Séminaire de Christopher M.Kube et de Andrea P. Arguelles

Advances in Ultrasonics Research at Penn State: From Additive Manufacturing Process Monitoring to Wave Propagation in Polycrystals

This presentation will introduce the Penn State Ultrasonics Laboratory and provide an overview of ongoing research activities. The laboratory, housed within the Department of Engineering Science and Mechanics, focuses on the development and application of acoustic and ultrasonic methods for nondestructive materials characterization. Faculty and students conduct fundamental and applied research spanning acoustic wave propagation, advanced signal processing, and novel sensing technologies, with applications ranging from advanced manufacturing to structural materials evaluation. Two representative research thrusts will be highlighted.
The first topic presents an alternative stress-based formulation for modeling ultrasonic waves in heterogeneous elastic materials. Traditionally, such waves are modeled using displacement fields through Cauchy's elastodynamics and Hooke's law, creating equations with spatial gradients on the stiffness tensor. The stress-based approach instead models elastic waves as propagating and scattering stress fields, yielding equations with spatial gradients on density rather than stiffness. For single-phase polycrystals with uniform density, this produces a standard tensorial wave equation without density heterogeneity complications. Numerical solutions using finite difference schemes on Dream3D-generated polycrystalline materials demonstrate the method's effectiveness in predicting grain scattering effects on wave attenuation and dispersion. Results are compared against analytical predictions from Weaver's model using Von Karman statistics. The stress-based approach offers significant advantages, particularly simplified Dirichlet-type boundary conditions compared to the complex Von Neumann conditions required in displacement-based simulations.
The second topic presents an acoustic technique for real-time detection, sizing, and localization of keyhole pores during laser powder bed fusion of AlSi10Mg. Three laser optical microphones employing Fabry-Perot interferometers detect the natural resonance modes of the vapor depression. A 1 MHz ultrasonic transducer mounted beneath the sample delivers broadband half-cycle pulses that actively excite the cavity and force incipient pores to announce themselves through amplified acoustic signatures. Measurements were conducted at APS 32-ID, where high-speed X-ray imaging synchronized with the acoustics provides direct visualization of the keyhole. This enables precise correlation between resonant frequency response and vapor depression dimensions, identifying acoustic signatures associated with keyhole pore pinch-off events. Coupled Flow3D and acoustic modeling supports experimental observations. Results demonstrate quantitative relationships between cavity geometry and acoustic response, revealing distinct frequency shifts as defects acoustically announce their presence during pore formation.