SEM analysis of PA38-T6 aluminum alloy thin-walled tubular specimen fatigue fracture, and comparison to surface replication results

The paper presents results of SEM analysis of fatigue fracture surface of thin-walled tubular specimen manufactured from PA38-T6 aluminum alloy. In the previous research specimens were subjected to multiaxial loadings. They were axial, torsional, in-phase, 90º out-of-phase and asynchronous loadings. During the fatigue experiments, the process of initiation and development of cracks was tracked using the surface replication technique, using cellulose acetate thin foils. In the present work, SEM observations of fatigue fracture surface were conducted and compared to previously obtained data. It was confirmed, that the dominating mechanism of fatigue crack formation was the coalescence of small crack, regardless of loading type and level

Особливості деформаційного рельефу в умовах багатоосьового втомного пошкодження

Наведено результати експериментального дослідження спрямовані на створення нового емпіричного критерію оцінювання багатоосьового втомного пошкодження. Запропонований критерій можна застосувати для матеріалів, на поверхні яких під дією циклічного навантаження формується деформаційний рельєф – екструзії, інтрузії, смуги ковзання. Випробування проводилися для зразків з алюмінієвого сплаву Д16АТ, алюмінієвий плакуючий шар якого реагує на циклічні навантаження у вигляді формування та еволюції кластерів рельєфу. Чутливість деформаційного рельєфу до видів навантажування була підтверджена порівнянням зображення рельєфу в умовах двоосьового навантаження, згин разом з крученням з рельєфом, котрий формувався на поверхні зразків, що випробовувалися лише при циклічному згині. Доказано, що багатоосьове навантаження має сильніший ефект на процес накопичення пошкодження. Також представлено результати дослідження еволюції деформаційного рельєфу поверхні зразків, що випробовувалися при комбінованому синхронному та несинхронному навантажуванні, показано чутливість поверхневої структури і до такої моди навантажування. Аналіз деформаційного рельєфу забезпечує достатньо точне прогнозування залишкової довговічності компонентів, які виготовлені з плакованих алюмінієвих сплавів. Прикладом застосування даного підходу є аналіз втомного пошкодження конструктивних елементів літака, що працюють в умовах багатоосьового навантаження. Іншою можливістю використання явища, що розглядається в статті, є розроблення нового покоління сенсорів втоми, призначених для оцінювання багатоосьової втоми. Сенсор втоми може мати хрестоподібний вигляд для втомних випробувань, закріплений тим чи іншим чином.The paper presents experimental results directed on the development of new empirical criterion for multiaxial fatigue. The proposed criterion may be applied for materials responding the cyclical loading by the formation of deformation relief, i.e. system of extrusions, intrusions, persistent slip bands. The tests were carried out on aluminum alloy D16AT, the aluminum cladding layer of which is able to react cyclical loading by formation and evolving of relief pattern

Fatigue behaviour of selected materials under multiaxial asynchronous loadings

Four types of materials: PA38 aluminium alloy, E235 steel, E355 steel and 1.4301 austenitic steel were subjected to low-cycle multiaxial loadings. All tests were strain-controlled and typical, thin-walled, hollow specimens were used. Various synchronous and asynchronous loadings were applied. The analysis of experimental results involved: cyclic stress-strain response, fatigue life and observation of microcracks behaviour on the surfaces of fatigued specimens. Obtained results indicate that the difference in the strain components frequency of the asynchronous loadings has a significant influence on the fatigue behaviour of the materials

The Shear Stress Determination in Tubular Specimens under Torsion in the Elastic–Plastic Strain Range from the Perspective of Fatigue Analysis

The comparison of shear stress determination methods in tubular specimens under torsion is presented in this paper. Four methods were analyzed: purely elastic solutions, purely plastic solutions, the midsection approach, and the Chaboche nonlinear kinematic hardening model. Using experimental data from self-designed and conducted fatigue experiments, an interesting insight on this problem was obtained that is not often tackled in the literature. It was shown that there are differences in determined shear stress values, and their level depends on a few factors. The midsection approach and purely plastic solution gave values of surface shear stress very close to the values obtained using the Chaboche nonlinear kinematic hardening model for high strain levels. The purely elastic solution gave proper results for the low strain ranges, close to the cyclic yield limit. Since none of the methods can be trusted in the full range of loading, an important conclusion from these analyses regards the formulated ranges of their applicability. It was also shown that the calculated values of shear stress and plastic and elastic strain energy density determined on this basis have a strong impact on fatigue life predictions. Finally, the influence of predicted values of shear stresses on the interpretation of cyclic hardening phenomena was also presented

