Improvement Of Fatigue Behavior Research Articles

Polarization response and thermally stimulated depolarization currents of the modified (Ba,Ca) (Zr,Ti)O3 piezoelectric ceramics

Polarization response and defect mechanism of the (Ba,Ca) (Zr,Ti)O3 piezoelectric ceramics by doping modification were intensively investigated. The Al-doping leads to the reduced piezoelectric polarization responses, but optimizes fatigue resistance and the quality factor, showing the acceptor characteristics. A way to test thermally stimulated depolarization currents (TSDC) is utilized to distinguish the role of defect dipole from oxygen vacancy and uncover the intrinsic reason of improving cyclic fatigue behavior. It has been confirmed that the improvement of fatigue behavior can be attributed to the formation of oxygen vacancies by Al doping. The present study gives a promising method to reveal the origin of piezoelectric characteristics based on defect configuration in advanced dielectrics.

Influence of Grain Orientation Distribution on the High Temperature Fatigue Behaviour of Notched Specimen Made of Polycrystalline Nickel-Base Superalloy

Two different material batches made of random and textured orientated polycrystalline nickel-base superalloy René80 were investigated under isothermal low cycle fatigue tests at 850 °C for a notched specimen geometry. In contrast to a smooth specimen geometry, no significant improvement in fatigue behaviour of the notched specimen could be observed for the textured material. Finite element simulations reveal an area along the notch where high stiffness evolves for the textured material, which lead to nearly similar shear stresses in the slip systems compared to a random orientation distribution and therefore to no distinct differences in the lifetime.

An improvement in fatigue behavior of AISI 4340 steel by shot peening and ultrasonic nanocrystal surface modification

Individual and synergy effects of shot peening (SP) and ultrasonic nanocrystal surface modification (UNSM) on rotary bending fatigue (RBF) behavior of AISI 4340 steel were systematically investigated at various bending stress levels in the range of 275–600 MPa. The results revealed that the fatigue behavior of the as-received sample was enhanced by SP and it was further enhanced by SP and UNSM combination, while the UNSM-treated one exhibited the highest enhancement in fatigue behavior. The fatigue behavior of the SP + UNSM sample was enhanced after SP, but it was found to be detrimental after UNSM. Apart from RBF experiments, individual and synergy effects of SP and UNSM on surface roughness, hardness, residual stresses and grain size were analyzed as well. Fractography of the samples was characterized by scanning electron microscope (SEM) and fatigue mechanisms of each sample were discussed based on the obtained observations. The finest grain size and highest compressive residual stress (CRS) obtained by UNSM in comparison with those of the SP and SP + UNSM was responsible for the best fatigue performance.

Improvement in Fatigue Behavior of Dental Implant Fixtures by Changing Internal Connection Design: An In Vitro Pilot Study.

(1) Background: The stability of the dental implant–abutment complex is necessary to minimize mechanical complications. The purpose of this study was to compare the behaviors of two internal connection type fixtures, manufactured by the same company, with different connection designs. (2) Methods: 15 implant–abutment complexes were prepared for each group of Osseospeed® TX (TX) and Osseospeed® EV (EV): 3 for single-load fracture tests and 12 for cyclic-loaded fatigue tests (nominal peak values as 80%, 60%, 50%, and 40% of the maximum breaking load) according to international standards (UNI EN ISO 14801:2013). They were assessed with micro-computed tomography (CT), and failure modes were analyzed by scanning electron microscope (SEM) images. (3) Results: The maximum breaking load [TX: 711 ± 36 N (95% CI; 670–752), EV: 791 ± 58 N (95% CI; 725–857)] and fatigue limit (TX: 285 N, EV: 316 N) were higher in EV than those in TX. There was no statistical difference in the fracture areas (P > 0.99). All specimens with 40% nominal peak value survived 5 × 106 cycles, while 50% specimens failed before 105 cycles. (4) Conclusions: EV has improved mechanical properties compared with TX. A loading regimen with a nominal peak value between 40% and 50% is ideal for future tests of implant cyclic loading.

Experimental study on fatigue properties of normal and rubberized self-compacting concrete under bending

Self-compacting concrete with the unique property of flowing under its own weight has great importance for the design of structures and pavements. The use of rubber aggregates in self-compacting concrete improves its working performance such as fatigue behavior and ductility. With respect to the influences of maximum stress level, loading frequency and stress ratio, the fatigue behavior of normal self-compacting concrete (NSCC) and rubberized self-compacting concrete (RSCC) under four-point bending was investigated. Considering the number of cycles to failure and the developments of strain and stiffness, RSCC shows improvements in fatigue behavior and resistance of deformation compared to NSCC. Furthermore, because the fatigue lives were discrete, the fatigue lives of NSCC and RSCC were modeled by the two-parameter Weibull distribution. Based on the results, double logarithmic fatigue equations which provide a way to predict the fatigue life were established.

