Decrease In Fatigue Strength Research Articles

Fatigue properties of a glass-fibre-reinforced polyester material used in wind turbine blades

Glass-fibre-reinforced polyester (GFRP) is a composite commonly used in the manufacture of wind turbine blades. In the present work, one such material has been subject to static and dynamic tests in order to obtain data that can be applied to the design of wind turbine blades and other machine elements. The results of the static tests established a basis for the determination of a set of tension-tension (constant amplitude and sinusoidal load) dynamic tests with the aim of establishing a mathematical model in order to predict life as a function of the load state and calculate the fatigue limit. The multiplicative model (y = axb) for y = log of life and x = transformed stress (a and b are characteristic parameters of the material obtained from data) matches the data quite well. The conclusion is that the GFRP studied has no fatigue limit. The possible decrease of fatigue strength of the material with solar radiation and moisture absorption was also investigated, with a negative result.

各種炭素鋼ばり付摩擦圧接継手の疲れ強さ

The fatigue strength of friction welded joint with flash decreased remarkably with the increase in upset pressure. This is caused by stress concentration with flash, as described in the previous report. In this study, the effect of flash on prevention of decrease in fatigue strength of friction welded joints was examined by means of rotating bending fatigue test using as welded joint specimens of S15C, S35C, S45C and S55C carbon steels under various friction welding conditions.The following experimental results were obtained:(1) The reduction ratio of fatigue strength was lowered with the decrease in upset pressure. It was small in the case of high carbon steel under the same pressure.(2) The decrease in fatigue strength of S45C and S55C welded joints with flash was not observed when upset pressure was 64 MPa.(3) Hardness of toe of flash increased remarkably by a friction weld thermal cycle in the case of high carbon steel.(4) The stress concentration factor of welded joint with flash became large with the increase of upset pressure. It was high in the case of high carbon steel under the same pressure.(5) The effect of hardening on the fatigue strength was larger than that of stress concentration in the case of high carbon steel.

Effects of aging on tensile fatigue strength of bovine dentin.

The effects of aging on the tensile fatigue strength of bovine dentin were investigated. Tensile tests and tensile fatigue tests were performed in 37 degrees C water with bovine dentin specimens of two different age groups. The tensile strengths of the young group and the adult group were 74.0 MPa and 72.7 MPa, respectively; there was no significant difference between the young and the adult group. However, the tensile fatigue strength of the adult group, 46.9 MPa, was significantly weaker than that of the young group, 51.0 MPa. There was a obvious difference between the tensile fracture surface and the tensile fatigue fracture surface; a smooth surface of the peritubular dentin and an uneven surface of the intertubular dentin were clearly distinct in the fatigue fracture surface. Plugged dentinal tubules and narrower dentin tubules were commonly observed in the adult group. These morphological changes were suggested to be the reason for the decrease in the tensile fatigue strength with aging.

疲労 Ｔｉ‐６Ａｌ‐４Ｖ合金ショット材の高温疲労特性

The effect of shot peening on the fatigue strength of Ti-6Al-4V alloy was investigated in the temperature range of 20°C to 450°C. In order to evaluate separately the individual effects of compressive residual stress, strain hardening and surface roughness induced by shot peening, the results obtained were discussed by the Murakami's √area parameter model which could predict fatigue strength using two parameters of defect size (√area) and Vicker's hardness (HV), and the effect of static mean stress (σm) could also be considered in the model. Shot peening increased fatigue strength at 20°C-350°C, but decreased at 450°C. The reduction at 450°C was due to surface roughness because compressive residual stress was decreased during fatigue loading. At 450°C, by comparing the fatigue strength of the rough surface specimens with that of the surface polished specimens, the defect size (√area) equivalent to the surface roughness was evaluated to be approximately 76μm. At 20°C-350°C, the fatigue strength observed could be regarded as that obtained by adding the contribution of compressive residual stress (static mean stress) to the fatigue strength of the specimens with the defect size of 76μm. The compressive residual stress predicted by the model showed reasonable values quantitatively. Drilling holes equivalent to √area of 93 and 185μm decreased fatigue strength markedly. The holes, however, were crushed by shot peening. For example, √area of 185μm was decreased down to about 55μm. Therefore, the fatigue strength of the drilled and shot-peened specimens increased drastically and was comparable to that of the shot-peened specimens without drilling holes but with the rough surface of 76μm.

