Investigation of strain rate on residual stress distribution

In production engineering, current research focuses on the induction of targeted residual stress states in components in order to improve their properties. Therein, the combination of experiment and simulation plays an important role. In this contribution, a focus is laid on the investigation of hot forming processes with subsequent cooling. A numerical approach is presented to analyze the distribution of residual stresses resulting from cooling of a cylinder with an eccentric hole made of chromium-alloyed steel. The occurring phase transformation, which is evoked by cooling, is considered in order to compute residual stress distributions inside the material.

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Residual stresses in hot bulk formed parts: two-scale approach for austenite-to-martensite phase transformation

Press-brake bending is one of the important sheet metal forming processes widely used to fabricate different angles and channels in automotive applications. Plastic deformation of material during bending generates elastic stress waves known as acoustic emission (AE), enabling online monitoring of the bending process. The analysis of these stress waves helps in process characterization and aids in the identification of defects. The non-homogeneous plastic deformation that occurs during bending leads to the development of residual stress (RS) in the bend regions after forming. The locked-in residual stresses that remain after bending are measured using the x-ray diffraction method. The present study aims to use the AE technique for online bend monitoring of austenitic stainless steel (SS) of type 304L and correlate the AE parameters acquired during bending with the final locked-in RS present in the bend regions. Toward this, SS 304L sheets were bent to various bend angles, viz. 45°, 60°, 90°, 108°, 120°, and 135°, along the rolling direction and perpendicular to the rolling direction. Among the many AE parameters derived from the analysis of AE signals, it is shown that normalized cumulative counts and energy increase with bend angle. The increase in AE is well correlated with the RS that develops due to plastic deformation. The observed correlations between AE parameters acquired during bending and RS measured after bending are discussed.

Acoustic Emission during Press-Brake Bending of SS 304L Sheets and its Correlation with Residual Stress Distribution after Bending

In this study, residual stresses in hot-extruded Al-6061 rods with different cross-sectional reduction were investigated using the contour method. The contour method was used to provide a twodimensional map of residual stresses. The residual stresses were evaluated along the radius of the rods with different cross-sectional reduction before and after annealing heat treatment, and the uncertainty of the contour method was estimated. The results indicate that in the extruded rods with high reduction of diameter, tensile residual stresses are generated in the rod core, which are balanced along the rod radius by compressive residual stresses at the surface. A decrease in the cross-sectional reduction or the rod diameter increase results in rise of residual stresses. The annealing heat treatment reduces residual stresses and creates a symmetrical balance between tensile and compressive residual stresses. The countour method application revealed that maximum and minimum uncertainties occurred at the rod center and perimeter, while the latter one had a greater effect on the residual stress estimation results.

Annealing Heat Treatment Effect on the Residual Stresses in Hot-Extruded Aluminum Alloy Rods with High Cross-Sectional Reduction

Novel approach to optimize the mechanical properties of Cu-Al composite wires

Metal-forming processes commonly generate non-homogeneous plastic deformation in the workpiece so that the final product is left in a state of residual stress. These stresses may be detrimental or beneficial to the strength and reliability of the product in subsequent service (certain metal-forming processes are designed to produce beneficial residual stresses). In order to be able to evaluate the residual stresses in the product, a complete stress analysis of the workpiece throughout the process is needed and this calls for elastic-plastic analysis, for the more common rigid-plastic theory of forming cannot in principle yield stresses in the rigid regions which include the product. The analysis of stress and deformation distributions on the basis of elastic-plastic theory at finite strain is described and applied to the extrusion process using the finite element method. Examples of residual stresses generated, including the important steady state situation, are presented which illustrate the influence of the configuration of the die and the influence of large and small area reductions.

E. H. Lee

Papers

The Generation of Residual Stresses in Metal-Forming Processes