FATIGUE CRACK GROWTH IN D16T A1-ALLOY SHEET SUBJECTED TO BIAXIAL OUT-OF-PHASE LOADING

Abstract— Fractographic peculiarities of fatigue crack development are studied in cruciform specimens of D16T aluminium alloy under out-of-phase biaxial tension and tension-compression. In the range of the biaxial load ratios λ from ?0.5 to +0.5 and an R-ratio of 0.3, fatigue striation formation took place beyond a crack growth rate near to 4 × 10^?8 m/cycle. The striation spacing and the crack growth rate increase as the φ-angle of the out-of-phase biaxial loads increases in the range of φ-angles from 0° to 180°. The ratio between the increment of crack growth, da/dN, and the striation spacing, δ, is approximately 1 to 1 when da/dN is greater than 4 × 10^?8 m/cycle. The relationship between the number of cycles from the beginning of a test up to the growth rate of 10^?6 m/cycle (N_d), and the crack growth period, N_P, from when the crack initiates up to the instant when that growth rate is reached, was determined for different λ ratios and φ angles. The value of N_d decreases as the φ angle is increased in the range from 0° to 1807deg;. Cycle loading parameters must be taken into account in order to describe the crack growth period when using a unified method that involves an equivalent stress intensity factor K_e=K_IF₁(λ, R)F₂(φ). The values of F₂(φ) were determined. The calculated fatigue crack growth period, N_c, applicable up to and including the stage of fatigue striation formation (predicted by using both of the F₁(λ, R) and F₂(φ) functions) is correlated with the experimental data and the error is of the order of 15%.     

Initiation of fatigue cracks in commercial aluminium alloys and the subsequent propagation of very short cracks

Experiments have been made on two commercial aluminium alloys (BS L65, Al-Cu-Mg-Si-Mn; DTD 5050, Al-Zn-Mg-Cu-Cr) to observe the initiation of fatigue cracks at a plane polished surface and the subsequent growth of very short cracks [0.006 mm (0.00025 in.)-0.5 mm (0.02 in.) deep]. It was found that cracks initiated at surface inclusions, either from the interface between an inclusion and the matrix or from a crack in an inclusion. In both cases the crack ran into the material in directions approximately perpendicular to the applied tensile stress. The growth rates of the short surface were compared, using linear elastic fracture mechanics, with the rates far long [>0.25 mm (0.01 in.)] through-section cracks. The growth rate for the short cracks tended towards that predicted from long cracks for creek depths greater than about 0.127 mm (0.005 in.) but the average crack rate in the early stages of growth was about 1.27 × 10^?6 mm/cycle (5 ×10^?8in./cycle) which is much faster than would be predicted from the long crack data. For cracks about 0.025 mm (0.001 in.) deep the rate varied approximately as the square root of the crack depth. The effect of stress on the proportion of the total life occupied by initiation and by propagation of the crack is discussed.     

Fracture toughness and growth of short and long fatigue cracks in ductile cast iron EN‐GJS‐400‐18‐LT

Heavy components of ductile cast iron frequently exhibit metallurgical defects that behave like cracks under cyclic loading. Thus, in order to decide whether a given defect is permissible, it is important to establish the fatigue crack growth properties of the material. In this paper, results from a comprehensive study of ductile cast iron EN‐GJS‐400‐18‐LT have been reported. Growth rates of fatigue cracks ranging from a few tenths of a millimetre (‘short’ cracks) to several millimetres (‘long’ cracks) have been measured for load ratios R=?1, R= 0 and R= 0.5 using a highly sensitive potential‐drop technique. Short cracks were observed to grow faster than long cracks. The threshold stress intensity range, ΔK_th, as a function of the load ratio was fitted to a simple crack closure model. Fatigue crack growth data were compared with data from other laboratories. Single plain fatigue tests at R=?1 and R= 0 were also carried out. Fracture toughness was measured at temperatures ranging from ?40 °C to room temperature.     