The influence of diamond chemical vapour deposition coating parameters on the microstructure and properties of titanium substrates

In contrast to their excellent mechanical properties, titanium alloys possess poor wear characteristics. Diamond coatings appearto be a promising solution for the wear problem. Using standard deposition parameters for silicon (diamond scratched surface; microwave chemical vapour deposition, 500 W, 800 °C for 4 h, 50 mbar with an atmosphere of 1% CH4 + 99%H2) good coatings on pure titanium were obtained. The mechanical properties of the titanium are strongly influenced by the processing temperature and the gas atmosphere. As a result of the hydrogen adsorption, grain coarsening and formation of titanium hydride occur and subsequently the low cycle fatigue strength decreases by an order of magnitude. However, by annealing in vacuum (800°C for 2 h, furnace cooled) the hydrogen can be removed again and the initial mechanical properties can almost completely be restored. Based on scanning electron microscopy micrographs, Auger analysis, X-ray diffraction and microhardness measurements a basic understanding of the microstructural changes was developed. The surface structure may be visualized as consisting of five zones, created during the deposition process (diamond layer/nucleation zone/reaction layer/gradient layer/affected substrate).

Ｔｉ‐６Ａｌ‐４Ｖ合金の高温疲労強度に及ぼすショットピーニングの影響

(1) The fatigue strength of the unpeened specimens of Ti-6Al-4V alloy was found to be 490MPa, 430MPa, 410MPa and 440MPa at 20°C, 250°C, 350°C and 450°C, respectively. The respective values for the shot peened specimens were 620MPa, 520MPa, 440MPa and 260MPa. The fatigue strength was definitely increased by shot peening at 20°C, 250°C and 350°C, and the increments were about 27%, 21% and 7%, respectively. The fatigue strength, however, was remarkably decreased by shot peening at 450°C, and its decrement reached about 41%.(2) The fatigue strength of the specimen shot-peened and then electro-polished was 470MPa at 450°C. The fatigue strength greatly increased from 260MPa to 470MPa after polishing. The value of 470MPa is comparable to 460MPa of the fatigue strength of the unpeened specimen.The decrease in fatigue strength due to shot peening at 450°C may be explained as follows. The compressive residual stress in the near-surface region induced by shot peening disappear during fatigue cycling at 450°C, but the great surface roughness due to shot peening still remains, which reduce the fatigue strength of the shot peened specimen.

Fatigue-Strength Distribution of a Cathodic-Protection Steel and Stainless Steels in Seawater.

The influence of a corrosive environment on fatigue-strength distribution was studied for a slightly notched specimen (stress concentration factor α=1.03) and a sharply notched specimen (α=4.60) of S35C carbon steel. Fatigue strength is markedly lower and the standard deviation is also lower for S35C steel in synthetic seawater than those in laboratory air, because many cracks initiate on the specimen surface. Cathodic protection restores the fatigue strength and increases the standard deviation to those obtained in laboratory air. Since many cracks initiate in the sharply notched specimen in laboratory air, the standard deviation of the fatigue strength is lowered to one-third that of slightly notched specimen. Cathodic protection in seawater reduces the number of cracks and increases the standard deviation to a value twice as large as in the air. The influence was also studied for three kinds of stainless steels with slight notch (SUS304, SUS316 and SUS403). Among stainless steels, low-corrosion-resistance SUS403 in seawater exhibits a large decrease in fatigue strength compared with that in air. However, the standard deviation is not lowered by the kind of stainless steel, because the number of cracks is extremely low compared with the carbon steel. The dispersion of the fatigue life has a good correlation with that of the fatigue strength in terms of standard deviation. Therefore, the standard deviation is a significant parameter for the corrosion fatigue. The standard deviation of the fatigue strength has a good correlation with the number of cracks which led to the final fracture.

