Experimental observation of crack propagation in 6063-T6 al-alloy under constant amplitude loading

Crack propagation experiments were performed on 6063-T6 Al-alloy for various load ranges and stress ratios. Experimental results show that for a constant load range, the life of specimens decreased as stress ratio increased. At constant maximum load, the life of the specimen increased as the load ratio increased. The crack growth data were analysed in terms of ΔK_eff as a function of stress ratio R. The data covered R values from 0 to 0·5 and a good relationship was obtained for K_eff/K = 0·55 + 0·12 R². A crack growth rate equation was developed.     

Effect of temperature on the threshold fatigue crack growth behaviour of spheroidal graphite cast iron

The influence of temperature at various R ratio values on the fatigue crack propagation response of a ferritic spheroidal graphitic cast iron has been studied. It has been established that the influence of R ratio on ΔK_th is strongly dependent upon the test temperature. At elevated temperatures the influence of R ratio is significantly less than that at ambient temperature. At low to intermediate R ratio values temperature initially decreased, then, with increasing temperature, increased the ΔK_th levels, causing a minimum in ΔK_th to occur at 250–300°C. At high R ratio, however, ΔK_th exhibited a small but consistent increase with temperature. The influence of temperature on the ΔK_th at various R ratio values could be adequately explained in terms of crack closure. Much intergranular failure was observed on the fatigue fracture surfaces at ambient temperature, whereas at elevated temperatures there was little evidence of this particular failure.     

A study of crack closure under constant amplitude loading for 6063-T6 Al-alloy

Crack closure experiments were performed on 6063-T6 Al-alloy, using a COD gauge for various load ranges (Δp) and stress ratios, R. On the basis of the experimental results a model for effective stress intensity range ratio U was developed. This model was found to be a function of the stress ratio, R, and was fitted to existing constant amplitude crack propagation data for 6063-T6 Al-alloy. The crack closure load stabilized after 1 mm initial crack growth.     

The influence of mean stress or R-Ratio on the fatigue crack threshold characteristics of steels—A review

The present review attempts to describe an assessment of the effects of the R-Ratio on ΔK_th values in steels and compares the predictions of the various approaches or models dealing with R-Ratio response, with experimentally determined threshold stress intensity range values. Essentially significant R-Ratio effects on ΔK_th have been observed and that the extent of such effects are highly dependent on microstructural considerations. The two classical approaches dealing with R-Ratio effects, viz., the Barsom and the Klesnil and Lukas models, can adequately describe the R-Ratio response when the data are considered in terms of microstructural characteristics. However, the other approaches of Mazumdar and Conrad and McEvily and Groeger fall short in predictive terms while the Musuva and Radon approach needs more detailed high R-Ratio ΔK_th data before it can be realistically assessed.     

Effect of stress ratio on effective stress range ratio and crack growth in 6061-T6 Al-alloy

Crack closure experiments using a COD gauge were performed on 6061-T6 Al-alloy, for various stress ratios. Experimental results show that for a given stress ratio, R, the crack length increases up to 1 mm after which it is stabilized. On the basis of the experimental results a model for effective stress range ratio U, which was found to be a function of stress ratio R, was developed. This model was fitted to existing constant amplitude crack propagation data for the 6061-T6 Al-alloy.     

Influence of carbon content and tempering temperature on fatigue threshold characteristics of a low alloy steel

This paper attempts to describe the effect of carbon content on the fatigue threshold characteristics ΔK_th in various heat treated conditions. Essentially it has been shown that a tempering treatment increased ΔK_th while increasing the carbon content of steels from 0·13% to 0·8% significantly decreased the ΔK_th value by over 100%. At intermediate fatigue crack growth rates all the data show a linear relationship with ΔK level.

In terms of yield strength σ_y, the threshold stress intensity level could be given by the expression: ΔK_th = 8·74 − 3·42 × 10⁻³(σ_y).

At near threshold fatigue crack growth levels significant amounts of isolated intergranular failure were observed in the 400°C tempered condition. In the other heat treated microstructures only a flat trans-granular ductile striated failure mode was evident. A maxima in the amount of intergranular facets occurred at ΔK values approaching 15 to 20 MPa√m. It has been shown the existence of intergranular failure resulted from environmentally induced fracture (through the diffusion of hydrogen) that occurred within the crack tip enclave.     

