Behavior Of Short Fatigue Cracks Research Articles

Ex-situ observation of the short crack initiation and propagation behaviour of nickel-base wrought alloy Inconel 718 for critical rotating components under fatigue conditions at elevated temperatures

In this paper, the initiation and propagation behaviour of short fatigue cracks in nickel-base alloy Inconel 718 is studied extensively in the fatigue loading regime by ex-situ scanning electron microscopy on component-near notched specimens in the temperature range between 400°C and 600°C. To investigate the influence of different surface treatments, both specimens with polished and shot-peened surfaces were tested. For polished specimens, most fatigue cracks initiated at oxidised non-metallic inclusions, whereas for shot-peened specimens, the process-induced surface roughness also served as crack starters. It was found, that the observed scatter in lifetime for repetitive tests is mainly caused within the short crack fatigue regime below 50μm, while cracks larger than 100μm showed similar growth characteristics.

Microstructural effects on short fatigue crack propagation in cast Al-Si alloys

The role of microstructure on the short fatigue crack behaviour of cast Al-Si alloy under high cycle loading conditions is investigated by tracking the progress of cracks through fine, medium and coarse microstructured samples. Generally, in the early stage of crack growth, short cracks mainly grow transgranularly, with cracks growth close to low index crystallographic planes (mainly {100}) in some grains and frequently inhibited at grain boundaries. In the late stage, the crack growth mode gradually shifts from crystallographic stage-I to non-crystallographic stage-II. The short cracks grow faster and fluctuate significantly according to local microstructure. Our results show that finer microstructures offer greater crack nucleation and propagation resistance.

Quantitative characterization of short fatigue crack and grain boundary interaction behavior in zirconium

Short fatigue crack growth behavior of zirconium was quantitatively investigated through post-mortem electron backscattering diffraction analyses. Results show that the short fatigue crack growth was mainly transgranular, sensitive to crystallographic orientation which determined the slip systems within grains and often deflected at grain boundaries. The cracks propagated preferentially on prismatic slip planes and pyramidal slip planes due to higher Schmid factor for each grain. A set of crack deflection at grain boundaries was quantitatively analyzed. A combination of the geometrical compatibility factor, m', and twist angle, α, as well as the Schmid factor, m, were proposed to evaluate short fatigue crack growth across grain boundaries. It was found the short fatigue crack could cross grain boundaries on the conditions of m > 0.3, α < 40°, and m' > 0.3.

Effect of microstructure on short fatigue crack growth of wire arc additive manufactured Ti-6Al-4V

In the present paper, the short fatigue crack behavior in wire arc additive manufactured (WAAMed) Ti-6Al-4V was investigated. It was found that the crack in the colony microstructure showed the highest fluctuation range (213 μm) because the crack deflected at the colony boundary. While the crack in the duplex microstructure showed the lowest fluctuation range (48.3 μm) because the crack cut through the αp (primary α) and αs (secondary α) directly. In addition, the short crack growth behavior was determined by the highest Schmid factor (M) and the burgers orientation relationship (BOR), which indicated that the short crack tended to pass through the α/β interface along with the basal slip systems within the shortest path distance. It was suggested that the short crack behavior with colony microstructure was determined by the α/β interface while the crack growth behavior with duplex microstructure was determined by the critical resolved stress (τCRSS) and the αs/β interface.

Growth behavior of short fatigue cracks in a unidirectional SiC fiber-reinforced titanium matrix composite under spectrum loading

In metal matrix composites, the propagation of matrix crack normally leads to load redistribution within the fiber and matrix. The variation of matrix load, on the other hand, affects the growth of the crack in the metal matrix. In this paper, an approach to predict the matrix crack growth rate of a unidirectional SiC fiber-reinforced titanium matrix composite (TMCs) during spectrum loading is developed. The matrix crack was regarded as a mode I central crack. The effects of crack closure and high load hysteresis caused by the variability of were considered. Based on the Forman model, the matrix crack growth rate was also obtained. The results show that the growth rate of matrix crack decreases with the increase of the number of cycles, which can be attributed to the decrease of the stress intensity factor of the matrix crack resulting from the decrease of the load exerted on the matrix. The decreased load of the matrix was caused by the increase of the fiber bridging stress and the redistribution of the stress within the composite when the matrix crack length increases.

