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

Evaluation of in-service surface cracking of rolls of continuous casting machines

The in-service damage in the material for rolls of continuous casting machines is investigated, in particular, the topology of surface cracks is studied, and their statistical analysis is made. Using the results of the microhardness measurements, the structural degradation of the material is evaluated depending on the distance to the roll surface

Research of SIF of two interacting semi-elliptical surface cracks using finite element method

Проаналізовано існуючі методи визначення коефіцієнтів інтенсивності напружень (КІН) для поверхневих непівеліптичних тріщин уздовж їх контуру та літературні дані щодо взаємного впливу кількох тріщин на зміну величини КІН. Для уточнення КІН уздовж фронту однієї та двох взаємодіючих поверхневих напівелептичних тріщин здійснено їх моделювання за допомогою методу скінчених елементів. Наведено результати досліджень коефіцієнтів інтенсивності напружень уздовж фронту поверхневої тріщини за одновісного розтягу пластини скінчених розмірів на основі методу скінченних елементів, які добре узгоджуються з відомими рішеннями. Здійснено моделювання взаємного впливу двох колінеарних тріщин на розподіл КІН уздовж фронту поверхневих тріщин.The authors present the results of the study of stress intensity factors along the front of surface crack and two collinear cracks under uniaxial tension of finite size plate basing on the finite element method. Surface cracks are a common phenomenon in structural elements of construction frequently as a result of high values of the stress in the body and availability of defects. Such defects can sufficiently reduce the service life of structural components causing their premature failure, especially under repeated loading. Definition of the stress intensity factors is one of the most important tasks for the residual life of structures estimation. In any linear elastic fracture mechanics (LEFM) problem, precise computation of stress intensity factors accurately is of great importance. The analysis of available methods for determining stress intensity factors for surface semi- elliptical cracks along their contour has been carried out and data on the mutual influence of several cracks on the change of SIF. In order to specify SIF along the front of one or two interacting semi-elliptical surface cracks, the problem of their evaluation using the finite element method was studied. First of all, the modeling of semi- elliptical surface crack in the prismatic sample under tensile pressure was carried out. A three-dimensional model of the sample with the global model elements and the model of crack area with a local mesh with tetrahedral elements were developed. Investigations were carried out for the surface semielliptical cracks with ratio a/t for the deepest crack point to the specimen thickness (a/t) in the range a/t=0,1...0,7 and different crack aspect ratios a/c = 0,2; 0,6; 1,0. For the chosen parameters the values of SIF K1 along the path of the semi- elliptical surface crack, well correlated with the known solutions, were obtained. The energy instability was found to be in the surface crack threshold of the contour areas when its semi-axes ratio amounts a/c=0,4...0,8. The authors modeled the mutual influence of two collinear cracks on the distribution of SIF along the surface cracks front basing on the similar research methods. The modeling of both commensurate cracks and those that are significantly different in size is also carried out. The mutual influence of cracks was found to start when the distances between the cracks are commensurate with the size of surface cracks. When the distance between the cracks is close to merger, the SIF K1 values for inside surface points of the crack contour significantly increase. The results obtained are of importance for the geometric schematization of defects and further estimation of the residual operating life of the construction parts

Fatigue crack growth. Overload and underload interactions

This article deals with the study of the overload- underload interactions effects on fatigue crack growth (FCG) rate. With the purpose of understanding of overload-underload interaction mechanisms the FEM modeling of stress-strain state at the crack tip was investigated. FCG tests with single tensile peak overloads and complex overloads- underloads have been performed in D16chT (analogue of American 2024 T3) aluminium alloy. Using the assumption of the principal role of residual stresses (as a result of plastic strain at crack tip) in crack growth retardation and acceleration, the FCG interaction model was developed. Based on the proposed FCG interaction model shows good correlation with calculated and experimental data

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

Influence of cold expansion of holes on crack initiation and growth in aluminum alloys

Досліджено вплив відносного натягу дорнування отворів на період зародження та швидкість поширення втомних тріщин в алюмінієвому сплаві Д16чТ. Виявлено, що незалежно від натягу дорнування (1–3%) тріщина зароджувалася від кромок отвору з боку входження дорна. На відміну від дорнованих зразків зародження тріщин в зразку у вихідному стані відбувається в середній ділянці отвору без виходу на бічні поверхні зразка. Графічно показано відмінності процесу руйнування залежно від величини натягу дорнування. За формулами Newman та Raju обчислено номінальні та ефективні коефіцієнти інтенсивності напружень для кутової тріщини у пластині з отвором.Influence of relative holes cold expansion on the fatigue crack initiation and growth rate in aluminium alloy D16chT has been researched. Irrespective of cold expansion rate (1–3%) the crack initiation was found to be initiated in the hole from the side of mandrel entrance. On the contrary to cold expanded samples the cracks initiation in an initial condition occurs in the middle area of the hole without penetration in the side surface of the specimen. Differences of fracture process depending on the cold expansion rate has been demonstrated graphically. Newman and Raju formulae were used to calculate stress intensity factor, geometry factor and correction function for angle crack in a plate with a hole. Stress intensity factors are calculated in the first approximation, where residual stresses caused by the cyclic plastic deformation at the crack tip are taken into account, technological residual stresses on the holes surface plastic cold expansion not being taken into account. Kinetic diagrams of specimens with a central hole fatigue fracture are constructed in terms of the effective stress intensity factor range. Fatigue crack growth rate in specimens made of D16chT alloy in double logarithmic coordinates is in the proportional dependence on the stress intensity factor range. However, after holes cold expansion fatigue crack growth rate is almost independent on the stress intensity factor range, when and is less than fatigue crack growth rate in samples with non cold expanded holes. The basic regularities of plastic cold worked holes with expansion rate 1–3% for fatigue cracks initiation and propagation in aluminum alloy D16chT are researched. Irrespective of cold expansion rate the cracks were found to start from the edge of plastically deformed holes in side, where the entrance of mandrel was. Number of cycles to surface fatigue crack length of 0,25 mm increases with the increasing holes cold expansion rate. This is due to the dominant influence of residual compressive stresses in the vicinity of hardened holes. With cold expansion rate increasing from 1% to 2% the effective stress intensity factor and fatigue crack propagation rate decreases

