Procedia Structural Integrity最新文献

15-5 PH stainless steel is a martensitic precipitation-hardenable stainless steel (0.07C-15Cr-5Ni-3.5Cu-Fe). Presently, high strength low alloy steel, AFNOR 15CDV6 is used for the fabrication of Thrust Vector Control system of satellite launch vehicle. These pressure vessels are used for storing strontium perchlorate, intended for attitude (pitch and yaw) control of the launch vehicle during first stage. The tanks are fabricated through welding of rolled plates, machined rings, and end domes from forgings. AFNOR 15CDV6 steel is prone for corrosion and requires alumina barrier coating to protect from strontium perchlorate. Therefore, as a superior alternative, 15-5PH steel with improved corrosion resistance, simple heat treatment avoiding distortion was selected for the fabrication of pressure vessel. In order to improve the structural integrity of the pressure vessel, flow forming technology was adopted for realization of cylindrical portion of the tank, which eliminated the long seam weld joint and reduced the number of circumferential weld joints. As a part of the developmental program, the tank was designed, shells were flow formed and welded with end domes and were directly aged. The fabricated tank was subjected to a series of pressure tests followed by final burst test for estimating structural margins and ensuring quality requirements.

This paper describes the design of the pressure vessel, process followed for manufacture, qualification program adopted and detailed metallurgical analysis carried out on the burst tested hardware. The 15-5PH pressure vessel has successfully withstood the proof pressures and test results are better than the predicted analysis results, indicating structural integrity of the pressure vessel for flight.

The objective of this work is to evaluate the parameters of strain rate dependent Johnson-Cook material model using a hybrid procedure which uses finite element analysis as well as experimental data. The large strain rate tests were conducted to using split Hopkinson pressure bar test setup, whereas the quasi-static test was carried out using conventional testing machines. Johnson-Cook material model has been used to simulate the plastic deformation behavior of material under the high strain loading. This model provides the flow stress of the material as a function of equivalent plastic strain, plastic strain rate and temperature. The parameters, i.e., A, B, n, C and m of the model have been determined using a hybrid procedure as mentioned earlier. One of the important issues in split Hopkinson pressure bar testing is the variation of strain rate during the duration of loading. Due to decrease in reflected strain magnitude, the strain rate continuously decreases (as strain rate is proportional to reflected strain signal) and it is a characteristic of the above test method. However, this creates problem in evaluation of parameters of Johnson-Cook material model using conventional procedure. In this work, a modified procedure has been developed in order to take into account of this variation in strain rate as function of applied strain. The results of the method has been validated with experimental data. The parameters have been evaluated only for room temperature data and the effect of temperature shall be studied in future.

Ceramic materials used in mechanical applications show variations in their properties due to the presence of cracks. Micro-cracks within the material (size, orientation and density) affect the ceramic material’s strength and other mechanical properties. This study developed a micro-mechanics-based model that accounts for the original orientation of micro-cracks and their propagation as wing cracks. Unlike other micromechanics-based models, the current model defines failure based on entropy associated with crack propagation within the material. Entropy is calculated from energy dissipation from crack propagation from the pre-existing flaws in the ceramic. The Unified Mechanics Theory (UMT) is used to define entropy-based damage in the ceramic material, in which a parameter called thermodynamic state index (TSI) is employed to describe the state of the material. A representative volume element (RVE) with a pre-existing flaw is used to calculate the energy dissipated during the wing crack propagation. The effect of various crack lengths and orientations is incorporated with a probability density function. The strain rate effects are implemented using dynamic crack growth law. The stress-strain curve at strain rate from quasi-static to high strain rate (10^-3-10⁶) is plotted for Alumina under dynamic compression.

Probable risks of the Corrosion Preventive Compounds (CPCs) for the aviation riveted joints fatigue are considered. It is shown that high penetration of the CPCs can lead to the reduction of the riveted joints fatigue life. Negative side effect assumed to be result of the reduction of the friction between the mating elements.

Various approaches for studying the jump-like deformation of AMg6 aluminum alloy are being compared. AMg6 alloy is characterized by instantaneous deformation increases during uniaxial stretching in the area of plasticity. It was assumed that the process of jump-like tensile deformation is caused by the cracking of dispersoids in the volume of the material. Based on that assumption, the methods that predict the initiation and magnitude of jump-like deformation depending on the proportion of destroyed inclusions were proposed. In particular, the ANSYS software complex was used to predict jump-like deformation, in which the groups of finite element models were developed to determine the main patterns of influence of structural heterogeneity parameters of the simulated environment on the stress-strain state. In addition, given the large amount of experimental data, it is important to learn how to solve such problems using machine learning (ML), particularly neural networks. It has been established that the prediction accuracy by one of the most common ML methods, that was neural networks, comprised more than 90%.

