Fatigue microcrack growth in a nickel-base superalloy

The growth of very small fatigue microcracks was studied in a powder metallurgy nickel-base superalloy. A novel specimen containing a small crack was used, with crack growth rates being measured optically at high magnification. Interaction between the crack and the material microstructure was observed in a cyclic loading stage within a scanning electron microscope.It was found that microcracks grew initially at rates more rapid than those corresponding to conventional fracture mechanics (large crack) specimens. The rate undergoes a transient decrease with increasing crack length, dropping below the corresponding plot for large cracks, before beginning to increase in accordance with large crack results, ultimately merging with the latter. These results are discussed in terms of microstructure and crack growth mode, and the findings considered in light of the few studies of cyclic microcrack growth which have previously been correlated with fracture mechanics.     

Bifurcation of crack pattern in arrays of two-dimensional cracks

Theoretical calculations based on simple arrays of two-dimensional cracks demonstrate that bifurcation of crack growth patterns may exist. The approximation used involves the dipole asymptotic or pseudo-traction method to estimate the local stress intensity factor. This leads to a crack interaction parametrized by the crack length/spacing ratio =a/h. For parallel and edge crack arrays under far field tension, uniform crack growth patterns (all cracks having same size) yield to nonuniform crack growth patterns (bifurcation) if is larger than a critical value _cr. However, no such bifurcation is found for a collinear crack array under tension. For parallel and edge crack arrays, respectively, the value of _cr decreases monotonically from (2/9)^1/2 and (2/15.096)^1/2 for arrays of 2 cracks, to (2/3)^1/2/ and (2/5.032)^1/2/ for infinite arrays of cracks. The critical parameter _cr is calculated numerically for arrays of up to 100 cracks, whilst discrete Fourier transform is used to obtain _cr for infinite crack arrays. For infinite parallel crack arrays under uniaxial compression, a simple shear-induced tensile crack model is formulated and compared to the modified Griffith theory. Based upon the model, _cr can be evaluated numerically depending on (the frictional coefficient) and c ₀/a (c ₀ and a are the sizes of the shear crack and tensile crack, respectively). As an iterative method is used, no closed form solution is presented. However, the numerical calculations do indicate that _cr decreases with the increase of both and c ₀/a.     

Grain size effects on cyclic fatigue and crack-growth resistance behaviour of partially stabilized zirconia

Cyclic fatigue crack growth and crack-resistance behaviour was studied in partially stabilized zirconia (PSZ) with three different cubic-phase grain sizes following sub-eutectoid heat treatments. Raman spectroscopy was used to determine the extent of phase transformation around the cracks for both cyclic and monotonic loading conditions. All tests were on long, through thickness cracks using compact-tension specimens. Predictions of crack-tip shielding were made following determination of toughening parameters using crackresistance data. It was found that the dominant factors affecting cyclic fatigue-crack growth were the level of crack-tip shielding, as a result of phase transformation, and the intrinsic toughness of the material. Grain size did not appear to significantly affect fatigue crack-growth behaviour.     

Coexistence of Cubic and Tetragonal Structures in Yttria-Stabilized Zirconia Nanoparticles

Experimental evidence is presented for the coexistence of cubic and tetragonal phases in yttria partially stabilized zirconia nanoparticles (centaurs).     

Fatigue crack propagation behaviour of unidirectionally solidified γ/γ′-δ eutectic alloys

Fatigue crack propagation (FCP) studies were conducted on a series of/- (Ni-Nb-Al) alloys by subjecting them to cyclic four-point bending loads at room temperature. The aluminium contents of the alloys investigated ranged from 1.5% to 2.5% by weight, with the niobium contents adjusted to maintain controlled eutectic microstructures. In addition to studies of as-grown alloys, heat treatments were performed on several of the alloys to determine the effect of resultant changes in microstructure on FCP behaviour. Crack growth rates from approximately 2×10^–6 to 10^–3 mm/cycle were recorded as a function of the crack tip stress intensity factor range. The growth rates for the heat-treated alloys differed little from the as-grown FCP behaviour. Comparison with other published results indicated that the addition of aluminium was beneficial to FCP resistance, although the level of aluminium addition (within the investigated range of 1.5 to 2.5% by weight) did not influence the crack growth rates. Based on a comparison with previously reported results, chromium additions were seen to have a detrimental effect on FCP behaviour. Fractographic studies revealed the superior fatigue behaviour of the/- eutectic composite to be a result of repeated grain boundary delamination as the crack progressed through the microstructure.     

