PSI - Issue 13

Ya. Khaburskyi et al. / Procedia Structural Integrity 13 (2018) 1651–1656 Author name / Structural Integrity Procedia 00 (2018) 000–000

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Stress intensity factor (SIF) K is used as a parameter which defines stress state in the crack tip and in a case of fatigue crack – a range of SIF Δ K . From the physical point of view a range of stresses determines a range of cyclic deformation which is responsible for fatigue fracture process. When the fatigue crack closure phenomenon was revealed by Elber W. (1971) effective range of SIF Δ K eff started to consider as a parameter which defines a real range of cyclic deformation in the crack tip. Therefore mechanical driving force of fatigue crack growth is only Δ K eff . Thus crack closure as intrinsic material feature which peculiar to fatigue crack growth process, plays a positive role in fracture retardation. It is shown by Suresh S. et al. (1981), Ritchie R. O. et al. (1982), that irrespective of the mechanisms which cause crack closure effect, it is more intensive for ductile materials and for near threshold low crack growth rates. Such positive crack closure effect on crack growth due to a decrease of a cyclic deformation range in the crack tip induces possibilities to consider an extrinsic interference in the Δ K eff level reduction and as a result – crack growth rate retardation. Earlier, the method of artificial crack closure creation was proposed by H. Nykyforchyn et. al. (1984) which is based on a crack cavity filling with fine dispersed powder. It prevents crack closure during semi-cycle of unloading and thus decreases Δ K eff . Another one by T. P. Venhrynyuk (2013) provides a filling of a crack cavity by liquid composition capable for solidification. A peculiarity of the mentioned method is a necessary of composition solidification at high level of loading close to maximum SIF K max . It prevents crack closure during semi-cycle of unloading and thus decreases Δ K eff level. These methods are complicated in the implementation, since it is often difficult or even impossible to fill the crack cavity with powder or to keep structure element under Δ K max . A novel method of fatigue crack growth retardation which is also based on artificial crack closure creation, but devoid of the above mentioned disadvantages, is described in the paper. A liquid matter as the special technological environment is proposed to use. After falling into crack cavity it chemically interacts with a metal of crack surfaces. The solid products of substantial volume, which form as a result of such interaction, fill crack cavity. The mechanism of this method is similar to intrinsic crack closure, caused by products of interaction between metal and humid air or corrosive environment due fretting-corrosion realization. However the mentioned crack closure mechanism like others is peculiar to crack growth at low Δ K . Therefore the task consists in a search of such substance which would rapidly provide much more intensive interaction with a metal of crack surfaces. High velocity and interaction intensity are necessary to provide an effectiveness of the method at high fatigue crack growth rate. Usage of this method should lead to such crack cavity filling by interaction products in order to decrease Δ K eff to a level sufficient for total stop of crack growth practically in all actual Δ K range. At the same time such substance is only the component of the technological environment which could be used for fatigue crack arrest. The environment should be enough fluid to easy fall into crack and fill it, and chemical substance should be dissoluble to concentrations which retard crack growth effectively. Such chemical substance was found, it is well dissoluble in water and alcohol. The role of solvent is at least in a transport of this substance into fatigue crack cavity. 2. Experimental procedure The experiments were performed on a low strength high ductile steel 20 (0.2С) in a state of supply, its mechanical properties: σ y = 245 MPa, σ uts = 410 MPa, elongation 21%. Specimen made of the organic glass “plexiglass” as inert material, which does not interact with the most chemical substances was used to compare with the steel one. Fatigue crack growth experiments were carried out on beam specimens of 10×20×160 mm in size with one side notch by cantilever bending with frequency of 1 Hz and under stress ratio R = 0. Side surfaces were polished using diamond paste of the different grain. Crack length a was measured on side surfaces of a specimen with an accuracy of 0.01 mm by using the optical microscope with the microdisplacement system. As a result of carried out experiments fatigue crack growth curves da / dN –Δ K were built. During fatigue crack growth tests the crack closure effect was periodically determined by the coefficient U as ratio Δ K eff / Δ K . The diagrams F –  (Fig. 1 а ) were registered during semi-cycle of loading: load on specimen F – mutual displacement  of two points which are symmetrically placed on both sides of crack somewhat higher than the crack tip. Two needle-shaped stems of strain-gauge 1 were fixed on the specimen 2 on both sides from the crack tip on the distance of ~4 mm using the fastening spring elements 3 (Fig. 1 b ).