THE INFLUENCE OF TEST VARIABLES ON THE FATIGUE CRACK GROWTH THRESHOLD

Abstract— A microcomputer controlled fatigue crack growth and threshold testing system has been used to investigate the influence of test variables on the measured values of Δ K _th, the threshold for fatigue crack growth, using a C-Mn steel. The work has examined: (1) the influence of crack length and test management; (2) the basic material scatter from repeated testing; (3) the effect of unloading rate C where C = (1/Δ/ K )(d Δ K /d a ); (4) the effect of step unloading; (5) the influence of minimum stress intensity factor, K _min . Comparisons have been made between the results of this computer controlled work and those published previously but made using a manual load shedding technique. The results of Δ K _th and fatigue crack growth rates are in general agreement with previous data and confirm the K _min dependence of Δ K _th and d a /d n. The value of Δ K _th is shown to be generally independent of the other test variables for a wide range of conditions and is reproducible with a low degree of scatter.     

A multiparameter approach to the prediction of fatigue crack growth in metallic materials

A multiparameter approach is proposed for the characterization of fatigue crack growth in metallic materials. The model assesses the combined effects of identifiable multiple variables that can contribute to fatigue crack growth. Mathematical expressions are presented for the determination of fatigue crack growth rates, d a /d N , as functions of multiple variables, including stress intensity factor range, Δ K , stress ratio, R , crack closure stress intensity factor, K _cl , the maximum stress intensity factor K _max , nominal specimen thickness, t , frequency, Ω , and temperature, T . A generalized empirical methodology is proposed for the estimation of fatigue crack growth rates as a function of these variables. The validity of the methodology is then verified by making appropriate comparisons between predicted and measured fatigue crack growth data obtained from experiments on Ti–6Al–4V. The effects of stress ratio and specimen thickness on fatigue crack growth rates are then rationalized by crack closure considerations. The multiparameter model is also shown to provide a good fit to experimental data obtained for HY-80 steel, Inconel 718 polycrystal and Inconel 718 single crystal. Finally, the implications of the results are discussed for the prediction of fatigue crack growth and fatigue life.     

NON-ISOTHERMAL FATIGUE CRACK GROWTH IN HASTELLOY-X

Abstract— Non-isothermal fatigue crack growth tests were performed on Hastelloy-X single edge notch specimens in which strain and temperature were varied simultaneously. Conditions were selected to include nominally elastic and nominally plastic conditions and temperatures up to 925°C. The crack growth rates were first reported as a function of the strain intensity factor (δ K _ε) derived from a crack compliance analysis. Out-of-phase (ε_max at T _max) cycling showed faster crack growth rates than isothermal or in-phase (ε_max at T _max) cycling under elastic straining. Under fully plastic cycling, the opposite results was observed, i.e. crack growth rates under isothermal cycling are faster than under TMF cycling. On a δ K _ε-basis, a strain range effect was observed. All the results were rationalized using a corrected stress-intensity factor (δ K _eff) computed from the actual load, the closing bending moment caused by the increase compliance with crack length, and with the effective opening stress. Each mode of fracture was found to be characterized by a unique crack growth rate vs δ K _eff curve. On a δ K _eff-basis, the isothermal crack growth rates at T _min and T _max provide an upper and a lower bound for the TMFCG rates. The effectiveness of δ K _eff to correlate crack growth rates under fully plastic cycling is discussed in detail.     

ASSESSMENT OF CRACK TIP CLOSURE IN AN ALUMINUM ALLOY BY ELECTRONFRACTOGRAPHY

Abstract— Crack tip closure measurements were performed in a 2124 T3 51 aluminum alloy by electronfractography. The technique makes use of high resolution fractography to correlate a closure in striation spacing with a sudden change of the stress intensity factor range following steady state crack growth. The crack growth tests under programmed loads were performed with a servohydraulic machine interfaced to a digital computer. An average value of K _op/ K _max equal to 0·15 was found in the range of 10 MPa√m < Δ K < 35 MPa√m. The higher closure developing after overloads was found a suitable explanation for crack growth retardation.     

