•Finite Element Analysis of fretting fatigue in Double Bolted Lap Joint.•Continuum Damage Mechanics.•High cycle fatigue.•Crack initiation analysis.•Effect of contact regime on fretting fatigue crack ...initiation and propagation.
In fretting fatigue, the combination of small oscillatory motion, normal pressure and cyclic axial loading develops a noticeable stress concentration at the contact zone leading to accumulation of damage in fretted region, which produces micro cracks, and consequently forms a leading crack that can lead to failure. In fretting fatigue experiments, it is very difficult to detect the crack initiation phase. Damages and cracks are always hidden between the counterpart surfaces. Therefore, numerical modeling techniques for analyzing fretting fatigue crack initiation provide a precious tool to study this phenomenon. This article gives an insight in fretting fatigue crack initiation. This is done by means of an experimental set up and numerical models developed with the Finite Element Analysis (FEA) software package ABAQUS. Using Continuum Damage Mechanics (CDM) approach in conjunction with FEA, an uncoupled damage evolution law is used to model fretting fatigue crack initiation lifetime of Double Bolted Lap Joint (DBLJ). The predicted fatigue lifetimes are in good agreement with the experimentally measured ones. This comparison provides insight to the contribution of damage initiation and crack propagation in the total fatigue lifetime of DBLJ test specimens.
This paper proposes a new methodology for fretting fatigue life assessment. It uses a combination of multiaxial fatigue criteria, the theory of critical distances and a node-displacement wear ...algorithm to account for the initiation life under fretting. Further, the extended finite element method is considered to compute the propagation life. A new method is also introduced to predict the crack propagation path under nonproportional loading conditions. To validate the analysis, available fretting fatigue data using an Al 2024-T3 alloy was considered. All the total lives estimated by the proposed approach fell within a scatter band 2, with an error between 11% and −39%. An analysis of the energy dissipated on the contact surface was performed and showed that for certain cases the inclusion of wear in the modeling of the problem is indeed essential to obtain more accurate fretting fatigue life assessments.
•Fretting fatigue in partial slip regime life assessment is carried out for Al2024-T3.•The numerical method proposed including wear effects and crack propagation life.•A new method based on the critical plane to predict the crack propagation angle.•The methodology proposed obtained more accurate predictions than simple methods.•The maximum energy dissipated has shown to be a parameter capable to identify when is necessary to compute wear or not.
In this paper we used a modified fluorine-doped tin oxide (FTO) as the substrate of a novel aptasensor for electrochemical determination of digoxin. For this purpose, a selective thiolated digoxin ...aptamer was immobilized onto the gold nanoparticles deposited FTO (GNPs/FTO) surface. Cyclic voltammetry (CV), electrochemical impedance spectroscopy, scanning electron microscopy (SEM) and atomic force microscopy (AFM) were used for the characterization of modified surfaces. The aptasensor response was based on a decrease in the methylene blue (MB) current, as an electrochemical probe, after incubating with digoxin. Digoxin can be determined in a linear concentration range from 0.02 to 0.2 μg L
−1
by differential pulse voltammetry (DPV). The detection limit of the proposed aptasensor was 0.01 μg L
−1
. Also, the fabricated aptasensor was applied to determine digoxin in urine and blood plasma samples with satisfactory results. Furthermore, high sensitivity and specificity, low detection limit and fast response, can make it possible to determine trace amounts of digoxin for routine clinical analysis.
Fabrication steps of digoxin aptasensor. The DPVs are for accumulated MB at FTO/GNPs/Ap, before (a) and after (b) incubation with digoxin.
This work presents methodologies for fretting fatigue life assessment that include the wear effect and crack propagation. The multiaxial fatigue parameter SWT and the theory of critical distance TCD ...were used to compute the damage, the node-displacement algorithm was applied to account wear, and the XFEM method to modeling the crack. Available fretting fatigue experiments conducted on a cylinder-on-flat contact configuration made of Al 2024-T351 alloy were used to evaluate the methodology. To apply the approach in the medium-cycle fatigue regime, the critical distance is assumed to depend on the fatigue life. All fatigue life estimates using TCD as a function of life were within factor-of-two boundaries.
•Fretting fatigue in partial slip regime life assessment is carried out for Al2024-T3.•The numerical method proposed including wear effects and crack propagation life.•A new method based on the critical plane to predict the crack propagation angle.•The methodology proposed obtained more accurate predictions than simple methods.•The energy dissipated can be used to know when computing wear or crack propagation.
•Crack initiation angle prediction in fretting fatigue tests under partial slip regime.•Comparison between two methods to predict the crack initiation angle.•Analysis of the influence of surface wear ...on crack nucleation angle.•Comparison of numerical results with tests carried out for A7050-T7451 available in the literature.
