Short Fatigue Crack Growth at Different Maintenance Times for LZ50 Steel

Based on the average fatigue life of LZ50 axle steel specimens without surface rolling, five maintenance times were determined. Accordingly, five groups of specimens were turned and rolled at above maintenance times and were fatigued using a replica technique. The results show that the crack growth rate is much lower than that before rolling at a given dominant short crack size. The effective short crack density of all specimens decreases significantly after maintenance. However, with the postponement of surface maintenance, the highest effective short crack density for the five studied groups of specimens increases continuously, while the average fatigue life decreases gradually. A maintenance time effect function is presented to refine a short crack growth model described previously. The revifined model can include a significant effect of the maintenance time on short crack growth and predict its patterns at different maintenance times with exiting test results for LZ50 axle steel.На основании средней усталостной долговечности образцов осевой стали LZ50, не подвергавшихся поверхностной обкатке роликами, были определены пять модельных межремонтных циклов. Соответственно пять групп образцов были обточены и обкатаны роликами при вышеуказанных межремонтных циклах и подвергнуты испытанию на усталость по методу реплик. Результаты показывают, что скорость роста трещин гораздо ниже таковой до обкатки роликами при заданном преобладающем размере коротких трещин. Эффективная плотность коротких трещин для всех образцов значительно уменьшается после обработки. Однако если поверхностную обработку вовремя не выполняют, наибольшая эффективная плотность коротких трещин для пяти исследуемых групп образцов непрерывно возрастает, тогда как средняя усталостная долговечность постепенно снижается. Приведена функция, описывающая эффект межремонтных циклов, для уточнения предложенной ранее модели роста коротких трещин. Уточненная модель может учитывать существенное влияние межремонтного цикла на рост коротких трещин и прогнозировать характер их роста при различных межремонтных циклах на основании полученных результатов испытаний осевой стали LZ50

A Probabilistic Model for Describing Short Fatigue Crack Growth Behavior of LZ50 Steel

Fatigue damage process of metal components and structures with smooth surface belongs primarily to stage of short fatigue crack. To characterize the random growth behavior of short fatigue crack and to apply the crack growth rate model for engineering safety assessment, a probabilistic model is proposed with consideration of the test data scattering regularity. This probabilistic model is based on the multi-microstructural barriers model and can describe the deceleration behavior of growth rate during the whole short fatigue crack propagation process. To take the statistical characteristics of whole test data into account, the idea from the general maximum likelihood method which is widely used in parameters estimation of fatigue S-N curves and ε-N curves is inherited. While estimating the parameters of the probabilistic model, conventional correlation coefficient optimization method is extended for calculating the parameters of both the mean curve and the standard deviation curve. Analysis on the test data of LZ50 steel indicates the reasonability and availability of present model

Electrical transport and magnetic properties of nanostructured La0.67Ca0.33MnO3

Nanostructured La0.67Ca0.33MnO3 (NS-LCMO) was formed by pulsed-laser deposition on the surface of porous Al2O3. The resistance peak temperature (Tp) of the NS-LCMO increases with increasing average thickness of the films, while their Curie temperatures (Tc) remain unchanged. The coercive field of the samples increases with decreasing film thickness and its temperature dependence can be well described by Hc(T) = Hc(0)[1-(T/TB)1/2]. A large magnetoresistance and strong memory effect were observed for the NS-LCMO. The results are discussed in terms of the size effect, Coulomb blockade and magnetic tunneling effect. This work also demonstrates a new way to get nanostructured manganites

Short Fatigue Crack Behavior of LZ50 Axle Steel under Rotating-Bending Cyclic Loading

The short fatigue short crack replica tests using LZ50 axle steel hourglass-shaped specimens of were carried out under rotating-bending cyclic load. Seven sets of effective test data were obtained. The analysis demonstrated that short fatigue cracks generally originate in ferrite grains or on their boundaries, and then short cracks start propagating, which is strongly influenced by the microstructure of the material. The crack growth rate increases serratically, followed by its decrease double-fold because of the limit of ferrite grain boundaries and the banded pearlite structure. Based on the three principles of the best fit, which include the overall fitting effect, correlation with fatigue physics, and safety of residual strength prediction, the statistical evolution of the dominant effective short fatigue crack length and life share were analyzed. The statistical results show that the minimum value distribution exhibit the best fit of the dominant effective fatigue short crack length and the life share. The short fatigue crack growth model, which includes various microstructure barriers, provides a good fitting result and reflects the cyclic effect of the microstructure on the short fatigue crack behaviorВыполнены испытания на репликах короткой усталостной трещины с использованием образцов осевой стали LZ50 в форме песочных часов в условиях циклического нагружения при изгибе с вращением. Выполнены семь серий испытаний. Показано, что короткие усталостные трещины возникают в ферритных зернах или на их границах, затем они начинают расти под влиянием микроструктуры материала. Скорость роста трещин увеличивается скачкообразно, затем снижается вдвое за счет барьера, создаваемого границами ферритных зерен и полосчатой структурой перлита. Проанализированы статистическая эволюция длины доминирующей эффективной короткой усталостной трещины и доля остаточного ресурса на основании трех принципов наилучшего соответствия, включающих общий эффект соответствия, соответствие принципам физики усталости и надежность прогнозирования остаточной прочности. Установлено, что статистическое распределение минимальных величин выявляет наилучшее соответствие для длины доминирующей эффективной короткой усталостной трещины и доли остаточного ресурса. Модель роста короткой усталостной трещины, включающая различные барьеры в микроструктуре, обеспечивает хороший результат подгонки и отражает циклическое влияние микроструктуры на ее поведение