Determination of da/dN-ΔK 1 curves for small cracks in alumina in alternating bending tests

Crack-growth relations under cyclic fatigue conditions are mostly determined for long cracks. In order to determine da/dN-K curves for small cracks from lifetimes under cyclic load a procedure has been derived which is based on a method usually applied to subcritical crack growth. To prove the cyclic effect and to demonstrate the procedure in detail, measurements were carried out on an Al₂O₃-ceramic in bending with anR-ratio ofR=–1 and two types of relatively small cracks, namely natural cracks and Knoop-cracks. It was found that both crack types exhibit the same da/dN-K relation. The exponent of the Paris law for fatigue crack growth is significantly different from the exponent of the power law for subcritical crack growth.     

Short cracks initiated in Al 6013-T6 with the focused ion beam (FIB)-technology

The fatigue crack growth behaviour of short corner cracks in the Aluminium alloy Al 6013-T6 was investigated. The aim was to determine the crack growth rates of small corner cracks at a stress ratio of R = 0.1, R = 0.7 and R = 0.8 and to find a possible way to predict these crack growth rates from fatigue crack growth curves determined for long cracks. Corner cracks were introduced into short crack specimens, similar to M(T) – specimens, at one side of a hole (Ø = 4.8 mm) by cyclic compression (R = 20). The precracks were smaller than 100 μm (notch + precrack). A completely new method was used to cut very small notches (10–50 μm) into the specimens with a focussed ion beam. The results of the fatigue crack growth tests with short corner cracks were compared with the long fatigue crack growth test data. The short cracks grew at ΔK-values below the threshold for long cracks at the same stress ratio. They also grew faster than long cracks at the same ΔK-values and the same stress ratios. A model was created on the basis of constant K_max-tests with long cracks that gives a good and conservative estimation of the short crack growth rates.     

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.     

Cyclic fatigue-crack propagation in a silicon carbide whisker-reinforced alumina composite: role of load ratio

The characteristics of subcritical crack growth by cyclic fatigue have been examined in a silicon carbide whisker-reinforced alumina composite, with specific reference to the role of load ratio (ratio of minimum to maximum applied stress intensity, R=K _min/K _max); results are compared with similar subcritical crack-growth data obtained under constant load conditions (static fatigue). Using compact-tension samples cycled at ambient temperatures, cyclic fatigue-crack growth has been measured over six orders of magnitude from ∼10⁻¹¹–10⁻⁵ m cycle⁻¹ at load ratios ranging from 0.05–0.5. Growth rates (da/dN) display an approximate Paris power-law dependence on the applied stress-intensity range (ΔK), with an exponent varying between 33 and 50. Growth-rate behaviour is found to be strongly dependent upon load ratio; the fatigue threshold, ΔK _TH, for example, is found to be increased by over 80% at R=0.05 compared to R=0.5. These results are rationalized in terms of a far greater dependency of growth rates on K _max(da/dN ∞ K _max ³⁰ ) compared to ΔK(da/dN ∞ ΔK ⁵), in contrast to fatigue behaviour in metallic materials where generally the reverse is true. Micromechanisms of crack advance underlying such behaviour are discussed in terms of timedependent crack bridging involving either matrix grains or unbroken whiskers.     

Fracture mechanics of surface fatigue crack growth by ductile striation space measurement in notched Waspaloy

Ductile striation space (DSS), a parameter to predict actual cracks in both direction of length and depth, is proposed for the surface fatigue crack behaviors on notched Waspaloy. Three different lengths (1, 2 and 4 mm) of artificial notches are formed as the initial surface crack for an applied maximum stress of 1,103 MPa at the stress ratio R of 0.05. These notches are similar with the appearance of the surface cracks found from the survey of compressor disk. The results show that, all initial crack sites in the depth direction started from the multiple origination sites. The DSS parameter was clearly confirmed, and it also proves the high effectiveness of the measurement in the range of the stress intensity factors for acquiring the crack growth rate on the fractured surface. The surface cracks on Waspaloy at room temperature in an atmosphere perfectly follow the relation of ΔK versus da/dN and db/dN, even though there are, respectively, earlier and later timing differences on the initiation of cracks for the notch sizes of 1 and 4 mm. The results of ΔK versus da/dN and db/dN relations show a similar slope for three different kinds of notches.     

