Test Geometry Research Articles

Effect of Insert Porosity on Combustion Instability in a Lean Premixed Combustor Analyzed by a Proper Orthogonal Decomposition-Based Phase Reconstruction Technique

Abstract Lean premixed (LPM) combustion is very effective at mitigating emissions but is vulnerable to strong thermoacoustic instabilities. A porous insert in the shape of an annular ring placed at the dump plane of the combustor has been proven to be an effective passive technique for mitigating these instabilities across a wide range of operating conditions. However, it is unclear if the change results from the insert geometry or porosity of the insert. In this study, swirl-stabilized LPM combustion is investigated for three configurations—without any insert, with a porous insert, and with a geometrically similar solid insert. Acoustics, flow, and heat release rate behavior of the three test geometries are investigated using diagnostics including dynamic pressure and acoustic probes, particle image velocimetry (PIV), and OH* chemiluminescence (OH*CL) imaging. Synchronized measurements at a fixed equivalence ratio were acquired at 40 kHz using sound probes and at 3.5 kHz using PIV and OH*CL. Results include time-series and spectral measurements of pressure, velocity, and OH*CL, and mode analysis by proper orthogonal decomposition (POD). In addition, the dynamics of the instability are investigated by high-resolution phase reconstructions of velocity and OH*CL data using a novel implementation of POD introduced in this work. Results show two different instability modes: a longitudinal instability for the solid insert case and a helical, precessing vortex driven instability for the no insert case. In both cases, the flow field and heat release rate oscillations are coupled to produce the instability. No such coupling or oscillations is observed for the porous insert case. These results ascertain the unique capabilities of the porous insert in protecting against instability from different, simultaneous driving mechanisms and demonstrate that the insert porosity and flow dynamics associated with it are the primary mitigating factors.

Model validation of hollow embossing rolling for bipolar plate forming

Hollow embossing rolling is a promising forming technology for metallic bipolar plates because of the high achievable production rates. However, the simulation-based process optimization is impeded by the incremental forming character and modeling of fine channel structures, which leads to large model sizes and long computation times. This paper presents a shell-based finite element approach validated by experimental forming tests using a miniaturized test geometry with typical discontinuities and varying channel orientations. The rolling experiments demonstrated that implementing restraining tension effectively decreases wrinkling, allowing successful forming of the selected test geometry by hollow embossing rolling. It was found that representing the manufacturing-related decreased rolling gap combined with the rolling gap changes due to roll system elasticity in the numerical model is essential for model accuracy. An optimized model approach with spring-controlled rollers was developed, which considers the effect of load-dependent rolling gap changes. With this approach the applied model achieves sufficient model accuracy for technological process simulation and optimization.

Effect of geometry on the evolution of DLOFC transients in high temperature helium loop

Generation IV high-temperature gas-cooled reactors (HTGR) are designed to exhibit passive safety under all off-normal circumstances. One such scenario, known as depressurized loss of forced circulation (DLOFC), occurs after a break in the inlet/outlet header, allowing helium to leak from the reactor. Upon depressurization, the onset of natural circulation (ONC) can occur causing bulk air ingress leading to the oxidation and degradation of core components. The Transformational Challenge Reactor (TCR) has similar features to those of an HTGR, but the primary difference is the use of a more complex, additively manufactured (AM) fuel geometry. The more compact, additively manufactured, ceramic fuel elements can be conveniently produced with optimally configured channels that suppress the air ingress progress and improve thermofluidic performance. DLOFC and air ingress are experimentally studied in a scaled HTGR flow test setup. The experiments use an AM test element embedded with a distributed temperature sensor as well as a test geometry comprised of spherical pebble bed elements. With the test geometries aligned within a lowercase h-shaped tube flow setup, further data was obtained to improve the understanding of DLOFC. The thermal transient and air ingress data gathered for both test geometries are compared. The results show that the AM part delayed ONC as compared to the pebble bed test piece at higher temperatures. The distributed temperature sensor shows intra-leg circulation at higher temperature tests.

Development approach of a workflow for 3D printing of living mycelium materials and their growth manipulation

As a viscous mycelium-substrate-composite, mycelia can be processed into more complex geometries in generative extrusion-based processes, even without molding tools. Biological transformation can also affect the properties of a printed object such that the mycelium still lives and grows after the 3D-build-process and changes the object over time. Depending on the substrate composition, type of fungus and growth conditions, different properties of the object can be manipulated. Different test geometries with corresponding preliminary characterization tests are shown. The present results demonstrate that test geometries could be successfully 3D printed with living mycelium material and manipulated during cultivation.

