Material properties are strongly influenced by their microstructure. Thus, the microstructure must be accurately quantified to predict material behavior. Although these properties are governed by their three-dimensional (3D) microstructure, most common characterization techniques rely on two-dimensional (2D) imaging. Serial sectioning methods can accurately capture 3D microstructural features, but they are time-consuming, expensive, and destructive. This work addresses 3D microstructure characterization using a power-law mapping derived from 2D images. To this end, a dataset of synthetic microstructures with equiaxed grains and six different grain size distributions is used. The extracted grain size distributions highlight the differences between 2D and 3D statistics. The study also quantifies the effect of the number of grains within a 2D image on the resulting statistics. The power-law mapping is also compared against the Saltykov stereological method. The mapping outperforms the Saltykov method for number-weighted statistics, where the Saltykov method exhibits a systematic underestimation of the mean of approximately 27

Abstract The goal of this study is to investigate the causes of skeletal fragility in Type 1 Diabetic (T1D) women. We hypothesize that bone fragility in diabetic individuals is partly due to changes in mineral and/or intrinsic material properties in the osteocyte lacunar/peri-lacunar regions of bone tissue. Studies of bone material properties in T1D are limited, and bone mineral density (BMD) alone does not explain the elevated fracture risk in T1D women (Cases). Innovative instruments with nanoscale resolution, including a laser scanning microscope (LSM), an atomic force microscope that is integrated with infrared spectroscopy (AFM-IR), and a nanoindenter were used for the characterization of the material properties surrounding osteocyte lacunae. In trabecular bone tissue, the compositional mineral matrix and Mineral Matrix Peak ratios (MMA, MMP for both near- & far-lacunae) along with material strength variables (Modulus, Hardness for both near- & far-lacunae) were lower in T1D compared to Controls. While material compositional ratio of mineral maturity crystallinity area (MMCA) was higher, the mineral maturity crystallinity peak (MMCP) and hardness tended to be higher in “lacunae-near” than “lacunae-far” for all the biopsies. Furthermore, the higher mineral matrix (MMA) and material strength (modulus, hardness) in peri-lacunar regions of Controls may suggest its increased brittleness or differences in properties as compared to T1D. We postulate that difference between Cases (T1D) and Controls in modulus and hardness could be due to the variation in mineral exchange (diffusion) rate within the peri lacunar space.

The manufacturing of billets of Ti6Al4V leads to a distribution of microtextured regions (MTRs) within the billet. The presence of MTRs is hypothesized by some to increase the risk of cold dwell fatigue in components forged from such billets. The position and orientation of a critical MTR within a component can increase the local stress state which may lead to fatigue crack initiation. An MTR orientation is characterized by its C-axis which defines the direction of the MTR which is mechanically the stiffest. Thus, it is imperative that critical MTRs within billets and forgings are detected and that their C-axis orientation is quantified. Ultrasonic inspections are currently used for Ti6Al4V billets during production, primarily to identify hard-alpha inclusions which also act as stress risers. These inspections have increased airline safety by ensuring that such inclusions are not present within billets prior to forging. Unfortunately, MTRs can result in false positives during these inspections, but little is known about the specific material state that leads to such indications. Here, we examine the ultrasonic scattering from suspected MTRs using a unique experiment to quantify the ultrasonic through-transmission polar scattering from an MTR. First, a Ti6Al4V billet was scanned using a cylindrically-focused transducer to identify potential MTRs. Possible MTRs were identified and 2″ cubes were cut from the billet with the goal to position specific MTRs near the center of each cube. The cubes were then scanned from all six faces using a spherically focused transducer to locate the primary MTR and any other interesting features. Scans of the six faces of the cubes provide partial information about the backscatter response for an MTR from different directions. These samples were used for additional through-transmission ultrasonic measurements that used a custom rotating fixture to reveal the strong dependency of the angular scattering from suspected MTR locations. Finally, the angular scattering profiles are compared with predictions of MTR scattering that are based on DREAM.3D models of textured microstructures. The results are expected to provide additional insight into the detection and characterization of MTRs, especially with respect to their C-axis orientation.

