Form Of Stressed State Research Articles

Vibrocreep of metals in uniaxial and complex stress states

The results of the creep tests with extension and torsion of tubular specimens made of D16AT and AD1 aluminum alloys, which were earlier obtained at the Institute of Mechanics, Lomonosov State University, were analyzed theoretically. Experimental data obtained with the author’s participation imply that a sharp increase in creep rate is observed under certain conditions when a vibration stress of small amplitude is added to the static stress. This effect (vibrocreep effect) is manifested only if the form of the stress state under the common action of static and dynamic stresses differs from the form of the static stress state. In this case, as the duration of the applied vibration stress increases, the vibrocreep effect gradually weakens.

Prediction of Strain Hardening and Fatigue Life of Cyclically Unstable Materials

The data on cyclic deformation of materials with different cyclic properties are analyzed. The relationship between the level of additional strain hardening and the mechanical characteristics of a material is established. The proposed models take into account the influence of the form of stressed state on strain hardening and durability under conditions of nonproportional cyclic deformation. The models are constructed on the basis of the analysis of the experimental data on cyclic deformation of 304 stainless steel. The prediction of strain hardening and durability is performed with separate analysis of the influence of shape of the cycle and the form of stressed state. To take into account the influence of shape of the cycle, we use the coefficient of disproportionality of the cycle and the parameter of sensitivity to disproportionality. To take into account the form of stressed state, we use the coefficient of the form of stressed state and the corresponding parameter of sensitivity to the form of stressed state. The results of prediction of both strain hardening and durability of materials on the basis of the proposed phenomenological models reveal their fairly high efficiency.

Two Failure Mechanisms and a Unified Strength Condition for Geomaterials

A unified failure criterion for geomaterials is examined in the development of Davydenkov-Fridman ideas. Experimental data are presented for a broad range of geomaterials (from water and ice to concrete and natural stone materials; these data confirm theoretical positions on failure by separation or shear as a function of physical parameters, load path, and form of stress state.

Viscoplastic properties of steel 10GN2MFA with a complex stressed state

Results are given for experimental studies of the viscoplastic properties of steel 10GN2MFA with different ratios of principal stresses in the temperature range 20–320°C performed under conditions of stepwise loading. The invariance of the generalized equilibrium deformation curve towards the forms of stressed state and also the material quasiviscosity characteristics towards the form of stressed state and the amount of accumulated irreversible strain for the test material is established by experiment.

Effect of the form of stressed state on the kinetics of damage accumulation and crack resistance of body steel 15kh2mfa in different conditions. Communication 3. Evaluation of steel crack resistance

The effect of the form of stressed state on energy transformation in deformed body steel 15Kh2MFA in different states connected with merging of defects and crack development is considered. It is shown that independent of the structural state of body steel there are physically substantiated dependences between steel crack resistance characteristics determined from the descending section of total deformation diagrams (parameter Kλ) and stiffness of the material stressed state at the instant of crack initiation (parameter $$K_\sigma = \sigma _m /\mathop \sigma \limits^\_ $$ ). These dependences with the maximum possible restraint in the zone of crack initiation tend towards asymptotes corresponding to the values of critical stress intensity factor KIc for steels. Experimental results confirm the scheme suggested previously by the authors according to which the whole region of possible values of crack resistance characteristics for a material corresponding to difference stiffness of the stressed state and operating time may be presented on coordinates Kλ−Kσ.

Effect of the form of stressed state on the kinetics of damage accumulation and crack resistance of body steel 15Kh2MFA under different conditions. Communication 2. Stagewise failure process for steels KP80 and KP100

The kinetics of body steels KP80 and KP100 failure are studied in the final stages of deformation with static tension for smooth specimens and specimens with stress concentrators. Attention is concentrated on features of the formation and initiation of separation macrocracks in test specimens. Common and distinctive features of damage formation in the test steels are noted taking account of the form of stressed state.

