Timothy Ngeru
Current Organizations / Institutes
The organizations/institutes from the scholar's latest article
Timothy Ngeru has a total of 7 co-authors. Here are the top co-authors by number of shared publications:
- Stefanie Hanke
Institute for Product Engineering and Materials Technology (IPM) and Center for Nanointegration Duisburg-Essen (CENIDE), University of Duisburg-Essen, Lotharstraße 1, 47057 Duisburg, Germany
4 shared publications - Dzhem Kurtulan
Chair of Materials Science and Engineering, Universität Duisburg-Essen, 47057 Duisburg, Germany
2 shared publications - Abhishek Biswas
ICAMS, Ruhr Universität Bochum, Universitätsstr. 150, Bochum, Germany
1 shared publication - Abril Azócar Guzmán
Institute for Advanced Simulations – Materials Data Science and Informatics (IAS‐9) Forschungszentrum Jülich GmbH Jülich Germany
1 shared publication - Ahmet Karkar
Materials Science and Engineering, Institute for Metal Technologies—ITM, University Duisburg-Essen, Lotharstraße 1, 47057 Duisburg, Germany
1 shared publication - Alexander Hartmaier
Ruhr University Bochum
1 shared publication
Latest Publications
- Open Access
Effect of Superimposed Static Compressive Stress on the Multiaxial Torsional Fatigue Behaviour and Crystallographic Deformation Response of P558 Austenitic Stainless Steel
Components loaded in cyclic torsion are often also subjected to a static compressive stress, but whether this stress extends or shortens the fatigue life is not resolved. Double-cone specimens of the nickel-free, nitrogen-alloyed austenitic stainless steel P558 were tested under angle-controlled cyclic torsion at (rotation ranges of 5–20°, and 22.5° in one test) with a superimposed static compressive stress of 0, 250 or 350 MPa, and were characterised by SEM, electron backscatter diffraction (EBSD), Taylor factor and hysteresis-energy analysis. Compression more than doubled the fatigue life at 5° but reduced it by up to 64% at larger rotation ranges; at 20°/350 MPa the test was stopped by axial ratcheting after 244 cycles. A repeat test and two tests at reproduced the lives within a factor of 1.25. The grain orientation spread (GOS) of the pure-torsion specimens remained at the level of the untested material, although the specimens at 10° and 15° were cycled plastically, and compression did not change it at 5°. At 10–22.5°, compression raised GOS to 2.4–4.4°, four to seven times the pure-torsion value, in line with the accumulated axial ratcheting strain (7–18%) rather than with the cyclic shear amplitude or the equivalent stress. The change in life followed the maximum principal stress: at 5°, compression lowered it by up to 47%, whereas at larger rotation ranges the compression-induced increase in the stabilised shear stress raised it by up to 79%. Whether superimposed compression is beneficial or detrimental therefore depends on the rotation range.Open AccessEffect of Superimposed Static Compressive Stress on the Multiaxial Torsional Fatigue Behaviour and Crystallographic Deformation Response of P558 Austenitic Stainless Steel
Components loaded in cyclic torsion are often also subjected to a static compressive stress, but whether this stress extends or shortens the fatigue life is not resolved. Double-cone specimens of the nickel-free, nitrogen-alloyed austenitic stainless steel P558 were tested under angle-controlled cyclic torsion at (rotation ranges of 5–20°, and 22.5° in one test) with a superimposed static compressive stress of 0, 250 or 350 MPa, and were characterised by SEM, electron backscatter diffraction (EBSD), Taylor factor and hysteresis-energy analysis. Compression more than doubled the fatigue life at 5° but reduced it by up to 64% at larger rotation ranges; at 20°/350 MPa the test was stopped by axial ratcheting after 244 cycles. A repeat test and two tests at reproduced the lives within a factor of 1.25. The grain orientation spread (GOS) of the pure-torsion specimens remained at the level of the untested material, although the specimens at 10° and 15° were cycled plastically, and compression did not change it at 5°. At 10–22.5°, compression raised GOS to 2.4–4.4°, four to seven times the pure-torsion value, in line with the accumulated axial ratcheting strain (7–18%) rather than with the cyclic shear amplitude or the equivalent stress. The change in life followed the maximum principal stress: at 5°, compression lowered it by up to 47%, whereas at larger rotation ranges the compression-induced increase in the stabilised shear stress raised it by up to 79%. Whether superimposed compression is beneficial or detrimental therefore depends on the rotation range. - 27 March 2024