V-I characteristics in the vicinity of order-disorder transition in vortex matter

The shape of the V-I characteristics leading to a peak in the differential resistance r_d=dV/dI in the vicinity of the order-disorder transition in NbSe2 is investigated. r_d is large when measured by dc current. However, for a small Iac on a dc bias r_d decreases rapidly with frequency, even at a few Hz, and displays a large out-of-phase signal. In contrast, the ac response increases with frequency in the absence of dc bias. These surprisingly opposite phenomena and the peak in r_d are shown to result from a dynamic coexistence of two vortex matter phases rather than from the commonly assumed plastic depinning.Comment: 12 pages 4 figures. Accepted for publication in PRB rapi

An Improved Projection Method for Determination of Fatigue Parameters of Metal Structures Based on Spherical Direction Cosine Group Construction

The key components of railway rolling stock are almost constantly in the multiaxial stress state in service. To evaluate the fatigue strength of the structure, stress parameters should be obtained beforehand. The traditional projection method assumes that the maximum principal stress is the maximum tensile stress, and then the projection process is performed without taking the tension/ compression states of principal stresses into account. To address these inadequacies, a method for fatigue parameter determination was proposed based on principal stress projection toward a spherical direction cosine group. This method can determine the direction of maximum tensile stress and preserve the tension/compression attribute of stress during calculating the maximum stress and the minimum stress. Therefore, the physical meaning of present improved method is more apparent than the traditional projection method, and its algorithm is simple and effective. The fatigue strength assessments of a welded bogie frame were performed utilizing the traditional projection method and present method respectively. Results show that some maximum stresses calculated by the improved method could be 19.4% higher than those obtained by the traditional one at some nodes, while the minimum stresses and the stress ratio deduced from these two methods could even be of opposite sign. The present method, which takes the tension/compression properties of principal stresses into consideration, is more reasonable, as compared with the traditional projection method, and thus the fatigue strength assessment results are more credible

Dark Energy and Neutrino CPT Violation

In this paper we study the dynamical CPT violation in the neutrino sector induced by the dark energy of the Universe. Specifically we consider a dark energy model where the dark energy scalar derivatively interacts with the right-handed neutrinos. This type of derivative coupling leads to a cosmological CPT violation during the evolution of the background field of the dark energy. We calculate the induced CPT violation of left-handed neutrinos and find the CPT violation produced in this way is consistent with the present experimental limit and sensitive to the future neutrino oscillation experiments, such as the neutrino factory.Comment: 10 pages, 2 figures. Typos corrected and references added. To be published in EPJ

Enhancing electron diffusion length in narrow-bandgap perovskites for efficient monolithic perovskite tandem solar cells

Developing multijunction perovskite solar cells (PSCs) is an attractive route to boost PSC efficiencies to above the single-junction Shockley-Queisser limit. However, commonly used tin-based narrow-bandgap perovskites have shorter carrier diffusion lengths and lower absorption coefficient than lead-based perovskites, limiting the efficiency of perovskite-perovskite tandem solar cells. In this work, we discover that the charge collection efficiency in tin-based PSCs is limited by a short diffusion length of electrons. Adding 0.03 molar percent of cadmium ions into tin-perovskite precursors reduce the background free hole concentration and electron trap density, yielding a long electron diffusion length of 2.72 ± 0.15 µm. It increases the optimized thickness of narrow-bandgap perovskite films to 1000 nm, yielding exceptional stabilized efficiencies of 20.2 and 22.7% for single junction narrow-bandgap PSCs and monolithic perovskite-perovskite tandem cells, respectively. This work provides a promising method to enhance the optoelectronic properties of narrow-bandgap perovskites and unleash the potential of perovskite-perovskite tandem solar cells

The PHENIX Experiment at RHIC

The physics emphases of the PHENIX collaboration and the design and current status of the PHENIX detector are discussed. The plan of the collaboration for making the most effective use of the available luminosity in the first years of RHIC operation is also presented.Comment: 5 pages, 1 figure. Further details of the PHENIX physics program available at http://www.rhic.bnl.gov/phenix