Gaussian Process for Machine Learning-Based Fatigue Life Prediction Model under Multiaxial Stress–Strain Conditions

In this paper, a new method for fatigue life prediction under multiaxial stress-strain conditions is developed. The method applies machine learning with the Gaussian process for regression to build a fatigue model. The fatigue failure mechanisms are reflected in the model by the application of the physics-based stress and strain invariants as input quantities. The application of the machine learning algorithm solved the problem of assigning an adequate parametric fatigue model to given material and loading conditions. The model was verified using the experimental data on the CuZn37 brass subjected to various cyclic loadings, including non-proportional multiaxial strain paths. The performance of the machine learning-based fatigue life prediction model is higher than the performance of the well-known parametric models

New Criterion for Aircraft Multiaxial Fatigue Analysis

The complexity of analytical and experimental estimation of aircraft components fatigue life is determined by the irregular character of the load’s sequence, a number of stress concentrators, multiaxial stress state. Proposed early multiaxial fatigue criteria are aimed to reduce the complex multi axial loading to an equivalent uniaxial loading. These criteria cover different categories of metals but taking into account the wide variety of constructional materials, modes of loading, environmental conditions, the instrumental structural health monitoring looks a reasonable alternative or at least a strong complement to existing multiaxial fatigue analysis procedures. The new criterion has been proposed as a result of multi-scale levels study of metal surface transformation under fatigue

Finite Element Analysis of Ventilation System Fire Damper Dynamic Time-History

The paper presents results of the numerical analysis of the fire damper used in ventilation systems under the earthquake loading. The research was conducted in accordance with the recommendations of the Nuclear Safety Standards Commission. The aim of the analysis was to examine the fire damper with respect to its resistance to service loadings, structural integrity, and capability to stay operative after an earthquake. The analysis was carried out using the Finite Element Method in LS-Dyna software. The earthquake loading was modelled as accelerations, measured in three directions during the earthquake. For modelling of the materials behaviour, material models taking into account the influence of strain rate on hardening were used. The analysis consisted of three stages, which were: loading the construction with the earth gravity, earthquake simulation by loading with accelerations in three directions, and, finally, closing the fire damper. The analysis has shown that some of the construction elements undergo plastic deformations. However, the performed simulation of fire damper closing showed that despite these deformations, the device remains capable to keep its functionality and the damper closes hermetically. The results of the analysis were important design indications for the fire damper prototype

Short cracks observations on surfaces of specimens made of three materials, subjected to synchronous and asynchronous multiaxial loadings

In this paper analysis of short cracks observed on surfaces of specimens subjected to axial, torsional, synchronous in-phase and out-of-phase as well as a number of asynchronous loadings was performed. The tests were conducted on PA38-T6 aluminum alloy, E235 non-alloy steel and 1.4301 austenitic stainless steel thin-walled specimens, in strain-control mode. It was concluded that for PA38-T6 aluminum alloy and E235 steel, fatigue cracks initiated and propagated on the plane of maximum shear strain, regardless of the loading case. Fatigue cracks observed on the surface of 1.4301 steel specimens developed predominantly on the levels of maximum normal strain. For this material, the surface condition resulting from the machining had a significant impact on the cracks growth. In the case of non-proportional loadings, at high levels of strain, cracks often propagated along the machining marks

Finite Element Analysis of Ventilation System Fire Damper Dynamic Time-History

The paper presents results of the numerical analysis of the fire damper used in ventilation systems under the earthquake loading. The research was conducted in accordance with the recommendations of the Nuclear Safety Standards Commission. The aim of the analysis was to examine the fire damper with respect to its resistance to service loadings, structural integrity, and capability to stay operative after an earthquake. The analysis was carried out using the Finite Element Method in LS-Dyna software. The earthquake loading was modelled as accelerations, measured in three directions during the earthquake. For modelling of the materials behaviour, material models taking into account the influence of strain rate on hardening were used. The analysis consisted of three stages, which were: loading the construction with the earth gravity, earthquake simulation by loading with accelerations in three directions, and, finally, closing the fire damper. The analysis has shown that some of the construction elements undergo plastic deformations. However, the performed simulation of fire damper closing showed that despite these deformations, the device remains capable to keep its functionality and the damper closes hermetically. The results of the analysis were important design indications for the fire damper prototype