Wolfgang Zinn

An Almost User-Independent Evaluation Formalism to Determine Arbitrary Residual Stress Depth Distributions with the Hole-Drilling Method

The incremental hole-drilling method is the method of choice to determine residual stress depth distributions with limited costs and minor destruction of the investigated component. With a spatial resolution of commonly two millimeters in diameter and one millimeter in depth especially the effects of frequently used surface treatments like e.g. shot peening or deep rolling can be reliably detected if the in depth residual stress gradients are relatively smooth. Nevertheless up to now the quantitative accuracy of the method is poor for residual stress analyses close to the materials surface up to depths of approximately 0.2 mm and in the case of steep in-depth residual stress gradients or oscillating residual stress depth distributions. In this paper, residual stress depth distributions of a broad range introduced by mechanical surface-treatments in flat specimens were analyzed with the hole-drilling method and compared with the results measured by X-ray diffraction as the reference. It comes out, that arbitrary residual stress depth distributions can be successfully determined with a modified differential evaluation formalism. For this purpose, often neglected well known weak points of the hole-drilling method were considered and improved, e.g. hole geometry, numerical calibration and data conditioning. Especially, the proposed strategy of data conditioning results in an almost user-independent evaluation formalism.

Corrosion and Fatigue of AL-Alloys AA359.0 and AA6060 in Different Surface Treatment States

The consequences of near surface materials properties and residual stress states produced by specific manufacturing operations on damage evolution during corrosion fatigue of the Al-base alloys AA359.0 (German grade G-AlSi9Cu3) and AA6060 (German grade AlMgSi0.5) were systematically investigated. Specimens were processed applying mechanical surface treatments like shot peening or deep rolling and investigated in comparison with turned states. Surface topographies as well as near surface work hardening states and residual stress depth distributions were analyzed. Tension-compression fatigue tests were carried out under laboratory air as well as under salt spray test conditions or in salt solution. Crack formation and crack propagation was studied and characteristic examples are presented. The influence of the mechanical surface treatments on the electrochemical behavior was also investigated. To assess the consequences of near surface materials properties on the corrosion fatigue behavior, their stability during fatigue, in particular the relaxation of residual stress distributions introduced by mechanical surface treatments, was taken into account.

Turning Residual Stresses in Functionally Graded Steel Components

Introduction Metallic components with graded materials properties are of particular importance for the realization of lightweight design. The scientific basis of such concepts was provided by the Collaborative Research Centre TRR 30, funded by the German Research Foundation DFG between 2006 and 2015. As part of the project, thermo-mechanically graded components were manufactured by hot metal forming processes [1, 2]. Their characteristic properties are to a large extent determined by final machining operations. In this context, residual stresses play an important role for strength and lifetime of the produced parts. A survey about near surface residual stress states after hard turning of differently heat treated quenched and tempered steel AISI 6150 (51CrV4) is given in [3]. In the present paper, results of similar investigations carried out of Jominy end quench samples are reported and compared with results of samples with homogeneous microstructures.

In Situ X-Ray Stress Analysis for the Highly Textured Mg-Base Wrought Alloy AZ31

The deformation behaviour of the highly textured Mg-base wrought alloy AZ31 subjected to tensile and compressive elasto-plastic loading was characterized by means of synchrotron radiation. In former publications it was shown that there exists an asymmetry in the deformation behaviour of the investigated alloy, which can be related to the deformation mechanism of the hexagonal structure due to the crystallographic texture relative to the loading direction. A local X-ray stress analysis was carried out on highly textured metal sheets for different {hkil}- planes of the hexagonal crystal structure. The load transfer was investigated within in-situ loading experiments in order to account for suitable XEC´s, thus ensuring accurate stress evaluations. An X-ray imaging method was applied in order to provide bending stress distributions with a high local resolution using synchrotron radiation (beamline G3, Hasylab (DESY)). Stress analyses were carried out on the side face of a bending bar being subjected to elasto-plastic bending up to total strains in the outer layers of approximately 2%. It is distinguished between loading stress distributions measured in in-situ loading experiments as well as residual stress distributions monitored after unloading of elasto-plastically bended bars. Furthermore the loading direction was alternated, in order to investigate the effect of the twin formation as well as the reversibility of the twinning on the results of X-ray stress analysis.