Neural approach to estimate the stress intensity factor of semi‐elliptical cracks in rotating cracked shafts in bending

In the last decades, neural network approach has often been used to study various and complex engineering problems, such as optimization or prediction. In this paper, a methodology founded on artificial neural networks (ANNs) was used to calculate the stress intensity factor (SIF) in different points of the front of a semi‐elliptical crack present in a rotating shaft, taking into account the shape and depth of the crack, the angle of rotation, and the location of the point in the front. In the event of rotating machines, such as shafts, it is crucial to know the SIF along the crack front because this parameter, according to the Paris Law, is related to the performance of the crack during its propagation. Previously, it was necessary to achieve the data for the ANN training, for this a quasi‐static numerical model was made, which simulates a rotating cracked shaft with a semi‐elliptical crack. The numerical solutions cover a wide range of crack depths and shapes, and rotation angles. The values of the SIF estimated by the ANNs were contrasted with other solutions available in the literature finding a good agreement between them. The proposed neural network methodology is an alternative that offers a very good option for the SIF estimation, because it is efficient and easy to use, does not require high computational costs, and can be used to analyse the propagation of cracks contained in rotating shafts by means of the Paris Law taking into account the nonlinear behaviour of the shaft.The authors would like to thank the Spanish Ministerio de Economía y Competitividad for the support for this work through the projects DPI2009-13264 and DPI2013-45406-

Crack identification in non-uniform rods by two frequency data

We consider the inverse problem of identifying a single open crack in a longitudinally vibrating rod having non-uniform smooth profile. Without any a priori assumption on the smallness of the damage and assuming that the rod profile is symmetric with respect to the mid-point of the rod axis, we present a constructive diagnostic algorithm from minimal frequency data. We show that the crack can be uniquely identified, up to a symmetric position, from the first two positive natural frequencies of the rod under free-free end conditions. We also show that the non-uniqueness of the damage location can be removed by using as data the first positive resonant frequency of the free-free rod and the first antiresonant frequency of the driving-point frequency response evaluated at one end of the rod. The results of numerical simulations and of applications of the method to experimental data agree well with the theory. (C) 2015 Elsevier Ltd. All rights reserved

Determination of dynamic fracture-initiation toughness using three-point bending tests in a modified Hopkinson pressure bar

8 pages, 12 figures.We present a procedure for measuring the dynamic fracture-initiation toughness of materials. The method is based on three-point bending tests at high loading rates, performed in an experimental device which is a modification of the classical split Hopkinson pressure bar. Coupled with the loading device, a high-speed photography system was used to measure the crack mouth opening displacement (CMOD) directly on the specimen. The stress intensity factor was calculated by three different simplified methods and the time to fracture was obtained from an appropriate specimen instrumentation. To evaluate the results derived from the simplified methods, a two-dimensional full-numerical analysis of the dynamic bending fracture test was made. The model includes the specimen, the input bar, the impacting projectile and the supporting device and takes into account the possible loss of contact during the experiment between the input bar and the specimen and between the specimen and its supports. From the tests and numerical results, it can be concluded that the CMOD procedure, together with the knowledge of the time to fracture determined using crack gages, seems to be the best method for measuring dynamic fracture-initiation toughness.This work was supported by the Comisión Interministerial de Ciencia y Tecnología under Grant PB98-0027.Publicad

A closed expression for the stress intensity factor of concave fatigue cracks in rotating shafts

A new analytical model is developed that allows us to obtain the Stress Intensity Factor (SIF) in a point on a concave shaped crack contained in a rotating shaft as a function of the characteristics of the crack (depth and aspect), the point position on the crack front, and the rotation angle. The model can be used for all kinds of linear elastic materials. The Finite Element Method (FEM) has been used to make a tridimensional quasi-static model of a shaft that contains a concave shaped crack in its central section, subjected to rotary bending. Different rotation angles are taken into account in order to simulate the rotation of the shaft. The SIF is determined in every position of the front of the concave crack, for each crack geometry and for each rotation angle. Then an analytical model is developed using all the results of SIF obtained with the numerical model, and is verified comparing its results with solutions found in the literature. The proposed model may be very useful to study the dynamic behavior of shafts with concave shaped cracks, and can be employed to analyze the propagation of these types of cracks.The authors would like to thank the Spanish Ministerio de Economía y Competitividad for the support for this work through the projects DPI2009-13264 and DPI2013-45406-

Elliptical Crack Identification in a Nonrotating Shaft

It is known that fatigue cracks are one of the most important problems of the mechanical components, since their propagation can cause severe loss, both personal and economic. So, it is essential to know deeply the behavior of the cracked element to have tools that allow predicting the breakage before it happens. The shafts are elements that are specially affected by the described problem, because they are subjected to alternative compression and tension stresses., is work presents, firstly, an analytical expression that allows determining the first four natural frequencies of bending vibration of a nonrotating cracked shaft, assumed as an Euler-Bernoulli beam, with circular cross section under pinned-pinned conditions, taking into account the elliptical shape of the crack. Second, once the direct problem is known, the inverse problem is approached. Genetic Algorithm technique has been used to estimate the crack parameters assuming known the natural frequencies of the cracked shaft.The authors would like to thank the Spanish Ministerio de Economía y Competitividad for the support for this work through the project DPI2013-45406-P

