Estimation of strength properties of functionally graded structures with elliptical stress concentrators

Purpose: The purpose of this paper is to elaborate new calculation schemes for evaluating the strength parameters of railway rolling stock parts with non-local properties of surface layers in the presence of elliptical stress concentrators. Design/methodology/approach: Using the proposed approaches of developed mathematical modelling and open software for calculating FEniCS, it were established the most dangerous angles of stress concentrator orientation and the required thickness of the hardened zones of parts, which ensures their minimum softening during operation. Findings: It is shown that for an elliptical stress concentrator with any orientation angle, there is a certain key size of surface hardening thickness, the exceeding the value of which does not have influence on the operational strength of the parts, but rise the price of technological operations. Research limitations/implications: In this paper proposes a method for computation the impact of the orientation of the surface elliptical stress concentrators on the contact strength of parts under conditions of dominate friction power loads. Practical implications: The obtained results were used to set the modes of plasma hardening, which increase the contact strength of railway parts with elliptical stress concentrators. Originality/value: Using the approaches of contact mechanics, mathematical and computer modelling, methods of controlling the contact strength of the parts with the surface elliptical stress concentrators were proposed for the first time.

Prediction of the Onset of Failure in Elastomeric Solids with Weld Lines being Represented as Localized Regions of Lower Density

We study the initiation of damage in a polymeric body in which there is a line defect due to the formation of a “weld line” that occurs when two polymer streams join together and then solidify. We show that damage initiates in the region of weakness, namely the “weld line” based on a criterion for damage that was developed earlier in [1]. We also show that if there are other stress concentrators also additionally present, such as a hole, then there is a competition between the stresses induced due to the weakness and the stress as a consequence of the stress concentrator (in this instance a hole). This study adds more credence to the criterion for the initiation of damage that is based completely on knowledge of information at the current configuration of the body, that is, the criterion for damage is not based on the value of quantities that also need information based on a reference configuration such as the stress or strain.

Features of the formation of structure and mechanical properties in rolled products of various thicknesses from low-carbon microalloyed steel produced by casting and rolling complex

The article considers results of the study of microstructure parameters effect on the impact strength in temperature range from 0 to –80 °C in 20 °C increments of Charpy samples with a sharp stress concentrator and Mesnager test pieces with a circular stress concentrator from rolled coils of low-carbon microalloyed steel with various thicknesses. The used roll products were produced in conditions of JSC “Vyksa Metallurgical Plant”. The tests were performed using optical and scanning electron microscopy. It is shown that with the same chemical composition and thermomechanical treatment modes, the metal of smaller thickness (6, 8 mm) is characterized by higher strength properties (on average, by 10 MPa for temporary resistance, by 30 MPa for yield strength) and a margin for viscous properties at negative temperatures at close values of grain score and average grain size corresponding to 10 – 11 numbers according to the State standard GOST 5639. The metal with a thickness of 12 mm has the lowest level of cold resistance, and the temperature of brittle transition is minus 50 °C. Structure of rolled products of various thicknesses has a variation in grain size. Rolled metal of smaller thicknesses have a smaller grains corresponding to number 14, rolled metal of larger thicknesses has a larger grains corresponding to number 8. By conducting electron microscopic studies using the backscattered electron method, it was found that a greater number of large-angle boundaries, which are barriers for brittle cracks propagation, are observed in the 6, 8 mm thick rolled products. The constructed orientation maps of the microstructure showed the presence of pronounced deformation texture corresponding to the orientations <110>||RD (rolling direction) and (<113>...<112>)||RD for rolled products with a thickness of 6 mm.

