Local Material Properties Research Articles

A Scan Strategy Based Compensation of Cumulative Heating Effects in Electron Beam Powder Bed Fusion

AbstractThe fabrication of complex geometries with uniform material properties in electron beam powder bed fusion (PBF-EB) remains a major challenge. Local material properties in PBF-EB are determined by the local thermal conditions and the spatio-temporal melt pool evolution. The local thermal conditions are governed by the cumulative heating effect on the hatch scale, which results from the superposition of temperature fields from adjacent hatch lines. The build-up of the cumulative heating effect at the beginning of a new hatch segment, without prior hatch lines, which results in regions with underdeveloped thermal conditions, is so far only rarely considered in the design of process strategies. This study introduces a numerical optimization scheme with the objective to minimize the extent of regions with underdeveloped thermal conditions at the beginning of line-based hatches, by means of scan strategy modifications. For this purpose, a simplified thermal solution is combined with an optimization approach to determine an optimal process strategy for line-based PBF-EB of a cuboid model geometry through the adaptation of individual hatch line spacing. Based on the approach determined for the model geometry, a generalized process strategy is derived for complex geometries and is numerically validated for different process parameter and geometry combinations.

Physical and Mechanical Properties of Local Carbon Materials and Warm Compaction Method on the Production of Current Collectors for Light Rail Transit (LRT) Applications

The purpose of this study is to investigate the physical and mechanical properties of local carbon materials and the warm compaction method in the production of current collectors for light rail transit (LRT) applications. The problem here is that the current collectors used by the railway industry in Malaysia for the LRT are still imported from foreign countries which is very costly. Furthermore, the production method of commercial current collectors, namely C-Cu composites, is compact and sintered, which also results in relatively high costs. Therefore, this study focuses on replacing the old materials and methods of commercial current collector production with new materials and methods. This study used a local carbon, which is the kernel shell of oil palm as the main carbon for the production of current collectors. The compact and sintered production methods are then replaced by warm compaction and post-heating methods. The current collectors that had been produced are then cut into several samples to allow them to be tested in anumber of physical and mechanical tests. Based on the test results, when using these new materials and methods, the density and hardness were observed to be slightly lower at 1.81 g/cm3 and 122.6 HRR compared to the commercial current collectors, which have a density of 2.2 g/cm3 and 126.9 HRR. Meanwhile, in the transverse rupture strength (TRS) and resistivity tests, the warm compaction method based on oil palm kernel shell demonstrated higher values of 175.1 MPa and 1631 Ω-cm, respectively, compared to the commercial current collectors which showed values of 58.71 MPa and 598.77 Ω-cm. In conclusion, there are various advantages and disadvantages between the new current collectors and the commercial current collectors in terms of mechanical as well as physical characteristics. Therefore, various improvements and suggestions need to be made in order to produce physically and mechanically better current collectors to be used in LRT applications.

Superdiffusion and heterogeneous dynamics in liquid crystals by microrheology

Microrheology (MR) has emerged as a powerful tool for unraveling the intricate local viscoelastic properties of various soft materials. By tracking the free (passive MR) or forced (active MR) diffusion of a tracer, valuable insights into the mechanical characteristics of the host system can be obtained. In this study, we investigate the forced diffusion of a spherical tracer within isotropic and smectic liquid crystal phases of hard rod-like particles. Our findings reveal superdiffusive behaviour induced by external forces, particularly pronounced when these are aligned parallel to the nematic director. Analysis of the dynamical susceptibility unveils heterogeneities strongly correlated with the magnitude and orientation of applied forces, highlighting the system's critical dependence on structural ordering. Intriguingly, we observe that tracer superdiffusion, driven by external forces and evident across all relevant system directions, does not demonstrate a strong correlation with resulting dynamical heterogeneities.

Small Punch Testing of a Ti6Al4V Titanium Alloy and Simulations under Different Stress Triaxialities.

