Papers by Keyword: Short Fatigue Crack

Abstract: The fatigue behavior of 2A12 Aluminum alloy was experimentally studied through different annular notched specimens under symmetrical triangle with the frequency of 0.5 Hz. The experimental result showed that the microstructure played an important role during the entire fatigue life of 2A12 Aluminum alloy. The short fatigue crack only initiated due largely to the second-phase particle such as the S phase (Al₂CuMg), the θ phase (CuAl₂) and especially the black impurity phases debonding from the basal body when the fatigue cycle was sufficient. The cracks propagated separately along the circumferential direction of the notch, crack coalescence and interaction of cracks were not common at early and middle stage of short crack’s fatigue life. Cracks tended to propagate along direction different from the original one after crack coalescence. Surface crack length at early and middle stage of short crack’s fatigue life were presented. These curves showed crack growth rate increased relatively as the reducing of notched radius or the increasing of nominal strain amplitude which implied that geometry and loading conditions were the factors of the crack propagation.

Abstract: 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.

Abstract: The collective evolution of short fatigue crack was experimentally studied through differently-notched cylindrical specimens under symmetrical triangle wave load. A simple and feasible method to measure the damage variable was introduced. The crack length and surface damage variable were obtained by the observation system, the measurement software and the Matlab program. The results show that: Grain boundary is the key factor leading to plateau regions phenomena during the increasing process of crack length; the group behavior of short fatigue crack is measurable and can be described by surface damage variable (D); the fitting curves of D-N/N_f present remarkable stage characteristics.

Abstract: Short fatigue crack density evolution on specimen surface of railway LZ50 axle steel is studied experimentally by local and overall viewpoints, respectively. Results indicate that the density of effective short fatigue cracks (ESFCs) around dominant crack, which results in the specimen failure, evolutes sensitively to the changes of the dominant crack size and tip locations. In accordance with the previous criterion of ESFCs, this density evolution is more suitable for describing the intrinsic and dynamic localization and randomization of fatigue damage. At the same time, statistical deriving is applied to address the randomness of the evolution behavior. It is revealed that the density data increases with the dominant crack growth in the micro-structural short crack (MSC) regime, and keeps declining in the physical short crack (PSC) regime. Coefficient-of-variations exhibit a contrary tendency. Appropriate distribution is further determined using a previous comprehensive statistical comparison method. Random characters and quantitative measurements of the density data have been well depicted.

Abstract: During high-cycle-fatigue loading of metastable austenitic steel AISI304L, the elastic anisotropy between neighboring grains causes the occurrence of stress peaks at grain boundaries, which again act as crack nucleation sites. This is in particular the case at twin boundaries. Cyclic crack tip plasticity leads to a transformation from  austenite to ´ martensite when different slip bands are activated, alternating during their operation. By means of in-situ fatigue testing in a scanning electron microscope (SEM) in combination with electron back-scattered diffraction (EBSD), the distributions of grain size, geometry, and crystallographic orientation relationship were correlated with the local occurrence of slip, martensite formation and fatigue-crack initiation and propagation. It was shown that the extent of martensite formation ahead of a propagating crack increases with increasing crack length and eventually, due to its higher specific volume, gives rise to transformation-induced crack-closure effects. The variation in the crack-propagation rate depending on the local microstructure was simulated by means of a short crack model, where the displacement fields within the crack, the adjacent plastic zone and the grain boundaries in combination with the martensite volume increase strain are superimposed by means of a boundary-element approach.

Abstract: As one of the key factors to induce crack closure, crack surface roughness plays a dominated role in evaluating the characteristics of short fatigue cracks. The crack retardation induced by the crack surface roughness was investigated by numerical simulation in this paper. The influence of crack surface roughness on short fatigue cracks was directly applied into the numerical simulation procedure with the help of friction coefficients on the contacted crack surfaces. The driving forces of cracks represented by the two indicators such as crack tip displacement and plastic strain range were studied and used to characterize the retardation induced by crack surface roughness. It is shown that the evidence of crack surface roughness affecting on crack retardation is obvious and the influence of crack surface roughness on the characteristics of short fatigue cracks must account for in evaluating the performance of short fatigue cracks.

Abstract: The short fatigue crack initiation of LZ50 axle steel for railway vehicles was investigated by numerical simulation in this paper. The microstructure of LZ50 steel was constructed with the application of 2D Voronoi tessellation. The stress and strain distributions in the microstructure were obtained by FEM under the boundary condition shifted from loading level applied in fatigue specimen of this steel. Finally, the probability of short fatigue crack initiation was given with different loading cycles to illustrate the process of crack initiation of LZ50 steel under the given loading cycles based on the S-N curve of the material. The further work on the research of crack growth and collective evolution of short fatigue cracks can be conducted with the simulated results of crack initiation in the microstructure of LZ50 steel.

Abstract: The behavior of short fatigue cracks under variable amplitude loading (VA) was investigated by FEM. The crack closure induced by the crack surface roughness was taken into consideration by using the contact between these crack surfaces. The effects of variable amplitude loading on the performance of short cracks are demonstrated with factors such as grain orientation and misorientation, crack length and the friction efficient between the contacted crack surfaces. Through the two indicators, crack tip opening displacement represented by "CTOD and "CTSD and the plastic strain range of crack tip, the characteristics of short cracks affected by loading blocks are discussed in detail. It is shown from the numerical results that the significance of the design of loading blocks in the fatigue experiments is evident and the performance of short cracks from the variable amplitude loading is more effective due to the closer to practice.

Abstract: Surface and depth morphology evodslutions of short crack propagation of 1Cr18Ni9Ti weld metal are investigated. In accordance with the previous effective short fatigue crack (ESFC) criterion, attention is paid on the formation zone of the dominant ESFC (DESFC), which resulted finally in the specimen failure, in micro-structural short crack (MSC) regime and then, the tip zone(s) ahead of the DESFC in physical short crack (PSC) regime. Results show that in MSC regime the surface ESFCs were imitated from the distributed randomly delta ferrite bounds separated from austenite matrix. The initiated ESFCs on surface propagated perpendicularly to loading axle. But in depth direction, the initiated ESFCs grew first similarly to the surface behaviour but lately, tended to be perpendicularly to the formation direction of the material columnar grain structure. When sizes of some longer ESFCs reached around the material maximum barrier size, coalescence occurred to form a true DESFC. In PSC regime the surface DESFC grew almost perpendicularly to loading axle. But in depth direction, it grew first perpendicularly to the formation direction of the columnar grain structure and then, tended to having 45 degree angle to the loading axle. Obviously, the evolutions were strongly affected by interactions between the load and the microstructures, especially, the columnar grain structure.

Abstract: Fatigue crack propagation is the chief style leading to the damage of engineering components. With the design change of modern engineering structure from conservative “safe-life” to “endurance/damage limit”, it brings new big contradiction. Bigger risk occurs when serving time of components extends to maximum. Regular NDT (Nondestructive Testing) methods cannot monitor the initiation and expansion process of short cracks inside components effectively. As an advanced NDT method, ICT (Industry Computed Tomography) has some particular advantages which regular NDT methods do not have. As a result, adopting the latest achievements on industry CT and fatigue fracture, this paper presented a new supervision method for short fatigue cracks propagation based on density field analysis. Experiments have proved that our proposed method could be a new way to inspect the components’ damage in service even to predict their life.