Date of Award

5-2007

Degree Type

Dissertation

Degree Name

Doctor of Philosophy

Major

Materials Science and Engineering

Major Professor

Peter K. Liaw

Committee Members

Hahn Choo, YanFei Gao, John D. Landes

Abstract

During the load-controlled high-cycle fatigue test, when the overload was applied, it is shown that from the crack-growth rate (da/dN) versus stress-intensity-factor range (K) curve, the crack-growth rate decreased, following the overload, which indicated the crack-closure phenomenon. The crack-growth-retardation period was observed after the overload. The goal of this study is to investigate the deformation evolution during tensile loading and unloading cycles using neutron diffraction.

Neutron diffraction is used to investigate the crack-closure phenomenon by measuring the changes in the elastic-lattice-strain profiles around the fatigue-crack tip in a compact-tension (CT) specimen during tensile loading and unloading cycles. Spatially-resolved-strain measurements were performed to determine the in-plane and through-thickness lattice-strain profiles ahead of the crack tip under a constant tensile load. The strain scanning was repeated under various applied loads ranging from 667 to 6,667 N. Subsequently, an overload at 8,889N was applied. The strain scans repeated. After the overload, large compressive strain fields were observed close to the crack tip, indicative of the crack-closure phenomena.

Residual strain/stress mapping using neutron diffraction was also designed to investigate the mechanism of the retardation phenomenon by mapping the changes in the lattice-strain profiles around the fatigue-crack tip in a series of compact-tension (CT) specimens, which were fatigued to various stages through the retardation period after the overload. Following the overload, compressive-strain fields were observed along the loading direction close to the crack tip. As the crack grows out of the retardation period, the residual compressive strains decreased. The results provide a microscopic understanding of the overload effect during cyclic loading.

The plastic deformation ahead of a fatigue-crack tip was measured from the diffraction-peak-width changes. The dislocation density was estimated from the fullwidth- half maximum (FWHM) of the diffraction peaks. High dislocation densities around the crack tip were observed after the overload. Furthermore, the plastic-zone size in front of the crack tip was estimated from the diffraction-peak broadening, which showed a good agreement with the calculated result. The plasticity-induced crack-closure phenomenon after an overload was observed.

The measured elastic strains and dislocation densities will be compared to the finite-element simulations that are based on an irreversible, hysteretic-cohesive interface model. The experimental and numerical results will be compared and discussed.

The deformation in the vicinity of the crack tip was not only studied with the neutron diffraction, but also the x-ray microbeam diffraction. The results help understand the overload effect, which induced a large plastic deformation causing dislocations inhomogeneously arranged around the crack tip. From neutron-diffraction measurements, the anisotropic line broadening was observed in front of the crack tip. Furthermore, Laue patterns, obtained from the microbeam diffraction at different locations near the crack, provide a better spatial resolution and show alternating regions with high and low dislocation densities. Overall, the dislocation density was found to decrease with the distance from the crack tip.

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