Fatigue characteristics of VT22 titanium alloy with wear-resistant coatings

We study the influence of wear-resistant gas-thermal plasma and ion-plasma nitride coatings, diffusive electrochemical treatment, and vacuum nitriding on the fatigue resistance of specimens made of VT22 titanium alloy. The experimental results demonstrate that the wear-resistant hard coatings decrease the fatigue limit, whereas the procedure of sand-blasting increases the fatigue resistance of the alloy. Unlike diffusive coatings, plasma coatings suffer cracking and exfoliate from the surface of titanium alloy, which reveals their insufficient adhesion and high stiffness. The obtained results are quite urgent for the aircraft industry, where the VT22 alloy is extensively used. __________ Translated from Fizyko-Khimichna Mekhanika Materialiv, Vol. 42, No. 6, pp. 119–122, November–December, 2006.     

Fatigue of beta titanium alloy at 20, 482 and 648 °C

Fatigue life of fibrous metal matrix composites is limited by the distribution of fibre strengths, the fibre‐matrix interfacial strength, and the fatigue resistance of the matrix. The aim of this work is to provide fatigue results for a beta titanium alloy over a range of temperatures and stresses that can be used as input for predicting fatigue life of a titanium matrix composite. Stress controlled tests having fatigue ratios between ?1 and ?0.2 were conducted on a limited number of samples machined from unreinforced laminated Ti‐15Mo‐3Al‐2.7Nb‐0.2Si (TIMETAL^®21S) sheets to represent as closely as possible the in situ matrix material. Stress control was used to enable quantification of strain ratcheting for tensile mean stresses and a fast loading rate was used to minimize time‐dependent (creep) deformation. Stress amplitude‐life data at 20, 482 and 648 °C for fully reversed loading are well fit by a power law. Normalizing the stress amplitude with respect to the power law coefficient appears to account for the temperature dependence of the S–N curves. As the tests had large strains and lives were in the low‐cycle fatigue range, strain range at the half‐life was also correlated to life. For tensile mean stress cycling at 482 and 648 °C, the rate of strain ratcheting per cycle increased to failure; shakedown was not observed.     

Fatigue and creep-fatigue crack growth in alloy 709 at elevated temperatures

Fatigue crack growth (FCG) and creep-fatigue crack growth (CFCG) in austenitic stainless-steel Fe-25Ni-20Cr (Alloy 709) were measured experimentally and simulated using the finite element method. Temperature conditions investigated were 550°C, 600°C and 700°C, with load hold times of 0 s, 60 s and 600 s. Fracture surface was investigated using scanning electron microscopy and optical imaging. Experiments indicated that crack growth exhibits minimal sensitivity to the various loading conditions evaluated. At 600°C, crack growth rates were independent of hold time or loading frequency. At 700°C, there was a small increase in crack growth rate as a function of hold time, with a 600s hold time causing a factor of 2 increase in crack growth rate over FCG. Finite element simulations were performed to compute plasticity-induced crack closure in the presence of creep deformations at the crack tip. The simulations produced FCG and CFCG rates similar to the experimental results.     

Ti-Al-Sn-Zr-W-Si高温钛合金的蠕变性能

为大幅度拓展Ti-Al-Sn-Zr-W-Si高温钛合金的工程应用,优化了Ti-Al-Sn-Zr-W-Si高温钛合金的热处理工艺,研究了该钛合金的蠕变性能,并采用扫描电子显微镜观察了蠕变实验前后合金的微观组织特征及断口形貌。研究表明,经合理热处理工艺后,该合金在550和600℃均体现出优异的蠕变性能。蠕变实验后,合金的微观组织发生了明显的变化:在颈缩区呈明显的纤维状,而在均匀塑性变形区,由于回复再结晶,合金的显微组织呈现细小的晶粒。     

Fatigue lifetime of ADI from ultimate tensile strength to permanent fatigue limit

S-N curve of austempered ductile iron was obtained in the range of lifetime including low cycle fatigue domain and high cycle fatigue domain up to 10⁸ cycles. Ultimate tensile strength is used as a limiting value of the curve. Symmetric push-pull fatigue and tensile tests were performed at room temperature on isothermally treated nodular cast iron alloyed with copper and nickel having positive impact on mechanical, technological and fatigue properties of austempered ductile iron. Suitable functions for the fit of experimentally determined points were tested and their parameters were calculated. The best results were obtained using the Palmgren function and the function introduced by Kohout and Věchet. Since the loading frequency in high-cycle region is two orders higher than in low-cycle region, the effect of loading cycle frequency on fatigue behavior of the studied material is also studied. A possibility of discontinuity of experimental data between low-cycle and high-cycle regions is discussed. __________ Translated from Problemy Prochnosti, No. 1, pp. 40–43, January–February, 2008.     

