Linear Fracture Mechanics Criteria Research Articles

К вопросу о допустимости дефектов в нижнем поясе подкрановых балок

Crane beams are among the most damaged structures in the buildings with heavy, and very heavy-duty overhead cranes. Their destruction can lead to the death of people working in the workshop, and to significant material losses. The durability of crane structures is determined by the intensity of work of the overhead cranes. The main defects and damages of metal crane beams are regulated by the standard STO-22-05—04, according to which all the cracks in the crane beams are divided into groups according to the risk of destruction. It is believed that the presence of cracks in the lower (stretched) chord of crane beams is unacceptable. Modern methods of non-destructive testing allow to detect defects with a minimum size from 2 mm (ultrasound) to 1 µm (capillary method). These cracks may be due to the inhomogeneity of the chemical composition of the alloy, or be of a structural nature, manifesting itself, in contrast to fatigue cracks that grow in the most loaded areas of the structure, in any part of the lower chord of the beam. If any crack is found in the lower chord of the crane beams, it is required to stop the operation of the crane and carry out a major overhaul of the crane runways. However, following these formal rules is not always economically and logically justified. The paper considers an example of rationing by the criterion of non-straining through cracks in the lower stretched chord (cracks of the first group). Based on the criteria of linear fracture mechanics, coefficients are proposed for all steels from which the crane beams are made, allowing for their technical inspection to give an express assessment of the admissibility of the detected through cracks. It is also shown that it is necessary to normalize the permissibility of the parameters of semi-elliptical cracks in the stretched zone (in the lower shelf of the I-beam crane beam), since these cracks can eventually develop into through defects.

Substantiation of two-criterial, failureestimation diagram for pressure vessels and pipelines with axial through cracks

A form of diagram for failure estimation is tested on the basis of results of full-scale experiments on the failure of pressure vessels and pipelines with axial through cracks. Use of the two-criterial approach to predict the failure of structural components containing a crack in the brittle, mixed, and ductile regions of failure is substantiated. For example, the brittle failure of cylindrical bodies with through cracks is monitored by criteria of linear fracture mechanics, and ductile failure by the criterion established when the load attains its limiting value (P=Pld).

Unstable fracture of rubber‐toughened polymers

AbstractThe goal of this investigation has been to obtain a useful criterion for the onset of ductile instability. Notched 3‐point bend samples of 4 impact‐modified plastics were tested under load‐controlled conditions to enhance the instability. Neither plastic collapse of the remaining ligament nor large scale yielding at the crack tip was observed, although significant damage zone development and sub‐critical crack growth were observed prior to crack instability. The stress intensity factor at instability (Kc) was found to be independent of crack length for the plastics tested. More surprisingly, when Kc was normalized by the corresponding yield stress, this normalized value was found to be experimentally the same for these four impact modified polymers. Calculations based on ductile tearing instability theory were ambiguous and inconclusive. These results suggest that linear fracture mechanics criteria in load controlled tests may still be applicable for engineering purposes under what has normally been considered ductile conditions.

Fracture of ductile materials in compression along two parallel disk-shaped cracks

The fracture criteria of linear fracture mechanics constructed by the Griffith-Irwin approach are inapplicable in the subcritical state in the case of loading of materials, weakened by parallel cracks, by compressive forces directed along the cracks [17, 20]. The monograph [3] examined a general approach to the study of such problems and presented the corresponding fracture criteria along with a detailed bibliography. The same publication examined problems of the fracture in compression of an infinite material with one crack or a system of cracks located in one plane. These problems dealt with elastic, composite, and plastic models of the materials. A three-dimensional axisymmetric problem for the same models in the case of a surface crack in a half-space was examined in [7, 8, 14-16, 21]. The studies [5, 6, 10] looked at a similar problem involving two parallel internal cracks in space for highly elastic and composite models.

Engineering method of predicting creep crack growth in materials

1. We worked out and substantiated an engineering method of temperature-time prediction of the kinetic characteristics of subcritical creep crack growth from the results of crack resistance tests of limited duration by using the criteria of linear fracture mechanics. 2. The suggested method of prediction takes integrally into account the change of the predominant mechanisms of failure in the metal of structural elements of thermal power generating equipment; this makes it possible to solve problems of extrapolation with better physical substantiation. 3. Our calculations make it possible to work out scientifically substantiated recommendations for determining the periods of maintenance and the remaining life of critical structural elements of large power units of thermal power stations operating under conditions of creep. 4. It is necessary to work in the field of experimental investigations of the temperature-time regularities of creep crack nucleation in the zone of a stress raiser with the object of working out scientifically substantiated methods of distant interpolation of crack nucleation; this will make it possible to predict the full life of objects operating under conditions of long-term thermal and mechanical loading.

