Fatigue Strength of Austenitic and Martensitic–Ferritic Steels for Nuclear Power Plants

We study the influence of boron-controlled media (12 g/liter H₃BO₃) with pH = 3 and 8 at temperatures of 293 and 353°K on the fatigue and cyclic crack resistance of 08Kh18N10T and 12Kh18N10T austenitic steels and 14Kh17N2 martensitic–ferritic steel. It is shown that 14Kh17N2 steel is characterized by the optimal combination of the long fatigue life with high cyclic crack resistance in inert and corrosive media at temperatures of 293 and 353°K, which gives it serious advantages over 08Kh18N10T and 12Kh18N10T austenitic steels.     

Cyclic fracture toughness of structural alloys with surface cracks at low temperatures

In the present paper, we study specific features of crack propagation from surface flaws in full-scale sheets used in manufacturing pressure vessels for cryogenic applications. The process of crack propagation consists of several stages and terminates in stages of surface through or central through cracks under conditions of low-frequency repeated tension. The effect of a decrease in temperature from 292 to 77°K on crack growth behavior was studied for sheets with a thickness of 2, 8, and 12mm. We describe a procedure for testing for crack-growth resistance at cryogenic temperatures and construct fatigue crack growth diagrams. It is shown that zones of influence of the front and rear faces of the specimen on the stress and strain fields near the crack front arise in the plane of a semielliptic crack. The shape of the interface of these zones can be approximated by a second-order curve. Variations in the thickness of the specimen and the test temperature affect the slope of the curve, i.e., the interface of the zones of influence. Specific features of the fracture process in the material of the plate with surface cracks manifest themselves most adequately at points of the crack front located on the indicated interface. We suggest a procedure for estimating the cyclic crack growth resistance of highly ductile stainless steels that is based on the use of the cyclic J-integral. We propose to regard the lengthl of the interface of the zones of influence of the front and rear faces of the specimen as a geometric parameter of the crack. It is used to construct kinetic fatigue crack growth diagrams for specimens with semielliptic surface cracks.Published in Fiziko-Khimicheskaya Mekhanika Materialov, Vol. 31, No. 1, pp. 9–19, January – February, 1995.     

Cryogenic-alloy strength and cracking resistance under dynamic loading

Test results are given on 12Kh18N10T and 03Kh20N16AG6 steels and on AMg6 alloy designed to give the dynamic strength and cracking resistance at strain rates of 0.36 m/sec in the temperature range 77–293 K. Cooling raises the strength, plasticity, cracking resistance, and resistance to crack growth in AMg6 alloy. Cooling the two steels increases the strength and cracking resistance, but reduces the plasticity and crack propagation resistance. Compact specimens with small natural thicknesses can be used to determine the correct critical values for the cracking resistance J_Ic for 03Kh20N16AG6 steel and AMg6 alloy. It is possible to determine correctly the critical values for the cracking resistance for 12Kh18N10T steel at 77 K.Translated from Problemy Prochnosti, No. 9, pp. 48–53, September, 1992.     

Crack resistance of chromium-nickel steels with impact loading under cryogenic temperature conditions

Results are presented for an experimental study of the effect of cooling to 77 K on resistance to crack development of chromium-nickel steels 12Kh18N10T and 03Kh20N16AG6 with impact loading. The crack resistance characteristics with a rate of crack growth of 1.5 m/sec were determined from the deformation diagram recorded during testing. Features are noted for crack advance with cooling under conditions of a dynamically applied load.Translated from Problemy Prochnosti, No. 7, pp. 26–29, July, 1992.     

Mechanical properties of martensitic steels in gaseous hydrogen

We present the regularities of hydrogen degradation of 03Kh12N10MT, 15Kh12N2MFAV and 13Kh11N2V2MF steels under a pressure of 30 MPa within the temperature range of 293–673 K. The minimum values for plasticity, low-cycle fatigue, and static crack resistance, which do not decrease with an increase in pressure of hydrogen atmosphere and content of the absorbed hydrogen, are found. The difference between temperature dependences of the coefficients of influence of hydrogen on static and cyclic crack resistance of martensitic steels with various content of austenite is established. The main fractographic features of the influence of hydrogen on the micromechanism of fracture of steels under different types of loading and temperatures are determined.     

