MODELING AND CALCULATION OF FATIGUE CRACK GROWTH LIFE IN STEELS

In this work, we studied the kinetics of fatigue crack growth on compact steel tensile specimens (C (T)-type), in the middle section of crack growth diagram under regular and irregular loading with different asymmetries and maximum load. The crack growth kinetics was obtained by the authors experimentally on modern servo-hydraulic testing machine. Irregular loading was carried out using samples of standard loading spectra characteristic of various technical objects experiencing variable loading during operation. The values of the crack growth rate were obtained. Parameters that evaluate the character of irregular loading and crack closure, namely, irregularity factor and crack closure ratio were suggested. When calculating the effective value of the magnitude of the stress intensity factor (SIF) at the crack mouth, it is proposed to consider in addition to the closure coefficient and cracks also measure irregular loading. The fatigue crack growth life was predicted taking into account its “closure” and the nature of loading according to the approach proposed by the authors and the cyclic calculation method (cycle-by-cycle), all the data obtained are tabulated and distributed according to the type of loading. The results obtained showed good convergence of the calculated and experimental data, which confirms the high values of the correlation.

Comparison between Ramberg-Osgood and Hardin-Drnevich soil models in Midas GTS NX

This paper studies the two widely used material models for predicting the dynamic behavior of soils, the Ramberg-Osgood and Hadrin-Drnevich models. Resonant column and torsional simple shear test results on dry sand were used to calibrate and evaluate the model built in the finite element software Midas GTS NX. Both material models are already implemented by the software. This study estimates the ability and efficiency of both soil models coupled with the Masing criteria to predict the behavior of soil when subjected to irregular loading patterns, (e.g., earthquakes), and measure the two most important dynamic properties, the dynamic shear modulus, and the damping ratio.

Comparative Analysis of Methods for Assessing Adhesion Strength of Thermal Spray Coatings

The paper presents the comparative analysis of existing methods for accessing the adhesive strength of thermally sprayed coatings on witness samples and products in research and industrial laboratories. The adhesive method of normal separation for cylindrical witness samples with a diameter of 25 mm, a height of 16...45 mm and a coating thickness of at least 0.2 mm is pointed out among the quantitative approaches described in both international and domestic standards. An alternative pin test having no restrictions on the ultimate strength of adhesion was used to evaluate conical witness samples with an end section diameter of 1.5...2.0 mm and a coating thickness of 0.3...0.5 mm. The shear method presents two possible adhesion measurement tests: with cylindrical witness samples having the diameter of 25 mm, the width of the annular coating section of 4 ... 30 mm and the thickness of the working layer not exceeding 0.1 mm and prismatic ones with linear dimensions of shear section of 2.5...5.0 mm and the coating thickness of at least 0.25 mm. During the tests all methods were characterized by irregular loading of the flat section of the coating. Mathematical models for calculating the actual value of the tensile and shear adhesion strength are presented, taking into account the appearance of additional bending and shear forces when loading the coating.

Modeling and calculation of the fatigue crack growth life in structural steels

The kinetics of fatigue crack growth has been studied in tensile testing of compact steel tensile specimens (S(T)-type) in the middle section of the kinetic diagram of fatigue fracture (fatigue crack growth diagram) under regular and irregular loading with different asymmetry and maximum load values. The samples were tested on a BISS Nano-25kN servo-hydraulic machine. Standard loading spectra typical for different technical objects exposed to alternating loading during operation were used. The values of the crack growth rate per cycle in the loading block were obtained. Parameters for assessing the character of irregular loading and crack closure, namely, the irregularity factor and crack closure coefficient were proposed. When calculating the effective value of the range of the stress intensity factor (SIF) at the crack mouth, we propose also to take into account the loading irregularity in addition to the closure coefficient. With this approach, the obtained fatigue crack growth diagrams can be grouped into one equivalent curve, which is characteristic of regular loading with R = 0. Moreover, grouping of the fatigue crack growth diagrams provided the use of unified parameters when calculating the crack growth kinetics, regardless of the type and parameters of loading, which rather simplified the crack growth determination. The fatigue crack growth life was predicted taking into account the crack «closure» and the nature of loading according both to the approach developed by the authors and by cyclic calculation method (cycle-by-cycle). All the data obtained are tabulated and classed according to the type of loading. The calculation results and experimental data showed good convergence, which was confirmed by the high values of the correlation coefficient.

Simulation of continuous random process according to the specified sequence of extremes

A method has been developed for converting a discrete sequence of extrema into a continuous process. The relevancy of the problem is attributed to the necessity of an approximate estimation of spectral density in in testing materials and structures under random (irregular) loading. A great bulk of available experimental data thus can be used in development and validation of calculation methods for assessing durability in the multi-cycle region. Postulating the continuity of random stress processes and their first derivative we propose to connect piecewise the available starting points (namely, the extrema of the random process) with half-cosine functions under the condition of compatibility at the points of extrema. A distinctive feature of the method is the provision of 100% coincidence of the values and sequences of extrema in the initial discrete and simulated continuous processes. The issue of choosing the magnitude of half-periods for these half-cosine functions is addressed on the basis of information obtained from the analysis of real stress records in the form of a regression equation linking half-periods and half-ranges for some realizations of the random process for transport vehicles. The regression dependences of the half-periods and semi-ranges of bending stresses (part of a railway train) and torsion (torsion shaft of a tracked vehicle) are shown as an example. An analysis of the correlation of two random variables (half-periods and half-ranges) according to empirical data has shown that the correlation exists and is significant for the observed number of points thus providing the basis for using the regression formula for an approximate choice of the frequency composition of the process. Moreover, the lower restrictions are imposed on the number of points (at least 5) in the half-period. Since the extrema of the initial and simulated processes coincide in accordance with the principle of the proposed simulation, the distribution of the amplitudes of complete cycles, as well as the results of schematization by other known methods are identical, therefore, the estimate of the durability by hypotheses based on a linear one is also identical. The validation of the method consists in consideration of the chain: 1) the initial continuous process; 2) the discrete process of extrema; 3) simulated continuous process according to the proposed method. Auxiliary distributions, such as distributions of maximum, minimum and average cycle values also coincide in accordance with the principle of modeling. The method is proposed to be used in analysis of the comparability of two competing approaches in assessing the loading in the problems of assessing durability, namely: those that use cycle-counting methods and methods based on the spectral density of processes. Since the spectral densities of the processes can differ due to an approximate choice of the frequencies on the basis of a regression formula, methods on their base can give estimates of the durability that differ from those obtained by schematization methods. To study this phenomenon, further computational experiments are required. The developed method can be very useful for the experiment design.

Modeling a random sequence of extremes of loads for fatigue tests under irregular loading

Methods and approaches to tests under random loading are considered, their role is characterized. To ensure the random nature of loading, a modeling method based on Markov transition matrices and real processes recorded in operation is proposed. Keywords: random loading process, Markov repetition matrices, resource estimation, corrected linear hypothesis, parameter of completeness of the loading spectrum. [email protected]