A New Model to Determine the Dispersion of Fatigue Damage Evaluations

Reliable predictions of remaining lives of civil or mechanical structures subjected to fatigue damage are very difficult to be made. In general, fatigue damage is extremely sensitive to the random variations of material mechanical properties, environment and loading. These variations may induce large dispersions when the structural fatigue life has to be predicted. Wirsching (1970) mentions dispersions of the order of 30 to 70 % of the mean calculated life. The presented paper introduces a model to estimate the fatigue damage dispersion based on known statistical distributions of the fatigue parameters (material properties and loading). The model is developed by expanding into Taylor series the set of equations that describe fatigue damage for crack initiation.

Effect of loading type on the fatigue strength of symmetric and asymmetric welded joints made of ultra high strength steel

In this study, finite element analyses and experimental tests are carried out in order to investigate the effect of loading type and symmetry on the fatigue strength of three different non-load carrying welded joints. The current codes and recommendations do not give explicit instructions how to consider degree of bending in loading and the effect of symmetry in the fatigue assessment of welded joints. The fatigue assessment is done by using effective notch stress method and linear elastic fracture mechanics. Transverse attachment and cover plate joints are analyzed by using 2D plane strain element models in FEMAP/NxNastran and Franc2D software and longitudinal gusset case is analyzed by using solid element models in Abaqus and Abaqus/XFEM software. By means of the evaluated effective notch stress range and stress intensity factor range, the nominal fatigue strength is assessed. Experimental tests consist of the fatigue tests of transverse attachment joints with total amount of 12 specimens. In the tests, the effect of both loading type and symmetry on the fatigue strength is studied. Finite element analyses showed that the fatigue strength of asymmetric joint is higher in tensile loading and the fatigue strength of symmetric joint is higher in bending loading in terms of nominal and hot spot stress methods. Linear elastic fracture mechanics indicated that bending reduces stress intensity factors when the crack size is relatively large since the normal stress decreases at the crack tip due to the stress gradient. Under tensile loading, experimental tests corresponded with finite element analyzes. Still, the fatigue tested joints subjected to bending showed the bending increased the fatigue strength of non-load carrying welded joints and the fatigue test results did not fully agree with the fatigue assessment. According to the results, it can be concluded that in tensile loading, the symmetry of joint distinctly affects on the fatigue strength. The fatigue life assessment of bending loaded joints is challenging since it depends on whether the crack initiation or propagation is predominant.

A crack on the interface of piezo-electric bodies

Singularities of elastic and electric fields are investigated at the tip of a crack on the interface of two anisotropic piezo-electric media under various boundary conditions on the crack surfaces. The Griffith formulae are obtained for increments of energy functionals due to growth of the crack and the notion of the energy release matrix is introduced. Normalization conditions for bases of singular solution are proposed to adapt them to the energy, stress, and deformation fracture criteria. Connections between these bases are determined and additional properties of the deformation basis related to the notion of electric surface enthalpy are established.

Simulation of delamination in composites under quasi-static and fatigue loading using cohesive zone models

Es desenvolupa una eina de disseny per l'anàlisi de la tolerància al dany en composites. L'eina pot predir el inici i la propagació de fisures interlaminars. També pot ser utilitzada per avaluar i planificar la necessitat de reparar o reemplaçar components durant la seva vida útil. El model desenvolupat pot ser utilitzat tan per simular càrregues estàtiques com de fatiga. El model proposat és un model de dany termodinàmicament consistent que permet simular la delaminació en composites sota càrregues variables. El model es formula dins el context de la Mecànica del Dany, fent ús dels models de zona cohesiva. Es presenta un metodologia per determinar els paràmetres del model constitutiu que permet utilitzar malles d'elements finits més bastes de les que es poden usar típicament. Finalment, el model és també capaç de simular la delaminació produïda per càrregues de fatiga.

Damage rate is a predictor of fatigue life and creep strain rate in fatigue of tensile human cortical bone samples

We present results on the growth of damage in 29 fatigue tests of human femoral cortical bone from four individuals, aged 53–79. In these tests we examine the interdependency of stress, cycles to failure, rate of creep strain, and rate of modulus loss. The behavior of creep rates has been reported recently for the same donors as an effect of stress and cycles (Cotton, J. R., Zioupos, P., Winwood, K., and Taylor, M., 2003, "Analysis of Creep Strain During Tensile Fatigue of Cortical Bone," J. Biomech. 36, pp. 943–949). In the present paper we first examine how the evolution of damage (drop in modulus per cycle) is associated with the stress level or the "normalized stress" level (stress divided by specimen modulus), and results show the rate of modulus loss fits better as a function of normalized stress. However, we find here that even better correlations can be established between either the cycles to failure or creep rates versus rates of damage than any of these three measures versus normalized stress. The data indicate that damage rates can be excellent predictors of fatigue life and creep strain rates in tensile fatigue of human cortical bone for use in practical problems and computer simulations.

