Optimization of a Novel Cruciform Marciniak Specimen Geometry Using Finite Element Method for Rapid Development of Forming Limit Curves

Abstract

Forming limit curve is one of the key measures of sheet formability used in industry. The process of generating this curve is long, costly and labour intensive. To date, there is no method in place to rapidly generate a forming limit curve. This research sought to apply a cruciform geometry to the commonly used in-plane conventional biaxial Marciniak forming test to obtain multiple strain paths from a single specimen and efficiently obtain a forming limit curve. The research involved simulating the cruciform Marciniak specimen geometries in Abaqus finite element (FE) code and revising specimen geometry iteratively to improve the range of strain paths and quality of generated forming limit curves. The FE optimization proved successful and has yielded two significant results; First, a specimen geometry has been arrived at that is both relatively simple and cost effective to manufacture and able to generate near complete FLCs. Second, an additional carrier blank, essential in the conventional Marciniak test, was determined to have no affect on the FLCs generated with the cruciform geometry. This blank has been eliminated allowing for further simplification of the cruciform Marciniak test. The optimized cruciform specimen was tested experimentally with three material conditions AA5754-O at room temperature, AA7075-O at room temperature and AA7075-O at 325°C. Strain paths were collected and FLCs were developed for each test condition and compared to those obtained from the FE models. The resulting breadth of strain paths showed good agreement between the model simulations and experiments. However, some variations in FLCs obtained from the experimental specimens and the FE models were observed. Explanations for the differences has been provided and path forward has been suggested.