The mechanical properties of local materials subjected to various stress triaxialities were investigated via self-designed small punch tests and corresponding simulations, which were tailored to the geometry and notch forms of the samples. The finite element model was developed on the basis of the actual test method. After verifying the accuracy of the simulation, the stress, strain, and void volume fraction distributions of the Ti6Al4V titanium alloy under different stress states were compared and analyzed. The results indicate that the mechanical properties of the local material significantly differ during downward pressing depending on the geometric shape. A three-dimensional tensile stress state was observed in the center area, where the void volume fraction was greater than the fracture void volume fraction. The fracture morphology of the samples further confirmed the presence of different stress states. Specifically, the fracture morphology of the globular head samples (with or without U-shaped notches) predominantly featured dimples. Modifying the specimen's geometry effectively increased stress triaxiality, facilitating the determination of the material's constitutive relationship under varying stress states.

Beam-shaping in laser-based powder bed fusion of metals: A computational analysis of point-ring intensity profiles

Laser-based powder bed fusion has the potential to enable manufacturing of local material properties and locally varying alloys. One promising approach under research is shaping of the applied laser beam. The current commercial standard using a Gaussian laser beam profile is associated with a high energy intensity peak and strong gradients. A potential alternative beam shape is the point-ring profile where laser energy is distributed between a central spot and an outer ring. We conduct a numerical study of beam shape impact on melt-pool shapes, surface temperatures, evaporation and flow dynamics of single-melt-tracks. To that end, we apply a Weakly Compressible Smoothed Particle Hydrodynamics solver coupled with a ray-tracing laser scheme. A point-ring profile is compared to a standard Gaussian profile. The profiles are studied at two different beam sizes to find the influence of beam size on the shape effect. We find good agreement between numerical results and experimental data for both narrow and deep, as well as wide and shallow melt-tracks. The simulation results show that beam size and thus energy density influence the effect beam-shaping has on the melt-pool dynamics. Both the distribution of evaporation and surface tension forces over the melt-pool surface are found to depend on the used beam shape. The study highlights the importance of distinguishing between size and shape effects when investigating laser beam shapes. Both energy density and its distribution need to be tuned for the desired melt-pool behavior.

Systematic Absences of Optical Phonon Modes in Phonon Dispersion Measured by Electron Microscopy.

Phonon dispersion relations are widely used to elucidate the vibrational properties of materials. As an emerging technique, momentum-resolved vibrational spectroscopy in scanning transmission electron microscopy offers an unparalleled approach to explore q-dependent phonon behavior at local structures. In this study, we systematically investigate the phonon dispersion of monolayer graphene across several Brillouin zones (BZs) using momentum-resolved vibrational spectroscopy and find that the optical phonon signals vanish at the Γ points with indices (hk0) satisfying h+2k=3n (n denoted integers). Theoretical analysis reveals that the observed phenomena arise from the complete destructive interference of the scattered waves from different basis atoms. This observation, corroborated by the study of diamond, should be a general characteristic of materials composed of symmetrically equivalent pairs of the same elements. Moreover, our results emphasize the importance of multiple scattering in interpreting the vibrational signals in bulk materials. We demonstrate that the systematic absences and dynamic effects, which have not been much appreciated before, offer new insights into the experimental assessment of local vibrational properties of materials.

Optimization of Sensor Placement for a Measurement System for the Determination of Local Magnetic Material Properties

The aim of this work is to optimize the sensor positions of a sensor–actuator measurement system for identifying local variations in the magnetic permeability of cut steel sheets. Before solving the actual identification problem, i.e., finding the material distribution, the sensor placement of the measurement setup should be improved in order to reduce the uncertainty of the identification of the material distribution. The Fisher information matrix (FIM), which allows one to quantify the amount of information that the measurements carry about the unknown parameters, is used as the main metric for the objective function of this design optimization. The forward problem is solved by the finite element method. The results show that the proposed method is able to find optimal sensor positions as well as the minimum number of sensors to achieve a desired maximum parameter uncertainty.

