Influence of cyclic to mean load ratio on creep/fatigue crack growth

Crack growth data under combined creep and fatigue loading conditions are presented on a nickel base superalloy and a brittle and ductile low alloy steel. The main variables that have been examined are minimum to maximum load ratioR and frequency. It is shown that at high frequencies transgranular fatigue failure dominates and at low frequencies time dependent mechanisms govern. Where fatigue processes control, it is demonstrated that crack growth/cycle can be described by the Paris law and that the influence ofR ratio can be accounted for by crack closure caused by fracture surface roughness, oxidation, and creep and plastic strain developed at the crack tip. At the low frequencies where time dependent processes dominate, it is shown that crack growth can be characterized satisfactorily in terms of the creep fracture mechanics parameterC * using a model of crack extension based on ductility exhaustion in a creep damage zone at the crack tip. This model leads to enhanced resistance to creep/fatigue crack growth with increase in material creep ductility. This paper is based on a presentation made in the symposium “Crack Propagation under Creep and Creep-Fatigue” presented at the TMS/AIME fall meeting in Orlando, FL, in October 1986, under the auspices of the ASM Flow and Fracture Committee.     

Effect of carbon content and helium gas environment on creep crack growth properties of Ni-26 pct Cr-17 pct W-0.5 pct Mo alloy at 1273 K

Creep crack growth tests were conducted on Ni-26 pct Cr-17 pct W-0.5 pct Mo alloys with different carbon contents in air and in helium gas environment at 1273 K using the compact-type (CT) specimen, and the effects of carbon content and environment on creep crack growth rate are discussed. Creep crack growth rateda/dt is evaluated by theC* parameter. Theda/dt is faster in higher-carbon alloys than in lower-carbon alloys in each environment. This effect of carbon content is attributed to the lower creep ductility due to the increase of fine trans-granular carbides in higher-carbon alloys. The environmental effect on theda/dt vs C* relations is scarcely observed in higher-carbon alloys. In the 0.003 pct C alloy, however,da/dt is much lower in the He gas environment than in air. Carburization is observed ahead of the crack tip in the He gas environment at 1273 K. The intergranular carbides precipitated due to carburi-zation have a granular configuration and are considered to prevent the grain boundary sliding in lower-carbon alloys.     

An intergranular creep crack growth model based on grain boundary sliding

An intergranular crack growth model is developed to describe the effect of microstructural features such as grain size, grain boundary precipitates, and serrated grain boundaries on creep crack growth under grain boundary sliding (GBS) conditions. The model considers quantitatively that several deformation mechanisms contribute to the stress redistribution ahead of the crack tip through a stress relaxation process. The crack tip region is divided into three zones: (a) the intragranular-deformation-controlled stress relaxation zone, (b) the GBS-controlled stress relaxation zone, and (c) the elastic region. Intergranular creep crack growth is considered to occur as a result of the GBS-controlled process in all cases. The derived creep crack growth model shows a complex dependence of the creep crack growth rate (CCGR) on fracture mechanics quantities, such as C(t) (the path-independent energy integral with its steady-state value as C*) and K (the stress intensity factor). For creep-brittle materials, the model predicts that the CCGR depends on K to the power of 2 and this is verified experimentally; however, when environmental effects contribute to the crack growth process, the power exponent will increase. A semiempirical factor is introduced to account for the effects of oxidation on CCGR.     

Creep crack growth simulation under transient stress fields

The validity of the C -integral for correlation of creep crack growth under transient and steady-state stress fields has been investigated, using FEM analysis. In the steady-state regime, crack growth rates can be correlated withC*, however only for limited amounts of crack extension. When a crack grows in a transient regime no correlation of crack growth is found with any of the conventional crack tip parameters. For both regimes the most relevant parameter is theC-integral values obtained from the near-field region ahead of the crack tip. This paper is based on a presentation made in the symposium “Crack Propagation under Creep and Creep-Fatigue” presented at the TMS/AIME fall meeting in Orlando, FL, in October 1986, under the auspices of the ASM Flow and Fracture Committee.     

Tensile and creep properties of the experimental oxide dispersion strengthened iron-base sheet alloy MA-956E at 1365 K

A study of the 1365 K tensile properties, creep characteristics and residual room temperature properties after creep testing of the experimental oxide dispersion strengthened iron-base alloy MA-956E (Fe-20Cr-4.5Al-0.5Ti-0.5Y₂O₃) was conducted. The 1365 K tensile properties, particularly ductility, are strongly dependent on strain rate. It appears that MA-956E does not easily undergo slow plastic deformation. Rather than deform under creep loading conditions, the alloy apparently fails by a crack nucleation and growth mechanism. Fortunately, there appears to be a threshold stress below which crack nucleation andJor growth does not occur.     

