Fatigue strength and cyclic crack resistance of titanium alloy VT3-1 in different structural states. Communication 2. Procedure for considering the effect of structure on fatigue limit

On the basis of experimental results presented in communication 1 for a study of the fatigue strength of two-phase titanium alloy VT3-1 in different structural states it is shown that the fatigue limit for the alloy increases with a reduction in the average size of a structural element critical for cyclic strength. A numerical model is developed by means of linear fracture mechanics approaches which makes it possible from available data about the size of structural parameters for the alloy to predict the fatigue limit of smooth specimens.Translated from Problemy Prochnosti, Nos. 5–6, pp. 12–17, May–June, 1995.     

Combination experimental and theoretical method for determining the stress state of structural elements with cracks

We propose a combination experimental and theoretical method for determining the stress state of sheet structural elements with cracks. The method involves estimating the load (the stress) acting on the structural element with a crack, using the experimentally measured displacement of the crack surfaces caused by action of that load. To do this, we use fracture mechanics equations connecting the indicated parameters. As an example, we solve several problems for plates of finite dimensions with cracks for two load application methods.Translated from Problemy Prochnosti, No. 8, pp. 29–34, August, 1994.     

Correlation of grain-boundary precipitates parameters with fracture toughness in an Al–Cu–Mg–Ag alloy subjected to long-term thermal exposure

The effect of thermal exposure on grain-boundary precipitation in Al–Cu–Mg–Ag alloy was studied using quantitative transmission electron microscopy. Grain-boundary precipitate parameters, such as average size, number density and precipitate-free zone width, were measured. The effective diameter of precipitates, number of precipitates per grain-boundary area and area fraction of precipitates on the grain boundary were calculated. These data were applied to a grain-boundary fracture model to calculate grain-boundary fracture strain. The calculated fracture strains, in turn, were used to check the validity of two existing models of fracture toughness, which are based on grain-boundary nucleation of cracks and their propagation through precipitate-free zones. The fracture toughness model of Hornbogen and Graf closely agrees with the experimental results.     

Fracture and progressive failure of defective graphene sheets and carbon nanotubes

An atomistic based finite bond element model for the prediction of fracture and progressive failure of graphene sheets and carbon nanotubes is developed by incorporating the modified Morse potential. The element formulation includes eight degrees of freedom reducing computational cost compared to the 12 degrees of freedom used in other FE type models. The coefficients of the elements are determined based on the analytical molecular structural mechanics model developed by the authors. The model is capable of predicting the mechanical properties (Young’s moduli, Poisson’s ratios and force–strain relationships) of both defect-free and defective carbon nanotubes under different loading conditions. In particular our approach is shown to more accurately predict Poisson’s ratio. The numerical prediction of nonlinear stress–strain relationships for defect-free nanotubes including ultimate strength and strain to failure of nanotubes is identical to our analytical molecular structural mechanics solution. An interaction based mechanics approach is introduced to model the formation of Stone–Wales (5-7-7-5) topological defect. The predicted formation energy is compared with ab initio calculations. The progressive failure of defective graphene sheets and nanotubes containing a 5-7-7-5 defect is studied, and the degradation of Young’s moduli, ultimate strength and failure strains of defective nanotubes is predicted.     

Prediction of the probability of nonfailure of structural elements of heat-and-power-generating equipment from the fracture-toughness characteristics of the material

Conclusions A method was developed to predict the life of important structural elements of heat-and-power-generating equipment from the parameter of the probability of nonfailure on the basis of statistical modeling of the vector of random parameters of crack-resistance characteristics of the metal using criteria of linear fracture mechanics.We also developed an algorithm and a program which automate the computing process with the use of modern computers to predict the life of a structure from the crack resistance of the material in a probabilistic formulation.We will subsequently conduct studies involving the construction and substantiation of the representativeness of samples of mechanical tests of materials for crack resistance, which is very important in predicting the safe life of a structure.Institute of Strength Problems, Academy of Sciences of the Ukrainian SSR, Kiev. Translated from Problemy Prochnosti, No. 1, pp. 7–13, January, 1987.     

Fiber-optic measurement grid for registration of the parameters of vibrational processes

We propose a design for a fiber-optic measurement grid for registration of the vibrational parameters of various structural elements. We consider the principle of operation and give an analysis of the output signal of the measurement grid, consisting of quasidistributed fiber-optic sensors of the integrating type. We also describe and experimentally study a model for a grid for registration of the vibrational intensity distribution for the surface of the test object.Translated from Izmeritel'naya Tekhnika, No. 3, pp. 32–33, March, 1995.     