FATIGUE CRACK GROWTH OF SILICON NITRIDE WITH CRACK WEDGING BY FINE FRAGMENTS

Abstract— Fatigue crack growth tests of silicon nitride Si₃N₄ were carried out under four-point bending using square bar specimens at room temperature. A pre-crack was introduced by a bridge indentation method. Decreasing K -type tests at stress ratios of R = 0.1 and 0.6 and also under static load were first carried out, and after observing the cessation of crack growth, K -increasing tests were performed except for the case of a static load. Crack closure was observed on most specimens by the elastic compliance method. Furthermore, SEM observations of the crack paths were made to see what was happening during crack growth. The threshold and the region of steady crack growth were observed more clearly under cyclic loading, and an effect of load cycling certainly existed which became more evident when the maximum stress intensity factor K _max approached the threshold. A wedge effect, caused by fine fragments on the crack surface, played an important role in crack closure behaviour of each specimen, and it is suggested that the crack growth rate is controlled by both the effective stress intensity range Δ K _eff and the effective mean stress intensity factor K _m,eff at least as a first approximation.     

FINITE ELEMENT ANALYSIS OF SPECIMEN GEOMETRY EFFECTS ON FATIGUE CRACK CLOSURE

Abstract— Elastic-plastic finite element analysis is used to study fatigue crack closure at three different crack length to width ratios for three plane stress specimen geometries: center-cracked plate, single-edge-cracked plate (tension), and single-edge-cracked plate (bend). The maximum stress to flow stress ratio, S_max/σ_O, which successfully describes closure results in many center-cracked plate configurations, does not correlate the effect of different geometries on the normalized opening stress, S _open/ S _max. Crack opening stresses for different geometries and crack lengths are successfully correlated by a normalized stress intensity parameter, K _max/ K ₀, where K ₀=σ₀φa. The quality of the correlation is very high at small K _max/ K ₀, and gradually deteriorates as K _max/ K ₀ increases beyond the small-scale yielding regime.     

CRACK CLOSURE AND FATIGUE CRACK GROWTH RATE IN THREE POINT BEND SPECIMENS OF DIFFERENT WIDTHS

Abstract— The crack closure stress intensity factor values and fatigue crack growth rates were determined in Three Point Single Edge Bend, SE(B), specimens prepared from rails manufactured using two different grades of rail steels. The width, and correspondingly the span, of the SE(B) specimens were varied eight fold; the thickness of all the specimens being the same. It is observed that the crack closure stress intensity factor values decrease with an increase in the width of SE(B) specimens. At a given value of Δ K _eff, the fatigue crack growth rate (FCGR) is independent of the width. However, at a given value of Δ K _eff, the FCGR is observed to decrease with increasing width. In view of the above results, the scope of application of the FCGR laws based on an effective stress intensity factor to the life prediction of components, requires careful examination.     

EVALUATION OF FATIGUE CRACK PROPAGATION PROPERTIES UNDER RANDOM LOADING AVOIDING CRACK CLOSURE

Abstract— Fatigue crack propagation rates and the fatigue threshold of HT80 steel were measured by maintaining the maximum load during the whole period of random loading in order to prevent fatigue crack closure. The random loading pattern involved 62 level block loadings in which the waveform was approximated to the Rayleigh distribution of peaks. The fatigue crack propagation rates under random loading were well predicted from those obtained from constant amplitude loading and assuming a linear cumulative damage law. That is, da/dn = C {Δ K ^m_eq−Δ K ^m_th} where the equivalent stress intensity factor, Δ K _eq={= n _iΔ K ^m_i/d n _i}^{1/ m} , where n_i = 0 for Δ K _i≤Δ K _th, or n_i = n_i for Δ K_i > Δ K _th.     

A GENERALIZED CONCEPT OF A FATIGUE THRESHOLD

Abstract— The conditions for non-propagating LEFM type fatigue cracks were investigated on an Inconel, 617 Alloy in the range of K _max between 10 and 50 MPa m^1/2 under four different types of fatigue loading conditions. In all tests, K _max was held constant during the fatigue cycling prior to determining the non-propagation condition. It was found that with decreasing range of applied K the fatigue tolerance range Δ K _{eff, th} increases. Furthermore, there is slight increase of Δ K _eff,th with decreasing K _max of approximately 20–30% when K _max is decreased from 50 to 10 MPa m^1/2. The results of the four types of tests are considered in respect to the damage in the near-region of the crack front, i.e. increasing K _max increases the damage zone and therefore decreases the fatigue tolerance range Δ K _eff,th.     