- Published by ASTM International in Materials Performance and Characterization
- Vol. 13 (1) , 121-134
Mechanical Behavior and Life Prediction of a Low Nickel/High Nitrogen Austenitic Steel under Pure Torsion, Uniaxial Tension-Compression, and Multiaxial Fatigue
Fatigue life prediction, crack growth, and propagation mechanisms are important details to consider for a structure to be used in service without fear of early failure. This is especially important for a new material for which very limited information is available. In this study, austenitic stainless steel P558 with low nickel content and high nitrogen content was subjected to fatigue testing at room temperature in multiaxial (static compression and cyclic torsion), pure torsion, and uniaxial tension-compression modes. The material generally depicts cyclic softening in the three loading conditions; although the multiaxial experiments experienced some initial hardening, those with a high shear stress and axial stress combination furthermore experienced a secondary hardening before softening. The Fatemi-Socie model predicts the pure torsion and multiaxial fatigue well but fails to predict fatigue lives for uniaxial tension-compression. On the other hand, the Smith-Watson-Topper model predicts fatigue lives for the uniaxial fatigue and pure torsion sufficiently but is poor in predicting fatigue lives for the multiaxial fatigue loading conditions. The cracking behavior was observed to be dependent on the loading condition and the strain amplitude with Mode II cracks or shear-oriented cracks observed for the high strain amplitudes (resulting in shorter lives) whereas Mode I cracks, or tensile-oriented cracks, were observed from low strain amplitudes (resulting in higher lives). A mixed mode was also observed for specimens in the midrange strain amplitudes, which exhibited a branching from shear to tensile cracks. The static compressive stress is believed to have a negative influence on the cracking behavior and the fatigue life by the rubbing of the crack surfaces, which in turn reduces the load-holding capability of the material. The same friction at the crack tip could also be the reason for the heating up of the specimens.
- 27 March 2024
- Published by ASTM International in Materials Performance and Characterization
- Vol. 13 (1) , 121-134
Mechanical Behavior and Life Prediction of a Low Nickel/High Nitrogen Austenitic Steel under Pure Torsion, Uniaxial Tension-Compression, and Multiaxial Fatigue
Fatigue life prediction, crack growth, and propagation mechanisms are important details to consider for a structure to be used in service without fear of early failure. This is especially important for a new material for which very limited information is available. In this study, austenitic stainless steel P558 with low nickel content and high nitrogen content was subjected to fatigue testing at room temperature in multiaxial (static compression and cyclic torsion), pure torsion, and uniaxial tension-compression modes. The material generally depicts cyclic softening in the three loading conditions; although the multiaxial experiments experienced some initial hardening, those with a high shear stress and axial stress combination furthermore experienced a secondary hardening before softening. The Fatemi-Socie model predicts the pure torsion and multiaxial fatigue well but fails to predict fatigue lives for uniaxial tension-compression. On the other hand, the Smith-Watson-Topper model predicts fatigue lives for the uniaxial fatigue and pure torsion sufficiently but is poor in predicting fatigue lives for the multiaxial fatigue loading conditions. The cracking behavior was observed to be dependent on the loading condition and the strain amplitude with Mode II cracks or shear-oriented cracks observed for the high strain amplitudes (resulting in shorter lives) whereas Mode I cracks, or tensile-oriented cracks, were observed from low strain amplitudes (resulting in higher lives). A mixed mode was also observed for specimens in the midrange strain amplitudes, which exhibited a branching from shear to tensile cracks. The static compressive stress is believed to have a negative influence on the cracking behavior and the fatigue life by the rubbing of the crack surfaces, which in turn reduces the load-holding capability of the material. The same friction at the crack tip could also be the reason for the heating up of the specimens.