Effect of the form of stressed state on the kinetics of damage accumulation and crack resistance of body steel 15Kh2MFA under different conditions. Communication 1. Stagewise process of steel KP60 failure

The kinetics of body steel KP60 failure are studied in the final stages of deformation with static tension for smooth specimens and with stress concentrators. Attention is concentrated on features of the generation and start of separation macrocracks in test specimens. Common and distinctive features of damage formation in the test steel are noted taking account of the form of stressed state.

Predicting the crack resistance of steels on small-scale specimens as a function of the structure's operating conditions

A fundamentally new scheme of evaluating the crack resistance of steels on small-scale specimens with allowance for service time and form of stress state, which is based on the method of complete strain diagrams, is presented.

Effect of loading prehistory on phase transformations in chromium-nickel steel with plastic deformation

Results are given for a study of the kinetics of phase transformations in chromium-nickel austenitic steel with repeated plastic deformation including under conditions when prior and repeated loading differ in temperature and form of stressed state. It is established that prior low-temperature deformation intensifies the process of martensite formation at room temperature whereas deformation under room temperature conditions has almost no effect on martensitic transformation kinetics at low temperature.

Curvilinear crack development trajectory in aluminum alloys with biaxial cyclic loading

A calculation and experimental study is provided for development of curvilinear cracks of different original orientation with uniaxial and biaxial tension. A marked dependence is noted for crack growth trajectory on the form of stressed state and aluminum alloy properties. It is established that the criterion considered for crack growth direction cannot be recommended as universal for a whole number of test materials. A method suggested previously for plotting calculation and experimental crack growth trajectories with a complex stressed state may be used in determining cyclic resistance characteristics for mixed forms of failure.

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.

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.

Deformation and failure of steel VNS25 at low temperatures taking account of the form of stressed state and stress concentrations

Deformation and failure of steel VNS25 at low temperatures taking account of the form of stressed state and stress concentrations

Effect of matrix type and form of stress state on mechanical properties of high-strength cast iron

Effect of matrix type and form of stress state on mechanical properties of high-strength cast iron

Influence of block cyclic loading on the character of deformation and strength of constructional materials under plane stressed conditions report 1. The resistance to elastoplastic deformation

1. For the high-strength alloy steel and D16T and VT6 alloys with the selected cycle of block loading, the character of accumulation of damages is close to the linear rule. 2. Both in the original condition and after preliminary cyclic loading, the studied constructional materials are anisotropic. Higher resistance to plastic deformation is characteristic of the steel and D16T alloy in the axial direction and of VT6 alloy in the tangential direction. 3. The generalized strain curves of the materials in stress intensity vs strain intensity coordinates are not invariant to the form of stressed state. 4. The influence of preliminary block cyclic loading on the resistance and general rules of deformation of materials under conditions of the complex stressed state is insignificant. At the same time, the resistance to plastic deformation of the steel decreases, while that of D16T and VT6 alloys increases, and the anisotropy of the materials decreases somewhat. The change in anisotropy of the materials is apparently related to the decrease in residual stresses as the result of preliminary cyclic loading.

Use of fracture mechanics to evaluate the bearing capacity and remaining life of rotors in turbomachinery