Current methods for measuring residual stress in additively manufactured components face limitations in sensitivity or are destructive. Resonant ultrasound spectroscopy (RUS) offers a promising approach for assessing residual stresses in hybrid additively manufactured components. Hybrid additive manufacturing (AM) involves using secondary energy sources or processes to create functionally graded components at specific locations. Each hybrid layer or step refines grain size, increases dislocation density, and consequently alters residual stresses. This study employs finite element models to simulate residual stress in hybrid AM components. Using machine learning (ML), we predict both stress levels and the location of hybrid layers. A total of 1000 simulations generated a sufficiently large dataset for ML models. Partial least squares regression (PLSR) predicts residual stress with a mean absolute error (MAE) of approximately 3 MPa, which is less than 1% of residual stresses found in hybrid AM layers. Simultaneously, the location of the hybrid layer exhibits degeneracies in the resonant modes, necessitating a more robust algorithm. The ML models are able to predict the location of a hybrid layer within 0.14 mm using 80% of the data as a training set. This work lays the groundwork for future noninvasive and nondestructive measurements of residual stresses through experimental RUS combined with finite element modeling and machine learning.

Microstructural characterization of polycrystalline materials is essential in order to predict the material properties and corresponding performance. Ultrasonic scattering has shown promise as a nondestructive tool to characterize polycrystalline materials. This study is focused on improving the fundamental understanding of ultrasonic scattering from different grain-size statistics for a single-phase polycrystalline material. The effect of microstructural grain-size distribution on ultrasonic scattering is studied using synthetic representative volume elements (RVE). The dataset consists of six different sets of thirty RVEs with different grain-size statistics created using DREAM.3D. The grain-size distribution spans from narrow to wide. The ultrasonic scattering is calculated numerically using all the different synthetic volumes. The results show a significant increase in ultrasonic scattering with the increase in the width of the grain-size distribution. Apart from the ultrasonic scattering, the dataset may be useful to researchers that would benefit from having access to multiple instantiations of synthetic grain ensembles with similar and/or systematically-varying lognormal size distribution.

Hybrid additive manufacturing (AM) generates materials with spatial variations by integrating additional manufacturing processes, such as milling, laser peening, or ultrasonic peening, at specific locations during the fabrication process. These material variations encompass alterations in average grain size, dislocation densities, and the introduction or alleviation of residual stresses. Importantly, these property changes extend beyond the applied layer, exhibiting a cumulative impact on preceding layers. The specifics of each hybrid process have been shown to improve mechanical properties, thereby enhancing the performance of components. However, there are distinctive challenges associated with the nondestructive validation of such samples. Traditional ultrasonic techniques have proven effective in mapping material variations with satisfactory spatial resolution in hybrid components. In this study, ultrasonic responses, i.e., wave speed, attenuation, and diffuse backscatter measurements, are used to differentiate between three different hybrid processes (milling, laser peening, and ultrasonic peening) in additively manufactured 316L stainless steel. This work is anticipated to impact the qualification of metal AM parts.

Current techniques for measuring residual stresses in additively manufactured components face certain limitations, often related to their sensitivity or the fact that they are destructive. Resonant ultrasound spectroscopy (RUS) has shown promise for evaluating residual stresses in additively manufactured components with single hybrid layers. Hybrid additive manufacturing (AM) incorporates additional energy sources or processes to create functionally graded components at specific locations. Each hybrid step or layer contributes to grain refinement and increased dislocation density which thereby alter residual stresses. In this presentation, the use of RUS to evaluate samples with multiple hybrid layers is described for samples created using three hybrid processes: milling, laser peening, and ultrasound peening. Prospects for determining residual stresses and layer locations are evaluated and issues related to non-uniqueness are discussed. This research lays the groundwork for measuring residual stress in hybrid components through experimental RUS measurements, finite element modeling, and machine learning algorithms.

Hybrid additive manufacturing (AM) involves secondary processes or energy sources to alter specified locations within the build volume. Each hybrid step can refine the grain size, increase dislocation density, or modify residual stresses. Typically, the changes in mechanical properties are not confined within a single layer but have a compounding effect on preceding layers. Existing methods of measuring AM residual stress are limited in terms of their sensitivity, or they are destructive measurements. We propose using resonant ultrasound spectroscopy (RUS) to measure the residual stress in hybrid-AM components noninvasively, based on changes to the resonances, compared to a stress-free component. In this paper, we use finite element models to simulate residual stress in hybrid-AM components and to examine the sensitivity of RUS measurements in terms of frequency shifts and mode shapes with respect to single hybrid layers. Then, the RUS results are used to predict stress for a layer at a known location with unknown stress. The approach highlights the capabilities of RUS to address an AM characterization challenge.

