Mechanical characterization of materials for bulk forming using a drop weight testing machine

The main limitation of mechanical testing equipments is nowadays centred in the characterization of materials at medium loading rates. This is particularly important in bulk forming because strain rate can easily reach values within the aforesaid range. The aim of this article is twofold: (a) to present the development of a low-cost, flexible drop weight testing equipment that can easily and effectively replicate the kinematic behaviour of presses and hammers and (b) to provide a new level of understanding about the mechanical characterization of materials for bulk forming at medium rates of loading. Special emphasis is placed on the adequacy of test operating conditions to the functional characteristics of the presses and hammers where bulk forming takes place and to its influence on the flow stress. This is needed because non-proportional loading paths during bulk forming are found to have significant influence on material response in terms of flow stress. The quality of the flow curves that were experimentally determined is evaluated through its implementation in a finite-element computer program and assessment is performed by means of axisymmetric upset compression with friction. Results show that mechanical characterization of materials under test operating conditions that are similar to real bulk forming conditions is capable of meeting the increasing demand of accurate and reliable flow stress data for the benefit of those who apply numerical modelling of process design in daily practice.