Evaluation of cyclic elastoplastic strains near notches

Ostash, О. P.; Kostyk, E. M.; Chepil, R. V.; Andreiko, I. M.; Makoviichuk, I. R.

doi:10.1007/bf02538514

Cited by 6 publications

(17 citation statements)

References 14 publications

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“…This agreement happens for low and high loading amplitudes, for all radii of the notch [5,17] in the case where the distribution of local strains in the vicinity of the tip of the notch is calculated by the Usami relations [20] or by the formulas proposed by ourselves in [17,21] under the assumption that p = Pelf for sharp notches [21]. For plastic materials (AMg-6 alloy and 08kp steel), we compared the experimental data for the range of local elastoplastic strains AE* with the values of Ae x calculated on the basis of the curve of cyclic strain of smooth specimens [22] using the Neuber [23] and Glinka [24] approaches as well as calculated by the Stadnik-Riznichuk formula [25] modified for conditions of low-and multi-cycle loadings [17]. It turns out [17] that the best agreement between the experimental data for AE* and the results calculated by the Glinka formula occurs for any of the mentioned laws of distribution of local strains if the quantity x = h + d* is used for the calculation of A~ x [20,21].…”

Section: Plastic Materialssupporting

confidence: 61%

“…We compared the experimental values for the range of local strains Ae* found by the procedure mentioned above with the strain range Ae x, calculated by the known formulas presented in [17], at the tip of the notch (x = h) and at the point (x = h + d* ), i.e., on the boundary of the characteristic zone d*.…”

Section: Plastic Materialsmentioning

confidence: 99%

“…For plastic materials (AMg-6 alloy and 08kp steel), we compared the experimental data for the range of local elastoplastic strains AE* with the values of Ae x calculated on the basis of the curve of cyclic strain of smooth specimens [22] using the Neuber [23] and Glinka [24] approaches as well as calculated by the Stadnik-Riznichuk formula [25] modified for conditions of low-and multi-cycle loadings [17]. It turns out [17] that the best agreement between the experimental data for AE* and the results calculated by the Glinka formula occurs for any of the mentioned laws of distribution of local strains if the quantity x = h + d* is used for the calculation of A~ x [20,21]. However, the agreement between the experimental and theoretical data for blunt notches (p > 4 mm) is observed for strains up to 1.5-2.0%, while for sharp notches (p < 0.75 mm), it is only for strains up to 1.0-1.5% [17].…”

Section: Plastic Materialsmentioning

confidence: 99%

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Unified model of nucleation and growth of fatigue macrocracks. Part 2. Application of deformational parameters of the fracture mechanics of materials in the stage of crack initiation

Ostash¹,

Panasyuk²,

Kostyk³

1998

Mater Sci

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We propose a new approach to the experimental estimation of local strains at the tip of a concentrator. The approach is based on the measurement of displacement of certain points in the vicinity of the tip of a notch, which is further associated with the effective radius of the notch. Various concentrators in structures are simulated within a wide range of variation in radii of notches (p = 0.1-6.5 mm ) and in the geometry of specimens. We establish the main dependences between the value of the range of local strains Ae* and the period N i prior to the nucleation of a fatigue macrocrack. Thus, by using the experimental quantity Ae*, we can estimate the period N i prior to the nucleation of a fatigue crack of length a i = ae for structural elements of a complicated geometry.As is known, the problem of fatigue fracture of materials is an actual problem of modern materials science and fracture mechanics of materials [ 1,2]. In solution of this problem, investigation of the nucleation of macrocracks near stress concentrators in structural elements plays an important role. For this purpose, the force (stress), strain, and energetic parameters of the stress-strain state of the material are mainly used. The force approach to solution of the problem was earlier considered in [2]. Owing to the difficulty of calculations of stresses (force parameters) in the vicinity of various concentrators in structural elements during their service, especially with regard for elastoplastic strains of material and the action of the environment (working medium), it is necessary to search for the most simple approaches to solution of the problem. To a certain extent, this can be realized by using the strain parameters, because strains can not only be calculated but measured directly on members as well.Local strains can be determined, for example, in terms of relative displacements of the lips of a notch [1-3] or a crack [6][7][8][9][10][11]. In particular, this was corroborated by numerous attempts (during the last 5-10 years) to use strain criteria on the basis of the characteristics of the relative displacement 8 in analysis of the kinetics of cracks in a deformable body. Among them, for example, the COD and CTOD [6,7], COA and CTOA [8], ASef f [9], and ~5 [I0] approaches as well as the method of marks [11] should be mentioned. Note that, contrary to the previously known methods regulated by BS (British Standards) [6], the estimation of the value of 8 immediately in the vicinity of the tip of a crack [10] or near the tip of a notch [4,5] is considered now as the main problem.In this work, we present the results of investigation of the possibility of use of strain criteria in determination of the period N i prior to the nucleation of fatigue macrocracks near stress concentrators in certain steels and aluminum alloys. In our investigations, we employ the concept [2] that the main parameters of the process of nucleation of a macrocrack are the range Ae* of local elastoplastic strains near the tip of a notch and the characteristic size a t* ...

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Section: Plastic Materialssupporting

confidence: 61%

Section: Plastic Materialsmentioning

confidence: 99%

Section: Plastic Materialsmentioning

confidence: 99%

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Unified model of nucleation and growth of fatigue macrocracks. Part 2. Application of deformational parameters of the fracture mechanics of materials in the stage of crack initiation

Ostash¹,

Panasyuk²,

Kostyk³

1998

Mater Sci

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“…It was taken that the period prior to the initial microcrack initiation N i was equal to the number of load cycles prior to the formation of a crack = d* * * with length a i [17]. Dependences ( AOy, Ni) and (d, Ni) were constructed by using the stress approach [17], and dependences (Ae*, Ni) and (d*, Ni) by using the strain approach [20,21]. The values of A~y and Ae* were used as the parameters of cyclic crack resistance at the stage of macrocrack initiation for N i = 3 9 105 and N i = 3 9 102 in the low-amplitude and high-amplitude regions of the diagrams, respectively (see Table 1).…”

Section: Experimental Techniquementioning

confidence: 99%

“…These parameters were determined within the frameworks of the stress [15,16] and strain [20,21] approaches. It was taken that the period prior to the initial microcrack initiation N i was equal to the number of load cycles prior to the formation of a crack = d* * * with length a i [17].…”

Section: Experimental Techniquementioning

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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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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