Determination of period of initiation of a fatigue macrocrack at stress raisers

Ostash, О. P.

doi:10.1007/bf00727057

Cited by 2 publications

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“…Consider the possibility of predicting the behavior of the dependences (v, A K) and (v, A E*), which describe the growth of a fatigue macrocrack, with the use of the data obtained at the stage of its initiation (curves ( ACYy, N i), (d*, Ni), and (Ae*, Ni)), and the opposite case where the period of initiation of the initial crack N i near a stress concentrator is calculated from the dependences (v, AK) and (v, Ae*) [16,20,26]. The inverse problem is especially important for a corrosive environment when the procedure of determination of the period N i on the basis of information about the kinetics of the system of microcracks in the vicinity of the concentrator tip is very laborious, especially for low-load amplitudes [ 14].…”

Section: Discussion Of the Resultsmentioning

confidence: 99%

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Initiation and growth of corrosion-fatigue cracks near stress concentrators in V95pchT2 aluminum alloy

Ostash¹,

Kostyk²,

Makoviichuk³

et al. 1999

Mater Sci

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We investigate the kinetics of small fatigue cracks starting from notches in samples of V95pchT2 aluminum alloy in air and in a 3.5% NaCI solution. The influence of salt water is manifested in the initiation of pits on the notch surface and subsequent accelerated growth of small cracks (2-3 times faster than in the laboratory air). The size of an initial macrocrack a i does not depend on the level of stress or strain, notch radius, or environment and is equal to the characteristic distance d* = 100 ~m of the prefailure zone which is a material constant. The effect of crack closure in the presence of a corrosive medium is observed at the crack length a > 100 ~m. Within the frameworks of the approach proposed earlier for the study of initiation of a macrocrack near a notch, the basic dependences of values of local stresses or strains on the duration of the period of initiation of a macrocrack with length a i = d* are established, and the standard diagrams of cyclic crack resistance are constructed for V95pchT2 aluminum alloy at the stage of macrocrack growth in air and in a 3.5% NaCI solution.A corrosive environment substantially accelerates fatigue processes in the vicinity of stress concentrators in the form of clinched openings, fillets, notches, etc. Such a fatigue failure is often observed in the long-term operation of structural parts, made of high-strength aluminum alloys, of aircrafts and ships. It was established that surface pits appear as a result of the action of the corrosive environment [1,2]. These pits serve as nuclei of microcracks and cause their further development. Moreover, in aluminum alloys, pits appear near inclusions of the secondary phases rich with iron and silicon [ 1 ]. The degree of corrosion damage depends on the acting stresses and type of corrosion [2]. According to the Rebinder effect, the adsorption action of the corrosive environment, in particular of C1-ions, promotes an increase of plastic deformation in the surface layer which is manifested in an increase in dislocation density [3]. It is obvious that a decrease of the macroplasticity parameters ~g and ~5 of aluminum alloys by 10-20% reflects the absorption influence of the corrosive environment [4]. In the neighborhood of micro-and macrocracks, the action of the environment is even more substantial. This effect is characterized by the mechanism of enhancement of the local microplasticity in the presence of hydrogen known as the HELP mechanism [5,6]. This mechanism is inherent to high-strength aluminum alloys.The corrosive environment intensifies the development of fatigue cracks. Investigations of the growth of long cracks (macrocracks) in the corrosive environment were performed for high-strength aluminum alloys [7][8][9][10][11][12]. The corrosive environment was modelled by a 3.5% NaC1 solution. It was established that this environment accelerates the growth of both fatigue macrocracks ( 2-3 times in comparison with the growth in air) and fatigue cracks which are short from the microstructural point of view and physically ...