Microstructural and chemical stability of Y-ZrO2 reinforced β″-alumina in molten sodium sulfide and sulfur

The stability of -alumina reinforced with 10 vol% of tetragonal partially stabilized 3 mol% Y₂O₃-ZrO₂ (3Y-ZrO₂) and with 10 vol% of cubic 8 mol% Y₂O₃-ZrO₂ (8Y-ZrO₂) in molten sulfur or molten Na₂S₄ has been examined using scanning electron microscopy (SEM) X-ray diffraction (XRD) and electron probe microanalysis (EPMA) both before and after immersion at 350 °C. Tetragonal partially stabilized 3 mol % Y₂O₃-ZrO₂ was destabilized when reinforced into -alumina and immersed in molten Na₂S₄. Destabilization without incorporation into -alumina or using molten S as the immersion medium was minor. EPMA analyses indicated that the presence of -alumina enhanced zirconia destabilization in that -alumina can react with the molten corrodants to form corrosion products which are known corrosion agents for the leaching of Y₂O₃ from partially stabilized 3Y-ZrO₂. From XRD analyses, changing from partially stabilized 3Y-ZrO₂ to cubic 8Y-ZrO₂ in the composite increased resistance against phase destabilization. EPMA analyses revealed that the depletion was almost halted for cubic 8Y-ZrO₂ suggesting that the change in the zirconia phase used had reduced the chemical reactivity between Y₂O₃ and the corrodants. In order to avoid depletion destabilization of zirconia in -alumina, corrosion resistance can be increased by reducing chemical reactivity by using fully stabilizing zirconia. In addition, partially stabilized tetragonal zirconia may still be considered for use if a less reactive stabilizer such as CeO₂ is used.     

Yttria-stabilized hafnia-zirconia thermal barrier coatings: The influence of hafnia addition on TBC structure and high-temperature behaviour

The search for more reliable and durable thermal barrier systems is a key factor for future aircraft turbine engines success. Hafnia is therefore an attractive ceramic component due to its similarity to zirconia and its elevated structural transformation temperatures. We report here structural and thermomechanical investigations of various plasma-sprayed coatings composed of ZrO₂+x mol% HfO₂ (x=0, 25, 50 and 100), partially stabilized by 4.53 mol% yttria. X-ray diffraction studies show that, a metastable, non-transformable, high yttrium content, tetragonal solid solution is the only phase observed on the as-sprayed samples. This phase is crystallographically equivalent to the t phase described for classical yttrium-partially stabilized zirconia (Y-PSZ) thermal barrier coatings (TBCs). Upon high-temperature annealing in air (T=1200C), however, the return of this t phase to equilibrium differs from the classical tt+c reaction. According to literature data, reactions of the type tt+c+m should prevail at the highest hafnia contents (x50). Indeed, important quantities of monoclinic phase are accordingly being observed upon cooling. Thermal cycling of TBC samples in air has been performed at 1100C. The Young's modulus of the ceramic coating, which progressively increases when hafnia is substituted for zirconia, has a strong influence on TBC thermomechanical resistance.     

