4,790 research outputs found
Characterisation of microstructure, defect and high-cycle-fatigue behaviour in a stainless steel joint processed by brazing
We report the characterisation of microstructures and high-cycle-fatigue (HCF) properties of Type 304 stainless steel joints processed by brazing. Pure copper was applied as the filler metal for brazing at 1120 °C. A two-phase microstructure was obtained within the joint region: the star-shaped precipitates and copper matrix. The precipitates with an average size of 0.43 μm were rich in iron and chromium. A fixed orientation relationship was found between the precipitates and copper matrix. The joint exhibited much higher tensile strength and HCF life when compared to pure copper. The strength enhancement can be attributed to the presence of precipitates. Furthermore, the effect of joint interface roughness as well as defects was critically investigated. The joint interface roughness showed little influence on the HCF lives. Post-examinations revealed that fatigue crack initiation and propagation occurred entirely within the joint region, hence being consistent with the similar HCF lives regardless of the pre-defined interface roughness conditions. In addition, it was found that the HCF lives decreased exponentially with the increase of initial defect area. Fractography analysis revealed that fatigue striation spacings near the crack initiation zone increased with the increase of defect area, suggesting that the larger defects result in higher crack growth rate, hence shorten the overall fatigue life.</div
A Life Prediction Model of Multilayered PTH Based on Fatigue Mechanism.
Plated through hole (PTH) plays a critical role in printed circuit board (PCB) reliability. Thermal fatigue deformation of the PTH material is regarded as the primary factor affecting the lifetime of electrical devices. Numerous research efforts have focused on the failure mechanism model of PTH. However, most of the existing models were based on the one-dimensional structure hypothesis without taking the multilayered structure and external pad into consideration. In this paper, the constitutive relation of multilayered PTH is developed to establish the stress equation, and finite element analysis (FEA) is performed to locate the maximum stress and simulate the influence of the material properties. Finally, thermal cycle tests are conducted to verify the accuracy of the life prediction results. This model could be used in fatigue failure portable diagnosis and for life prediction of multilayered PCB
Strong ETH Breaks With Merlin and Arthur: Short Non-Interactive Proofs of Batch Evaluation
We present an efficient proof system for Multipoint Arithmetic Circuit
Evaluation: for every arithmetic circuit of size and
degree over a field , and any inputs ,
the Prover sends the Verifier the values and a proof of length, and
the Verifier tosses coins and can check the proof in about time, with probability of error less than .
For small degree , this "Merlin-Arthur" proof system (a.k.a. MA-proof
system) runs in nearly-linear time, and has many applications. For example, we
obtain MA-proof systems that run in time (for various ) for the
Permanent, Circuit-SAT for all sublinear-depth circuits, counting
Hamiltonian cycles, and infeasibility of - linear programs. In general,
the value of any polynomial in Valiant's class can be certified
faster than "exhaustive summation" over all possible assignments. These results
strongly refute a Merlin-Arthur Strong ETH and Arthur-Merlin Strong ETH posed
by Russell Impagliazzo and others.
We also give a three-round (AMA) proof system for quantified Boolean formulas
running in time, nearly-linear time MA-proof systems for
counting orthogonal vectors in a collection and finding Closest Pairs in the
Hamming metric, and a MA-proof system running in -time for
counting -cliques in graphs.