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Section: Discussion Of the Resultsmentioning

confidence: 99%

“…2b). The well-known technique [26] was used in this calculation. Comparison of the calculated and experimental values of N i (see Table 2) showed good agreement.…”

Section: Discussion Of the Resultsmentioning

confidence: 99%

Initiation and growth of corrosion-fatigue cracks near stress concentrators in V95pchT2 aluminum alloy

Ostash¹,

Kostyk²,

Makoviichuk³

et al. 1999

Mater Sci

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“…3) enables one to solve the inverse problem, namely, to determine the period of initiation of a fatigue macrocrack near a concentrator by using the diagram (v, AK) or (v, Ae*) concerning growth of a macrocrack. We consider this problem within the force approach [6].…”

Section: Determination Of the Period Of Initiation Of A Fatigue Macromentioning

confidence: 99%

“…The structure of this function was established at the stage of growth of a macrocrack [3]. However, it was not taken into account that the effect of crack closure is revealed at this stage, which was absent at the stage of crack initiation [4,5].Further, we proposed a model in [4][5][6], within whose framework the process of initiation of a fatigue macrocrack is regarded as primary and defining. The subsequent growth of the macrocrack is considered as a sequence of the same processes repeated in the vicinity of the tip of the original macrocrack.…”

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A unified model of initiation and growth of fatigue macrocracks. part 3. stage of growth of a macrocrack

Ostash¹,

Panasyuk²

1999

Mater Sci

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From a common viewpoint, fatigue fracture of materials is simulated by the process of initiation of an initial macrocrack of length a i = d* (d* is the constant of the material), which is successively (stepwise) repeated at the stage of its growth. As a result, the diagram "range of local stresses -period of initiation of an initial macrocrack" or "range of local strains -period of initiation of an initial macrocrack," which was determined for notched specimens, can be used for the construction of the diagram "growth rate of a macrocrack -range of the stress intensity factor" or "growth rate of a macrocrack -range of local strains," respectively, if the crack is presented as a sharp notch with effective rounded radius Peff = d* of the tip.For estimation of the endurance of a structural material, the endurance limits ~R and ~g of smooth and notched specimens, respectively, and the threshold of cyclic crack resistance Kth of specimens with a macrocrack are the most widespread characteristics. It often occurs that these characteristics correlate with other mechanical and physical parameters of materials in different ways. The reason for this lies in the fact that the characteristics are determined at various stages of fatigue fracture on the basis of fracture criteria for the limiting state of macro-or microvolumes of the material. At present, the opinion exists that the regularities of the processes of initiation and growth of a macrocrack are significantly different and, for this reason, are separately studied. To our mind, the process of fatigue of materials (the initiation of a macrocrack and its propagation) can be studied from a common viewpoint by using unified approaches and parameters and by interpreting the growth of a fatigue macrocrack as the stepwise repeated act of its initiation.In this context, it is worth noting the models of growth of fatigue cracks, which are based on the assumption that a crack can be regarded as a sharp notch [1, 2] with a rounded radius p = P* of the tip, where p* is the constant of the material [2]. The growth of a fatigue macrocrack is considered as a sequence of acts of its reinitiation in the vicinity of the tip of a sharp notch. This growth is described by the parameters that were found with smooth and notched specimens by using the well-known Neuber and Manson-Coffin approaches [1,2]. However, the parameter p* has no clear physical meaning, and methods for direct determination of its value are unknown up to now [2]. The problem of initiation of fatigue cracks was not considered in the works mentioned above.Let us postulate that the formation rate of the unit surface of a fatigue crack is a single-valued function of local strain both in the process of initiation of a macrocrack and in the course of its growth. From the viewpoint of the unified approach to initiation and propagation of fatigue macrocracks, investigations based on the above statement are of high priority. The structure of this function was established at the stage of growth of a macrocrack [3]. However, it w...

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Determination of period of initiation of a fatigue macrocrack at stress raisers

Cited by 2 publications

References 2 publications

Initiation and growth of corrosion-fatigue cracks near stress concentrators in V95pchT2 aluminum alloy

Initiation and growth of corrosion-fatigue cracks near stress concentrators in V95pchT2 aluminum alloy

A unified model of initiation and growth of fatigue macrocracks. part 3. stage of growth of a macrocrack

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