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

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

Assessment of holes damageability for countersunk rivets

Досліджено основні дефекти виготовлення отворів під потайні заклепки за допомогою аналізу поверхні на растровому електронному мікроскопі. Проведено оцінювання параметрів небезпечних мікродефектів на циліндричній та конусній поверхні отвору. Проведено порівняльний аналіз різних способів виготовлення отворів під потайні заклепки.Main manufacturing defects of countersunk rivets holes are researched with scanning electron microscope. Characteristic of dangerous defects are qualified on conical and cylindrical surface. Comparative analysis of different holes manufacture methods is done. Main methods of surface quantity improvement are described

Research of stress strain state of power structural elements with functional holes under uniaxial cyclic loading

Методом скінченних елементів обчислено одновісне пружно-пластичне деформування пластини з центральним деформаційно зміцненим отвором діаметром 8 мм, 10 мм після дорнування з натягом із алюмінієвого сплаву Д16чТ 1%, 2%, 3%. Побудовано розподіл розмаху локальних напружень, максимальних та мінімальних напружень та в околі отвору залежно від кількості циклів навантаження та відносного натягу дорнування. У середній ділянці циліндричної поверхні отвору ( ) розрахункові локальні коефіцієнти асиметрії циклу у другому півциклі навантаження знижуються зі збільшенням відстані від краю отвору та зі збільшенням відносного натягу дорнування. Найменші локальні коефіцієнти асиметрії циклу навантаження R=-4…-5 для натягу дорнування 3% і сталі по товщині зразка ( мм).Using finite element method taking advantage of the software module ANSYS Explicit Dynamics elastic-plastic deformation of the plate with 8 mm, 10 mm central strain hardened hole diameter with burnishing tension 1%, 2%, 3% have been calculated. To calculate the stress strain state in the threshold of the functional hole after burnishing with different tension and cyclic loading finite-element models of a quarter of the sample with a central hole were built. The load was applied to one surface of the edge of sample model and the other surface was fixed and limited by vertical movement (along the direction of application of force). The cyclic elastic-plastic deformation of aluminum D16chT alloy plate under of soft load at and has been studied. Characteristics of mechanical properties are and . Scale distribution of local stresses , maximum and minimum local stress and in the threshold of the hole depending on the number of loading cycles and burnishing tension has been built. Local stresses and , while calculating taking advantage of the finite element method has been stabilized already for two load half-cycles. The largest range of local stresses occur in the middle part of the hole thickness without burnishing (i = 0%, ) and on the edge of the mandrel entrance after burnishing with the tension (1% ... 3%, ). For burnished holes initiation and propagation of crack from the edge of the mandrel entrance are the characteristics. It is caused by the fact, that the compressive residual stresses caused by burnishing and the range of local strains for study of tightened burnishing (i = 1% ... 3%) on the hole edge are much smaller than in the middle part. Distribution of local coefficients of asymmetry of loading cycles for burnishing tension 1%, 2%, 3% and without burnishing obtained by finite element calculation has been built. In the middle section of the cylindrical surface of the hole ( ) calculation local coefficients of asymmetry in the second half cycle load decrease with the increase of distance from the edge of the hole and with the increase of burnishing tension. Local load cycle asymmetry coefficients decrease with the threshold of depth of the cylindrical surface of the hole along the X axis for the average in depth section along the axis Z ( ) The smallest local load cycles of asymmetry coefficients R = -4 ... -5 are for burnishing tension 3% and are stable for all thickness of the sample ( mm)