Effects of Stress Amplitude Ratio and Phase Angle on Short Fatigue Crack Behavior and Fracture Mode of EA4T Steel

The loads on railway axles in actual service conditions are complex and variable, which may lead to local changes in the fatigue performance, which then affect the fatigue reliability and safety evaluation results of the components and structures. At present, studies on the short fatigue crack behavior and fracture failure mode of EA4T steel, a common material for high-speed trains, are limited. Therefore, fatigue tests on smooth funnel solid round bar specimens under different stress amplitude ratios and different phase angles were performed. Replica technology was used to record the initiation and propagation information of short fatigue cracks. Due to the homogenous and fine microstructure of all the specimens, the same short fatigue crack behavior was observed regardless of the loading condition. The fracture morphology observed by a scanning electron microscope shows that the cracks initiated on the surface of the specimens, and the specific fracture morphology was exceedingly distinct in each region. In addition, the short fatigue crack growth rate and fatigue life were compared and analyzed. The results show that the stress amplitude ratio and phase angle have a considerable influence on the fatigue life and short crack growth rate of the specimens. The results of this research can provide a reference for practical engineering applications.

Effect of Periodic Overloads on Short Fatigue Crack Behavior in CuNi2Si Alloy under Rotating Bending Load

In this study, the short fatigue crack behavior in a precipitation-strengthened CuNi2Si alloy was investigated using a replica technique under rotating bending loads with periodic overloads, an overload ratio of 1.5, and the stress ratio of both was R = −1. The results show that all the fatigue cracks originated from the surface of the specimen and displayed a trend of slow initiation and then rapid propagation. The introduction of overloads significantly reduced the fatigue crack initiation time and the fatigue life of the sample. The average life of the overloaded samples was only 31% that of the constant load samples. For overload specimens, multiple cracks grew at the same time and merged at different stages, causing the crack length to increase instantaneously after they merged, thereby considerably reducing the fatigue life. Fractographical analysis and observation of the surface-etched sample replica film showed that cracks in samples with and without overload both propagated along the grain boundaries.

Short fatigue crack behaviour of LZ50 railway axle steel under multi-axial loading in low-cycle fatigue

An experimental study was carried out on the low-cycle fatigue of LZ50 railway axle steel under multi-axial loading. The initiation and propagation behaviour of a short fatigue crack was observed and analysed as per the principles of effective short fatigue crack. Our results showed that in the microstructurally short crack (MSC) stage, the crack propagation was strongly influenced by the microstructural features of the material and did not satisfy the linear elastic fracture mechanics. At the physically short crack (PSC) stage, the crack was long enough to not be directly influenced by the microstructures; nevertheless, linear elastic fracture mechanics still could not explain the law of crack propagation properly owing to the large-scale yield range of the crack tip. Based on these observations, a crack growth rate model under multi-axial loading was presented to explain the short fatigue crack behaviour. The non-linear elastic-plastic fracture mechanics parameter, J-integral (cyclic), was adopted as the driving parameter in this model and the crack closure effect was also taken into account. In the MSC stage, the crack growth rate fluctuated significantly when the cyclic J-integral was used owing to the influence of the microstructure. However, at the PSC stage, the crack growth rate fluctuations were small. A linear correlation for the PSC was obtained with R = 0.934 and this model could be applied to describe the short fatigue crack behaviour in the PSC stage.