Simulated flight fatigue of an alpha/beta titanium alloy

Simulated flight (FALSTAFF) fatigue tests have been carried out on precracked single edgenotch test-pieces of (Ti4Al4Mo2Sn0.5Si) IMI 550 titanium alloy. Predictions of simulated flight fatigue behaviour have been made from constant amplitude fatigue data, using a damage accumulation approach, with no allowance for load history. The predicted lives were conservative compared with the measured lives, and accurate within a factor of approximately two. Retardation of fatigue crack growth increased with increasing load amplitude. The microstructure produced by β-solution heat treatment at 1010°C, followed by ageing, was found to improve simulated flight fatigue lives by up to approximately 100% compared with standard solution treatment at 900°C, followed by ageing.     

Theory of superconductivity at high temperatures

Superconductivity is generally explained by an electron-lattice interaction which results in the pairing of electrons and the condensation of these pairs into a state of lower entropy. In high-temperature superconductivity the pairs consist of hybrids in which the top of the oxygen band of the crowded perovskite layer is mixed with the bottom of unoccupiedd- orf-bands from monoxide layers in the crystal. Only electrons and phonons with low quasi-momentum (k) values participate. This makes it possible to localize the lattice perturbation into broad regions in which the van der Waals forces are reduced and the perovskite planes are contracted. The low entropy state associated with superconductivity manifests itself as the formation of a superlattice of lattice distortions which is in actual motion in the current-carrying states. The observability of this superlattice is discussed.     

Fatigue and creep crack growth of Alloy 800 and Alloy 617 at high temperatures

A common evaluation is given for creep crack growth and fatigue crack growth experiments which have been performed at the companies ABB, Siemens-KWU and KFA. The materials under investigation were X10NiCrAlTi32 20 (Alloy 800) and NiCr22Col2Mo (Alloy 617). Several production lots and semi-finished materials as well as welded materials have been tested. Testing techniques differed at the different labs. In order to eliminate the influence of individual testing techniques, material from some production lots was investigated at different labs. The given data cover fatigue crack growth (the materials were tested between room temperature and 1050°C; the influence of temperature, R?ratio, and frequency was investigated) and creep crack growth (Alloy 800 was tested between 550°C and 900°C, Inconel 617 between 800°C and 1000°C; the evaluation was done on the basis of the fracture mechanics parameters K₁ and C*).     

Fatigue life behaviour of TiAlN and TiAlN/CrN coated notched P20 steel specimens

The performance of single layer TiAlN and multilayered TiAlN/CrN physical vapour deposition coatings deposited on AISI P20 steel substrate in affecting overall fatigue resistance of notched specimens was assessed and compared with the performance of the uncoated counterparts. V‐shaped circumferential notches on cylindrical specimens were adopted for fatigue tests. Surface coating characteristics such as hardness, elastic modulus and microstrains were measured and found to be different and often larger than those of the steel substrate. Unlike the un‐notched (smooth) coated specimens, which are known to exhibit large improvements in fatigue life in the high cycle fatigue regime, considerable reductions in fatigue life of the coated notched samples were observed. This was understood to be because of the coating's brittleness, which induces at the notch tip early and frequent fatigue crack initiations, especially in the case of multiple layered coatings. Scanning electron microscope images showed more crack initiation sites in both the coated specimens compared with the uncoated specimen. Also, presence of dimples on the surface confirmed dimple rupture mechanism in the ductile steel substrate in the coated and uncoated specimens.     

