Poster Session: Tuesday, June 13, 6-7PM

Maraging steels are a class of precipitation hardened steels wherein different micro-mechanisms of deformation such as planar slip, interaction with coherent/incoherent precipitates, and reverted austenite controls the overall mechanical behavior of the material. High Pressure Torsion (HPT) processing adds to this complexity by pumping in a large density of dislocations that form sub-grain boundaries and cellular structures, leading to changes in precipitate morphology and stability upon ageing. This results in a drastic change in the deformation accommodation mechanism. While these steels are known to display high fracture toughness in the hot rolled condition (hereafter referred to as: as-received), this study reports for the first time their KIC values after deformation processing, including the effect of grain size refinement, dislocation density and texture induced anisotropy. To accomplish these measurements in the small volume discs that are produced by HPT, small-scale clamped beam bend geometries were utilized for the first time. KIC measurements were carried out for both cases in the unaged, peak-aged and over-aged conditions. DIC strain mapping has been made use of to quantify the crack tip opening displacement and process zone evolution ahead of the crack tip.
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In this study, static fracture experiments under mode-I and mixed mode loading, and fatigue testing under mode-I loading were carried out on double cantilever beam (DCB) specimens, and subsequent healing of the delamination was investigated. Thermoplastic healants dispersed in a thermoset CFRP composite were used to perform the healing, triggered through brief heating in an oven. It was observed from the test results that delaminations can be healed efficiently and the healing was found to be repeatable. As a result of healing, significant crack closure was observed and the fatigue crack growth rate was considerably reduced. These findings can be helpful in extending the service life of laminated composites.
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A wavelet-enriched adaptive hierarchical, coupled crystal plasticity - phase-field finite element model is developed in this work to simulate crack propagation in complex polycrystalline microstructures. The model accommodates initial material anisotropy and crack tension-compression asymmetry through orthogonal decomposition of stored elastic strain energy into tensile and compressive counterparts. The crack evolution is driven by stored elastic and defect energies, resulting from slip and hardening of crystallographic slips systems. A FE model is used to simulate the fracture process in a statistically equivalent representative volume element reconstructed from electron backscattered diffraction scans of experimental microstructures. Multiple numerical simulations with the model exhibits microstructurally sensitive crack propagation characteristics.
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Most studies on the fracture of bulk or ribbon superconductors are based on superconducting critical state models that do not consider temperature changes. Most of the research objects of the thermal-mechanical-electric-magnetic model only focus on the distribution of magnetic field current and stress, while the thermal-mechanical-electric-magnetic model with cracks is rarely involved. The research in this paper will be based on a generalized critical state model that considers both temperature and magnetic field effects to investigate the effects of thermal and magnetic effects on cracks in superconducting structures.
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A complete constitutive theory is presented to enable ductile fracture simulations under complex loadings that may involve shear-dominated stress states or even negative triaxialities. The yield criteria accounting for various forms of anisotropy is supplemented with evolution equations to complete the constitutive theory formulation. State-of-the-art ductile fracture theory can only be fully exploited when a robust implementation enabling structural computations is available. This work set out to address the latter within a multisurface framework. A complete constitutive theory of plastic porous materials incorporating homogeneous (HY) and multiple (n) unhomogeneous yieldings (UY), named HUNnY is developed. The capabilities of the new formulation and its implementation are demonstrated by simulating fracture in tension, fracture in shear of top hat specimen and fracture by shear banding. The predictive theory promises to completely change our understanding of some of these most challenging problems that remained elusive for decades.
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Understanding the remaining life of a component is critical to maintaining safe operation and is necessary for budgeting repairs. This paper uses Finite Element Analysis to predict the performance of a steam header under realistic loading scenarios, comparing the difference between life expectancies of service-aged material to that of virgin material.