Advanced X-ray absorption spectroscopy under high pressures at the ESRF beamline ID12

ABSTRACT This paper reports on advanced instrumentation for high pressure research developed for the ESRF beamline ID12 which is dedicated to polarization-dependent X-ray absorption spectroscopy at photon energies from 2 to 15 keV. Emphasis is laid on X-ray optics, the design of diamond anvil cell adapted for tender X-ray range and compatible with low temperatures and with high magnetic field as well as a highly efficient X-ray fluorescence detection system. Several examples have been selected to illustrate the performance of the beamline and show what type of advanced spectroscopic techniques can be used to study the local electronic and magnetic properties of materials under pressure.

Effect of nonlocality on the dispersion relations of mechanical metamaterials

The advent of mechanical metamaterials, with their unconventional and programmable properties, has revolutionised the field of engineering across length scales. Their design remains an area of interest to researchers requiring careful analysis of unit cells so as to achieve desired mechanical and wave dispersion behaviour. While metamaterials are known to extract their unique properties from the internal geometry of the unit cells, the contributions of the constituent materials are typically accounted for via their local material properties in the form of Young’s modulus and Poisson’s ratio. In this article, we demonstrate that material nonlocality of the constituents can also play a crucial role in modulating the nontrivial properties of metamaterials. Such nonlocality can arise from various sources, such as finer scale inhomogeneity like defects, inclusions etc. Towards this, we propose a nonlocal continuum model by upscaling a discrete potential through a stochastic gradient estimator (SGE) interpreted herein as a generalised Cauchy–Born rule. The nonlocal parameters can also be estimated from experimental data by solving an inverse problem, similar to the estimation of constitutive parameters in classical continuum mechanics. The proposed model accurately captures the dispersion behaviour at the continuum level as compared to its discrete counterpart. We further incorporate the model within the transfer matrix method to derive the dispersion relations of a metabar as a function of the constituent material nonlocality. We envisage that the present study introduces a new handle in the form of constituent material nonlocality for designing architected metamaterials with unprecedented mechanical functionalities.

Harvesting deformation modes for micromorphic homogenization from experiments on mechanical metamaterials

A micromorphic computational homogenization framework has recently been developed to deal with materials showing long-range correlated interactions, i.e. displaying patterning modes. Typical examples of such materials are elastomeric mechanical metamaterials, in which patterning emerges from local buckling of the underlying microstructure. Because pattern transformations significantly influence the resulting effective behaviour, it is vital to distinguish them from the overall deformation. To this end, the following kinematic decomposition into three parts was introduced in the micromorphic scheme: (i) a smooth mean displacement field, corresponding to the slowly varying deformation at the macro-scale, (ii) a long-range correlated fluctuation field, related to the buckling pattern at the meso-scale, and (iii) the remaining uncorrelated local microfluctuation field at the micro-scale. The micromorphic framework has proven to be capable of predicting relevant mechanical behaviour, including size effects and spatial as well as temporal mixing of patterns in elastomeric metamaterials, making it a powerful tool to design metamaterials for engineering applications. The long-range correlated fluctuation fields need to be, however, provided a priori as input parameters. The main goal of this study is experimental identification of the decomposed kinematics in cellular metamaterials based on the three-part ansatz. To this end, a full-field micromorphic Integrated Digital Image Correlation (IDIC) technique has been developed. The methodology is formulated for finite-size cellular elastomeric metamaterial specimens deformed in (i) virtually generated images and (ii) experimental images attained during in-situ compression of specimens with millimetre sized microstructure using optical microscopy. The proposed IDIC method identifies the different kinematic fields, both before and after the microstructural buckling, and without any prior knowledge determines correctly the relevant patterning modes required by the homogenization scheme. It is further argued that patterning modes are independent of the unit cell size, the hole diameter to cell size ratio, as well as local material properties, allowing for modelling and design of (finite- and infinite-size) metamaterials and specimens with graded microstructures in terms of geometry and/or material properties. It is shown that the proposed methodology is also applicable to cellular metamaterials and structures with different microstructural designs.