On the Statistical Theory of Long-Term Strength of Glass

We present new methods for the evaluation of the statistical parameters of long-term strength of glass and describe the procedure of preparation of specimens (glass disks) for testing, the method of loading, and the experimental procedure. The statistical model of fracture of glass is presented and the conclusions concerning the evaluation of the parameters of strength are made.__________Translated from Problemy Prochnosti, No. 3, pp. 75 – 85, May – June, 2005.     

Strength Analysis of Structural Elements of Hydrogen Power-Generating Equipment

The strength of materials and structural elements operating in contact with high-pressure gaseous hydrogen is studied as applied to the problems of hydrogen power-generating industry. The requirements to the design of structural elements operating in hydrogen are formulated. We propose a model of influence of high-pressure gaseous hydrogen on the strength and plasticity characteristics of structural materials and apply this model to the development of the corresponding procedures of strength analysis and evaluation of the cyclic fatigue life and durability of structural elements of hydrogen power-generating equipment taking into account the dislocation and diffusion mechanisms of hydrogen transport in the metal. The data on the properties of materials in hydrogen, the choice of the proper safety and durability margins, and the evaluation of admissible stresses in the structures are presented. The analysis of strength, cyclic fatigue life, and the number of cycles to failure are illustrated by special examples.__________Translated from Fizyko-Khimichna Mekhanika Materialiv, Vol. 40, No. 6, pp. 89–94, November–December, 2004.     

Numerical Simulation of Brittle Damage in Concrete Specimens

A framework for damage mechanics of concrete is applied to simulate the nonlinear elastic deformation behavior of concrete using finite element method (FEM). A rather simple isotropic damage model containing essentially no adjustable parameters is shown to produce results in remarkably good agreement with sample experimental data: the damage law requires only the fracture energy to be defined completely. The model is achieved by introducing a damage surface that is similar to the yield function in the conventional theory of plasticity. A special form of damage surfaces is constructed to illustrate the application of the model. A new damage criterion, defined as an equivalent strain norm, is proposed, in order to take into consideration the asymmetric behavior of concrete. For verifying the FEM program including the model, deformations predicted by this model are compared with both the experimental ones for the concrete structural model and the ones calculated without application of the continuum damage mechanics.__________Translated from Problemy Prochnosti, No. 3, pp. 57 – 74, May – June, 2005.     

Interrelation between Micromechanisms of Structural Rearrangement of VT18U Titanium Alloy in Fatigue Fracture

We examine the dislocation structure of a pseudo--alloy of the Ti–Al–Zr–Sn–Mo–Nb–Si system upon fatigue and cyclic crack-resistance testing under identical conditions of symmetrical tensile–compressive loading with a frequency of 100, 500, 3000, and 10,000 Hz. It is shown that the structural development of the alloy during the fatigue-damage accumulation occurs in a limited number of microvolume zones near local stress concentrators, although a considerable number of structural alteration zones originate but stay undeveloped, while the rest of the material remains almost unchanged. At the stage of main-crack development, the structure of the crack-tip plastic zone presents a further logical development of the structure that was formed during the fatigue-damage accumulation. However, the stress level, which is much higher than that in the bulk volume of the material, can be responsible for additional structural elements and a diminishing role of local stress concentrators. These mechanisms are true for the entire range of cyclic loading frequencies studied. A variation of the rate of cyclic loading leads only to a slight decrease in the size of the microvolumes of maximum structural changes during fatigue-damage accumulation and in the honeycomb structural elements during the main crack propagation.     

Relaxation processes near the tip of a notch in porous powder iron

Conclusions We discovered the plasticity zone near the tip of a notch in porous iron tested by concentrated bending at 77°K and demonstrated that porosity affects the parameters of the plasticity zone. It was shown that the size of the zone increases with the porosity of materials whose fracture obeys the mechanism of quasishear (up to 12.5% for the powder iron under investigation). Porosity promotes yield of materials at low temperatures and, thus, affects the mechanism of fracture. These effects are most pronounced in the field of stresses near the tip of the notch and, consequently, must be taken into account in the analysis of structural sensitivity of the crack-growth resistance of porous iron.Institute for Problems in Materials Science, Ukrainian Academy of Science, Kiev. Translated from Fiziko-Khimicheskaya Mekhanika Materialov, Vol. 31, No. 4, pp. 113–116, July – August, 1995.     

