MEASUREMENT OF FRICTION UNDER SHEET FORMING CONDITIONS

Among the tests used to evaluate friction in sheet forming processes, bending-under-tension (BUT) tests have received considerable attention . A new test was developed incorporating a smooth increase of wrap ang le during deformation, even at high deformation rates, thus replicatin g typical conditions at die radii. A corresponding data analysis proce dure was introduced to separate the effects of bending and friction an d to account for the strain-rate sensitivity of the test material. The new test and analytical procedure were used to investigate the fricti onal behavior for four sheet alloys: interstitial-free (IF) steel, hig h-strength galvanized (HSG) steel, 2008-T4 aluminum, and 70/30 brass. A range of testing rates, pin radii, and lubrication conditions were e mployed, and the variation of friction coefficient (mu) as a function of contact radius, contact angle, and punch speed was measured. Comple x variations of mu with respect to tool radius and wrap angle were obs erved, consistent with previous studies. No significant, systematic va riation of mu was found for punch speeds ranging from 1.7 to 170 mm/s, contrary to reports in the literature. Failures in 2008-T4 aluminum s hifted from tensile localization to bending as the tool radius was red uced. The other materials consistently failed by tensile localization.