New Hampshire Center for Multiscale Modeling and Manufacturing of Biomaterials (NH Bio-MADE)

Orthopedic implants are often required to replace or support damaged skeletal structures. For this application and many others, an individualized component, determined, through, for example, an MRI scan, is required for repairs. Thus, the ability to rapid prototype biomedical implants for mass customization is desirable. Furthermore, the final product properties and characteristics are critical to, for example, reduce the gauge thickness and, thus, the weight of the component. Computational modeling is necessary to optimize the final product. Cost is important so creating the component from stock sheet metal, as opposed to from metal powder, machining from a large workpiece, or casting the component, is advantageous. Furthermore, by work hardening of the material through forming, increased strength is achieved.   

With respect to sheet metal, characterizing the material behavior thoroughly is essential to produce an optimal final product. Tests to characterize the material, e.g. uniaxial tension/compression (Marciniak tests), are typically performed under proportional loading and follow a linear deformation path. Such testing can produce a failure criterion for the material, e.g., a forming limit curve (FLC), which represents the planar strain values in sheet metal above which the deformation localizes into a diffuse neck. The strain path dependence of FLCs has been documented by a number of workers. During forming, the deformation path can be intentionally manipulated to improve the forming process. For example, if a sharp corner feature is desired in a component, a generous radius is formed first, then a restrike operation is performed in the corner with the desired sharp punch tool. The variation in the deformation path allows the final strain values to be below the FLC and, thus, avoids failure. While such industrial applications are well known, the phenomenon which allows this increased formability during non-linear deformation path loading is poorly understood. This project will test the hypothesis that the increased formability exhibited by metallic alloys during non-linear deformation paths results from dislocation mechanisms, i.e., recombination and change of glide systems upon strain-path change, which prevents strain localizations due to pile-ups at grain or phase boundaries or other interfaces in the microstructure. Increment forming of stainless steel and titanium parts is being performed at the University of New Hampshire alongside numerical modeling of the process, while analysis of the dislocation structures using transmission electron microscopy and scanning electron microscopy is being performed at Dartmouth.

Funded by the U.S. National Science Foundation EPSCOR program.

Faculty contact: Ian Baker