ばり付鋼材摩擦圧接継手の疲れ強さに及ぼす圧接条件の影響

The effect of flash on fatigue strength of friction welded joints are examined by means of rotating bending fatigue test, using as-welded joint specimens of hot and cold drawn mild steel round bar.The following experimental results were obtained ;(1) Fatigue strength of hot and cold drawn steel decreased with the increase of upset pressure.(2) In welded joints with flash, fatigue cracks were initiated at toe of flash.(3) Effect of upset pressure on fatigue strength was remarkable, but effect of friction pressure and total loss were slightly.(4) Toe radius of flash decreased with the increase of upset pressure and toe angle increased with the increase of this pressure. Therefore, it was considered that the main cause of the decrease of fatigue strength was stress concentration of toe of flash.(5) In the case of hot drawn steel, fatigue strength of the joint under any welding condition was recovered up to that of base metal, when flash was removed. In the case of cold drawn steel, however, that was not recovered, even when flash was removed.(6) Fatigue strength of cold drawn steel joints was remarkably decreased, when fatigue failure occurred in duplicate zone of stress concentration and softening. Therefore, this material required sufficient care, when friction welding condition was selected.

Effects of Water Absorption upon Fatigue Characteristics of Carbon-Fiber-Reinforced Epoxy Resin Composites.

This paper discusses the environmental degradation behavior of carbon-fiber-reinforced plastic (CFRP) laminated material, due to water absorption. In order that the CFRP materials can be used more effectively as structural members with higher safety and reliability for factories, it is considered indispensable to understand in detail the influence of environment factors that accelerate dynamic degradation, such as fatigue. A summary of the results is as follows. Fick's diffusion law is applicable to analysis of water desorption and absorption characteristics of the uni-directional fiber reinforced (UD) and plain woven reinforced (CL) materials. The desorption and absorption processes are diffusion-controlled processes within matrix resins, including the skin layer. The properties of the skin layer significantly influence the fatigue life of the CFRP laminates. Furthermore, the decrease of fatigue strength is caused by the lowering of molecular weight in the regions of resin close to the reinforced fibers in the skin layer, when a small amount of absorbed water and repeated stress coexist.

単発放電痕におけるき裂発生域の解析

In Electrical Discharge Machining(EDM), the residual stress generates on the surface of the workpiece because of repetitive heating and cooling process. The stress causes decrease of fatigue strength, delayed fracture and stress corrosion which affect quality of the workpiece. In the previous papers, the distributions of temperature and thermal stress in impulsive EDM has been analyzed by means of Finite Element Method taking into consideration the temperature dependency of mechanical properties and thermal physical properties of workpiece material. In this paper, the procedure for analysis of the region in which the crack must be generated is developed and time variation of the region is investigated under various machining conditions. The result shows that, on the machined surface, it is possible to generate cracks along radial and circumferential directions, because the residual stress exceeds the tensile strength.

燃焼合成法によるＴｉＡｌ基金属間化合物粉の焼結組織と疾労特性

Microstructure, fatigue properties, tensile strength, density and phase identification have been investigated for sintered compacts of TiAl base intermetallic compound powders prepared by combustion synthesis from pure titanium and aluminum powders. The results obtained are as follows: Both the TiAl powder prepared by the combustion synthesis and the compacts sintered at 1523-1648K for 25.2ks consist of γ-TiAl and α2-Ti3Al. The microstructure of the sintered compacts is dependent on the sintering temperature. The compacts sintered at 1523 and 1553K show equiaxed grains of the γ-and α2-single phase, and the compacts sintered at 1573 and 1648K contains α2/γ lamellar grains in addition to the γ-and α2-single phase grains. The amount of voids in the sintered compacts decreases with increasing sintering temperature and the relative density increases with increasing sintering temperature. The compact sintered at 1523K which contains most voids shows the lowest fatigue strength and the compact sintered at 1553K consisting of the (γ+α2) fine microduplex structure shows the highest fatigue strength. Preferential nucleation of cracks at theα2/γ lamellar interfaces in the compacts sintered at 1573 and 1648K brings about decrease in fatigue strength. The ratio of fatigue strength (at 107 cycles)to the tensile strength (UTS) is about 0.6 to 1.0, and decreases with increasing temperature. Large value of the fatigue ratio is characteristic of TiAl base intermetallic compounds.

Effect of Anisotropy on Fatigue Properties of Notched CFRP Laminates.