Short fatigue crack behavior of carbon structural steel under tension–torsion loading

Fatigue short crack replica tests of a carbon structural steel, i.e., LZ50 steel, were carried out under tension–torsion loading using MTS electro hydraulic servo machine. Totally eight specimens with smooth axial hourglass shape were tested. The results show that short fatigue cracks generally initiate from the ferrite grain boundary or in the ferrite grain. In the microstructural short crack (MSC) stage, the crack grows slowly, but the growth rate will decrease when the crack encounters microstructural barriers, such as the ferrite grain boundary and the rich pearlite banded structure. In the physical short crack (PSC) stage, the crack breaks through the rich pearlite banded structure and meanwhile the dominant effective short fatigue crack (DESFC) has established its dominant position. As a result, the size and the growth rate of the crack increase rapidly. Based on the fatigue failure mechanism and the safety in tail region predictions, the statistical evolution of the DESFC size and the life fraction was analyzed respectively by comparing six commonly used statistical distributions. The statistical results show that the extreme minimum value (EMV1) distribution is a good assumed distribution for both the DESFC size and the life fraction.

Short Fatigue Crack Behavior of LZ50 Axle Steel Under Rotating-Bending Cyclic Loading

The short fatigue short crack replica tests using LZ50 axle steel hourglass-shaped specimens of were carried out under rotating-bending cyclic load. Seven sets of effective test data were obtained. The analysis demonstrated that short fatigue cracks generally originate in ferrite grains or on their boundaries, and then short cracks start propagating, which is strongly influenced by the microstructure of the material. The crack growth rate increases serratically, followed by its decrease double-fold because of the limit of ferrite grain boundaries and the banded pearlite structure. Based on the three principles of the best fit, which include the overall fitting effect, correlation with fatigue physics, and safety of residual strength prediction, the statistical evolution of the dominant effective short fatigue crack length and life share were analyzed. The statistical results show that the minimum value distribution exhibit the best fit of the dominant effective fatigue short crack length and the life share. The short fatigue crack growth model, which includes various microstructure barriers, provides a good fitting result and reflects the cyclic effect of the microstructure on the short fatigue crack behavior.

Tracking hydrogen embrittlement using short fatigue crack behavior of metals

Understanding hydrogen embrittlement phenomenon that leads to deterioration of mechanical properties of metallic components is vital for applications involving hydrogen environment. Among these, understanding the influence of hydrogen on the fatigue behaviour of metals is of great interest. Total fatigue life of a material can be divided into fatigue crack initiation and fatigue crack growth phase. While fatigue crack initiation can be linked with the propagation of short fatigue cracks, the size of which is of the order of grain size (few tens of microns), that are generally not detectable by conventional crack detection techniques applicable for the long fatigue crack growth behaviour using conventional CT specimens. Extensive literature is available on hydrogen effect on long fatigue crack growth behaviour of metals that leads to the change in crack growth rate and the threshold stress intensity factor range (ΔKth). However, it is the short fatigue crack growth behaviour that provides the fundamental understanding and correlation of the metallic microstructure with hydrogen embrittlement phenomenon. Short fatigue crack growth behaviour is characteristically different from long crack growth behaviour showing high propagation rate at much lower values than threshold stress intensity factor range as well as a strong dependency on the microstructural features such as grain boundaries, phase boundaries, and inclusions. To this end, a novel experimental framework is developed to investigate the short fatigue crack behaviour of hydrogen charged materials involving in-situ observation of propagating short cracks coupled with image processing to obtain their da/dN vs a curves. Various metallic materials ranging from austenitic stainless steel (AISI 316L) to reactor pressure vessel steel (SA508 Grade 3 Class I low alloy steel) and line pipe steels (API 5L X65 & X80) are studied in this work.

Short fatigue crack behaviour under low cycle fatigue regime

This paper addresses the important issue of the description of short fatigue crack behaviour. It is typical for these cracks that they propagate under large scale yielding conditions at the crack tip, which means that the non-linear fracture mechanics has to be applied. This paper presents results of experiments designed to measure the short crack growth rates in five different materials – 316L steel, Eurofer 97 steel, ODS Eurofer steel, Duplex 2205 steel and Al 6082 alloy. The crack growth rates of these materials are described using different fracture mechanics parameters – the stress intensity factor, the J-integral and the plastic part of the J-integral. These approaches are evaluated and compared. The comparison revealed that the plastic part of the J-integral is the parameter governing the short crack growth rate in large scale yielding conditions. Moreover, crack growth rate data from all the tested materials measured at various loading levels lies on a unique curve. This remarkable observation suggests that the crack growth rate is determined by the extent of energy spent to plastic deformation, irrespective of the other materials properties.

