The sudden appearance of a material bulge beneath the roller during tube spinning operation (referred to as “plastic instability”) is considered experimentally and examined quantitatively via an upper bound analysis. The phenomenon is explained by a sudden transfer of flow from one admissible mode to its competing mode when an external condition (or few conditions) is changed in a certain way to be discussed (e.g., increasing the roller angle of attack, increasing the initial tube thickness, etc.). As a by-product, the interrelation between the dominant variables of the process emerges. The associated formulation with its inherent idealizations enables one to mark the line between favorable and unfavorable working conditions of the spinning process. Experimental evidence demonstrates the utility of the proposed approach.
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.
A fundamental new approach of deep drawing processes for uniform wall thickness is suggested and proved experimentally. It is based on imposition of a back-up fluid pressure which is prescheduled to vary with respect to the punch position during the drawing stroke in the hydroforming process. The formula for scheduling this fluid pressure path is generated via the plastic limit analysis and presented in terms of the geometry of the product, the work-hardening of the material, and the friction coefficient. It leads systematically to better results than empirical procedures reported in the current literature and industrial technical reports. Experiments in hydroforming sheets to cups and hemispheres performed on different materials (Al, Cu, and stainless steel) substantiate the improvement suggested. The associated overall punch load is overestimated (at most by 15 percent) with the unique feature of including Coulomb friction into the rigorous upper bound analysis.
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