Stage-I-fatigue-cracks are used as highly localized dislocation sources with well-known Burger’s ... more Stage-I-fatigue-cracks are used as highly localized dislocation sources with well-known Burger’s vectors to study the interaction between dislocations and grain boundaries. This interaction in the plastic zone is of particular interest to understand the fluctuating crack growth in the very short crack regime. In the case of a blocked slip band the dislocations pile up at the grain boundary causing a local stress concentration. The resulting local stress distribution is calculated based on measurements of the dislocation density distribution in the plastic zone. For this purpose the slip line profiles were measured by AFM, the dislocation density distribution was determined and the dislocation-free zone model of fracture (DFZ) was validated. With this it is possible to quantify the grain boundary resistance and to combine geometric and stress approach for grain boundary resistance against slip transfer.
Fatigue induced fracture is the number one reason for failure of technical systems. Thereby there... more Fatigue induced fracture is the number one reason for failure of technical systems. Thereby there are a lot of approaches to strengthen a material against fatigue cracks for instance by using the strengthening effect of grain boundaries. Especially in the stage of short crack growth it is well known that grain boundaries lead to a fluctuating crack propagation rate with a deceleration of the crack growth rate in front of the grain boundaries sometimes combined with a complete stop for a large number of cycles. We identified some of the interaction mechanisms which are responsible for this behaviour by focused ion beam (FIB) artificial crack initiation combined with in-situ fatigue testing in the scanning electron microscope and FIB tomography. The results include useful aspects for fatigue life calculation and to make materials fatigue resistant. Introduction Materials and components exposed to cyclic loading spent up to 90 % of their lifetime by the growth of short cracks which nuc...
Fatigue-crack propagation is, by far, the most crucial failure mechanism of technical systems. Th... more Fatigue-crack propagation is, by far, the most crucial failure mechanism of technical systems. The key to understanding microscopic processes that lead to failure lies in the knowledge of local stresses, the driving force of cracks. We present the mapping of stress and strain fields induced by a single overload on a fatigue crack and their influence on transient crack-growth retardation. In contrast to former work, in which synchrotron x-ray diffraction (XRD) was used, this investigation was performed by using a calibrated magnetic Barkhausen noise microscope in combination with digital image correlation based on in situ scanning electron microscope imaging. The underlying mechanisms, residual stress effects in front of the crack tip and plasticity-induced crack closure caused by the plastic wake, have been studied. Specifically, a crack in S960Q steel has been followed through the overload (OL) region while examining measurements at distinctive overload points: before OL, after OL, at maximum retardation, and at recovery. We observe a strong correlation of the local fatigue-crack-growth rate with the local residual stress distribution obtained from magnetic Barkhausen noise, which remains nearly unchanged after the crack has passed by. Digital image correlation results reveal the influence of these residual stresses on crack-tip opening reactions and strain fields under external loads. Although strain fields show a strong decrease because of the OL event, differences in crack opening stresses remain rather low at first, but prevail in the second part of the OL region. The applicability of new measurement methods and their results regarding the dominating retardation mechanism are discussed. These indicate that the residual stress effect on the strain fields can be associated to be more significant than plasticity-induced crack closure at maximum retardation with a change of mechanisms on reacceleration.
The article focuses on the fatigue performance after a moderate heat treatment of nanocrystalline... more The article focuses on the fatigue performance after a moderate heat treatment of nanocrystalline (nc) nickel, which leads to the formation of a bimodal microstructure in the nc to ultrafine grained (ufg) regime. Electrodeposition was used to produce nc macro nickel samples with grain sizes of about 40 nm for mechanical testing. The thermal stability of the material as well as the influence on the mechanical properties and the fatigue crack propagation behavior was investigated. The results of tensile and fatigue tests are discussed in respect to the chosen production method and boundary conditions. In this context, the influence of the bath additives used during the plating process was investigated and rated as the major challenge for a further improvement of the thermal stability and mechanical properties of the material. Finally, a co-deposition of nickel and metal oxides with enhanced thermal stability is presented.
