Ductility Enhancement in Pulsed Uniaxial Tension of Stainless Steel 304

It was recently demonstrated that significant increases in formability in tube hydroforming (THF) can be achieved if the load is applied in a pulsed, as opposed to the monotonic, manner. The frequency of such loading is of the order of 1 Hz, and the benefits can be observed in the case of free inflation of a tube (i.e., without a die), as well. Hence the mechanism of such improvements is not related to enhanced tribological conditions between the tube and the die, as is the case in the ultrasonic THF. In an effort to understand if and how the pulsed application of the load affects the material ductility, specimens from stainless steel 304 were subjected to uniaxial tension, with the load applied in a pulsed manner. The results of these experiments were compared to the corresponding monotonic cases under the same strain rates. Significant increases in ductility can be noted between the two types of loadings. Subsequently, further experiments described in the paper revealed that the deformation-induced heating of the stainless steel is often responsible for this behavior. However, if the loading is rather slow, the ductility enhancement is still observed, but cannot be attributed to the specimen heating.