Unraveling dynamic Jahn-Teller effect and magnetism in FeTiF6×6H2O single crystal

Hydrated iron fluoridotitanate (FeTiF6 × 6 H2O) single crystals are fascinating magnetic materials with unique properties. To understand the underlying mechanisms, this study combines X-ray absorption near-edge structure (XANES) and X-ray magnetic circular dichroism (XMCD) techniques, complemented by density functional theory (DFT) calculations. Polarization-dependent X-ray absorption spectroscopy, encompassing XANES and XMCD, is a powerful technique for probing the local structures and magnetic properties of materials. It is element-selective, bulk-sensitive, and compatible with a wide range of experimental conditions. In this study, we used XANES and XMCD spectroscopies to investigate the local structures and magnetic properties of Fe and Ti in FeTiF6 × 6 H2O single crystals. XANES analysis revealed distinct local environments around Fe and Ti, providing insights into their coordination environments. Element-selective magnetization measurement at the Fe K-edge demonstrated that iron sites in the oxidation state Fe2+ have an unambiguous paramagnetic contribution to the magnetization along the b-axis. Notably, the absence of an XMCD signal at the Ti K-edge confirmed the absence of a magnetic moment in Ti atoms within the crystal. DFT calculations corroborate the experimental findings and provide insights into the electronic structure and magnetic interactions. The combined results provide a comprehensive understanding of the dynamic Jahn-Teller effect in FeTiF6 × 6 H2O single crystals, highlighting the significance of polarization-dependent X-ray absorption spectroscopy in unraveling the intricate magnetic behavior of such materials. This study contributes to the fundamental understanding of magnetism in these materials and paves the way for the development of novel magnetic materials with tailored properties.

Methods for characterization and continuum modeling of inhomogeneous properties of paper and paperboard materials: A review

The potential of paper and paperboard as fiber-based materials capable of replacing conventional polymer-based materials has been widely investigated and evaluated. Due to paper’s limited extensibility and inherent heterogeneity, local structural variations lead to unpredictable local mechanical behavior and instability during processing, such as mechanical forming. To gain a deeper understanding of the impact of mechanical behavior and heterogeneity on the paper forming process, the Finite Element Method (FEM) coupled with continuum modeling is being explored as a potential approach to enhance comprehension. To achieve this goal, utilizing experimentally derived material parameters alongside stochastic finite element methods allows for more precise modeling of material behavior, considering the local material properties. This work first introduces the approach of modeling heterogeneity or local material structure within continuum models, such as the Stochastic Finite Element Method (SFEM). A fundamental challenge lies in accurately measuring these local material properties. Experimental investigations are being conducted to numerically simulate mechanical behavior. An overview is provided of experimental methods for material characterization, as found in literature, with a specific focus on measuring local mechanical material structure. By doing so, it enables the characterization of the global material structure and mechanical behavior of paper and paperboard.

Elasto-viscoplastic model for rayon yarns

AbstractIn this paper we develop a new model for the simulation of the mechanical behavior of rayon twisted yarns, at macroscopic level. A yarn with its continuous filaments is represented by an equivalent three-dimensional solid of cylindrical shape, discretized by finite elements, with properly defined local anisotropic material properties. The new constitutive model, inspired by experimental results on rayon untwisted yarns, is formulated in the framework of the thermodynamics of irreversible processes and includes visco-elastic and visco-plastic dissipation mechanisms. The effect of twist is taken into account by including the direction of the fibers in the free energy definition. The overall model is validated comparing numerical and experimental results on twisted rayon yarns.

