OPTIMIZATION OF NANOPOROUS METALLIC ACTUATORS BY COMBINING MULTISCALE CALCULATIONS AND MACHINE LEARNING [Poster #18]

Nanoporous materials (NMs) in electrolytes can achieve approximately 1% deformation under low operating voltages of about 1 V, making them promising for use in artificial muscles. The multi-field and multi-scale nature of the NM electrochemical actuator makes simulation-based optimization extremely challenging. A computational framework was developed that combines joint density functional theory (JDFT), surface eigenstress model, symbolic regression, finite element methods (FEM), and surrogate modeling to perform both concurrent and sequential multi-scale calculations. Specifically, JDFT calculations were performed on Au thin films to obtain in-plane strain as a function of charge density and film thickness. The surface eigenstress and surface Young’s modulus of the Au nanofilm were then determined by fitting the surface eigenstress model to the JDFT data. Additionally, symbolic regression was used to obtain the constitutive equation of surface eigenstrain versus surface charge density, which realized macroscale FEM calculations. Finally, a mapping scheme was established between a given sequence of numbers and a particular structure of nanoporous Au, which allowed for the employment of Gaussian process regression surrogate models. These surrogate models were employed to accelerate the evaluation of actuation strain and effective Young’s modulus, and hence enable the exploration of the entire design space.
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COMPETITION BETWEEN NECKING AND PRE-CUT PROPAGATION IN FRACTURE OF HIGH-DENSITY POLYETHYLENE REVEALED BY TIME COURSES OF STRAINS [Poster #19]

High-density polyethylene pipes are widely used in pressure pipe applications such as water and gas transportation, but both necking and pre-crack effects are still poorly understood. This paper presents experimental observations to highlight strain field evolutions to necking and effects of pre-crack on strain field evolutions in a high density polyethylene material deformed in tension through analyzing spatial distributions of time histories of strains. Necking and its growth along the tension direction dominate the failure behavior of the intact specimen. Necking and crack propagation are both observed in the pre-cut specimen, but the crack propagation eliminates the necking propagation along the tension direction. Energy releases from positions outsides the crack zone lead to the macroscopic load-displacement curve deviates from the trend of the intact specimen. These findings present new recognitions on strain fields evolving to necking and failure induced by the pre-crack that are significant for designing of theoretical models and simulations of polymeric materials and structures.
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TRANS-SCALE PROPERITES OF PRECURSORY ACCELERATING DEFORMATION IN CATASTROPHIC FAILURE OF UNIAXIALLY COMPRESSED SANDSTONES [Poster #20]

The power law acceleration has been validated as an effective method for predicting catastrophic failure time, however, the precursory acceleration distribution in local monitoring signals is still unclear. This paper experimental results to show the variable properties of durations, onset times and critical power law exponents of precursory accelerating deformation with monitoring positions and sizes. Our results declare that precursory strain acceleration at different positions and size windows can provide consistent and stable prediction that agree well with the actual failure time. Our findings suggest that there is an optimal size and monitoring position that present earlier alarm and higher accurate prediction, because of heterogeneity of precursory accelerations in amplitudes and durations.
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INFLUENCE OF PRINT PARAMETERS ON FRACTURE RESPONSE OF PLAIN AND FIBER-REINFORCED 3D-PRINTED BEAMS [Poster #21]

While much attention has been given to developing concrete mixtures for digital manufacturing (3D printing) and their associated rheological and mechanical properties, selecting appropriate printing parameters is also crucial for extrusion-based layered manufacturing. This paper explores the impact of layer height, a key parameter affecting rheology requirements, print quality, overall printing time, and interlayer bonding, on the flexural strength and fracture properties of 3D printed beams. This study investigates three-layer heights (LH) (5, 10, and 15 mm) corresponding to 25, 50, and 75% of the nozzle diameter (ND) (20 mm). The results show that smaller layer heights are more beneficial for both unreinforced and fiber-reinforced 3D printed mortars, despite the longer printing times and increased number of interfaces. Furthermore, adding a small amount of steel fiber reinforcement mitigates the adverse effects of weak interfaces on bulk properties. On average, flexural strengths are 30-40% higher, and fracture toughness and crack tip opening displacement are almost 30% higher than plain mixtures. The study employs strain energy release rates, digital image correlation, and optical images/micrographs to explain crack propagation in layered 3D printed mortars under unnotched four-point and notched three-point bending.
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AN INTEGRATED APPROACH TO DIGITAL IMAGE CORRELATION APPLIED TO A NOVEL THREE ACTUATORS FRETTING FATIGUE RIG [Poster #22]

