CORRELATION OF FATIGUE CRACK GROWTH RATE AT DIFFERENT STRESS RATIOS FOR QUENCHED AND TEMPERED STEELS AND OTHER ALLOYS

Abstract— Measurements of the effect of stress ratio on the constant amplitude fatigue crack growth rates in four quenched and tempered steels in the Paris regime are reported. This data and published data for other alloys (including lower strength steels and non-ferrous alloys) are evaluated, and a correlation function suitable for practical fatigue life calculations is derived. In addition to stress intensity factor range and stress ratio, other significant parameters are the yield stress of the material and its thickness. For the four steels on which new measurements were made, the degree of dependence of the crack growth rate on stress ratio may be related to sensitivity to environmental conditions.     

SLOW FATIGUE CRACK GROWTH AND THRESHOLD BEHAVIOUR IN IMI 685

Abstract— Fatigue thresholds and crack growth rates up to 10⁻⁴ mm cycle ⁻¹ have been measured in β processed IMI 685. The results obtained in laboratory air for material having an aligned α microstructure and a random basketweave microstructure displayed a pronounced load ratio dependence which increased with decreasing ΔK. This sensitivity to mean load was also apparent from the threshold results determined in a vacuum of 5 ± 10⁻⁶ torr.
Fractographic observations, compliance measurements, pd output and crack path replication have indicated that contacts can be made between the fracture faces at a number of points behind the crack tip during the load cycle. These contacts wedge the crack open, thus preventing the stress intensity from falling to the value associated with the minimum applied load. A critical stress intensity, K _op, has been determined which relates to the crack being fully "open" and the results are reanalysed and discussed in terms of an effective stress intensity range, Δ K _eff.     

THE INFLUENCE OF TEST TEMPERATURE ON IMPACT FATIGUE CRACK GROWTH BEHAVIOR IN QUENCHED AND TEMPERED Cr-Mo ALLOY STEELS WITH DIFFERENT PRIOR AUSTENITE GRAIN SIZES

Abstract— Impact fatigue tests were carried out using a rotating-disk type impact fatigue testing machine. The influence of prior austenite grain size, ductile-brittle transition temperature and test temperature on impact fatigue crack growth rate was investigated by means of fracture mechanics and fractography in quenched and tempered Cr-Mo alloy steel in which the prior austenite grain size was varied from 8–3 to 25-4 μm. The results in impact fatigue tests were compared to those under non-impact conditions. The crack growth rates associated with striation formation were insensitive to the change in prior austenite grain size, ductile-brittle transition temperature and test temperature regardless of impact and non-impact fatigue. When the material was in the brittle condition, impact fatigue gave rise to a transition from striation formation to intergranular and cleavage cracking. Such a transition will result in the acceleration of crack growth rate. The Paris Law exponent values in impact fatigue were reasonably expressed by the ratio of test temperature to ductile-brittle transition temperature.     

SLOW FATIGUE CRACK GROWTH AND THRESHOLD BEHAVIOUR IN AIR AND VACUUM OF COMMERCIAL ALUMINIUM ALLOYS

Abstract— Fatigue crack growth and threshold behaviour have been examined in three commercial aluminium alloys in both air and vacuum environments. It was observed that, in air, the threshold stress intensity range Δ K _t, varied linearly with the Δ K _t ratio. In contrast Δ K , in vacuum was found to be independent of R. Over the whole growth rate range examined fatigue crack growth in vacuum was Δ K controlled and failure occurred by a dimple and ductile striation mechanism. This also applied to failure in the intermediate growth rate ranges in air. However, at slow growth rates in air, fatigue crack growth was structure sensitive and crystallographic facets were formed during the crack propagation process.     

A REVIEW OF FATIGUE CRACK GROWTH IN STEELS UNDER MIXED MODE I AND II LOADING

The majority of methods for predicting the direction of propagation of mixed mode cracks have assumed that they branch to grow as mode I cracks. However, under some circumstances mode II crack growth occurs. Rolling contact fatigue cracks are one example of an industrial problem where cracks appear to grow under predominantly mode II loading without branching. This paper reviews the available models and the experimental studies in the literature on mixed mode I and II loading, and discusses the parameters that affect the mode of crack growth.     

A NEW METHOD FOR THE MEASUREMENT OF MODE II FATIGUE THRESHOLD STRESS INTENSITY FACTOR RANGE ΔKτth

Abstract— In order to evaluate the threshold value Δ K _τth for mode II fatigue crack growth, a new measurement method of mode II fatigue crack growth has been developed. This method uses a conventional closed-loop tension—compression fatigue testing machine without additional loading attachments. Mode II fatigue tests for structural steel and rail steel have been carried out. This method has proved successful and has reproduced mode II fatigue fracture surfaces similar to those found in the spalling of industrial steel-making rolls. The crack length during testing was measured by an AC potential method. The relationships between d a /d N and Δ K _τ and AK _τth for several materials have been obtained.     

