Plasticity, localization, and damage in ferritic-pearlitic steel studied by nanoscale digital image correlation

Crystallographic slip activity fields identified automatically from DIC data, even for intersecting, diffuse and cross slip

“…The 4 single slip displacement fields are then superposed to form a combined displacement field. To simulate experimental conditions, Gaussian noise with a standard deviation of 0.05 pixel is added (which is relatively large considering that the noise in the DIC displacement data is often as low as 0.01 pixel) and subsequent Gaussian filtering is applied to emulate SEM-DIC conditions under which we recently showed to achieve high spatial strain resolutions [7,21]. See Table 1 for details on the generation of this virtual experiment.…”

A Nanomechanical Testing Framework Yielding Front&Rear-Sided, High-Resolution, Microstructure-Correlated SEM-DIC Strain Fields

“…Particularly, the development of Digital Image Correlation (DIC) patterning methods for in-situ Scanning Electron Microscopy based DIC (SEM-DIC) has lead to the capability of observing plastic deformation mechanisms from micrometer to nanometer scales [13][14][15][16][17][18]. In practice, SEM-DIC testing of these metals is commonly performed on bulk samples that are (mechanically) polished, of which the microstructure is characterized (with, e.g., Electron Backscatter Diffraction (EBSD)), that are subsequently decorated with a DIC speckle pattern, and are finally tested by means of in-situ SEM-DIC, resulting in plastic strain fields over a certain millimeter or micrometer sized area in the microstructure [10,11,15,[19][20][21][22][23][24][25][26]. Such approaches have provided valuable insights into plasticity mechanisms in the microstructure, strain partitioning between phases [19,20], how plasticity leads to damage [15,23,25], and on qualitative deformation patterns at multiphase interfaces [11,24].…”

“…In practice, SEM-DIC testing of these metals is commonly performed on bulk samples that are (mechanically) polished, of which the microstructure is characterized (with, e.g., Electron Backscatter Diffraction (EBSD)), that are subsequently decorated with a DIC speckle pattern, and are finally tested by means of in-situ SEM-DIC, resulting in plastic strain fields over a certain millimeter or micrometer sized area in the microstructure [10,11,15,[19][20][21][22][23][24][25][26]. Such approaches have provided valuable insights into plasticity mechanisms in the microstructure, strain partitioning between phases [19,20], how plasticity leads to damage [15,23,25], and on qualitative deformation patterns at multiphase interfaces [11,24]. However, these experiments have severe limitations, as information on the subsurface 3D microstructure and on the true local stresses and boundary conditions are often unknown.…”

“…Aspects that complicate such experiments are (i) the inherent complexity and small scale of the microstructure, especially when complex phases such as martensite and bainite are involved, leading to a highly complex, intangible combination of deformation mechanisms near and at the interfaces and (ii) the inherent nanoscale dimensions of these microstructures and their mechanisms (e.g. individual slip traces and substructure sliding [27]), requiring nanoscale spatial strain resolution and high-resolution microstructure to strain alignments [25].…”

“…In this work, we present a new nanomechanical testing framework that addresses all these challenges as this framework allows to combine, for the first time, such a large number of state-of-the-art micro-mechanical testing and microscopic characterization tools to enable highly detailed investigation of nanoscale deformation mechanisms. These tools include (i) nano-tensile testing of "1D" ( 3.5 * 2.5 * 10 µm) specimens [33], isolated from the bulk microstructure at specific regions of interest, (ii) multi-modal microstructure measurements at the front and rear of the micro-specimens, (iii) front&rear-sided SEM-DIC yielding very high-resolution strain maps by (iv) application of a recently proposed nanoscale DIC speckle patterning technique [16,25], (v) SEM scanning artefact correction [39] and (vi) DIC deconvolution correction [40], while (vii) all of this front&rear-sided data is aligned using a novel data alignment framework. We will demonstrate that the combination of all these measurement modalities will yield front&rear-sided nanoscale and microstructure-resolved strain fields that push the limits of SEM-DIC.…”