TY - BOOK

T1 - Strain Localization under Fatigue Loading

AU - Reiser, Johannes

N1 - no embargo

PY - 2013

Y1 - 2013

N2 - Since the first systematical fatigue tests performed by August Wöhler in the 19th century significant progress on studying fatigue phenomena was made. Especially in the second half of the last century fatigue mechanisms of single crystals have been studied intensively to get knowledge of fatigue from the micro- to the macroscale. For the bare eye on a single crystal the well known persistent slip bands PSB or persistent slip markings PSM on the surface are the main indicator for mechanisms leading to fatigue of the material. These PSBs are a result of dislocation motion and accumulation below the surface on an active glide system and are said to be the origin for first fatigue cracks. But single crystals except for special applications are not used in mechanical engineering. Common construction materials are polycrystals. The big difference between single and polycrystals is, that in a polycrystal never only one distinct slip system is activated and therefore fatigue is a result of many factors. One main point of fatigue limit prediction is the accumulation of plastic strains within the grains of the material. Although PSBs or PSMs are noticed in grains of polycrystals too, regions in the material that accumulate plastic strains also show a band like strucure but bigger in size as they occupy many grains. These bands seem to be the origin of fatigue cracks in polycrystals. For the purpose of studying plastic local strain accumulation and phenomena like local ratcheting within the microstructure and i.e. fatigue behaviour at the grain scale, a new testing rig for SEM in-situ fatigue testing was developed. Till now low testing frequencies of SEM in-situ test rigs limited tests at higher load cycles. The developed in-situ tension-compression testing apparatus for in-situ SEM use allows testing at higher frequencies and therefore to perform even higher load cycles with less time consumption. Further it is capable of testing in tension compression regime with zero mean stress. To study strain evolution and strain accumulation during the test cycle digital image correlation DIC technique in combination with local deformation analysis was used. For better comparison with literature data the classical model material oxygen free high conductivity copper which is a face centered cubic metal was used. Different specimen geometries were developed to find the best for in-situ testing. In-situ tests of up to 1000 load cycles for evaluation of function of test bench were performed. Final tests were conducted with two different constant and one stepwise increasing strain amplitude. SEM micrographs were taken at different magnifications to compare results from the grain scale up to the total geometry of the specimen. The results of local deformation analysis showed, that strain localizations started in distinct regions in the early cycles of the test and progressed further to higher load cycles showing higher values of local strain. Distinct regions show high strains for tension as well as for compression and ratcheting on the grain scale was observed, while other regions experience nearly no strain during the whole cycle. The strain concentrations are arranged in a band like structure and were independent of grain boundaries as they crossed many grains. The found experimental results are the base for calibration of cyclic polycrystal plasticity finite element simulations and improvement of the understanding of the local phenomena during fatigue loading.

AB - Since the first systematical fatigue tests performed by August Wöhler in the 19th century significant progress on studying fatigue phenomena was made. Especially in the second half of the last century fatigue mechanisms of single crystals have been studied intensively to get knowledge of fatigue from the micro- to the macroscale. For the bare eye on a single crystal the well known persistent slip bands PSB or persistent slip markings PSM on the surface are the main indicator for mechanisms leading to fatigue of the material. These PSBs are a result of dislocation motion and accumulation below the surface on an active glide system and are said to be the origin for first fatigue cracks. But single crystals except for special applications are not used in mechanical engineering. Common construction materials are polycrystals. The big difference between single and polycrystals is, that in a polycrystal never only one distinct slip system is activated and therefore fatigue is a result of many factors. One main point of fatigue limit prediction is the accumulation of plastic strains within the grains of the material. Although PSBs or PSMs are noticed in grains of polycrystals too, regions in the material that accumulate plastic strains also show a band like strucure but bigger in size as they occupy many grains. These bands seem to be the origin of fatigue cracks in polycrystals. For the purpose of studying plastic local strain accumulation and phenomena like local ratcheting within the microstructure and i.e. fatigue behaviour at the grain scale, a new testing rig for SEM in-situ fatigue testing was developed. Till now low testing frequencies of SEM in-situ test rigs limited tests at higher load cycles. The developed in-situ tension-compression testing apparatus for in-situ SEM use allows testing at higher frequencies and therefore to perform even higher load cycles with less time consumption. Further it is capable of testing in tension compression regime with zero mean stress. To study strain evolution and strain accumulation during the test cycle digital image correlation DIC technique in combination with local deformation analysis was used. For better comparison with literature data the classical model material oxygen free high conductivity copper which is a face centered cubic metal was used. Different specimen geometries were developed to find the best for in-situ testing. In-situ tests of up to 1000 load cycles for evaluation of function of test bench were performed. Final tests were conducted with two different constant and one stepwise increasing strain amplitude. SEM micrographs were taken at different magnifications to compare results from the grain scale up to the total geometry of the specimen. The results of local deformation analysis showed, that strain localizations started in distinct regions in the early cycles of the test and progressed further to higher load cycles showing higher values of local strain. Distinct regions show high strains for tension as well as for compression and ratcheting on the grain scale was observed, while other regions experience nearly no strain during the whole cycle. The strain concentrations are arranged in a band like structure and were independent of grain boundaries as they crossed many grains. The found experimental results are the base for calibration of cyclic polycrystal plasticity finite element simulations and improvement of the understanding of the local phenomena during fatigue loading.

KW - in-situ

KW - SEM

KW - DIC

KW - Dehnungslokalisierungen

KW - Ermüdung

KW - strain localization

KW - in-situ

KW - SEM

KW - fatigue

KW - DIC

M3 - Doctoral Thesis

ER -