THE GROWTH OF SMALL CORROSION FATIGUE CRACKS IN ALLOY 2024

Abstract— The corrosion fatigue crack growth characteristics of small surface and corner cracks in aluminium alloy 2024 is established. The damaging effect of salt water on the early stages of small crack growth is characterized by: (1) crack initiation at constituent particle pits, (2) intergranular microcracking for a≤100μm, and (3) transgranular small crack growth for a≥100μm. In aqueous 1% NaCl and at a constant anodic potential of −700 mV_SCE, small cracks exhibit a factor of three increase in fatigue crack growth rates compared to laboratory air. Small cracks exhibit accelerated corrosion fatigue crack growth rates at low levels of Δ K (< 1 MPa√m) below the long crack Δ K _th value. When exposed to Paris regime levels of crack tip stress intensity, small corrosion fatigue cracks exhibit growth rates similar to that observed for long cracks. Similar small and long crack growth behavior at various levels of R suggest that crack closure effects influence the corrosion fatigue crack growth rates of small cracks for a≥100 μm. Contrary to the corrosion fatigue characteristics of small cracks in high strength steels, no pronounced chemical crack length effect is observed for alloy 2024 exposed to salt water.     

THE EFFECT OF STRESS RATIO ON THE FATIGUE THRESHOLD CONDITION OF PHYSICALLY SMALL CRACKS

An approach is proposed to predict the intrinsic threshold of physically small cracks without invoking crack closure considerations. The basic assumption invoked is that a Δ K representation is valid for short cracks, hence the lower-bound threshold value, ΔK_0(s)(min) for short cracks can be numerically equated with the lower-bound threshold value of long cracks, ΔK*_{0(l)(min), s} of the same material. Several experimental observations provide a basis for this rationalization. The approach allows a quantitative prediction of stress ratio and crack length dependence of Δ K _0(S) which provides good agreement with experimental data for several low-strength steels and aluminium alloys. This alternative procedure may be found useful in design applications.     

Mechanisms of fatigue-crack propagation in ductile and brittle solids

The mechanisms of fatigue-crack propagation are examined with particular emphasis on the similarities and differences between cyclic crack growth in ductile materials, such as metals, and corresponding behavior in brittle materials, such as intermetallics and ceramics. This is achieved by considering the process of fatigue-crack growth as a mutual competition between intrinsic mechanisms of crack advance ahead of the crack tip (e.g., alternating crack-tip blunting and resharpening), which promote crack growth, and extrinsic mechanisms of crack-tip shielding behind the tip (e.g., crack closure and bridging), which impede it. The widely differing nature of these mechanisms in ductile and brittle materials and their specific dependence upon the alternating and maximum driving forces (e.g., ΔK andK _max) provide a useful distinction of the process of fatigue-crack propagation in different classes of materials; moreover, it provides a rationalization for the effect of such factors as load ratio and crack size. Finally, the differing susceptibility of ductile and brittle materials to cyclic degradation has broad implications for their potential structural application; this is briefly discussed with reference to lifetime prediction. This revised version was published online in July 2006 with corrections to the Cover Date.     

AN EXPERIMENTAL PARAMETER Jmax IN ELASTIC-PLASTIC FATIGUE CRACK GROWTH

Abstract— Imitating Garwood's 3-parameter technique, an experimental parameter J _max was introduced to predict fatigue crack growth rate (d a /d N ) over a wide range including small scale yielding and large scale yielding. It was found that for a Δ K -increasing fatigue test condition, J _max is a valid parameter. A significant crack growth acceleration, caused by a transition of fracture mechanism, occurs when J _max= J _IC The fracture mechanism involving striation formation when J _max < J _IC becomes ductile tearing when J _max > J _IC Equations to predict the effect of stress-ratio on J _max as well as on d a /d N are given.     

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.     

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.     

The preferred fatigue crack propagation mode in a M250 maraging steel loaded in shear

Macroscopic torsional fatigue cracks are shown to propagate in shear, in plain tubular specimens, in the M250 maraging steel, for stress ranges from 90% down to 40% of the yield stress. This cannot be explained in terms of microcrack coalescence for the smallest stress range, for which microcracks are scarce. The kinetics and mechanisms of mode II fatigue crack growth are thus investigated, using precracked CTS or tubular specimens. For a high Δ K _II , slowly decelerating mode II propagation takes place for a distance that increases with Δ K _II before branching occurs. Friction stresses along the crack flanks shield the applied load and explain this deceleration. An inverse analytical procedure is used to derive the effective stress intensity factor, allowance being made for friction effects, from displacement profiles measured from microgrids using a scanning electron microscope. The measured crack growth rates correlate much better with the effective stress intensity factor than with the nominal Δ K _II value. The crack paths observed in torsion are discussed in terms of maximum crack velocity.     

