Small Crack Growth Rates Research Articles

IN SITU SEM MEASUREMENTS OF CRACK CLOSURE FOR SMALL FATIGUE CRACKS IN ALUMINIUM 2024-T351

Abstract Crack closure has been measured for a range of small, self-initiated fatigue cracks using in situ SEM loading. Cracks were grown at positive R ratios in the aluminium alloy 2024-T351 and at nominal ΔK levels that extend substantially below the corresponding long crack threshold. The crack closure stress of the small cracks decreased and the Kcl level increased with increasing crack size until the long crack value near threshold was reached. For cracks of depth larger than about one grain size, a good correlation was obtained between small and long crack growth rate data in terms of ΔKeff

β型チタン合金Ｔｉ‐１５Ｍｏ‐５Ｚｒ‐３Ａｌの疲労挙動に及ぼす組織の影響

The fatigue behaviour has been studied under rotating bending using the materials with three different microstructures (β grain sizes) of a Ti-15Mo-5Zr-3Al alloy which were solution-treated at 735°C (STA735), 850°C (STA850) and 1000°C (STA1000) followed by aging at 500°C. It was found that STA735 showed the highest fatigue strength, and in the materials solution-treated above the β transus, STA850 and STA1000, the fatigue lives decreased with increasing solution temperature i.e. β grain size but the fatigue limit was almost the same in both materials. Fatigue cracks were initiated as a consequence of slip deformation in STA735, while at β grain boundaries in both STA850 and STA1000. In particular, STA850 exhibited an interesting initiation behaviour in which cracks were initiated at the specimen surface at high stress levels, but subsurface crack initiation was observed at low stress levels. The growth rate of small cracks was almost identical for all the materials, indicating no influence of solution temperature. Therefore, the observed difference in fatigue strength was attributed to the resistance to crack, initiation of the microstructures studied.

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 mVSCE, 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 ΔK(< 1 MPa√m) below the long crack ΔKthvalue. 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 ofRsuggest 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.

Fatigue behaviour of a commercial aluminium alloy in sea water at different temperatures: Bayoumi, M.R. Eng. Fract. Mech. (June 1993) 45 (3), 297–307

This paper reviews the capabilities of a plasticity-induced crack-closure model to predict fatigue lives of metallic materials using `small-crack theory' for various materials and loading conditions. Crack-tip constraint factors, to account for three-dimensional state-of-stress effects, were selected to correlate large-crack growth rate data as a function of the effective-stress-intensity factor range (ΔKeff) under constant-amplitude loading. Some modifications to the ΔKeff rate relations were needed in the near-threshold regime to fit measured small-crack growth rate behavior and fatigue endurance limits. The model was then used to calculate small- and large-crack growth rates, and to predict total fatigue lives, for notched and un-notched specimens made of two aluminum alloys and a steel under constant-amplitude and spectrum loading. Fatigue lives were calculated using the crack-growth relations and microstructural features like those that initiated cracks for the aluminum alloys and steel for edge-notched specimens. An equivalent-initial-flaw-size concept was used to calculate fatigue lives in other cases. Results from the tests and analyses agreed well.

STATISTICAL INVESTIGATION OF THE BEHAVIOUR OF SMALL CRACKS AND FATIGUE LIFE IN CARBON STEELS WITH DIFFERENT FERRITE GRAIN SIZES

Abstract— In order to study the relation between the scatter characteristics of small crack growth behaviour and fatigue life, rotatory bending fatigue tests of smooth specimens were carried out using 0.21% carbon steels of different ferrite grain sizes. Fifteen to eighteen specimens were fatigued at each stress amplitude, and the initiation and propagation behaviour of the cracks which led to the final fractures were examined for all the specimens. The physical basis of scatter in fatigue life was investigated, based on the successive observation of fatigue damage on the surface using the plastic replica technique, followed by an analysis of the data assuming a Weibull distribution. A statistical investigation of the physical basis of scatter in relation to the ferrite grain size was performed, i.e. the distributions for crack initiation life, crack propagation life, fatigue life and growth rate of small cracks. Finally, the fluctuation of crack growth rate was studied in relation to the application of a crack growth law for microstructurally small cracks.

Investigation of small fatigue cracks—I. Plastic deformation associated with small fatigue cracks

Small fatigue crack growth has been studied in two 7000 series aluminium alloys possessing different grain sizes. As it has been suggested that the relatively large plastic deformation associated with small fatigue cracks may explain thier fast growth rates compared to long cracks, the work focused on experimental measurement of the plastic zone size (PZS) accompanying small cracks. The characteristics of plastic zone development and the relationship between PZS and crack growth rate have been studied through periodic plastic zone size measurement of growing small fatigue cracks. It was found that small fatigue crack growth is indeed accompanied with comparatively large plastic zone sizes. It is suggested that this is mainly due to a near surface effect. It was also found that an exponential relation exists between small crack growth rates and their plastic zone sizes for a given material.

