Papers by Keyword: Short Crack

Abstract: The fatigue crack propagation tests for circular notched compact tension specimens of GH4133B superalloy used in a turbine disk of aero-engine are carried out at room temperature. The test data shows that in the transition region from short to long crack growth, the crack growth rate appears as a fluctuation form with acceleration following deceleration. A quasi harmonic function is constructed to model the fatigue crack growth in the transition region, and the result indicates that such function is suitable to describe the crack growth behavior. A metallographic analysis with respect to microscopic observation for the specimen’s surface suggests that the wave period, that is, a step length of a cycle fluctuation from acceleration to deceleration on the crack growth rate curve is agree well with the intrinsic scale of grain size, which indicates that in the transition region from short to long crack growth, grain boundary plays a dominant role in crack growth rate. Finally, the fracture surfaces of specimen are observed using a scanning electron microscopy. It can be found from the fracture surface morphologies that in the transition region from short to long crack growth, the stress intensity factor range is still lower than the fatigue crack growth threshold ΔK_th, the effect of microstructure, such as grain size, grain boundary, secondary phase particle, and inclusion ahead of the crack tip, and the effect of crack closure on crack growth behavior should be considered, the local further plastic deformation at crack tip will meet larger resistance, so some twin bands are observed on the fracture surface. As crack length increases with increasing fatigue cycle, the crack propagation is noted to change from a transgranular crack propagation mode, a mixed transgranular and intergranular crack propagation nature, to intergranular crack propagation manner.

Abstract: In general, aluminum alloy does not exhibit distinct fatigue limit (knee point) in the S-N diagram. The growth of a small fatigue crack of precipitation-hardened Al-Mg-Si system alloy (6061-T6) was investigated to clarify the mechanism of non-appearance of distinct fatigue limit (knee point) in the S-N diagram. The small crack was analysed in detail by replica method, scanning electron microscope (SEM), and Electron Back Scatter Diffraction Patterns (EBSD). On the other hand, the existence of distinct fatigue limit (knee point) of new developed aluminum alloy by adding excess Mg to the 6061 alloy was found. In this study, the resistance of small crack growth of the developed alloy was compared with standard 6061 alloy. It was revealed that the resistance of crack growth of new developed alloy was higher than that of standard 6061 alloy in short crack region (l<1.0 mm).

Abstract: The mechanisms of fatigue crack initiation due to second phase particles are studied in 2050-T8 and 7050-T74 plate material. The particles in the specimens gauge lengths are imaged using SEM at the initial state. In 7050-T74, Mg₂Si particles are very often cracked before any loading, whereas Al₇Cu₂Fe particles are not. In 2050-T8, the fraction of (Al, Cu, Fe, Mn) particles initially cracked is larger than that of Al₇Cu₂Fe in 7050-alloy, but lower than that of Mg₂Si particles for similar sizes. For (Al, Cu, Fe, Mn) particles, the proportion of cracked particles increases when the modified shape ratio (aspect ratio including orientation versus rolling direction) increases. This effect is present but less pronounced for Mg₂Si particles in 7050-T74. Fatigue cracks initiate at cracked (Al, Cu, Fe, Mn) particles in 2050-T8 alloy, and at both Al₇Cu₂Fe (cracked during cycling) and Mg₂Si in 7050-T74 alloy.

Abstract: The corrections incorporated in fatigue crack growth prediction programs for crack closure are usually tested by their ability to predict retardation following an overload and for the accuracy of their prediction lives for long cracks greater than about 1mm. They should, however, be examined on their ability to predict the life of cracks that grow from small sizes, such as small inherent material discontinuities, to failure, which is more typical of service situations and the growth produced by small cycles as well as the larger cycles. To examine the extent of crack closure in aluminium alloy 7050-T7451 and the prediction of that growth, quantitative fractography measurements of short periods of fatigue crack growth produced with a specially engineered spectrum were conducted and are reported here. The spectrum contained bands of constant amplitude loads with diminishing mean stress designed to examine the extent of closure. The quantitative fractography results are compared to predictions by the common analytical programs FASTRAN and AFGROW and further with a basic effective stress intensity calculation method at a crack depth of about 1mm. The results showed that the analytical programs were able to predict the presence of closure; however, the extent of the closure was not accurately predicted.

