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

Abstract

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* ...

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

“…The existence of such a region is reflected in the initial deceleration or delay and subsequent sharp acceleration of the crack development. For each material, this instant is detected for a certain length of the crack [17].…”

“…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]. 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].…”

“…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).…”

“…The size of this zone a e is the material constant for specified conditions of the experiment. Therefore, the initial size a 0 of a macrocrack is determined by the criterion a 0 = d* [17]. Both approaches are joined by the presence of the experimentally determined transient region of the transformation of a microcrack into a macrocrack.…”

A model of fatigue crack propagation in metals

“…The second stage is characterized by the crack propagation up to a com plete body failure. N owadays, there are attem pts to describe the entire process o f fatigue fracture from a single perspective, w ith the leading role given to the process zones w hich are form ed both during the first (incubation) period and at the crack grow th stage [1][2][3][4][5][6]. The grow ing fatigue crack is regarded as a sharp notch and its grow th is m odeled as repeated crack initiation events w hich follow the sam e law s as those governing the initiation o f the prim ary crack.…”