Giuseppe Pellegrini

The Effect of Sheet and Material Properties on Springback in Air Bending

Angle control in air bending is achieved either by exploiting direct angle measurement (adaptive forming) or by controlling punch displacement. In general, the desired angle is supplied as input to CNC press brakes and the choice of punch stroke relies on either analytical or empirical models. Process geometry and material properties affect the outcome, therefore, full knowledge of these values is critical. Since a major source of inaccuracies is due to errors in material description (sheet thickness is critical as well), material data (or, more generally, sheet behavior information) collection in process is advisable. In air bending, process control can be improved by studying the total load as a function of punch displacement. This approach becomes more and more interesting since devices for load measurement are now available on the market. The aim of this work is to analyze some experimental load measures, collected in actual working conditions, to evaluate the accuracy of such technique and its potential for in process applications. Several sheet materials (ferrous and non-ferrous alloys) are studied through both bending and tensile tests; the resulting material properties (tensile and bending) are evaluated and compared. After data treatment of punch force signal, the ability of predicting punch displacement needed to reach a defined bending angle (after springback) is discussed.

The Formability of Aluminum Foam Sandwiches: Experimental and FEM Analysis

This work deals with the formability of metal foams and it is focused on three point bending of aluminum foam sandwich panels. In this study bending can be considered as both a process for shaping of foamed panels and a mean for metal foam testing. Several tests were carried out by varying bending conditions and collecting load-displacement data. Specimens showed an interesting behaviour during bending since the sample deformation was related to the occurrence of foam cells failure or collapse. Moreover, once the process is terminated specimens retained a significant bending strength. During the experimental campaign several samples showed irregular fracture behaviour. In these cases cells fracture often starts in a generic part of the specimen (in correspondence of some non-eligible material defects) and propagates through the foam. The reasons of this problem and the possibility of failure prediction were investigated using different approaches. In particular, thickness measurements (using a ultrasound feeler) and X-ray analysis were carried out for this purpose. In addition, a study based on foam density showed a remarkable data scatter that can be considered as a characteristic of the state of the art in foamed panels manufacturing (in terms of process control and product variability). Finally, load-stroke curves were taken into account for this purpose. Computer simulations of the experiments were performed using the commercial FEM code (Deform 2D). Foam compressibility was simulated using a porous material model and the onset of foam instability was simulated by means of a specific damage criterion. Good agreement between simulative and experimental results was found.

Surface alteration induced by machining

This paper, dealing with strain hardening in turning, is a comparison of numerical (Finite Element Method (FEM)) and experimental results. Orthogonal cutting was assumed as the reference working condition. A FEM model was set up to simulate experimental conditions and to evaluate the effects of cutting parameters on surfacial strain hardening. Experimental data were recorded by machining thin disks of C20 steel (1CD20 UNI EN 10017: 2005) with radial feed; several values of both cutting speeds and feed were tested. After machining, work hardening was evaluated by micro Vickers test as a function of the distance from the machined surface. Regression techniques were used to fit experimental data and to find out work hardening patterns (depth of the affected zone, maximum hardness of the surface). FEM simulations of the experiments were performed using a commercial code for forming processes. A plane strain model of a rigid-plastic workpiece was set up with suitable material and interface properties. Good agreement between numerical and experimental results was found, therefore FEM was used to model a wider range of process parameters.