Fatigue damage detection in 2024 aluminum alloy by optical correlation

The use of optical correlation techniques to monitor fatigue damage in 2024-T3 aluminum alloy is described. Topographical information from the surface of a sheet specimen is recorded holographically and compared with the actual surface by measuring correlation intensity as fatigue damage accumulates. The hologram is recorded on a thermoplastic - photoconductor device that can be developed and erasedin situ, so that repeated recordings can be made easily during a fatigue test. The results are presented as curves of correlation intensity (I _c) vs fatigue cycles(N) for unnotched specimens fatigued in tension-tension. The curves exhibit three regions, namely a decelerating loss of log I_c vs N over the first portion of the fatigue life (Region A), followed by a linear loss (RegionB), and finally by an accelerating loss over the final portion of the life (Region C). Changes in the initial surface finish of the specimen do not affect the general form of the correlation curve. Metallographic evidence indicates that the three regions correspond to fatigue-induced changes in the specimen surface, although dimensional changes in the specimen may also contribute to correlation losses. RegionA appears to correspond to a period of initial cyclic strain adjustment, and Region B to a period of crack initiation and/or early growth. RegionC corresponds to the presence of growing fatigue cracks, and the transition fromB to C occurs when the cracks exceed about 20 μm in length. Thus, the correlation intensity data provide a sensitive indication of accumulating fatigue damage and impending failure in individual specimens. formerly Graduate Student in the Department of Metallurgical Engineering, Wayne State University, Detroit, MI 48202 .     

The Role of Local Microstructure on Small Fatigue Crack Propagation in an α?+?β Titanium Alloy, Ti-6Al-2Sn-4Zr-6Mo

Microstructural origins of the variability in fatigue lifetime observed in the high- and very-high-cycle fatigue regimes in titanium alloys were explored by examining the role of microstructural heterogeneity (neighborhoods of grains with similar crystallographic orientations or microtexture) on the initiation and early growth of fatigue cracks in Ti-6246. Ultrasonic fatigue of focused ion beam (FIB) micronotched samples was used to investigate long lifetime (10⁷ to 10⁹) behavior for two microstructural conditions: one with microtexture and one without microtexture. For specimens containing notches of nominally 20???m in length, fatigue crack initiation in the microtextured material was most likely to occur from notches placed in neighborhoods with a microtexture favorably oriented for easy basal slip. Initiation lifetimes in the untextured material with similar sized notches were, on average, slightly greater than those for the microtextured condition. In both materials, the crack-initiation lifetime from micronotches of length 2c?>?20???m was a very small fraction (<1?pct) of the measured fatigue lifetime for unnotched specimens. Furthermore, in the microtextured condition, small fatigue crack propagation rates did not correlate with the microtextured regions and did not statistically differ from average small crack growth rates in the untextured material. As the micronotch size was reduced below 20???m, fatigue crack initiation was controlled by microstructure rather than by FIB-machined defects. Finally, predictions of the fraction of life consumed in small and long fatigue crack growth from preexisting cracks nominally equivalent in size to the micronotches was compared with the measured fatigue life of unnotched specimens. The predicted range of lifetimes when factoring in the experimentally observed variability in small fatigue crack growth, only accounted for 0.1?pct of the observed fatigue lifetime variability. These findings indicate that in the high-and very-high-cycle fatigue regimes, fatigue life is dominated by crack initiation and that the variation in the initiation lifetime is responsible for the observed variation in total fatigue life.     

Low-Cycle fatigue properties of a SiC Whisker-reinforced 2124 aluminum alloy

Low-cycle fatigue microcracking leading to failure of smooth specimens of a powder metallurgy (PM) 2124 aluminum alloy reinforced with 20 vol pct SiC whiskers was studied. The crack size near the onset of unstable growth was inferred to be 50 to 70 μm in the stress amplitude range of the present study (400 to 600 MPa,R = −1) from observations of the fracture surfaces of the specimens. This corresponds to stress intensities between 1/3 to 1/2 typical values ofK _1c or 1/4 to 1/9 the critical length predicted fromK _1c values of 12 to 14 MPa√m. The microcrack size distributions and growth data were obtained from the low-cycle fatigue specimens at various stages of fatigue, using a surface replica technique. During continued cycling, microcracks formed and were lost through linkage with other cracks. At the same time, the fraction of small cracks (<5 μm) decreased, while that of larger cracks (>5 μm) increased. The total number of cracks increased with increasing numbers of cycles. Typical microcrack growth rates were determined to bedb/dn = (3.57 to 6.11) × 10⁻¹⁰ (Δ/K)^2.2to2.48 in the lateral direction of the crack, andda/dn = (5.83 to 13.0) × 10⁻¹¹ (ΔK)^{1.54 to 1.60} in the depth direction of the crack.     

