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Degree

Doctor of Philosophy, Ohio State University, Welding Engineering, 2004.

Abstract

A state-of-the art microradiographic system including an imaging chain and a micro-focus X-ray source has been used for development of a quantitative nondestructive evaluation (NDE) tool in characterizing material microdamages. In contrast to conventional radiographic systems, where the performance is well understood, there is limited scientific data available for the evaluation of microradiographic system performance in terms of the effect of projection magnification and the geometric unsharpness on system detectability. Evaluating the detectability of a microradiographic system is a further challenge in this research. Quantitative methods are developed to characterize the detectability of a microradiographic system. The concept of optimum magnification and the effect of unsharpness on detectability of the system are studied. The enlarged and high resolution images are used to develop a microradiographic method for microdamage depth measurement. The method was applied to quantify micro-damage due to pitting corrosion in Al 2024-T3 alloy. The accuracy of the developed technique was then validated using other methods such as comparison with optical microscopy data, comparison with the 3-D images taken by profilometry, direct reading of the thickness lost by cutting the actual corroded samples, radiography of samples with two perpendicular shots, and also quantitative pixel-by-pixel comparison was done between data from profilometry and those from the microradiographic method. Hidden microdamage in materials is a challenging area for NDE; in order to characterize this sort of damage, the effect of contrast change on detectability was studied and compared with the theoretical predictions. Also a microradiographic technique has been developed to study initiation, growth and locations of fine primary and secondary cracks in dwell fatigue testing on titanium samples. Microradiographic data then has been compared with in-situ ultrasonic signals for further evaluation. This research also involves the study of the phase-contrast imaging applicability for microradiographic characterization of damage in structural materials. It has been shown that by optimization of the exposure parameters, the X-ray beam from a micro-focus system can be used as a phase contrast imaging technique in engineering settings. For the first time the phase-contrast imaging is done with a micro-focus system on an aluminum sample with a pit. Optimization of the radiographic parameters to receive the enhanced phase-contrast images of microdamages has been done quantitatively on an epoxy sample with pores and an aluminum specimen with an artificial pit. The experimental data from the obtained images has been compared with the theoretical results based on the refraction model. This research shows some successful efforts in how quantitative microradiography has been applied as an in-situ NDE technique for different applications like: studying the localized microdamage growth in aluminum in corrosive solutions, the study of real-time microradiography of keyhole formation in laser beam welding combined with a high-speed camera and porosity formation in welding of titanium.

Keywords

Microradiography; micro-focus; X-ray imaging; corrosion damage; Phase-contrast imaging

Advisor

Stanislav I Rokhlin

Pages

274p.

Document number: osu1092867821
Permalink: http://rave.ohiolink.edu/etdc/view?acc_num=osu1092867821

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