Abstract Increasing the resistance of a material against fatigue crack growth by optimizing the m... more Abstract Increasing the resistance of a material against fatigue crack growth by optimizing the microstructure is one of the major tasks of modern materials science. Thereby grain and phase boundaries are microstructural elements which can decelerate the propagation rate especially of short cracks. However, in different materials cracks and grain boundaries interact differently. For instance in some steels the blocking effect was only found for large angle grain boundaries while small angle boundaries showed nearly no effect. On the other hand in nickel based superalloys a retardation of cracks was found even for small angle boundaries when the crack was propagating in stage I. Even in front of the same grain boundary, the blocking effect varies for different cracks. To investigate this behaviour systematically focused ion beam (FIB) initiated cracks were used. By this method of artificial crack initiation the crack parameters like crack length and distance to the obstacle can be varied separately. Additionally special grain boundaries can be selected after a microstructural characterisation by electron backscatter diffraction (EBSD). Finally FIB tomography helps to understand the process how a crack overcomes a grain boundary.
Lightweight construction is one of the most demanded technologies in many engineering systems. In... more Lightweight construction is one of the most demanded technologies in many engineering systems. In order to guarantee the safety of the whole system, it is mandatory to improve models that describe and predict its behavior under load. Fatigue, the damaging of materials under cyclic loading, is the main phenomenon leading to failure in e.g. automobile and aerospace components. Cyclic loading during service does usually not happen with constants amplitudes, rather there are complex patterns of different load levels. High load variations in these patterns lead to deviations from the linear Paris behavior. Strong decelerations occur as consequence of a single increased tensile load, which is known as the overload effect. Nevertheless, this effect does not affect all materials the same, there are materials that show a strong overload sensitivity and others on which overloads only have a minor influence. Reasons for this can be seen in the interplay of the underlying mechanisms of the overload effect: plasticity induced crack closure and compressive residual stresses. While both effects lead to crack tip shielding and a reduction of stress intensity, crack closure delays the opening of the crack tip and thereby reduces the effective ΔK range, whereas compressive residual stresses superimpose with crack tip stresses and thereby reduce Kmax. Possible reasons for differences in the sensitivity can be differences in the strain hardening, both in the static and in the dynamic case, as well as in changes of the sign of stresses (Bauschinger effect). Since crack propagation is driven by local stresses and strains, measurements to examine differences in them have to be performed on a microscopic scale. We could show that by the combination of modern measurement techniques – magnetic Barkhausen noise and digital image correlation in scanning electron microscope – we were able to image, separate and evaluate the mechanisms of the overload effect quantitatively. The calibrated magnetic Barkhausen noise microscope allows us measurements of residual stresses with a spatial resolution of 10 µm. From the digital image correlation results we could evaluate the crack tip driving forces namely the crack opening behavior, changes in the stress intensity Kand in the strain energy release rate via the J-integral. Using a simple model based on these results, we were furthermore able to predict the crack growth behavior due to the overload effect. These results will be used to extend crack growth models, while taking the interaction of materials´ properties with the mentioned mechanisms into account. This should enable a physically based, improved lifetime prediction and material selection for certain load patterns.
Fracture of Nano and Engineering Materials and Structures, 2006
Fatigue is based on the initiation and propagation of microcracks. This period can occupy up to 9... more Fatigue is based on the initiation and propagation of microcracks. This period can occupy up to 90% of the lifetime of a cyclic loaded structure. It is well known that short fatigue cracks propagate faster than long cracks at the same level of the stress intensity factor ΔK [1,2,3]. Deshpande, Needleman and Van der Giessen showed that the behavior of
Increasing the resistance of a material to fatigue crack growth by optimizing the microstructure ... more Increasing the resistance of a material to fatigue crack growth by optimizing the microstructure is a major task of materials science. In this regard, grain boundaries and precipitates are well known to decelerate short cracks. Thereby the strength of the interaction is influenced by the crack parameters crack length and distance to the obstacles, the grain boundary parameters like orientation of the adjacent grains and the precipitate parameters like size and distance. A comprehensive understanding of the underlying physical principles is missing. The focused ion beam (FIB) microscope offers new possibilities for systematic experiments and three dimensional investigations to quantify the microstructural impact. The ion beam is used to cut micro-notches as initiation sites for cracks. Contrary to natural cracks the influencing parameters can be varied independently for a systematic investigation of the mechanisms. Additionally, the ion beam is used to make a 3D image of the crack pa...
