Fatigue Strength Degradation Research Articles

Evaluation of static tensile strength and fatigue strength degradation of plain woven CFRP laminates under seawater immersion

Evaluation of static tensile strength and fatigue strength degradation of plain woven CFRP laminates under seawater immersion

Dynamic Characteristic and Fatigue Accumulative Damage of a Cross Shield Tunnel Structure under Vibration Load

This study presents an improved constitutive model for concrete under uniaxial cyclic loading which considers the fatigue stiffness degradation, fatigue strength degradation, and fatigue residual strain increment of concrete fatigue damage. According to the constitutive model, the dynamic response and cumulative damage of the tunnel cross structure under various train operation years were analyzed. The results show that the vibration in the middle of the main tunnel is most violent. With the increase of train operation period, the acceleration in the middle of the transverse passage floor, both sides of the wall corner and the vault increase significantly, and the maximum principal stress increases significantly only in both sides of the wall corner. The compressive damage is mainly distributed at both sides of the wall corner, while tensile damage is distributed in both sides of the inner wall corner. The accumulative damage of the cross structure exhibits a two‐stage profile. The size and range of accumulative tensile damage of the connecting transverse passage are greater than those of accumulative compressive damage.

Fatigue strength degradation of metals in corrosive environments

Structures exposed to aggressive environmental conditions are often subjected to time-dependent loss of coating and loss of material due to corrosion; this causes reduction in the cross-sectional properties of the members, increased surface roughness, surface irregularities and corrosion pits, and degradation of material strengths. These effects have been identified and simulated in different research studies. However, time and corrosive media dependent fatigue strength curves for materials have not been discussed in the design or assessment guidelines for structures. This paper attempts to review the corrosion degradation process and available approaches/models used to determine the fatigue strength of corroded materials and to interpolate corrosion deterioration data. High cycle fatigue and full range fatigue life formulae for fatigue strength of corroded materials are proposed. The above formulae depend on the endurance limit of corroded material, in addition to the stress-life fatigue curve parameters of the uncorroded material. The endurance limit of corroded material can either be determined by a limited number of tests in the very high-cycle fatigue region or predicted by an analytical approach. Comparison with experimentally measured corrosion fatigue behavior of several materials is provided and discussed.

English

The most important target of this study is to develop a method to compare the materials’ degradation due to thermal fatigue and temperature variation between the carbon nanotube wire and unidirectional carbon fiber/epoxy composite (UCFEC). In the presented study, by using experimental results data, thermal fatigue life of carbon nanotube wire and unidirectional carbon fiber/epoxy was compared after being exposed to temperature variation in space. This process was done by using a new analytical method that models the material strength deterioration as a function of temperature variation in low Earth orbit thermal cycles. Based on the relations that were obtained by using recommended analytical method, the carbon nanotube wire offered less thermal fatigue strength degradation equal to 6.608% after being subjected to 1,200,000°C temperature variation in comparison with unidirectional carbon fiber/epoxy composite thermal fatigue strength degradation. Therefore, it seems that carbon nanotube wire shows better thermal fatigue resistance. The results of this comparison may be used to determine the suitable carbon material for space missions based on thermal fatigue life.   Key words: Carbon nanotube, low earth orbit, thermal cycles, temperature variation, material degradation.

An approach to structural reliability evaluation under fatigue degradation and shocks

Abstract Fatigue is one of the most typical failure mechanisms, which can be regarded as a degradation process. The non-linearity in fatigue damage due to shocks cannot be disregarded. The mechanical shock is a transient physical excitation, in which the structure is nonequilibrium caused by a sudden applied load with high stress level or an abrupt change in the direction or magnitude of velocity. After the shock, the rate of fatigue damage accumulated decreases for a long time. When shock exists, the degradation could be generally retarded. The retardation phenomenon has been proven by the experimental observation. Therefore, a new approach is proposed to evaluate the structural reliability, in which the fatigue strength degradation process considering the retardation effect is explained by a crack-closure based fatigue model. The lifetime will be extended due to the applied shock loads. This fatigue crack growth model is validated by the experimental data of aluminum alloy D16. In addition, considering the material parameter and the shock loads are indeterminate, the reliability assessment under the fatigue loading with shocks is performed. A case study is given and two shock scenarios are discussed. In the first scenario, shocks occur in a fixed time period; while in the second scenario, shock occurs with varying time periods. The results indicate that the proposed approach can account for the retardation and evaluated the fatigue damage more appropriately.