Comparison of a quasi Newton method using Broyden’s update formula and an adjoint method for determining local magnetic material properties of electrical steel sheets

PurposePerforming accurate numerical simulations of electrical drives, the precise knowledge of the local magnetic material properties is of utmost importance. Due to the various manufacturing steps, e.g. heat treatment or cutting techniques, the magnetic material properties can strongly vary locally, and the assumption of homogenized global material parameters is no longer feasible. This paper aims to present the general methodology and two different solution strategies for determining the local magnetic material properties using reference and simulation data.Design/methodology/approachThe general methodology combines methods based on measurement, numerical simulation and solving an inverse problem. Therefore, a sensor-actuator system is used to characterize electrical steel sheets locally. Based on the measurement data and results from the finite element simulation, the inverse problem is solved with two different solution strategies. The first one is a quasi Newton method (QNM) using Broyden's update formula to approximate the Jacobian and the second is an adjoint method. For comparison of both methods regarding convergence and efficiency, an artificial example with a linear material model is considered.FindingsThe QNM and the adjoint method show similar convergence behavior for two different cutting-edge effects. Furthermore, considering a priori information improved the convergence rate. However, no impact on the stability and the remaining error is observed.Originality/valueThe presented methodology enables a fast and simple determination of the local magnetic material properties of electrical steel sheets without the need for a large number of samples or special preparation procedures.

Using miniature experiments to reveal strength gradients in battery casings

Complex metallic structures are often manufactured through multi-step forming or casting processes that induce severe mechanical property variations within the final part. Instead of using the virgin material properties to predict the final part performance, the local material properties after manufacturing need to be known. A novel comprehensive testing methodology is presented allowing the resolution of spatial gradients in the plasticity and fracture properties in metallic structures. It involves using a recently-developed miniature linear-motion system that probes the material within 2 mm wide gage sections (uniaxial, notched and central hole tension, in-plane shear). Additionally, a novel mini-bulge testing system is presented along with a mini-punch system to provide further insight into the local material response for biaxial loading conditions. A full testing campaign comprising 60 miniature samples is performed on all sides of a prismatic aluminum casing for Li-ion battery cells. The results demonstrate significant property variations with more than 30 % higher yield stresses for the side walls as compared to the bottom section. The miniature specimens allow identifying a location-dependent anisotropy, which would otherwise be missed by standard-sized samples. Furthermore, it is shown that the anisotropic Yld2000 plasticity model and the stress-state dependent Hosford-Coulomb fracture initiation model can be fully identified using results from experiments on miniature specimens only.

Development of a piezo stack – Laser doppler vibrometer sensing approach for characterizing shear wave dispersion and local viscoelastic property distributions

Laser Doppler vibrometry and wavefield analysis have recently shown great potential for nondestructive evaluation, structural health monitoring, and studying wave physics. However, there are limited studies on these approaches for viscoelastic soft materials, especially, very few studies on the laser Doppler vibrometer (LDV)-based acquisition of time–space wavefields of dispersive shear waves in viscoelastic materials and the analysis of these wavefields for characterizing shear wave dispersion and evaluating local viscoelastic property distributions. Therefore, this research focuses on developing a piezo stack-LDV system and shear wave time–space wavefield analysis methods for enabling the functions of characterizing the shear wave dispersion and the distributions of local viscoelastic material properties. Our system leverages a piezo stack to generate shear waves in viscoelastic materials and an LDV to acquire time–space wavefields. We introduced space-frequency-wavenumber analysis and least square regression-based dispersion comparison to analyze shear wave time–space wavefields and offer functions including extracting shear wave dispersion relations from wavefields and characterizing the spatial distributions of local wavenumbers and viscoelastic properties (e.g., shear elasticity and viscosity). Proof-of-concept experiments were performed using a synthetic gelatin phantom. The results show that our system can successfully generate shear waves and acquire time–space wavefields. They also prove that our wavefield analysis methods can reveal the shear wave dispersion relation and show the spatial distributions of local wavenumbers and viscoelastic properties. We expect this research to benefit engineering and biomedical research communities and inspire researchers interested in developing shear wave-based technologies for characterizing viscoelastic materials.

