Abstract
This paper is focused on the utilisation of double edge notched tension, staggered and shear tests to determine fracture toughness and the formability limits by fracture in principal strain space. The experiments were performed in test specimens with different geometries and ligament angles, and the influence of strain hardening was taken into consideration by selecting two materials (aluminium AA1050-H111 and pure copper), with very different strain hardening exponents. Results are plotted in principal strain space, and the discussion is focused on the link between formability limits, fracture toughness and macroscopic fractography characteristics of the specimens that fail by mode I, mode II or mixed-mode.
Highlights
Failure in sheet metal forming can occur by necking, fracture or wrinkling
Definition of the fracture forming limits by tension and in-plane shear in principal strain space must be accompanied by macroscopic scanning electron microscope (SEM) observations of the fracture surfaces, in order to validate the crack opening modes of the different tests that are used to obtain the fracture strains
The same connection is needed to associate the determination of fracture toughness to crack opening by tension, in-plane shear or mixed-mode, consisting of tension and in-plane shear
Summary
Failure in sheet metal forming can occur by necking, fracture or wrinkling. The fracture forming limit line (FFL) defines failure by fracture in mode I, and is plotted as a straight line falling from left to right with a slope of ‘−1’ in principal strain space. The physics of the FFL is the condition of maximum admissible thickness reduction at fracture [4]. The shear fracture forming limit line (SFFL) defines failure by fracture in mode II, and is plotted as a straight line rising from left to right with a slope of ‘+1’ in principal strain space. The physics of the SFFL is the condition of maximum in-plane shear work per unit of volume, as it was recently shown by Isik et al [5].
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