One problem of the quantitative description of small fatigue crack propagation is the fluctuating... more One problem of the quantitative description of small fatigue crack propagation is the fluctuating crack growth rate induced by obstacles like grain or phase boundaries. Sometimes cracks stop completely for a large number of cycles sometimes cracks only decelerate, both resulting in an additional number of life time cycles. However, so far it is not clear, what actually determines the resistance of a grain boundary against fatigue cracks. Therefore we investigate small crack propagation through grain boundaries systematically by in-situ imaging in the scanning electron microscope and focused ion beam (FIB) crack initiation. By this unique technique, artificial stage I cracks with constant crack parameters can be observed while interacting with different grain boundaries which gives detailed information on the interaction mechanisms. We identified different useful aspects of the interaction between microcracks and microstructural barriers on the microscopic scale. 3D-tomographs reveal...
ABSTRACT The article describes the micromagnetic behavior of non- and pre-plastically deformed hi... more ABSTRACT The article describes the micromagnetic behavior of non- and pre-plastically deformed high strength steel samples under applied stress using different magnetic nondestructive methods such as magnetic Barkhausen noise analysis and hysteresis measurements. It was found that the maximum amplitude of Barkhausen noise (MMAX) increases with applied stress up to a certain point and then decreases again (so-called MMAX(σ)-curve). Changes of magnetostriction, hysteresis curves and magnetic domain structures have been measured and have been further investigated to find out the reasons with respect to macro- and microscopic material behavior. The results obtained are mainly discussed on the basis of the Villari effect and the relation between applied stress and the Barkhausen noise parameters is described. It is concluded that the interaction between crystal and stress anisotropy is the main reason of the specific MMAX(σ)-curve observed.
Stage-I-fatigue-cracks are used as highly localized dislocation sources with well-known Burger’s ... more Stage-I-fatigue-cracks are used as highly localized dislocation sources with well-known Burger’s vectors to study the interaction between dislocations and grain boundaries. This interaction in the plastic zone is of particular interest to understand the fluctuating crack growth in the very short crack regime. In the case of a blocked slip band the dislocations pile up at the grain boundary causing a local stress concentration. The resulting local stress distribution is calculated based on measurements of the dislocation density distribution in the plastic zone. For this purpose the slip line profiles were measured by AFM, the dislocation density distribution was determined and the dislocation-free zone model of fracture (DFZ) was validated. With this it is possible to quantify the grain boundary resistance and to combine geometric and stress approach for grain boundary resistance against slip transfer.
Fatigue induced fracture is the number one reason for failure of technical systems. Thereby there... more Fatigue induced fracture is the number one reason for failure of technical systems. Thereby there are a lot of approaches to strengthen a material against fatigue cracks for instance by using the strengthening effect of grain boundaries. Especially in the stage of short crack growth it is well known that grain boundaries lead to a fluctuating crack propagation rate with a deceleration of the crack growth rate in front of the grain boundaries sometimes combined with a complete stop for a large number of cycles. We identified some of the interaction mechanisms which are responsible for this behaviour by focused ion beam (FIB) artificial crack initiation combined with in-situ fatigue testing in the scanning electron microscope and FIB tomography. The results include useful aspects for fatigue life calculation and to make materials fatigue resistant. Introduction Materials and components exposed to cyclic loading spent up to 90 % of their lifetime by the growth of short cracks which nuc...
