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1. Experimental Characterization of Plastic Hinge Behavior from Flexure and Axial Effects

   https://doi.org/10.21838/uhpc.16690  

   Sthapit, Ronit; Bandelt, Matthew (June 2023, International Interactive Symposium on UHPC)

   Aaleti, Sriram; Okumus, Pinar (Ed.)

   The unique mechanical properties of ultra-high performance concrete (UHPC) causes changes in failure modes and ductility in reinforced components. Numerous experiments have shown these materials, and others with similar ductile characteristics in tension, can improve the damage tolerance, strength, and ductility of members subjected to large deformations from seismic loading and similar extreme conditions. The use of these materials, however, has not been systematically studied to understand their application at a system-level performance and design procedures have been complicated due to their unconventional failure mechanism. This project aims to fill this gap by testing a targeted set of components subjected to combined effects of axial loads and bending with variations in axial load ratio and longitudinal reinforcement ratio. Additional experiments are planned to compare performance across other ductile concrete materials with variations in mechanical properties. The experimental results including load-deformation, reinforcement strain, concrete surface strain will be used to understand the parameters that have the highest influence on plastic hinge length and moment-rotation response which can ultimately help to validate analytical models against experiments based on these key parameters. 

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2. Micromechanics based discrete damage model with multiple non-smooth yield surfaces: Theoretical formulation, numerical implementation and engineering applications

   https://doi.org/10.1177/1056789517695872  

   Jin, Wencheng; Arson, Chloé (May 2018, International Journal of Damage Mechanics)

   null (Ed.)

   The discrete damage model presented in this paper accounts for 42 non-interacting crack microplanes directions. At the scale of the representative volume element, the free enthalpy is the sum of the elastic energy stored in the non-damaged bulk material and in the displacement jumps at crack faces. Closed cracks propagate in the pure mode II, whereas open cracks propagate in the mixed mode (I/II). The elastic domain is at the intersection of the yield surfaces of the activated crack families, and thus describes a non-smooth surface. In order to solve for the 42 crack densities, a Closest Point Projection algorithm is adopted locally. The representative volume element inelastic strain is calculated iteratively using the Newton–Raphson method. The proposed damage model was rigorously calibrated for both compressive and tensile stress paths. Finite element method simulations of triaxial compression tests showed that the transition between brittle and ductile behavior at increasing confining pressure can be captured. The cracks’ density, orientation, and location predicted in the simulations are in agreement with experimental observations made during compression and tension tests, and accurately show the difference between tensile and compressive strength. Plane stress tension tests simulated for a fiber-reinforced brittle material also demonstrated that the model can be used to interpret crack patterns, design composite structures and recommend reparation techniques for structural elements subjected to multiple damage mechanisms. 

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3. Artificial neural network enhanced plasticity modelling and ductile fracture characterization of grade-1 commercial pure titanium

   https://doi.org/10.1016/j.ijplas.2024.104044  

   Ebrahim, Abrar Salam; Zhang, Qi; Ha, Jinjin (June 2024, International Journal of Plasticity)

   This study primarily aims to develop a robust modelling approach to capture complex material behavior of CP-Ti, appeared by high anisotropy, differential hardening due to anisotropy evolution, and flow behavior sensitive to strain rate and temperature, using artificial neural networks (ANNs). Plasticity is characterized by uniaxial tension and in-plane biaxial tension tests at temperatures of 0°C and 20°C with strain rates of 0.001 /s and 0.01 /s, and the results are used to calibrate the non-quadratic anisotropic Yld2000-3d yield function with respect to the plastic work. In order to predict the intricate plastic deformation with the temperature and strain rate effects, two distinct ANN models are developed; one is to capture the strain hardening behavior and the other to predict the anisotropic parameters in the chosen yield function. The developed ANN models predict an unseen dataset well, which is intermediate testing conditions at a temperature of 10°C and strain rate of 0.005 /s. The ANN models, being computationally stable and adhering to conventional constitutive equations, are implemented into a user material subroutine for the ductile fracture characterization of CP-Ti sheet using the hybrid experimental-numerical analysis. The favorable agreement between experimental data and numerical predictions, particularly using the ANN models with evolving anisotropic material parameters for the Yld2000-3d yield function, underscores the significance of differential hardening effect on the ductile fracture behavior and highlights the capabilities of ANN models to capture the complex plastic behavior of CP-Ti. The key parameters including stress triaxiality, Lode angle parameter, and equivalent plastic strain at the fracture location are extracted from the simulations, enabling the calibration of ductile fracture models, namely Johnson-Cook, Hosford-Coulomb, and Lou-2014, and construction of fracture envelopes. 

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4. Ductile fracture model describing the impact of internal pores: Model development and validation for additively manufactured Ti-6Al-4V

   https://doi.org/10.1016/j.addma.2025.104722  

   Furton, Erik T; Beese, Allison M (March 2025, Additive Manufacturing)

   Additively manufactured metals often contain pores, which limit the strength and ductility of resulting components. In this study, a ductile fracture model was developed to describe the effect of pore size, in terms of absolute and relative metrics, on fracture strain under uniaxial tension. The model approximates lack of fusion (LoF) pores as penny-shaped cracks, and damage accumulation was based on the J-integral and secondary Q parameter. The model was calibrated with Ti-6Al-4V samples with intentionally introduced pores fabricated by laser powder bed fusion (PBF-LB) additive manufacturing (AM) in as-built and heat-treated conditions. The model captures the experimentally observed size effect, where for a given pore area fraction, larger samples fracture at smaller strains. By identifying the critical pore size for a single, isolated pore for either load or displacement-controlled applications, the model developed in this study is a crucial step to developing a comprehensive fracture model for establishing confidence in the structural capability of pore-containing additively manufactured components. 

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5. Viscoplastic modeling of elastic and creep deformation of fractured Berea sandstone.

   Talukdar, M.; Sone, H.; Griffith, W. A. (June 2021, 55th US Rock Mechanics/Geomechanics Symposium)

   null (Ed.)

   We observed and modeled the elastic, inelastic and time-dependent viscous properties of damaged Berea Sandstone samples to investigate the impact of damage on the rheological properties of rocks. Cylindrical Berea Sandstone plugs were prepared both parallel and perpendicular to bedding. We impacted the samples with Split Hopkinson Pressure Bar to pervasively fracture the specimens at different strain rates. Longitudinal mode-I fractures are dominant in specimens impacted at relatively low strain rates (about 130 /s), whereas shear fractures also form in specimens deformed at high strain rates (up to 250 /s). The damaged rocks were subjected to multiple steps of differential stress loading and hold stages under 15 MPa confining pressure. A key observation is that higher damaged specimens showed greater axial and volumetric creep strain deformation during loading and hold stages. Poisson ratio also increase with increasing damage. We modeled the volumetric strain of the sandstone specimens using a Perzyna viscoplasticity law that employs the Modified Cam Clay model as the yield criterion (Haghighat et al. 2020). We deduced that fractured rocks undergo substantial bulk time-dependent deformation due to volumetric compaction and fracture closure. Damage increase results in decrease of the effective viscosity of the material. 

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