Crack propagation rate as a function of crack tip characteristics

The dependence of crack growth rate on various crack tip parameters was studied. Experiments were performed on thin sheets of 6063-T6 Al-alloy having a central notch, to find crack tip opening displacement, total strain range, plastic strain range, crack opening stress and crack growth rate. Crack tip opening displacement and crack opening stress were measured, using a surface measurement technique, with small crack opening displacement gauges. The theoretical predictions of crack tip opening displacement compare fairly well with the experimental values. It is found that crack propagation rate vs total strain range-plastic strain range gives a straight-line fit on a log-log graph and, for positive stress ratios, the fatigue crack growth rates are found to be independent of R.

Experimental results show that the crack opening stress is not affected by the position of the gauge when it is mounted behind and near the crack tip.

The effect of mechanical properties and loading on crack growth were also studied. The specimens were fatigue cracked to a predetermined length and some specimens were annealed and again loaded cyclically. The application of cyclic loads to annealed specimens caused significant increase in crack propagation rates in comparison with the specimens having no heat-treatment. The load-displacement record was found to stabilize in about 10 cycles; the crack then extended slowly as a fatigue crack. Crack propagation rates for different values of R for annealed and work-hardened material were plotted against a crack tip parameter, ΔK*, based on notional crack lengths. Since the results of da/dN vs ΔK* for both states of material (as-received and annealed) seem to lie on the same straight line on a log-log graph, the study provides a hope that the results for a material tested in any state (annealed or work-hardened) for positive values of R (0·0≤R≤0·3) will lie on this line, thus eliminating fatigue tests on the same material under different work-hardening conditions for different values of R. Models for da/dN have been developed using various crack tip parameters.     

The near-threshold high R-ratio fatigue crack growth characteristics of SA508 cl III reactor pressure vessel steel

This paper describes the effect of frequency and environment on the near-threshold fatigue crack growth behaviour of SA508 cl III reactor pressure vessel (RPV) steel. The study has shown that in the near-threshold regime microstructure and environment markedly affect fatigue crack growth behaviour. In an aqueous environment, fatigue crack growth behaviour became even more sensitive to microstructure, and the fatigue crack growth rate increased by a factor of four in the case of the 3 Hz test, while that for the 0·3 Hz test was increased by a factor of approximately sixteen. This environmental enhancement manifested itself in the form of intergranular failure. For the 0·3 Hz test the percentage intergranular failure decreased from 18% to <1% with an increase in ΔK level. The transition from microstructure-sensitive to microstructure-insensitive occurs when the cyclic plastic zone size is of the order of the prior austenite grain size.     

A comparison of methods of calculating stress intensity factors for cracks in pipes and thin walled cylinders

This paper presents results of a study undertaken to compare stress intensity factor solutions for various crack geometries in pipes and thin walled cylinders against the equivalent flat plate K solutions. The exercise was restricted to cylinders and pipes with wall thickness to radius ratios (t/R) of 0·1.

The results of the exercise indicate that structural integrity assessments of pipes and thin walled cylinders which contain flaws should ideally incorporate representative stress intensity factor solutions. Nevertheless there are a number of crack geometries for which flat plate K solutions can provide reasonable estimates of the stress intensity factor.     

Measurement of crack propagation velocity in nuclear pressure vessel steel (SA516 gr. 70)

An attempt has been made to develop a simple, reliable and cost-effective device for measuring the dynamic crack propagation velocity in a nuclear pressure vessel steel (SA516 gr. 70). The experimental method is described and a simple digital approach is proposed. The experimentally determined dynamic crack velocity has been utilized to obtain elastic dynamic stress intensity factors by INSAMCR (a two-dimensional dynamic finite element code which is a modified version of SAMCR developed by Dr Schwartz at the University of Maryland). A relationship between instantaneous crack tip velocities and dynamic stress intensity factors for pressure vessel steels is estimated using dynamic crack propagation velocities determined by a proposed measuring device. The relationship between the dynamic stress intensity factor and time history and the dynamic arrest toughness for each test are obtained using the generation mode dynamic finite element analysis. A function ƒ(å) = 1·356 − 2·672å + 6·494å² − 4·539å³ + 1·461å⁴ is suggested which may be useful to predict the relationship between the dynamic fracture toughness (K(å)) and the dynamic crack arrest toughness (K_Ia) for SA516 gr. 70 steel (say K(å) = K_Ia ƒ(å) where å is the dynamic crack propagation velocity).     

Calculation of stress intensity factors for semielliptical cracks in a thick-wall cylinder

Weight functions for the surface and the deepest point of an internal semielliptical crack in a thick-wall cylinder were derived from a general weight function and two reference stress intensity factors. For several linear and nonlinear crack face stress fields, the weight functions were validated against finite element data. Stress intensity factors were also calculated for the Lamé through the thickness stress distribution induced by internal pressure. The weight functions appear to be particularly suitable for fatigue and fracture analysis of surface semielliptical cracks in complex stress fields. All stress intensity factor expressions given in the paper are valid for cylinders with an inner radius to wall thickness ratio, R_i/t = 4.     