Fatigue-crack propagation is, by far, the most crucial failure mechanism of technical systems. Th... more Fatigue-crack propagation is, by far, the most crucial failure mechanism of technical systems. The key to understanding microscopic processes that lead to failure lies in the knowledge of local stresses, the driving force of cracks. We present the mapping of stress and strain fields induced by a single overload on a fatigue crack and their influence on transient crack-growth retardation. In contrast to former work, in which synchrotron x-ray diffraction (XRD) was used, this investigation was performed by using a calibrated magnetic Barkhausen noise microscope in combination with digital image correlation based on in situ scanning electron microscope imaging. The underlying mechanisms, residual stress effects in front of the crack tip and plasticity-induced crack closure caused by the plastic wake, have been studied. Specifically, a crack in S960Q steel has been followed through the overload (OL) region while examining measurements at distinctive overload points: before OL, after OL, at maximum retardation, and at recovery. We observe a strong correlation of the local fatigue-crack-growth rate with the local residual stress distribution obtained from magnetic Barkhausen noise, which remains nearly unchanged after the crack has passed by. Digital image correlation results reveal the influence of these residual stresses on crack-tip opening reactions and strain fields under external loads. Although strain fields show a strong decrease because of the OL event, differences in crack opening stresses remain rather low at first, but prevail in the second part of the OL region. The applicability of new measurement methods and their results regarding the dominating retardation mechanism are discussed. These indicate that the residual stress effect on the strain fields can be associated to be more significant than plasticity-induced crack closure at maximum retardation with a change of mechanisms on reacceleration.
The article focuses on the fatigue performance after a moderate heat treatment of nanocrystalline... more The article focuses on the fatigue performance after a moderate heat treatment of nanocrystalline (nc) nickel, which leads to the formation of a bimodal microstructure in the nc to ultrafine grained (ufg) regime. Electrodeposition was used to produce nc macro nickel samples with grain sizes of about 40 nm for mechanical testing. The thermal stability of the material as well as the influence on the mechanical properties and the fatigue crack propagation behavior was investigated. The results of tensile and fatigue tests are discussed in respect to the chosen production method and boundary conditions. In this context, the influence of the bath additives used during the plating process was investigated and rated as the major challenge for a further improvement of the thermal stability and mechanical properties of the material. Finally, a co-deposition of nickel and metal oxides with enhanced thermal stability is presented.
Abstract Increasing the resistance of a material against fatigue crack growth by optimizing the m... more Abstract Increasing the resistance of a material against fatigue crack growth by optimizing the microstructure is one of the major tasks of modern materials science. Thereby grain and phase boundaries are microstructural elements which can decelerate the propagation rate especially of short cracks. However, in different materials cracks and grain boundaries interact differently. For instance in some steels the blocking effect was only found for large angle grain boundaries while small angle boundaries showed nearly no effect. On the other hand in nickel based superalloys a retardation of cracks was found even for small angle boundaries when the crack was propagating in stage I. Even in front of the same grain boundary, the blocking effect varies for different cracks. To investigate this behaviour systematically focused ion beam (FIB) initiated cracks were used. By this method of artificial crack initiation the crack parameters like crack length and distance to the obstacle can be varied separately. Additionally special grain boundaries can be selected after a microstructural characterisation by electron backscatter diffraction (EBSD). Finally FIB tomography helps to understand the process how a crack overcomes a grain boundary.
Lightweight construction is one of the most demanded technologies in many engineering systems. In... more Lightweight construction is one of the most demanded technologies in many engineering systems. In order to guarantee the safety of the whole system, it is mandatory to improve models that describe and predict its behavior under load. Fatigue, the damaging of materials under cyclic loading, is the main phenomenon leading to failure in e.g. automobile and aerospace components. Cyclic loading during service does usually not happen with constants amplitudes, rather there are complex patterns of different load levels. High load variations in these patterns lead to deviations from the linear Paris behavior. Strong decelerations occur as consequence of a single increased tensile load, which is known as the overload effect. Nevertheless, this effect does not affect all materials the same, there are materials that show a strong overload sensitivity and others on which overloads only have a minor influence. Reasons for this can be seen in the interplay of the underlying mechanisms of the overload effect: plasticity induced crack closure and compressive residual stresses. While both effects lead to crack tip shielding and a reduction of stress intensity, crack closure delays the opening of the crack tip and thereby reduces the effective ΔK range, whereas compressive residual stresses superimpose with crack tip stresses and thereby reduce Kmax. Possible reasons for differences in the sensitivity can be differences in the strain hardening, both in the static and in the dynamic case, as well as in changes of the sign of stresses (Bauschinger effect). Since crack propagation is driven by local stresses and strains, measurements to examine differences in them have to be performed on a microscopic scale. We could show that by the combination of modern measurement techniques – magnetic Barkhausen noise and digital image correlation in scanning electron microscope – we were able to image, separate and evaluate the mechanisms of the overload effect quantitatively. The calibrated magnetic Barkhausen noise microscope allows us measurements of residual stresses with a spatial resolution of 10 µm. From the digital image correlation results we could evaluate the crack tip driving forces namely the crack opening behavior, changes in the stress intensity Kand in the strain energy release rate via the J-integral. Using a simple model based on these results, we were furthermore able to predict the crack growth behavior due to the overload effect. These results will be used to extend crack growth models, while taking the interaction of materials´ properties with the mentioned mechanisms into account. This should enable a physically based, improved lifetime prediction and material selection for certain load patterns.
