W.J. Evans

Dwell sensitive fatigue in a near alpha titanium alloy at ambient temperature

The ambient temperature fatigue performance of the near alpha titanium alloy IMI834 was evaluated using laboratory specimens manufactured from two material sources: rolled bar stock and an isothermally forged compressor disc. The effect of dwell periods imposed at peak stress and R value were assessed. Significant differences were defined between the two variants with respect to their sensitivity to dwell loading. Variations in microstructural form together with a localised texture within the disc material are considered responsible for controlling the dwell performance in each case. The findings are consistent with a previously proposed model for facet development in this class of material.

Impact of texture on mechanical properties in an advanced titanium alloy

Abstract The mechanical response of highly textured Ti 6/4 plate material is evaluated under monotonic and cyclic loading. Significant variations in yield and ultimate tensile strength, bend ductility and total fatigue life to failure are all noted, depending on the orientation of the principal stress axis with respect to the dominant basal plane texture. These effects are discussed with reference to the inherent, anisotropic mechanical response of α + β and near α titanium alloys that results from the hexagonal crystallographic form of the α phase and the availability of preferential slip systems. It is argued that the anisotropic response could be utilised to an engineering advantage by matching critical stressing directions to the specific properties offered by the texture.

Electron back scattered diffraction (EBSD) analysis of quasi-cleavage and hydrogen induced fractures under cyclic and dwell loading in titanium alloys

Evidence for sub-surface fatigue crack initiation is often reported for near alpha titanium alloys such as the coarse grained IMI685 and the fine duplex structured IMI834. In such materials with a typical as received hydrogen concentration of 40–60 ppm the initiation site is invariably characterized by quasi-cleavage facetting. Similar facetting is also associated with the low temperature dwell sensitive fatigue response in the same alloys. For IMI685, it is reported that this failure mechanism is replaced by α/β interface cracking when the alloy contains a relatively high concentration of interstitial hydrogen. The present paper characterises the local grain orientation and microstructural conditions associated with these various forms of failure through the use of a microtextural analysis technique based upon electron back scattered diffraction (EBSD) measurements. The observations are related to an existing model to account for facet formation based upon the pile-up of dislocations at grain-boundaries. The implications for further use of this technique with titanium alloys are discussed.

Optimising mechanical properties in alpha + beta titanium alloys

The alpha/beta titanium alloys have played a major role in the development of gas turbine technology but it is doubtful that the growth in mechanical properties that contributed to their current dominance can be maintained into the next century. Future applications will require a more efficient utilisation of their strength and fatigue capability through a clearer understanding of deformation and crack development and by matching their inherent anisotropy with principal load direction. As a contribution to this process, the paper explores the factors involved in the formation of the quasi-cleavage facets that play such an important role in cyclic and time dependent fracture modes and examines influencing parameters such as stress redistribution, microstructure and notch geometry.

The effects of texture in titanium alloys for engineering components under fatigue

Abstract The mechanical response of textured Ti 6/4 plate material is assessed through an evaluation of monotonic properties under tension and torsion loading and fatigue testing of plain section and notched specimen geometries. Significant variations in modulus, yield strength, ultimate tensile strength and ductility are demonstrated for testpieces taken from the plate materials parallel to either the transverse or longitudinal rolling direction. Cyclic performance is also shown to be sensitive to orientation with different cyclic stress–strain curves defined in the two orientations. The relationship between the principal stress axis and the dominant basal plane texture is shown to control fatigue crack initiation lives and the ultimate mode of fracture. Whilst loading parallel to the transverse direction offers the strongest monotonic and cyclic stress–strain response, fatigue tests performed on specimens orientated parallel to the longitudinal rolling direction provide the optimum cyclic life. These effects are discussed with reference to the inherent, anisotropic mechanical response of α+β titanium alloys, which results from the hexagonal crystallographic form of the α phase and the availability of preferential slip systems. It is argued that the anisotropic response could be utilised to an engineering advantage by matching critical stressing directions to the specific properties offered by the texture.

The effects of environment and internal oxygen on fatigue crack propagation in Ti-6Al-4V

Abstract The role of environmental and internal oxygen on fatigue crack propagation is assessed for the titanium alloy Ti-6Al-4V at laboratory temperature. Growth rates for part through corner cracks are demonstrated to be highly dependent on the amount of oxygen present. Compared to data for conventional plate material at atmospheric conditions, tests conducted under a vacuum of 10 −6 torr illustrate a marked reduction in the rates of growth for the facet dominated phase at low values of Δ K. Intermediate vacuum levels and partial pressures of argon or hydrogen lead to growth rates that are similar to or slightly lower than the conventional atmospheric baseline. Significant growth rate accelerations are observed, however, for 6/4 material containing internal oxygen concentrations in excess of the standard 1700–2300 ppm. This acceleration is enhanced by the application of a high load ratio, R = 0.5. The implications with respect to current ideas on environmental interactions are discussed.

