Sheet metal forming is an important manufacturing process that allows for complex geometries to be fabricated from a simple sheet of material. It has many applications in the automotive, aerospace, appliance, and biomedical industries, including the fabrication of biomedical hardware such as trauma fixation plates to secure bones during the healing process. The forming process causes the sheet metal to experience high levels of stress and strain which leads to significant changes in the material. Understanding the material changes that occur during and as a result of sheet metal forming operations will allow for optimized, efficient forming methods that can produce stronger, yet lighter weight, parts than previously possible. To reach this level of understanding, investigation of specimens subjected to various linear and non-linear deformation paths is required.

A cruciform specimen, which has the shape of a plus sign with unique features to prevent stress concentrations in the corner areas, was designed in collaboration with the US National Institute of Standards and Technology (NIST) and tested in a biaxial loading frame at the John Olson Advanced Manufacturing Center at the University of New Hampshire. This specimen contains a central “pocket” region with reduced thickness where the stress concentrates, enabling a larger amount of plastic deformation and therefore martensitic transformation for austenitic stainless steels (SS), prior to fracture. The purpose of this testing was to characterize the material properties of SS316L, which has a thickness of 1.2 mm, when it undergoes various deformation paths and to understand the austenite to martensite transformation that occurs.

A model of this specimen was created using Abaqus 2019 finite element analysis software to predict the stress and strain contours during deformation. This model was simulated with six different displacement ratios in the perpendicular arms along the x- and y-axes to subject the cross section to various biaxial stress states. The x:y displacement ratios were 4:4, 3:4, 2:4, 1:4, and 0:4 plus an additional condition without a constraint in the x-direction, i.e., a uniaxial tension test along the y-axis.

Initial results show that when strain values along the x- and y-axes are extracted from the surface element in the center of the cruciform geometry and plotted against each other, the resulting strain paths are approximately linear for each of the different loading paths. Some of the strain paths develop non-linearity towards the end of the simulation. Stress paths were also plotted at the central element using a similar process. The stress paths are non-linear, with the exception of the 4:4 stress path, which represents equibiaxial tension along the x- and y-axes.

To understand the stress and strain distributions throughout the pocket area, stress and strain values at each element along the x-and y-axes of the pocket, as well as the 45-degree diagonal, were plotted at different levels of deformation for each of the loading paths. This provides an evaluation of where the highest stress and strain values are located on the geometry and how these values vary at different locations in the pocket.

The results from these simulations were compared with experimental data using the physical specimen at the Olson Center in order to assess the accuracy of the simulations. Non-linear deformation paths will also be investigated, where different paths are taken to achieve the same final strain state. Since the austenite to martensite transformation is path dependent, further research will allow for the optimization of loading paths for achieving different levels of transformation in different areas of a component. This is critical for varying the strength and weight to meet the heterogeneous requirements for patient-specific, biomedical trauma fixation hardware.


Finite Element Analysis of Cruciform Testing of SS316l Sheet Metal With Non-Linear Deformation Paths

Division / Sponsors: 
Aerospace, Bioengineering, Applied Mechanics, Design Engineering Division, Advanced Energy Systems, Manufacturing Engineering Division, Noise Control and Acoustics, Nondestructive Evaluation, Diagnosis, and Prognosis (NDPD), Nuclear Engineering Division, Process Industries Division, Fluids Engineering, Heat Transfer, Materials, Microelectromechanical Systems, Safety Engineering and Risk Analysis, Computers & Information in Engineering


Paper Proceedings: https://asmedigitalcollection.asme.org/IMECE/IMECE2020/volume/84478

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