Fatigue crack nucleation and propagation in aluminum alloy plates with cold expanded holes

AbstractAnalysis of the literature shows that in some cases existing technological methods of processing, including cold expansion, are the effective means to improve fatigue durability of structural elements with holes. The effect of cold expansion (1−3 %) on the stress state and fatigue durability of aluminum plates with functional holes under uniaxial cyclic loading was studied. The fatigue crack growth in the plates with a thickness t = 6 mm and width of 60 mm with a central hole diameter of 8 mm and 10 mm of aluminum alloy D16chТ (2024-T3) was investigated. Fatigue test was carried out under constant amplitude loading at stress ratio R = σmin/σmax = 0 and room temperature. Here σmin, σmax are the minimum and maximum stresses, respectively. The stress range was 147 MPa, loading frequency was 25 Hz. The mechanical properties of the alloy at room temperature were: yield strength σY = 300 MPa, tensile strength σU = 430 MPa. For all tested specimens with cold expanded holes the cracks initiation from the edges of holes on the entrance faces mandrel and from middle part for plain hole. Using the finite element method, the distribution of local residual stresses in the vicinity of the expanded hole was calculated. For specimens with cold expanded holes the least compressive stresses were near the entrance face and the largest were in the middle of the hole and near the exit face. With the increase of the cold expansion hole, the difference in values of compressive residual stress along the plate thickness is reduced. The width zone of residual compressive stresses increase with the increase of cold expansion. The dependencies of the number of cycles to crack initiation in aluminum specimens on the local maximum stress and local stresses range in the vicinity of surface hole were obtained. Cold expansion degree of 1% increases the lifetime of the plates to the initiation of fatigue crack length of 0.25 mm in 1.5-3 times as compared with plane plates. A similar dependence is observed for cold expansion of 2%. With further increase of cold expansion degree up to 3% the lifetime to fatigue crack initiation is increased in 7−10 times as compared with plane plate. By using the finite element method, the distributions of local stress range, maximum and minimum local stress near the hole depending on the number of loading cycles and cold expansion were built. In the middle section of the specimen the local stress ratio in the second half-cycle of load decrease with increasing of distance from the edge of the hole and with the increase of cold expansion. The lowest value of local stress ratio was obtained at cold expansion degree of 3% and it is constant across thickness of plate

Methodology and some results of study of frequency and waveform effect on the fatigue crack growth resistance of heat-resistant steel

Investigation of frequency and waveform effect on the fatigue crack growth resistance of heat-resistant 12Cr1MoV steel of a thermal power plant header after exploitation is presented. The studies showed that the crack growth is very non even. The increase of fatigue crack growth rate was 0.2-0.65 mm. The decrease of loading frequency at 500 C decreases significantly the FCG rate on the right part of FCG diagram

Specific features of deformation of the nitinol alloy after electrolytic hydrogenation

Specific features of the effect of hydrogenation on the susceptibility of a Ni–Ti alloy with shape memory to deformation are determined with the use of metallographic, electrochemical, and mechanical studies. Three sections are detected in the tensile curves of the specimens of nickel–titanium alloy in the initial state. The first section is linear due to the elastic deformation of the alloy with initial austenitic struc-ture. The second section is nonlinear and associated with pseudoelastic structural transformations of the original austenitic structure into a martensitic structure. The third section is also linear and caused by the elastic deformation of martensite formed in the course of deformation of austenite. After hydro-genation of the Ni–Ti alloy, the pseudoelastic structural transformation starts at a somewhat lower level of stresses than without hydrogenation. In this case, the specimens are destroyed after the termination of this transformation for a much lower level of plasticity than in the nonhydrogenated alloy. It is assumed that the electrolytic hydrogenation of the alloy promotes the formation of a very brittle hydride phase on the surface of Ti-type inclusions revealed in the structure of alloy in the initial state. Its thickness is determined by the duration of the process of hydrogenation rather than by the current used for hydro-genation