Effect of cooling to 4.2 K on the strength and cracking resistance of the parent metal in welded joints in 07Kh13N4AG20 steel

The results are presented of experimental examination of short-term strength and cracking resistance of the parent metal and welded joints in 07Kh13N4AG20 steel in the temperature range 293–4.2 K. The mechanical characteristics of the welded joint in the examined steel are compared with the characteristics of 12Kh18N10T and 03Kh20N16AG6 steels. The results show that the mechanical properties and crack propagation resistance of the weld metal in 07Kh13N4AG20 steel are similar to those of the parent metal.Translated from Problemy Prochnosti, No. 9, pp. 45–48, September, 1990.     

Analysis of the stress-strained state in the zone of a semi-elliptical crack with low-cycle loading

Results are given for analysis of the stress-strained state in the zone of a surface semi-elliptical crack in plane specimens. Specimens prepared from steels 12Kh18N10T and 07Kh13N4AG20 were tested in a soft cyclic loading regime, maximum stresses in the cycle were in the range _max=(0.6-0.9)_0.2, and the temperature of the atmosphere T was 293 and 77 K. The analysis was carried out using an x-ray method for studying residual stresses and strains, and also by means of fractography. A correlation is established between the form of stress-strained state, the degree of its inhomogeneity and failure micromechanism, and also the microrelief of fatigue fractures corresponding to them. Differences are detected in the mechanisms for occurrence of plastic deformation and strengthening mechanisms for the two test steels with cyclic deformation. It is shown necessary to consider the effect of the body boundaries (specimen front and back) on the stress and strain fields in the zone of a surface fatigue crack.Translated from Problemy Prochnosti, No. 3, pp. 3–12, March, 1992.     

Effect of aqueous media on the cyclic cracking resistance of steels at 300°C

We investigate the influence of reactor water (of the boron control system) on the fatigue crack growth rates in 15Kh2NMFA and 08Kh18N10T steels at 300°C. Corrosion-fatigue cracking is found to be typical of 15Kh2NMFA steel. The kinetics of the potentials of newly formed steel surfaces in high-temperature reactor water is studied. A phenomenological model that describes the influence of high-temperature aqueous media on the fatigue crack growth rates in steels is developed on the basis of data of corrosion-mechanical and electrochemical investigations. The results obtained by this model are in good agreement with experimental data.Karpenko Physicomechanical Institute, Ukrainian Academy of Science, L'viv. Translated from Fiziko-Khimicheskaya Mekhanika Materialov, Vol. 30, No. 1, pp. 34–42, January–February, 1994.     

Strength and crack resistance of chromium-nickel-manganese 03Kh19AG3N10 steel in the temperature range 293–4.2 K

We present the results of the experimental investigation of the characteristics of strength and crack resistance of chromium-nickel-manganese 03Kh19AG3N10 steel in the temperature range 293–4.2 K under static (in the initial state and under the action of pulses of electric current) and dynamic loads. We consider some distinctive features of the process of the discontinuous yield of steel at a temperature of 4.2 K and its influence on the kinetics of fracture processes. The comparative analysis of temperature dependences of the characteristics of strength, plasticity, and crack resistance of 03Kh20N16AG6, 03Kh20N16AG6Sh, and 07Kh13N4AG20 steels extensively used in cryogenic engineering and 03Kh19AG3N10 steel demonstrates that 03Kh19AG3N10 steel can be used in the temperature range 293–4.2 K. Translated from Problemy Prochnosti, No. 3, pp. 90–95, May–June, 1997.     

