The geometrical and material constraints which limit the quality of hydroforming products in regard to failure by wrinkling (buckling) and/or rupture (tensile instability) are investigated in a unified framework. The analysis is based on limit theorems of plasticity (with a power-law hardening and Mises-Hill normal anisotropy) and resulted in distinct bounds for the permissible operating fluid pressure path. The parameteric study which follows includes a wide range of physical variables, some of which (not considered hitherto) show substantial effects on anticipated failure. Experiments with copper, aluminum, steel, and stainless steel agree very well with the supposition that premature failure (up to certain situations) is avoidable if the fluid pressure path is restricted to travel only within the suggested bounds.
The concept of Maximum Drawing Ratio (MDR), supplementary to the well-known Limit Drawing Ratio (LDR), is defined, examined, and illustrated by experiments. In essence the MDR is reached when the two basic failure modes, namely: rupture (due to tensile instability) and wrinkling (due to buckling instability) are delayed till they occur simultaneously. Thus the process is beneficially utilized for higher drawing ratio by postponing earlier interception of either one of the above failures alone. The ability to suppress (up to a certain extent) the appearance of these failure modes depends heavily on the fluid-pressure path which controls the hydroforming process. The effect of the material properties, like the strain hardening exponent, the normal anisotropy of the blank, etc., as well as the geometrical properties (i.e., the thickness of the blank, the radius of curvature at the lip, etc.) on the MDR, are considered here in some detail. The nature of the solutions by which MDR is reached is discussed.
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