Ultrasonic nondestructive evaluation of microstructures and defects of metals at different length scales

The stress perturbations generated by a propagating ultrasonic wave interact with microstructures and internal defects of different length scales e.g. grains, cracks, pores, and dislocation structures. The interaction of the wave with microstructure/defects causes changes to ultrasonic parameters such as amplitude, velocity, and frequency contents of the propagating wave, which can be measured with different ultrasound measurement techniques. Thus, ultrasound has been extensively studied as a nondestructive evaluation (NDE) method to evaluate the change of microstructure/defects. Among many types of ultrasonic measurements, we particularly focus on two: phase velocity and acoustic nonlinearity parameter (β) measurements. Phase velocity of the propagating wave depends on the linear interaction between the propagating wave and microscale features such as average grain orientation (texture), porosity, and pore aspect ratio. Thus, phase velocity can be used to evaluate the effective stiffness tensor of the material and its change associated with these microstructural features. The second parameter, β, depends on the nonlinear stress-strain response of nano- and micro-scale features, e.g. dislocations, precipitates, and microcracks. β has been proposed as a metric to evaluate the damage state of a material, since nonlinearity gets stronger as the dislocation structures evolve with accumulated damage. The first goal of my research was to characterize the dependence of phase velocity on microstructural features such as texture, porosity, and pore aspect ratio of additively manufactured (AM) metals. In prior work, these microstructural features were only studied individually, however it is important to consider them together as they both change as a result of different printing parameters. In this work, the measured phase velocity in additively manufactured stainless steel (SS) 316L was compared with electron backscattered diffraction iii (EBSD) to characterize texture and with X-ray computed tomography (XCT) and optical microscopy to characterize pores. The measured phase velocity is dominated by texture, pores, or both texture and pores depending on the printing parameter, which further confirms the combined effect of these parameters on phase velocity and gives more insights about ultrasonic wave interactions in AM components. The second goal is to study the dependence of β on fundamental characteristics of cyclic deformation of SS316L instead of dislocation parameters as in prior work. We relate β to material hardening and slip irreversibility, which are macroscopic phenomena that represent the average dislocation activities. A new in situ ultrasonic measurement setup was developed and β was measured in situ w.r.t. the load frame after the tension and the compression portion of fatigue cycles. We observe that the change of β was compared with material hardening and the difference β between the tension and the compression cycles were related to the Coffin-Manson law. This phenomenological approach is useful to understand how β relates to the bulk material response as well as corresponding dislocation structure changes.

Name thesis:degree_name

Ph.D.

Level thesis:degree_level

Dissertation

Discipline thesis:degree_discipline

Mechanical Engineering

Grantor

University of Illinois at Urbana-Champaign

Year dc:date

2022

Author dc:creator

• Kim, Changgong

Contributors dc:contributor

• Matlack, Kathryn H
• Popovics, John S
• Johnson, Harley T
• Admal, Nikhil C
• Krogstad, Jessica A

• Ultrasound
• Fatigue
• Additive manufacturing
• Fatigue life
• Wave speed
• Nonlinear ultrasound

Statement dc:rights

• Copyright 2022 Changgong Kim

Language dc:language

en, eng

Handle dc:identifier

https://hdl.handle.net/2142/115691

OAI record oai:identifier

https://www.ideals.illinois.edu/oai-pmh?verb=GetRecord&metadataPrefix=etdms&identifier=oai%3Awww.ideals.illinois.edu%3A2142%2F115691