FRACTOGRAPHIC ANALYSES OF FATIGUE CRACK GROWTH IN D16T ALLOY SUBJECTED TO BIAXIAL CYCLIC LOADS AT VARIOUS R-RATIOS

Abstract Fractographic peculiarities of fatigue crack development are studied in cruciform specimens of D16T aluminium alloy under biaxial tension and tension-compression. In the range of the biaxial load ratios λ from - 1 to +1.5, and in the range of R-ratios 0.05 to 0.8, fatigue striation formation took place over a crack growth rate near to 4×10^?8 m/cycle. The striation spacing and the crack growth rate decrease as the ratios λ and R increase. The ratio between the increment of crack growth, da/dN, and the striation spacing, δ, is approximately 1:1 when da/dN is greater than 4×10-^?8 m/cycle. The relationship between the number of cycles from the beginning of a test up to the growth rate of 2.14×10^?7 m/cycle (N_d), and the crack growth period, _Np, from when the crack initiates up to the instant when that growth rate is reached, was determined for different λ and R-ratios. The value of N_d increases as the stress ratio, λ, is increased. Cycle loading parameters must be taken into account in order to describe the crack growth period when using a unified method involving an equivalent stress intensity factor K_e, =K₁,F(λ, R_s). The values of F(λ, R) for the growth rate (F(λ, R)_s) and for the striation spacing (F(λ, R_s) were determined and compared. The fatigue crack growth period, N^t_p, applicable to the stage of fatigue striation formation, (predicted by using both of the F(λ, R) values) is correlated with the experimental data and the error is of the order of 15%.     

Cyclic fatigue crack growth tests on disc specimens

A ring has been designed and built in which four disc specimens containing bore cracks were subjected to fatigue conditions by continuous cycling between variable upper and lower speed limits. The behaviour of the individual cracks was found to be inconsistent. While in the majority of cases the crack growth rate was always less than that predicted by the expression da/dN = C(ΔK)ⁿ, where C and n constants which were found from crack growth tests on standard fracture toughness specimens, an important exception was found in which a pair of cracks grew slowly over a large number of cycles before growing at a very much faster rate than that predicted. Some cracks grew much more slowly than predicted and some did not grow at all, even though minimum values of ΔK were always greater than threshold values. The circumstances which govern the different types of crack growth rate behaviour are examined using finite element analysis. It appears that, for a given specimen geometry and rotational speed, the controlling influences are the number and relative lengths of the bore cracks.     

Initiation and growth behaviour of small internal fatigue cracks in Ti‐6Al‐4V via synchrotron radiation microcomputed tomography

Small internal fatigue cracks initiated in Ti‐6Al‐4V in the very high cycle regime were detected by synchrotron radiation microcomputed tomography (SR‐μCT) at SPring‐8 in Japan. The initiation and growth behaviours of the cracks were nondestructively observed, and the da/dN‐ΔK relationship was measured and compared with that obtained in a high vacuum environment. SR‐μCT revealed that more than 20 cracks were initiated in one specimen. The crack initiation life varied widely from 20% to 70% of the average fatigue life and had little influence on the growth behaviour that followed. The initiation site size of each internal crack detected in one specimen was comparable with the size of the fracture origins obtained in ordinary fatigue tests. These results suggest that the surrounding microstructures around the initiation site are likely a dominant factor on the internal fracture rather than the potential initiation site itself. The internal crack growth rates were lower than 10^?10 m/cycle, and extremely slow rates ranging from 10^?13 to 10^?11 m/cycle were measured in a lower ΔK regime below 5 MPa√m. The internal crack growth rate closely matched that of surface cracks in a high vacuum, and the reason for the very long life of internal fatigue fractures was believed to result from the vacuum‐like environment inside the internal cracks.     

Nucleation and propagation of fatigue cracks in Al-304 stainless steel laminate composite