FATIGUE CRACK GROWTH IN RESIDUAL STRESS FIELDS: EXPERIMENTAL RESULTS AND MODELLING

Abstract— The paper discusses the results of fatigue crack growth rate tests conducted in the presence of residual stresses. Three different residual stress distributions, obtained by laser welds, were employed in order to characterize the crack propagation behaviour under different conditions, producing either an increase or a reduction of the stress intensity factor due to external loads. Test results are analysed by means of a non-linear numerical model (based on the weight function method) and a knowledge of the fatigue crack growth properties of the base material, free from residual stresses. The results of the analysis are discussed with reference to experimental trends, in order to clarify the predictive capabilities of the method and aspects needing further investigation.     

THE EFFECTS OF CATHODIC POTENTIAL AND CALCAREOUS DEPOSITS ON CORROSION FATIGUE CRACK GROWTH RATE IN SEA WATER FOR TWO OFFSHORE STRUCTURAL STEELS

The effects of cathodic protection potential, corrosion products and stress ratio on corrosion fatigue crack growth rate have been studied on offshore structural steels. These materials were cathodically polarised in seawater and 3% sodium chloride solution at three potentials of -0.8, -1.0 and -1.1 V(SCE). The corrosion fatigue crack growth rate in seawater was greater than that in air and increased with more negative potentials. The maximum acceleration of crack growth rate in seawater was observed at the crack growth plateau which was independent of ΔK. Calcareous deposits precipitated within the cracks resulted in an increase of crack opening level and contributed to a reduction of the corrosion fatigue crack growth rate. Such a corrosion-product-wedging effect could be evaluated by using an effective stress intensity range, ΔK_eff. The estimation of corrosion fatigue crack growth rate in terms of ΔK_eff clarified the effect of hydrogen embrittlement under a cathodic potential. Thus the processes of cracking in seawater at cathodic potentials resulted from mechanical fatigue and hydrogen embrittlement with calcareous deposits reducing the crack growth rate. All these three mechanisms were mutually competitive.     

NON-PROPAGATION CONDITIONS (ΔKth) AND FATIGUE CRACK PROPAGATION THRESHOLD (δKT)

Abstract— Fatigue crack propagation threshold values have been determined with two experimental methods, it., the constant R method and the constant K_max method. Three materials, namely A17075-T7351 and Ti6A14V STA in the LT- and TL-orientations, and a Ti-turbine disk material (IMI 685) in the CR-orientation, were investigated. The paper is divided into 3 parts. In the first part the test conditions, the experimental results and the conclusions drawn from the experimental results are presented, namely that the three different functional dependencies of ΔK_th on R cannot be reconciled with present continuum mechanics concepts. In the second part, some facts used in conjunction with the da/dN–ΔK_eff methodology are applied to the non-propagation condition ΔK_th. Parameters such as K_Op, the threshold ΔK_T, and a parameter “K_LL” are investigated by numerical modelling of their individual influence on the ΔK_th versus R curves. This modelling work shows that the individual ΔK_th versus R curves are primarily dependent on the K_op behavior of the respective material. Further, it is shown that the threshold ΔK_T is a constant value, independent of any particular cyclic loading condition. In the third part of the paper, the ΔK_eff concept is applied to the experimental results obtained in the first part. Using either experimentally or semi-empirically determined K_op functions and the measured ΔK_T values, the ΔK_th versus R curves of the three materials investigated were accurately reconstructed. It follows that the ΔK_th versus R curves of the individual materials are the natural consequence of the driving force for fatigue crack propagation, namely ΔK_eff     

FATIGUE LIFE ESTIMATION IN RESISTANCE SPOT WELDS: INITIATION AND EARLY GROWTH PHASE

Abstract— The main objective of this research is to develop a model of fatigue crack initiation and early crack growth in resistance spot welds that is specimen independent. This objective is achieved by examining the stress state around a resistance spot weld. A general expression for the structural stress around the weld is formulated that is dependent only on the loading immediately surrounding the weld. As such, it is specimen independent.
An additional objective is to explore the feasibility of applying this fatigue crack initiation model of life estimation using structural response data from finite element analysis (FEA). This numerical technique is often used for evaluating structural integrity of assemblies. Limited verification examples show that the structural stress range as calculated from FEA reaction load data is capable of describing fatigue crack initiation and early crack growth in cyclically loaded resistance spot welds.     

STRESS INTENSITY AND FATIGUE CRACK GROWTH IN MULTIPLY-CRACKED,PRESSURIZED, PARTIALLY AUTOFRETTAGED THICK CYLINDERS*

Stress intensity factors are determined for internally and externally cracked, pressurized thick cylinders with partial autofrettage (less than 100% overstrain). The solutions are based on a superposition of existing solutions which does not involve any loss of accuracy. Implications of the stress intensity factor results for the safe-life design of pressure vessels subjected to fatigue are discussed.     