The effect of anisotropy on fatigue properties of notched carbon fiber reinforced plastic laminates has been studied. The off-axis angle of carbon fiber was varied from 0° to 45°. The ultimate tensile strength and the tensile elastic modulus drastically decreased when increasing the off-axis angle of carbon fiber to 30°. When the off-axis angle of carbon fiber increased over 30°, these values approached approximately constant values. The maximum tensile strain also increased and saturated to a constant value when the off-axis angle of the carbon fiber increased. The fatigue strength of notched CFRP laminates showed a similar dependence of ultimate tensile strength on the off-axis angle. The decrease of fatigue strength results from the fatigue damage in relation to the off-axis angle. The fatigue damage process was analyzed in terms of AE measurement and fractography. These results indicate that the fatigue damage process of CFRP laminates was significantly affected by the off-axis angle of carbon fiber.

Fatigue Resistance of Large‐Diameter Cable for Cable‐Stayed Bridges

Fatigue tests on a parallel‐wire large‐diameter cable system for cable‐stayed bridges and a detailed review of other fatigue test results are conducted. As a result, it is made clear that as the diameter of a cable increases, its fatigue strength decreases, stress concentration at the cable anchor portion is the principal factor influencing the fatigue strength of large‐diameter cables within working stress range in fatigue design of cables, and when the cable is grouted with cement for anticorrosion purposes, nongalvanized wires may have their life extremely shortened by corrosion fatigue due to excess water after a chemical reaction, which depends on the quality of material and on the manner of execution. Furthermore, the design S‐N curves, which are based on 5% wire breakage life, are proposed. The results of this study are used for fatigue design of Honshu‐Shikoku railway/highway cable‐stayed bridges, which are constructed over the sea (severely affecting durability) and on which a large repeated load due to railway acts.

Static and Fatigue Fracture Behavior of a Carbon Fiber Reinforced Epoxy Composite under Combined Tensile and Torsional Loading and Influences of Water Absorption.

Static and fatigue tests of thin-walled, [±45] filament-wound tubular specimens of a carbon fiber reinforced plastic, T300/827, under combined axial tensile and torsional loading have been conducted, and the influences of water absorption on the failure behavior have been investigated. The tests were conducted under a load-controlled condition, keeping a constant value of the combined stress ratio, α=γ/σ. The static strength of dry specimens, which had been held in air, showed good agreement with the Tsai-Hill failure criterion. The static strength of wet specimens, which had been preconditioned in water for two months, atα= 0 - 1 decreased by water absorption, whereas atα= 2 - ∞, where compressive strength in the fiber direction dominates over the failure, the decrease in the strength of wet specimens from dry ones was small. Fatigue strength of dry specimens atα= 0 - 1 was smaller than those of α= 2 - ∞. This indicates that a combined loading mode influences the fatigue strength. The decrease in fatigue strength of wet specimens from air data at α= 0 - 0.5 was higher than those at α= 2 - ∞.

アラミド／エポキシ複合材料の両振疲労強度に及ぼす水の影響

An investigation has been carried out concerning the influence of water absorption on the fatigue properties under completely reversed stress of 0° unidirectional and ±45° cross-ply composites of a Technola (HM50, Teijin)/epoxy and a Kevlar 49 (Dupont)/epoxy. The influence of water on the fracture mechanism was observed by a SEM.The ratio of fatigue strength under reversed tension and compression at N=106 in water to the one in air of the ±45° cross-ply composite was 0.61-0.66, and a remarkable decrease in fatigue strength was observed in water. In addition to the weakening of a fiber/matrix interface, local buckling of reinforcing fibers under compression due to the softening of water absorbed resin decreased the fatigue strength in water. The ratio of the fatigue strength in water to the one in air of the 0° unidirectional composite was 0.86-0.94, and the reduction of fatigue strength was moderate compared to that of the cross-ply one. This was due to the fact that the softening of resin not only increased the fatigue strength under tension but also decreased that under compression. Under reversed tension and compression, fiber splitting was decreased in water than in air in both types of composite. This was quite opposite to the case of pulsating tension, where the fiber splitting caused by water absorption reduced the fatigue strength in water.

Ａｌ‐Ｓｉ‐Ｃｕ‐Ｎｉ‐Ｍｇ系合金鋳物の中高温における疲労特性

Rotating bending fatigue tests of modified Al-Si-Cu-Ni-Mg eutectic casting alloys (AC8A) and hypereutectic casting alloys (AC9A and AC9B) were carried out in a temperature range between room temperature and 400°C. The fatigue strength of sodium modified AC8A is superior to that of phosphorus modified AC8A. The fatigue strength of AC9B with lower content of silicon is greater than that of AC9A. The decrease in fatigue strength is due to a coarse primary silicon. It is concluded that the fatigue properties of Al-Si-Cu-Ni-Mg casting alloys are influenced by the primary silicon grains.

