Decrease In Fatigue Strength Research Articles

1705 発電用設備規格委員会疲労評価タスクの活動状況(OS17-2 低サイクル疲労とその応用-設計疲労線図の検討-)

Fatigue failure is one of the essential phenomena to be considered in the design and integrity assessment of light water reactor plants and the methods for assessing its significance are given in the structural design standards. Decrease in fatigue strength due to environmental effect of high-temperature water ns reported and its introduction into the design standards gains a large importance. This issue and others related to fatigue evaluation for light water reactor plants have been discussed in a task group formed in the committee for power equipments codes organized in JSME. This report summarizes the updated infonnation on the activities in the task group including those on design fatigue curves, environmental effect as well e flaw tolerance approach.

Influence of Surface Treatments on Fatigue Strength of Ti6Al4V Alloy

Titanium alloys do not exhibit good seizure toughness. In general, a surface treatment must be applied if the component is subjected to sliding contact. For surface hardening, it is well known that oxygen and nitrogen treatments can be used because they are easily available, inexpensive elements. However, these treatments cause the fatigue strength to decrease which is a problem.The reason for the fatigue strength decrease due to oxygen diffusion treatment is residual tensile stress on the surface. Shot peening is an effective technique to improve the fatigue strength of oxygen diffusion treated titanium. This report details the effects on fatigue strength due to surface treatments such as notch and shot peening.

オーステナイト系ステンレス鋼の0.9wt%食塩水中における疲労特性におよぼす溶存酸素の影響

Fatigue life evaluations in low dissolved oxygen conditions have been found to be conservative regarding the prediction of the fatigue strength of biomaterials. However, dissolved oxygen is considered to increase crack propagation rates. It is necessary to observe whether the strength in low dissolved oxygen conditions is higher. In this paper, the authors observed the effects of dissolved gas on the corrosion fatigue characteristics of austenitic stainless steels. Fatigue strength was higher with low dissolved O2 NaCl aq at a stress ratio of R= 0.1. However, fatigue strength did not change with low dissolved O2 NaCl aq at a stress ratio of R= 0.5 because of the effect of work hardening. Cycles to crack initiation were longer and crack growth rates were lower in the low dissolved O2 NaCl aq. The mechanism of decreasing fatigue strength can be explained by the oxidation process during slipping. In conclusion, dissolved oxygen has the effect of accelerating crack propagation processes.

Effects of LCF Loadings on the HCF Life of Notched Specimens in Ferritic-Bainitic Steel*

Abstract Fatigue tests were performed on ferritic bainitic steel notched specimens (Kt = 2.5) under load controlled constant amplitude loading. These tests show that under constant amplitude tension compression loading, periodical overloads application have a detrimental effect on the fatigue crack initiation strength for fully reversed load ratio (Rσ = −1), while they have no influence under pulsating loading (Rσ = 0). A finite element analysis shows that in the fully reversed tension (Rσ = −1), the stabilized cyclic behaviour at the notch root is an elastic-plastic shakedown while elastic shakedown is obtained under pulsated regime (Rσ = 0), so that we can consider that the local cyclic behaviour has an influence on the overload effect. However, the overload application does not imply a remarkable modification of the stress and strain field under the subsequent constant amplitude loading and can not explain such a fatigue strength decrease in fully reversed tension.

Definition of the Influence of Pore Size on the Fatigue Limit Using Short Crack Propagation Experiments

Abstract Aluminium high pressure die casting is used to reduce costs and weight of various components in the automotive industry. The main problem of components made by pressure die casting is connected with inherent flaws (porosity, oxide skins, etc.), which can hardly be avoided. Today the fatigue calculation of aluminium high pressure die cast parts is usually performed using two S/N curves—one account for the flawed basic material and one for the pore-free surface layer. This does not provide an accurate estimate for the computation of the lifetime or safety against failure of the component. In a cast component the number and size of pores increases towards the center. As a consequence the fatigue strength decreases. This inhomogeneous distribution throughout the component has to be taken into account for a realistic estimation of the fatigue strength. To do so, two models are required—one to compute the pore size distribution in the component [Oberwinkler, C., Leitner, H., and Eichlseder, W. “Improvement of an Existing Model to Estimate the Pore Distribution for A Fatigue Proof Design of Al HPDC Components,” TMS 2009, Shape Casting: 3rd Int. Symposium, 2009] and a material model [Oberwinkler, C., Leitner, H., and Eichlseder, W. “Computation of Fatigue Safety Factors for High-Pressure Die Cast (HPDC) Aluminium Components Taking into Account the Pore Size Distribution,” SAE World Congress 2009], which describes the influence of the defect size on the fatigue strength. This paper focuses on the definition of the correlation between the fatigue strength and the pore size. The lifetime of a component is defined by the crack growth if the crack initiation period can be neglected. Casting defects usually show high stress concentration factors due to their irregular shapes where cracks initiate. In this case the defect size is a part of the total crack length. Pores can be considered as physically short cracks because of their size of approximately 10–500 μm. The El-Haddad equation [El Haddad, M. H., Smith, K. N., and Topper, T. H., “Fatigue Crack Propagation of Short Cracks,” ASME Transactions, Vol. 101, 1979, pp. 42–46] has been used to describe the influence of the defect size on the fatigue strength. Short crack experiments have been performed to show that due to the shorter cracks the threshold stress intensity factor decreases below the threshold stress intensity factor of long cracks.