Diffusion and deformation controlled creep crack growth

A mechanism for creep crack growth is proposed by which the crack grows by formation of grain boundary cavities ahead of the crack tip. Two cases are considered; firstly, when cavity growth is diffusion controlled and secondly, where growth is deformation controlled. The resultant crack growth rates predicted by these theoretical models are compared with experimental data.     

Crack growth in a nearly fully-lamellar gamma TiAl alloy at 650 °C and 800 °C under constant load conditions

The crack growth behavior of a gamma titanium aluminide alloy, K5S, was investigated at 650 °C and 800 °C under constant load conditions in a nearly fully-lamellar microstructural form. Crack growth at both temperatures ensues at stress intensities (K) much higher than anticipated from the R curves. At 650 °C, creep crack extension occurs through the formation of microcracks (interlamellar (IL) separation) and their joining to the main crack tip through ligament fracture. This results in a mainly transgranular (TG) fracture with occasional IL separation. This process features a rapid initial crack growth but at decreasing growth rate, followed by a nearly no-growth stage. At 800 °C, crack extension is accompanied by extensive plastic deformation and consists of an initial rapid transition period and a subsequent steady state. For similar K’s, crack extension and growth rate are greater at 800 °C than at 650 °C, but even these are very slow processes for this alloy. The resistance to crack propagation at 650 °C is explained in terms of work hardening that arises during the extended primary creep deformation occurring ahead of the crack tip. Increased crack propagation at 800 °C is accredited to grain boundary and lamellar-interface weakening and extensive post primary creep damage in the plastic zone. The resulting fracture at 800 °C is mainly boundary fracture, which consists of IG fracture involving formation and coalescence of voids, and IL separation.     

Study of creep crack growth in 2618 and 8009 aluminum alloys

Creep crack growth (CCG) has been investigated in an 8009 (Al-Fe-V-S) P/M alloy at 175 °, 250 °, and 316 ° and in a 2618 ingot alloy at 150 °, 175 °, and 200 °. Under sustained load, subcritical crack growth is observed at stress intensity levels lower thanK _ic ; for 2618, at 200 °, crack growth is observed at stress intensities more than 40 pct lower thanK _ic . Alloys 8009 and 2618 exhibit creep brittle behavior,i.e., very limited creep deformation, during CCG. The CCG rates do not correlate with CCG parameters C* and C but correlate with the stress intensity factor,K, and theJ integral. Generally, crack growth rates increase with increasing temperature. Micromechanisms of CCG have been studied with regard to microstructural deg-radation, environmental attack, and creep damage. Although theoretical estimation indicates that CCG resistance decreases with second-phase coarsening, such coarsening has not been observed at the crack tip. Also, no evidence is found for hydrogen- or oxygen-induced crack growth in comparing test results in moist air and in vacuum. Creep deformation and cavitation ahead of crack tip are responsible for observed time-dependent crack growth. Based on the cavitation damage in the elastic field, a micromechanical model is proposed which semiquantitatively explains the correlations between the creep crack growth rate and stress intensity factor,K.     

Simulations of stable crack propagation based on cavity growth by coupled diffusional and creep processes

A model based on cavity growth by coupled diffusional and creep processes has been developed to describe the stable crack growth behavior that has been observed experimentally in pure copper and Cu + 1 wt% Sb. Favorable comparisons of the predictions with experimental data for different possible stress distributions suggest that cavity growth and coalescence on transverse grain boundaries limit the rate of crack growth once cavitation has become extensive. These comparisons also indicate that stable crack growth can occur in the presence of a gradual, nonsingular stress distribution once a stable cavity size gradient exists ahead of the crack tip.     

On macroscopic and microscopic analyses for crack initiation and crack growth toughness in ductile alloys

Relationships between crack initiation and crack growth toughness are reviewed by examining the crack tip fields and microscopic (local) and macroscopic (continuum) fracture criteria for the onset and continued quasi-static extension of cracks in ductile materials. By comparison of the micromechanisms of crack initiationvia transgranular cleavage and crack initiation and subsequent growthvia microvoid coalescence, expressions are shown for the fracture toughness of materials in terms of microstructural parameters, including those deduced from fractographic measurements. In particular the distinction between the deformation fields directly ahead of stationary and nonstationary cracks are explored and used to explain why microstructure may have a more significant role in influencing the toughness of slowly growing, as opposed to initiating, cracks. Utilizing the exact asymptotic crack tip deformation fields recently presented by Rice and his co-workers for the nonstationary plane strain Mode I crack and evoking various microscopic failure criteria for such stable crack growth, a relationship between the tearing modulusT _R and the nondimensionalized crack initiation fracture toughnessJ _Ic is described and shown to yield a good fit to experimental toughness data for a wide range of steels.     