Fatigue crack growth of PMMA in organic agents

Crack growth tests under cyclic loading were executed at 295 K in various organic agents using compact tension and pure bending specimens of polymethylmethacrylate (PMMA). The cyclic frequencies f for the two kinds of test were 0.4 to 1 and 33 Hz, respectively. Two interesting features are pointed out: (i) transitional behaviour is observed on a crack growth rate against stress intensity factor range K diagram, and (ii) the fatigue fracture surfaces tested in highly viscous agents are covered with a new type of striation named wavy striation, as reported previously. The crack growth rate at the transition was analysed based on fluid flow through the pores within the craze forming at the crack tip. The wavy striation was also investigated by use of the theory of meniscus instability. It is found that both the phenomena may be well described by a parameter P = T(K) ²/f where T and are the surface tension and viscosity of the organic agents, respectively.     

The morphology of α′ martensite in a two-phase (α+γ) Fe-Cr-Ni stainless steel

Both lath-shaped and martensites are induced by tensile deformation within of a two-phase ( + ) Fe-Cr-Ni stainless steel, forms from the through the at an intersection of two crystals. These are observed both when is surrounded by and when borders . The amount of strain at which both and nucleate, increases with test temperatures in the range –196 to 50° C. Adjacent laths are either twin-related, or 5, 9, 15 or 19° off the twin relationship, as found by analysing electron diffraction patterns.     

Crack-growth phenomena in fatigued poly(methylmethacrylate)

The conditions under which crack-growth occurs from a natural crack induced in one side of a tensile specimen of poly(methylmethacryiate), when subject to a cyclic tensile load, at 20±3° C and 166 c/min, have been examined. The fracture surface can show three distinct regions, viz. a fissured rough growth area, a smooth growth area in which crack profiles extend across the width of the specimen, and a final area where catastrophic rupture has occurred. Under the conditions used, the fatigue life is mainly determined by the rate of growth of the crack through the rough-growth region. The number of cycles to failure has been determined for various values of the maximum bulk stress applied per cycle and of the initial crack length. The results appear to be consistent with a relationship put forward by Thomas, based on a tearing energy concept, which successfully predicts the dynamic cut-growth behaviour of certain natural and synthetic (SBR) gum rubbers. The conditions for the onset of catastrophic failure appear to be in accord with the Griffith criterion for brittle fracture.     

The effect of crystallization of the grain-boundary phase on ambient temperature fatigue short-crack growth behaviour in Y-α′-β′-Sialon

The cyclic fatigue short-crack growth behaviour of Y---Sialons with both crystallized and amorphous grain-boundary phases, were investigated to determine whether crystallization of grain boundaries affected crack-growth behaviour under cyclic and monotonic loads. Micromechanisms for fatigue-crack growth in Y---Sialon were examined by scanning electron microscopy and high-resolution electron microscopy. These results show that the wear debris on the fatigue fracture surfaces gave evidence of a frictional wear crack-growth mechanism. Comparison of fatigue short-crack growth rates for Sialon of crystallized grain-boundary phases with that for the amorphous grain-boundary phases indicated that crystallization of grain-boundary phases does not appear to affect cyclic fatigue growth behaviour, similar to long-crack growth behaviour. The similarity of fatigue short-crack growth behaviour in both the crystallized and amorphous grain boundaries sialons is rationalized in terms of the thin residual amorphous grain-boundary regions.     

R-curves of an yttria- and alumina-doped hot-pressed silicon nitride ceramic at 1200°C and room temperature

An energy approach has been utilized to measure theR-curves of an Y2O3~A3-doped hot-pressed silicon nitride ceramic at 1200C in an argon atmosphere in three-point bending. In order to evaluate theR-curves at 1200C, a low constant displacement rate of =5 m min^–1 was applied in cyclic loading to obtain the cyclic loading/ unloading-displacement curves during controlled-crack propagation. Propagated crack lengths were measured directly by a microscope and they were compared to compliance-calculated crack lengths. After digitizing the cyclic load-displacement and crack length-displacement curves, crack-resistance parameters,R-curves andK-curves, were calculated by computer. At 1200C this material behaved non-elastically and the crack parameters, obtained here, represent the non-elastic ones. For comparison, at room temperature, continuous loading was applied to obtain the load-displacement curves. At room temperature, linear-elastic fracture mechanics behaviour was observed.     