ステンレス鋼 SUS304 の微小疲労き裂の進展下限界に及ぼす水素の影響

For the safety use of hydrogen utilization systems, it is necessary to understand properly the effect of hydrogen on the fatigue properties of structural materials. In the present study, fatigue life tests of 304 stainless steel with small artificial defects were carried out under the presence of hydrogen; the tests in hydrogen gas environment and the tests using hydrogen-charged specimens, and the effect of hydrogen on the fatigue limit of the material was investigated by paying attention to the behavior of short fatigue cracks. As a result, the fatigue limit was not degraded in hydrogen gas environment, whereas there was slight degradation of fatigue life in short life regime (i.e. Nf < 105 ). On the other hand, fatigue limit of hydrogen-charged specimens was increased compared with that of non-charged specimens. It is noteworthy that non-propagating cracks existed in the unbroken specimens in all environmental conditions. As is well known, fatigue limit of steels with low or moderate strength is determined by the non-propagation limit of small fatigue cracks. Consequently, it was revealed that the fatigue limit of the specimens was determined by the arresting limit of fatigue cracks, which was not degraded in the tests in hydrogen gas as well as in the tests using hydrogen-charged specimen.

The behavior of short fatigue cracks during Very High Cycle (VHCF) Fatigue of duplex stainless steel

During VHCF loading of duplex steels fatigue damage starts within the softer austenite phase. Depending on the spatial mismatch between the slip systems of neighboring grains, intergranular or transgranular fatigue cracks are initiated. If these cracks can propagate or are blocked depends on the interactions with the local microstructure. These interactions have been studied by ultrasonic fatigue testing in combination with electron microscopy and synchrotron diffraction experiments. It was found that the strength of the first microstructural barrier is decisive for VHCF life. The barrier strength as a function of the microstructural arrangement has been implemented in a micromechanical finite element approach that shall be used to predict the VHCF life or the existence of a fatigue limit for given microstructure parameters.

Short fatigue cracks nucleation and growth in lean duplex stainless steel LDX 2101

This work is focused on the fatigue damage of lean duplex stainless steels (LDSSs) LDX 2101. Special interest is placed on analyzing short fatigue crack behavior. In this sense, short crack initiation and growth during low cycle fatigue (LCF) and short crack nucleation during high cycle fatigue (HCF) of this LDSS have been studied. The active slip systems and their associated Schmid factors (SF) are determined using electron backscattered diffraction (EBSD). Additionally, the dislocation structure developed during cycling is observed by transmission electron microscopy (TEM). Regardless of the fatigue regime, LCF and HCF, short cracks nucleate along intrusion/extrusions in ferritic grains. Moreover, during the LCF phase boundaries decelerate short crack propagation. These results are rationalized by the hardness of the constitutive phases and the dependence of screw dislocation mobility in the ferrite phase on strain rate and stress amplitude.

A Fractal Study on Collective Evolution of Short Fatigue Cracks under Various Strain Amplitudes

The fractal dimension is a basic parameter to indicate the random self-similar shape and phenomena. The damage process is the result of co-effect of all the cracks, which shows a good collective behavior and random statistical complexity. The collective evolution of short fatigue cracks was experimentally studied through cylindrical specimens with annular notches with respect to the variation of nominal strain amplitudes. The maximum between-class variance method (Otsu's method) was adopted for the denoised image binarization and the fractal dimensions were obtained. The results show that: the collective behavior of short fatigue cracks possesses good fractal characteristics; as the evolution of the short fatigue cracks, fractal dimensions underwent two stages: a primary stage of high growth speed, a relatively stable stage of almost to zero growth speed; the critical cut-off point at about 30% of the fatigue life according to the experimental results can be used to represent a threshold value of the MSC and PSC of short fatigue cracks.