Experimental Investigation on the 3D Printing of Nylon Reinforced by Carbon Fiber through Fused Filament Fabrication Process, Effects of Extruder Temperature, and Printing Speed

This study investigated how the extruder temperature, printing speed, and specimen geometry interact during a tensile test of continuous carbon fiber‐reinforced nylon matrix composites produced by the fused deposition modelling (FDM) process. The investigation utilized statistical techniques. For this purpose, tensile examinations were done on manufactured samples using a testing apparatus. The study’s objective is to identify the most efficient specimen geometry for tensile testing result optimization and to maximize the 3D printing process’s capability for producing complex, freeform patterns in these composites. In this study, the input parameters required for the response surface methodology (RSM) were varying extruder temperature (240‐255°C) and printing speed (60‐80 mm/s), and experimental responses included modulus, elongation at break, and weight. The findings of the regression analysis showed output responses are influenced by both input variables. The results showed that the strength of the samples was significantly influenced by the input parameters. To draw the surface and residual plots, the software of design expert software was used. The interaction between the two input variables suggests raising the extruder temperature and decreasing printing speed, which leads to printing heavier samples. Inversely, the diversity between the forecasted and real responses for the optimal specimens is less than 10% which is assumed to be acceptable for the design of experiments (DOE). The analysis took into account the lower and upper ranges of the input variable with the goal of enhancing both the most modulus and fracture elongation while simultaneously degrading the weight of the specimens. To achieve this objective, the extruder temperature and printing speed are between 240 and 250°C and 65 and 75 mm/s, respectively.

Description of component distortion in aluminum gravity die casting as a result of hindered shrinkage and evaluation of numerical predictions

AbstractDeviation from the nominal dimension in the sense of component distortion is a quality‐reducing phenomenon in the foundry. This distortion problem as a result of impeded solidification and cooling shrinkage is of particular importance for permanent mold casting with its mechanically stiff and thermally well‐conducting molds. In studies, the temperature when the mold constraint is removed has already been identified as one of the dominant factors influencing the dimensional deviations of the casting. So far, however, no clear dependencies have been described. In the present work, the temperature up to the removal of the mold constraint is varied from near solidus to ambient temperature on a specially designed permanent mold casting made of AlSi7Mg0.3. This is compared with thermally and mechanically coupled finite elements method calculations of the investigated test geometry and the resulting component distortions. Finally, the applicability of the distortion prediction for the development of compensation strategies is evaluated.

Fracture forming limit curve prediction by ductile fracture models

The fracture forming limit curve is an essential tool giving information about fracture initiation in metal forming applications. In order to obtain a fracture-forming limit curve, the Nakajima test should be carried out. However, these tests are costly and time-consuming processes. Therefore, companies in the automotive industry tended to use non-expensive numerical approaches to determine the material's formability limits. In this work, an anisotropic polynomial yield criterion was implemented to calibrate the different ductile fracture models, including only or both the stress triaxiality and the Lode parameter's effect. Analyses of different uniaxial tensile test geometries were carried out, and the two-dimensional fracture loci were constructed by different ductile fracture models. Correspondingly, the fracture-forming limit curves were predicted by the two-dimensional fracture loci. The results showed that the ductile fracture models, including only the stress triaxiality effect, provide an acceptable performance covering the range from uniaxial tension and the plane strain tension lines, while the models, including the stress triaxiality and Lode parameter, provide a plausible prediction performance throughout the whole range for forming limit diagram.

Estimation of Temperature Effects on Uncertainties of Dimensional Measurements in Industrial Computed Tomography by Finite Element Modeling

Using industrial computed tomography (CT) in measuring dimensional parameters (such as diameter, distance) of manufactured parts becomes more and more popular due to its advantages such as being a nondestructive method and possibility to complete complex shape measurements rapidly. However, the accuracy and precision of the measured values are required to be validated by quality control. The complicated structure of multistep processing contribute to measurement errors and uncertainties through various factors. Among them, temperature change during the operation of CT machine possesses a high potential influence. Two test geometries created in Catia V5 were modelled in Marc Mentat Finite Element software considering different materials (PMMA, Al, AlMgSi1) to study the deformation behavior against the change of temperature during the CT measurement process. The results of simulations provided an appropriate point of view about the effect of the temperature parameter to the measurement accuracy and uncertainties