The role of silicon in the formation of the (Al5Cu6Mg2) σ phase in Al-Cu-Mg alloys

The role of silicon in the precipitation of the phase (Al₅Cu₆Mg₂) has been investigated through comparative studies on Al-3.63Cu-1.67Mg (wt%) and Al-3.63Cu-1.67Mg-0.5Si alloys. Both alloys were extensively examined after solution treating at 525°C for 2.5 h followed by ageing at 265°C for times up to 650 h. Limited studies were also undertaken on both alloys after ageing at 200 and 305°C. Precipitation of was observed in Al-3.62%Cu-1.66%Mg-0.5%Si for all ageing conditions studied but was absent in Si-free Al-3.62%Cu-1.66%Mg. In addition, S and phases were observed in both alloys. The volume fraction of phase in the Si containing alloy was substantially reduced by a pre-age stretch followed by ageing for 24 h at 265°C with S being the dominant precipitate type. The volume fraction of phase in the Si containing alloy was lower after ageing 24 h at 200°C than after 24 h at 265 and 305°C. Peak hardness was higher for the Si free alloy on ageing at 200 and 265°C, but the Si free alloy softened more rapidly, reflecting the more rapid coarsening kinetics of S compared with .     

Fatigue crack growth behaviour of tungsten carbide-cobalt hardmetals

Fatigue crack propagation studies have been carried out on a range of WC-Co hardmetals of varying cobalt content and grain size using a constant-stress intensity factor double torsion test specimen geometry. Results have confirmed the marked influence of mean stress (throughK _max), which is interpreted in terms of static modes of fracture occurring in conjunction with a true fatigue process, the existence of which can be rationalized through the absence of any frequency effect. Dramatic increases in fatigue crack growth rate are found asK _max approaches that value of stress intensity factor ( 0.9K_IC) for which static crack growth under monotonic load (or static fatigue) occurs in these materials. Lower crack growth rates, however, produce fractographic features indistinguishable from those resulting from fast fracture. These observations, and the important effect of increasing mean free path of the cobalt binder in reducing fatigue crack growth rate, can reasonably be explained through a consideration of the mechanism of fatigue crack advance through ligament rupture of the cobalt binder at the tip of a propagating crack.     

Reverse cyclic fatigue of a hot isostatically pressed silicon nitride at 1370 °C

The uniaxial, reverse cyclic fatigue performance of a commercially available hot isostatically pressed silicon nitride was examined at 1370 °C in air and with a 1 Hz sinusoidal waveform using button-head tensile specimens. Specimens did not fail in less than 10⁶ cycles when the applied stress amplitude was less than 280 MPa. Slow crack growth occurred at stress amplitudes 280 MPa and failure always occurred during the tensile stroke of the waveform. Multi-grain junction cavities resulted (i.e., the accumulation of net tensile creep strain) as a consequence of the reverse cyclic loading even though the specimens endured half their life under tensile stresses and the other half under compressive stresses. The presence of multi-grain junction cavities was a consequence of the stress exponent of tensile creep strain being greater than the stress exponent of compressive creep strain. Lastly, it was observed that the static creep resistance of this material improved when it was first subjected to reverse cyclic loading at 1370°C for at least 10⁶ cycles at 1 Hz. Silicon nitride grain coarsening (which was a consequence of the completion of the to silicon nitride solution/reprecipitation process that occurred during the history of the reverse cyclic loading) lessened the capacity for grain boundary sliding resulting in an improved static creep resistance.     