We point to some potential future directions for refuting the
Nondeterministic Strong ETH.Comment: 17 page
Dynamical Isometry is Achieved in Residual Networks in a Universal Way for any Activation Function
We demonstrate that in residual neural networks (ResNets) dynamical isometry
is achievable irrespectively of the activation function used. We do that by
deriving, with the help of Free Probability and Random Matrix Theories, a
universal formula for the spectral density of the input-output Jacobian at
initialization, in the large network width and depth limit. The resulting
singular value spectrum depends on a single parameter, which we calculate for a
variety of popular activation functions, by analyzing the signal propagation in
the artificial neural network. We corroborate our results with numerical
simulations of both random matrices and ResNets applied to the CIFAR-10
classification problem. Moreover, we study the consequence of this universal
behavior for the initial and late phases of the learning processes. We conclude
by drawing attention to the simple fact, that initialization acts as a
confounding factor between the choice of activation function and the rate of
learning. We propose that in ResNets this can be resolved based on our results,
by ensuring the same level of dynamical isometry at initialization
A simulation method for fatigue-driven delamination in layered structures involving non-negligible fracture process zones and arbitrarily shaped crack fronts
Most of the existing methods for fatigue-driven delamination are limited to
two-dimensional (2D) applications or their predictive capabilities have not
been validated in three-dimensional (3D) problems. This work presents a new
cohesive zone-based computational method for simulating fatigue-driven
delamination in the analysis of 3D structures without crack migration. The
method accurately predicts fatigue propagation of non-nelgigible fracture
process zones with arbitrarily shaped delamination fronts. The model does not
require any kind of fitting parameter since all the input parameters are
obtained experimentally from coupon tests. The evaluation of the energy release
rate is done using two new techniques recently developed by the authors (the
growth driving direction and the mode-decomposed J-integral) leading to an
accurate prediction of delamination propagation under mixed-mode and
non-self-similar growing conditions. The new method has been implemented as a
UEL for Abaqus and validated against an experimental benchmark case with
varying crack growth rate and shape and extension of the fracture process zone.Comment: 37 pages, 14 figures, 7 table
Aggregate effect on the concrete cone capacity of an undercut anchor under quasi-static tensile load
In the last decades, fastening systems have become an essential part of the construction industry. Post-installed mechanical anchors are frequently used in concrete members to connect them with other load bearing structural members, or to attach appliances. Their performance is limited by the concrete related failure modes which are highly influenced by the concrete mix design. This paper aims at investigating the effect that different aggregates used in the concrete mix have on the capacity of an undercut anchor under tensile quasi-static loading. Three concrete batches were cast utilising three different aggregate types. For two concrete ages (28 and 70 days), anchor tensile capacity and concrete properties were obtained. Concrete compressive strength, fracture energy and elastic modulus are used to normalize and compare the undercut anchor concrete tensile capacity employing some of the most widely used prediction models. For a more insightful comparison, a statistical method that yields also scatter information is introduced. Finally, the height and shape of the concrete cones are compared by highly precise and objective photogrammetric means
Non-destructive testing of rock bolt fastening as an element of monitoring the state of mine workings
Purpose. To substantiate the informative parameter, to develop the method and equipment for non-destructive testing (NDT) of rock bolt fastening, as an element of geomechanical monitoring of the mine workings state.
Methods. Analytical and bench experimental testing of shock-wave processes in the “rock bolt – bonding layer – rock massif” system.
Findings. It has been established, that the non-destructive shock-wave method is the most satisfying for monitoring the geomechanical state of the “rock bolt – bonding layer – rock massif” system. This method is based on the registration and analysis of rock bolt oscillations, caused by non-normalized hit to the rock bolt end. It has been confirmed, that the most informative parameter is the relaxation time, that is the time over which the amplitude of oscillations decreases by “e” times. It has been shown, that clamping of the resin-grouted rock bolt in massif and its tensioning affects the relaxation time of damped oscillations. Furthermore, the relaxation time is inversely proportional to the degree of the rock bolt clamping, and is linked linearly with an increase in the rock bolt tensioning. The spectral composition of wedge shaped rock bolts self-oscillations, which are caused by impact excitation, depends both on the nature of clamping and on the rock bolt length, and makes it possible to identify the rock bolts by their length.
Originality. The existence of a special type of damped longitudinal oscillations of the rock bolt in viscoelastic medium of the bonding layer has been established. The relaxation time of the specified type of oscillations was chosen as an informative parameter. An analytical link has been established between the informative parameter of shock-wave method of monitoring and the main indicators of rock bolt fastening quality: the degree of adhesion with massif and tension value.
Practical implications. The method has been improved and equipment has been developed for non-destructive testing of rock bolt fastening, which makes it possible to control in-situ the rock bolt fastening quality, including the geomechanical state monitoring of mine workings.Мета. Обґрунтування інформативного параметра, розробка способу та апаратури неруйнівного контролю анкерного кріплення як елементів геомеханічного моніторингу стану гірничих виробок.
Методика. Аналітичні та стендові експериментальні дослідження ударно-хвильових процесів в системі
“анкер – закріплюючий шар – масив гірських порід”, апробація розробки в умовах шахт і підземних об’єктів цивільного та промислового призначення.