Diffuse ultrasonic backscatter techniques are used to characterize microstructures and work well when grain scattering is confined within the single-scattering regime. When effects from higher-order scattering are present within the measurements, grain sizes are often overestimated. These effects have been primarily examined with single-element transducers, but phased array ultrasonic transducers (PAUTs) offer several measurement advantages, particularly for samples with complex geometry. In this presentation, the influence of higher-order scattering on PAUT experiments is discussed for samples of weakly scattering aluminum and strongly scattering steel. Spatial variance measurements from PAUT signals show differences in scattering content that are dependent on focal depth and material scattering strength when compared with experiments using single-element transducers. We hypothesize that the multiple transducer elements within PAUTs increase the probability of double or multiple scattering effects within backscatter measurements. These effects are dependent on the material single-crystal anisotropy, the grain size, frequency, experimental configuration, and specifics of the PAUT. Models to describe the PAUT experimental results are also discussed. Accurate theoretical models are critical for validation in order to provide insight into the limits of such measurements for microstructure quantification.

Diffuse ultrasonic backscatter, which results from the interaction of elastic waves with material heterogeneity, can be used to characterize microstructural information. The scattering that occurs within the material can be complex and accurate models are needed to interpret measurements quantitatively, particularly for higher-scattering materials. Recently, a double scattering model was derived in which it was assumed that the energy scatters at most twice within the microstructure prior to detection. The significance of this model, with a focus on valid ranges of applicability, has not been thoroughly explored. In this article, theoretical and experimental results are examined to compare the single and double scattering models. The theoretical results show that single scattering models are appropriate for weakly scattering materials for a wide range of experimental setups while stronger scattering materials predict significant second-order scattering for certain measurement parameters, including transducer frequency and material path. Experimental results for steel, a strongly scattering material, are presented for frequencies of 5, 7.5, 10, and 15 MHz and show the domain for which the doubly scattered response becomes significant, as well as model limitations, due to selected experimental parameters. Finally, guidelines are provided for experiment design in order for measurements to remain within the desired scattering regime.

Stomata are pores at the leaf surface that enable gas exchange and transpiration. The signaling pathways that regulate the differentiation of stomatal guard cells and the mechanisms of stomatal pore formation have been characterized in Arabidopsis thaliana. However, the process by which stomatal complexes develop after pore formation into fully mature complexes is poorly understood. We tracked the morphogenesis of young stomatal complexes over time to establish characteristic geometric milestones along the path of stomatal maturation. Using 3D-nanoindentation coupled with finite element modeling of young and mature stomata, we found that despite having thicker cell walls than young guard cells, mature guard cells are more energy efficient with respect to stomatal opening, potentially attributable to the increased mechanical anisotropy of their cell walls and smaller changes in turgor pressure between the closed and open states. Comparing geometric changes in young and mature guard cells of wild-type and cellulose-deficient plants revealed that although cellulose is required for normal stomatal maturation, mechanical anisotropy appears to be achieved by the collective influence of cellulose and additional wall components. Together, these data elucidate the dynamic geometric and biomechanical mechanisms underlying the development process of stomatal maturation.

Metal additive manufacturing (AM) processes result in complex microstructures, porosity, material texture, and residual stresses. These properties may vary throughout AM parts. Ultrasonic nondestructive evaluation (NDE) is essential for understanding the processes leading to these variations. However, the use of single-element transducers to scan AM parts with complex geometries can be challenging. Such measurements may require multiple scans to level a surface or excite various wave types within a part. On the contrary, phased array ultrasonic transducers (PAUTs) can be much more efficient for these studies. In this presentation, custom AM sample designs are discussed which allow multiple ultrasonic wave speeds to be measured using a PAUT. The manufactured samples were assessed with the sectorial scan feature of the PAUT to scan multiple positions and directions quickly without moving the probe. Data from these scans were then used to estimate material wave speeds, material texture, and residual stress along multiple directions within the samples. The results from these scans were also compared with experiments that used single-element ultrasonic transducers. This presentation will describe the sample designs, the experiments, and the analysis in order to highlight the usefulness of PAUTs for scanning AM parts.