Finite element based evaluation of stress intensity factors for interactive semi-elliptic surface cracks

A finite thickness plate with two coplanar self-same shallow and deep semi-elliptical surface cracks subjected to remote tensile surface traction is considered for fracture analysis. Based on three-dimensional (3D) finite element solutions, stress intensity factors (SIFs) are evaluated along the entire crack front using a force method. The line spring model has also been used to evaluate crack depth point SIFs using shell finite element analysis. A wide range of geometric dimensions and crack configurations viz. crack shape aspect ratio (0.3≤a/c≤1.2), crack depth ratio (1.25≤t/a≤6), relative crack location (0.33≤2c/d≤0.9) and normalized location on the crack front (0≤2φ/π≤2) are considered for numerical estimation of crack interaction factors. SIFs evaluated at the depth point using the force method from the 3D finite element results are compared with SIFs evaluated using the line spring model. Finally, using finite element results, an empirical relation is proposed for the evaluation of crack interaction factors. For the ranges considered, the proposed empirical relation predicts crack interaction factors at critical locations within ±2% of the 3D finite element solutions.     

A weight function approach to stress-intensity factors for half-elliptical surface cracks in cylindrical pressure vessels subjected to thermal shock

Root-mean-square averaged (RMS-averaged) stress-intensity factors were calculated for internal half-elliptical surface cracks in cylindrical vessels using a weight function method. The weight function was derived based on an approximate crack surface displacement representation. Stress-intensity factors for longitudinal half-elliptical inner surface cracks subjected to polynomial stress distribution have been presented and compared favorably with the existing numerical solutions. Superposition of the polynomial stress-intensity factors has provided an extremely efficient solution to the thermal shock crack problems. The crack geometries analyzed were R_i/R₀ = 10/11 and 4/5, a/c = 0·3333 and 0·8, the ratio of crack depth to wall thickness ranged from 0 to 0·8. The results, as well as the proposed method, offer a very powerful and economic way for the safety assessment of pressure vessels subjected to complex and varying load conditions.     

The determination of Jq, the initiation elastic-plastic toughness parameter, using pre-cracked Charpy specimens

In order to assess the structural integrity of pressure vessel steels in the nuclear and chemical industries a need has arisen to measure fracture toughness using Charpy V-notch surveillance specimens.

This paper describes a procedure for determining the initiation fracture resistance, J_q of a material using pre-cracked Charpy specimens loaded in three-point bend. The size restrictions imposed by the geometry of a Charpy specimen on the measurement of the plane strain fracture toughness, K_IC, and the elastic-plastic toughness parameter, J_IC, are discussed for typical low- and medium-strength steels.

Consideration is given to the measurement of crack growth, side-grooving and the use of exclusion lines to interpret the fracture resistance, J, versus crack growth, Δa, curves.     

Numerical simulation of tearing–fatigue interactions in 316l(N) austenitic stainless steel

The loading history of engineering components can influence the behaviour of defects in service. This paper presents, the results of a numerical study aimed at using the Gurson ductile damage model, calibrated against J R-curve data, to simulate load-history effects on ductile tearing behaviour in austenitic materials. The work has demonstrated that ductile crack growth resistance is influenced by sub-critical crack growth by an intervening mechanism such as fatigue. Fatigue crack growth under a positive R-ratio leads to increase in subsequent tearing resistance through three main mechanisms: (i) re-sharpening of the crack tip; (ii) crack extension through the fracture process zone; and (iii) cyclic loading effects on void development. The ratio of minimum to maximum stress during fatigue loading (R-ratio) has been shown to influence subsequent tearing resistance, with an R-ratio of 0.2 generally leading to a greater enhancement in tearing resistance than an R-ratio of 0.1. This behaviour is due to the influence of R-ratio on void development ahead of the fatigue crack tip. Finally, relevant experimental data compare favourably with the predicted J R-curves.     

Investigation of yield strength and single cycle overload on crack closure

Crack closure experiments were performed on 6063-T6 and 6061-T6 aluminium alloys, using a crack opening displacement gauge, for various overload ratios (1·67, 1·88 and 2·06). On the basis of these experiments some relationships are developed. The delay period after application of a single overload increases with increase in the magnitude of overload. The retardation is decreased with increase in prestrain for the same overload ratio. After the overload cycle the increase in U values is less in prestrained material than in the as-received material. Crack growth rate also decreases after application of an overload cycle; it attains a constant amplitude crack growth rate and crack closure value after a certain number of cycles. For the same stress ratio, the delay period in the 6063-T6 alloy was found to be more than in the 6061-T6 alloy. The experimental results when plotted on log-log graph paper, show that N_D/N_CAL vs overload ratio and crack length (when U = U_min) vs overload ratio fit a straight line, from which the power laws are developed.     