DEVICE FOR CREATING SLOTS-VOLTAGE CONCENTRATORS IN THE BOUNDARY AREA OF WELLS IN FINE ROCKS

It is shown that in order to expand the field of practical application of directional measuring hydraulic fracturing, it is necessary to create technical means that provide cutting of initiating slots on the walls of boreholes in rock masses of different strength. A new slotter design is proposed, the distinctive feature of which is using carbide cutters that are extended during cutting slots along the normal to the device axis. The device is brought into operation due to its normalized movement tagging the bottom. The slotter dimensions allow using it in diamond drill holes of 76 mm diameter. The device is designed to create stress concentrator slots in adjoining zones of boreholes in hard rocks with a coefficient of 5-8 according to M. M. Protodyakonov’s scale.

Hydrogen-Assisted Fatigue Propagation in Corner Cracks at Holes Located in Plates under Tensile Loading

In this study, hydrogen-assisted fatigue propagation (a kind of corrosion fatigue phenomenon) in corner cracks at holes located in plates under tensile loading was studied, the results compared with those obtained for propagation by fatigue in air. To this end, numerical modeling was carried out for the case studied to evaluate the advance of the crack front based on the Paris equation and the stress intensity factors (SIFs) obtained by Raju and Newman. The results showed that the cracks tended toward a preferential propagation path in their growth, the effect of the presence of the stress concentrator on the preferential fatigue propagation path being more pronounced in the crack growth by fatigue in air than in the crack growth by corrosion fatigue.

Обоснование основных положений деформационной модели подготовки очага корового землетрясения

The article reasonably shows that the uppermost layer of the Earth's crust up to 25 kilometers is seismogenic. Aim. The article provides the evidence that crustal seismicity is generated not by regional stress fields of a homogeneous shear, as it was adopted in the strategy for solving the problem of earthquake forecast, but by local fields of exponential elastic stress. Such fields arise in one or another section of a seismogenic fault due to the occurrence of a stress concentrator in this section. According to the Saint Venant principle, such a stress concentrator (an additional load in the system) generates a local stress field of an exponential form. In this field the maximum stress is localized in the areas of an increment load application (in the fault) and decreases very quickly (exponentially) on both sides of the fault. Such stress concentrators arise in those areas of a seismogenic fault, where displacements along the fault stop due to various reasons. G.A. Gamburtsev foresaw this situation and very precisely called such concentrators as “seams”. The origin of a local stress field at the place, where a seam appears, is caused by the following fact: the power impulse generated by the seam is small compared to the linear momentum of the entire system of blocks of the considered fault and, therefore, it will stop the displacement of blocks only within the seam; but the displacements of blocks outside the seam will continue in the same mode. One can single out the following reasons causing stress concentrators in the fault: variations in different stress fields, changing the value of the friction coefficient in the fault; variations in fluid processes; the influence of temperature and pressure; mechanical “hooks” of blocks due to irregularities of their contacting surfaces, etc. Methods. The fact of the existence of the considered local stress fields is confirmed by geodetic studies, i.e. the results of repeated geodetic measurements in the epicentral zones of strong earthquakes. Results.These results allow drawing the following conclusions: 1) the sign of the preparation of a crustal earthquake source was reliably determined. This sign means the increasing deformation of the elastic bending of rocks in the source in the course of time; 2) from the standpoint of solving the problem of earthquake forecast, the main and decisive result of these studies is that the deformation processes occurring in the impending source also capture the Earth's surface, because this is precisely what opens up great opportunities in solving this problem; 3) with the help of special geodetic systems (forecast profiles), one can detect the places of the impending earthquake source preparation, i.e. make an accurate forecast of the site of a future earthquake; 4) since the energy of the earthquake source is functionally related to its size, one can realize the correct prediction of the maximum possible intensity of the future earthquake by determining the length of the seismogenic fault section, elastically deformed by the preparation of the earthquake using the forecast profiles В статье обоснованно показано, что сейсмогенным является самый верхний слой земной коры мощностью до 25 километров. Цель работы. В статье приведены доказательства того, что коровая сейсмичность порождается вовсе не региональными полями напряжений однородного сдвига, как это было принято в стратегии решения проблемы прогноза землетрясений, а локальными полями упругих напряжений экспоненциального вида. Такие поля возникают в том или ином участке сейсмогенного разлома из-за появления на этом участке концентратора напряжений. Согласно принципу Сен-Венана такой концентратор напряжений (дополнительная нагрузка в системе) порождает локальное поле напряжений экспоненциального вида. Максимальная величина напряжения в этом поле расположена в месте приложения дополнительной нагрузки (в разломе) и очень быстро (экспоненциально) убывает в обе стороны от разлома. Такие концентраторы напряжений возникают на тех участках сейсмогенного разлома, на которых в силу тех или иных причин прекращаются смещения по разлому. Г.А. Гамбурцев провидчески предвидел данную ситуацию и очень метко такие концентраторы назвал «спайками»». Возникновение локального поля напряжений в месте появления спайки обусловлено тем, что импульс силы, порождаемый спайкой мал по сравнению с количеством движения всей системы блоков рассматриваемого разлома и, следовательно, он остановит смещение блоков лишь в пределах спайки, но смещения блоков вне спайки будут продолжаться в прежнем режиме. Среди причин, порождающих концентраторы напряжений в разломе можно назвать следующие: вариации различных полей напряжений, изменяющие величину коэффициента трения в разломе; влияние температуры и давления; вариации флюидных процессов; механические «зацепы» блоков из-за неровностей их соприкасающихся поверхностей и др. Методы исследования. Факт существования рассматриваемых локальных полей напряжений подтвержден геодезическими исследованиями – результатами повторных геодезических измерений в эпицентральных зонах сильных землетрясений. Результаты работы. Эти результаты позволяют сделать следующие выводы: 1) достоверно определен признак подготовки очага корового землетрясения, которым является нарастающая во времени деформация упругого изгиба горных пород в его очаге; 2) с позиций решения проблемы прогноза землетрясений главным и определяющим результатом этих исследований является то, что происходящие в готовящемся очаге деформационные процессы захватывают и земную поверхность, ибо именно это открывает большие возможности в решении этой проблемы; 3) с помощью специальных геодезических систем (прогнозных профилей) можно обнаруживать места подготовки очагов готовящихся землетрясений, т.е. осуществлять точный прогноз места будущего землетрясения; 4) так как энергия очага землетрясения функционально связана с его размерами, то определив с помощью прогнозных профилей длину участка сейсмогенного разлома, упруго деформированного подготовкой землетрясения, можно осуществить и точный прогноз максимально возможной силы будущего землетрясения