Application of design of experiments for laser shock peening process optimization

Laser shock peening—a very promising life enhancement technique—has demonstrated great success regarding the improvement of fatigue behavior via deep compressive residual stresses. However, the prediction and adaption of residual stress fields on basis of the laser peening parameters are still not comprehensively established. The aim of the current work is to investigate the effects of the laser pulse energy, the number of treatment overlaps as well as the laser spot size on the resulting residual stress distribution, characterized by following quantities: the residual stress close to the surface, the maximum compressive residual stress, and the integral compressive stress area over the specimen depth. For a systematic investigation of all main and interaction-based process parameter effects, and a subsequent parameter optimization, the general full factorial design is employed. The results show that laser shock peening with different process parameter combinations, inducing residual stresses with comparable integral stress area, can lead to a minimum fatigue life extension of approx. 100,000 cycles, representing a minimum fatigue life of 250% of the base material. The experimental scatter in the number of cycles to failure follows the Weibull distribution which qualitatively correlates with the standard deviation of the integral stress area.

Fatigue Behavior of Friction Stir-Welded Joints Repaired by Grinding

Fatigue is undoubtedly the most important design criterion in aeronautic structures. Although friction stir-welded joints are characterized by a high mechanical performance, they can enclose some defects, especially in their root. These defects along with the relatively low residual stresses of the friction stir-welding thermomechanical cycle can turn into primary sources of crack initiation. In this context, this article deals with the fatigue behavior of friction stir-welded joints subjected to surface smoothing by grinding improvement technique. The 4-mm-thick aluminum alloy 2024-T351 was used in this study. The fatigue strength of the base material, joints in the as-welded condition, and the sound and defective friction stir-welded joints improved by grinding were investigated in detail. The tests were carried out with a constant amplitude loading and with a stress ratio of R = 0. The fatigue results show that an improvement in fatigue behavior was obtained in the joints repaired by superficial grinding technique. The weld grinding technique is better especially for lower loads and increases the high cycle fatigue strength. The fatigue strength of the improved welded joints was higher than that of the base material.

Experimental and numerical investigation into effect of elevated temperature on fretting fatigue behavior

t Fretting fatigue damage occurs in contacting parts when they are subjected to fluctuating loadingsand sliding movements at the same time. This phenomenon may occur in many applications such asbearings/ shafts, bolted and riveted connections, steel cables, and steam and gas turbines. In this paper,the effect of elevated temperature on fretting fatigue life of Al7075-T6 is investigated using a new device forfretting fatigue tests with variable crank shaft mechanism. Also a finite element modeling method was usedto estimate crack propagation lifetime in aluminum alloy, Al7075-T6 specimens at elevated temperatureunder fretting condition. In this method, shear and normal stresses that are caused by contact load areupdated at each crack growth increment. Finally, a comparison between the experimental and numericalresults is done in order to evaluate the FE simulation.Department of mechanical engineering, Islamic Azad University, Takestan Branch, Takestan, IranThe experimental results show that: (i) fretting fatigue life of the material increases with temperature up to350°C by 180% for low stresses and decreases by 40% for high stresses, (ii) this fashion of variation offretting fatigue life versus temperature is believed to be due to degradation of material properties whichoccurs by overaging and wear resistance increase due to oxidation of aluminum alloy. While overaginggives rise to degradation of mechanical strength of material and hence the reduction of its fretting fatiguelife, surface oxidation of the specimens brings some improvement of fatigue behavior of the material.Metallurgical examination of the specimens reveals that temperature results in precipitation of impurities ofal-7075-T6. The size of precipitated impurities and their distances gets bigger as temperature increases.This could be a reason for material degradation of specimens which are exposed to heating for longer timeduration.