Lower-Dimensional Model of the Flow and Transport Processes in Thin Domains by Numerical Averaging Technique

In this work, we present a lower-dimensional model for flow and transport problems in thin domains with rough walls. The full-order model is given for a fully resolved geometry, wherein we consider Stokes flow and a time-dependent diffusion–convection equation with inlet and outlet boundary conditions and zero-flux boundary conditions for both the flow and transport problems on domain walls. Generally, discretizations of a full-order model by classical numerical schemes result in very large discrete problems, which are computationally expensive given that sufficiently fine grids are needed for the approximation. To construct a computationally efficient numerical method, we propose a model-order-reduction numerical technique to reduce the full-order model to a lower-dimensional model. The construction of the lower-dimensional model for the flow and the transport problem is based on the finite volume method and the concept of numerical averaging. Numerical results are presented for three test geometries with varying roughness of walls and thickness of the two-dimensional domain to show the accuracy and applicability of the proposed scheme. In our numerical simulations, we use solutions obtained from the finite element method on a fine grid that can resolve the complex geometry at the grid level as the reference solution to the problem.

Analysis of Geometric and Material Nonlinearity in the Column Bending Test

The column bending test (CBT) is an experimental method to measure the moment–curvature behavior and failure curvature of high-strain composites (HSCs), designed to apply a relatively constant bending moment through a sample up to very high curvatures. In this paper, we first calculate the errors involved in the simplified geometric analysis used to model the CBT by considering two different effects: variations in the bending moment through the sample and nonlinearities in the material response of the fibers. The first effect, modeled using Euler’s elastica, results in a set of design guidelines for the test geometry (i.e., length of the grips and the specimens) in order to achieve moderate errors in moment and curvature predictions. The nonlinearity in the fiber behavior (tension stiffening and compression softening) does not have a strong effect on the curvature but produces significant variations in the bending stiffness of HSCs as well as the maximum strain observed in the fibers due to the shift in the neutral axis. This indicates that accounting for the nonlinear behavior of fibers is necessary in order to accurately predict the failure properties of HSCs under bending.

Formability characterization of steel tailor welded blanks formed through single point incremental forming

TWBs are being well formed by a simple, highly innovative method known as Incremental Sheet Forming (ISF). Both aluminum and steel sheets welded through friction stir welding (FSW), laser beam welding (LBW) or electron beam welding (EBW) have been explored, extensively, by researchers. However, no research is available on the formability characterization of selected steel sheets having thickness and strength differential, manually welded using Tungsten Inert Gas (TIG) welding technique and formed through Single Point Incremental Forming (SPIF). The same is addressed in this study. Variable wall angle truncated pyramid was used as test geometry. Tensile testing, microhardness and scanning electron microscope (SEM)/Energy Dispersive X-ray (EDX) analysis were performed before forming. Material selected for the study included deep drawing quality (DDQ) steel (DC06) and stainless steel (SS) (AISI 201). Because of huge difference in both strength and hardness values of DDQ steel and SS, low formability was achieved in case of strength differential with minimum and maximum forming depths of 1.8 mm and 3.6 mm, respectively. Whereas formability achieved in case of thickness differential was quite high with minimum and maximum values of forming depths of 27.8 mm and 36.6 mm, respectively since strength and hardness values were not varied much. Percentage thinning was used to assess formability achieved. Final thicknesses achieved during forming were verified by cosine law also. Research investigations may be applied in similar fashion for precise study of material characterization of various kinds of TWBs being used in multiple industries including automotive, vessel, medical, etc.

A novel method to evaluate the mechanical properties of resistance spot welds under dynamic loading

The mechanical properties of resistance spot welds are integral in dictating joint performance in service. However, previous studies have evaluated the joint response to loading under the quasi-static condition which is not a true indication of the loading condition in service. This work has developed a novel method to evaluate spot welds under dynamic loading using two test geometries (opening and shear mode). The testing technique was used to compare the mechanical properties of a conventional and in-situ post-weld heat treatment (PWHT) schedule. Our developed method can expand the testing capability of single-spot welds under dynamic loading.