Investigation of the thermal shock resistance of ceramic materials by the sensitivity of their structure to the stress concentrator

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Modeling the deformation process of a plate with a stress concentrator taking into account the postcritical stage of material deformation

Postcritical deformation of a material is a process which is characterized by a decrease of stress during growing deformations as a result of accumulation of structural damage. The design becomes unable to withstand the external load only when zones with weakened connections are developed enough. Evolution of postcritical deformation zones can occur with an increase of the external load applied to the construction. It means that taking into account the softening of the material allows determining the strength and deformation reserves of constructions more accurately. The mathematical formulation of the boundary value problem of supercritical deformation mechanics is given in the paper. The features of the experimental study of the postcritical stage of material deformation are listed. Strain curves of various steels with a long section of softening are obtained. Numerical solutions for the problems of deforming a thin plate with stress concentrators of different geometries under kinematic loading are obtained. Piecewise linear approximations and real strain curves of steel 20 and steel 40Cr4 obtained experimentally are considered. The evolution of zones of postcritical deformation in the material is considered. The correspondence between the value of the decline modulus and the nature of the evolution of the softening zones is determined. A stress plot is constructed that reflects how the complete material deformation diagram is realized near the concentrator. The calculated loading diagrams are constructed. It is noted that even after the appearance of softening zones, an increase in external load is possible. The strength and deformation resources of structures are determined, and the influence of the geometry of the stress concentrator on their values is considered. It is noted that the consideration of softening in modeling the behavior of structures with stress concentrators is appropriate.