Comparison of structural and electric properties of PbZr0.2Ti0.8O3 and CoFe2O4/PbZr0.2Ti0.8O3 films on (100)LaAlO3

In this work, we investigated structural, electrical, and magnetic properties of ferroelectric PbZr0.2Ti0.8O3 (PZT) and ferrimagnetic/ferroelectric [CoFe2O4(CFO)/PZT] bilayers grown on (100)LaAlO3 (LAO) substrates supplied with bottom 50 nm thick LaNiO3 electrodes. Interestingly, structural and electrical properties of the PZT layer exhibited remarkable changes after the top-layer CFO deposition. X-ray diffraction data suggested that both the c- and a-domains exist in the PZT layer and the tetragonality of the PZT decreases upon the top-layer deposition. A variation in the electrical properties of the PZT layer upon the CFO deposition was investigated by polarization versus voltage (P-V), capacitance versus voltage (C-V), and capacitance versus frequency (C-f) measurements. The CFO deposition induced a slight decrease of the remnant polarization and more symmetric behavior of P-V loops as well as led to the improvement of fatigue behavior. The tentative origin of enhanced fatigue endurance is discussed based on the measurement results. These results were corroborated by local piezoelectric measurements. Ferrimagnetic property of the CFO/PZT bilayer was confirmed by magnetic measurement at room temperature.

FATIGUE-FREE MULTIFERROIC Sc-DOPED BiFeO3 THIN FILMS PROCESSED BY A CHEMICAL SOLUTION DEPOSITION METHOD

ABSTRACT Sc-modified (0.3 mol%) multiferroic BiFeO3 thin films (BFS) were synthesized on Pt/Ti/SiO2/Si substrates through a chemical deposition method. The effect of Sc doping on the fatigue and retention properties, and polarization switching with magnetic poling of the films were investigated. Through Sc-doping, the film exhibited marked improvement in fatigue behavior at least up to 1011 cycles in comparison with the BFO film. The stable polarization retention property of the BSF film over the range of 1–105 seconds demonstrated that Sc doping in BFO improved the retention characteristics compared to BFO. This supports that Sc addition can reduce oxygen vacancies and charge defects in the BSF system which results in the excellent fatigue and retention properties. Magnetic field poling of 0.8T improved the polarization of the BSF film by about 48% which decreased slowly to ∼10% after 14 days. These results suggest that the BSF film has potential in ferroelectric and magnetic devices applications.

Ferroelectric and fatigue behavior of Pb(Zr0.52Ti0.48)O3∕(Bi3.15Nd0.85)Ti3O12 bilayered thin films

Bilayered ferroelectric thin films consisting of Pb(Zr0.52Ti0.48)O3 (PZT) and (Bi3.15Nd0.85)Ti3O12 (BNT) layers have been successfully fabricated via a synthesis route of combining sol-gel and rf sputtering. Both ferroelectric layers are well retained in both PZT/BNT and BNT/PZT bilayered films as suggested by x-ray diffraction and secondary ion mass spectroscopy analyses. Their ferroelectric and dielectric properties are largely dependent on the thicknesses of the constituent layers. An anomalous enhancement in polarization is demonstrated by the PZT/BNT bilayered thin film, whereby the switchable polarization increases dramatically upon >106cycles of polarization switching. The fatigue anomaly observed for the PZT/BNT bilayered thin film is related to the space charges that are accumulated at the interfaces in the heterolayered structure. By comparison, the BNT/PZT bilayered thin film exhibited better ferroelectric behavior than that of the PZT/BNT film where the P-E hysteresis loops were relatively well saturated with higher polarization. The BNT/PZT bilayered thin film also demonstrated a much improvement in fatigue behavior as compared to that of the single layer PZT film.

Improvement of Fatigue Behaviour of High Strength Aluminium Alloys by Fluidized Bed Peening (FBP)

This paper deals with a definition of a relatively novel technique to improve the fatigue behavior of high strength aluminum alloys, namely, Fluidized Bed Peening (FBP). Fatigue samples made from AA 6082 T6 alloy were chosen according to ASTM regulation about rotating bending fatigue test and, subsequently, treated by varying FBP operational parameters and fatigue testing conditions. First, a full factorial experimental plan was performed to assess the trend of number of cycles to rupture of fatigue samples varying among several experimental levels the factors peening time and maximum amplitude of alternating stress applied to fatigue samples during rotating bending fatigue tests. Second, design of experiment (DOE) technique was used to analyze the influence of FBP operational parameters on fatigue life of AA 6082 T6 alloy. Finally, ruptures of FB treated samples and untreated samples were discussed in order to evaluate the influence of operational parameters on the effectiveness of FBP process and to understand the leading process mechanisms. At any rate, the fatigue behavior of processed components was found to be significantly improved, thereby proving the suitability of FBP process as alternative mechanical technique to enhance fatigue life of components made from high strength aluminum alloy.