Instrumented drop-weight test results for normalised and tempered 9Cr1Mo steel

From instrumented drop-weight tests, the nil ductility transition temperature (T_NDT), and a conservative estimate of dynamic fracture toughness (K_Id), at T_NDT for normalised and tempered 9Cr---1Mo steel, are determined to be −25°C and 70 MPa√m, respectively. The latter value agrees well with that determined from pre-cracked Charpy tests. The K_Id/σ_Yd (σ_Yd is the dynamic yield stress) ratio at T_NDT is estimated to be 0·076 √m, in agreement with previous estimates. The uncertainties in crack profile measurement and effect of microstructural variation in the heat affected zone on fracture loads are also discussed.     

Emerging technology for component life assessment

Operating plant component damage and failure experience is reviewed. Loading conditions such as thermal stratification and striping, turbulent flow and flow-induced vibrations are often found to limit useful life, even though such loadings were typically not considered when the components were designed. High cycle thermal and mechanical fatigue are identified as important damage mechanisms. A new method of correlating fatigue data and extrapolating to the very high cycle regime is described.

The results of environmental degradation testing during the past fifteen years have shown that such effects are much more deleterious than previously assumed. Therefore environmental and aging effects must be taken into account in evaluating the reliability and dependability of components for extended periods of operation. Since most of the available data on environmental effects focus on measured crack growth rates, methods of developing improved fatigue life evaluation methods which include environmental effects on crack growth rates are now being developed.

Fatigue tests on polished specimens are characterized by nominal stress amplitudes over yield, where linear elastic fracture mechanics (da/dn vs. ΔK) methods, such as those used in the ASME Code, are not valid. The small plastic zone corrections used in the Code do not account for the plastic crack-driving energies encountered in low-cycle fatigue testing. J-integral solutions are used herein to evaluate the growth of cracks in these specimens. This approach can be shown to correlate the growth of cracks over the entire range of loading from elastic to grossly plastic conditions in widely different geometries and sizes, including the growth of very short cracks for materials of major interest in pressure vessels and piping. It can be used to correct S-N fatigue life evaluation, curves for known differences in crack growth rates whether they are due to corrosion-assisted fatigue or other variables.

Environmental effects on the crack initiation phase of fatigue failure can be directly incorporated into S-N life evaluation curves. Once the crack propagation effects are included, the resulting improved S-N curves provide a means for plant operators to evaluate the current condition of these components and systems, taking into account the cumulative damage from operating transients and cycles which the plant has experienced. The safe residual life can then be evaluated using the S-N curves to include cumulative damage for the anticipated future period of operation. This plant life evaluation approach is applicable even where in-service inspections are not feasible. It provides a sound quantitative basis for making repair/replacement decisions.     

Thermodynamic evaluation of water splitting by a cation-excessive (Ni, Mn) ferrite

Water-splitting potential by cation-excessive (Ni, Mn) Ferrite, Ni_{0.5(1 + )}Fe_{1.99(1 + )}O₄ was evaluated using the standard Gibbs free energy change (ΔG°) for the cation-excessive ferrite formation in different O₂ partial pressures. The cation-excessive degree ranged from 0.026 to 0.16 in pO₂ values of 7.9 × 10⁻⁷ to 1.0 × 10⁻¹. From the relation between value of (Ni, Mn) ferrite and oxygen partial pressure, equilibrium constant K(th) was determined. Furthermore ΔH°s for O₂ releasing and water-splitting reactions with cation-excessive (Ni, Mn) ferrite were evaluated from the van't Hoff plot and compared with that for magnetite-wüstite system; where ΔH°s were assumed to be the same values for both (Ni, Mn) ferrite and magnetite-wüstite system: +300 kJ for O₂ releasing and −35 −63 kJ for water-splitting. ΔG°s evaluated for water-splitting with cation-excessive (Ni, Mn) ferrite and wüstite were −38 kJ and −35 kJ, respectively, at 298K. It suggests that water splitting with cation-excessive (Ni, Mn) ferrite proceed easily compared with that with wüstite. ΔS°s for water-splitting are −0.93 kJ K⁻¹ for the former and −0.83 kJ K⁻¹ for the latter. H₂ generation rates by reaction with H₂O for (Ni, Mn) ferrite were studied at temperatures of 573 K-1073 K. It reached the maximum at 1000 K while it proceeds preferentially below 830 K from thermodynamics.