Effect of low dosage fiber reinforcement on cracking performance of asphalt concrete

Fiber reinforcement is one of the methods used to improve cracking resistance of quasi-brittle materials such as asphalt concrete (AC). Despite the widespread use of fibers in AC, there is a lack of fundamental understanding of fracture behavior of fiber reinforced asphalt concrete (FRAC). The main goal of this research is comprehensive fracture characterization of low dosage applications of two synthetic fibers (polyethylene-aramid and nylon) and understand the performance bias of low dosage fiber reinforcement at mixture level in different test conditions. Two laboratory prepared mixtures (fine and coarse-graded) were designed with consistent volumetrics. Two crack mouth opening displacement (CMOD) – controlled experiments (SCB-CMOD and DCT) were performed to facilitate a controlled crack propagation at low and intermediate temperatures. Alternative load-line displacement (LLD) control cracking tests (I-FIT and IDEAL-CT) with different geometries, testing parameters, and loading modes were also evaluated. It was observed that the mixes with nylon fibers had a distinctive and better fracture response in all of the experiments. Some improvements were observed with aramid fiber configuration dependent on the test performed and mixture type. The key test features that resulted in more distinction between the low dosage fiber mixes and the control appeared to be ligament length, affected damage area, and cracking patterns observed in the experiments. The tests with larger ligament length (i.e., larger affected damage zones) were sometimes visible with more tortuous cracking patterns with many branches resulted in more consistent and noticeable outcomes with the fibers. Further research is needed to identify potential test bias and develop an ideal test protocol and test geometry that can objectively evaluate the performance of low dosage fibers.

A microtiter peg lid with ziggurat geometry for medium-throughput antibiotic testing and in situ imaging of biofilms

Bacteria biofilm responses to disinfectants and antibiotics are quantified and observed using multiple methods, though microscopy, particularly confocal laser scanning microscopy (CLSM) is preferred due to speed, a reduction in user error, and in situ analysis. CLSM can resolve biological and spatial heterogeneity of biofilms in 3D with limited throughput. The microplate peg-lid-based assay, described in ASTM E2799-22, is a medium-throughput method for testing biofilms but does not permit in situ imaging. Breaking off the peg, as recommended by the manufacturer, risks sample damage, and is limited to easily accessible pegs. Here we report modifications to the peg optimized for in situ visualization and visualization of all pegs. We report similar antibiotic challenge recovery via colony formation following the ASTM E2799-22 protocol and in situ imaging. We report novel quantifiable effects of antibiotics on biofilm morphologies, specifically biofilm streamers. The new design bridges the MBEC® assays design that selects for biofilm phenotypes with in situ imaging needs.

Heat transfer and pressure drops during high heat flux flow boiling of low-GWP refrigerants in 200 μm-wide multi-microchannels

Thermal management of electronics is continuously challenged by increasing heat fluxes to be dissipated at minimum power consumption. This study investigated the pressure drops and the heat transfer coefficient of two low-GWP refrigerants, R1234yf and R1234ze(E), during flow boiling at high heat fluxes. The test geometry comprised 25 parallel microchannels, 200 μm wide, 1200 μm deep and 1 cm long, nominally. Inlet orifices were used to stabilize the flow. A parametric analysis involving the mass flux and heat flux was conducted for nominal saturation temperatures of 30.5C∘ and 40.5C∘, resulting in a maximum confinement number NConf of 2.16 for R1234yf and 2.52 for R1234ze(E). As the applied heat flux was increased from low to near-critical values, the two-phase heat transfer transitioned from a heat-flux-dominated mode to a convection-dominated one. The flow finally approached dryout, which led to the critical heat flux upon further heating. The experimental results were analysed through a regression with relevant non-dimensional groups, in particular the confinement number, NConf, the equivalent wall Biot number, Bi, the boiling number, Bo, the convection number, Co, and the Weber number, We. It was shown that Bo and Bi had a major influence in the heat-flux-dominated region, while Co and NConf determined the performance in the convective region, mainly due to the establishment of an intermittent annular flow. The total channel pressure drops were evaluated through the experimental measurement of the pressure drops in the inlet-outlet manifold, and the orifices. Due to the short channel length and the high heat fluxes, the pressure drops were dominated by the momentum change contribution, which accounted for at least 60% of the total pressure drops in the lowest case. The flow-wise heat transfer coefficients and the channel pressure drops were compared with the predictions of existing correlations, showing generally a fair agreement.