The Effect of Gas Nitriding on Fatigue Behaviour of Ti-6Al-4V Alloy.

The fatigue behaviour of a gas-nitrided Ti-6Al-4V alloy has been studied under rotating bending fatigue test. The results were compared to those of annealed materials on the basis of detailed observations on crack initiation, growth and fracture surfaces. Nitriding was performed using smooth specimens for 4 h and 15 h at 850°C in pure nitrogen gas, and the depths of the nitrided layer obtained were approximately 25 μm and 65 μm, respectively. The fatigue lives of material nitrided for 4 h were shorter than those of annealed material, but the fatigue limit increased. On the other hand, fatigue strength was reduced by nitriding for 15 h. It is concluded that the decrease in fatigue strength of nitrided materials is mainly attributed to inferior crack initiation resistance of the nitrided layer.

溶融硝酸塩中における鋼材の腐食疲労挙動

Corrosion fatigue behaviour of plant materials such as 21/4;Cr-1Mo, SUH309 and S20C was investigated in a molten salt environment of (NaNO240%, NaNO37%, KNO353%) at 723K. The rotating bending fatigue strength at 4×107 cycles of 21/4;Cr-1Mo steel in the molten salt environment was 8 percent lower than the fatigue limit in air. The fatigue strength at 4×107 cycles of SUH309 in the molten salt environment was deteriorated by 27 percent. Corrosion fatigue cracks were initiated from corrosion pits and propagated accompanying striation predominantly in the molten salt environment. The morphology of the corrosion products on the specimen surface was examined by X ray-diffractions. The major corrosion product of these steels was Fe3O4. It may be concluded that the formation of corrosion pits at the initiation of fracture surface is the major cause of fatigue strength decrease of these steels in a molten salt environment containing Cl-.The corrosion fatigue strength of S20C steel was higher than the fatigue limit in air. This may be caused by the retardation of crack initiation from the hardened surface layer due to nitriding in a molten salt environment. Neither the initiation nor the propagation of stress corrosion cracking of these steels was observed in the molten salt environment during the initiation test up to 2880hrs and the propagation test up to 2160hrs in the stress intensity factor range KIO of 248 and 372MPa√m, respectively.

微粒等方性黒鉛の低サイクル疲労寿命

Low cycle fatigue tests of isotropic fine-grained graphite IG-11 were performed at the stress combination of pull-pull, pull-push and push-push. The best fit S-N curves obtained by several statistic analysis methods, were compared and the effect of stress ratio and specimen size on fatigue strength were discussed. Following conclusions were obtained:(1) The best fit S-N curves obtained by Price's method showed comparatively better in data fitting than the other methods.(2) The fatigue strength decreased as decrease of stress ratio R. The drastic decrease of fatigue strength was observed in the case of R=-3.5. This is because that the effect of pre-compressive stress in first fatigue cycle induced the decrease of tensile strength.(3) The effect of specimen size on fatigue strength in the case of R=+∞ and compressive strength were observed.(4) The mechanism of fatigue in graphite was well understood assuming that fatigue fracture were occurred when the stress intensity reach to the fracture toughness value in the process of crack extension.

Corrosion fatigue crack initiation and growth in 18 Ni maraging steel

Nucleation of fatigue cracks in air and 3.5 wt% NaCl solution has been studied in an 18 wt% Ni maraging steel. Specimens tested on reverse bending fatigue machine showed a marked decrease in fatigue strength of the steel in NaCl solution reducing the 107 cycles endurance limit from 410 MPa in air to 120 MPa. Microscopic studies revealed crack initiation to be predominantly associated with non-metallic silicate inclusions in both cases. In air, initiation is caused by decohesion of the inclusion/matrix interface, while in NaCl solution complete detachment of inclusions from the matrix results due to the dissolution of the interface. 70% more inclusions are quantitatively shown to be associated with cracks in NaCl solution than in air at the same stress levels. Experimental and theoreticalS-N curves and inclusion cracking sensitivity data are consistent with the mechanism suggested. The final fracture occurs by the main crack consuming the inclusions ahead of it by the “unzipping” of the shear band produced between the crack tip and the inclusion ahead.