Fatigue behavior of basalt fiber-reinforced polymer tendons under a marine environment

Due to their advantageous mechanical behaviors and anti-salt properties, basalt fiber-reinforced polymer (BFRP) tendons are a promising prestressing component under a marine environment. However, the degradation law of fatigue behavior of BFRP tendons under a marine environment is still unclear. This paper studies the fatigue behavior of BFRP tendons after aging in a salt solution by temperature-accelerated experiments. The degradation mechanism of fatigue behavior is clarified based on previous studies and analysis of the failure mode. The effect of aging temperature on fatigue strength degradation is analyzed using both experimental data and scanning electron microscopy (SEM) images. Moreover, the fatigue strength degradation of BFRP tendons at different local temperatures was predicted according to the Arrhenius equation. The results show that the fatigue strength degradation of BFRP tendons is governed by the weakened fiber-matrix interface resulting from the hydrolyzation of Si–O–Si chemical bonds in salt solution. SEM showed that specimens aged at 55°C for 63days demonstrate increased corrosion than those in other groups. Through the Arrhenius equation, the fatigue strengths of BFRP tendons are predicted to be 0.41, 0.43 and 0.45fu, respectively, at three representative northern latitudes after 100years of aging in marine environments. The results provide guidance for fatigue design of BFRP tendons in a marine environment.

Effect of hydrogen on very high cycle fatigue behavior of a low-strength Cr-Ni-Mo-V steel containing micro-defects

The role of hydrogen in fatigue failure of low strength steels is not as well understood as of high strength steels in very high cycle fatigue regime. In this work, axially cyclic tests on a low strength Cr-Ni-Mo-V steel with charged hydrogen were carried out up to the very high cycle fatigue regime under ultrasonic frequency to examine the degradation of fatigue strength and associated failure mechanisms. Results show that the S-N curves show a continuously decreasing mode and hydrogen-charged specimens have lower fatigue strength and shorter fatigue lifetime, as compared with as-received specimens. It is concluded that the hydrogen trapped by inclusions drives interior micro-defects as dominant crack initiation site, and has a clear link to the initiation and early growth of interior fatigue cracks.

Very-high-cycle fatigue behavior of a structural steel with and without induced surface defects

Fatigue tests via rotary bending were performed on the specimens with and without induced surface defects for a structural steel of medium carbon content to investigate the effect of surface defects on fatigue behavior in high-cycle fatigue and very-high-cycle fatigue (VHCF) regimes. The S–N data showed that induced surface defects substantially degraded the related fatigue strength. For the specimens without surface defects failed in VHCF regime, crack initiated from the interior of specimens with inclusions or matrix inhomogeneities as crack origin and the initiation regions were of different extents of rough surface. The observations on the profile samples cut from crack initiation regions revealed that the region was a layer of nanograins for the case of inclusion as crack origin, and was without grain refinement for the case of grain boundary as crack origin. For the specimens with induced surface defects, crack initiated from surface defects and the initiation region was without grain refinement. The characteristics of crack initiation were carefully examined and the effect of surface defects on fatigue strength degradation was analyzed by available models.

Fatigue Behavior of Extruded Mg-Al-Ca-Mn Alloy with T6 Treatment at Elevated Temperature

Fatigue behavior of Mg-Al-Ca-Mn alloy with T6 treatment was studied at room temperature and 150°C by conduction rotating bending fatigue test. Fatigue strength at high temperature was lower than that at room temperature in the alloys with and without heat treatment. However, degradation of fatigue strength at high temperature in the T6 treated alloy was not significant compared to the as-extruded alloy. Fatigue crack initiation and propagation behavior was observed with replication technique by conducting interrupted fatigue test at room temperature and 150°C. Multiple cracking was significantly observed at 150°C in both as-received and T6 treated alloys. Change in grain size and randomization of crystal orientation due to the heat treatment could affect the fatigue property.