1. The risk of damage or brittle failure of rotors owing to the growth of crack-type macroscopic defects in them increases as the effective time to overhaul of turbomachinery increases with the expanded use of steels possessing increased creep resistance. According to statistical data, rotors of low and moderate pressure in steam turbines are especially subject to this risk. 2. Nondestructuve methods should provide for the reliable detection of defects with characteristic sizes\(\Delta<< {{K_{1_C }^2 } \mathord{\left/ {\vphantom {{K_{1_C }^2 } {n\beta \sigma _y^2 }}} \right. \kern-\nulldelimiterspace} {n\beta \sigma _y^2 }}\), where n is the safety factor, and β depends on the form of stress state at the most critical point of the rotor. The following methods of flaw detection are promising for practical use: magnetoscopic (external defects), ultrasound (internal defects), and vibrodiagnostics (for continuous in-service). 3. For equal characteristic dimensions, defects located in the axial channel or near it, and also on the surface of stress concentrators are the most dangerous: the first may give rise to brittle failure, and the second to vibration and curvature of the rotor. 4. To evaluate the stress intensity factors near cracks located on the surface of the axial channel, it is expedient to use Eqs. (2) and (4). Such a relation between the material's crack resistance KIc and the loading parameters ω, Δ, T, and p, which reduces the risk of the rotor's brittle failure, may exist for a number of rotor designs and loading conditions. 5. The selection of the material for turbomachine rotors should ensure the best compromise relationship between the material's capacity to resist creep and crack development. This selection should be made on the basis of analysis from positions of fracture mechanics and with allowance for the effective time to overhaul, as well as to the potentials of flaw-detection methods. The trend toward increased creep resistance for rotor steels increases the risk of brittle failure with decreasing KIc.

Effect of reactor irradiation on the long-term strength and creep of 0Kh16N15M3B steel in a plane stressed state. Part III. Experimental study of creep resistance

1. Reactor irradiation over the range of experimental parameters investigated does not have a substantial effect on the nature of the creep curves and long-term ductility of 0Kh16N15M3B steel but increases the intensity of the minimum rates of creep by almost two orders of magnitude. Here the extent of the radiation-induced increase in the rate of creep is reduced with an increase in stress intensity. 2. The ratio of the principal stresses in the experiments being discussed affects the form of the creep and long-term ductility curves appreciably in all the experiments conducted. Approximately, it can be considered that the\(\mathop {\varepsilon _i }\limits^ \bullet\) min (σi) relationship is invariant to the form of stressed state. An exception is the control experiments for σ1/σ3=−1.5. 3. The grain size of the initial structure of the steel investigated affects all the characteristics of the creep process substantially, both under normal and reactor test conditions. With an increase in the grain size the stage of accelerated creep preceding fracture is shortened, and the rate of steady-state creep and long-term ductility are lowered.

Equilong-term strength surfaces of organic textolite in the plane stressed state

Long-term strength tests of textolite were conducted for seven different particular cases of the plane stressed state. Maximum times to failure for which experimental data were obtained reach 2500 h. It is established from analysis of the test results that the experimental long-term strength curves for the types of stressed state under consideration are closely similar. The condition of long-term strength in the general case of the plane stressed state for constant levels of stresses is taken as the equation of the short-term strength surface in which the time factor is introduced parametrically; in this case, the apparent observance of similarity between equilong-term strength surfaces makes it possible to express the tensor components characterizing the long-term strength by tensor components of the short-term strength surface and a certain monotonically decreasing time function, which is independent of the form of stressed state.

Effect of a cyclic hold on the ductility and strength of 45 steel in the complex stressed state at low temperatures

1. The effect of preliminary cyclic loading by pulsating tension at room temperature on the low-temperature strength and ductility of steel depends upon the test temperature and the form of stressed state. 2. Preliminary cycling at normal temperature with the maximum stress not exceeding the yield strength by more than 10% has practically no effect on the strength of steel in the +20 to −100°C range. At lower temperatures it reduces the strength properties of the steel, especially in uniform biaxial tension. 3. As a result of cyclic loading, the ductility of steels decreases, and this decrease is more substantial in the direction of preliminary loading. With a decrease in temperature the effect of cyclic loading on the ductility increases.

Failure of sterically cross-linked polymers

A method is suggested for describing local long-term strength which is based on specific concepts about failure mechanisms of sterically cross-linked polymers. Microstructural characteristics of the material are used for a detailed description of the local failure process; from these one can go over to a damage macroparameter by use of the method proposed in [2]. A kinetic equation is obtained, from which simple formulas follow, for a number of cases, for determining time to failure as a function of the form of stressed state or the like. These formulas agree qualitatively with similar empirical relationships. A definition of the concept of local failure and a condition (criterion) for failure are given within the framework of the approach developed.