Bone fracture and the healing mechanisms. Pathophysiology and classification of osteoporotic fractures

Bone provides momentary strength and fatigue strength, and bone strength decreases with age. In elderly men and women with fragile bones osteoporotic fractures frequently occur. Fragility fracture occurs as a consequence of the decrease in momentary strength, and fragility fracture is one of the pathological fractures. In patients with the decrease in fatigue strength, insufficiency fractures frequently occurs. Insufficiency fracture is the same term as stress or fatigue fracture.

Backplate의 유무에 따른 맞대기 용접 시험편의 피로강도 평가

In this study, a series of fatigue test was performed to evaluate the fatigue strength of butt welded specimens. Effective notch stress through finite element analysis was conducted to analyze the fatigue results. As a results, no significant decrease in fatigue strength was observed when backplates were present. The S-N curve that based on effective notch stress appeared a similar fatigue lift to FAT 225 curve without reference to existence of backplates.

Effect of Humidity on Fatigue Strength of Maraging Steel with Different Hardness

In order to investigate the effect of humidity on initiation and propagation properties of a fatigue crack of 18%Ni maraging steels with different grain size and hardness, rotating bending fatigue tests were carried out in relative humidity of 25% and 85%. Fatigue strength was largely decreased by high humidity and the decrease in fatigue strength due to high humidity was increased with increases in grain size and specimen hardness. In high humidity, both of initiation and propagation of a crack were accelerated. The acceleration of a crack growth was caused by intergranular crack. Although the proportional relation between fatigue limit and hardness held up to HV600 in high humidity of 85%, the limiting hardness in the relation increased to HV 750 in low humidity of 25%. The decrease in fatigue strength due to high humidity was suppressed by double aging, which was aged at 673K for 3.6Ks after aging at 753K for 150ks.

予ひずみを付与したオーステナイト系ステンレス鋼SUS304の疲労挙動に及ぼす加工誘起マルテンサイト変態の影響

The fatigue behavior of prestrained type 304 stainless steel was studied. Rotating bending fatigue tests have been conducted in laboratory air and in 3%NaCl solution using specimens subjected to the tensile-prestrains of 15%, 30% and 60%. A particular attention was paid to the stress-induced martensite transformation during stress cycling. The fatigue strength of the prestrained specimens increased with increasing prestrain in both environments, but decreased significantly in 3%NaCl solution compared with in laboratory air. The strain-induced martensite transformation occurred in the prestrained specimens, and martensite phase increased with increasing prestrain and with stress cycling at the fatigue limit stress in the 30% and 60% prestrained specimens. The coaxing effect took place remarkably in the unprestrained specimen, but decreased with increasing prestrain. The increase in fatigue strength of the prestrained specimens in laboratory air and the coaxing effect were attributed to both work hardening and the strain-induced martensite transformation, where the contributions of the former and the latter decreased and increased with increasing prestrain, respectively. Since corrosion pits were present at the crack initiation site in the large prestrained specimens, the decrease in fatigue strength in 3%NaCl solution was due to larger environmental susceptibility of martensite phase.

Effects of Microstructure and Humidity on Fatigue Strength of Maraging Steel

Rotating bending fatigue tests were carried out for maraging steels with different grain size in moist air in order to investigate the effects of humidity, grain size and reversion austenite on fatigue strength of the steel. Fatigue strength was decreased by humidity, and the decrease in fatigue strength was large in larger grain sized steel. Both of the initiation and propagation of a crack were promoted by humidity. Many cracks initiated at the specimen surface and intergranular cracks were observed at specimen surface and on fracture surface when humidity was high. The decrease in fatigue strength by humidity was suppressed by formation of reversion austenite.