This paper investigates two different methods to predict crack initiation direction in fretting problems subjected to partial slip conditions. One of them is based on the Critical Direction Method (CDM) combined with the Smith Watson and Topper (SWT) fatigue damage parameter. The other consists in simulating early crack propagation using a Linear Elastic Fracture Mechanics (LEFM) approach associated with the SWT parameter. In addition, this paper also investigates the influence of considering wear in the crack initiation direction estimates. For validation of the methods, experimental data from fretting tests conducted on an AA7050- T7451 alloy were used. Results have shown that the CDM-based model provides very accurate estimates, whereas the LEFM-based approach failed to properly reproduce experimental observations. Besides, it was also verified that the inclusion of wear has little effect on the estimates despite the method considered.
The main aim of this work was to numerically investigate the effect of sequences of variable amplitude shear loading on contact tractions/stresses and on the development of life estimation ...methodologies for these more challenging loading histories. In this setting, a finite element (FE) model has been implemented in order to simulate a cylinder on plane contact problem for an aeronautical Al 7075-T651 alloy. High-Low (H-L) and Low-High (L-H) shear amplitude loading sequences under partial and gross slip conditions were considered in three different case studies. For the case studies where gross slip was involved, the proposed life estimation procedure was enhanced to include the effect of material loss due to wear. More explicitly, such procedure was based on the use of (i) the SWT (Smith-Watson-Topper) multiaxial fatigue model, (ii) the Theory of Critical Distances, (iii) Miner's cumulative damage rule and (iv) the numerical update of the contact surfaces profiles (for loading blocks under gross slip). The numerical analyses for the case studies under partial slip revealed that, when the presence of wear is neglected in the modelling, the Miner's rule provided a divergence between the expected life for the H-L and L-H loading blocks. This difference was considerably larger when gross slip took place. For the case study involving the presence of gross slip in one of the shear loading blocks, the calculated life tended to infinite when the damage generated by the high amplitude block is greater than a certain critical value. These studies are the basis for an experimental programme which will be carried out in the 4 actuators fretting fatigue rig of the University of Brasilia.
•Fretting fatigue under variable amplitude loading life assessment is carried out for Al 7075-T651.•Life estimates considering and neglecting wear are compared for gross slip regime.•The results showed that the sequence of loading can influence on life prediction.•When considering wear for gross slip regime the damage has a plateau and life tends to infinite value.
The risk of severe coronavirus disease-2019 (COVID-19) disease seems to be higher in individuals with solid organ transplantation. Therefore, the purpose of the present research is to investigate the ...incidence of COVID-19 and laboratory data and epidemiologic factors in liver transplant recipients and the patients on the waiting list for liver transplantation. In this study, we evaluated the records of patients on the waiting list for liver transplantation and of recipients of a liver transplant. Demographic data, underlying disease, history of drug use and participants’ outcomes were collected. The diagnosis of SARS-CoV-2 infection for all patients was confirmed using a nasopharyngeal swab specimen with real-time RT-PCR. During the study period, 172 patients were enrolled, among whom 85 patients (49.4%) were on the waiting list for liver transplantation, and 87 patients (50.6%) were recipients of a liver transplant. Out of them, 10 (5.8%) had a positive result for SARS-CoV-2. Of these patients, 7.05% (6/85) and 4.6% (4/87) of patients on the waiting list and recipients of liver transplants were positive for SARS-CoV-2, respectively. Patients on the waiting list with COVID-19 infection had a higher median of albumin, ALT, AST, TBIL, DBIL, HDL and LDL value. In summary, the incidence of COVID-19 in liver transplant patients was slightly higher. The existence of underlying liver diseases should be well known as one of the poor predictive factors for worse outcomes in patients with COVID-19. So, comparative studies are recommended to identify risk factors for COVID-19 in patients with liver injury.
Application of high‐strength steel on different structural components is becoming more attractive. In spite of their great advantages of high yield strength, the use of these steel grades faces some ...important challenges as well. There are many formed steel components of different structures that are subjected to fatigue loading conditions. The main objective of the present study is to investigate the effect of pre‐bending process of high‐strength steel subjected to low‐cycle fatigue loading conditions. For this purpose, a new test set‐up has been designed to take into consideration the effect of pre‐bending process when the fatigue load is applied. To detect fatigue crack initiation onset, lock‐in thermography technique is used to monitor the incremental temperature variation during fatigue cycling. Furthermore, to estimate fatigue lifetime of the formed fatigue sample, continuum damage mechanics approach is applied by means of numerical modelling.
Digital image correlation (DIC) is a full field three dimensional measurement technique that can quantify displacements and strains of a surface. In this paper, digital image correlation is used as a ...slip measurement technique during coupon scale fretting fatigue experiments. Slip measured with the novel DIC technique is compared to conventional slip measurement techniques as clip gauges and modified clip gauge measurements proposed by Wittkowsky et al. Slip measurements with the DIC system show lower slip values and higher tangential contact stiffness’s compared to (modified) clip gauge measurements. Slip measured with DIC is obtained closer to the contact compared to clip gauges, eliminating the influence of elastic deformations or fitting parameters. During the fretting fatigue experiments are two equal contacts simultaneously tested. However, the slip of both contacts is not identical with outliers of more than 10% difference in slip amplitude.
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