A machine learning approach to automate ductile damage parameter selection using finite element simulations

Ductile damage models require constitutive model parameter values that are difficult to derive experimentally or analytically. The calibration procedure for ductile damage model parameters, typically performed manually, is labour-intensive. In this work we detail a fully autonomous framework that integrates Bayesian optimisation and finite element analysis to identify ductile damage model parameters. The framework detailed here selects ductile damage model parameters from inputs that can be derived from a simple tensile test. This framework has been successfully deployed to three datasets of P91 material tested at ambient (20°C) and higher (500°C) temperatures. The Bayesian optimisation derived material model parameters result in simulated output with less than 2% error compared to experimental data. This research demonstrates that algorithm hyperparameters can significantly affect the Bayesian optimised ductile damage parameter values resulting in non-unique ductile damage parameters. We show that the non-unique solutions can be further assessed using a second test geometry.

Orientation-dependent extreme shear strain in single-crystalline silicon - from elasticity to fracture

Measuring strain accurately at small length scales poses a significant challenge, making it difficult to obtain precise elastic properties of small materials. This becomes particularly pronounced for test geometries beyond micro-pillars and for materials with high elastic limits and high Peierls stresses. This study investigates the elastic strain limits and strain distribution in micro double shear tests conducted on single-crystalline silicon with different crystallographic orientations. In situ scanning electron microscopy images were used to obtain full-field strain maps using digital image correlation. This local strain analysis approach revealed that the shear zones of the test geometry are not solely under pure shear conditions, but also experience superimposed bending. The local strain analysis approach increases the precision of measured elastic properties to ±15% of the literature value compared to deviations of 75-80% using the traditional global strain analysis approach. This study highlights the limitations of the global strain analysis approach in complex specimen geometries and demonstrates the effectiveness of digital image correlation in accurately determining elastic strain at the small length scale. Furthermore, both the low defect density in the samples as well as the small length scales allow for the exploration of orientation dependent strength levels close to the theoretical limit.

An Empirical Model and Feedforward Control of Laser Powder Bed Fusion

Abstract Reliable process control for the laser powder bed fusion process, especially at the melt pool scale, remains an open challenge. One of the reasons for this is the lack of suitable control-oriented models and associated control design strategies. To address this issue, this paper (1) identifies an empirical control-oriented model of geometry-dependent melt pool behavior and (2) experimentally demonstrates melt pool regulation with a feedforward controller for laser power based on this model. First, the study establishes that the melt pool signature increases as the scan lines decrease in length. An empirical model of this behavior is developed and validated on different geometries at varying laser power levels. Second, the model is used to design a line-to-line feedforward controller that provides an optimal laser power sequence for a given geometry. Finally, this controller is validated experimentally and is demonstrated to suppress the in-layer geometry-related melt pool signal deviations for different test geometries.

Fresh properties of fly ash-based geopolymer: the role of the testing conditions on the rheological measurements

Rheology is a powerful tool to assess the fresh properties of cementitious systems accurately. However, uncertainties and systematic errors in measurements can impact the obtained values, even though the rheological properties should not depend on the measurement procedure. Contrary to Portland-based systems, few reports systematically assessed the role of the testing conditions on the rheological properties of geopolymers. This study evaluates the effect of testing conditions on the measurement of the rheological properties of fly ash-based geopolymer paste. Different testing geometries, including two sets of parallel plates, two concentric cylinders, and a four-blade vane spindle, were used to obtain flow curves by continuous and stepwise shear rate increments. The results show that reducing the ratio between the spindle and container radii for coaxial geometries can induce wrong shear stress/shear rate calculations and local instability. Additionally, the differences in shear stress measured with ramp and steps protocols increase as the Ri/Ro ratio (i.e., the ratio between the spindle and the container radii) increases. Parallel plates require an extra ring to keep the paste between the plates during shearing. Increasing the maximum shear rate applied reduces yield stress and viscosity values, indicating improved particle dispersion. Comparative tests with a Portland paste demostrated that the mini-slump test was suitable for assessing the yield stress of the geopolymeric system. However, its limitation regarding viscosity assessment was evidenced. Therefore, the measurement setup must be carefully chosen for the geopolymer under evaluation, and preliminary tests are highly recommended to support this selection.

Spacecraft Charging Test Considerations for Composite Materials

Composite materials present a growing challenge for spacecraft charging assessments. We review some recent lessons learned for charging tests of composite materials using both parallel-plate and electron beam test geometries. We also discuss examples of materials that exhibit significant variations between samples, despite them all having the same trade name.