Strain Analysis of the Hemispherical Stretch-Bend Test M.Y. DEMERI A number of simulative tests have been developed over the years to assess sheet metal formability. Two of the tests, namely, the Forming Limit Diagram (FLD) and the Limiting Dome Height (LDH), are being used as standard tests for the evaluation of stretch-type forming operations. The tests do not include the effect of bending on formability and hence are limited in their application. Recently, the Hemispherical Stretch-Bend Test (HSBT) was introduced to assess the combined effect of material and design variables on stretch- bend type forming. This paper presents a strain data base for the HSBT and gives the influence of punch curvature and sheet thickness on the limit strains and the strain distributions after forming. Curvature and thickness alter the strain distribution so much that one punch cannot be representative of all stretch-bend forming operations. INTRODUCTION Sheet metal stamping involves the interaction among three types of variables: material, design, and process. In produc- tion, processs variables are usually determined during die tryout to give maximum productivity for a stamping. In the laboratory, process variables are difficult to simulate, but they can be controlled and duplicated by the establishment of a standard test procedure. Simulative tests for forma- bility, therefore, should assess the interaction between mate- rial and design variables. Most stampings are too complex to be simulated by a single test; hence, a number of such tests have been developed over the years. Metal forming operations are usually characterized as mainly stretching or mainly drawing. Simulative tests for drawing have been fairly successful in assessing the performance of drawn sheet metal. For complex stretching, the tests were not as successful. Some sheet metal users and producers employ a standard dome test to assess and predict sheet stretchability. The test M. Y. DEMERI is a Member of the Research Staff at Ford Motor Company, Dearborn, MI 48120. involves the stretching of clamped strips of sheet metal over a 4-inch (102 mm) diameter punch until the formed sheet fails. If the blanks are gridded, the test produces data for the construction of the Forming Limit Diagram (FLD) ~ and Limiting Dome Height (LDH). 2 The LDH is similar to the FLD with the dome height (H) replacing the major strain. While FLDs give forming limit strains, LDH type test incor- porates both the limit strain as well as the strain distribution after forming. Stretching tests are performed using a punch radius-to-thickness ratio of 20 or more. Within this range the effect of bending is minimal and data obtained are of limited application because shapes and contours are not included. FLDs and LDHs are, therefore, applicable only to stretch- type forming with negligible bending. As such, they give only the material's contribution to formability and neglect the interaction between material and design variables. In many stampings, the sheet metal is drawn in from the flange, then stretched and simultaneously bent over small radii. The relative amount of stretching and bending varies from part to part and from location to location in the same part. With bending, sheet thickness becomes important be- cause it controls the strain distribution both in the plane and through the thickness of the sheet metal. To account for the J. APPLIED METALWORKING 1986 AMERICANSOCIETYFOR METALS VOL. 4, NO. 2, JANUARY 1986 183