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This poster presentation summarizes the results from several projects in this field conducted at the TU Darmstadt to identify and describe the various influences on crack growth under thermo-mechanical fatigue (TMF) loading. The activation of damage mechanisms under TMF loading and interactions between them are dependent of the temperature cycle and the respective load phasing. Depending on the type of loading (force- vs. strain-control), contrary influences of the phase shift on the TMF crack growth rates are found. To describe crack growth under creep-fatigue and TMF conditions, the linear accumulation model ‘O.C.F.’ was developed - based on the contributions of fatigue, creep and oxidation to crack growth per load cycle. This model is capable to reproduce the effects of time-dependent damage, different load ratios and TMF phase shifts, as well as component geometries. The model’s linear formulation allows assessing the dominant driver of crack growth at each stage of an experiment. These predictions are compared with fractographic investigations and in-situ observations of crack paths to identify the mechanisms of crack growth under different TMF load cycle forms.
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Existing ductile failure models such as the Gurson-Tvergaard-Needleman (GTN) model as well as more recent physics-based models (for instance, the Benzerga-Leblond coalescence model from 2014) were all derived for perfectly plastic porous materials using classical limit analysis, with plastic flow in the matrix being described by J2 flow theory. When extended heuristically to hardenable materials, these models do not account for the heterogeneity of plastic strain in the matrix, and are unable to capture the effect of hardening on the evolution of porosity, the primary damage variable.

This work uses “sequential limit analysis” (SLA) to first derive a hardening-sensitive void coalescence criterion for a cylindrical cell containing a coaxial cylindrical void of finite height, by discretizing the intervoid ligament into a finite number of shells in each of which the quantities characterizing isotropic hardening are considered to be homogeneous. Next, this new criterion is combined with a recently formulated hardening-sensitive void growth criterion (also derived using SLA) to obtain a hybrid model of ductile failure. The new constitutive formulation’s ability to remedy the two aforementioned shortcomings of existing models is examined, and a set of finite-element micromechanical unit cell calculations is used to further assess the model's predictive capabilities.
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An analysis of the fatigue performance of 316L stainless steel bar drilled and tapped is performed. The effect of flow drilling and flow tapping on the material microstructure, microhardness and fatigue life is compared to the characteristics of conventional cutting processes.
The hardness recorded at the surface of flow formed holes is 62% higher than that of the raw material. In addition, grains are refined and plastically deformed by the flow processes.
Four-points bending fatigue tests were performed at 3 stress amplitudes and with a stress ratio of 0.1. The results revealed no significant differences in fatigue life for tests performed bending moment is equal to 75% and 60% of the yield bending moment. Nevertheless, when the maximum bending moment applied was limited to 50% of the yield bending moment, the specimens containing holes manufactured by the cutting endure more cycles. Fractographic observations revealed, for both specimens, that the failure initiated from the thread beneath the surface of maximum tensile stress. On the fracture surfaces of flow processed specimens, cracks initiated from the discontinuities observed at the peak of threads. In addition, secondary cracks are observed at the thread roots where to material is hard and the grains are refined.
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In applications requiring high velocity interactions of energetic materials, the shock response of the crystal-binder interface is of great importance. We demonstrate a technique for capturing the high localized deformation of the crystal-binder interface using time resolved Raman spectroscopy at nanosecond intervals. A bi-crystal interface of polydimethylsiloxane (PDMS) sandwiched between sucrose crystals is used in the method, with the sample as a whole put on a glass surface and impacted from the opposite end. Aluminum cylindrical flyers with thicknesses of 25-50 um and diameters of 1 mm were accelerated utilizing the Laser Induced Projectile Impact Test (LIPIT) to create high velocity shock compression loads. The velocity of the projectiles was determined using heterodyne photon doppler velocimetry (het-PDV) and ranged from 0.5 to 1.5 km/s. Full field measurements of the 532nm Raman spectroscopic response were acquired using an in-house designed laser array configuration with 27 discrete laser subsets. The pressure and temperature distributions over the interface were calculated using the pre-calibrated peak shifts of the sucrose CH and CH2 bonds. The highly localized deformation generated by pressure and temperature rise as the shock front travels across the interface were measured in-situ by the time resolved Raman spectroscopic response. The results showed
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Creep in future long-term space technology materials is a critical concern due to the duration of potential missions to Mars and beyond. Structural and skin components in long-term mission spacecraft will undergo creep deformation and eventual failure if not designed to be sufficiently creep resistant. The microstructural deformation mechanisms that control the creep behavior must be understood to intelligently inform the design of new creep resistant alloys and enhance those already in service. Using lightweight single phase β Ti alloys, an analysis tool was developed to measure grain boundary sliding (GBS) and intragranular slip in-situ via a Heaviside function-based algorithm. The data needed for the analysis tool includes an electron backscattered diffraction generated microstructural map and high-resolution digital image correlation (HRDIC) strain fields. This testing technique advances the state of the art by facilitating in-situ measurement of these microstructural deformation mechanisms without the need to interrupt creep testing and introduce unwanted thermic cyclic effects. Proof-of-concept experiments utilizing this analysis tool on a single phase β Ti alloy in room temperature creep rapidly identified the dominant deformation mechanism to be intragranular slip and glide creep without the need for destructive and expensive post-mortem testing.