The problem of plate bending, weakened by rectilinear through crack, taking into account the contact of defect faces is considered. A detailed analysis of the analytical solution obtained previously by the classical theories of the plates within the framework of a model of crack closure along the line is performed. According to the data for intensity factors of forces and moments, the distribution of stress intensity factor across the plate thickness has been restored. We pay particular attention to the problem of determining the value of the fracture bending load. Process of propagation of the crack with non-uniformly stressed front in conditions of bending consists of two stages: irregular over its thickness consistent growth of the crack when load reaches its lower limiting value and unstable propagation of the crack when load reaches its upper limiting value. Analytical estimates of such limiting values are made using of the force and energy criteria of linear mechanics of fracture. The authors’ experience with the determine the limiting bending loads for some contact cracks systems in infinite and semi-infinite plate sis illustrated too.

Experimental studies of the influence of harmful elements (hydrogen, sulphur, oxygen) on the degradation of structural steels of underground sewer systems and the nature of their distribution in the metal of pipelines operated for a long time in aggressive environments have been carried out. It is shown that with an increase in the service life, the surface layers of the metal are enriched by hydrogen, oxygen and sulfur, adversely affecting the metal’s corrosion-mechanical properties. Combined with structural transformations, this leads to local metal embrittlement and, under favourable conditions (under alternating cyclic loads), to the formation of micropores, the coagulation of which leads to crack formation. An increase in the service life of sewage steel pipe structures leads to an increase in the values of the parameter of the body-centred cubic crystal lattice of the α-solid solution and an increase in microstresses in the structure. At the same time, part of the carbon from the decomposed cementite passes to the interface of the α-matrix. The second part, apparently, remains on dislocations, turning into microcracks, and also goes to the formation of new finely dispersed carbide particles; relatively large carbide particles form at the grain boundaries between pearlite and ferrite.

The stress-strain state data for various types of intermediate supports in two-span continuous perforated beams were obtained through calculations using the finite element method within the "Lira" software complex. The calculations were performed under the influence of a symmetrical evenly distributed load applied across the spans. It was determined that the design of the intermediate support has minimal influence on the magnitude of maximum deflections and stresses within the beam span. However, it somewhat changes the nature of its operation, especially in the case of using a support part without transverse stiffeners and with unwelded holes to the left and right of the axis of the support. The maximum normal stresses were observed at the characteristic design point of the cross-section with the hole in the supporting part of the beam, specifically without the stiffening rib and with unwelded holes on both sides of the support. Additionally, comparable stress values were noted with the inclusion of stiffeners, albeit still with unwelded holes. In all cases, these stresses remain below the calculated yield point resistance of the steel. The inclusion of welded holes and support stiffeners proves beneficial in enhancing the performance of the support components. This is attributed to a significant reduction in the concentration of both tangential and normal stresses, despite stress values in all scenarios remaining below the calculated resistance of steel.

This study is devoted to the determination of the effect of hydrogen concentration in metal C_H on the residual durability N_f of defected pipelines. Three different cases of defected pipes were considered, namely: pipe with longitudinal semi-elliptical crack at outer surface; pipe with longitudinal semi-elliptical crack at inner surface and pipe with circumferential semi-elliptical crack at inner surface. It has been shown that the presence of the crack-like defects in the pipelines is very dangerous from the point of view of their further reliable operation. This danger significantly increases with the increase of the hydrogen concentration in the metal. The increase of the danger of crack-like defects in the pipes may be expressed by the sequence: longitudinal semi-elliptical crack at outer surface – longitudinal semi-elliptical crack at inner surface – circumferential semi-elliptical crack at inner surface. The residual durability of defected pipe N_f was chosen as the basic parameter for the assessment of its further serviceability. Thus, all defects detected in the pipeline after inspection can be compared with received data and on this base the expert conclusion for the evaluation of the potential risk of each defect can be made.

Strain aging of structural steels is generally considered as an important factor in their embrittlement. In the laboratory, it is initiated by preliminary plastic deformation of a metal for the generation of dislocations, followed by short-term heating. In the study, a new hypothesis about the possibility of strain ageing occurring without any initial plastic deformation at the micro-scale under the presence of hydrogen in the process that induces internal stresses in steel was proposed. It was presumed that areas with local plastic deformation induced by hydrogen would be the preferred locations for steel to undergo strain aging during subsequent heating. This assumption was substantiated through experimental tests using the low-alloy pipe steel in different states (as-delivered state, after low-temperature tempering and after preliminary electrochemical hydrogen charging followed by low-temperature tempering). The mechanical properties of the steel, including strength, plasticity, impact strength, fracture toughness, and resistance to stress corrosion cracking, were assessed. Low-temperature tempering did not affect the mechanical behaviour of the steel. However, the steel subjected to the procedure of combining preliminary hydrogen charging with subsequent low-temperature tempering was characterized by a significant decrease in fracture toughness and resistance to stress corrosion cracking. Hydrogen had an impact on the embrittlement of the steel through the strain aging at local sites being preferable for hydrogen diffusion.