Metal ductility evaluation by flattening test: The geometry dependence

Highlights: • The processes of flattening test with platens and indenters by using the Gurson-Tvergaard-Needleman damage model were studied systematically. • Two laws of the failure compressible displacement versus tube length curve were revealed. • The appropriate dimension requirements for effective evaluation of metal ductility with flattening tests were proposed. -- Abstract: The reliability of pipes, tubes and liquid tanks (cylinders) is of paramount significance to our life. The typical way to examine the ability of those structures to undergo plastic deformation is to apply a flattening test. In this paper, we used the Gurson-Tvergaard-Needleman damage model and finite element simulations to capture the flattening of a tube made of typical pipe materials. We demonstrated how the tube thickness, radius, and length would affect the critical displacement where tube failure initiates. For flattening with platens, the failure displacement first increases with tube length and reaches a peak value, and it then decreases and converges to a constant value for a particular geometry. The failure initiates at the two edges of short tubes, but shifts to the center in intermediate tubes where the failure displacement maximizes. Failure then always starts from the middle in even longer tubes. In contrast, flattening with indenters exhibits two peaks in the compressible displacement vs. tube length curve. In the end, we proposed effective experimental strategies to obtain the intrinsic ability of tubes under plastic deformation. The results reported here could be employed to characterize the mechanical properties of materials for pipes, tubes and tanks, and they could also be applied to guide the engineering design of such structures.