Energy method of longevity calculation under an irregular loading. Report 2. Longevity under programmed block loading

A method of calculating longevity to macrocrack development under a programmed loading in the form of a block repeated many times to failure is proposed. Relationships are derived for calculation of longevity under blocks with stepwise variation in stress amplitude, and also blocks assigned by the probability-distribution density of the stress amplitudes and tables of the differential frequency of repetition of peak and average stresses. Consideration of variation in the material's endurance limit under an irregular cyclic loading is demonstrated. It is shown that a specific type of relationships can be derived for the material under investigation with allowance for its kinetic cyclic-strain diagrams. An example of the longevity calculation, which is compared with experimental data for the block loading of steel 40Kh specimens, is cited.Translated from Problemy Prochnosti, No. 8, pp. 3–11, August, 1995.     

Fatigue lifetime of welded joints under random loading: rainflow cycle vs. cycle sequence method

The evaluation of fatigue lifetime is a complex problem due to the loading cycle sequence effect influencing the fatigue damage process. In the literature two different approaches are usually adopted to solve the fatigue crack growth under time varying loading: the rainflow cycle (RFC) and the cycle sequence (CSQ) model. In this paper the performance of the recently proposed stochastic version of these methods is compared and applied to the evaluation of the fatigue lifetime of the welded cruciform tested in the literature. Non-Gaussianity and bandwidth effect are correctly captured by both methods but only the CSQ model enables us to examine the influence of the load sequence effect on the fatigue lifetime. Numerical results show that the retardation effect can be neglected for Gaussian and non-Gaussian loading as the bandwidth increases. Finally, this work shows the greater performance and flexibility of the CSQ model compared to the RFC model.     

Consideration of defects in calculating the failure probability of ceramic articles with short-term loading

On the basis of analyzing current approaches it is concluded that the optimum approach to estimating the failure probability for ceramic articles with short-term loading should be a combination of explicit and implicit consideration of different features of material defects and the stressed state. Calculation dependences developed by the authors are given in explicit form which consider separately the effect of surface and volume defects, the form of stressed state, curvature of the loading path, and the nature of defect orientation distribution. The effect of loading path curvature on the calculated failure probability is demonstrated by a numerical example. Methods are considered for determining the parameters of material surface and volume defects used in the suggested relationships.Translated from Problemy Pochnosti, No. 4, pp. 25–30. April, 1991.     

Relationship of fracture mechanics criteria. Report 1. Force loading

Fracture mechanics critria, used in calculations in practice, are presented in the form in which they can be compared on the plane of loading parameters. Equations are derived for determining the critical stress intensity factors in relation to the fracture toughness of the material, mechanical properties and temperature. The temerature dependences of the critical stress intensity factors calculated from different criteria, are compared.Translated from Problemy Prochnosti, No. 4, pp. 3–7, April, 1990.     

Investigation into threshold stress intensity factors of materials under cyclic loading. Report 1. Methods and experimental results

On the basis of the results of studying threshold stress intensity factors of stainless and heat-resistant steels and titanium and nickel alloys, the author considers the correlation between numerical values of the threshold stress intensity factors and the yield stress, ultimate strength, and fatigue limit, as well as the influence of temperature, environment, cycle asymmetry, and overloads on the threshold stress intensity factor. Institute for Problems of Strength, National Academy of Sciences of Ukraine, Kiev, Ukraine. Translated from Problemy Prochnosti, No. 4, pp. 5–15, July–August, 1998.     

Computational modelling of irregular open‐cell foam behaviour under impact loading

Cellular structures represent an important class of engineering materials. Typical representative of such structures are metallic foams, which are being increasingly used in many advanced engineering applications due to their low specific weight, appropriate mechanical properties and excellent energy absorption capacity. For optimal design of cellular structures it is necessary to develop proper computational models for use in computational simulations of their behaviour under impact loading. The paper studies the effects of open‐cell metallic foam irregularity on deformation behaviour and impact energy absorption during impact loading by means of parametric computational simulations, using the lattice‐type modelling of open‐cell material structure. The 3D Voronoi technique is used for the reproduction of real, irregular open‐cell structure of metallic foams. The method uses as a reference a regular mesh structure and utilises an irregularity parameter to reproduce the irregularity of real open‐cell structure. A smoothing technique is introduced to assure proper stability and accuracy of explicit dynamic simulations using the produced lattice models. The effects of the smoothing technique were determined by comparative simulations of smoothed and unsmoothed lattices subjected to dynamic loading.     

Diffractive optical element designed by use of an irregular etching-depth sequence

In fabricating a diffractive optical element the ratio of the etching depth between the (n - 1)th and the nth mask is usually 1/2. We found that the diffraction efficiency of a diffractive optical element can be improved by as much as 7.8% if the above ratio (1/2) is not kept constant. For achieving this improvement the difference between the desired and the actual diffraction pattern is also used as an objective function for phase quantization.     

Numeric modeling of the strength and longevity of structures with allowance for scale effect. Report 1. Substantiation of strength and longevity criteria

The nonlocal criterion of the static strength of structurally inhomogeneous construction materials, whose basic mechanical properties depend heavily on the loaded volume in a complex stress-strain state, is developed. A statistical interpretation of the proposed criterion is given. The case where the criterion is used for an isotropic material with volume-dependent ultimate tensile, compressive, and torsional strengths is analyzed as an example. A longevity criterion, which makes it possible to account for the different character of the material's rupture strength as the form of the stress state changes, in addition to the scale effect, is proposed for structural components that operate under asymmetric low-cycle loading. This criterion is in accord with the static criterion and is obtained on the assumption of the invariance of the limiting-stress diagrams, which apply to the ultimate strength relative to the form of stress state.Translated from Problemy Prochnosti, No. 5, pp. 37–43, May, 1991.     

An interpolation method for an irregular net of nodes

A local interpolation method for an irregular mesh of nodal points is proposed. The method is based on a Taylor expansion of the unknown function combined with the minimization of errors. Some numerical tests as well as a computer program are presented. Applicability and stability of the method are shown. By the appropirate definition of weighting coefficients, this method may be viewed as an interpolation or approximation in the sense of minimum deviation from given values. Applications in finite element and finite difference methods are shown.     

Comparative analysis of models of accumulation of dispersed damages in metals under irregular variable loading

We analyze models of summation of fatigue damages under irregular variable loading on the basis of linear and nonlinear interpretation of the damage function. We show the necessity of taking into account the main physicomechanical processes in the metal under cyclic loading, which can improve the accuracy of computations.     

Correlation of residual stresses and fatigue limit with torsion under conditions of stress concentration Report 1. Substantiation of strength and longevity criteria

Results are given for a calculation and experimental study of the effect of residual stresses on fatigue limit with torsion for specimens with circular notches of semicircular and rectangular shape made of steels 40, 30KhGSA, EI961, and aluminum alloy V95. The possibility is demonstrated of a criterion for mean integral residual stresses through the thickness of a surface layer equal in depth to a nonpropagating fatigue crack in order to estimate the influence of residual compressive stresses on fatigue limit in the case of torsion with stress concentration.Translated from Probleray Prochnosti, No. 5, pp. 43–46, May, 1991.