An analysis of non-propagating fatigue cracks

An analysis for the formation of nonpropagating fatigue cracks at (the base of V-shaped) notch roots, based on the considerations of the extent of the critically stressed region ahead of a notch or a crack tip, and the resulting volumetric strength effect, is developed. Assuming that the minimum local cyclic stress required for crack initiation from a notch root is equal to the unnotched fatigue limit, σ_e, and that the minimum local cyclic stress required for the propagation of the crack is equal to the theoretical strength of the material, σ_e, a model of notch fatigue limit is proposed that shows that nonpropagating cracks should form at the notch base if ρ≤ ρ⁰, a critical root radius, provided the notch is sufficiently deep,i.e. d ≥ ρ₀. The radius ρ₀ is a material constant and can be estimated from known material properties. The estimated values of ρ₀ are in fairly good agreement with available experimental values for steels and pure copper. For stresses near the notch fatigue limit it is suggested that p₀ be regarded as a radius above which notch fatigue limit is essentially initiation controlled and below which essentially propagation controlled. The notch fatigue limit based on complete fracture can then be estimated more accurately with mild as well as sharp notches.     

Gel electrode imaging of metal fatigue: Part i. cracks in 6061-T6 aluminum

A simple electrochemical technique is described which produces high-resolution, visible images of fatigue cracks in aluminum alloys, without conventional metallographic surface preparation. The only preparation required is the formation of a thin (14 nm) anodic oxide film on the surface. After fatigue cycling, a semisolid electrolyte is placed in contact with the specimen and a voltage applied. As the current flows to the fatigue cracks a clearly defined image is formed. The capabilities of the technique are demonstrated by measurements on6061-T6 aluminum, and the images are correlated with scanning electron micrographs of the specimens. The images are reproducible and record features of the cracks which are barely discernible with a scanning electron microscope. Fatigue cracks only ∼30 μm long have been detected. Measurement of the charge flow during imaging is a quantitative measure of the crack length. These measurements indicate that considerably smaller cracks should also be detectable.     

Effect of strain wave shape on low-cycle fatigue crack propagation of SUS 304 stainless steel at elevated temperatures

Effect of strain wave shape on strain-controlled low-cycle fatigue crack propagation of SUS 304 stainless steel was investigated at 600 and 700 °C. It was found that the rate of crack propagation in a cycle-dependent region was successfully correlated with the range of cyclicJ-integral, ΔJf, regardless of the strain wave shape, frequency, and test temperature. It was also shown that the rate of crack propagation gradually increased from cycle-dependent curve to time-dependent one with decreasing frequency and slow-fast strain wave shape, and that one of the factors governing the rate of crack propagation in such a region was the ratio of the range of creepJ-integral to that of totalJ-integral, ΔJ _c/ΔJT. Based on the results thus obtained, an interaction damage rule proposed semi-empirically was interpreted, with regard to crack propagation. Furthermore, fatigue crack initiation mechanism in slow-fast strain wave shape was studied, and it was shown that grain boundary sliding took an important role in it.     

Fatigue crack propagation in 4140 steel

The fatigue crack propagation rates, da/dN, of 4140 steel were measured in dry argonvs tempering temperature. In specimens 3.2 mm thick at a given ΔK between 15 and 30 MN/ m^3/2, da/dN decreases with increasing tempering temperature, reaches a shallow minimum for tempering at 400°C. The rate for as-quenched specimens increases withR ratio; this is not the case for the 400, 550 and 650°C tempers. Reducing the specimen thickness to 1.3 mm has little effect on the 650°C temper but causes a large decrease in da/dN for the asquenched condition and 200°C temper. Edge notch specimens tempered at 550 and 650°C are subject to crack arrest from cycling prior to crack initiation. The results are discussed in terms of the metallurgical structures and various fatigue crack propagation equations which have been proposed. The results cannot be explained on the basis of da/dN being determined only by Young’s modulus andK _c.     

Fatigue Crack Prognostics by Optical Quantification of Defect Frequency

Defect frequency, a fatigue crack prognostics indicator, is defined as the number of microcracks per second detected using a laser beam that is scanned across a surface at a constant predetermined frequency. In the present article, a mechanistic approach was taken to develop a methodology for deducing crack length and crack growth information from defect frequency data generated from laser scanning measurements made on fatigued surfaces. The method was developed by considering a defect frequency vs fatigue cycle curve that comprised three regions: (i) a crack initiation regime of rising defect frequency, (ii) a plateau region of a relatively constant defect frequency, and (iii) a region of rapid rising defect frequency due to crack growth. Relations between defect frequency and fatigue cycle were developed for each of these three regions and utilized to deduce crack depth information from laser scanning data of 7075-T6 notched specimens. The proposed method was validated using experimental data of crack density and crack length data from the literature for a structural steel. The proposed approach was successful in predicting the length or depth of small fatigue cracks in notched 7075-T6 specimens and in smooth fatigue specimens of a structural steel.     