Objective To determine the sensitivity of vascular endothelial cells to long durations of low-intensity pulsed ultrasound (LIPUS) compared to normal flow and identify the duration that maximizes expression of two mechanosensitive genes related to healthy endothelial function, endothelial nitric oxide synthase (eNOS) and Krüppel-like factor 2 (KLF2). Methods Custom ultrasound exposure tanks were developed and the acoustic field was characterized. Human umbilical vein endothelial cells were seeded into culture plates and exposed to LIPUS at a frequency of 1 MHz and acoustic pressure of 217 kPa for 20 min, 1 h, 6 h, 9 h, or 24 h. As a comparator, other cells were exposed to normal flow. RT-qPCR was used to assess mRNA expression of eNOS and KLF2. Results Maximum eNOS and KLF2 expression occurred at 6 h and was localized to the beam path. Both genes exhibited qualitatively similar patterns of expression under LIPUS compared to normal flow. LIPUS induced a more rapid beneficial response compared to normal flow, but flow induced higher expression of both genes. eNOS expression after 6 h of LIPUS was dependent on RNA yield and culture duration prior to experiments. Conclusion Endothelial cells exposed to longer durations of LIPUS than typically employed exhibited greater expression of beneficial genes. The temporal gene expression patterns resulting from LIPUS and normal flow suggest activation of similar signaling pathways. However, LIPUS also caused increased RNA yield that may be linked to proliferation, which would suggest more of a wound healing than atheroprotective phenotype.

Metal additive manufacturing (AM) based on laser powder bed fusion (LPBF) generates components by melting metal powder on a layer-by-layer basis. The melting and cooling process often generates samples with a preferred material symmetry aligned with the build direction. This anisotropy affects mechanical performance and can be challenging to characterize nondestructively. Here, LPBF was used to create samples of AlSi10Mg with specific geometries to affect the overall anisotropy. In addition, the hybrid AM process of interlayer milling was used to impact the microstructure and residual stress of some samples. Ultrasonic measurements were used to characterize the samples using both coherent wave and diffuse wave experiments to capture the anisotropic nature of the wave speed and scattering. The material symmetry and morphology of the grains affect the wave speed, attenuation, and backscatter with respect to direction. Furthermore, spatially resolved acoustic spectroscopy was used to provide insight regarding the localized wave speeds with respect to sample location and propagation direction. The experimental data were used collectively to quantify differences between the AM processes used to create the samples. Such information can be used to guide AM process parameters to optimize sample performance. Finally, prospects for characterization of residual stresses will be discussed.

During laser-based metal additive manufacturing (AM), the melted powder is subjected to rapid cooling and the resulting microstructures often have material texture. In many cases, the texture is uniaxial (i.e., transversely isotropic) such that one symmetry axis defines the material response. Therefore, ultrasonic inspection methods that exploit the scattering from the microstructure are complicated by the resulting texture which affects the coherent propagation and scattering. In this presentation, a generalized approach is described in which the covariance of the elastic modulus tensor for a uniaxial ensemble of cubic crystals is expressed in terms of a fundamental set of constants. These constants are determined for an arbitrary texture from synthetic polycrystals created using DREAM.3D. With this information, calculations for wave velocity, attenuation, and diffuse scattering can be made efficiently for any wave type and propagation direction relative to the material symmetry axis. Results are compared with analytical expressions for simplified cases and then more generalized textures are examined. Finally, prospects for characterization of components created using metal AM are discussed within the context of input data from electron backscatter diffraction measurements. These results are expected to provide insight regarding the inversion of measurement data for texture characterization.

Journal Article Graphene Reinforced 316L Stainless Steel Prepared via Laser Powder Bed Fusion Get access Wen Qian, Wen Qian Department of Mechanical and Materials Engineering, University of Nebraska-Lincoln, Lincoln, NE, United States Search for other works by this author on: Oxford Academic Google Scholar Maxwyll McConnell, Maxwyll McConnell Department of Biological System Engineering, University of Nebraska-Lincoln, Lincoln, NE, United States Search for other works by this author on: Oxford Academic Google Scholar Joseph A Turner, Joseph A Turner Department of Mechanical and Materials Engineering, University of Nebraska-Lincoln, Lincoln, NE, United States Search for other works by this author on: Oxford Academic Google Scholar Xin Chen, Xin Chen Department of Mechanical and Materials Engineering, University of Nebraska-Lincoln, Lincoln, NE, United States Search for other works by this author on: Oxford Academic Google Scholar Bai Cui Bai Cui Department of Mechanical and Materials Engineering, University of Nebraska-Lincoln, Lincoln, NE, United States Search for other works by this author on: Oxford Academic Google Scholar Microscopy and Microanalysis, Volume 29, Issue Supplement_1, 1 August 2023, Pages 1430–1431, https://doi.org/10.1093/micmic/ozad067.736 Published: 22 July 2023