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

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.

Method for determining the temperature dependence of subcritical creep crack growth of a material

A calculation-experimental engineering method is proposed for determining an adequate temperature dependence of the kinetic characteristics of subcritical creep crack growth in the material by using the criteria of linear fracture mechanics.

Accuracy of prediction of the endurance of structural members in creep conditions on the basis of cracking resistance characteristics of the material

1. Equations were derived for the quantitative determination of the accuracy of the method of prediction of the endurance of the structural member in the creep conditions. The method is based on the application of the criteria of linear fracture mechanics. 2. The sensitivity of the predicted endurance of the structural member to the cracking resistance characteristics of the material, the temperature-force regime of loading, and defect size was determined. 3. The error theory [2] was used as a basis for deriving equations for the numerical calculation of the required accuracy of determination of the cracking resistance characteristics of the material and also the accuracy of maintenance of the regimes of temperatureforce loading in the course of laboratory creep experiments for the given value of the relative error of the predicted endurance of the structural member.

A specimen geometry for plane mixed modes

A new specimen geometry and an appropriate loading equipment is propounded to apply plane mixed mode on a crack. One can obtain any value of the ratio K 1/K 11 which remains constant during loading. The load is imposed by a standard testing machine. The specimen is precracked in opening mode like a CT one and the thickness of 25 mm is sufficient to assure plane strain in the mid-thickness. An application of the test to fatigue studies of tough steel points out differences in behavior between plane stress and plane strain. Number of cycles at the onset of kinking and also branching direction is found at different ratios of K I/K II . The discrepancies with linear fracture mechanics criteria are analysed.

Low temperature fatigue fracture of metals and alloys

Using the data available in the literature and the present author's previous results the present-day studies of low temperature fatigue fracture of metals and alloys are reviewed in terms of the physics of metals. The effect of low temperatures on the fatigue life and fatigue limit of different crystal lattice types of metals and alloys is considered. The correlation between an increase in fatigue life and a reduction in temperature is analysed and certain exceptions to this correlation are pointed out. The problem of both the simultaneous influence of temperature and environment and the separate effects of each of these factors on cyclic fatigue life is discussed. It has been found that a superconducting transition affects the cyclic fatigue life of certain superconductors and that the effect of the superconducting state on fatigue life is ambiguous; this is accounted for by the varying plasticity of the superconductor. The experimental data on dislocation structure and fatigue crack nucleation at temperatures down to 4.2 K are discussed. Particular attention is given to the effect of low temperatures on the rate of fatigue crack growth and the threshold points of this stage. In this paper it is attempted to correlate the macroscopic characteristics of fatigue crack growth found by the criteria of linear fracture mechanics with the microscopic parameters of plastic strain and the fracture micromechanisms and to consider modern trends and perspectives in a further investigation of the low temperature fatigue problem.

Method and some results of investigating the crack resistance of steels 15Kh2MFA and 15Kh2NMFA in the 293-77�K temperature range

1. A good agreement was found between the values of KQ and KIc for steels 15Kh2MFA and 15Kh2NMFA determined in the case of advance of the crack by the secant method and calculated from the JIc values. 2. Disregard of the three dimensionality of the SSS can lead to large errors, especially for curvilinear cracks, when determining KIc on CT specimens and flat plates. 3. For steels 15Kh2MFA and 15Kh2NMFA the temperature characterizing transition from ductile to brittle fracture (\(T_{br_1 } \)), determined according to the type of fracture surface and fracture diagram, lies not below 293°K. 4. The conditions of plane strain at the mouth of the crack according to the criteria of linear fracture mechanics are reached at 183°K and lower. 5. A decrease of the test temperature from 293 to 213°K does not lead to a decrease of the values of KQ and JIc, and at a temperature below 213°K the values of KIc and JIc drop substantially for both investigated steels. 6. Consideration of the three-dimensionality of the SSS when determining KIc permits assuming the possibility of spontaneous arrest of the crack after its advance (jerky propagation of the crack) in the case of a change in the shape of its front from straight to curvilinear during monotonic loading of the specimens with cracks and explaining the cutoffs of the load on the P-v diagrams, which occur when determining the characteristics of the fracture toughness of certain materials under static loading conditions.