An analysis of non-propagating fatigue cracks

An analysis for the formation of nonpropagating fatigue cracks at (the base of V-shaped) notch roots, based on the considerations of the extent of the critically stressed region ahead of a notch or a crack tip, and the resulting volumetric strength effect, is developed. Assuming that the minimum local cyclic stress required for crack initiation from a notch root is equal to the unnotched fatigue limit, σ_e, and that the minimum local cyclic stress required for the propagation of the crack is equal to the theoretical strength of the material, σ_e, a model of notch fatigue limit is proposed that shows that nonpropagating cracks should form at the notch base if ρ≤ ρ⁰, a critical root radius, provided the notch is sufficiently deep,i.e. d ≥ ρ₀. The radius ρ₀ is a material constant and can be estimated from known material properties. The estimated values of ρ₀ are in fairly good agreement with available experimental values for steels and pure copper. For stresses near the notch fatigue limit it is suggested that p₀ be regarded as a radius above which notch fatigue limit is essentially initiation controlled and below which essentially propagation controlled. The notch fatigue limit based on complete fracture can then be estimated more accurately with mild as well as sharp notches. D. N. LAL, formerly a Graduate Assistant in Materials Science, Syracuse University     

Fatigue Strength and Crack Initiation Mechanism of Very-High-Cycle Fatigue for Low Alloy Steels

The fatigue strength and crack initiation mechanisms of very-high-cycle fatigue (VHCF) for two low alloy steels were investigated. Rotary bending tests at 52.5?Hz with hour-glass type specimens were carried out to obtain the fatigue propensity of the test steels, for which the failure occurred up to the VHCF regime of 10⁸ cycles with the S-N curves of stepwise tendency. Fractography observations show that the crack initiation of VHCF is at subsurface inclusion with ??fish-eye?? pattern. The fish-eye is of equiaxed shape and tends to tangent the specimen surface. The size of the fish-eye becomes large with the increasing depth of related inclusion from the surface. The fish-eye crack grows faster outward to the specimen surface than inward. The values of the stress intensity factor (K _I ) at different regions of fracture surface were calculated, indicating that the K _I value of fish-eye crack is close to the value of relevant fatigue threshold (??K _th ). A new parameter was proposed to interpret the competition mechanism of fatigue crack initiation at the specimen surface or at the subsurface. The simulation results indicate that large inclusion size, small grain size, and high strength of material will promote fatigue crack initiation at the specimen subsurface, which are in agreement with experimental observations.     

Crack deflection: Implications for the growth of long and short fatigue cracks

The influences of crack deflection on the growth rates ofnominally Mode I fatigue cracks are examined. Previous theoretical analyses of stress intensity solutions for kinked elastic cracks are reviewed. Simple elastic deflection models are developed to estimate the growth rates of nonlinear fatigue cracks subjected to various degrees of deflection, by incorporating changes in the effective driving force and in the apparent propagation rates. Experimental data are presented for intermediate-quenched and step-quenched conditions of Fe/2Si/0.1C ferrite-martensite dual phase steel, where variations in crack morphology alone influence considerably the fatigue crack propagation rates and threshold stress intensity range values. Such results are found to be in good quantitative agreement with the deflection model predictions of propagation rates for nonlinear cracks. Experimental information on crack deflection, induced by variable amplitude loading, is also provided for 2020-T651 aluminum alloy. It is demonstrated with the aid of elastic analyses and experiments that crack deflection models offer a physically-appealing rationale for the apparently slower growth rates of long fatigue cracks subjected to constant and variable amplitude loading and for the apparent deceleration and/or arrest of short cracks. The changes in the propagation rates of deflected fatigue cracks are discussed in terms of thelocal mode of crack advance, microstructure, effective driving force, growth mechanisms, mean stress, slip characteristics, and crack closure.     

The influence of crack length on fatigue crack growth in deep sharp notches

The fatigue crack growth rateda/dN of short cracks and the transition to long crack behavior were investigated for ARMCO-iron. Deep notched specimens with very small notch radius (between 1.5 and 4 μm) were used. The experiments were performed with constant stress intensity ranges for various stress ratios; the fatigue crack growth rate was measured as a function of the crack length. The results permit a discussion of the mechanisms responsible for the different behavior of “short” and “long” cracks.     