INFLUENCE OF SURFACE ROLLING ON SHORT FATIGUE CRACK BEHAVIOR FOR LZ50 AXLE STEEL

Crack initiation, coalescence and propagation normally occupy more than 70% the fatigue life of most engineering structure in the whole fatigue damage process. Investigation on the influence of final machining methods, such as surface rolling, on short fatigue crack behavior for LZ50 axle is beneficial to the manufacture and maintenance of corresponding railway axles. After rolling, the surface hardness increased from 201.68 HV0.1 to 222.90 HV0.1. Much higher residual compressive stress was also engendered in surface and sub-surface by this machining method. Totally six smooth hourglass shaped specimens with surface rolling were tested by a replica technique. The characteristic two-stages behavior, that is, the micro-structural short crack (MSC) stage and the physical short crack (PSC) stage, during crack initiation and propagation was revealed. In MSC stage, the growth rate of dominant short crack for all specimens decelerated twice clearly due to different microstructural barriers. This behavior was related to the restraint of ferrite grain boundary firstly and then to the constraint of pearlite banded structure. While in PSC stage, the decelerating trend was no longer obvious with the increasing dominant crack size. With a given dominant crack size, the crack growth rates of surface rolled specimens were much slower than those of specimens without surface rolling. This difference could reach 1 order of magnitude in MSC stage. Meanwhile, the average fatigue life of the former was about 5.4 times longer than that of the latter. The effective short fatigue crack density

Study on Group Behavior of Short Crack for Low Cycle at High Temperature Based on Fractal Theory

The propagation of fatigue short crack is a fractal curve, the irregular path can be described by fractal dimension. The fractal dimension of the crack implicit in many of the physical factors, such as microstructure of material, load conditions and so on. In this paper, the short crack for low cycle at high temperature is studied. The material 20# steel in the various stress state is experimentally studied, the initiation, propagation and coalescence influence of short crack are observed. By analyzing experiment results, characters of crack fractal dimensions evolving with the cycle process are as follows, the behavior of short fatigue cracks satisfies fractal feature, with the development of the fatigue process, the fractal dimension is increased.

Experimental research on dominant effective short fatigue crack behavior for railway LZ50 axle steel

Behavior of dominant effective short fatigue crack, the crack results in the final failure of specimen, was experimentally investigated by a replica technique with seven smooth hourglass shaped specimens of railway LZ50 axle steel. Character of two-stages, i.e. the micro-structural short crack stage and the physical short crack stage, was revealed for the crack initiation and growth. Most importantly, the crack growth rate exhibited decelerations twice in micro-structural short crack stage. This behavior was related to the ferrite grain boundary firstly and then to the pearlite banded structure. The boundary appeared a barrier because there were pearlites around with significant higher micro-hardness values. Also because of the higher hardness values, the layered pearlite banded structure acted as another barrier for the crack to overcome. In physical short crack stage, the crack propagated with a decreasing resistance of micro-structural barriers as the crack length increased. Hence, the process of crack initiation and growth are subjected to the competition between the intrinsic resistances from the barriers and the increasing driving force from the growing crack size. Finally, a new short crack growth model is presented to describe the periodic influence of micro-structural barriers. Description to the test results of LZ50 steel has indicated the availability and reasonability of present model.

Experimental Observations of Dominant Effective Short Fatigue Crack Behavior for Railway LZ50 Axle Steel

, was experimentally investigated by a replica technique to seven smooth hourglass shaped specimens for railway LZ50 axle steel. Character of two-stages, i.e. the micro-structural short crack (MSC) stage and the physical short crack (PSC) stage, was revealed for the crack initiation and growth. Most importantly, the crack growth rate exhibited decelerations twice in MSC stage. This behavior was corresponding to the ferrite grain boundary firstly and then to the pearlite banded structure. The boundary appeared a barrier because there were pearlites around with significant higher micro-hardness values. The banded structure appeared a barrier because each band was rich in hard layered pearlites for the crack to cross. In PSC stage, the crack propagated with a decreasing resistance of micro-structural barriers as the crack length increased. The two barriers are inherent in the material and the crack initiation and growth are subjected to an evolutionary process under competition between the inherent resistances from the barriers and the increasing driving force from the growing crack size. This provides a prehensive understanding of the crack initiation and growth.