CREEP, FATIGUE AND CREEP-FATIGUE CRACK GROWTH RATES IN PARENT AND SIMULATED HAZ TYPE 321 STAINLESS STEEL

Abstract— Crack growth rate data are presented from a range of fully reversed displacement-controlled fatigue and creep-Fatigue tests and from static load-controlled creep crack growth tests on aged 321 stainless steel (parent and simulated HAZ) at 650 ° C. In the creep fatigue tests, constant displacement tensile hold periods of 12–192 h were used. Crack growth rates comprised both cyclic and dwell period contributions. Cyclic growth contributions are described by a Paris-type law and give faster crack growth rates than those associated with pure fatigue tests. Dwell period contributions are described by the C* parameter. The total cyclic crack growth rates are given by summing the cyclic and dwell period contributions. Estimates of C* using a reference stress approach together with the appropriate stress relaxation creep data are shown to correlate well with experimentally measured C* values. Crack growth rates during static load-controlled tests correlate well with C* . Good agreement is obtained between crack growth rates during the static tests and those produced during the hold period of the creep-fatigue tests.     

SMALL FATIGUE CRACK GROWTH IN A LOW CARBON STEEL UNDER TENSION–COMPRESSION AND PULSATING-TENSION LOADING

The growth behaviour of small fatigue cracks has been investigated in a low carbon steel under axial loading at the stress ratios R of –1 (tension-compression) and 0 (pulsating-tension). Crack closure was measured to evaluate the effects of stress ratio and stress level on small crack growth. Except for the accelerated growth at stress levels close to the yield stress of the material, at R=–1 small cracks grow faster than large cracks below a certain crack length, but at R= 0 the crack growth rates for small cracks are coincident with those for large cracks in the whole region of crack length investigated. The critical crack length, 2c_c, above which the growth behaviour of small cracks is similar to that of large cracks depends on stress ratio, being 1–2 mm at R=–1 and smaller than 0.7 mm at R=0. The 2c_c value at R=–1 agrees with that obtained under rotating bending (R=–1). The small crack data are closely correlated with large crack growth rates in terms of the effective stress intensity range, ΔK_eff; thus ΔK_eff is found to be a characterizing parameter for small crack growth including the growth at the higher stress levels.     

CRACK GROWTH RATES DURING CREEP, FATIGUE AND CREEP-FATIGUE IN AN AUSTENITIC FEATURE WELD SPECIMEN

Abstract— Static creep crack growth tests and displacement controlled fatigue and creep-fatigue crack growth tests have been performed on austenitic feature weld specimens at 650°C. The creep-fatigue tests incorporated hold times of up to 96 h. During these tests, crack growth appeared to comprise cyclic and dwell components. Cyclic crack growth components were characterised by the fracture mechanics parameter K whilst creep crack growth contributions were correlated with C *. In order to determine K and C * for the non-standard feature weld specimen, elastic and elastic-plastic creep finite element analyses were conducted. Good correspondence is shown between the feature weld data and comparable data from compact tension specimen tests on similar materials. Equations obtained from the compact tension specimen results, which describe total crack growth rates as the sum of the cyclic and dwell contributions, are shown to adequately describe the features test results also. Furthermore, it is demonstrated that a reference stress approach can be used to estimate C * for the features specimens.     

CRACK GROWTH AND PREDICTION OF ENDURANCE IN THERMAL-MECHANICAL FATIGUE OF 12Cr—Mo—V—W STEEL

Abstract Fractography and fracture mechanics were used to study the behaviour of a crack that emanated from a starter notch on a specimen surface subjected to thermal cycles, and which grew in the direction perpendicular to the surface, under out-of-phase loading conditions. A 12Cr–Mo–V–W steel was used as representative of an elevated temperature material, and one that is frequently employed for vessels or heat-exchanger tubes. A low power travelling microscope observed crack length changes on the specimen surface. It was shown that the growth rate of a part-through crack could be expressed in terms of a strain intensity factor range and two constants, and that the striation spacings agreed with the microscopically observed crack growth rate. The integration of the crack growth rate equation gave an approximate evaluation of thermal fatigue life of smooth specimens.     

FATIGUE CRACK GROWTH RATES IN Al–Li ALLOY, 2090. INFLUENCE OF ORIENTATION, SHEET THICKNESS AND SPECIMEN GEOMETRY

.The fatigue crack growth (FCG) behaviour of the aluminium–lithium (Al–Li) alloy 2090-T84 has been investigated from a series of constant amplitude FCG tests. The influence of in-plane orientation (L–T, T–L, L–T + 45°) and sheet thickness (1.6 and 6  mm) on the FCG rates for the rolled product has been examined. In general, the T–L orientation possesses superior FCG resistance for both thicknesses and the 6  mm thick sheet material showed marginally improved FCG resistance when compared to the 1.6  mm thick material, for all orientations. Closure-corrected FCG data suggests that much of the difference between the L–T and T–L orientation for the 6  mm thick sheet arises from differences in crack closure levels. When comparing the crack closure levels for C(T) and M(T) specimens, a significant difference is shown as ΔK increases. Fatigue crack growth rates for ΔK less than 15  MPa m were significantly higher in the M(T) specimens compared to the C(T) specimens. Compared with other factors examined the influence of specimen geometry appears to be a dominant factor.     

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.     

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.