Endurance training stimulates growth and survival pathways and the redox balance in rat pancreatic islets

Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)

Influence of shot peening and hard chromium electroplating on the fatigue strength of 7050-T7451 aluminum alloy

Chromium electrodeposition is a technique for the production of functional coatings on engineering components. These coatings are extensively micro-cracked and present high level of hardness, resistance to corrosion and wear and low coefficient of friction. In this paper the shot peening influence on the fatigue strength of aluminum 7050-T7451 alloy chromium electroplated, was investigated.The shot peening process was carried out to create residual stresses using ceramic and glass shots. A hard chromium electroplated coating of 100 mu m thickness was performed on the base material and the shot peened base material surfaces. S-N curves were obtained in axial and bending fatigue tests and compared with the 7050-T7451 aluminum alloy. In order to study the influence of residual stresses on fatigue life, the behavior of compressive residual stress field was measured by an X-ray tensometry.An increase in the axial fatigue strength of 25% and 50% of ceramic and glass shots, respectively, was observed. The lower performance in fatigue life for ceramic-shot peening may be attributed to higher surface damage, as a consequence of the overpeening intensity performed. However, in bending fatigue the behavior was practically equivalent for both processes. Fracture surface analysis by scanning electron microscopy was used to observe crack origin sites from shot peened and chromium electroplated samples. (C) 2006 Elsevier Ltd. All rights reserved.

Effect of electroless nickel interlayer on the fatigue strength of chromium electroplated AISI 4340 steel

Deposition of wear-resistant hard chromium plating leads to a decrease in the fatigue strength of the base material. Despite the effective protection against wear and corrosion, fatigue life and environmental requirements result in pressure to identify alternatives or to improve conventional chromium electroplating mechanical characteristics. An interesting, environmentally safer and cleaner alternative for the replacement of hard chronic plating is tungsten carbide thermal spray coating, applied by high velocity oxyfuel (HVOF) process.To improve the fatigue strength of aeronautical steel chromium electroplated, shot peening is a successfully used method. Multiple lacer systems of coatings are considered to have larger resistance to crack propagation in comparison with simple layer.The aim of this study was to analyze the effect of nickel underplate on the fatigue strength of hard chromium plated AISI 4340 steel in two mechanical conditions: HRc 39 and HRc 52.Rotating bending fatigue tests results indicate that the clectroless nickel plating underlayer is responsible for the increase in fatigue strength of AISI 4340 steel chromium electroplated. This behavior may be attributed to the largest toughness/ductility and compressive residual stresses which, probably, arrested or delayed the inicrocrack propagation from the hard chromium external layer. The compressive residual stress field (CRSF) induced by the electroplating process was determined by X-ray diffraction method. The evolution of fatigue strength compressive residual stress field CRSF and crack sources are discussed and analyzed by SEM. (c) 2006 Elsevier Ltd. All rights reserved.

Growth of SrBi4Ti4O15 thin films in a microwave oven by the polymeric precursor method

SrBi4Ti4O15 (SBTi) thin films were obtained by the polymeric precursor method and crystallized in a domestic microwave oven. For comparison, films were also crystallized in a conventional furnace at 700 degrees C for 2 h. Structural and morphological characterization of the SBTi thin films was investigated by X-ray diffraction (XRD) and atomic force microscopy (AFM), respectively. Using platinum coated silicon substrates configuration, ferroelectric properties of the films were determined with remanent\polarization P-r and a coercive field E-c of 5.1 mu C/cm(2) and 135 kV/cm for the film thermally treated in the microwave oven and 5.4 mu C/cm(2) and 85 kV/cm for the film thermally treated in conventional furnace, respectively. The films thermally treated in the conventional furnace exhibited excellent fatigue-free characteristics up to 10(10) switching cycles indicating that SBTi thin films can be a promise material for use in non-volatile memories. (C) 2007 Elsevier B.V. All rights reserved.

