Röntgenographische Bestimmung von Mikrospannungen in polykristallinenen Werkstoffen und ihr Verhalten bei statischer Verformung und schwingender Beanspruchung

Residual stresses are mechanical stresses in components, which are free from outside forces, moments and temperature gradients. They can considerably influence failure limits since they overimpose with external stresses. Two categories of residual stresses are macro residual stresses and micro residual stresses varying on a microscopic scale. The latter encompasses stresses between different phases of a material and stresses between differently oriented crystallites. The aim of the thesis is the investigation of microstresses in technical metal alloys after rolling, plastic axial deformation and cyclic loading. A substantial focus is laid on the possibilities to determine the residual stress state in a material with orientation-dependent microstresses. So far different evaluation methods already exist. Each of these procedures is based on restrictive conditions concerning the crystallite coupling, the kind of the residual stress state or on idealized conditions of the investigated material. In recent years a new promising evaluation method has been presented which apart from the macrostress state enables the analysis of the microstress state as a function of the crystal orientation. This so called “harmonic method” displays the strain or stress components depending on the crystal orientations by a series of symmetric harmonics. The unknown coefficients are derived with regression analysis from the experimental strain measurements. The generated strain/stress orientation functions result in macroscopic values by averaging over all orientations. So far the procedure has been rarely used in practice. As part of this work it was applied to Cu basis alloys. As comparative evaluation methods an advanced crystallite group method and the relatively simple method of linearization of averaged D(sin²psi) distributions are selected. The methods are applied to experimental data and tested concerning their restrictive conditions. Apart from the direct comparison of the evaluation methods, the sensitivity of the procedures was examined in relation to the inputs, i.e. the size of the measuring expenditure and possible measuring errors. Apart from the macro residual stresses the harmonic method provides the orientation-dependent micro stresses. Thus, a tool is available with which the development of these orientation-dependent micro stresses can be analyzed under plastic deformation. As part of this thesis Cu basis alloys were selected as exemplary material for analysis and examination. Apart from the evaluation of residual stresses the assessment of their influence on the endurance limits of components under static and dynamic loading is of particular interest. The positive and negative effects of macro residual stresses during cyclic loading have already been thoroughly studied. However, only few investigations have been conducted on the behavior and the influence of microstresses during cyclic loading. The microstresses can deviate severely from the macro residual stresses and can reach high absolute values compared to the yield points of the material. With a relaxation during cyclic loading they have rather little impact on failure limits. On the other hand rising or stable high initial values are more likely to influence failure limits. Therefore an examination of the development of the micro residual stresses is necessary and was conducted on a ferrite/austenitic duplex stainless steel with approximately balanced portions of the phases. A pulsating bending load was selected to apply pure tension on one side of the specimen and pure compression on the opposite side. Both phases can be measured well, so that apart form the development of the orientation-dependent microstresses primarily the stresses between the phases could be analyzed. The influence of a high anisotropy was examined with comparative measurements of a brass alloy.

Grantor dc:publisher

Mainz

Year dc:date

2007

Author dc:creator

• Possel-Dölken, Inken

Contributors dc:contributor

• Beiss, Paul

• info:eu-repo/classification/ddc/620
• Eigenspannung
• Röntgenbeugung
• Zyklische Belastung
• Deformation
• Ingenieurwissenschaften
• Metallischer Werkstoff
• Polykristall
• Schwingungsbelastung
• Mikrostruktur
• Röntgenographie
• microstress
• static deformation
• cyclic loading
• duplex stainless steel
• Cu basis alloy

Statement dc:rights

• info:eu-repo/semantics/openAccess

Language dc:language

ger

Identifier

https://publications.rwth-aachen.de/search?p=id:%22RWTH-CONV-124058%22

OAI identifier oai:identifier

oai:publications.rwth-aachen.de:62492