Propagation of microcracks at stress amplitudes below the conventional fatigue limit in Ti–6Al–4V

Fatigue crack growth below the conventional fatigue limit was examined in Ti–6Al–4V in two different microstructural conditions, bi-modal and fully lamellar. Tests conducted at R = −1, at room temperature and in air showed that there is a stress dependency in the d a /d N –Δ K behaviour in both microstructures. The increasing crack front roughness associated with increasing crack size results in a decrease in the crack growth rate relative to the crack growth rate in a single grain. The d a /d N vs. Δ K lines were drawn for each crack size and a 'threshold'Δ K was determined using the intercept of the lines with d a /d N = 10⁻¹⁰ m cycle⁻¹. These values were used to construct modified Takahashi–Kitagawa diagrams to predict microcrack growth below the fatigue limit for each microstructure. A comparison of the two microstructures indicated differences in behaviour in microcrack and macrocrack growth that were explained by differences in crack front roughness at a given crack size.     

Characteristics of static and fatigue strength and of crack resistance of welded joints in structural steels at low temperatures. Communication 2

The article presents the characteristics of cyclic crack resistance and the critical values of the SIF under static, cyclic, and dynamic loading of the main zones of welded joints in steels 20K, 09G2S, 12KhGDAF, 10KhSND, 06G2AF, 20KhGSA, 07Kh3GNMYua. and ON6 in the temperature range 293-113°K. The obtained results are the initial data for the stress analysis of metal structures at the stage of crack growth.Translated from Problemy Prochnosti, No. 2, pp. 22–27, February, 1992.     

Experimental study of variation of 08Kh18N12T steel properties in the npp main circulating pipeline during its operation

We present results of experimental studies of crack growth resistance characteristics and hardness of 08Kh18N12Tsteel specimens in the initial condition and after 100 thousand hours of operation in NPP main circulating pipeline.     

The Influence of the Cyclic Loading Rate on the Plastic Zone Depth in VNS-25 Alloy

We have studied the plastic zone depth at the fatigue crack tip in VNS-25 (03Kh12N10MT) alloy specimens, previously subjected to the cyclic fracture toughness tests under symmetrical push-pull loading with a frequency of 20, 170, 600 Hz, 3 and 10 kHz. For the same values of the stress intensity factor, an increase in the loading frequency is shown to slow down the fatigue crack growth and reduce the plastic zone depth under the fracture surface. However, the dependence of the plastic zone depth on the crack growth rate is invariant with respect to the loading frequency.     

Structure of the plastic zone at the crack tip and the endurance of steel during low-cycle loading

From microhardness, metallographic, and also layered and sight x-ray analyses, the mechanisms controlling changes in the phase composition, structure, and size of the plastic zone at the crack tip during low-cycle loading of steels 12Kh18N10T and Kh11N10M2T have been established. These steels had various initial structures due to directional changes in their strength and ductility. It was shown that with increase in the maximum uniform elongation, there is an increase in the amount of intense structural changes in the plastic zone, an increase in the number of load cycles to failure, and a decrease in the rate of stable crack growth. These mechanical effects can be explained by the positive influence of the martensitic transformation and of dislocation mobility on the energy intensity of failure activation in the plastic zone. In particular, dislocation mobility leads to a partial relaxation of microdistortion in the crystallographic lattice of the matrix phase.Translated from Problemy Prochnosti, No. 8, pp. 23–31, August, 1993.     

Nonuniform cleavage cracking across persistent grain boundary

The nonuniform characteristics of cleavage cracking across high-angle grain boundaries are analyzed in considerable detail. To break through a grain boundary, a cleavage front would first penetrate across the boundary at its central part, with the side sections being locally arrested. Such a front behavior causes a strong crack trapping effect and a large increase in required crack growth driving force. Eventually, as the persistent grain boundary areas are separated apart, the crack front bypasses the grain boundary. The critical condition of the unstable crack propagation is determined by both the local fracture resistance and its increase rate with respect to the expansion of the break-through window. The grain boundary toughness is dominated by the effective grain boundary ductility.     