Abstract: It has often been observed that the growth of short fatigue cracks under variable amplitude (VA) cyclic loading is not well predicted when utilising standard constant amplitude (CA) crack growth rate/stress intensity data (da/dN v DK). This paper outlines a coupon fatigue test program and analyses, investigating a possible cause of crack growth retardation from CA-only testing. Various test loading spectra were developed with sub-blocks of VA and CA cycles, then using quantitative fractography (QF) the sub-block crack growth increments were measured. Comparison of these results found that, after establishing a consistent uniform crack front using a VA load sequence, the average crack growth rate then progressively slowed down with the number of subsequent CA load cycles applied. Further fractographic investigation of the fracture surface at the end of each CA and VA sub-block crack growth, identified significant crack front morphology differences. Thus it is postulated that a variation or deviation from an efficient crack path is a driver of local retardation in short crack growth during CA loading. This may be a source of error in analytical predictions of crack growth under VA spectra loading that may need to be considered in addition to other potential effects such as less closure whilst cracks are small. For aircraft designers, using solely CA data for fatigue life predictions this may result in non-conservative estimates of total crack fatigue life, producing unexpected failures or an increased maintenance burden.

Abstract: The behavior of short cracks, focusing on the study of small manufacturing flaws in a critical structure working under a high frequency load spectrum, is a complex and very current topic and its investigated in this study. The modeling and predicting of the effect of these flaws is a matter of interest also for industrial manufacturers with regards to the future design of high stressed components. Aeronautical components in general, but particularly helicopter components, are subjected to a high number of fatigue cycles that potentially cause the propagation of very small flaws and thus require careful analysis. The application of multi-axial loading conditions such as axial and torsional load complicate the assessment by provoking a multimode propagation. A high cycle multi-axial fatigue loading condition (torsional and axial) on a tube that is representative of a helicopter main transmission shaft has thus been investigated in this work. The presence of a small flaw has been artificially created on the surface of the tube and the behavior of the defect in terms of the threshold of the propagation of a crack emanating from it under high cycles fatigue due to axial and torsional stresses has been investigated by means of analytical and FE models. Experimental test have also been carried out to validate the results of the simulations.

Abstract: The simultaneous effect of crack length and crack front shape on plasticity-induced crack closure (PICC) for a 304L austenitic stainless steel is simulated through 3D numerical modelling using finite element software Abaqus for through-thickness cracks with different curved crack fronts in CT specimens in comparison with bidimensional through crack with a straight front. The influence of possible loading history effect is avoided by applying constant K amplitude. The local stress intensity factor range for crack opening K_op is evaluated from the simulation of the loss of the last local contact between the crack lips near the crack tip. The pertinence of the different crack front shapes is discussed in term of the effective stress intensity factor range K_eff and in comparison with the experimental crack front observations.

Abstract: Short cracks play the dominant role in material fatigue damage process, from the physical point of view, the solid containing cracks is actually a non-linear dissipation system, so it can be described by fractal theory. The evolution of short fatigue crack is a fractal curve, the irregular process can be described by fractal dimension. The material 20 steel in high temperature low fatigue is experimentally studied, the evolution of short cracks are observed. By analyzing experiment results, the fractal dimensions evolving with the evolution process are as follows, with the development of the damage degree, the fractal dimension is increased, and the dimension of the crack implicates many physical parameters, it can provide a reference for understanding the damage degree of material, it also provide reference for the life of short crack stage of material.

Abstract: Collective short fatigue behaviour nearby material micro-structural barrier threshold is experimentally observed for the smooth specimens of Chinese railway LZ50 axle steel. Effective micro-cracks which result in the specimens’ failure were initiated from the material ferrite particles on the surfaces of the specimens. Four elements i.e. crack size, orientation angle, and the two crack tip orientation angles should be contained for each micro-crack. The collective micro-crack behaviour should be described by an equivalent dominant effective short fatigue crack (ESFC) concept using an ESFC theory. And an equivalent method is then developed with an equivalent growth driving energy concept. A random modeling of the equivalent dominant ESFC growth rates is further constructed for describing the random behaviour of the collective ESFCs nearby the material micro-structure barrier threshold. And the kinetic threshold is subsequently conducted from the modeling.

Abstract: The effect of recrystallization on the low cycle fatigue life of DZ4 directionally solidified superalloy was investigated for specimens with three different recrystallized layers, which were generated by shot peening (0.1MPa, 0.3MPa and 0.5MPa respectively) and a subsequent annealing heat treatment. The fatigue life showed a decrease for recrystallized specimens with shot-peening of 0.1 MPa and 0.3 MPa, and an unusual increase for that of 0.5MPa, in comparison with the original DZ4 specimen. In-situ SEM observations were performed on the short crack growth behaviors for both original and recrystallized specimens, which revealed the fracture mechanism and the interaction with microstructure. Quantitative analysis of fatigue crack growth rates rationalized the influence of recrystallization on the low-cycle fatigue life of DZ4.