Modelling of stress strain state of type TP-100 boiler superheater collector under complex thermo-mechanical loading using the finite element method

Оцінено напружено-деформований стан колектора пароперегрівача котла ТП-100 із урахуванням впливу внутрішнього тиску пари та різниці температур на його внутрішній та зовнішній стінках. Основну увагу зосереджено на дослідженні НДС найбільш ймовірної ділянки руйнування − містка між отворами колектора пароперегрівача. Виявлено, що основне зростання напружень на цій ділянці спричиняє градієнт напружень по товщині стінки колектора (різниця між температурами зовнішньої та внутрішньої поверхонь). Досліджено, що найнебезпечнішими режимами експлуатації колектора є умови, коли температура зовнішньої поверхні перевищує температуру внутрішньої. Отримано залежності розподілу напружень у різних ділянках містка між отворами колектора від температурних режимів експлуатації та зроблено висновки щодо вірогідних причин виникнення пошкоджень.The power facilities, in particular, thermal power plants (TPPs) belong to the important objects, the failure of which can leads to accidents with severe consequences. The uninterrupted operation of TPPs greatly depends on the reliability of main elements of steam power systems. Therefore, one of the crucial tasks to be provided the structural integrity and life of the superheater collector is to increase the accuracy of its stress strain state estimation taking into account the thermo-mechanical loading. In the study the stress strain state of type TP-100 boiler superheater collector was evaluated, taking into account the effect of internal steam pressure and temperature difference on its internal and external surfaces. The main attention is focused on the study of the stress strain state of the most probable collector failure area that is the ligament between the superheater collector holes. The impact of internal pressure and temperature changes on the internal and external surfaces on the stress-strain state of thermal power plant superheater collector during the starts and stops was modeled in elastic formulation using finite element method. It was found out that the major stress increase in this area is caused by stress gradient through the thickness of the collector wall (the temperature difference between the external and internal surfaces). It was determined that the most dangerous modes of operation of the collector are the conditions when the temperature of the external surface exceeds the temperature of the internal surface. The dependences of the stress distribution on the temperature regimes of collector operation in different parts of the ligament between the holes and the conclusions about the likely causes of damage emergence were done. For the studied temperature range (20-600°C) the stresses that arise in superheater depend only on the temperature difference between the internal and external surfaces and do not depend on their maximum values. The most dangerous operation modes of superheater collector are when the temperature on its internal surface is lower than on the external. With the increase of this difference from 0 to 100°C the normal stress on the internal surface of the ligament between the superheater holes increases in 5 times and reaches 230 MPa, which exceeds the yield strength of the material at the temperature of 500°C

The influence of selected factors on the strenght of wood adhesive joints

This paper examines the effect of selected factors on the wood bonded joint strength. The structural factor under scrutiny was the geometry of the surface area; however, other major dimensions of adherends were compared as well. The tests were per-formed on 6 joint types: butt, v-shaped, interlocking, scarf, tongue & groove, and single lap joints, which were formed on two types of substrate materials – pine and oak wood, and adhesively bonded. The joints were bonded with Prefere 6312 wood adhesive and Loctite 3430 two-component epoxy adhesive. Strength testing of the joints was performed on Zwick/Roell Z150 material testing machine, according to DIN EN 1465 standard. The results obtained from the tests indicate that bonded joints of oak wood exhibit higher strength properties than those of pine wood adherends, which was confirmed in nearly all strength tests

The effect of stress ratio on functional behavior and structural fatigue of pseudoelastic niti alloy

The influence of stress ratio on functional behavior and structural fatigue of pseudoelastic NiTi alloy are studied. With the change of the stress ratio from 0 to 0.5 the residual strain in the first and next cycles increases significantly even at lower values of maximal stress. The fatigue life of NiTi alloy increases with the decrease of stress ratio from 0.5 to 0 in the case of presenting the results depending on the stress range