This poster showcases a novel three-actuator fretting fatigue rig that features a horizontal contact orientation. The machine is equipped to conduct tests under lubrication and enables independent control of all loads in terms of intensity and angle phase. To validate this new rig, we performed fretting fatigue tests on a Ti-6Al-4V alloy couple in a cylinder-plane configuration, instrumented with an integrated approach to digital image correlation.
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FRACTURE OF MULTI-PRINCIPAL ELEMENT ALLOYS [Poster #23]

Yield strength and fracture toughness are often mutually exclusive properties in metals and their alloys. The CrCoNi-based face-centered cubic (fcc) multi-principal element alloys (MPEAs) are known to possess extraordinary high fracture toughness that is enhanced at cryogenic temperatures; however, their relatively low yield strengths limit their engineering applications. This study investigates the role of sub-grain cellular structures in CrCoNi introduced by laser powder bed fusion (LPBF) that enhance its strength, with small compromise to the fracture toughness.
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FRACTURE PROPERTIES OF TETRAGRAPHENE UNDER MIXED MODE LOADING [Poster #24]

Tetragraphene (TG) is a quasi-2D semiconductor carbon allotrope composed of hexagonal and tetragonal rings and shows metallic or semiconducting behaviors. This study uses molecular dynamics (MD) simulations to understand fracture properties of triple-layered TG sheets with two different structures under mixed mode I and II loading using the Tersoff–Erhart potential. We investigate the effect of crack edge chirality, loading phase angle, and temperature on the crack propagation path and critical stress intensity factors.
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A FINITE ELEMENT METHOD FOR EVALUATING DISBONDS AND THEIR IMPACT ON SINGLE LAP JOINTS [Poster #25]

Adhesive technologies are widely employed in the aerospace and automobile industries due to its advantages over the conventional fasteners. However, the adhesive technologies come with its own shortcomings in bonding two materials together. One of the key challenges in using composites is the occurrence of disbonds. A disbond refers to the failure of an adhesive to fully cure or attach to the adherend surface, leading to a lack of stress transfer at the interface. Achieving a strong bond in such situations can be challenging because it’s difficult to spread the adhesive evenly over the surface. In this study, a numerical framework is considered to evaluate the quantitative and qualitative effect of disbonds on the single lap joints. Finite element technique showed that there was a reduction in the strength of the lap joints as different discontinuities were applied at the joint area.
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FRACTURE PROPERTIES OF MULTIDIMENSIONAL CARBON-BASED MATERIALS [Poster #16]

We perform steered molecular dynamics tensile studies [1] on carbon-based low dimensional materials including carbyne, cyclo[18]carbon, carbon nanotubes, and hybrid structures. We study the response of these materials to quantify the maximum stress, strain, and force required for fracture. We then use density functional theory to study the electron density distributions at different strains in low-dimensional materials to validate the molecular dynamics fracture predictions. This study predicts the fracture and mechanical properties of carbon-based low dimensional materials that will help with applications such as nanodevices and nanocomposites.

[1] Eaton, A. L., Fielder, M., and Nair, A. K., 2022, “Mechanical and thermal properties of carbon-based low-dimensional materials,” MRS Bulletin, 47, pp. 1001-1010.
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UNUSUAL STRESS SERRATIONS AND PLC BANDS IN HIGH MANGANESE AUSTENITIC FE-MN-C TWIP STEEL [Poster #17]

Deformation twinning and dynamic strain aging (DSA) are two major phenomena occurring in Fe-Mn-C twinning-induced plasticity (TWIP) steels. DSA is manifested with serrated plastic flow, with stress serrations appearing on stress-strain or stress-time curves. TWIP steels, especially Fe-Mn-C TWIP steels, show apparent serrated plastic flow. However, the stress serrations and associated Portevin-Le Chatelier (PLC) band behavior of such steels reported in several publications, especially at very low strain rates, are not consistent. This paper is to investigate the serrated plastic flow and the spatio-temporal behaviors of PLC bands in a Fe-Mn-C TWIP steel at very low strain rates, by means of in-situ tensile tests, in conjunction of digital image correlation (DIC) technique.
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