The effect of grain size on small fatigue crack growth in pure titanium

The growth behaviour of small fatigue cracks has been studied in both fine- and coarse-grained versions of a pure titanium under axial loading at stress ratio, R, of −1. The growth behaviour and its statistical properties in a coarse-grained version of a different pure titanium have also been investigated under rotating bending ( R = −1), and the results obtained were compared with those of a fine-grained version of this titanium in a previous report. Under both loading conditions, small cracks grew faster than large cracks. As the growth data were plotted in terms of the effective stress intensity factor range Δ K eff (after allowing for crack closure, the growth rates could be well correlated with large-crack data in a large-crack regime. In a small-crack regime, however, small cracks still grew faster than large cracks. Small cracks in coarse-grained material showed higher growth rates than those in fine-grained material owing to a much smaller effect of microstructure such as grain boundaries and crack deflection. Stage I facets were observed in all the specimens tested, and their depths were less than the maximum grain size estimated by the statistics of the extreme values, but the distribution of stage I facet depths approximately corresponded to the maximum value distributions of grain size of the materials. The growth rates of small cracks followed log-normal distributions independent of grain size. The coefficients of variation, η, of growth rate in coarse-grained material were smaller than those in fine-grained material. The η values were significantly large at a/ d ⩽ 3 ( a = crack depth, d = grain size), indicating that the relative size of microstructurally small cracks was not dependent on grain size.

A MODEL FOR SMALL FATIGUE CRACK GROWTH

Abstract— Based on the assumption that normalized Kitagawa‐Takahashi diagrams for different materials are the same, a unified model for microstructurally small fatigue crack and physically small fatigue crack growth rates was developed to describe their behaviour under different fatigue stress ranges. The stress‐sensitive blocking effect of microstructural barriers to small fatigue crack growth is satisfactorily simulated by the model. Incorporated with the materials fatigue limit and microstructural barrier spacing, this model can be easily used in the prediction of small fatigue crack lifetime. Small fatigue crack growth rates of previous experimental studies in 7075‐T6 Aluminium alloy and HT60 steel under different stress ranges are in an envelope between two boundary prediction curves corresponding to the largest and smallest stress ranges applied in the experiments. Problems concerning model accuracy and model application are also discussed in the present paper.

破壊力学 Ｔｉ‐６Ａｌ‐４Ｖ合金の微小疲労き裂進展における微視組織の影響とき裂開閉口 β域熱処理材を用いた検討

The effect of microstructure on the small fatigue crack growth behavior was studied in Ti-6Al-4V alloy, to which three kinds of heat treatments were conducted in beta field, in comparison with the long crack growth properties. The small crack opening-closing behavior relevant to microstructure was also investigated. The crack closing ratio exhibited a noteworthy behavior near crystallographic orientation boundaries: the ratio gradually increased as the crack tip approached to the grain boundary, and decreased after passing through. This experimental result indicates that the change of the crack closure is one of the important factors which lead to the characteristic behavior of small crack growth near grain boundaries. It was shown from these experimental evidences that the microstructural influence on the small crack growth rate and the difference in growth rate between small and long cracks were intrinsic and not compensated only by the crack closure phenomenon.

き裂開閉口測定に基づくＮｉ基超合金の高温における微小疲労き裂進展の調査

Fatigue small crack growth behavior was studied in three kinds of Ni-base superalloys at high temperature and was compared with the physically long crack behavior; a single crystal alloy, CMSX-2, a directionally-solidified alloy, CM247LC-DS and a polycrystalline alloy, CM247LC-CC. The crack opening-closing behavior was also measured by means of a new device originally developed and improved. The experimental results indicated that the small cracks exhibited a higher growth rate than the long cracks at the same stress intensity factor range in all of the materials. However, the rates of small crack growth in the CMSX-2 and CM247LC-DS agreed with those of the long crack, when they were correlated with the effective stress intensity factor. This shows that the crack opening-closing effect is mainly responsible for difference of small and long crack growth rates. In contrast to these two materias, the lack of similitude between small and long crack propagation rates was still found in CM247LC-CC, even if the crack closure effect was taken into account.

Effect of Grain Size on Fatigue Life and Fatigue Crack Growth Mechanism of IN 718.