On macroscopic and microscopic analyses for crack initiation and crack growth toughness in ductile alloys

Relationships between crack initiation and crack growth toughness are reviewed by examining the crack tip fields and microscopic (local) and macroscopic (continuum) fracture criteria for the onset and continued quasi-static extension of cracks in ductile materials. By comparison of the micromechanisms of crack initiationvia transgranular cleavage and crack initiation and subsequent growthvia microvoid coalescence, expressions are shown for the fracture toughness of materials in terms of microstructural parameters, including those deduced from fractographic measurements. In particular the distinction between the deformation fields directly ahead of stationary and nonstationary cracks are explored and used to explain why microstructure may have a more significant role in influencing the toughness of slowly growing, as opposed to initiating, cracks. Utilizing the exact asymptotic crack tip deformation fields recently presented by Rice and his co-workers for the nonstationary plane strain Mode I crack and evoking various microscopic failure criteria for such stable crack growth, a relationship between the tearing modulusT _R and the nondimensionalized crack initiation fracture toughnessJ _Ic is described and shown to yield a good fit to experimental toughness data for a wide range of steels. An erratum to this article is available at .     

Characteristics of crack tip deformation during high temperature straining of austenitic steels

Deformation behavior, local distribution of mechanical properties and the microstructure near crack tips originating from notches were studied in 316 stainless steel and in Incoloy 800 H, which had been deformed in tension at 923 K at a strain rate of 1.3 × 10⁻⁶ s⁻¹. Local strain measurement showed that strain is concentrated in a rather large region ahead of the crack, several 100 μm in size; here, a multiaxial stress condition is established and severe cavitation is observed. By means of micro-hardness measurements, work-hardening in this region was found to resemble that which is obtained in the steady state for a uniaxial stress state at a 10 times higher strain rate. The average stress concentration in the crack tip area may be estimated to amount to 30%. It is proposed that a prerequisite for crack growth is the establishement of a critical state with respect to both the creep damage (cavitation) and the degree of work-hardening ahead of the crack. Differences in the crack growth behavior in notched and unnotched specimens are ascribed to whether the critical state is established locally ahead of the crack or homogeneously within the specimen.     

Theoretical model for FCGR near the threshold

A theoretical model for fatigue crack growth rate at low and near threshold stress intensity factor is developed. The crack tip is assumed to be a semicircular notch of radius ρ and incremental crack growth occurs along a distance 4ρ ahead of the crack tip. After analysis of the stress and strain distribution ahead of the crack tip, a relationship between the strain range and the stress intensity range is proposed. It is then assumed that Manson-Coffin cumulative rule can be applied to a region of length 4ρ from the crack tip, where strain reversal occurs. Finally, a theoretical equation giving the fatigue crack growth rate is obtained and applied to several materials (316L stainless steel, 300M alloy steel, 70-30 α brass, 2618A and 7025 aluminum alloys). It is found that the model can be used to correlate fatigue crack growth rates with the mechanical properties of the materials, and to determine the threshold stress intensity factor, once the crack tip radius α is obtained from the previous data.     

Time-Dependent Crack Growth Thresholds of Ni-Base Superalloys

A micromechanical model has been developed for predicting the time-dependent crack growth threshold and its variability by considering oxide formation or cavity formation ahead of an elastic crack subjected to a sustained load at a stress intensity factor, K, at elevated temperatures in air. It is demonstrated that stress relaxation associated with a volume-expansion process such as the formation of creep cavities or oxides with a positive transformation strain can induce residual stresses at the tip of the elastic crack. The near-tip residual stresses must be overcome by the external load, thereby instigating a growth threshold, K _th, for the onset of time-dependent crack growth. This micromechanical framework provides the basis for developing appropriate predictive models for the time-dependent crack growth thresholds associated with several damage processes, including (1) oxidation-assisted intergranular crack growth, (2) K-controlled creep crack growth along an intergranular path, and (3) stress corrosion cracking. The micromechanical threshold models have been utilized to predict the time-dependent crack growth thresholds of a variety of Ni-base superalloys. The material parameters that contribute to the variability of the time-dependent crack growth thresholds have been identified and related to variations of mixed oxides or creep cavities formed near the crack tip. A size scale effect is also predicted for the transformation toughening phenomenon, which is largest at or below K _th but diminishes at increasing K levels above the threshold. Finally, the micromechanical models are utilized to identify means for suppressing time-dependent crack growth in Ni-base alloys.     