강 봉(SM45C) 맞대기 용접부의 피로수명 평가

SM45C steel rods being used generally for power transmission shafts and machine components was selected and welded by Butt-GMAW(Gas Metal Arc Welding) method. An estimation of fatigue life was studied by constructing S-N curve. Fatigue strength of base metal zone showed higher values than one of weld zone in low cycles between <TEX>$10^4$</TEX> and <TEX>$10^6$</TEX>cycles. However, significant decrease in fatigue strength of base metal was found around <TEX>$10^6$</TEX>cycles, which were almost same as one of heat affected zone. This decrease was attributed that initial residual stress of the steel rods distributed by drawing process was diminished by continually applied load, and resulted in softening of base metal. The fatigue limit of the weld zone was highest in the boundary between deposited metal zone and heat affected zone, and followed by in the order of deposited metal zone, base metal zone, and heat affected zone. Based on these results, it is revealed that the stress for safety design of machine components using SM45C butt-welded steel rods must be selected within the region of the lowest fatigue limit of heat affected zone.

A STUDY ON THE FRETTING FATIGUE LIFE OF ZIRCALOY ALLOYS

Studies on the strength and fatigue life of machines and structures have been conducted in accordance with the development of modern industries. In particular, fine and repetitive cyclic damage occurring in contact regions has been known to have an impact on fretting fatigue fractures. The main component of zircaloy alloy is Zr , and it possesses good mechanical characteristics at high temperatures. This alloy is used in the fuel rod material of nuclear power plants because of its excellent resistance. In this paper, the effect of the fretting damage on the fatigue behavior of the zircaloy alloy is studied. Further, various types of mechanical tests such as tension and plain fatigue tests are performed. Fretting fatigue tests are performed with a flat-flat contact configuration using a bridge-type contact pad and plate-type specimen. Through these experiments, it is found that the fretting fatigue strength decreases by about 80% as compared to the plain fatigue strength. Oblique cracks are observed in the initial stage of the fretting fatigue, in which damaged areas are found. These results can be used as the basic data for the structural integrity evaluation of corrosion-resisting alloys considering the fretting damages.

Relationship Between the Activation Energy of Hydrogen Trap Site and a Decrease in Fatigue Strength by Absorption of Irreversible Hydrogen in Cold-Drawn High Strength Steels

Fatigue tests have been conducted to investigate the influence of irreversible hydrogen and its activation energy for hydrogen desorption from trap site on the fatigue strength for cold-drawn high strength steels with different activation energy, but having the same chemical composition. Specimens were cathodically hydrogen charged, and internal hydrogen states were changed as follows : (a) non-charged sample and (b) the one that contained only irreversible hydrogen. Since the fatigue crack was initiated at internal inclusion or microstructural inhomogeneity, the fatigue strength was discussed based on the stress intensity factor calculated from stress and defect size. In the case of the sample with higher activation energy, 71.4 kJ/mol, irreversible hydrogen had no influence on the fatigue strength. In this case, the fracture surfaces of the sample with and without irreversible hydrogen were the same. However, for the sample with lower activation energy, 60.3 kJ/ mol, the fatigue strength of the specimen having only irreversible hydrogen decreased compared with non-charged specimens. The fracture surface of the sample with irreversible hydrogen was brittle compared with that of non-charged sample. This indicates that the influence of irreversible hydrogen on the fatigue strength tends to become small with increasing activation energy for irreversible hydrogen desorption.

冷間にて弾加工したオーステナイト系ステンレス鋼ＳＵＳ３０５の高温強度とミクロ組織

The high temperature strength and the microstructural change of strongly cold worked austenitic stainless steel SUS305 were investigated. 60% cold rolled specimens suggested strong decrease of fatigue strength, creep rupture strength and Vickers hardness (hereafter; hardness) after aging, corresponding to the increase of recrystallized grains. On the other hand, 30% cold rolled specimens suggested less decrease of strength and hardness, and revealed no recrystallization.In this study, the microstructure and hardness of strongly deep drawn SUS305 at high temperature were also investigated. On this specimen, similar to 60% cold rolled SUS305, decrease of hardness corresponded to the increase of recrystallized grains. But, since M23C6 precipitations were fine and closely spaced in the material, the recrystallization was delayed. On the other hand, the both materials suggested same relativeness between the softening tendency and recrystallization. Therefore, fatigue strength and creep rupture strength of deep drawn SUS305 should decrease with the recrystallization at high temperature.