Low cycle fatigue tests and fatigue crack growth rate tests were conducted at ambient temperature on fine and coarse-grained IN 718 of which grain size numbers were ASTM No. 10 and ASTM No. 5, respectively. In the low cycle fatigue tests on fine-grained material at maximum stress of 1180 MPa and R=0, fatigue lives of ASTM standard cylindrical specimens ranged from 50000 to 70000 cycles, and every fatigue crack was initiated from the defect on the surface of specimen, i.e., nonmetallic inclusion or carbide. The Fatigue crack growth rate test showed that small surface cracks in the coarse-grained material were propagated considerably faster than those in the fine-grained material. If the small surface cracks started to be propagated from the defects immediately after the start of cyclic loading, the difference in fatigue life between fine-grained material and coarse-grained material resulted from the difference in the small surface crack growth rate.

Statistical Property of Small-Crack Growth Rate in 0.21% C Steel Smooth Speciments Having Different Ferrite Grain Sizes.

In order to investigate the scatter characteristics of growth rate for small cracks, rotating bending fatigue tests of smooth specimens were carried out using 0.21% carbon steel with different ferrite grain sizes. Fifteen to eighteen specimens were fatigued at each stress range, and the growth rate of a crack which led to the fracture was measured for all the specimens. The growth data were analyzed statistically. The results show that the growth rate for each crack length can be expressed by a three-or two-parameter Weibull distribution. The CV, coefficient of variance, for crack growth rate distribution was calculated, and using the CV the relations between the scatter in growth rate and the ferrite grain size and stress level were investigated.

Nondestructive detection of fatigue cracks in PM 304 stainless steel by internal friction and elasticity

A resonant frequency mechanical vibration method was used to nondestructively detect fatigue crack initiation in notched 304 stainless steel samples prepared by powder metallurgy. This method allowed the determination of an effective elastic modulus and the direct measurement of internal friction. Changes in the modulus and internal friction were found to correlate well with the presence of 50 μm long fatigue cracks. The length of the through cracks initiated at the notch was measured using surface replicas, which were examined in a scanning electron microscope. Small crack growth rate data were also obtained and plotted versus the stress intensity factor. The crack growth rates were compared with long crack data performed on compact tension samples of this material. The short cracks grew at intensity factors below the long crack threshold stress intensity factor.

SCATTER IN SMALL CRACK PROPAGATION AND FATIGUE BEHAVIOUR IN CARBON STEELS

AbstractIn order to investigate the physical basis of scatter in fatigue behaviour, rotatory bending of smooth fatigue specimens was carried out using two steels with different microstructures (ferrite/pearlite and tempered martensite). Fifteen or sixteen specimens were fatigued at each stress, and the initiation and propagation behaviour of a crack which led to the final fracture were examined for all the specimens. The emphasis was to investigate the scatter characteristics of fatigue behaviour based on the scatter of small crack propagation data. The data were analysed by assuming the Weibull distribution, and a statistical investigation of the physical basis of scatter was performed, i.e. the distributions for fatigue life, crack propagation life and growth rate of small cracks were established and the relation between the scatter in those distributions was studied.

The growth of small fatigue cracks in A286 steel

Fatigue crack propagation in high-strength A286 steel was studied by comparing crack growth rates determined from: (1) conventional long-crack propagation tests, (2) closure-free long-crack tests at constant Kmax, and (3) small-crack propagation tests. Small-crack growth rates were measured by following the growth of surface cracks in samples cycled from near-zero stress to 0.5 or 0.8σy. While most of the surface cracks became dormant shortly after nucleation, some grew into long cracks, and some of these propagated at cyclic stress intensities below the long-crack threshold, ΔKth (or ΔK th eff , the threshold cyclic stress intensity after crack closure effects have been removed). Surface cracks grew more rapidly than long cracks at the same ΔKor ΔKeff. The small-crack effect disappeared when the crack-tip plastic zone size became greater than the grain size. The results show that the absence of crack closure is only one of several factors that influence short-crack growth in A286 steel. Both peak stress and microstructural effects are important. Microstructural effects are apparently responsible for subthreshold crack growth; the cracks that grow at ΔK < ΔK th eff form and grow in statistically weak regions of the microstructure.

Fatigue crack growth retardation in plane stress and plane strain: Damri, D. and Knott, J.F. J. Mater. Sci. 15 Oct. 1991 26, (2), 5613–5617

This paper reviews the capabilities of a plasticity-induced crack-closure model to predict fatigue lives of metallic materials using `small-crack theory' for various materials and loading conditions. Crack-tip constraint factors, to account for three-dimensional state-of-stress effects, were selected to correlate large-crack growth rate data as a function of the effective-stress-intensity factor range (ΔKeff) under constant-amplitude loading. Some modifications to the ΔKeff rate relations were needed in the near-threshold regime to fit measured small-crack growth rate behavior and fatigue endurance limits. The model was then used to calculate small- and large-crack growth rates, and to predict total fatigue lives, for notched and un-notched specimens made of two aluminum alloys and a steel under constant-amplitude and spectrum loading. Fatigue lives were calculated using the crack-growth relations and microstructural features like those that initiated cracks for the aluminum alloys and steel for edge-notched specimens. An equivalent-initial-flaw-size concept was used to calculate fatigue lives in other cases. Results from the tests and analyses agreed well.