Fatigue strength and cyclic crack resistance of titanium alloy VT3-1 in different structural states. Communication 1. Study procedure and experimental results

Procedures are described for studying the fatigue strength and cyclic crack resistance characteristics of alloy VT3-1 in different structural states achieved by varying thermomechanical treatment (TMT) parameters. TMT schedules, alloy structure and texture characteristics, and also the size of structural elements typical for an alloy structural state are given. Results are given for static tensile, fatigue, and crack resistance tests on alloy VT32-1 in different structural and textural states, and they are analyzed.Translated from Problemy Prochnosti, Nos. 5–6, pp. 3–11, May–June, 1995.     

Residual strength of aluminum–lithium alloy center surface crack tension specimens at cryogenic temperatures

A three parameter fracture criterion which correlates the stress and the stress intensity factor at failure, is followed for the residual or fracture strength estimations of cracked configurations made of aluminum–lithium (Al–Li) alloys. The three fracture parameters are determined from the fracture data of Al–Li alloy center surface crack tension (SCT) specimens at cryogenic temperatures. It is found that the estimated fracture strength values compare well with the test results.     

Fatigue tests of asymmetric structural elements by axial loading

An examination is made of the loading of asymmetric structural elements in axial loading machines during fatigue tests. It is shown that it is necessary to normalize stiffness with respect to the angle of clamp rotation for machines used to fatigue-test complex structural elements of the given type.Translated from Problemy Prochnosti, Nos. 5–6, pp. 62–68, May–June, 1995.     

Simulating crack initiation and failure in a multilayer composite: Effective-strength distribution in a unidirectional reinforced composite

Summary A unidirectionally reinforced composite is considered, whose components are homogeneous in elastic properties but stochastically inhomogeneous in strength parameters. The material is represented as a continuous set of structural elements composed of fiber segments, matrix, and adhesive layers. A combined theoretical and experimental approach has been used to determine the microstrength distribution parameters for rectangular specimens having thin notches, which includes an analytical calculation on the state of strain and the determination of structural element dimensions, as well as experimental determination of the failure load and statistical processing.Translated from Fiziko-Khimicheskaya Mekhanika Materialov, Vol. 26, No. 2, pp 86–91, March–April, 1990.     

Creep crack growth under history-dependent loading

We discuss the influence of loading history on creep crack growth. Our attention is mainly focused on the following three aspects of this problem: (i) principal laws of history-dependent creep strain of materials; (ii) creep behavior of cracks, including the choice of suitable fracture parameters that may help to predict cracking; (iii) the importance of taking the history-dependent response of the material into account. We performed numerical calculations based on the use of an appropriate constitutive model and fracture theory for (1) and (2), respectively, to analyze results of tests for (3).Battelle, Columbus, Ohio. UES Incorporated, Dayton, Ohio. Published in Fiziko-Khimicheskaya Mekhanika Materialov, Vol. 30, No. 4, pp. 37–45, July – Augus, 1994.     

Hydrogen permeability of niobium during cathodic polarization in hydrochloric acid

Conclusions For high-strength chilled steel, the limit value of the stress intensity factor K^IIIp attained in antiplane strain during short-period fracture by shear is a characteristic that is not sensitive to the effect of an ambient medium such as water and aqueous solutions of sulfuric acid. The possible reduction of short-period bearing capacity of specimens with a crack during loading by mode III takes place only as a consequence of reorientation of the crack into the plane in which the maximum normal stresses act and is associated with reduction in fracture toughness during tear (K_1c) because of the effect of the medium.During long-term corrosion tests in antiplane strain of specimens with a crack, the final act of fracture is almost always completed by tear. In addition, preceding spontaneous fracture, subcritical crack growth is possible by both tear and shear. For certain structural states of steel and ambient media, the latter mechanism and the corresponding high rate of subcritical crack growth may become dominant at low levels of K^III0. This explains the anomalous character of curves of long-term crack resistance observed in these cases.Translated from Fiziko-Khimicheskaya Mekhanika Materialov, Vol. 13, No. 1, pp. 42–46, January–February, 1977.