4046 Effect of anodized layer on fatigue mechanism of magnesium alloy

Anodic coatings generally degrade the stress-life (S-N) fatigue performance of a material. So far many research works have been carried out on the effect of anodic coating on fatigue performance but very few of them deal with the possible effect of anodizing thickness on the fatigue behavior. Hence, rotating bending fatigue test was conducted in laboratory air under the stress ratio, R, of 0.1 using smooth specimen with anodized film thickness of 15 and 5μm. Fatigue strength of anodized specimen decreased considerably as compared with that of the untreated one. But the degradation of fatigue life was much more pronounced in 15μm coating compared to the 5μm coated samples. It was pointed out through study that the fatigue strength of anodized alloy was controlled by the crack initiation behavior in the coating and the result of the interrupted fatigue test showed a much faster crack initiation time in anodized materials compared with that of the base metal. Crack imitation time increased with decreasing coating thickness causing an improvement in fatigue behavior. Fractographic observation also revealed the presence of many flaws in 15μm coating compared to that of the 5μm one that also lead to better fatigue performance of the later under low humidity condition.

인접 홀의 영향을 받는 홀 확장 잔류응력의 유한요소해석

홀 확장법은 홀 주위에 압축 잔류응력층을 생성시킴으로서 균열 발생을 지연시키는 방법으로 항공산업 분야에서 널리 사용되고 있다. 홀 확장 잔류응력의 분포는 홀 확장률, 맨드럴 삽입방향, 재료 물성치 등 여러 가지 홀 확장 피라미터에 따라 달라진다. 홀 확장 잔류응력층의 정확한 파악은 항공산업의 실제 설계에 있어서 매우 중요함에도 불구하고 이에 대한 연구는 상대적으로 부족하였다. 본 연구에서는 유한요소해석을 통하여 두 개의 인전한 홀에 홀 확장법을 적용하는 경우 대한 잔류응력 분포를 구하였다. 연구 결과 두개 홀에 동시에 홀 확장을 하는 경우가 순차적으로 홀 확장으로 하는 경우보다 더 유리한 잔류응력 분포를 얻을 수 있었다. Cold expansion of fastener holes is a mechanical process widely used in the aerospace industry. This treatment leads to an improvement of fatigue behavior due to the developed compressive residual stresses on the hole surface. Despite its importance to aerospace industries, little attention has been devoted to the accurate modeling of the process. This study is devoted to the modeling and simulation of the residual stress resulting from the cold expansion of two adjacent fastener holes. Simultaneous cold expansion of two adjacent holes lead to much higher compressive residual stress than sequential cold expansion.

Matrix effects on the photochemical fatigue resistance of fulgide-doped polymers

Fulgide Aberchrome 670-doped polymer films are studied. The closed form of this chromophore presents a maximum of absorption centered at about 525 nm and is formed upon irradiation of its colorless open form at 365 nm. We have determined the photoreaction rate constants, kUV and kVIS, respectively, for the coloring and bleaching processes. It was found that photochemical fatigue of fulgide doped in the mixture of a cyanoacrylate adhesive with 10% of dioctoepoxy monomer is essentially absent after 40 UV/visible irradiation cycles. We thus report, for the first time, a simple and rapid preparation of dye-doped polymer films in a solvent-free process that shows significant improvements in fatigue behavior. Our previous investigation of conventional polymer matrices prepared by gravity deposition of dilute dye-polymer solutions found losses of 9, 11, 13, and 35% after only 8 UV/visible irradiation cycles, respectively, in polystyrene (PS), cellulose acetate (CA), poly(methyl methacrylate) (PMMA), and polyvinyl carbazole (PVK).

유한요소법을 이용한 홀 확장 잔류응력 해석

홀 확장법은 홀 주위에 압축 잔류응력 층을 생성시킴으로서 균열 발생을 지연시키는 방법으로 항공산업 분야에서 널리 사용되고 있다. 홀 확장잔류응력의 분포는 홀 확장률, 맨드럴 삽입방향, 재료 물성치 등 여러 가지 홀 확장 파라미터에 따라 달리진다. 홀 확장 잔류응력 층의 정확한 파악이 항공사업의 실제 설계에 있어서 매우 중요함에도 불구하고 이에 대한 연구는 상대적으로 부족하였다. 본 연구에서는 3차원 유한요소해석을 통하여 알루미늄 평판에 홀 확장법을 적용하는 경우의 잔류응력 분포를 예측하고자 하였다. 유한요소해석의 타당성을 검증하기 위하여 스트레인 게이지를 이용한 홀 확장 잔류변형률을 측정하였으며, 홀 확장률과 2단 홀 확장이 잔류응력 분포에 미치는 영향을 파악하였다. 2단 홀 확장을 적용함으로써 최대 압축 잔류응력의 크기가 약 7% 향상됨을 알 수 있었다. Cold expansion of fastener holes is a mechanical process widely used in the aerospace industry. This treatment leads to an improvement of fatigue behavior due to the developed compressive residual stresses on the hole surface. The residual stress profile depends on the parameters of cold expansion, which are expanding rate, inserting direction of mandrel, material properties dtc. Despite its importance to aerospace industiries, little attention has been devoted to the accurate modeling of the process. In this paper, three-dimensional finite element simulations have been conducted for the cold expansion in an aluminium plate in order to predict the magnitude and distribution of the residual stress. To prove the results of FE analysis, the residual strain was measured by strain gage in cold expansion test. Maximum compressive residual stress could be increase about 7 percentage using the 2-step cold expansion method.

