Brian Wisner

Acoustic Emission of Deformation Twinning in Magnesium

The Acoustic Emission of deformation twinning in Magnesium is investigated in this article. Single crystal testing with combined full field deformation measurements, as well as polycrystalline testing inside the scanning electron microscope with simultaneous monitoring of texture evolution and twin nucleation were compared to testing at the laboratory scale with respect to recordings of Acoustic Emission activity. Single crystal testing revealed the formation of layered twin boundaries in areas of strain localization which was accompanied by distinct changes in the acoustic data. Testing inside the microscope directly showed twin nucleation, proliferation and growth as well as associated crystallographic reorientations. A post processing approach of the Acoustic Emission activity revealed the existence of a class of signals that appears in a strain range in which twinning is profuse, as validated by the in situ and ex situ microscopy observations. Features extracted from such activity were cross-correlated both with the available mechanical and microscopy data, as well as with the Acoustic Emission activity recorded at the laboratory scale for similarly prepared specimens. The overall approach demonstrates that the method of Acoustic Emission could provide real time volumetric information related to the activation of deformation twinning in Magnesium alloys, in spite of the complexity of the propagation phenomena, the possible activation of several deformation modes and the challenges posed by the sensing approach itself when applied in this type of materials evaluation approach.

Data-driven Damage Model based on Nondestructive Evaluation

A computational damage model which is driven by material, mechanical behavior and nondestructive evaluation data is presented in this study. To collect material and mechanical behavior damage data, an aerospace grade precipitate-hardened aluminum alloy was mechanically loaded under monotonic conditions inside a Scanning Electron Microscope, while acoustic and optical methods were used to track the damage accumulation process. In addition, to obtain experimental information about damage accumulation at the laboratory scale, a set of cyclic loading experiments was completed using 3-point bending specimens made out of the same aluminum alloy and by employing the same nondestructive methods. The ensemble of recorded data for both cases was then used in a post-processing scheme based on outlier analysis to form damage progression curves which were subsequently used as custom damage laws in finite element simulations. Specifically, a plasticity model coupled with stiffness degradation triggered by the experimentally defined damage curves was used in custom subroutines. The results highlight the effect of the data-driven damage model on the simulated mechanical response of the geometries considered and provide an information workflow that is capable of coupling experiments with simulations that can be used for remaining useful life estimations.

Damage Precursor Indicator for Aluminum 7075-T6 Based on Nonlinear Dynamics

In this work, a damage precursor indicator for aluminum 7075-T6 is proposed based on the nonlinear dynamics of a cantilevered beam system. A shouldered beam specimen is used to move the region of highest stress away from the clamped end. The beam is subject to harmonic base excitation in a uniaxial shaker. Fatigue damage accumulates in the beam with damage forming most quickly in the region of highest stress. By monitoring the tip deflection, changes in the natural frequency and response to a given excitation are correlated to fatigue life. Despite the absence of large-scale cracks, detectable changes in the nonlinear dynamics are discovered. The nonlinear dynamic parameters are estimated capturing the change in the forward and backward nonlinear sine sweeps. These changes could lend themselves to being a trackable damage precursor. The presence of microstructural precursors are confirmed using nanoindentation.

Fatigue Damage Precursor Identification Using Nondestructive Evaluation Coupled with Electron Microscopy

Several fatigue failure modes originate at the microstructural level by the activation, interactions and development of what are referred to as “damage precursors” long before the formation of dominant cracks that grow as a function of loading and crystallographic parameters. In this context, this work presents new developments of an in-house developed experimental mechanics approach to evaluate aspects of microstructure evolution and identify validated damage precursors that are active during fatigue loading by combining Nondestructive Evaluation (NDE) methods with ex situ and in situ Scanning Electron Microscopy (SEM). The used NDE methods include real time Acoustic Emission (AE) monitoring from inside the SEM chamber and Digital Image Correlation (DIC) for strain evolution directly at the grain scale. The coupling between quasi in situ microscopy with actual in situ nondestructive evaluation falls into the ICME framework and the idea of quantitative data-driven and multiscale characterization of material behavior. To demonstrate this approach, Aluminum 2024-T3 specimens were tested using a SEM mechanical testing stage under low cycle fatigue to identify and validate the presence of damage precursors, while correlating their presence with specific parameters extracted by the available NDE data. The reported results show how load information could be correlated with both AE activity, DIC strain maps, and microscopic observations of precipitate fracture and microcracks.