloys using state-of-the-art diffraction-based tools, nanoindentation and continuum mechanics modeling. For aerospace aluminum alloys, environmental fatigue crack advance most likely involves the interaction of environment-produced H, crack tip plastic strain accumulation, and local normal stress. In particular, we focus explored the possibility that strain gradients elevate crack tip stresses to a level where significant H accumulation would be expected. The interaction of this H with the level of accumulated plasticity, and the crystallographic characteristics of the resulting damage were probed experimentally. The understanding developed within this program will advance the management of fatigue durability of airframe components; by providing guidance to future alloy development, chemical-environment control schemes, and fracture mechanics-based performance prognosis.