A study has been made of fatigue crack nucleation and propagation in Al-stainless steel (30 vol%) laminate composites. A Paris type power relationship between the crack growth rate, da/dN, and the alternating stress intensity, K, was obtained over the crack growth rates ranging from 10^–7 to 10^–4 mm/cycle, with an exponentm of 2.7. The cracks nucleated first in Al strips and then in stainless steel strips accompanied by some interface decohesion. The fatigue crack propagated in two stages. In the first stage, where the Al-steel interface was largely intact, the crack propagated in a plane strain mode (flat fracture surface with striations, each striation consisting of a cluster of interstriations). In the second stage, where there occurred extensive Al-steel interface delamination and the concomitant loss of mutual constraint, the crack propagated in the plane stress mode (slant fracture with voids). The crack growth was faster in Al than that in steel since the apparent striation spacing was larger in the former than in the latter. No one to one correspondence existed between the apparent striation spacing and the macroscopic crack growth rate.Thus, although, microscopically, the crack front was not planar; macroscopically, it could be regarded as planar, and a Paris type power relationship did characterize the macroscopic fatigue crack growth in this laminate system over the applied stress amplitude studied. Comparing the fatigue crack growth rates among Al-steel laminate, commercial or pure aluminium and 304 stainless steel, the Al-steel laminate has the lowest crack growth rate. This plus the weight and cost saving benefits make Al-steel laminate quite attractive.     

Influence of stress ratio on the threshold level for fatigue crack propagation in high strength steels

Centrally cracked specimens of JIS SM58Q and HT80 steels were fatigued. The fatigue crack growth rates, da/dn, and the stress intensity threshold levels, ΔK_th were measured over the range of stress ratio, R, from ?1 to 0.8 by the use of an automatic method of continuously decreasing stress intensity factor with crack extension. The measured ΔK_th was well represented as |ΔK_th/2|_R=(1?R)γ|ΔK_th/2|_R=0; and the propagation rate, as da/dn = A(1?R)^γm[(ΔK/2)^m ? {(1 ? R)^γ|ΔK_th/2|R=0}^m] for ?1≦R≦0.33 or da/dn = A(1 ? 0.33)^?γm [(ΔK/2)^m {(1 ? R)^γ |ΔK_th/2|R=0}^m] for 0.33 < ≦ 0.8.     

Acoustic emission study of fatigue crack closure of physical short and long cracks for aluminum alloy LY12CZ

Crack closure of physical short and long cracks of LY12CZ aluminum alloy during fatigue process was investigated using acoustic emission (AE) technique. Results showed that the effective fatigue crack growth curve (da/dN vs. ΔK_eff) of physical short and long cracks obtained by the AE technique was consistent with the effective fatigue crack growth curve at high stress ratio (R = 0.8), which implied that the AE technique could measure the crack closure level, especially for physical short crack. The growth rate of physical short crack was much higher than that of long crack at the same ΔK, and the lower crack closure level of short crack was the main reason.     

Transverse fatigue crack propagation behavior in equine cortical bone

Stress fractures stem from the initiation and propagation of fatigue cracks through bone, and fatigue damage may play a role in many other orthopaedic problems, such as hip fractures in the elderly. The objective of this investigation was to measure fatigue crack propagation rates in cortical bone. Specific aims were to determine fatigue crack growth rate, da/dN, as a function of alternating stress intensity factor, K, for equine third metacarpal cortical bone tissue; to determine whether the resulting data followed the Paris law; and to test the hypothesis that crack growth rates differ between dorsal and lateral regions. Compact type specimens oriented for transverse crack growth were subjected to fatigue under Mode I loading. The da/dN vs. K data for the dorsal specimens revealed a Paris law exponent of 10.4 (R ² = 0.82), comparable to that for ceramics. These data also exhibited an apparent threshold stress intensity factor of 2.0 MPa · m^1/2. It was not possible to obtain similar results for lateral specimens because all cracks deviated from the desired transverse path and ran longitudinally in spite of the use of side grooves to constrain the crack path. However, the results for lateral specimens were not due to a failure of the test method, but reflect dramatic differences in fatigue crack propagation resistance between the two cortical regions. These results are consistent with clinical observations that stress fractures in the third metacarpus typically occur in the mid-diaphysis of the dorsal cortex, but not in the lateral cortex.     