Результати. Встановлено, що вимогам оперативності контролю з можливістю моніторингу геомеханічного стану системи “анкер – закріплюючий шар – масив” найбільш задовольняє ударно-хвильовий метод неруйнівного контролю, що реалізується шляхом нанесення ненормованого удару в торець анкера з реєстрацією та аналізом його коливань, при цьому найбільш інформативним та таким, що задовольняє ергономіці контролю, є параметр – час релаксації, за який амплітуда коливань зменшується в “е” разів. Показано, що ступінь защемлення сталеполімерного анкера в масиві та його натяг однозначно впливають на час релаксації затухаючого коливального процесу в системі “анкер – закріплюючий шар – масив”, при цьому зв’язок параметра з защемленням описується обернено пропорційною, а зв’язок з натягом анкера – лінійною залежністю. При ударному збудженні спектральний склад власних коливань клиновидних анкерів залежить як від характеру защемлення, так і від довжини анкера, що дозволяє при контролі виробок ідентифікувати анкери за їх довжиною.
Наукова новизна. Встановлено існування особливого виду затухаючих поздовжніх коливань анкера в пружно-в’язкому середовищі закріплюючого шару. Як інформативний параметр вибрано час релаксації вказаного виду коливань, що в ергономічному діапазоні практично не залежить від сили збуджуючого удару. Встановлено аналітичний зв’язок між інформативним параметром ударно-хвильового методу контролю системи “анкер – закріплюючий шар – масив” і основними показниками якості металополімерного анкерного кріплення гірничих виробок: ступенем зчеплення з масивом та величиною натягу.
Практична значимість. Вдосконалено метод і розроблено апаратуру неруйнівного контролю анкерного кріплення, які дозволяють оперативно у режимі моніторингу оцінювати якість закріплення анкера в масиві та його навантаження в умовах гірничих виробок діючого виробництва, зокрема, і у режимі системного геомеханічного моніторингу стану підземних виробок.Цель. Обоснование информативного параметра, разработка способа и аппаратуры неразрушающего контроля анкерного крепления как элементов геомеханического мониторинга состояния горных выработок.
Методика. Аналитические и стендовые экспериментальные исследования ударно-волновых процессов в системе “анкер – закрепляющий слой – массив горных пород”, апробация разработки в условиях шахт и подземных объектов гражданского и промышленного назначения.
Результаты. Установлено, что требованиям оперативности контроля с возможностью мониторинга геомеханического состояния системы “анкер – закрепляющий слой – массив” наиболее удовлетворяет ударно-волновой метод неразрушающего контроля, который реализуется путем нанесения ненормированного удара в торец анкера с регистрацией и анализом его колебаний, при этом наиболее информативным и таким, что удовлетворяет эргономике контроля, является параметр – время релаксации, за которое амплитуда колебаний уменьшается в “е” раз. Показано, что степень защемления сталеполимерного анкера в массиве и его натяжение однозначно влияют на время релаксации затухающего колебательного процесса в системе “анкер – закрепляющий слой – массив”, при этом связь параметра с защемлением описывается обратно пропорциональной, а связь с натяжением анкера – линейной зависимостью. При ударном возбуждении спектральный состав собственных колебаний клиновидных анкеров зависит как от характера защемление, так и от длины анкера, позволяет при контроле выработок идентифицировать анкеры по их длине.
Научная новизна. Установлено существование особого вида затухающих продольных колебаний анкера в упруго-вязкой среде закрепляющего слоя. В качестве информативного параметра выбрано время релаксации указанного вида колебаний, которое в эргономичном диапазоне практически не зависит от силы возбуждающего удара. Установлена аналитическая связь между информативным параметром ударно-волнового метода контроля системы “анкер – закрепляющий слой – массив” и основными показателями качества металлополимерного анкерного крепления горных выработок: степенью сцепления с массивом и величиной натяжения.
Практическая значимость. Усовершенствован метод и разработана аппаратура неразрушающего контроля анкерного крепления, которые позволяют оперативно в режиме мониторинга оценивать качество закрепления анкера в массиве и его нагружение в условиях горных выработок действующего производства, в том числе и в режиме системного геомеханического мониторинга состояния подземных выработок.The work results are part of the “Theories and me-thods development for geotechnical systems state ma-nagement to ensure mines performance intensification” research under National Academy of Sciences of Ukraine funding program
- …