Bulk wave nonlinear acoustics evaluation of crept Inconel 617

Open Access
Author:
Yao, Xiaochu
Graduate Program:
Engineering Science and Mechanics
Degree:
Master of Science
Document Type:
Master Thesis
Date of Defense:
November 10, 2014
Committee Members:
  • Clifford Jesse Lissenden Iii, Thesis Advisor
Keywords:
  • nonlinear acoustics
  • creep
  • Inconel 617
  • bulk wave
  • microstructure evaluation
Abstract:
The urgent demand for nondestructive evaluation (NDE) and structural health monitoring (SHM) systems for Next Generation Nuclear Power Plants accelerates the development of nonlinear acoustics for material characterization. Taking advantage of its high sensitivity to microstructure changes, the nonlinear acoustics method utilizes higher harmonic generation due to nonlinearity in structures to characterize microstructure damage prior to the development of macroscale damage. The very high temperature reactor (VHTR) is designed to survive up to 1000oC and last for 60 years. Inconel 617 is a primary candidate material for the VHTR. The aim of this thesis was to use nonlinear acoustics approaches to evaluate microstructure changes in crept Inconel 617 specimens and compare them with microscopy results. Control mechanical creep tests were first conducted on seven Inconel 617 specimens. Then these seven specimens, as well as an additional uncrept specimen, went through experimental nonlinear acoustic bulk wave second harmonic measurements with the wave propagation direction along the loading axis. Furthermore, scanning electron microscopy and transmission electron microscopy were both used to characterize the microstructure degradation, such as microvoids, dislocations, grain refinements, and precipitation in the specimen. Last but not least, correlation between nonlinear acoustic response and microstructure degradation was analyzed and discussed. Experiment results show that the nonlinear parameter first increases and then decreases as creep strain to rupture life percentage grows, which correlates with microscopy observations of the precipitate distribution. Therefore, nonlinear acoustic measurements are more sensitive to precipitation in crept Inconel 617 than other microstructure evolution, such as microvoids and dislocation density.