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1. Prediction of yield surface of single crystal copper from discrete dislocation dynamics and geometric learning

   https://doi.org/10.1016/j.jmps.2024.105577  

   Jian, Wu-Rong; Xiao, Mian; Sun, WaiChing; Cai, Wei (May 2024, Journal of the Mechanics and Physics of Solids)

   Deshpande, Vikram (Ed.)

   The yield surface of a material is a criterion at which macroscopic plastic deformation begins. For crystalline solids, plastic deformation occurs through the motion of dislocations, which can be captured by discrete dislocation dynamics (DDD) simulations. In this paper, we predict the yield surfaces and strain-hardening behaviors using DDD simulations and a geometric manifold learning approach. The yield surfaces in the three-dimensional space of plane stress are constructed for single-crystal copper subjected to uniaxial loading along the [100] and [110] directions, respectively. With increasing plastic deformation under loading, the yield surface expands nearly uniformly in all directions, corresponding to isotropic hardening. In contrast, under [110] loading, latent hardening is observed, where the yield surface remains nearly unchanged in the orientations in the vicinity of the loading direction itself but expands in other directions, resulting in an asymmetric shape. This difference in hardening behaviors is attributed to the different dislocation multiplication behaviors on various slip systems under the two loading conditions. 

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2. Fabric anisotropy effects on static liquefaction under constant shear drained loading

   https://doi.org/10.1016/j.compgeo.2023.105724  

   Bokkisa, Srinivas Vivek; Macedo, Jorge; Petalas, Alexandros L (November 2023, Computers and Geotechnics)

   Several case history failures of slope systems have highlighted that the instability onset in loose materials can be triggered under prevailed drained conditions and stress paths that can be represented by constant shear drained (CSD) loading. This study uses the anisotropic critical state theory (ACST) to assess the effect of fabric anisotropy and loading characteristics (e.g., Lode angle and principal stress direction) on the instability onset under CSD stress paths, comparing our numerical-based observations with available experimental information. Towards this end, the ACST-based SANISAND-F model’s performance under CSD stress paths is also assessed. In addition, multiaxial conditions are incorporated through the estimation of instability surfaces. The numerical simulations are useful in explaining that the instability onset under CSD loading is dictated by a trade-off of volumetric strain components. Moreover, the results show an important effect of fabric anisotropy on the instability stress ratio (𝜂𝑓 ). For conditions representative of common experimental setups, 𝜂𝑓 decreases with the increase of the Lode angle and the major principal stress inclination, and 𝜂𝑓 increases with the increase of initial fabric intensity, consistent with available experimental evidence. However, these trends can change based on the interaction between the Lode angle and loading/fabric directions; hence, departing from typical experimental observations. Finally, we discuss the potential of a simplified approach to estimate 𝜂𝑓 analytically, including fabric effects. 

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3. A novel three-dimensional notch fracture criterion via effective critical distances

   https://doi.org/10.1016/j.ijmecsci.2024.109149  

   Mirsayar, Mirmilad; Shahbazian, Behnam (June 2024, International Journal of Mechanical Sciences)

   Many well-known fracture criteria rely on a mode-independent parameter measured under pure mode I loading conditions, called the critical distance, which is traditionally considered a material property representing the size of the fracture processing zone. Recent studies have unveiled the potential for significantly increased accuracy in fracture criteria by utilizing a mode-dependent critical distance in calculations. In response to this revelation, the concept of effective critical distance (ECD) was recently introduced and successfully examined in cracked components under in-plane and out-of-plane loading conditions, both theoretically and experimentally. In this work, for the first time, the concept of ECD is introduced for V- and U-shaped notches to form a new three-dimensional notch fracture criterion based on the maximum principal strain (MPSN). The fracture angle and the onset of fracture predicted by the proposed criterion are theoretically compared to other existing criteria, and experimentally, to the test data presented in the literature. It is shown that the developed criterion can more accurately predict the mixed-mode I/II/III fracture behavior of V- and U-notched components which accentuates the profound significance of embracing the ECD concept in constructing three-dimensional fracture criteria. 

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4. Experimental Investigation and Plasticity Modeling of SS316 L Microtubes Under Varying Deformation Paths

   https://doi.org/10.1115/1.4049364  

   Mamros, Elizabeth M.; Ha, Jinjin; Korkolis, Yannis P.; Kinsey, Brad L. (December 2020, Journal of Micro and Nano-Manufacturing)

   null (Ed.)

   Abstract In this paper, results for SS316 L microtube experiments under combined inflation and axial loading for single and multiloading segment deformation paths are presented along with a plasticity model to predict the associated stress and strain paths. The microtube inflation/tension machine, utilized for these experiments, creates biaxial stress states by applying axial tension or compression and internal pressure simultaneously. Two types of loading paths are considered in this paper, proportional (where a single loading path with a given axial:hoop stress ratio is followed) and corner (where an initial pure loading segment, i.e., axial or hoop, is followed by a secondary loading segment in the transverse direction, i.e., either hoop or axial, respectively). The experiments are designed to produce the same final strain state under different deformation paths, resulting in different final stress states. This difference in stress state can affect the material properties of the final part, which can be varied for the intended application, e.g., biomedical hardware, while maintaining the desired geometry. The experiments are replicated in a reasonable way by a material model that combines the Hill 1948 anisotropic yield function and the Hockett–Sherby hardening law. Discussion of the grain size effects during microforming impacting the ability to achieve consistent deformation path results is included. 

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5. Experimental Implementation of SS 316L Cruciform Testing to Achieve Various Deformation Paths

   https://doi.org/10.1007/978-3-030-75381-8_166  

   Mamros E.M., Mayer S.M. (July 2021, Forming the Future. The Minerals, Metals & Materials Series)

   Daehn G., Cao J. (Ed.)

   By following varying deformation paths, e.g., a linear path to equibiaxial loading versus a bilinear path of uniaxial loading followed by biaxial loading, the same final strain state can be achieved. However, the stress state that the material is subjected to is considerably different due to the varying deformation. This is of interest in a growing field of stress superposition to improve formability and manipulate final part properties in metal forming applications. One potential application is forming patient-specific, trauma fixation hardware with differing strength and weight reduction requirements in various regions. In this paper, experiments were performed on a custom fabricated cruciform machine with the goal of subjecting stainless steel 316L to various deformation paths. A novel cruciform specimen geometry was designed in collaboration with the US National Institute of Standards and Technology to achieve large strain values in the gauge region. Digital Image Correlation was utilized to measure surface strain fields in real time. 

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