J.A. Araújo

The effect of rapidly varying contact stress fields on fretting fatigue

An experimental and analytical study of the effect of varying contact stress fields on the initiation life of fretted specimens is conducted for two high strength alloys commonly used in the aerospace industry, namely Al4%Cu and Ti-6Al-4V. The experiments reveal there is a contact size effect in fretting fatigue life. Two critical plane models are used to predict the observed experimental lives. The results show that these models may provide over-conservative life estimates for fretting tests subjected to more rapidly varying contact stress fields. The existence of a critical stressed layer, or volume, is used to explain these results qualitatively, and averaging methods are then developed to allow the extension of the critical plane approach to cases of rapidly varying contact stress fields. It is shown that a critical averaging dimension of the order of the grain size of the material appears to give realistic estimates of fatigue life and predicts the observed size effect reported in the experimental work.

Fatigue limit under multiaxial loadings: on the definition of the equivalent shear stress

Abstract The goal of the present paper is to propose a criterion to predict the fatigue strength of hard metals under conditions of multiaxial, non-proportional loadings. It is very simple to compute, but still provides very good results for a wide range of in-phase and out-of-phase cycling loads.

Surface Treatment of Coconut Fiber and its Application in Composite Materials for Reinforcement of Polypropylene

This work describes the surface treatment of coconut fiber and its application in composite materials for reinforcement of polypropylene. Before testing coconut fiber in composite materials, its susceptibility to chemical treatment was characterized by exposure to the following: (1) hot water; (2) aqueous NaOH 2% (w/v); and (3) a sequence of chemical treatments in hot water, extran 20% (v/v), acetone/water 1:1 (v/v), and aqueous NaOH 10% (w/v).The efficiency of each treatment was evaluated by Fourier transform infrared spectrometry analysis, indicating a reduction in the C=O peak. Besides that the scanning electron microscope revealed that treatment changed the morphology of the fibers. Thermogravimetric analysis showed that the insertion of fibers into PP caused a decrease in the thermal stability and differential scanning calorimetry exhibited that crystallization of PP was favored in the presence of fibers.

Estimation of fretting fatigue life using a multiaxial stress-based critical distance methodology

This work presents a methodology for life estimation of mechanical couplings subjected to fretting fatigue. In this approach, a stress-based multiaxial fatigue parameter is evaluated at a critical distance below the contact surface. The fatigue parameter is based on an improved formulation of the Modified Wohler Curve Method, in which the shear stress amplitude is measured via the Maximum Rectangular Hull method. To apply the Theory of Critical Distances in the medium-cycle fatigue regime, the critical distance is assumed to depend on the number of cycles to failure. Available fretting fatigue data, conducted on a cylinder-plane contact configuration made of Al alloy 4% Cu, were used to assess the methodology. Most of the life estimates were within an error band given by a factor of 2.

Mechanical evaluation of elastic tubes used in physical therapy

Abstract Purpose: To quantify the force of elastic tubes at different elongation patterns used in exercises. Methods: A tensile test of elastic tubes which had seven levels of resistance identified by colors was performed. All samples (n = 105; 15 samples for each color) were 7 cm of length and had two interfaces for fixation. The samples were tested in a tensile testing hydraulic machine, under elongation control, up to 200% of the original length (21 cm). The force values corresponding to an increase of 50%, 100%, 150% and 200% of the samples initial length were recorded. Results: A strong linear association between force and elongation for all colors was found. There were different forces for each elongation investigated for the same tube and between different tubes in almost all percentage of elongation investigated, except for the red and green tubes (all elongations), and for the blue and black tubes at 50% elongation. Conclusion: Mechanical tests revealed different elastic forces for different levels of elongation of each tube. The replication of the results in clinical situations is recommended, so the elastic resistance in clinical routine could be evaluated with more propriety.