Effect of grain size on high-cycle fatigue properties in alpha-type titanium alloy at cryogenic temperatures

High-cycle fatigue properties were investigated at 4, 77 and 293 K in Ti-5%Al-2.5%Sn ELI alloy which was used for liquid hydrogen turbo-pumps of Japanese-built launch vehicles. Mean grain size of specimens was controlled to be about 30 or 80 μm. In the specimens with a grain size of 30 μm, fatigue strengths at 10⁶ cycles at 4 and 77 K are 1.6 and 1.5 times higher than that at 293 K, respectively. On the other hand, in the specimen with a grain size of 80 μm, fatigue strengths at 10⁶ cycles at 4 and 77 K get lower to the same level as that at 293 K. Thus, it is concluded that refinement of α grains is one of important factors to obtain the good high-cycle fatigue properties for Ti-5%Al-2.5%Sn ELI alloy at cryogenic temperature.     

超声载荷下TC4钛合金的疲劳寿命分析

通过计算裂纹尖端应力强度因子及疲劳裂纹扩展速率da/d N,由C.Paris模型推导出安全寿命Nf,由Bathias公式计算"哑铃"状钛合金试样的裂纹扩展寿命。通过理论计算和有限元分析超声疲劳"哑铃"状试样,得出应力最大位置。利用有限元仿真和实验数据分析TC4钛合金疲劳寿命。在20 k Hz的超声疲劳试验中,试样的断口位置表明:TC4钛合金材料内部缺陷是试样萌生裂纹使断裂位置偏离最大应力处的主要原因。并得出疲劳裂纹萌生阶段寿命决定"哑铃"状试样的疲劳寿命。     

Interrelation between Micromechanisms of Structural Rearrangement of VT18U Titanium Alloy in Fatigue Fracture

We examine the dislocation structure of a pseudo--alloy of the Ti–Al–Zr–Sn–Mo–Nb–Si system upon fatigue and cyclic crack-resistance testing under identical conditions of symmetrical tensile–compressive loading with a frequency of 100, 500, 3000, and 10,000 Hz. It is shown that the structural development of the alloy during the fatigue-damage accumulation occurs in a limited number of microvolume zones near local stress concentrators, although a considerable number of structural alteration zones originate but stay undeveloped, while the rest of the material remains almost unchanged. At the stage of main-crack development, the structure of the crack-tip plastic zone presents a further logical development of the structure that was formed during the fatigue-damage accumulation. However, the stress level, which is much higher than that in the bulk volume of the material, can be responsible for additional structural elements and a diminishing role of local stress concentrators. These mechanisms are true for the entire range of cyclic loading frequencies studied. A variation of the rate of cyclic loading leads only to a slight decrease in the size of the microvolumes of maximum structural changes during fatigue-damage accumulation and in the honeycomb structural elements during the main crack propagation.     

Fatigue behaviour of 16Mo3 steel at elevated temperatures under uniaxial as well as biaxial‐planar loading

The isothermal fatigue behaviour of the ferritic steel 16Mo3 was investigated at 200 and 500 °C under uniaxial and biaxial‐planar loading. Furthermore, thermo‐mechanical fatigue behaviour under uniaxial loading was characterized under in‐phase (IP) and out‐of‐phase (OP) conditions between 200 and 500 °C. The fatigue lives of uniaxial and biaxial loading are in a good agreement to each other by using the distortion energy hypothesis according to von Mises. Under IP and OP thermo‐mechanical loading, nearly the same lifetimes were determined. They agree well with those of the isothermal tests at 500 °C. A recently developed fatigue lifetime model was applied on all tests and shows an excellent agreement within a scatter of two.     

Fatigue strength of fillet‐welded joints at subzero temperatures

Ships and offshore structures may be operated in areas with seasonal freezing temperatures and extreme environmental conditions. While current standards state that attention should be given to the validity of fatigue design curves at subzero temperatures, studies on fatigue strength of structural steel at subzero temperatures are scarce. This study addresses the issue by analysing the fatigue strength of welded steel joints under subzero temperatures. Although critical weld details in large welded structures are mostly fillet‐welded joints, most published data are based on fatigue crack growth rate specimens cut out of butt‐welded joints. This study analyses fillet‐welded specimens at ?20°C and ?50°C against controls at room temperature. Significantly higher fatigue strength was measured in comparison to estimates based on international standards and data from design codes even at temperatures far below the allowed service temperature based on fracture toughness results.