Fatigue life estimation of existing bridges under vehicle and non-stationary hurricane wind

During hurricane events, additional damages to existing bridge details might be accumulated from the large stress cycles generated by the non-stationary wind and dynamic vehicle loads. The environmental corrosion could also reduce the structural fatigue strength. Based on the non-stationary wind field modeling techniques and the vehicle–bridge–wind dynamic analysis framework, this paper initiates a fatigue damage assessment of existing bridges with corrosion under vehicle and non-stationary extreme hurricane wind loads. The non-stationary hurricane wind is simulated as a summation of time-varying mean and fluctuating non-stationary components with a time-varying spectrum. Dynamic stress histories are obtained by solving the vehicle–bridge–wind dynamic system. The rain flow counting method is used to obtain the stress ranges and numbers of stress cycles. The corrosion induced fatigue strength degradation is included in the analysis. Numerical examples of an existing long span bridge under simulated non-stationary hurricane winds and vehicles are presented to demonstrate the effects of the dynamic loads and corrosions on the fatigue life estimation of existing bridges.

Degradation of titanium 6Al–4V fatigue strength due to electrical discharge machining

Electrical discharge machining (EDM) has been known for many years to induce degradation of fatigue strength. Outdated or coarse EDM processing methods can reduce the fatigue strength of titanium alloy Ti–6Al–4V by as much as factors of 2–5, but a paucity of fatigue data exists for this important aerospace alloy processed with modern EDM techniques. An experimental study is presented in which the degradation of fatigue strength of Ti–6Al–4V due to improved EDM processing is measured in axial tension at a load ratio of R=0.1. It is shown that, relative to specimens finely milled, state-of-the-art EDM processing causes a reduction of fatigue strength by 15–30%. This strength degradation is found to correlate directly to the thickness and roughness of recast layers created by the solidification of melt material formed during EDM. Post-processing with either electrochemical polishing or bead blasting is demonstrated to remove the deleterious effects of EDM, such that specimens possessed intrinsic fatigue behavior as indicated by crack initiation at interior locations.

Fatigue and Mechanical Properties of Aluminium-Copper Bi-Metal Tubes

Metallurgical bonded aluminium-copper bi-metal tube has a future prospect as an alternative material to copper in the heat, ventilation and air-conditioning (HVAC) industries. The application of aluminum-copper bi-metallic material is seems practical in maintaining the quality of existing products. However, the mechanical strength and fatigue properties of the bi-metal tubes are unknown. In this study, metallurgical, mechanical and fatigue properties of the copper phosphorous alloy and aluminium-copper bi-metal tubes were characterized through metallographic analysis, tensile, bending and fatigue tests. The results show that there observed a weak and brittle Al-Cu intermetallic compound at the interlayer between Al and Cu. Tensile fracture surface observation revealed that separation of Cu from Al occurred at the interlayer. The bending properties of the tubes were influenced by the amount of volume fraction of Cu in the materials. Three point bending fatigue test results showed that a critical buckling stress is presents for tubes with diameter less than 12.7mm. The aluminium-copper bi-metal tubes show degradation of fatigue strength almost 55% as compared to that of Cu alloy tube.

Mechanical fatigue degradation of ceramics versus resin composites for dental restorations

ObjectivesFor posterior partial restorations an overlap of indication exists where either ceramic or resin-based composite materials can be successfully applied. The aim of this study was to compare the fatigue resistance of modern dental ceramic materials versus dental resin composites in order to address such conflicts. MethodsBar specimens of five ceramic materials and resin composites were produced according to ISO 4049 and stored for 14 days in distilled water at 37°C. The following ceramic materials were selected for testing: a high-strength zirconium dioxide (e.max ZirCAD, Ivoclar), a machinable lithium disilicate (e.max CAD, Ivoclar), a pressable lithium disilicate ceramic (e-max Press, Ivoclar), a fluorapatite-based glass-ceramic (e.max Ceram, Ivoclar), and a machinable color-graded feldspathic porcelain (Trilux Forte, Vita). The composite materials selected were: an indirect machinable composite (Lava Ultimate, 3M ESPE) and four direct composites with varying filler nature (Clearfil Majesty Posterior, Kuraray; GrandioSO, Voco; Tetric EvoCeram, Ivoclar-Vivadent; and CeramX Duo, Dentsply). Fifteen specimens were tested in water for initial strength (σin) in 4-point bending. Using the same test set-up, the residual flexural fatigue strength (σff) was determined using the staircase approach after 104 cycles at 0.5Hz (n=25). Weibull parameters σ0 and m were calculated for the σin specimens, whereas the σff and strength loss in percentage were obtained from the fatigue experiment. ResultsThe zirconium oxide ceramic showed the highest σin and σff (768 and 440MPa, respectively). Although both lithium disilicate ceramics were similar in the static test, the pressable version showed a significantly higher fatigue resistance after cyclic loading. Both the fluorapatite-based and the feldspathic porcelain showed equivalent initial and cyclic fatigue properties. From the composites, the highest filled direct material Clearfil Majesty Posterior showed superior fatigue performance. From all materials, e.max Press and Clearfil Majesty Posterior showed the lowest strength loss (29.6% and 32%, respectively), whereas the other materials lost between 41% and 62% of their flexural strength after cyclic loading. ConclusionsDental ceramics and resin composite materials show equivalent fatigue strength degradation at loads around 0.5σin values. Apart from the zirconium oxide and the lithium disilicate ceramics, resin composites generally showed better σff after 10,000 cycles than the fluorapatite glass-ceramic and the feldspathic porcelain. Resin composite restorations may be used as an equivalent alternative to glass-rich-ceramic inlays regarding mechanical performance.

