FATIGUE CRACK GROWTH IN NICKEL-BASED SUPERALLOYS AT 500-700°C. I: WASPALOY

Abstract— The effects of cyclic frequency, hold time, and stress-intensity-factor range (δ K ) on rates of fatigue crack growth in air at 500-700°C have been studied for Waspaloy—a nickel-based superalloy used for gas-turbine engine discs. The main effects observed were: (i) higher rates of crack growth for lower cyclic frequencies at high δ K at 600 and 700°C. and (ii) lower rates of crack growth at low δ K (and higher δ K thresholds) for longer hold times at 700°C, compared with those at a baseline frequency of 2 Hz. Metallographic and fractographic observations suggested that the effects of cyclic frequency and hold time could be rationalised in terms of the competing effects of enhancement of cracking due to creep and inhibition of cracking caused by oxide-induced crack closure, fracture-surface-roughness induced crack closure, and crack-branching/deflection. Possible mechanisms for promoting intergranular and transgranular cracking at low cyclic frequencies or long hold times are discussed.     

CREEP CRACK INITIATION AND GROWTH IN INCONEL 718 ALLOY AT 650°C

Creep cracking tests were performed on a nickel base superalloy, Inconel 718, using CT type specimens at 650°C in an air environment. Both time to initiate creep crack propagation (T¹) and creep crack growth rate (?) were measured as a function of the stress-intensity factor, K. It is shown that crack initiation behaviour and early crack growth (stage I) are strongly dependent on test procedures. Furthermore there is no unique correlation between ? and K, except in the regime corresponding to relatively high crack growth rates (stage II). This conclusion was reached by conducting tests both under increasing and under decreasing K. The reasons for this behaviour are briefly discussed in relation to environmental effects. A simple model (based upon a theoretical stress-strain field for creeping cracked solids and the ductility exhaustion concept) is used in an attempt to explain the results. The limitations for the application of this model are discussed. It is shown that this approach can explain a number of experimental results provided that it is accepted that creep ductility is time-dependent because of environmental effects. Finally, the results of these crack propagation tests are used in conjunction with the theoretical results derived from the Riedel and Rice analysis to model both crack initiation and crack growth behaviour of Inconel 718 under sustained load.     

OXIDATION-ASSISTED FATIGUE CRACK GROWTH BEHAVIOR IN ALLOY 718-PART II. APPLICATIONS

In previous work by the authors, a quantitative model based on a two-stage oxidation mechanism was developed to describe the oxidation-assisted crack growth behavior in alloy 718. This model is used here to predict the crack growth rate in this alloy at 650°C for two different loading conditions: one is a continuous cycle with a hold time duration of 300 s imposed at minimum load, the other is a continuous cycle without a hold time duration. The results obtained from applying the model to these loading profiles were then compared with those obtained experimentally. Good agreement was observed between the two data sets. Details of the model calculations are discussed, and suggestions to further extend the model capabilities are made in this paper.     

OXIDATION-ASSISTED FATIGUE CRACK GROWTH BEHAVIOR IN ALLOY 718-PART I. QUANTITATIVE MODELING

This paper presents a new model to predict the high temperature, intergranular crack growth rate behavior in alloy 718 at 650°C. The model is based on the concept that the crack tip oxidation-fatigue damage is a nonlinear process governed by the chemical-mechanical interaction along the grain boundary fracture path at the crack tip. A concept of two-stage oxidation mechanism was used here. This mechanism depends on the rate of formation of the chromia layer in relation to the build-up of other oxide types at the crack tip. The saturation of the Cr₂O₃ build-up signifies the occurrence of the oxide passivation effect. The determination of the amount of Cr₂O₃ depends on the amount of both oxygen diffused along the affected grain boundary and chromium transported via a mobile dislocation network. Here, both the grain boundary and effective dislocation pipe are treated as short-circuit diffusion paths along which the diffusion process can be described using Whipple's solution. The model yields sufficient information to correlate the amount of Cr₂O₃ with the reduction in the grain boundary ductility within the affected oxide zone. The grain boundary ductility is balanced by the effective strain at the crack tip resulting from the external loading. This balance defines the fracture criterion of the model and permits the calculation of the crack advance per cycle which is the ultimate goal of this work.     

