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* ...
According to the phenomenological model of nucleation of a fatigue macrocrack, the process is considered as a two-parameter process. The process is described by the local stress or strain range and a certain linear parameter of the material. We propose the corresponding parameters, namely, the local stress range At~. and the characteristic size d" of the prefracture zone. The formation of this zone is caused by the anomaly of the yield strength of the material in subsurface layers, the microstructure, the loading amplitude, the cyclic strain hardening, and the environment. The quantity d* is a constant of the material, which is independent of the geometry of notch and specimen. The boundary of the prefracture zone is considered as a macrobarrier that determines the growth of microstructuraliy short and physically small cracks. The moment when a physically small crack oversteps the boundary of the prefracture zone is defined by the quantitative criterion (ao= d* ) of the initial size a 0 of a macrocrack in the material. The proposed dependences of ( A~., At,.) and (d*, Ni) can be regarded as a basis for the determination of characteristics of resistance of the material to the nucleation of a fatigue macrocrack.Due to the rapid development of fracture mechanics, especially structural fracture mechanics, in the past ten years considerable progress has been made in understanding the phenomena of fatigue of metals. A detailed analysis of successes and failures concerning the solution of this problem is given in [1,2]. By summing our own data and literature data, we propose to consider fatigue of materials as a process that consists of the following main stages: 1) nucleation and growth of microstructurally short cracks, 2) growth of physically small cracks and formation of a macrocrack, and 3) growth of the macrocrack up to total fracture of the body.Within the framework of this approach, the endurance limit of the material is estimated by the maximum stresses for which a crack in the material does not grow, i.e., the crack is retarded by certain physical barriers (crack resistance of the material). These barriers are caused by the microstructure of the material (boundaries of grains and twins, sizes of pearlitic colonies and martensitic packs, etc.). Thus, the endurance limit is, to a great extent, connected with a certain linear parameter of the structure of the material, in particular, with the distance between structural barriers [2]. At the same time, it is necessary to take into account the following fact: if the microstructural barrier of the material is considered as the main factor for its endurance limit, then it is impossible to explain some experimental data. For example, there exist qualitatively opposite dependences of the fatigue limit t~ R of a smooth specimen and the fatigue threshold Kth of a cracked specimen on the grain size for the same material [3]. It is also known [4] that for a given loading amplitude, the growth rate of microstructuraily short and physically small cracks is greater than that ...
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