Effect of WC-10%Co-4%Cr coating on the Ti-6Al-4V alloy fatigue strength

Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES)

Evaluation of WC-10Ni thermal spraying coating by HVOF on the fatigue and corrosion AISI 4340 steel

Shot peening is a surface process widely used to improve the fatigue strength of materials, through compressive residual stresses induced in their surface layers. Considering mechanical components for high responsible applications, wear and corrosion control is currently accomplished by the use of coated materials.In the case of chrome plating or hard anodizing, lower fatigue strength in comparison to uncoated parts are associated to high residual tensile stresses and microcracks density. Under constant or variable amplitude loading microcracks will propagate and cross the interface coating substrate without impediment.The aim of the present study is to analyze the influence of WC-10Ni coating applied by HVOF process on the axial fatigue strength of AISI 4340 steel. The shot peening effect on the fatigue performance of coated AISI 4340 steel was also evaluated. The fractured fatigue specimens were investigated using a scanning electron microscope in order to obtain information about the crack initiation points. (C) 2010 Published by Elsevier Ltd.

Evaluation of WC-10Ni thermal spray coating with shot peening on the fatigue strength of AISI 4340 steel

Fatigue failure is a result of a crack initiation and propagation, in consequence of a cyclical load. In aeronautical components as landing gear the fatigue strength is an important parameter to be considered in project, as well as the corrosion and wear resistance.The thermal sprayed HVOF technology it's normally used to protect components against wear and corrosion, and are being considerate an alternative to replace chromium by the aeronautical industry. With respect to fatigue life, the HVOF technique induces residual stress on the interface. In the case of tensile residual stresses, the initiation and propagation phases of fatigue process are accelerated; on the other hand, compressive residual stresses close to the surface may increase fatigue life. The technique to improve the coated materials fatigue strength is the shot peening process, which induces residual stress in the surface in order to delay the nucleation and propagation process.The aim of present study is to compare the influence of WC-10 Ni coating applied by HVOF on the fatigue strength of AISI 4340 steel, with and without shot peening. S-N curves were obtained in axial fatigue tests for material base, and tungsten carbide coated specimens. (C) 2010 Published by Elsevier Ltd.

Fatigue in AISI 4340 steel thermal spray coating by HVOF for aeronautic application

Currently, high-strength materials, particularly AISI 4340 steel, are used in several landing gear components. Due to the high resistance to wear and corrosion required, the components are usually coating by hard chromium. This treatment produces waste, such as Cr+ 6 (hexavalent chromium), generally after applying the coating of hard chromium which is harmful to health and the environment. The process HVOF (High-velocity-oxygen-fuel) is considered a promising technique for deposition of hard chromium alternative coatings, for example, coatings based on tungsten carbide. This technique provides high hardness and good wear strength and more resistance to fatigue when compared to AISI 4340 hard chromium coated. To minimize loss fatigue due to the process of deposition, shot peening is used to obtain a compressive residual stress. The aim of this study was to analyze the effects of the tungsten carbide thermal spray coating applied by the HVOF, in comparison to the conventional hard chromium electroplating on the AISI 4340 high strength steel behavior in fatigue. Optical microscopy and scanning electron microscopy were used to observe crack origin sites, thickness and adhesion of the coating. (C) 2010 Published by Elsevier Ltd.

Fatigue fracture behavior of Ti-6Al-4V PVD coated

Fatigue, corrosion and wear resistance are important parameters in aircraft components development as landing gear. High strength/weight ratio and effective corrosion resistance make of titanium alloys an alternative choice to replace steel and aluminum alloys. However, titanium alloys have poor tribological properties, which reduce devices performance under friction. PVD coatings tribological systems has been increased due to their attractive mechanical properties as low environmental impact, low friction coefficient, low wear rate and hardness up to 2000 HV.In this study the influence of TiN deposited by PVD on the fatigue strength of Ti-6Al-4V alloy was evaluated. Comparison of fatigue strength of coated specimens and base material shows also a decrease when parts are coated. It was observed that the influence is more significant in high cycle fatigue tests. Scanning electron microscopy technique (SEM) was used to observe crack origin sites and fracture features. (C) 2010 Published by Elsevier Ltd.

An evaluation of shot peening, residual stress and stress relaxation on the fatigue life of AISI 4340 steel

Shot peening is a method widely used to improve the fatigue strength of materials. through the creation of a compressive residual stress field (CRSF) in their surface layers. In the present research the gain in fatigue life of AISI 4340 steel, Used in landing gear. is evaluated under four Shot peening conditions. Rotating bending fatigue tests were conducted and the CRSF was measured by an X-ray tensometry prior and during fatigue tests. It was observed that relaxation of the CRSF occurred due to the fatigue process. In addition, the fractured fatigue specimens were investigated using a scanning electron microscope in order to obtain information about the crack initiation points. The evaluation of fatigue life, relaxation of CRSF and crack sources are discussed. (C) 2002 Elsevier B.V. Ltd. All rights reserved.