P.S. Pao

Corrosion–fatigue crack growth in friction stir welded Al 7050

Fatigue crack growth rates in friction stir welds in air are slightly higher than those in base metal. In both air and 3.5% NaCl solution, fatigue crack growth rates in the heat affected zone are significantly lower and ΔKth significantly higher than those in the base metal.

Chemical and metallurgical aspects of environmentally assisted fatigue crack growth in 7075-T651 aluminum alloy

A comprehensive study has been carried out on a 7075-T651 alloy to examine the influence of water vapor on fatigue crack growth. The kinetics of fatigue crack growth were determined as a function of water vapor pressure at room temperature and at 353 K. Detailed fractographic analyses and surface chemistry studies were carried out to identify the micromechanisms and to quantify the chemical interactions for corrosion fatigue crack growth in this alloy. Experiments were also carried out in ultra-high vacuum and in oxygen to provide for comparisons. Two regions of fatigue crack growth response were identified. In the low pressure region (below 67 Pa at 5 Hz), crack growth is controlled by the rate of transport of water vapor to the crack tip, and the response can be described by a model for transport controlled crack growth. At pressures above 67 Pa, additional increases in crack growth rate occurred, which are attributed to the further reactions of water vapor with segregated magnesium in this alloy. Different micromechanisms for crack growth have been identified for vacuum, oxygen, and water vapor. These micromechanisms are considered in relation to the environmental parameters through a modified superposition model for corrosion fatigue.

Hydrogen-assisted fatigue crack growth in beta-annealed Ti-6Al-4V

In the present study, the effect of internal hydrogen on beta-annealed Ti-6Al-4V subjected to a very small amplitude fatigue loading (a cyclic amplitude equal to 5% of the maximum load or a stress ratio, R, of 0.90) was determined. The fatigue crack growth rates and fatigue crack growth threshold stress intensities were determined and compared for beta-annealed Ti-6Al-4V with hydrogen contents of 40 ppm, 370 ppm, and 1000 ppm. The observed secondary cracking and unique fatigue crack growth kinetics in 1000 ppm hydrogen-charged Ti-6Al-4V are discussed in relation to stress-assisted hydride information in the crack tip region.

Indentation of titanium processed by equal channel angular pressing

Commercially pure titanium rods were processed by equal channel angular pressing (ECAP) to obtain an ultrafine-grained microstructure. The titanium rods were extruded for 2 passes at 623 K, using three different bending angles of 90°, 120° and 150°. For the bending angle of 90°, the titanium rods were extruded for 4 and 8 passes in addition to 2 passes. The microstructure of the ECAP-processed titanium was evaluated using the TEM technique. The bending angle had a strong effect on the grain refinement, while the number of pressing passes did not have a noteworthy effect on the evolution of the microstructure. Microindentations were conducted with the indentation load in the range 100–2000 mN. The indentation hardness decreased slightly with the increase in the indentation load. The titanium processed by the die with a bending angle of 90° had the highest indentation hardness. The energy ratio of the plastic energy to the total energy was found to be proportional to the ratio of the residual indentation depth to the maximum indentation depth.

On ripple-load, stress-corrosion, and sustained-load cracking behavior in a high strength beta titanium alloy

Both overaged and peakaged TIMETAL 21S beta titanium alloys exhibit significant ripple-load cracking susceptibility in salt water and in ambient air environments. At R = 0.90, the ripple-load cracking thresholds of the overaged alloy are 67% and 72% lower than the stress-corrosion cracking and sustained-load cracking thresholds. For the peakaged alloy, the reductions are 55% and 61%. The stress-corrosion cracking threshold in salt water and the sustained-load cracking threshold in air of peakaged TIMETAL 21S are significantly lower while the ripple-load cracking threshold is slightly lower than those of the overaged alloy. The stress-corrosion cracking, sustained-load cracking, and ripple-load cracking resistance of peakaged TIMETAL 21S are significantly inferior to those of both beta-annealed Ti-6Al-4V and Ti-15V-3Cr-3Al-3Sn. The ripple-load cracking resistance of overaged TIMETAL 21S, though better than Ti-15V-3Cr-3Al-3Sn, is still inferior than that of beta-annealed Ti-6Al-4V.

Stress-Corrosion Cracking and Fatigue Crack Growth Behavior of Ti-6Al-4V Plates Consolidated from Low Cost Powders

Titanium is highly desirable for a wide range of applications because of its combination of high strength, low density and outstanding corrosion characteristics. However, the cost of titanium, produced by conventional technology, is high compared to steel and aluminum, which is a result of high extraction and processing costs. New approaches are being investigated maintaining required quality while lowering the cost of finished products. Ti alloy powder, Ti-6Al-4V, manufactured by a low cost hydride-process and consolidated into flat products (sheet, plate), were studied. The results of the study were compared with the properties obtained from plates of Armstrong Titanium consolidated powder. To remove the prior history of consolidation, the plates are beta annealed and the test results are compared with “as received” condition. The mechanism of the fatigue crack growth rate difference, fracture toughness, and stress-corrosion cracking resistance in terms of the respective Ti-6Al-4V microstructure differences will be discussed.

Tensile Deformation and Fatigue Crack Growth in Bulk Nanocrystalline Al-7.5Mg

Abstract : The fatigue crack growth kinetics and tensile deformation of bulk nanocrystalline Al-7.5Mg were investigated. Nanocrystalline particulates were first prepared by mechanically ball milling spray atomized Al-7.5Mg powders in liquid nitrogen. These particulates were then degassed, consolidated by hot isostatic pressing and extruded into rods. Bulk nanocrystalline Al-7.5Mg has significantly higher fatigue crack growth rates and lower fatigue crack growth thresholds than those of ingot-processed 7050-T7451. The fatigue crack growth thresholds exhibit only a weak stress ratio dependency and can be identified as having a Class I behavior when using the fatigue classification proposed by Vasudevan and Sadananda. In 3.5% NaCl solution, fatigue crack growth rates of bulk nanocrystalline Al-7.5Mg are as much as three times higher than those obtained in air. Tensile fracture of bulk nanocrystalline Al-7.5Mg is preceded by the formation of a localized shear band. In contrast to the low dislocation density in the as-extruded material, the gage section and the shear band region both exhibited a high dislocation density and dislocation cell structure.

Effect of Ripple Load on the Stress-Corrosion Cracking in Structural Steels

The presence of small ripple loading can, under certain circumstances, significantly reduce time-to-failure and threshold stress intensity for stress-corrosion cracking (SCC) of steels. A predictive framework for such ripple-loading effects (RLE) is developed from concepts and descriptors used in SCC and corrosion fatigue characterisation. The proposed framework is capable of defining critical conditions required for the occurrence of RLE and predicting time-to-failure curves. The agreement between the predicted and laboratory data is excellent.