Neutron Radiation-induced Damage to the Local Structure and Optical Properties of Amorphous Phase-Change Materials

Neutron Radiation-induced Damage to the Local Structure and Optical Properties of Amorphous Phase-Change Materials

Assessment of the nano-mechanical properties of healthy and atherosclerotic coronary arteries by atomic force microscopy.

Nano-indentation techniques might be better equipped to assess the heterogeneous material properties of plaques than macroscopic methods but there are no bespoke protocols for this kind of material testing for coronary arteries. Therefore, we developed a measurement protocol to extract mechanical properties from healthy and atherosclerotic coronary artery tissue sections. Young's modulus was derived from force-indentation data. Metrics of collagen fibre density were extracted from the same tissue, and the local material properties were co-registered to the local collagen microstructure with a robust framework. The locations of the indentation were retrospectively classified by histological category (healthy, plaque, lipid-rich, fibrous cap) according to Picrosirius Red stain and adjacent Hematoxylin & Eosin and Oil-Red-O stains. Plaque tissue was softer (p < 0.001) than the healthy coronary wall. Areas rich in collagen within the plaque (fibrous cap) were significantly (p < 0.001) stiffer than areas poor in collagen/lipid-rich, but less than half as stiff as the healthy coronary media. Young's moduli correlated (Pearson's ρ = 0.53, p < 0.05) with collagen content. Atomic force microscopy (AFM) is capable of detecting tissue stiffness changes related to collagen density in healthy and diseased cardiovascular tissue. Mechanical characterization of atherosclerotic plaques with nano-indentation techniques could refine constitutive models for computational modelling.

An efficient method for multiscale modelling of the mechanical properties of additively manufactured parts with site-specific microstructures

The ability to modulate microstructures at different locations within a part (i.e., site-specific microstructures) can lead to tailored properties and enhanced performance. However, it remains challenging to efficiently model the mechanical response of such materials due to the need for extensive multiscale analysis. To address this issue, we demonstrate through microstructure-based finite element simulations, that the mechanical properties of a randomly distributed two-phase microstructure are primarily dependent on the phase fraction. Therefore, by establishing a mathematical relationship between the phase fraction and mechanical properties, the local material properties can be efficiently calculated from the local microstructure. Later, these site-specific properties were incorporated into a finite element structure to predict the global mechanical properties of the sample part. To validate the effectiveness of the proposed method, low alloy steel with site-specific microstructures was fabricated using binder jet additive manufacturing by depositing varying concentrations of carbon binder at different locations inside the part. We observed a strong agreement between the experimental and simulation stress-strain responses of the steel samples. This method can potentially pave the way for the development of a non-destructive qualification method for additively manufactured parts, as well as provide an efficient approach to multiscale modeling of material behavior, which may help accelerate the design and innovation of novel material systems.

Quantifying local stiffness and forces in soft biological tissues using droplet optical microcavities

Mechanical properties of biological tissues fundamentally underlie various biological processes and noncontact, local, and microscopic methods can provide fundamental insights. Here, we present an approach for quantifying the local mechanical properties of biological materials at the microscale, based on measuring the spectral shifts of the optical resonances in droplet microcavities. Specifically, the developed method allows for measurements of deformations in dye-doped oil droplets embedded in soft materials or biological tissues with an error of only 1 nm, which in turn enables measurements of anisotropic stress inside tissues as small as a few pN/μm2. Furthermore, by applying an external strain, Young's modulus can be measured in the range from 1 Pa to 35 kPa, which covers most human soft tissues. Using multiple droplet microcavities, our approach could enable mapping of stiffness and forces in inhomogeneous soft tissues and could also be applied to in vivo and single-cell experiments. The developed method can potentially lead to insights into the mechanics of biological tissues.