Remaining fatigue life estimation of corroded bridge members

ABSTRACTThis paper presents a method to estimate remaining fatigue lives of railway bridge members subjected to time‐dependent corrosion. The method addresses effects of material loss due to general (uniform) corrosion and fatigue strength degradation of material due to corrosive environment. The method mainly consists of stress history, which is obtained by considering the effect of time‐dependent loss of material, full‐range S–N curve, which represents the corrosive environment, and sequential law, which takes the loading sequence effect more precisely in to account than Miner's rule. Initially, nonlinear behaviour of material loss over time (i.e. time‐dependent growth of corrosion wastage) is discussed, and hence, necessary formulae to calculate time‐dependent cross‐sectional properties are comprehensively presented. Then finite element analysis‐based procedure is clearly proposed to predict stress histories of corroded members. A technique is introduced to obtain the full‐range S–N curve for the corroded structural detail. The concept of sequential law is summarized with the algorithm, and then the proposed method is applied to predict the remaining fatigue lives of the corroded members of a railway bridge. The predicted remaining lives were compared with the previous method‐based estimations, and comparisons reveal the range of 16–47% reduction of fatigue lives of critical members when time‐dependent corrosion is taken into account. Also, the results reveal that the corroded members of smaller cross‐sectional area are most vulnerable for fatigue damage. Finally, significance of the proposed method is confirmed.

Fatigue Strength Degradation of AlSi12CuNiMg Alloy Due to High Temperature Exposure: A Structural Investigation

The strengthening mechanism based on the formation of precipitates (Guinier-Preston (GP) zones) during decomposition of a metastable supersaturated solid solution is stable and effective below temperatures, which are typically overcome in the piston region facing the combustion chamber. Therefore, pistons experience a progressive loss of strength due to over-aging during service. This contribution reports of a structural investigation on a eutectic AlSi12CuNiMg alloy before and after fatigue testing at high temperatures (up to 350°C). Metallography, color etching, SEM and TEM are used to elucidate the following aspects: 1) the structural features of the alloy (dendrites of α−phase, primary Si particle size and distribution, morphology and distribution of intermetallic phases); 2) the evolution of the strengthening mechanism (GP zones) with high temperature exposure.

Fatigue life evaluation of aligned plant fibre composites through S–N curves and constant-life diagrams

There is a noticeable lack of fatigue data on plant fibre composites (PFCs) which seriously limits their prospective use in fatigue critical components. The objective of this manuscript is to provide a complete set of fatigue data on vacuum infused aligned PFCs. S–N lifetime diagrams have been constructed to investigate the effect of (i) plant fibre type/quality, (ii) fibre content, (iii) textile architecture, and (iv) stress ratio, on PFC cyclic loading behaviour. At each stage, the fatigue performance of PFCs is compared to that of E-glass/polyester (GFRP) composites. A comprehensive constant-life diagram has been generated to facilitate life prediction of PFC components. While plant fibre type and quality, textile architecture and composite fibre content have a significant impact on the static properties, they have little impact on the material fatigue strength coefficient b (which dictates the slope of the S–N curve). In essence, higher static properties are a sign of superior fatigue loading capacities throughout their lifetime. Increasing stress ratios lead to improved fatigue performance in PFCs. Fracture mechanisms and modes are the same for composites made from three different bast fibres, but depend on fibre content, textile architecture and stress ratios. Although the absolute fatigue performance of GFRPs is far superior to PFCs, it is a revelation to find that fatigue strength degradation rates are lower in PFCs than in GFRP.

