Life Of Structural Elements Research Articles

Probabilistic forecasting of durability and residual life of structural elements

The method of computational forecasting of durability and residual life of structural elements based on the results of failure analysis at a small initial time interval of their operation is proposed. Keywords forecasting, durability, residual life, failure, structural element, reliability sopr@mami.ru

Crack Resistance of a Shaft in Flexure with Rotation

For more reliable prediction of the life of structural elements, an energy criterion of crack resistance corresponding to nonlinear failure mechanics is proposed: the critical effective shape energy. On the basis of fatigue tests of 15 model shaft elements in flexure with rotation, the crack resistance of the shaft is assessed as a function of the depth of the annular cracks.

Contribuição para estimativa do grau de saturação a partir das condições ambientais aplicada à análise de confiabilidade de estruturas de concreto sujeitas ao ataque por cloretos

A durabilidade de elementos estruturais de concreto armado sujeitos a condições ambientais de severa agressividade apresenta-se como um dos principais problemas enfrentados na construção civil. Dentre os agentes de deterioração dessas estruturas destacam-se os íons cloretos, amplamente difundidos em ambiente marinho, que causam a corrosão de armaduras. A estimativa de vida útil de elementos de concreto armado surge como uma temática fundamental na avaliação da durabilidade das estruturas. Embora outros processos causem penetração de íons cloreto no concreto, é praticamente consenso que a difusão é o mecanismo predominante. Um modelo clássico de caracterização da penetração de cloretos é a 2ª Lei de Fick. Guimarães (2000) demonstrou que o grau de saturação (GS) do concreto é um parâmetro que exerce forte influência sobre o coeficiente de difusão de cloretos (D). Embora tenham sido desenvolvidas metodologias de medição do GS, estas se apresentam onerosas em termos de tempo e recursos. O objetivo deste trabalho é apresentar estimativas de GS com base nas condições ambientais a que o material é exposto e posteriormente analisar a influência deste parâmetro na avaliação da estimativa de vida de elementos estruturais através da confiabilidade. São utilizados dados meteorológicos e de medição do GS obtidas em duas regiões litorâneas: Rio Grande, Brasil e Santa Pola, Espanha. Obteve-se modelos de estimativa do GS a partir das variáveis ambientais de cada região e também de ambas em conjunto. Avaliou-se então a influência destes modelos na estimativa de vida útil através da probabilidade de falha da estrutura.

Effect of Coatings on the Fatigue Characteristics of Steels Under Contact Load

A new type of technological process is proposed for increasing the life of structural elements under rolling friction. Results of comparative experimental studies on increasing the life of steels under rolling friction, using hydrogen-free integral techniques for the application of gradient coatings with optimal properties, residual stresses and substrate hardness and using ion nitriding and heat treatment.

Failure of structural elements made of polymer supported composite materials during the multiyear natural aging

Modern high-rise construction introduces a number of limitations and tasks. In addition to durability, comfort and profitability, projects should take into account energy efficiency and environmental problems. Polymer building materials are used as substitutes for materials such as brick, concrete, metal, wood and glass, and in addition to traditional materials. Plastic materials are light, can be formed into complex shapes, durable and low, and also possess a wide range of properties. Plastic materials are available in various forms, colors and textures and require minimal or no color. They are resistant to heat transfer and diffusion of moisture and do not suffer from metal corrosion or microbial attack. Polymeric materials, including thermoplastics, thermoset materials and wood-polymer composites, have many structural and non-structural applications in the construction industry. They provide unique and innovative solutions at a low cost, and their use is likely to grow in the future. A number of polymer composite materials form complex material compositions, which are applied in the construction in order to analyze the processes of damage accumulation under the conditions of complex nonstationary loading modes, and to determine the life of structural elements considering the material aging. This paper present the results of tests on short-term compression loading with a deformation rate of v = 2 mm/min using composite samples of various shapes and sizes.

COMPARATIVE ANALYSIS OF NONLINEAR MODELS FOR PREDICTING RESIDUAL OPERATING LIFE AND OPERABILITY OF THE STRUCTURAL ELEMENTS OF RESIDENTIAL BUILDINGS

A number of researchers are working on the creation of a general method for forecasting operability and residual life of engineering constructions of buildings and structures of different purposes using the models of nonlinear physical wear development. The authors analyzed two approaches to forecasting operability and estimating residual operating life of structural elements of residential houses using nonlinear models of the changes in physical wear of building structures basing on forecast logistic model. The description of the differences in the approaches to forecasting and advantages of one method compared to another are given. As a result of the conducted analysis the authors recommend the most accurate model for making predictions and estimation of residual operating life of building structures.

Life of Structural Elements of Centrifuges with Allowance for Damage

The paper presents the procedure for calculating and predicting the state of critical structural elements in low-cycle fatigue with allowance for features of the operating conditions. The procedure is based on the nonlocalized damage concept and the allowance for the damage parameter in the system of constitutive equations using fast-rotating parts of laboratory centrifuges as an example. A macro- and microstructural analysis of the material is made, and the static and cyclic tests of the specimens of V95 (7075) aluminum alloy are carried out. The results of the experimental investigations are compared with the theoretical calculations performed based on the finite element method (the ANSYS software package).

Calculation of strength reliability and fatigue life of structural elements of mobile machines, loaded with random bending and twisting moments

The authors propose a new technique of calculation of strength reliability and fatigue life of metal elements of mobile machines experiencing a simultaneous loading by non-synchronous and out-of-phase random bending and twisting moments.

Fatigue life of structural elements in multiaxial loading

The fatigue life of structural elements in multiaxial loading may be assessed on the basis of the balance of fatigue strength, taking account of the change in stress-strain state in multiaxial and uniaxial loading.