Damage tolerance analysis of continuous caster roll containing a circumferential semi-elliptical crack

Проведено аналіз існуючих методів прогнозування залишкового ресурсу металургійного обладнання. Оцінено тримку здатність ролика машини безперервного лиття заготовок із напівеліптичною поверхневою тріщиною на основі використання підходів SINTAP з урахуванням розкиду розмірів тріщини, в'язкості руйнування, характеристик механічних властивостей матеріалу та величини прикладеного навантаження через випадкові змінні з відповідними функціями розподілу. Одержані результати свідчать, що метод може бути рекомендовано для практичного використання.The procedure for damage tolerance analysis of metallurgical equipment is presented and illustrated by a case study on caster roll. The scheme is based on the recently developed prosedure SINTAP taking into account the scatter of crack’s geometry, fracture toughness, material properties and applied loadings as random variables with corresponding distribution functions. The results show that the method can be used in practice

Crack emanation from defects modelled by extremely small drilled holes in two conditions

Fatigue is the primary reason for the failure of structural components. Resistivity of a coarse grain heat affected zone against fatigue crack initiation in presence of micro-defects is discussed in the present article. Samples of material with a martensitic coarse grain heat affected zone microstructure were prepared by proper thermal treatments. Microstructurally small holes were used as artificial micro-defects. They were created by drilling. Compressive residual stresses appear in the material due to the irreversibility of plastic deformation. Moment of hole drilling enables to prepare samples with and without effects of residual stresses. Critical stress level for fatigue crack initiation depends on the actual size of the hole. The location and the way of crack initiation is affected by the presence and character of residual stresses

Simulation of fatigue crack growth in d16t alloy using finite element method

Створено модель плоского зразка з центральною тріщиною в пружно-пластичній постановці за допомогою методу скінченних елементів у програмному комплексі ANSYS. Обґрунтовано довжину проростання тріщини для усталення пластичної зони у вістрі тріщини. Змодельовано ріст втомної тріщини за регулярного циклічного навантаження з урахуванням залишкових деформацій та напружень в околі вістря тріщини та контакту її берегів. Розраховано швидкість росту втомної тріщини з використанням ефективного коефіцієнта інтенсивності напружень, отриманого при моделюванні розкриття тріщини. Отримано задовільне узгодження результатів моделювання та експериментальних даних.The aim of this study was to simulate fatigue crack growth using ANSYS software, determine crack growth rate and compare it with experimental data. In order to simulate the fatigue crack propagation along the direction of its growth, contact conditions were applied on two separate surfaces – cracks faces. This made it possible to simulate the processes that occur at the crack closure due to the formation of residual plastic deformations, contact and compressive stresses in the crack tip. In order to simulate crack propagation, the rate of crack growth was determined by the Paris formula. Crack growth rate was determined after application of each cycle of regular loading and summarized using of the «cycle by cycle» technique. When the overall crack growth reached finite element size, the limitation of displacements in Y axis were removed from a pair of nodes that were at one point and modeled crack tip at that time. The minimal crack growth length which is required for stabilization of plastic zone, stress-strain state and crack tip opening level was determined. For realistic simulation of fatigue crack growth the crack should be grown for 2 mm at least. This can be explained by the stabilization of stress-strain state in the crack tip and plastic deformations on crack faces (behind crack tip). Results of stress-strain state simulation for each load step, including plastic deformations, were considered during simulation of next load step (cycle). During crack growth on its faces residual plastic deformations (extensions) were formed. Therefore during unloading the crack faces closed up prematurely, before unload if finished. This leads to the crack closure effect. In addition, after premature crack closer, residual contact compressive stress appears on the crack faces. Thus, the created model made possible to take into account the effect of residual plastic deformation (extensions) on crack faces and consequently residual compressive stresses. The series of FEM simulation experiments of fatigue crack growth that corresponded to the experimental data at different values of maximum stress intensity factor (SIF) and SIF ratios were modeled. It was determined that values of effective SIF ranges are by 15–20% higher than obtained experimentally or by calculations using formulas. Therefore, it was proposed to introduce correction coefficient in order to eliminate these differences in simulation algorithm of effective SIF range calculation. Corrected values of calculated SIF are in satisfactory conforming to experimentally obtained results. The reasons of overestimation of simulated effective SIF ranges relatively experimentally obtained are – neglecting roughness and accumulation of corrosion products at crack faces. These factors cause increasing of the level of crack tip opening stress and decreasing of effective SIF range. Proposed finite element model takes into account only plastic deformations of material and residual compressive stresses ahead and behind the crack tip. Proposed correction factor integrally takes into account an increasing of roughness and accumulation of corrosion products on crack tip faces. To compare the simulated and experimental results, the V Keff diagram of D16T alloy in effective Keff values was built. The diagram shows satisfactory agreement of experimental and simulated results for each experiment