Thermal degradation of chlorosulfonated polyethylene rubber and ethylene propylene diene terpolymer

In this work, the thermal degradation of chlorosulfonated polyethylene rubber (CSM) and ethylene propylene diene terpolymer (EPDM) were studied. The thermogravimetry of CSM and EPDM were firstly analyzed, and the loss in elongation and fatigue strength of CSM and EPDM due to thermal and humidity conditions was measured. To evaluate the degradation of these properties, the Arrhenius equation was used to confirm the relationship between temperature and exposure time. From these evaluation methods, the thermal reduction of fatigue strength can be predicted by taking into account the temperature and exposure time. The acceleration in the thermal degradation of elongation and fatigue strength were almost equal. In addition, the activation energy of thermogravimetry agreed well with that of fatigue strength. Next, the viscoelastic behavior of virgin CSM, EPDM and their thermal degradation were measured. The thermal degradations of mechanical loss tangent were evaluated by using the Arrhenius equation to confirm the relationship between the temperature and the exposure time. The acceleration in the thermal degradation of a mechanical loss tangent agreed well with the acceleration in the thermal degradation of elongation and of fatigue strength.

Failure behaviour of the superalloy MAR-M247 LC under LCF, HCF and combined LCF/HCF loading

Abstract Materials for turbine blades experience in service a combined loading of low and high cycle fatigue at high temperatures. In order to understand the failure behaviour under these loading conditions, systematic investigations were carried out. Low cycle fatigue, high cycle fatigue and combined low and high cycle fatigue tests were realised on MAR-M247 LC at 650 °C in an air environment under total strain control. Surface damage and fracture surfaces were analysed. Under combined low and high cycle fatigue, the lifetime is reduced if the low cycle fatigue leads to a degradation of the high cycle fatigue strength caused by crack initiation and crack growth. By analysing the fracture surface, the crack growth rate under combined cycle fatigue loading could be determined and it was significantly higher than under pure low cycle fatigue loading. The accelerated crack growth mainly causes the lifetime reduction.

Fatigue Strength Degradation by Irreversible Hydrogen in Cold Drawn Eutectoid Steels

The fatigue tests were conducted for cold-drawn eutectoid steels having different activation energies and microstructures to investigate the mechanism of fatigue strength degradation by irreversible hydrogen. The fatigue strength of a sample with low activation energy was decreased by irreversible hydrogen, whereas the fatigue strength of a sample with high activation energy was not. When the activation energy for irreversible hydrogen becomes small, the desorption of irreversible hydrogen from its trap site is easily induced by cyclic loading, which results in a decrease in fatigue strength. When a sample having high activation energy was annealed at 473 K, the fatigue strength was decreased by irreversible hydrogen although the activation energy barely changed. This is because the precipitation of fine carbides due to annealing causes higher sensitivity to hydrogen embrittlement. High sensitivity to hydrogen embrittlement was concluded to induce the fatigue strength degradation by irreversible hydrogen. Therefore, the degradation of fatigue strength by irreversible hydrogen depends on the activation energy and sensitivity to hydrogen embrittlement.

OS2215 水素チャージした水素ステーション蓄圧器用SCM435鋼のギガサイクル疲労特性(OS22-4 水素と疲労,OS-22 水素環境の材料強度特性への影響)

Gigacycle fatigue properties of a storage cylinder in hydrogen station, which had been subjected to 4 years verification tests, was examined in both longitudinal and circumferential directions. The storage cylinder was made of SCM435 steel. Fatigue tests were carried out on hydrogen-charged and uncharged specimens by using ultrasonic and servo-hydraulic fatigue testings. In the uncharged specimens, only surface fracture occuned in both directions. On the other hand, the hydrogen-charged specimens showed internal fracture and degradation of fatigue strength. The crack initiation sites of internal fracture were different in the two directions: globular Al_2O_3 and slender MnS inclusions in the longitudinal and circumferential directions, respectively. The fatigue strength of the hydrogen-charged specimens was lower in the circumferential direction than in the longitudinal direction, while the difference was small.