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1. 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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2. 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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3. Double Interpolation to Achieve Linear Strain Path for AISI 1008 Steel Cruciform Specimen

   https://doi.org/10.1115/MSEC2022-85595  

   Hoffman, Jordan ; Ha, Jinjin ; Kinsey, Brad ; Banerjee, Dilip ; Iadicola, Mark A. ( June 2022 , ASME 2022 17th International Manufacturing Science and Engineering Conference: Volume 2: Manufacturing Processes; Manufacturing Systems)

   The automotive industry relies heavily on sheet metal forming processes for many components. Material data solely from uniaxial testing is insufficient to fully define the material behavior of the complex plastic deformation during numerical simulations of the forming processes. In-plane biaxial testing using a cruciform type specimen is a more comprehensive representation than the traditional uniaxial testing alone. Wide ranging biaxial stress states can be imposed by applying different loading conditions on each cruciform axis. However, this can create a challenge to achieve desired deformation paths due to the non-linear relationship between the control parameter, e.g., displacement, and the output of interest, e.g., strain path. In this paper, an interpolation method to develop the displacement control that produces a linear strain path with a desired strain ratio is revisited and expanded upon from the authors’ previous work [1,2]. In the first iteration, linear biaxial displacements were applied to the specimen and the corresponding strain paths were obtained from the numerical simulations. The non-linear strain paths, due to geometry effects of the specimen, were used to reverse engineer a new displacement path that results in a linear strain path. Interpolation is revisited to show increased success with a second iteration. Analysis of the simulation results shows that linear strain paths of a given model can be determined and improved by successive iterations of interpolating the strain data from adjacent deformation paths.

    

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4. Manipulating martensite transformation of SS304L during double-sided incremental forming by varying temperature and deformation path

   https://doi.org/10.1016/j.cirp.2023.04.015  

   Darzi, Shayan ; Adams, Matt D. ; Roth, John T. ; Kinsey, Brad L. ; Ha, Jinjin ( January 2023 , CIRP Annals)

   Double-sided incremental forming (DSIF) is a die-less sheet metal forming process capable of fabricating complex parts. The flexibility of DSIF can be used for in-situ mechanical properties alteration, e.g., by controlling deformation-induced martensite transformation of austenitic stainless steels. In this paper, SS304L is deformed using DSIF at three different cooling conditions and two different tool paths to affect the martensite transformation. Additionally, finite element analyses were used to understandthe effect of tool paths on springback and plastic strain. Implementing a reforming tool path at the lowest achievable temperature resulted in a martensite volume fraction as high as 95%. 

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5. Evolution of Force Chains Explains the Onset of Strain Stiffening in Fiber Networks

   https://doi.org/10.1115/1.4055586  

   Sarkar, Mainak ; Notbohm, Jacob ( November 2022 , Journal of Applied Mechanics)

   Abstract Fiber networks are the primary structural components of many biological structures, including the cell cytoskeleton and the extracellular matrix. These materials exhibit global nonlinearities, such as stiffening in extension and shear, during which the fibers bend and align with the direction of applied loading. Precise details of deformations at the scale of the fibers during strain stiffening are still lacking, however, as prior work has studied fiber alignment primarily from a qualitative perspective, which leaves incomplete the understanding of how the local microstructural evolution leads to the global mechanical behavior. To fill this gap, we studied how axial forces are transmitted inside the fiber network along paths called force chains, which continuously evolve during the course of deformation. We performed numerical simulations on two-dimensional networks of random fibers under uniaxial extension and shear, modeling the fibers using beam elements in finite element software. To quantify the force chains, we identified all chains of connected fibers for which the axial force was larger than a preset threshold and computed the total length of all such chains. To study the evolution of force chains during loading, we computed the derivative of the total length of all force chains with respect to the applied engineering strain. Results showed that the highest rate of evolution of force chains coincided with the global critical strain for strain stiffening of the fiber network. Therefore, force chains are an important factor connecting understanding of the local kinematics and force transmission to the macroscale stiffness of the fiber network. 

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