Damage of structural materials under complex low-cycle loading

On the basis of the concepts of the continuum mechanics of damage, we propose an engineering method for the analysis of the kinetics of accumulation of scattered defects in metallic structural materials under conditions of elastoplastic deformation and low-cycle fatigue. It is shown that, in the general case of complex loading for the complex stress state, it is reasonable to use the specific energy of additional stresses (with regard for the arc of plastic strains in a loading cycle) as a parameter of damage for two types of fracture (rupture and shear). __________ Translated from Problemy Prochnosti, No. 6, pp. 25–34, November–December, 2007.     

Kinetics of scattered fracture in structural metals under elastoplastic deformation

The paper presents a procedure whereby the damage accumulation kinetics in structural materials, such as steel 45, stainless steel 12Kh18N10T, aluminum alloy D16T, and titanium alloy VT22, under elastoplastic deformation is studied based on variation parameters of elastic modulus and resistivity. For complex stress conditions, a continuum model for damage accumulation is proposed which relates the damage parameter to the intensity of accumulated plastic strains. The data calculated by the proposed continuum model are compared to the experimental findings of the investigation of the damage accumulation kinetics for some structural metals. __________ Translated from Problemy Prochnosti, No. 3, pp. 23–34, May–June, 2007.     

Analysis of the effect of biaxial loading on the fracture toughness of reactor pressure-vessel steels

The stress-strain state at the crack tip and its relation to the crack opening displacement and the J-integral under biaxial loading have been studied by solving elastoplastic problems in a geometrically nonlinear formulation by the finite-element method. Numerical investigations have been performed for various cracks and two modes of biaxial loading (tension and bending) under conditions of both small-scale and large-scale yielding. For prediction of the influence of biaxial loading on fracture toughness (at brittle fracture) a procedure has been developed that is based on established laws of stress-strain state formation at the crack tip under biaxial loading and a criterion of brittle fracture proposed earlier. The effect of biaxial loading on fracture toughness is predicted as applied to reactor pressure-vessel steels. Calculated results are compared with avilable experimental data. Alternative approaches to prediction of the effect of biaxial loading on fracture toughness are discussed. TsNII KM “Prometei,” St. Petersburg, Russia. Translated from Problemy Prochnosti, No. 5, pp. 5–26, September–October, 1999.     

Method for the evaluation of the service life under random loading based on the energy criterion of fatigue fracture

We propose a method aimed at the assessment of the service life of structural elements under random loads based on the energy criterion of fatigue fracture and the model of cyclic deformation of materials after overloading. The possibility of determination of the maximum amplitude of stresses according to the number of loading cycles prior to fracture is substantiated. The results of evaluation of the service life under conditions of uniform stressed state are compared with the experimental data obtained by Swanson, Raikher spectral hypothesis of summation of defects, and linear hypothesis of summation. The dependence of the right-hand side of the linear hypothesis equation on the dispersion of the random loading process is established. __________ Translated from Problemy Prochnosti, No. 2, pp. 82–97, March–April, 2008.     

Determination of the conditions of fracture of plutonium and its alloy with gallium under impact loading

We present the results of numerical analysis of the conditions of cleavage fracture of plutonium and its alloy containing 1.6 wt.% of gallium under impact loading. On the basis of the experimental data obtained earlier, we determine the critical tensile stresses corresponding to the initial stage of macroscopic cleavage fracture of specimens. In our calculations, we take into account the elastoplastic properties of materials and polymorphic transformations in the alloy at normal and elevated (315°C) temperatures. All-Russian Scientific-Research Institute of Experimental Physics, Sarov, Russia. Translated from Problemy Prochnosti, No. 5, pp. 100–105, September–October, 1998.     

A study of deformation and damage accumulation processes in 10GN2MFa steel under low-cycle loading

The paper presents results of the experimental investigation of deformation behavior and damage accumulation kinetics in a heat-resistant reactor pressure vessel steel at high stress levels under stress-controlled (pulsating), low-cycle loading up to 10⁴ cycles. Hardness measurements have been performed upon various operating time periods under elastoplastic deformation. It has been found that hardness and strength of the steel exhibit qualitatively different behaviors throughout the lifetime. The damage accumulation process in steels under static and cyclic loadings has been studied by means of the LM-hardness method. The curves of Weibull homogeneity coefficient vs. cycling stress and accumulated strains are obtained. __________ Translated from Problemy Prochnosti, No. 2, pp. 5–10, March–April, 2008.     