Fracture of Nano and Engineering Materials and Structures, 2006
Fatigue is based on the initiation and propagation of microcracks. This period can occupy up to 9... more Fatigue is based on the initiation and propagation of microcracks. This period can occupy up to 90% of the lifetime of a cyclic loaded structure. It is well known that short fatigue cracks propagate faster than long cracks at the same level of the stress intensity factor ΔK [1,2,3]. Deshpande, Needleman and Van der Giessen showed that the behavior of
Increasing the resistance of a material to fatigue crack growth by optimizing the microstructure ... more Increasing the resistance of a material to fatigue crack growth by optimizing the microstructure is a major task of materials science. In this regard, grain boundaries and precipitates are well known to decelerate short cracks. Thereby the strength of the interaction is influenced by the crack parameters crack length and distance to the obstacles, the grain boundary parameters like orientation of the adjacent grains and the precipitate parameters like size and distance. A comprehensive understanding of the underlying physical principles is missing. The focused ion beam (FIB) microscope offers new possibilities for systematic experiments and three dimensional investigations to quantify the microstructural impact. The ion beam is used to cut micro-notches as initiation sites for cracks. Contrary to natural cracks the influencing parameters can be varied independently for a systematic investigation of the mechanisms. Additionally, the ion beam is used to make a 3D image of the crack pa...
One problem of the quantitative description of small fatigue crack propagation is the fluctuating... more One problem of the quantitative description of small fatigue crack propagation is the fluctuating crack growth rate induced by obstacles like grain or phase boundaries. Sometimes cracks stop completely for a large number of cycles sometimes cracks only decelerate, both resulting in an additional number of life time cycles. However, so far it is not clear, what actually determines the resistance of a grain boundary against fatigue cracks. Therefore we investigate small crack propagation through grain boundaries systematically by in-situ imaging in the scanning electron microscope and focused ion beam (FIB) crack initiation. By this unique technique, artificial stage I cracks with constant crack parameters can be observed while interacting with different grain boundaries which gives detailed information on the interaction mechanisms. We identified different useful aspects of the interaction between microcracks and microstructural barriers on the microscopic scale. 3D-tomographs reveal...
ABSTRACT The article describes the micromagnetic behavior of non- and pre-plastically deformed hi... more ABSTRACT The article describes the micromagnetic behavior of non- and pre-plastically deformed high strength steel samples under applied stress using different magnetic nondestructive methods such as magnetic Barkhausen noise analysis and hysteresis measurements. It was found that the maximum amplitude of Barkhausen noise (MMAX) increases with applied stress up to a certain point and then decreases again (so-called MMAX(σ)-curve). Changes of magnetostriction, hysteresis curves and magnetic domain structures have been measured and have been further investigated to find out the reasons with respect to macro- and microscopic material behavior. The results obtained are mainly discussed on the basis of the Villari effect and the relation between applied stress and the Barkhausen noise parameters is described. It is concluded that the interaction between crystal and stress anisotropy is the main reason of the specific MMAX(σ)-curve observed.
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