ELEVATED TEMPERATURE FATIGUE CRACK GROWTH IN ALLOY 718—PART II: EFFECTS OF ENVIRONMENTAL AND MATERIAL VARIABLES

Observations concerning the effects of the environment and material variables on the crack growth process in alloy 718 are reviewed and analyzed on the basis of deformation characteristics in the crack tip region. The review of the role of material variables has focused on the effects of chemical composition and microstructure parameters including precipitate size and morphology as well as grain size and morphology. These analyses have suggested that the governing mechanism at the crack tip is the degree of homogeneity of plastic deformation and associated slip density. For conditions promoting homogeneous plastic deformation, with a high degree of slip density, the environmental damage contribution is shown to be limited, thus permitting the dominance of cyclic damage effects which are characterized by a transgranular crack growth mode and a lower crack growth rate. Under conditions leading to inhomogeneous plastic deformation and lower slip density the crack tip damage is described in terms of grain boundary oxidation and related intergranular fracture mode. Considering that the crack growth damage mechanism in alloy 718 ranges from fully cycle dependent to fully environment dependent, conflicting experimental observations under different operating conditions are examined and a sensitizing approach is suggested to increase the alloy resistance to environmental effects.     

ELEVATED TEMPERATURE FATIGUE CRACK GROWTH IN ALLOY 718—PART I: EFFECTS OF MECHANICAL VARIABLES

Abstract— In this paper observations concerning the effects of mechanical variables on the crack growth process in alloy 718 are reviewed and analyzed on the basis of the related deformation characteristics in the crack tip region. The variables included temperature, frequency, wave shape, hold time, load ratio and load interaction. These analyses have suggested that the role of each parameter in the acceleration of crack tip damage is governed mainly by their relative influence on the nature of the corresponding plastic deformation and associated slip line density. On the basis of this view (which assumes crack growth damage covers the range from cyclic- to fully time-dependent processes), the interactive effects of loading parameters are discussed when considering the corresponding fracture mode. Conflicting experimental observations under different operating conditions are examined.     

FATIGUE CRACK GROWTH IN A SINTERED TUNGSTEN ALLOY

Abstract— Room temperature fatigue crack propagation in a sintered tungsten alloy was studied. The fatigue crack growth rates were found to be identical for the material in the sintered and forged and as sintered conditions. The propagation rates are slower when compared with other metals due to the relatively high Young's modulus of tungsten. The value of the exponent m in Paris' power law equation was found to be 12 which is higher than for most metals. This was ascribed to the activity of a cleavage mechanism through some of the tungsten grains along with the ductile decohesion fatigue mechanism.     

FATIGUE CRACK GROWTH RATE BEHAVIOUR OF STAINLESS STEEL CLADDINGS IN AIR AND UNDER VACUUM AT 300°C

CT type specimens containing two layers of 309L and 308L cladding stainless steels welded to A508 carbon steel and 316 stainless steel were specially devised to test the influence of R ratio and environment on the crack propagation rate behaviour of cladding materials at 300°C. Large effects are shown. The crack growth rate under vacuum can be smaller by more than one order of magnitude as compared to air. Large differences are also shown on the fracture surfaces, where it is observed than vacuum promotes the formation of large crystallographic {111} facets. The effect of environment is briefly discussed on the basis of existing gas adsorption models.     

EFFECT OF CYCLE MEAN STRAIN ON SMALL CRACK GROWTH IN ALLOY 718 AT ELEVATED TEMPERATURES

The influence of the cyclic compressive excursion on the fatigue crack growth behavior of small surface cracks in Alloy 718 at 650°C is experimentally studied. Test conditions were chosen to simulate the cyclic plasticity found at notch locations in high temperature structural components. During cycling, the crack lengths were continuously monitored using the direct current potential drop method while the near field crack mouth opening displacement and global cyclic stressstrain behavior were measured using a laser interferometry technique and mechanical extensometry, respectively. Two aspects related to cyclic compressive excursion have been studied; crack closure and crack tip plasticity. Attempts have been made to use several modified ΔK expressions as well as ΔJ_eff to account for the effects of closure and decrease crack tip plasticity. It was concluded that the compressive excursion is most prominent in this alloy in its effect on the global plasticity and the subsequent loss of constraint. Closure was not found to be significant in the consolidation of test data.     

