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Investigation of Contact Pressure Distribution on Sheet Metal Stamping Tooling Interfaces: Surface Modeling, Simulations, and Experriments
Master of Science (M.S.)
Year Degree Awarded
Month Degree Awarded
Embedded Sensing, Sheet Metal Stamping, Contact Pressure Sensing, Process Monitoring
In stamping operations, sheet metal is formed into a desired shape by pressing it in a hydraulic or mechanical press between suitably shaped dies. As a predominant manufacturing process, sheet metal forming has been widely used for the production of automobiles, aircraft, home appliances, beverage cans and many other industrial and commercial products. A major effort till date on stamping processes monitoring has been focused on investigating variations in the press force. Given that the press force itself is an integral of the contact pressure distribution over the die and binder contact interfaces, it is conceivable that defects may be better identified by analyzing the contact pressure distribution directly at the tooling-workpiece interface, instead of measuring the press force, which is less reflective of the localized forming process due to its nature as a secondary effect. It is thus desirable that a new, integrated sensing method capable of directly assimilating forming pressure distribution in the tooling structure be devised for improved stamping process monitoring. Designing such a distributed sensing scheme and analyzing the feasibility of its structural integration into a stamping tooling structure is the objective of this reported work. In this context, four research tasks have been identified and examined during the course of this work: 1) Devising a New, Embedded Sensing Method The new sensing method monitors stamping processes by means of an array of force sensors structurally integrated into the stamping tooling. The ability to directly measure local forming events by means of such an integrated and distributed sensing provides a new means of performing defect detection and process monitoring. Such a distributed sensing system overcomes the limitations of traditional tonnage and acceleration sensing systems which are focused on the measurement of indirect, global parameters. The new method is based on the evaluation of spatially continuous pressure surfaces from spatially discrete sensor measurements that are directly related to the local events at the stamping interface. To evaluate the effectiveness of this method, a panel stamping test bed equipped with an array of embedded force sensors has been designed, modeled and fabricated. Data obtained from experiments conducted on the test bed indicates that the new sensing method can be highly effective in process monitoring of stamping operations. 2) Reconstruction of Spatio-Temporal Distribution of Contact Pressure Structurally integrating sensors under tooling surfaces reduces the surface rigidity of the tool, thus limiting the number of sensors and the locations at which they can be embedded. This in turn affects the reconstruction of contact pressure distribution on the tooling surface. Numeric surface generation methods, such as Bezier surfaces and Thin Plate Spline surfaces offer a method for estimating the contact pressure distributions on the tooling surfaces from a sparse distribution of sensors. The concept of interpolating force distributions using surfaces has been investigated by researchers previously. However, selection of the surface generation method has remained largely an ad hoc process. The work presented here addresses this issue by using tooling interface contact pressure distribution information obtained from FE simulations as the basis for evaluating the accuracy of two commonly employed surface methods mentioned above. In order to reach a generic conclusion, the mathematical background of these schemes has been examined in light of the purpose at hand. The results indicate that an interpolative scheme such as the Thin Plate Spline surfaces (TPS), which can estimate the contact pressure distributions more accurately in a multi-sensor environment. The local and global accuracies of the Thin Plate Spline surface modeling technique have been experimentally evaluated using a sensor embedded stamping test bed designed for the purpose. 3) Modeling of Contact Pressure Distribution at the Sheet Metal-Tooling Interface Information about the contact pressure distribution at the tooling interface is critical to identifying the accuracy of numeric schemes that estimate by interpolation or approximation the contact pressure at any point on the tooling surface, based on a limited number of spatially distributed sensors. Furthermore, such knowledge is valuable in identifying operational parameters for the sensors to be integrated into the stamping tooling structure. In the absence of a tractable analytic method of determining the contact pressure distribution on stamping tooling surfaces, Finite Element models of a stamping operation have been created. Furthermore the drilling of sensor cavities under the working surfaces of the dies adversely affects the working life of stamping dies and their strength. The accuracy of analytic fatigue failure mechanics in evaluating the effect of parameters, such as embedding depth and sensor rigidity, on the operational life of the die, suffers from uncertainty in the estimation of stress concentrations around sharp geometric features of the sensor cavity. This shortcoming has been circumvented by the creation of FE models of the sensor cavity for more accurate estimation of stress concentrations around sharp geometries. The effect of different embedding materials on the sensitivity of embedded sensors has also been evaluated based on these models. 4) Defect Detection in Stamping Operation The ultimate goal of this thesis research was to study the feasibility of identifying defects in a stamping process based on the contact pressure distribution surfaces. This was achieved in this reported work by spatio-temporal decomposition of ‘parameters’ derived from the contact pressure distribution surfaces. Here ‘parameters’ refers to quantities such as the minimum, maximum, and mean contact pressures. These parameters have a time-varying spatial location as well as magnitude value associated with them. The feasibility of defect detection in stamping operations based on such parameters has been investigated. In addition to these focal areas, the design and implementation of a stamping test bed equipped for distributed contact pressure sensing has also been researched. This test bed was utilized for experimental verification of the developed theories and numerical models. Design of the proposed test bed required research into additional topics like the design of a protective package for embedded sensors and the effect of sensor embedding depth on contact pressure measurements. These issues have been addressed in this work, culminating in the experimental demonstration of the embedded pressure sensing system for process monitoring in the sheet metal stamping processes.