Microstructure and the development of fatigue cracks at notches

Abstract The paper focuses on the α/β titanium alloy, Ti–6Al–4V, in a range of microstructural conditions from extensive equiaxed alpha with retained beta or transformed beta through bimodal morphologies consisting of primary alpha and transformed beta to coarse fully transformed forms produced by heat treatment above the beta transus. The material is taken from several processing routes encompassing cross-rolled plate, hot-rolled bar stock and small disc forgings. The work brings together strain control low cycle fatigue behaviour obtained on plain testpieces and data obtained on a variety of notch geometries with K t factors in the range 1.4–4.7. A correlation technique based on the parameter K t Δ σ for notches and Δ σ for plain specimens, with Δ σ the appropriate stress range, is used to relate the datasets. Notch behaviour that is not consistent with this correlation is related to the microstructural condition, the cyclic stress–strain behaviour of that condition and the presence of ‘fatigue sensitive’ features. Finally, the titanium characteristics are compared with a competing particle reinforced aluminium based metal matrix system, AMC 225.

Environmental interactions with fatigue crack growth in alpha/beta titanium alloys

The impact of different environments on the growth of short through cracks (0.25 mm < a < 3 mm) in thin section DEN test pieces (t = 2 mm) is evaluated for the titanium alloy IMI318 (Ti-6Al-4V) at atmosphere. Both oxidising and reducing atmospheres have been considered in the form of air, chlorine and hydrogen gas at various levels of partial pressure. Data are presented for crack growth under cyclic and dwell waveforms and for R values of 0.01 and 0.625. It is demonstrated that the measured rates are extremely sensitive to the gaseous species. This sensitivity is reflected in fracture surface features and quasi-cleavage facetting in particular. The implications for engineering component life predictions are discussed.

Biaxial strength of advanced materials

The concentric ring test has been proposed as the preferred method for testing the flexural strength of bioceramics. If the apparatus used for such tests is proved to be reliable and accurate, it may be adapted to enable cyclic fatigue testing. However, before such adaptions are made, it is necessary to ensure the apparatus operates satisfactorily. The assessment programme involved strain gauge and finite element analyses of both an advanced titanium alloy (IMI 834) and 3 mol% yttria-stabilized zirconia disc specimens and preliminary fatigue tests on IMI 834 discs. The results of the analyses indicate that concentrically loaded discs are exposed to peak biaxial stresses on the tensile surface of the specimen and that these stresses are uniformly distributed within the peak loaded region of the disc. The fatigue response of IMI 834 discs under concentric loading was similar to that under uniaxial tension and torsional loading. The success of this programme will now enable fatigue tests on bioceramic discs to be performed with confidence in the reliability of the test apparatus.

Crack growth response of IMI 834 under variable amplitude loading

Abstract The paper addresses the application of constant load amplitude fatigue crack growth data in the prediction of crack development at stress concentrations subjected to variable amplitude loading. The variable amplitude cycles were applied in repeated blocks. Several loading forms were evaluated in which each block included a single major cycle and either 5, 20 or 100 minor cycles with amplitudes between 30 and 70% that for the major cycle. The relative position of the minor cycle is also varied. The basic behaviour of the IMI 834 alloy was measured on corner crack specimens at R values of 0, 0.3, 0.5 and 0.7. It is shown that the resultant growth rate response can be correlated either by the Walker relationship or by a closure correction approach. Closure values for the latter were measured by a D.C. potential drop monitoring system and illustrate a similar dependence on load ratio, R , as reported by Elber (with U = 0.72 + 0.1 R + 0.45 R 2 ). The closure corrected growth rate data follow a power law relationship but display two regimes of behaviour with an exponent of 1.9 at low stress intensity factors and 3.6 above a transition point. The closure corrected growth rates are used in a numerical model to predict crack development within two notch geometries. For a double edge notch configuration, ‘ a against N ’ response is predicted to a high level of accuracy for several distinct major/minor cyclic combinations. However, discrepancies are observed for a more highly stressed cylindrical notch specimen. These are attributed to geometry induced closure. and interactions from overload/underload excursions.