Fatigue behaviour of Ti sputtered Al composites

Remarkable enhancement of fatigue life and endurance limit of Al matrix composites has been achieved by thin coatings of Ti sputtered at room temperature. Three materials have been investigated. They differ for matrix (6061, A359, 2618), reinforcement particles (SiC and Al2O3) and preparation process (casting and powder metallurgy). Fatigue tests employing a rotating bending machine showed that the method works on all of them, higher effectiveness has been observed in the composite (6061/ 20%SiCp) which has more homogeneous reinforcement distribution. An improvement of fatigue behaviour has been observed also in tests performed on the composite A359/20%SiCp at 200°C, typical working temperature for some industrial applications of Al matrix composites. Coatings seal the defects present on the external surface of the composites, retard crack growth thus lead to an improvement of fatigue behaviour. The effect of coating on fatigue behaviour is discussed with reference to the surface state, which has been investigated by scanning electron microscopy (SEM), scanning tunnelling microscopy (STM), X-ray diffraction (XRD) and microhardness tests.

Improvement of the fatigue behaviour of Al 6061/20% SiC p composites by means of titanium coatings

The present work shows that it is possible to increase the fatigue life and endurance limit of Al 6061/20% SiC p composite by means of coatings of titanium sputtered at room temperature. Coatings seal defects always present on the external surface of the composite, retard crack growth thus lead to a remarkable improvement of fatigue behaviour. The effect of coating thickness on fatigue is discussed with reference to surface state, which has been investigated by scanning tunnelling microscopy (STM).

Numerical simulation of shot peening

Shot peening leads to an improvement of fatigue behavior due to the compressive residual stresses developed. There are many parameters. mamlv the velocity and the radius of the shot as well as the material parameters, influencing the results in a very complcx manner. In order to minimize the effort required to adjust the process. - besides experimental work -, analytical methods as well as the Finite Element Method (FEIvQ have been used to investigate the process. In this paper a numerical approach will be introduced in order to predict the development. magnitude and distribution of the residual stress state in accordance with the above named parameters. As a first step. axi-jsymmetric models are used to simulate the very short term elastic-plastic deformation process caused by normal impacts of up to 5 sphcrcs on the same location of the surface of a work-piece. In a second step, the effects of adjacent shots are investigated applying a 3D-FE model. With this model, it can be shown how adjacent shots influence the residual stress state developed by former shots, and that the final residual stress state in the surface of the work-piece is inhomogeneous. Simulating a stepwise etching process of the surface of the work-piece reveals that this procedure leads to an increasingly homogenous residual stress state. Etching the surface is necessav to measure the residual stresses experimentally using the x-ray diffraction method. The residual stresses calculated show good agreement with experimental results obtained from peening samples made of spring steel,

Improvement of fatigue behaviour of mechanically surface treated materials by annealing

The positive effects of mechanical surface optimization methods for fatigue lifetime and strength are generally attributed to the formation of compressive residual stresses and strain hardening in near surface layers. This work concentrates on the cyclic deformation behavior of three different commercial, widely used alloys (steels SAE 1045 and AISI 304, magnesium wrought alloy AZ31) subjected to thermal treatment after mechanical surface optimization. In the case of SAE 1045 fatigue lifetime as well as fatigue endurance strength were shown to be affected positively by different heat treatments in spite of residual macro and micro stress relaxation. Macroscopically according to Manson-Coffin`s law this improvement can be explained by the reduction of plastic strain amplitude. Microscopically, strain ageing can be identified as the responsible process. For all three materials, optimum heat treatment temperatures and times are suggested, derived from hysteresis measurements and cyclic lifetimes. Finally, it will be shown that further surface optimization can be achieved by simultaneously applying mechanical and thermal treatments (thermomechanical rolling or peening).