Influence of fretting corrosion on the fatigue life of structural elements made of VT6 alloy

We present results of an experimental investigation of fatigue resistance characteristics of VT6 titanium alloy plates with washers made of D16AT aluminum alloy, VT3-1 titanium alloy, and 30KhGSA steel and pressed to them. It is established that, under operating loads, the fatigue life of titanium specimens decreases 400 – 750 times as a result of fretting corrosion.

Evaluating the life of structural elements in complex loading

Methods of estimating the life of structural elements in cyclical and quasi-random loading are considered. The sum of the relative numbers of cycles to failure is found to be more stable when using failure characteristics for structural elements in narrow-band quasi-random loading than when using characteristics for cyclic loading.

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.

Residual service life of thin-walled structural elements under biaxial cyclic loading

We generalize the energy approach to the evaluation of the residual service life of structural elements under biaxial loading proposed somewhat earlier. On this basis, we construct a mathematical model aimed at the evaluation of the period of subcritical growth of a fatigue crack in the plate subjected to biaxial cyclic loading. The model is used to analyze the influence of the parameters of biaxial cyclic loading on the period of subcritical growth of the fatigue crack.

Evaluation of the residual service life of structural elements in hydrogen-containing media

We propose an algorithm for the evaluation of the residual service life of structural elements with cracks (hydrogen-fueled engines and petroleum-hydrocracking reactors) under the combined action of hydrogen-containing media and long-term mechanical loads.

Modelling of residual stress field in spherical mandrelling process

Fatigue life of structural elements with bolt holes depends mainly on residual stress distribution law around these holes. Residual compressive circumferential normal stresses around the hole reduce operating stress magnitudes to minimum values for cyclic tension and this enhances fatigue life and load-carrying capacity of structures. The presence of residual stresses is a result of the manufacturing process. Residual stresses can also be induced deliberately around the holes by means of appropriate working with suitably chosen parameters. Quantitative knowledge of residual stresses is necessary to model the stress field after applying an external load to a structural element in order to assess static or dynamic strength, fatigue strength including. This paper presents a combined approach consisting of experimental and numerical modelling of residual circumferential normal stress distribution when forming holes in workpieces of medium carbon steel by spherical mandrelling (SM). Since the object of study is residual macrostresses (stresses of first type), a mechanical method of their determination has been employed. On the basis of experimental outcomes, a mathematical model has been built and it predicts mean integral value distribution of residual circumferential normal stresses. Since the range of the experimental technique employed is limited by the wall thickness of the bushing being worked, numerical modelling of residual stresses by means of FE simulation has been performed. The numerical results obtained allow this mathematical model to be applied to various wall thicknesses by introducing correction factors for the polynomial coefficients. At the same time, the adequacy of the proposed FE model can be evaluated only by the experimentally obtained mathematical model. The SM efficiency for enhancing the load-carrying capacity of structural elements with cylindrical holes subjected to tension has been proved by means of FE simulation.

Corrections to B-models for fatigue life prediction of metals during crack propagation

In this paper a new model for the prediction of the Cumulative Distribution Function (CDF) of fatigue life of structural elements during the crack propagation stage is described. This problem is considered as a cumulative damage process following the probabilistic approach of Bogdanoff and Kozin [1] (B-models). The initial and final crack lengths, the crack propagation angle, the material fracture and elastic parameters and the external loads may be considered as random variables. In this initial approach, a linear approximation of the random variable ‘fatigue life’ and a truncated uniform distribution for the crack length variable are considered. Two corrections to this model are discussed: a second-order approximation of the fatigue life to compute its variance, and a modification of the Probability Density Distribution (PDD) of the crack length, which is now derived from the truncated uniform distributions of the initial and final crack lengths. Some examples for mode I are compared to the ones obtained using a Monte Carlo scheme with 400 000 samples, showing a good agreement and a much better performance of the corrected version of the model, specially for big standard deviations. Copyright © 1999 John Wiley & Sons, Ltd.

Method of evaluating the residual life of structural elements with crack-like surface defects. Report no. 1. Modeling the growth of a surface fatigue crack

A method has been developed for predicting the growth of surface fatigue cracks with allowance for the shape of the crack front under regular cyclic loading and the presence of tensile overloads in the loading spectrum. The crack model is constructed on the basis of consideration of the specific contributions of the mechanisms that determine the advance of the crack. It is shown that allowance for such important factors as the variation in the severity of the stress state along the crack front is essential in model representations.

Fatigue Damage Prediction for Combined Random and Static Mean Stresses

Closed form analytical expressions have been derived and are proposed for use to predict accumulated fatigue damage and fatigue life of structural elements subjected to a combination of fully reversed narrow-band Gaussian random and static mean stresses. Such mean stresses can significantly alter fatigue life. The proposed method of combining random alternating and mean stresses shows excellent agreement with published experimental data for a steel alloy. Reasonable agreement is maintained, surprisingly, even for static tensile stress values up to near the material's yield stress where the failure mode shifts from that of typical brittle fatigue to that of stress rupture (i.c.,creep). Numerical examples are provided to illustratc the application.

Propagation of physically short fatigue cracks and the life of structural elements. Report 1

A relationship for describing the kinetics of short fatigue cracks is proposed and experimenteally confirmed on the basis of the energy two-parameter criterion of fracture. It is established that with use of this relationship it is possible to quantitatively describe the influence of the concentration and asymmetry of the stress cycle and the role of the compressive portion of the cycle and the surface layer on the kinetics of a crack and the threshold values ΔKth.

Propagation of physically short fatigue cracks and the life of structural elements. Report 2

Propagation of physically short fatigue cracks and the life of structural elements. Report 2