Fatigue crack growth and crack closure in an AlMgSi alloy

This study reports an experimental investigation of fatigue crack propagation in AlMgSi1-T6 aluminium alloy using both constant and variable load amplitudes. Crack closure was monitored in all tests by the compliance technique using a pin microgauge. For the constant amplitude tests four different stress ratios were analysed. The crack closure parameter U was calculated and related with Δ K and the stress ratio, R . The threshold of the stress intensity factor range, Δ K _th , was also obtained. Fatigue crack propagation tests with single tensile peak overloads have been performed at constant load amplitude conditions. The observed transient post overload behaviour is discussed in terms of the overload ratio, Δ K baseline level and R . The crack closure parameter U trends are compared with the crack growth transients. Experimental support is given for the hypothesis that crack closure is the main factor determining the transient crack growth behaviour following overloads on AlMgSi1-T6 alloy for plane stress conditions.     

A SIMPLIFIED LABORATORY APPROACH FOR THE PREDICTION OF SHORT CRACK BEHAVIOR IN ENGINEERING STRUCTURES

Abstract— Conventionally determined fatigue threshold information (ASTM E647) can lead to non-conservative estimates of fatigue lifetimes when these data are utilized in damage tolerant design assessments. The non-conservative nature of such data can be attributed primarily to the development of excessively large amounts of crack closure at low R -ratios, particularly at near threshold stress intensity factor levels. These high closure levels attenuate the effective stress intensity condition prevailing at the crack tip and confound attempts to predict the behavior of short cracks that exhibit limited crack closure. A modified test procedure, involving constant maximum stress intensity factor ( K ^c_max) test conditions, is described which identifies fatigue crack propagation (FCP) threshold behavior in the absence of detectable amounts of crack closure. These data have been generated with conventional long crack specimens for several aluminum, iron, and nickel-based alloys and which are shown to closely simulate the FCP response of short cracks in these engineering materials. As such, the modified threshold test procedure, incorporating constant K _max loading conditions, represents a valuable tool in the prediction of the cyclic lifetime of engineering components. The stress-cyclic lifetime (S-N) curve for aluminum butt-welded beams was computed based on K ^c_max data and found to be in excellent agreement with actual test results.     

THE INFLUENCE OF STRENGTH AND STRESS RATIO ON SHORT-CRACK THRESHOLDS AND NON-PROPAGATING FATIGUE CRACKS

Quantitative predictions of the influence of yield strength and stress ratio, R , on the physically small crack fatigue threshold stress intensity, Δ K _0(s), are presented. It is shown that at R = 0 to -1, although the threshold stress Δ₀ increases, the threshold stress intensity, Δ K _0(s), decreases with increasing yield strength. Moreover, a lower bound value, Δ K _0(s)(min) is shown to have a constant value, irrespective of the strength and stress ratio. For a given strength, Δ K _0(s), decreases with increasing R in the range -1 R 0.6 and attains a constant low value for R > 0.6. Predicted values of Δ K _0(s) are in good agreement with experimental data for steels. The formation and length of non-propagating fatigue cracks, a _np, are also discussed. The methods suggested for estimating Δ K _0(s) and a _np may be found useful in design procedures.     

THE EFFECTS OF FREQUENCY AND R-RATIO ON FATIGUE CRACK GROWTH BEHAVIOUR IN Al-Li-Cu-Mg ALLOY (8090) PLATE

Abstract— The effects of frequency and R -ratio on the fatigue crack growth rate of the Al-Li-Cu-Mg alloy 8090 have been assessed in air. Compact tension test pieces ( B = 25 mm) were tested at constant P _max using a triangular waveform (CA) and the crack length was monitored by a four-wire pulsed potential drop technique. Crack closure measurements were made during the test by a front face compliance technique.
The crack growth rate was found to be dominated by the high level of crack closure and the fatigue fracture morphology. Two fracture types were identified; a highly deviated "shear" fracture mode which was in competition with a macroscopically flat "tensile" fracture mode. The tensile fracture mode was predominant at low test frequency, high R -ratio and short crack lengths whereas the shear mode was predominant at high frequency, low R -ratio and long crack lengths. The transition from tensile to shear mode was primarily controlled by the effective stress intensity factor range, Δ K _eff.
The effect of loading variables on the transition was explained in terms of the diffusion distance of hydrogen relative to the effective cyclic plastic zone diameter. Test piece thickness had a secondary effect on the transition and this was explained in terms of gas transport considerations.