- Open Access
Mechanical Behavior of Austenitic Steel under Multi-Axial Cyclic Loading
Low-nickel austenitic steel is subjected to high-pressure torsion fatigue (HPTF) loading, where a constant axial compression is overlaid with a cyclic torsion. The focus of this work lies on investigating whether isotropic J2 plasticity or crystal plasticity can describe the mechanical behavior during HPTF loading, particularly focusing on the axial creep deformation seen in the experiment. The results indicate that a J2 plasticity model with an associated flow rule fails to describe the axial creep behavior. In contrast, a micromechanical model based on an empirical crystal plasticity law with kinematic hardening described by the Ohno–Wang rule can match the HPTF experiments quite accurately. Hence, our results confirm the versatility of crystal plasticity in combination with microstructural models to describe the mechanical behavior of materials under reversing multiaxial loading situations.
Open AccessMechanical Behavior of Austenitic Steel under Multi-Axial Cyclic Loading
Low-nickel austenitic steel is subjected to high-pressure torsion fatigue (HPTF) loading, where a constant axial compression is overlaid with a cyclic torsion. The focus of this work lies on investigating whether isotropic J2 plasticity or crystal plasticity can describe the mechanical behavior during HPTF loading, particularly focusing on the axial creep deformation seen in the experiment. The results indicate that a J2 plasticity model with an associated flow rule fails to describe the axial creep behavior. In contrast, a micromechanical model based on an empirical crystal plasticity law with kinematic hardening described by the Ohno–Wang rule can match the HPTF experiments quite accurately. Hence, our results confirm the versatility of crystal plasticity in combination with microstructural models to describe the mechanical behavior of materials under reversing multiaxial loading situations.
- Open Access
Mechanical Behaviour and Failure Mode of High Interstitially Alloyed Austenite under Combined Compression and Cyclic Torsion
Multiaxial stress states frequently occur in technical components and, due to the multitude of possible load situations and variations in behaviour of different materials, are to date not fully predictable. This is particularly the case when loads lie in the plastic range, when strain accumulation, hardening and softening play a decisive role for the material reaction. This study therefore aims at adding to the understanding of material behaviour under complex load conditions. Fatigue tests conducted under cyclic torsional angles (5°, 7.5°, 10° and 15°), with superimposed axial static compression loads (250 MPa and 350 MPa), were carried out using smooth specimens at room temperature. A high nitrogen alloyed austenitic stainless steel (nickel free), was employed to determine not only the number of cycles to failure but particularly to aid in the understanding of the mechanical material reaction to the multiaxial stresses as well as modes of crack formation and growth. Experimental test results indicate that strain hardening occurs under the compressive strain, while at the same time cyclic softening is observable in the torsional shear stresses. Furthermore, the cracks’ nature is unusual with multiple branching and presence of cracks perpendicular in direction to the surface cracks, indicative of the varying multiaxial stress states across the samples’ cross section as cross slip is activated in different directions. In addition, it is believed that the static compressive stress facilitated the Stage I (mode II) crack to change direction from the axial direction to a plane perpendicular to the specimen’s axis.
Open AccessMechanical Behaviour and Failure Mode of High Interstitially Alloyed Austenite under Combined Compression and Cyclic Torsion
Multiaxial stress states frequently occur in technical components and, due to the multitude of possible load situations and variations in behaviour of different materials, are to date not fully predictable. This is particularly the case when loads lie in the plastic range, when strain accumulation, hardening and softening play a decisive role for the material reaction. This study therefore aims at adding to the understanding of material behaviour under complex load conditions. Fatigue tests conducted under cyclic torsional angles (5°, 7.5°, 10° and 15°), with superimposed axial static compression loads (250 MPa and 350 MPa), were carried out using smooth specimens at room temperature. A high nitrogen alloyed austenitic stainless steel (nickel free), was employed to determine not only the number of cycles to failure but particularly to aid in the understanding of the mechanical material reaction to the multiaxial stresses as well as modes of crack formation and growth. Experimental test results indicate that strain hardening occurs under the compressive strain, while at the same time cyclic softening is observable in the torsional shear stresses. Furthermore, the cracks’ nature is unusual with multiple branching and presence of cracks perpendicular in direction to the surface cracks, indicative of the varying multiaxial stress states across the samples’ cross section as cross slip is activated in different directions. In addition, it is believed that the static compressive stress facilitated the Stage I (mode II) crack to change direction from the axial direction to a plane perpendicular to the specimen’s axis.