A Three-dimensional Numerical Study of Mixed Mode (I and II) Crack Tip Fields in Elastic–plastic Solids

The stress fields near a crack front in a ductile solid are essentially three-dimensional (3D) in nature. The objective of this paper is to investigate the structure of these fields and to establish the validity of two-dimensional (2D) plane stress and plane strain approximations near the crack front under mixed mode (combined modes I and II) loading. To this end, detailed 3D and 2D small strain, elastic–plastic finite element simulations are carried out using a boundary layer (small scale yielding) formulation. The plastic zones and radial, angular and thickness variations of the stresses are studied corresponding to different levels of remote elastic mode mixity and applied load, as measured by the plastic zone size with respect to the plate thickness. The 3D results are compared with those obtained from 2D simulations and asymptotic solutions. It is found that, in general, plane stress conditions prevail at a distance from the crack front exceeding half the plate thickness, although it could be slightly smaller for mode II predominant loading. The implications of the 3D stress distribution on micro-void growth near the crack front are briefly discussed.     

MODE II STABLE CRACK GROWTH

Abstract Mode II stable crack extension has been examined for an aircraft grade aluminium alloy D16AT. Both theoretical and experimental results are presented. The experimental observations include load displacement diagrams, plastic wake, crack front tunnelling and scanning electron micrographs of the fracture surfaces. The crack shows a tendency for in-plane extension, and the fracture surface is very flat, smooth and free of any dimples. The crack front advances with neghgible tunnelling at all stages of extension. The span of mode II stable crack growth (SCG) is longer than in the case of mode I SCG reported earlier for the same material and there is also more extensive plastic deformation. In the presence of a slight mode I load, the crack grows out-of-plane and the fractured surface facets resemble that of a mode I or mixed-mode dimpled fracture. The theoretical study is based on a finite element analysis using small deformation theory and incremental plasticity. Some of the experimental results have been theoretically predicted using the COA criterion as the governing criterion. The theoretical results include load-displacement diagrams, crack edge displacement curves, plastic zones and the J resistance curves. There is good agreement between the load-displacement diagrams. The initiation and maximum loads differ by less than 15%. The J resistance curve has a constant slope over the whole span of stable crack growth.     

Fem prediction of the influence of warm prestressing on fracture toughness of heat-resistant steel

We present a procedure of prediction of the influence of warm prestressing combined with cycling on the brittle strength of steel 15Kh2MFA. Using a finite-element method, the effect of the combined warm prestressing on the stress-strain state at a fatigue crack tip is studied in an elastic-plastic statement. Electron microscopic observations of fracture surfaces have revealed that fracture is initiated at some distance from the fatigue crack front. Based on the pattern of influence of the plastic prestrain level on the cleavage stress of steel 15Kh2MFA and the experimental CID value, a method is put forward for finite-element modeling of the stress-strain state at a crack tip during the specimen fracture. Using the results of the finite-element modeling, the relevant curves have been plotted and an approximating formula has been proposed to represent the influence of the combined warm prestress level on the fracture toughness of steel 15Kh2MFA.     

Numerical simulation study of fatigue crack growth behavior of cracked aluminum panels repaired with a FRP composite patch using combined BEM/FEM

A combined boundary element method and finite element method (BEM/FEM) is employed to investigate the fatigue crack growth behavior of cracked aluminum panels repaired with an adhesively bonded fiber-reinforced polymer (FRP) composite patch. Numerical simulation of crack growth process of a cracked aluminum panel repaired with a FRP composite patch under uniaxial cyclic loading has been carried out. The curve of crack length on unpatched side of the cracked panel versus the number of cyclic loading is determined by the numerical simulation, and it agrees well with experimental data. Furthermore, the crack front profiles of the cracked panel during fatigue crack growth and the distributions of stress intensity factors along crack fronts are also numerically simulated.