Residual stress influence on fatigue lifetimes of electroplated AISI 4340 high strength steel

ABSTRACTResidual stresses play an important role in the fatigue lives of structural engineering components. In the case of near surface 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. In both decorative and functional applications, chromium electroplating results in excellent wear and corrosion resistance. However, it is well known that it reduces the fatigue strength of a component. This is due to high tensile internal stresses and microcrack density. Efforts to improve hard chromium properties have increased in recent years. In this study, the effect of a nickel layer sulphamate process, as simple layer and interlayer, on fatigue strength of hard chromium electroplated AISI 4340 steel hardness – HRc 53, was analysed. The analysis was performed by rotating bending fatigue tests on AISI 4340 steel specimens with the following experimental groups: base material, hard chromium electroplated, sulphamate nickel electroplated, sulphamate nickel interlayer on hard chromium electroplated and electroless nickel interlayer on hard chromium electroplated. Results showed a decrease in fatigue strength in coated specimens and that both nickel plating interlayers were responsible for the increase in fatigue life of AISI 4340 chromium electroplated steel. The shot peening pre‐treatment was efficient in reducing fatigue loss in the alternatives studied.

Design of Electrical Panels of Naval Vessels for Improved Fatigue, Shock and Vibration Performance

Structural reliability of electrical panels installed in naval vessels is of critical importance from the structural performance viewpoint. The electrical panels may be exposed to vibration and fatigue loadings from internal and external sources as well as wave loadings that result into fatigue crack and fracture due to the decrease of fatigue strength. In this regard, electrical outfitting equipments on naval vessels are required to comply with capabilities against vibration and shock tests, normally tested according to Military Standard. This study introduces the effect of post welding treatment, such as toe grinding as well as post welding heat treatment (PWHT), are examined through a series of fatigue and impact tests according to military standards. On the other hand, the use of elastic mounts that are capable of isolating and mitigating shock and vibration toward electrical panels are investigated. Both experimental and numerical results are presented based on the analysis according to military standards.

Influence of surface defects and corrosive medium on the fatigue resistance of ST17G1S steel

We study the influence of surface defects and corrosive medium on the characteristics of fatigue resistance of specimens of ST17G1S steel extensively used for the production of pipes in the oil-and-gas industry. The accumulated results enable us to estimate the degree of decrease in the limited fatigue strength for specimens of ST17G1S steel in the presence of surface defects and under the action of corrosive medium.

The effect of pitting on the bending fatigue performance of high-strength aluminum alloy

The effect of pitting on the bending fatigue performance of high-strength aluminum alloy has been investigated by conducting a series of rotary bending fatigue tests at 95 Hz. From pitting corrosion studies, it was observed that coarse constituent particles play the main role in the pitting process. The results obtained from fatigue tests revealed that the fatigue strength, based on a fatigue life of 107 cycles, was dramatically reduced by the presence of pitting. The reduction in fatigue life depended on the severity of pitting; an approximately 60% decrease in fatigue strength was obtained.

A Probabilistic Model of Fatigue Strength Controlled by Porosity Population in a 319-Type Cast Aluminum Alloy: Part II. Monte-Carlo Simulation

In Part I, the fatigue strength of a cast 319-type aluminum alloy at 108 cycles was evaluated using an ultrasonic testing system, and a simple probabilistic model was developed to establish the relationship between the porosity population and the resultant fatigue strength of the alloy. In Part II, a detailed analysis and comprehensive simulation based on this model were performed to examine the effects of casting porosity characteristics on fatigue strength in cast aluminum alloys. The results predict that, when fatigue life is controlled by porosity population, the mean and standard deviation of the fatigue strength decrease with increasing mean pore size, pore size standard deviation, and porosity number density in the castings. In addition, the specimen size and shape are predicted to influence the fatigue strength by affecting the number of pores and the probability of intersection of pores with the specimen surface within the stressed volume. In general, a large specimen volume containing a large number of pores (>1000) or surface over volume ratio in the range of 5.0 to 6.0 can lead to a decrease in fatigue strength up to 10 pct, as compared with the counterparts. The applicability of the model to a cast W319-T7 aluminum alloy is demonstrated.

Course of SN-curves especially in the high-cycle fatigue regime with regard to component design and safety

Conventional design codes base their recommendations still on the common prejudice that an “endurance limit” exists. However, several investigations prove clearly that in the high-cycle regime a decrease of fatigue strength with increased number of cycles still occurs, even if corrosion or temperature effects are excluded. Therefore, the fatigue design of components submitted to loadings below the knee point of the SN-curve must consider this fact in order to avoid failures. With regard to the course of the SN-curve in the very high-cycle area, material and manufacturing dependent recommendations are given.