Effect of a Deep Cryogenic Treatment on Wear and Microstructure of a 6101 Aluminum Alloy

The aim of this work is to evaluate the effect of a deep cryogenic treatment (DCT) on the wear behavior and on the microstructure of an aluminum alloy. In order to compare the level of improvement on the wear resistance provided by the DCT with a more traditional technique, a test matrix which included DCT, CrN coated specimens, and combinations of both modification methods was conducted. The wear behavior was investigated using microabrasive wear tests. The cryogenic treated specimens proved to have similar low wear rates as the specimens coated with CrN. The most distinct improvement was reached with a combination of both techniques. In the case of the DCT, the performed microstructural analysis identified the generation of additional GP-zones as the reason for the improved wear resistance.

Equivalent configurations for notch and fretting fatigue

Under the typical partial slip conditions under which fretting fatigue takes place, the amount of superficial damage is small. Therefore, the substantial reduction in fatigue life caused by fretting, when compared to plain fatigue, may well be more associated with the stress concentration and the stress gradient phenomena generated by the contact problem than to the superficial loss of material. In this setting, notch stress-based methodologies could, in principle, be applied to fretting in the medium/high cycle fatigue regime. The aim of this work was to investigate whether it is possible to design fretting and notch fatigue configurations, which are nominally identical in terms of damage measured by a multiaxial fatigue model. The methodology adopted to carry out this search considered a cylindrical on flat contact and a V-notch. Load and geometry dimensions of both configurations were adjusted in order to try to obtain the “same” decay of the Multiaxial Fatigue Index from the hot spot up to a critical distance. Positive results of such simulations can lead us to design an experimental program that can bring more firm conclusions on the use of pure stress-based approaches, which do not include the wear damage, in the modeling of fretting fatigue.

High-cycle notch sensitivity of alloy steel ASTM A743 CA6NM used in hydrogenator turbine components

The presence of notches and other stress concentrations in turbine blades and other notch hydraulic components is a current problem in engineering. It causes a reduction of endurance limit of material. In that sense, specimens of the ASTM A743 CA6NM alloy steel using in several hydrogenator turbine components was tested. The specimens were tested under uniaxial fatigue loading with a load ratio equal to -1, and the considered stress concentration factors, Kt, values, calculated with respect to net area, were 1.55, 2.04 and 2.42. In order to determine the fatigue limit for such notch type, a reduction data method by Dixon and Mood, Staircase method was used. This approach is based on the assumed target distribution of the fatigue limit. For such geometry at least 8 specimens were tested. In addition, the Peterson and Neuber’s notch fatigue factor were compared through fatigue notch reduction factor, Kf, obtained from experimental data. According to results obtained it was possible to conclude that the tested material is less sensitive to notches than the prediction of the Peterson and Neuber’s empirical models.

The application of a mesoscopic scale approach in fretting fatigue

This work concerns the application of a mesoscopic scale fatigue criterion to predict the initiation of cracks in components under fretting conditions. A verification of the analysis is carried out by considering available experimental data for fretting fatigue in a high strength Aluminium alloy commonly used in the aerospace industry. These experiments revealed there is a contact size effect in fretting fatigue life. The results show that the mesoscopic scale fatigue criterion can correctly predict the initiation of fretting cracks for larger contact configurations. It is concluded that the reason for the poor performance of the criterion in predicting failure at smaller contacts may well be related to the effect of the stress gradient, a variable not accounted for in the mesoscopic criterion here assessed.

Crack path simulation for cylindrical contact under fretting conditions

In this work different strategies to estimate crack path for cylindrical contacts under fretting conditions are carried out. The main goal is to propose and to evaluate methodologies not only to estimate the direction of crack initiation but also the subsequent propagation in its earlier stages, where the stress field is multiaxial, non-proportional and decays very fast due to the proximity with the contact interface. Such complex conditions pose a substantial challenge to the modelling of crack path. The numerical simulations are provided by a 2D Finite Element Analysis taking into account interactions between the crack faces. The results show that, under fretting conditions, models based on the critical plane method are not effective to estimate the crack initiation orientation, while models based on a so called “critical direction” applied along a critical distance provide better results. Regarding the subsequent crack propagation orientation, it was possible to see that stress intensity factor based models where one considers an infinitesimal virtual crack emerging from an original preexistent crack are powerful mechanisms of crack orientation estimation