FATIGUE CRACK GROWTH BEHAVIOUR OF A ZINC-ALUMINIUM ALLOY AT HIGH HOMOLOGOUS TEMPERATURES

Abstract— The fatigue crack growth behaviour and crack closure response of a zinc base die casting alloy at high homologous temperature were studied. The crack growth rate was both frequency and temperature dependent. The frequency dependence of crack growth rate, which has been commonly attributed to creep-fatigue interaction, can be rationalized by the crack closure phenomenon. The temperature dependence is contrary to that observed in other materials and cannot be simply explained in terms of the interaction between creep and fatigue damage. The effect of a single tensile overload on the crack growth behaviour at high homologous temperatures has also been investigated.     

FATIGUE CRACK GROWTH IN TWO ADVANCED TITANIUM ALLOYS UNDER CONSTANT AND VARIABLE AMPLITUDE LOADING AT 25 AND 175 °C

Fatigue crack growth of β-21S and Ti-62222 in sheet form was investigated under constant and miniTWIST flight spectra loading conditions at 25 and 175 °C. Variable amplitude results were compared with life calculations performed using NASA/FLAGRO software and constant amplitude fatigue crack growth results. Single tensile overloads under constant Δ K were performed to evaluate load interaction effects. Constant amplitude results showed that fatigue crack growth resistance was slightly better for Ti-62222 than β-21S at 25 and 175 °C. The presence of crack closure under various conditions caused moderate shifts in the fatigue crack growth data. Under miniTWIST flight spectra loading, Ti-62222 exhibited a greater extension in life in comparison to the β-21S at elevated temperature, consistent with the NASA/FLAGRO calculations. This was also consistent with the single tensile overloads where 25 °C tests were comparable for both materials, while at 175 °C, delay cycles were greater by a factor of almost three for Ti-62222. Extensive secondary cracking in Ti-62222 at elevated temperature accounted for the extended fatigue lives.     

A SIMULATION OF THE FATIGUE CRACK PROCESS IN TYPE 304 STAINLESS STEEL AT 538°C

Abstract— Quantitative information, such as the initiation period, growth and coalescence behaviour, statistical distributions of crack length, density of cracks, distribution patterns and crack growth properties, were obtained from fatigue tests on type 304 stainless steel at 538°C in a previous study. Using this information as input data and a condition for the connection and interactions of cracks, a statistical simulation and life prediction due to the high temperature fatigue crack growth process was performed and illustrated as output on a two-dimensional graphical display.     

FATIGUE, CREEP AND CREEP/FATIGUE BEHAVIOUR OF A NICKEL BASE SUPERALLOY AT 700°C

Abstract— This paper presents the results of an experimental testing programme to examine the uniaxial creep, low cycle fatigue and creep/fatigue interaction behaviour of a Ni-base superalloy at 700°C. The material is used in the manufacture of aeroengine turbine discs. A creep continuum damage mechanics model is shown to be capable of accurately predicting the creep and creep rupture behaviour of the material. A healing term has been incorporated into the damage mechanics model to allow the behaviour under creep/fatigue conditions to be described.     

CREEP CRACK GROWTH MICROMECHANISMS AT 405°C IN COLD BENT TUBES OF A C-Mn-Mo STEEL

Abstract— Creep Crack Growth (CCG) tests were performed at 405°C on specimens cut out of the cold bent extrados of five tubes of a C-Mn-Mo steel. Intergranular fracture and grain boundary cavitation was less in the C-Mn-Mo than in the C-Mn steels, in accordance with better CCG resistance of the former material. The dimensions and hardness variation across the crack tip process zone were measured by microhardness profiles performed on metallographic sections of the broken samples. TEM analysis of the dislocation patterns close to the fracture surface confirmed the presence of temperature- and stress-induced plasticity phenomena. A significant enrichment of N at grain boundaries (GB) inside the process zone was detected by Auger spectroscopy; N not only inhibits dislocation motion and stress field relaxation at the crack tip but also causes a decrease in GB cohesion ahead of the crack tip. These results help in understanding the micromechanisms which reduce the creep ductility of C-Mn-Mo and C-Mn cold bent tubes and the role of chemical composition in improving CCG resistance.     

CRACK CLOSURE EFFECT ON CRACK GROWTH RATE AT 650°C IN DOUBLE NOTCHED SPECIMENS OF A NICKEL BASE SUPERALLOY

Abstract— Creep-fatigue tests were performed at 650°C in air on a N18 nickel base superalloy, using double notched and smooth specimens. The deformation mechanisms observed by TEM at the notch root are shown to be compatible with the constitutive set of equations used in the finite element analysis which is presented. For a given K _max at the notch root, the crack growth rate is much higher in a notched specimen than in a smooth one. This effect can be explained by a variation of the crack closure stress level with the local R ratio and the local stress. A strong accelerating effect of the R ratio, especially for negative values, is found in smooth specimens. Introducing a K _op correction in the experimental results leads to a good agreement between the measured crack growth rate plotted versus K _eff in notched and smooth samples.     