Prediction of fracture characteristics of metals from high-frequency test data

A method is proposed for the prediction of cyclic crack resistance characteristics of metallic materials under low-frequency loading from high-frequency test data, which is based on a model of development of local plastic deformation regions during the accumulation of fatigue damages and fatigue crack growth with allowance for cyclic loading rate. We performed a comparative analysis of the results of prediction of fatigue fracture diagrams with test data for VT22, VT18U, VNS-25, and AMg6N alloys in a frequency range of 20 Hz–10 kHz. Report on International Conference “Dynamics, Strength, and Life of Machines and Structures” (1–4 November 2005, Kiev, Ukraine). __________ Translated from Problemy Prochnosti, No. 2, pp. 121–128, March–April, 2007.     

Shape of a mechanical hysteresis loop for metallic materials under harmonic stresses below the fatigue limit. Part 1. Experimental method

Experimental studies of the shape of a mechanical hysteresis loop for different metallic materials were carried out at stress amplitudes which exceeded the fatigue limit, i.e., at a relatively high level of inelasticity and under conditions of active fatigue damage accumulation. The experimentalanalytical method proposed for the construction of hysteresis loops at harmonic loading has a higher resolution. This allows testing at stress amplitudes well below the fatigue limit of the material under study. The initial data for the construction of a loop are the results of harmonic analysis of cyclic strain dependence on time, as well as the shift between cyclic stresses and strains in time. A number of criteria are presented for objective assessment of the hysteresis loop shape and their efficiency is evaluated. Translated from Problemy Prochnosti, No. 2, pp. 57–66, March–April, 1997.     

Numerical Investigation of the Effect of Loading Multiaxiality and Deformation Process Parameters on the Fatigue Life of Metals

Known experimental data on the effect of loading multiaxiality and the type of deformation path on the fatigue damage accumulation process have been analyzed. In terms of the mechanics of a damaged medium, a variant of the defining relations has been developed that describes the processes of elastoplastic deformation and damage accumulation in structural materials (metals and their alloys) in the case of multiaxial nonproportional low-cycle loading paths. The damaged-medium model consists of three interrelated components: relations defining elastoplastic behavior of the material with allowance for the influence of the fracture process, equations describing the damage accumulation kinetics, and a strength criterion for damaged material. To qualitatively assess the defining relations, a numerical experiment to construct equal-damage surfaces has been carried out. The calculated data are compared with the experimental ones. The effect of the stress-state multiaxiality and the type of the deformation path on the low-cycle fatigue life of metals has been investigated. It has been shown that the developed variant of defining relations for a damaged medium reflects correctly the main effects of cyclic elastoplastic deformation and damage accumulation at a multiaxial stress-state and arbitrary material deformation paths.     

Effect of the grain size on the accumulation of damage in a metal subjected to plastic deformation

The integrated assessment of the accumulation of damage in D16 aluminum alloy with coarse grains (≈3 and 8 mm) grown by a special thermal treatment shows that the grain size can strongly affect the rate of accumulation of damage in metallic materials under static loading. The degree of damage to the metal correlates well with some parameters of its damping ability and acoustic emission. These results are important from the scientific point of view and can be used for the solution of practical problems, in particular, for the improvement of existing methods and developing new methods and equipment for nondestructive testing of deformed metals. Translated from Problemy Prochnosti, No. 5, pp. 5–13, September–October, 1997.     

Experimental and Numerical Investigation of Mixed-Mode Interlaminar Fracture of Carbon-Polyester Laminated Woven Composite by Using Arcan Set-up

Composite materials are widely used in marine, aerospace and automobile industries. These materials are often subjected to defects and damages from both in-service and manufacturing process. Delamination is the most important of these defects. This paper reports investigation of mixed-mode fracture toughness in carbon–polyester composite by using numerical and experimental methods. All tests were performed by Arcan set-up. By changing the loading angle, α, from 0° to 90° at 15° intervals, mode-I, mixed-mode and mode-II fracture data were obtained. Correction factors for various conditions were obtained by using ABAQUS software. Effects of the crack length and the loading angle on fracture were also studied. The interaction j-integral method was used to separate the mixed–mode stress intensity factors at the crack tip under different loading conditions. As the result, it can be seen that the shearing mode interlaminar fracture toughness is larger than the opening mode interlaminar fracture toughness. This means that interlaminar cracked specimen is tougher in shear loading condition and weaker in tensile loading condition.     

Modeling of scattered damage accumulation kinetics under combined stress

An analysis of the reliability and range of application of a number of phenomenological damage models has been performed on the basis of experimental research carried out into the kinetics of damage accumulation in metallic structural materials under plane stress and active loading. Damage models are based on the main postulates of irreversible thermodynamics and continuum damage mechanics. The laws governing the effect of the type of stress state on scattered damage accumulation kinetics have been established.     