Vorhersagemodell für die Wachstumsraten kurzer Risse auf der Basis von Kmax‐konstant‐Versuchen an M(T)‐Proben

Prediction model for the growth rates of short cracks based on K_max‐constant tests with M(T) specimens The fatigue crack growth behaviour of short corner cracks in the Aluminium alloys Al 6013‐T6 and Al 2524‐T351 was investigated. The aim was to determine the crack growth rates of small corner cracks at stress ratios of R = 0.1, R = 0.7 and R = 0.8 and to develop a method to predict these crack growth rates from fatigue crack growth curves determined for long cracks. Corner cracks were introduced into short crack specimens, similar to M(T)‐specimens, at one side of a hole (Ø = 4.8 mm) by cyclic compression (R = 20). The pre‐cracks were smaller than 100 μm (notch + precrack). A completely new method was used to cut very small notches (10–50 μm) into the specimens with a Focussed Ion Beam. The results of the fatigue crack growth tests with short corner cracks were compared with long fatigue crack growth test data. The short cracks grew at ΔK‐values below the threshold for long cracks at the same stress ratio. They also grew faster than long cracks at the same ΔK‐values and the same stress ratios. A model was developed on the basis of K_max‐constant tests with long cracks that gives a good and conservative prediction of the short crack growth rates.     

The influence of crack depth on the fatigue crack propagation rate for a marine steel in seawater

It has been shown that, for QIN steel specimens tested in seawater at a zero-tension cyclic frequency of 30 cycles per hour, crack growth rates are influenced by crack depth at low values of the stress intensity range (K). Crack growth rates are faster for shallow cracks in the crack depth range 0.5 to 2.0 mm when K is less than about 30 MN m^–3/2. For deeper cracks at low stress intensity ranges and for all crack depths at high stress intensity ranges, crack depth does not exert a significant influence on crack growth rate. The implications of such factors in relation to the provision of corrosion fatigue crack growth rate data for use in engineering design are discussed.     

SMALL CRACK GROWTH IN THE ALUMINIUM–LITHIUM ALLOYS 8090 AND 8091

The relationship between microstructure and the fatigue behaviour of small cracks has been examined for the aluminium–lithium alloys 8090 and 8091 after peak ageing at 170°C. Duplex ageing and pre-stretching were used to vary the distribution of S'precipitates and thus the distribution of slip. No effect of S'distribution an small crack growth was observed in either alloy. This is thought to be due to a combination of the lack of closure and lower overall slip reversibility in small cracks. Small cracks in 8091 were found to grow slower than in 8090 due to differences in grain shape rather than texture. Small cracks in both alloys were observed to grow much faster than long cracks for equivalent ΔKs. This difference was reduced when small crack data were compared with long crack data generated at R= 0.7 due to the reduced closure. The use of ΔJ made long and small crack growth rates still more comparable.     

Effects of various environments on fatigue crack growth in Laser formed and IM Ti–6Al–4V alloys

The fatigue crack growth behaviors of Laser formed and ingot metallurgy (IM) Ti–6Al–4V alloys were studied in three environments: vacuum, air and 3.5% NaCl solution. Taking the Unified Fatigue Damage Approach, the fatigue crack growth data were analyzed with two intrinsic parameters, stress intensity amplitude ΔK and maximum stress intensity K_max, and their limiting values ΔK^* and . Fatigue crack growth rates da/dN were found increase with stress ratio R, highest in 3.5% NaCl solution, somewhat less in air and lowest in vacuum, and higher in IM alloy than in Laser formed one. In 3.5% NaCl solution, stress corrosion cracking (SCC) was superimposed on fatigue at R=0.9 for where K_max>K_ISCC, the threshold stress intensity for SCC. This and environment-assisted fatigue crack growth were evidenced by the deviation in fatigue crack growth trajectory (ΔK^* vs. curve) from the pure fatigue line where . Furthermore, the fractographic features, identified along the trajectory path, reflected the fatigue crack growth behaviors of both alloys in a given environment.     

Three-dimensional finite element simulations of microstructurally small fatigue crack growth in 7075 aluminium alloy

Three‐dimensional finite element simulations were performed to study the growth of microstructurally small fatigue cracks in aluminium alloy 7075‐T651. Fatigue crack propagation through five different crystallographic orientations was simulated using crystal plasticity theory, and plasticity‐induced crack opening stresses were calculated. The computed crack opening stresses were used to construct small crack da/dN‐ΔK diagrams. The generated da/dN‐ΔK curves compared well with experimental small crack data from the literature. The variance observed among the da/dN‐ΔK results, which occurred as a consequence of the different crystallographic orientations employed, was found to be of the same order of magnitude as commonly observed variability in small fatigue crack growth data. This suggests that grain orientation is a major contributor to observed small fatigue crack data scatter.