Elevated temperature creep-fatigue crack propagation in nickel-base alloys and 1 Cr-Mo-V steel

The crack growth behavior of several high temperature nickel-base alloys, under cyclic and static loading, is studied and reviewed. In the oxide dispersion strengthened (ODS) MA 6000 and MA 754 alloys, the high temperature crack propagation exhibited orientation dependence under cyclic as well as under static loading. The creep crack growth (CCG) behavior of cast nickel-base IN-738 and IN-939* superalloys at 850 °C could be characterized by the stress intensity factor,K ₁. In the case of the alloy IN-901 at 500 °C and 600 °C,K ₁ was found to be the relevant parameter to characterize the creep crack growth behavior. The energy rate line integral,C*, may be the appropriate loading parameter to describe the creep crack growth behavior of the nickel-iron base IN-800H alloy at 800 °C. The creep crack growth data of 1 Cr-Mo-V steel, with bainitic microstructure, at 550 °C could be correlated better by C * than byK ₁. This paper is based on a presentation made in the symposium “Crack Propagation under Creep and Creep-Fatigue” presented at the TMS/AIME fall meeting in Orlando, FL, in October 1986, under the auspices of the ASM Flow and Fracture Committee.     

Creep-fatigue life prediction in terms of nucleation and growth of fatigue crack and creep cavities

In low cycle fatigue at elevated temperature, the interaction between fatigue crack and creep damages is known to be responsible for the significant reduction of the fatigue life. In this investigation, a model for the life prediction for low cycle fatigue with hold time at tensile peak strain is suggested for the temperature range of 0.5T _m. This model is formulated on the basis of the assumptions that the creep cavities are formed due to the vacancies generated during fatigue, and are grown during the hold period. The fatigue crack nucleated at the surface due to fatigue loading is affected by the creep damages for its propagation. The model is checked by experimental results with various hold time periods. The predicted creep-fatigue lives are in good agreement with experimentally observed ones for 304 stainless steel and 13CrMo44 steel. Formerly Graduat3e student, Department of Materials Science and Engeneering ,KAIST, Seoul, Korea.     

Surface Crack Growth Behavior of Pipeline Steel Under Disbonded Coating at Free Corrosion Potential in Near-Neutral pH Soil Environments

Crack growth behavior of X65 pipeline steel at free corrosion potential in near-neutral pH soil environment under a CO₂ concentration gradient inside a disbonded coating was studied. Growth rates were found to be highest at the open mouth of the simulated disbondment where CO₂ concentrations, hence local hydrogen concentration in the local environment, was highest. Careful analysis of growth rate data using a corrosion-fatigue model of the form ΔK ^α /K _max ^β /f ^γ , where (1/f ^γ ) models environmental contribution to growth, revealed that environmental contribution could vary by up to a factor of three. Such intense environmental contribution at the open mouth kept the crack tip atomically sharp despite the simultaneous occurrence of low-temperature creep and crack tip dissolution, which are the factors that blunt the crack tip. At other locations where environmental enhancement was lower, significant crack tip blunting attributed to both low-temperature creep and crack tip dissolution was observed. These factors both led to lower crack growth rates away from the open mouth.     

A theoretical model for creep crack growth

A theory of creep crack growth has been developed with the presumption that the crack growth occurs by the diffusion of vacancies along the grain boundaries. This is consistent with many experimental results that show that creep fracture is generally of intergranular type and the activation energies for crack growth rates fall within the range of grain boundary diffusion energies. The theory is based on the concept that creep crack growth results from a balance of two competing processes-the diffusion of point defects that contributes to the growth and the creep deformation process that retards the growth and causes even its arrest. The present analysis shows that crack growth via grain boundary diffusion occurs within some temperature range. The upper limiting temperature is determined by the bulk diffusion process which disperses the vacancies, that are diffusing to the crack tip, to the plastic zone ahead of the crack front. The lower temperature limit is set by the fact that the grain boundary diffusion rates decrease with the decrease in temperature and thus large stress intensities approaching the fracture toughness value are required to accomplish crack growth by the grain boundary diffusion. Outside these limits creep crack growth occurs via deformation which is significantly slower than growth by the grain boundary diffusion process. The importance of the present analysis rests on the fact that service conditions for many high temperature structural materials fall within the regime wherein creep crack growth occurs via grain boundary diffusion.     

A transgranular fatigue crack growth model based on restricted slip reversibility

This article presents a transgranular fatigue crack growth model based on a restricted slip reversal process where the transgranular crack growth rate is related to the cyclic plastic strain range ahead of the crack tip. Upon applying deformation and fracture kinetics theories, a physically based constitutive law for fatigue crack growth rate is derived. In the absence of any environmental contributions to crack growth, the model takes the form of the Paris equation with a power law exponent of 3 at positiveR values. The model expresses the fatigue crack growth rate explicitly in terms of material properties, such as yield strength, work-hardening coefficient, microstructural quantities such as activation energy, activation volume, and work factor, as well as test constraints such asΔK andR. The absence of a fatigue threshold is predicted for test conditions where environment does not influence the crack growth process and the material microstructure remains stable. Formerly Graduate Student, Department of Mechanical Engineering, University of Ottawa.