Study of ductile propagation of cracks in low-carbon steels under static loading

For the analysis of various micromechanisms of fracture in the process of ductile crack growth in elastoplastic materials under the conditions of static loading and their correlation, we studied the process of stable propagation of main cracks in low-carbon steels. The methods of acoustic emission and fractographic analysis were used as the basic methods of investigation. We establish the dependence of the parameters of acoustic emission on the stress intensity factor and their correlation with the processes of brittle and ductile microfracture initiated by the main crack and forming the basis of its ductile jumplike growth and propose an explanation of the nature of jumplike crack propagation. Translated from Problemy Prochnosti, No. 3, pp. 64–73, May–June, 1997.     

Kinetics of damage accumulation in sheet materials under biaxial deformation along nonlinear trajectories

We study elastoplastic deformation of a sheet material subjected to loading along curved trajectories in a two-dimensional deformation space. The influence of the deformation trajectory parameters on the damage accumulation process in a sheet material is established. It is demonstrated that express assessment of crack growth resistance of material can be performed by using a hardness scatter parameter. __________ Translated from Problemy Prochnosti, No. 1, pp. 5–12, January–February, 2007.     

Evaluation of crack resistance based on the numerical modeling of the plane strained state

We propose a method for the nondestructive evaluation of the ultimate strength and fracture toughness K _Ic of structural steels based on the high-precision data on strains and stresses obtained as a result of the improved numerical solution of the problem of plane strained state in the nonstationary elastoplastic formulation. The developed method satisfactorily describes the degradation of strength of structural materials intended for long-term operation at high or low temperatures in aggressive media or under the action of radiation, in particular, of 15Kh2NMFA, 10GN2MFA, and 2Cr–Ni–Mo–V reactor steels.     

Tensile strength of the brittle materials, probabilistic or deterministic approach?

For various loading rates we estimated the activated defect localization in Modified Brazilian Disk type glass specimens in comparison with standard spherical glass specimens. Specimen geometry can considerably affect the mechanical response of material, especially brittle ones, which are very sensitive to the distribution of defects. High and low loading rates of Modified Brazilian Disk lead glass specimens have been investigated using universal Instron test machine and compressive Hopkinson pressure bars. The experimental results obtained have been compared using the Weibull distribution for scatter strength variation. Stress distribution in the above specimens was calculated using the finite element method, which provided detailed analysis of the macromechanical brittle fracture mechanism. In static tests of spherical glass specimens, we observed generation of contact stresses, which result in activation of defects in the working parts of specimens, whereas no activated defects were observed in Modified Brazilian Disk specimens neither under static, nor under dynamic loading conditions. For specimens of various geometries and type of load application it is recommended to apply probabilistic approaches, e.g., the Weibull approach, insofar as contact stresses in brittle materials induce activation of defects, location of which depends on the specimen geometry and loading type. __________ Translated from Problemy Prochnosti, No. 1, pp. 100–115, January–February, 2006.     

Localized shear in sheet materials

Procedures and results of numerical simulations and experimental investigations of sheet materials under localized plastic deformation in shear are presented. Under dynamic loading these materials are considered to be viscoelastoplastic solids. Localized deformation and stress-strain distributions resulting from plastic shear are examined as a one-dimensional problem with account for nonlinear viscosity, damage, and temperature effects. Mild steel and high-strength aluminum alloy strips were tested under multiple impact loading with a small strain increment per impact (which corresponds to isothermal loading with sufficient accuracy). The experimental relations of localized shear deformation under such loading allow one to make the conclusion of a prevailing stress concentration effect on the kinetics of localized shear at the edge of the strip. Institute of Problems of Strength, National Academy of Sciences of Ukraine, Kiev, Ukraine. Translated from Problemy Prochnosti, No. 5, pp. 53–63, September–October, 1999.     

The construction of constitutive relations for isotropic strain-hardening elastoplastic materials of the differential type of complexity <Emphasis Type="Italic">n</Emphasis>. Part 2. Infinitesimal strains

A method of specialization of the physical equations constructed earlier by the author for isotropic hardening elastoplastic materials of the differential type of complexity n for finite strains is developed within the theory of infinitesimal strains. With this method, a number of constitutive relations are derived in the form of the hierarchy according to the level of complexity of the material response to deformation. For n = 1, the conditions for the existence of a loading surface are established. Translated from Problemy Prochnosti, No. 4, pp. 29–47, July–August, 2009.