Optical observations of fatigue crack growth behaviour of a low-alloy pressure vessel steel in high-temperature pressurized water: Katada, Y., Nagata, N. and Sato, S. Int. J. pressure Vessels Piping Oct. 1991 48 (1) 37–52

This paper reviews the capabilities of a plasticity-induced crack-closure model to predict fatigue lives of metallic materials using `small-crack theory' for various materials and loading conditions. Crack-tip constraint factors, to account for three-dimensional state-of-stress effects, were selected to correlate large-crack growth rate data as a function of the effective-stress-intensity factor range (ΔKeff) under constant-amplitude loading. Some modifications to the ΔKeff rate relations were needed in the near-threshold regime to fit measured small-crack growth rate behavior and fatigue endurance limits. The model was then used to calculate small- and large-crack growth rates, and to predict total fatigue lives, for notched and un-notched specimens made of two aluminum alloys and a steel under constant-amplitude and spectrum loading. Fatigue lives were calculated using the crack-growth relations and microstructural features like those that initiated cracks for the aluminum alloys and steel for edge-notched specimens. An equivalent-initial-flaw-size concept was used to calculate fatigue lives in other cases. Results from the tests and analyses agreed well.

Modeling of Notch Effects on Stress Corrosion Cracking

The intergranular stress corrosion cracking (IGSCC) of sensitized Type 304 stainless steel (SS) has been investigated by slow strain rate tests (SSRTs) in 289°C water containing sulfate impurity. Both smooth and circumferentially notched specimens were used to assess the effects of strain concentrations on stress corrosion cracking (SCC). Experiments were conducted over a range of nominal strain rates of 10−5 to 10−7 s−1. A comparison of the results observed for the smooth and notched specimens suggests that the estimated growth rates of small cracks in SSRT specimen geometry is influenced by the presence of strain concentrations. In particular, the average crack growth rates estimated from tests performed at the same nominal strain rate are observed to increase with the notch depth, and power-law relationships exist between strain rate and SCC parameters such as failure time and crack growth rate. The strain concentration factors at the notch roots of Type 304 specimens subjected to axial load have been estimated by finite-element elastic-plastic stress analyses, as well as by Neuber’s rule. The nominal and crack-tip strain rate effects on SCC in both smooth and notched specimens are interpreted in terms of a model based on elastic-plastic fracture mechanics and film-rupture mechanisms that invoke diffusion-controlled SCC growth kinetics.

Damping characteristics of flexural vibration for partially covered beams with constrained viscoelastic layers: Dewa, H., Okada, Y. and Nagai, B. JSME Int. J. Ser. III, June 1991 34, (2), 210–217

For a wide range of stress amplitudes, the fatigue life of cast aluminum specimens is dominated by propagation of small cracks that initiate predominantly from microshrinkage pores. Therefore, an understanding of the small-crack behavior and knowledge of the pore size distribution enables the prediction of fatigue properties. In order to understand the influence of processing parameters on fatigue performance, small-crack growth was monitored for three different solidification times in both peak-aged (T6) and over-aged (T7) conditions of cast W319 aluminum, a commercial Al–Si–Cu alloy used in automotive castings. Results indicate that small cracks in the T6 microstructures grow only slightly slower than those in the T7 material. A correlating parameter of the form [(εσ/σy)na] is shown to effectively characterize small-crack growth rates in both the T6 and T7 conditions for a wide range of applied stresses and solidification times.

Microstructure and crack-shape effects on the growth behavior of small fatigue cracks in Ti24Al11Nb

The effects of four different microstructures in the titanium aluminide alloy Ti24Al11Nb (at.%) on the fatigue crack growth behavior of small surface cracks and large cracks have been investigated. The four microstructures were a Widmanstätten basketweave, a Widmanstätten aligned colony, an equiaxed primary α 2 in a Widmanstätten matrix and a completely equiaxed α 2 structure. Small cracks were found to develop arbitrary shapes owing to the effects of microstructure. The crack shapes (aspect ratios) were measured using a laser interferometric and photomicroscopic system, and these measurements allowed accurate calculation of crack growth rates at the surface position as well as at the depth position for the surface cracks. After accounting for the continuous variation in crack shape in crack growth rate calculations, the trends in small-crack growth rates agreed reasonably well with the corresponding large-crack growth rates. While the crack growth rates at depth positions for small cracks correlated well with large-crack data in the basketweave microstructure, crack growth rates at surface positions correlated well with the corresponding large-crack data in the other microstructures. The microstructural factors that may be responsible for this behavior are discussed.