SHORT AND LONG FATIGUE CRACK GROWTH IN A SiC REINFORCED ALUMINIUM ALLOY

Fatigue crack growth behaviour in a 15 wt% SiC particulate reinforced 6061 aluminium alloy has been examined using pre-cracked specimens. Crack initiation and early growth of fatigue cracks in smooth specimens has also been investigated using the technique of periodic replication. The composite contained a bimodal distribution of SiC particle sizes, and detailed attention was paid to interactions between the SiC particles and the growing fatigue-crack tip. At low stress intensity levels, the proportion of coarse SiC particles on the fatigue surfaces was much smaller than that on the metallographic sections, indicating that the fatigue crack tends to run through the matrix avoiding SiC particles. As the stress intensity level increases, the SiC particles ahead of the growing fatigue crack tip are fractured and the fatigue crack then links the fractured particles. The contribution of this monotonic fracture mode resulted in a higher growth rate for the composite than for the unreinforced alloy. An increase in the proportion of cracked, coarse SiC particles on the fatigue surface was observed for specimens tested at a higher stress ratio.     

EFFECTS OF AIR AND INERT ENVIRONMENTS ON THE NEAR THRESHOLD FATIGUE CRACK GROWTH BEHAVIOR OF ALLOY 718

The near threshold fatigue crack growth behavior of alloy 718 was studied in air and helium environments at room temperature and at 538°C. Tests were performed at 100 Hz and at load ratios of 0.1 and 0.5. At room temperature and at 538°C, the ΔK_th values in helium were lower than in air. The ΔK_th values in air decreased with increasing load ratio. These results can be explained with a model that involves the accumulation of oxide in the crack which enhances crack closure. In the air tests, the oxide build-up on the fatigue fracture surfaces at ΔK_th was of the order of magnitude as the crack tip opening displacement. In the helium tests, no significant build-up of oxide on the fracture surface at threshold was found.     

AN INVESTIGATION OF FATIGUE CRACK GROWTH IN A DUCTILE MATERIAL AT HIGH GROWTH RATES

Abstract— Investigations into tearing-fatigue have been performed using three point bend specimens made of mild steel. A computer controlled testing machine was used which could maintain constant cyclic displacement ranges, or constant cyclic energy input ranges, and hence provide a range of crack tip driving force conditions. Fatigue crack growth rates were measured and compared with the predictions of a model based on the linear summation and non-interaction of fatigue and ductile tearing growth rates. The effects of fatigue crack growth on monotonic crack growth resistance properties were investigated. It is concluded that there is no significant interaction between these two crack propagation mechanisms in this steel. Crack growth rate equations governing propagation in the tearing-fatigue regime are given.     

THE EFFECT OF HYDROGEN ON SHORT FATIGUE CRACK GROWTH IN AN Al-Li ALLOY

Abstract— Experimental results are presented to illustrate the degree of susceptibility to hydrogen embrittlement in an Al-Li 8090 alloy. The ductility of the alloy (as recorded in slow strain rate tests) is reduced by hydrogen charging which induces microcrack formation on the surface of the hydrogen treated specimens. Results of fatigue tests show that small fatigue cracks propagate 1.5 to 10 times faster in specimens charged with hydrogen and fractographic evidence shows that short crack growth in hydrogen precharged specimens is transgranular, along persistent slip bands. The effect of hydrogen is reversible, the embrittlement effect being eliminated by holding the specimens for 24 hr at 470°C.     

SHORT FATIGUE CRACK GROWTH BEHAVIOUR IN WASPALOY AT ROOM AND ELEVATED TEMPERATURES

Short fatigue crack growth tests have been carried out at room and elevated temperatures using the nickel-based superalloy known as Waspaloy. A fully automated computer controlled system has been developed and employed for controlling the testing and monitoring of the growth of freely initiated surface short cracks on smooth specimens. Surface cracks as small as 10 um in length have been detected and recorded at temperatures up to 700°C. Anomalous short crack propagation behaviour was observed when comparisons were made with the corresponding long crack behaviour. Some aspects of mechanical, microstructural and environmental effects on the short fatigue crack growth behaviour of the material are discussed.