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Pre-Northridge moment frames with PJP Welded Column Splices (WCS) are highly vulnerable to brittle fracture much before the connection develops the strength of the upper connected column due to the inherent crack-like flaw (unfused region of the weld) and the low toughness of the weld material. Given that the consequences of fracture are catastrophic and that retrofitting these splices can be highly disruptive to building operations, accurately estimating their fracture risk is of great importance. To achieve this, a probabilistic quantification of splice fracture is necessary, along with tools that simulate splice fracture and post-fracture response in a global frame assessment framework.

A framework to probabilistically assess the fracture strength of these splices is presented which addresses shortcomings of previous research and performance assessment guidance that do not consider key mechanistic or statistical effects. A new element model (in OpenSees), which is informed by the fracture mechanics-based estimates of splice strength and existing material models in OpenSees, is developed to simulate the splice fracture and post-fracture response. Application of the new splice element in assessment of a 20-story building to scaled ground motions is demonstrated.
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Ductile fracture is affected by the state of stress, which is commonly described by two parameters, stress triaxiality and Lode parameter. While the effects of triaxiality are well known, the effect of the Lode parameter are uncertain. This uncertainty results in particular from the difficulty to vary the Lode parameter at controlled triaxiality. Recent experiments by the authors suggest that the Lode parameter does indeed affect ductile fracture to some extent. The aim of this work is to analyze the mechanisms behind these apparent effects of the Lode parameter.To accomplish this, an advanced multi-surface porous-plasticity model that accounts for both homogeneous and inhomogeneous yielding is used in an Abaqus Umat to simulate proportional loading of a single integration point. Within this modeling framework, the effect of Lode parameter is inherently captured through the competition between the two main modes of inhomogeneous yielding: internal necking and internal shearing of the intervoid ligament. The ability of this constitutive formulation to capture the effects of the Lode parameter that were observed in experiments is examined.
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High cycle fatigue (HCF) in composite structures leads to damage accumulation and associated stiffness
degradation, which are challenging to quantify. This work uses a medium wave infrared to monitor selfheating in chopped carbon fiber/acrylonitrile butadiene styrene specimens subjected to tension-tension
fatigue loading. An innovative rapid testing protocol that correlates the generated full-field temperature
maps and stiffness degradation data has been developed providing a comprehensive understanding of
material behavior under cyclic loading. Results contribute to the fundamental understanding of HCF in
composite materials and develop more accurate predictive models for fatigue life. Rapid testing has allowed
correlating process parameters with the microstructure and structural integrity of additively manufactured
(AM) composites.
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This study investigated the accumulation of damage in periodic, engineered disbond arrays and its effect on the shear strength and failure mode of single lap joints. The impact of surface contamination on shear strength was also analyzed. Experimental results showed that surface contamination had a significant negative impact on shear strength, with a reduction of up to 98% in specimens with 100% contamination. The use of a disbond stripe resulted in a slight reduction of only 3.89% in shear strength. However, no progressive accumulation of damage in bonds was observed in the current set of experiments. Further investigation is required to examine the relationship between crack mode and design configuration. This study highlights the importance of addressing these factors in the design and analysis of bonded structures to ensure their lifetime and durability.
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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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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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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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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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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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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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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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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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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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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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