Microstructural development and mechanical properties of pressureless-sintered SiC with plate-like grains using Al2O3-Y2O3 additives

Dense SiC ceramics with plate-like grains were obtained by pressureless sintering using -SiC powder with the addition of 6 wt% Al₂O₃ and 4 wt% Y₂O₃. The relationships between sintering conditions, microstructural development, and mechanical properties for the obtained ceramics were established. During sintering of the -SiC powder compact the equiaxed grain structure gradually changed into the plate-like grain structure that is closely entangled and linked together through the grain growth associated with the phase transformation. With increasing holding time, the fraction of phase transformation, the grain size, and the aspect ratio of grains, increased. Fracture toughness increased from 4.5 MPa m^1/2 to 8.3 MPa m^1/2 with increasing size and aspect ratio of the grains. Crack deflection and crack bridging were considered to be the main operative mechanisms that led to improved fracture toughness.     

Determination of the unsafe fatigue crack growth rate in a stringer with a hole

The limitation on strength of a structural element with a hole from the edges of which without danger of brittle fracture a corner crack develops to a length specified in advance is obtained. A uniaxial tensile load changing cyclically with time acts on the structural element. Each of the cycles consists of five characteristic portions imitating a typical flight load which act on the upper portion of a passenger airplane fuselage. Small random white noise type loads are accumulated in the load of each portion.Translated from Problemy Prochnosti, No. 9, pp. 38–43, September, 1993.     

Crack growth in lamellar titanium aluminides containing beta phase precipitates

Mode-I fracture behavior of fully-lamellar polycrystalline -TiAl + ₂-Ti₃Al intermetallic alloys and the role of Ti-V base -phase precipitates of different thermodynamic stability have been studied using a finite element method. A rate-dependent, finite-strain, crystal-plasticity based materials constitutive model is used to represent the deformation behavior of both the -TiAl + ₂-Ti₃Al lamellar matrix and the -phase precipitates. Within the matrix colonies, fracture is assumed to take place throughout the ₂-Ti₃Al lamellae. In addition, fracture along colony boundaries and matrix/precipitate interfaces is considered. The constitutive behavior of all fracture interfaces is modeled using a cohesive-zone formulation. The analysis is carried out using the commercial finite element program Abaqus/Standard within which the material state is integrated using an Euler-backward implicit formulation. The results obtained show that the main mechanism of crack growth is nucleation of secondary cracks along ₂-Ti₃Al lamellae ahead of the main crack and their subsequent link-up with the tip of the main crack. The resulting fracture resistance curve acquires the characteristic step-wise shape. Both stable and metastable -phase precipitates are found to have a beneficial effect on the fracture resistance of the material. However, the effect is not very significant and metastable -phase precipitates appear to be a little bit more beneficial. All these findings are consistent with their experimental counterparts.     

Formation of metastable t′ -phase in solute-redistributed Y-PSZ

Owing to solute redistribution by a liquid phase analogue of diffusion-induced grain-boundary migration metastable and anti-phase domain boundary-bearing tetragonal (t) phase was seen to occur in the cubic (c) matrix grain of sintered (1400 °C, 2 h) and furnace-cooled yttriapartially stabilized zirconia (Y-PSZ) specimens (TZ3Y + 12Y-PSZ in molar ratios of 4:1, 1:1 and 14). In contrast to that formed by rapid cooling, t-phase formed by slow cooling shows no deformation accommodation twins. Subsequent ageing at 1100 °C for up to 240 h caused the formation of finely tweed (c) and tetragonal (t) assemblages at the expense of coarse tweed t-phase in the 12Y-PSZ grains. The absence of secondary deformation twins in the t-phase formed by slow cooling is discussed.     

Structure and Magnetism of the Composite Crystal Ca0.824CuO2

We have studied the magnetic state from a viewpoint of crystallographic features of the 1-D chain compound Ca_0.824CuO₂. A possible spin-hole arrangement in the magnetically coexisting state was determined by analyzing the local structural distortion in the CuO₂ chain by means of a modulated-crystal-structure analysis. The essential periodic sequence expected is (: up- and down-spin, : hole), which can be regarded as a kind of spin-1/2 ferromagnetic-antiferromagnetic alternating Heisenberg chain.