A compact bending device for in situ three-point bending tests with a scanning electron microscope
Material-specific forming limit curves are widely used for the prediction of cracks that may occur during cold-forming processes with sheet metal. The reason for the initiation of these cracks is the localization of material flow that leads to local necking. In bending operations, tensile strain occurs only in the extrados. Therefore, cracks on the extrados occur at much higher strains compared to the forming limit curve. To improve the prediction of crack initiation in the outermost layers of bending components, a model test with similar material flow is required.
By in situ bending in a scanning electron microscope (SEM), we are able not only to detect the initiation of a crack at a very early stage on the nm-scale and to monitor its propagation. Moreover, it is possible to measure the strain distribution on the specimen’s surface with high lateral resolution by image-correlation techniques and to quantitatively monitor deformation-induced phase transformations, e.g., in austenitic steel by EBSD and SEM imaging.
Consequently, a novel miniaturized device for 3-point bending was developed and built to perform such quantitative in situ tests. The machine allows for the massive deformation of to 2 mm thick sheet metals with a maximum load of more than 2 kN. The bending head is driven by a DC motor with a planetary reduction gear followed by a worm gear and two spindles. This self-locking drive concept allows continuous as well as interrupted bending tests. A force sensor as well as a displacement sensor are fitted and allow the measurement of temporal curves of bending force versus the travel of the head. The traveling speed is held constant with a closed loop control.
First experiments in our FEI Helios 600 FIB/SEM microscope were performed with the aluminium alloy (EN AW 7075) and the austenitic steel sheet AISI 304 (X5 CrNi 1810). The thickness of the specimens was 1.5 mm. Crack initiation and growth were monitored for the aluminium alloy, while the steel specimen didn’t show cracks but martensitic phase transformation to a large extend. The experiments will be complemented by finite element simulations. Aim of this work is to develop parameters to precisely predict crack initiation in plasto-mechanical calculations of bending processes.