Influence of Inhomogeneous Deformation on Tensile Behavior of Sheets Processed Through Constrained Groove Pressing

Abstract

Constrained groove pressing (CGP) is a severe plastic deformation technique to produce the ultra-fine grained sheet. The inhomogeneous strain distribution and geometry variation induce differential mechanical properties in the processed sheet. The improved mechanical properties of CGP sheets is due to the composite effect of weak and strong regions formed by geometric and strain inhomogeneities. Weaker regions exhibit large strain, lower yield strength, and higher strain hardening compared to stronger regions. The estimation of mechanical properties is influenced by these defects leading to the difference in the mechanical properties along different orientations. Experimental investigation revealed that the commonly used tensile samples cut perpendicular to the groove orientation exhibit variation in thickness along the gauge length affecting the results from tensile tests. To further understand the effect of geometric variation, a typical CGP specimen was reverse engineered and finite element (FE) simulation was performed using the actual geometry of the CGP processed specimen. The strain distribution from FE simulation was validated experimentally using the digital image correlation data. Based on the numerical and experimental studies, miniature specimens were designed to eliminate the geometric effects from the standard parallel specimen. Miniature parallel specimens showed lower yield strength and total elongation compared to the standard specimens. However, the statistical scatter of total elongation of the miniature specimens was much less than that of the standard specimens, indicating better repeatability. Probably this is the first study to quantify the contribution of composite geometric effect in the mechanical properties of CGP.

Developing methods of constrained groove pressing technique: A review

“…28 Geometric and microstructural inhomogeneity (variation in grain size, surface unevenness, and microcracks) in the processed sample produces variance in mechanical properties along different orientations. It is investigated in the literature 73 that the processed sample consists of the composite effect of weak and strong regions where the weaker regions show a higher strain hardening ability, large deformation, and smaller yield strength during tensile tests. Therefore, a variation in mechanical properties is exhibited in parallel and perpendicular to the groove orientations.…”

“…Therefore, material anisotropy occurs after CGP due to the difference in strength in the two orientations. 70,73,82 Researchers have used numerical techniques to simulate the deformation behavior and strain distribution during CGP. Lee and Park 54 first demonstrated the FE simulation of CGP and CGR processes and reported nonuniform strain distribution throughout the sample.…”

“…Mechanical properties like tensile strength of parallel tensile samples are better due to homogeneous strain distribution compared to the perpendicular tensile samples. 73 The tensile behavior and strain hardening of CGP-processed nickel samples are investigated 59,74 and shown in Figure 11. On tensile loading, the strain hardening occurs along with pre-existing shear bands, and the deformation mechanism changes to crystallographic glides along with pure shear.…”

Re-examination and comparison of activation volume, strain rate sensitivity, and dislocation density for coarse grain and CGP-processed UFG materials

“…Constrained groove pressing (CGP) is one of the famous SPD techniques for sheet metal processing to impose large plastic strain and thereby produce UFG microstructure. [4][5][6] However, an effective and sufficient way to process the material is merely through one or two CGP passes since additional grain refinement through higher passes is not economical due to lower grain refinement rate. 7 Moreover, inhomogeneous grain refinement, sheet defects like surface unevenness and cracks, and lower grain refinement are the common limitations in the CGP process.…”

“…7 Moreover, inhomogeneous grain refinement, sheet defects like surface unevenness and cracks, and lower grain refinement are the common limitations in the CGP process. 5,8 Apart from CGP, inhomogeneous grain refinement is always present in all the SPD processes though it is lower in the last passes. 9 Sometimes highly distorted microstructure, which is difficult to analyze optically, 10 is observed due to a large strain imposed during the process.…”