Deformation approach to the evaluation of the period of initiation and growth of fatigue macrocracks

We performed experimental investigation of the opening displacements of the contours of stress concentrators (notches and cracks) for various amplitudes of cyclic loading. On the basis of experimental results, we propose a new deformation parameter _t which is a function of the notch (crack) tip opening displacement , namely, _t /(+d*), where is the radius of the tip of the notch andd* is the characteristic size of the prefracture zone. It is shown that this parameter uniquely determines the number of cyclesN _l to the initiation of a fatigue macrocrack independently of the geometry of the specimens and stress concentrators in elastic and elastoplastic materials, i.e., the dependence of _t onN ₁ is a characteristic of the material. It is experimentally demonstrated that this dependence enables one to quantitatively describe the process of fatigue fracture both in the stage of initiation of macrocracks and their propagation.Karpenko Physicomechanical Institute, Ukrainian Academy of Sciences, L'viv. Translated from Fiziko-Khimicheskaya Mekhanika Materialov, Vol. 31, No. 5, pp. 7–21, September – October, 1995.     

New approaches in fatigue fracture mechanics

It is shown that the size d* of the process zone is a basic parameter of the material depending on its microstructure, mechanical properties, and testing conditions and specifying various stages of the fatigue fracture of materials and structural elements. The proposed unified model of fatigue fracture based on the parameter d* makes it possible to determine the service life of cyclically loaded structural elements with stress concentrators solely according to the diagrams of fatigue macrocrack growth rate. It is assumed that the size d* of the process zone determines the specific features of the well-known regularities of fatigue fracture, including the characteristic points in the Kitagawa-Takahashi and Smith-Miller diagrams. __________ Translated from Fizyko-Khimichna Mekhanika Materialiv, Vol. 42, No. 1, pp. 13–25, January–February, 2006.     

Prediction of the Durability of Cyclically Loaded Structural Elements

A procedure of determination of the durability of cyclically loaded notched specimens is proposed and experimentally verified. The procedure is based on the concepts of the unified model of fatigue fracture treating the processes of initiation and propagation of a fatigue macrocrack from the common point of view. For specimens with structural stress concentrators of two types, we compute the periods of initiation and growth of a fatigue macrocrack and the number of loading cycles to failure on the basis of the diagrams of fatigue crack growth rates. The numerical results agree with the experimental data with an error of at most 38% depending on the method of calculations and durability. __________ Translated from Fizyko-Khimichna Mekhanika Materialiv, Vol. 41, No. 4, pp. 39–44, July–August, 2005.     

Effect of copper on the cyclic crack resistance and heat resistance of graphitic steels

We study the effect of copper content (0.05–3.45 mass %) on the structure of the matrix and graphitic phase of graphitic steels (GS) annealed for globular perlite. It has been established that the effect of copper content on the physicomechanical characteristics of GS is ambiguous. An increase in the copper content up to 2.3–2.9% is accompanied by an increase in the characteristics of thermal conductivity, strength, cyclic crack resistance, and heat resistance. The thermal conductivity of GS grows with increase in the copper content (if it is more than 3%), but the mechanical characteristics fall. Thus, the heat resistance of GS is mainly determined by their crack resistance.Translated from Fizyko-Khimichna Mekhanika Materialiv, Vol. 40, No. 3, pp. 109–112, May–June, 2004.     

Influence of the Structure and Asymmetry of Loading Cycles on the Cyclic Crack Resistance of Ti–Si Composites

We analyze fatigue-crack growth rate diagrams for various modifications of a Ti–Si cermet alloy in the initial (as-cast) state and after thermomechanical treatment plotted for beam specimens subjected to three-point bending. It is shown that, for different asymmetries of the loading cycle, the maximum stress intensity factor K _max better describes the behavior of the growth rate of fatigue macrocracks in the high-amplitude part of the diagrams than the range of the stress intensity factor K. The threshold crack resistance of the Ti–Si composite under cyclic loading is 2–5 times lower than under long-term static loading. For highly asymmetric loading cycles (R = 0.6), the maximum cyclic crack-growth resistance is exhibited by a modification with the structure of grains of pseudo--titanium matrix 20–40 m in size and 15–30 wt.% of dispersed precipitations of titanium and intermetallic silicides 5–10 m in size.     

Influence of Microstructure on the Strength and Cyclic Crack Resistance of Cast Irons

We study the influence of microstructure of high-strength cast irons of ferritic, ferritic-pearlitic, and pearlitic classes on the characteristics of strength and cyclic crack resistance and establish the relationship between the characteristics of strength and cyclic crack resistance and the chemical composition and microstructural parameters of the matrix and graphite inclusions of high-strength cast irons. Indeed, the ultimate strength _u = 750–850 MPa, fatigue threshold K _th = 8–10 MPa , and cyclic fracture toughness K _fc = 50–60 MPa are guaranteed by the pearlitic matrix and the following parameters of the graphite phase: the content of graphite f _p 10%, the degree of its spheroidization of about 95%, the size of globules d _gl 20–50 m, and the distance between them 40–70 m, obtained in high-strength cast irons containing (wt.%): 3.2–3.5 C, 1.9–2.5 Si, 0.35–0.4 Mn, and 0.05 Mg.     

Prediction of the Period of Initiation of Macrocracks in Notched Specimens

We propose an experimental–numerical procedure for the prediction the number of cycles N _i to the initiation of a fatigue crack near stress concentrators in specimens of complex shape. This procedure is based on direct measurements of the opening displacements of the concentrator and the numerical evaluation of the total range of local strains * or its elastic component near the tip of the notch. The applicability of this procedure to the prediction of N _i is experimentally checked for specimens in the form of strips cut out from rolled sheets of V95pchT2 alloy with stress concentrators of different geometries.