Natural frequencies for bending vibrations of Timoshenko cracked beams

The natural frequencies for bending vibrations of Timoshenko cracked beams with simple boundary conditions have been obtained. The beam is modelled as two segments connected by two massless springs (one extensional and another one rotational). This model promotes discontinuities in both vertical displacement and rotation due to bending, which are proportional to shear force and bending moment transmitted by the cracked section, respectively. The differential equations for the free bending vibrations are established and then solved individually for each segment with the corresponding boundary conditions and the appropriated compatibility conditions at the cracked section. The problem is also solved by the perturbation method and both procedures are applied to the case of a simply supported cracked beam. The results show that the perturbation method provides simple expressions for the natural frequencies of cracked beams and it gives good results for shallow cracks.Publicad

Aproximación del alumno al diseño por ordenador de conjuntos mecánicos reales

Proyecto de Innovación Docente en la asignatura Diseño Asistido por ComputadorEste Proyecto de Innovación Docente ha sido llevado a cabo en el curso académico 2004-05con los alumnos de la asignatura de“Diseño Asistido por Computador” de 2º de IngenieríaTécnica Industrial (Mecánica), que se imparte en el primer cuatrimestre. En él se ha inducido alos alumnos al trabajo en grupo sobre un problema de modelado virtual de un conjunto mecánico real, para el que es necesarioaprender y aplicar las herramientas básicas propiasde la asignatura. Además, el alumno puede, como ha ocurrido en muchos casos, profundizar de forma autodidacta en el empleo de algunas herramientas derivadas que por su complejidad o extensión, no se desarrollan en el marco teórico de la asignatura. La consecución de este doble objetivo ha supuestoun acercamiento al crédito europeo ECTS, basado en el trabajorealizado por el estudiante en su proceso de aprendizaje. El análisis de losresultados académicos y de las encuestas realizadas a los alumnos tras la experiencia, permiten emitir una valoración positiva de la metodología docente propuesta

Improvement of chatter stability in boring operations with passive vibration absorbers

This paper is focused on the behavior of boring bars with a passive dynamic vibration absorber (DVA) for chatter suppression. The boring bar was modeled as a cantilever Euler–Bernoulli beam and only its first mode of vibration was considered. The stability of the two-degree-of-freedom model was analyzed constructing the stability diagram, dependent on the bar characteristics and on the absorber parameters (mass, stiffness, damping, and position). Two analytical approaches for tuning the absorber parameters were compared. The selection criterion consisted on the maximization of the minimum values of the stability-lobes diagram. Subsequent analysis performed in this work, allowed formulating of new analytical expressions for the tuning frequency improving the behavior of the system against chatter.Publicad

Optimization of passive vibration absorbers to reduce chatter in boring

This paper is focused on the optimal selection of the parameters of a passive dynamic vibration absorber (DVA) attached to a boring bar. The boring bar was modeled as an Euler-Bernoulli cantilever beam and the stability of the system was analyzed in terms of the bar and the absorber characteristics. To obtain the optimum parameters of the absorber, a classical method for unconstrained optimization problems has been used. The selection criterion consisted of the maximization of the minimum values of the stability lobe diagram. Empirically fitted expressions for the frequency and damping ratio of the DVA (which permit to obtain its stiffness and damping) are proposed. These expressions are fully applicable when the damping ratio of the boring bar is non-none as it is in practical operations. The computed results show a clear improvement in the stability performance regarding other methodologies previously used

Treatment with tocilizumab or corticosteroids for COVID-19 patients with hyperinflammatory state: a multicentre cohort study (SAM-COVID-19)

Objectives: The objective of this study was to estimate the association between tocilizumab or corticosteroids and the risk of intubation or death in patients with coronavirus disease 19 (COVID-19) with a hyperinflammatory state according to clinical and laboratory parameters. Methods: A cohort study was performed in 60 Spanish hospitals including 778 patients with COVID-19 and clinical and laboratory data indicative of a hyperinflammatory state. Treatment was mainly with tocilizumab, an intermediate-high dose of corticosteroids (IHDC), a pulse dose of corticosteroids (PDC), combination therapy, or no treatment. Primary outcome was intubation or death; follow-up was 21 days. Propensity score-adjusted estimations using Cox regression (logistic regression if needed) were calculated. Propensity scores were used as confounders, matching variables and for the inverse probability of treatment weights (IPTWs). Results: In all, 88, 117, 78 and 151 patients treated with tocilizumab, IHDC, PDC, and combination therapy, respectively, were compared with 344 untreated patients. The primary endpoint occurred in 10 (11.4%), 27 (23.1%), 12 (15.4%), 40 (25.6%) and 69 (21.1%), respectively. The IPTW-based hazard ratios (odds ratio for combination therapy) for the primary endpoint were 0.32 (95%CI 0.22-0.47; p < 0.001) for tocilizumab, 0.82 (0.71-1.30; p 0.82) for IHDC, 0.61 (0.43-0.86; p 0.006) for PDC, and 1.17 (0.86-1.58; p 0.30) for combination therapy. Other applications of the propensity score provided similar results, but were not significant for PDC. Tocilizumab was also associated with lower hazard of death alone in IPTW analysis (0.07; 0.02-0.17; p < 0.001). Conclusions: Tocilizumab might be useful in COVID-19 patients with a hyperinflammatory state and should be prioritized for randomized trials in this situatio