Stress-corrosion cracking of Ti-8Al-lMo-lV in aqueous environments: 2. Plastic zones,crack morphology,and fractography

Optical and electron metallographic studies of stress-corrosion cracks in Ti-8Al-lMo-lV have verified that the principal crack extension mechanism is cleavage of theα grains. There are two distinct crack morphologies which correspond to the two regimes of subcritical crack velocity. At low stress intensities(a ∞ K _I) the microscopic crack front consists of small cleavage facets approximately 1 to 4α grain diameters in size, and ligaments of material which fracture by ductile rupture and corrosion. At high stress intensities (a ≅ constant), the crack front consists of large cleavage “fingers”, 20 to 50α grain diameters in length, separated by regions which fracture by a combination of cleavage (on a much smaller scale), ductile rupture, and corrosion. The transition from Stage I to Stage II crack propagation apparently occurs when the strain-energy release rate is sufficient to support two crack branches,i.e., K_I≥ √2K _Iscc. Thereafter, the diameter of the plastic zone at the crack tip remains constant, suggesting that the effective stress intensity at the tip of each branch is also invariant. The slip within the plastic zone is markedly nonhomogeneous, and trenches are often observed along the slip steps. Formerly with the Metal Science Group, Battelle Columbus Laboratories, Columbus, Ohio.     

Sliding mode crack closure and mode III fatigue crack growth in mild steel

Mode III fatigue crack growth behaviour in AISI C1018 steel (R = −1, 1 Hz) is investigated in circumferentially notched cylindrical specimens. The crack surfaces in contact glide against each other, the friction, abrasion and mutual support between them reducing the effective stress at the crack tip (“sliding mode crack closure”). Crack growth rates under the effective stressing (so-called “true” crack growth rates) can be obtained by extrapolation to zero crack length (c → 0), when no friction can be present. This way the effect of “sliding mode crack closure” can be quantified. It increases with increased plastic strain intensity, reaches a maximum for ΔΓ₁₁₁~1.5 × 10⁻²mm for the tested specimens and decreases drastically for stressing beyond that value. For these large loadings the large plastic zones formed in mode III open the crack in mode I and eliminate friction. Fractographic evidence for the processes involved is obtained.     

Initiation and growth of small fatigue cracks in a Ni-base superalloy

This article reports research on the initiation and growth of small fatigue cracks in a nickel-base superalloy (produced commercially by INCO as INCOLOY* 908) at 298 and 77 K. The experimental samples were square-bar specimens with polished surfaces, loaded in fourpoint bending. The crack initiation sites, crack growth rates, and microstructural crack paths were determined, as was the large-crack growth behavior, both at constant load ratio (R) and at constant maximum stress intensity (K _max). Small surface cracks initiated predominantly at (Nb,Ti)_xC_y, inclusion particles, and, less frequently, at grain boundaries. Small cracks grew predominantly along {111} planes in individual grains and were perturbed or arrested at grain boundaries. For values of ΔK above the large-crack threshold, ΔK _th, the average rate of smallcrack growth was reasonably close to that of large cracks tested under closure-free conditions. However, short-crack growth rates varied widely, reflecting the local heterogeneity of the microstructure. The threshold cyclic stress (Δσ_th) and the threshold cyclic stress intensity (ΔKσ_th) for small surface cracks were measured as functions of the crack size, 2c. The results suggest that a combination of the fatigue endurance limit and the threshold stress intensity for closure-free growth of large cracks can be used to define a fatigue-safe load regime. formerly with Lawrence Berkeley Laboratory     

Crack propagation near cylindrical voids

Stress intensity factorsK _I andK _II are presented for a planar, sharp-ended crack subjected to nearby line forces and line force doublets. The resulting near crack tip stress field is used to predict the influence of such singularities upon the crack propagation direction. The concept of the criticality of the angle of crack departure from symmetric propagation is introduced and used to compare computer predictions with experiments performed on double cantilever beam (DCB) specimens of 7075 aluminum alloy. The form of the near crack tip elastic equations and the criticality are verified. The critical angle parameter is found to be a material and experimental constant, independent of the strength of the centers of stress.     

Fatigue crack initiation and early propagation in Al 2219-T851

Fatigue crack initiation in Al 2219-T851 for fully reversed loading(R = σ/σ_max =?1) parallel to the material rolling direction is found to occur at intermetallic inclusions at the specimen surface. The inclusions are not involved in crack initiation for fatigue perpendicular to the rolling direction, and for this orientation crack initiation is at grain boundaries and specimens have an increased fatigue life. Except for fatigue at low peak stress, multiple numbers of microcracks are formed and for selected failed specimens the number of cracks has been determined as a function of crack length. Such crack length distribution measurements show that there is significant retardation of microcracks by interaction with grain boundaries. Furthermore it is found that the coalescence of microcracks provides a mechanism for cracking to “jump“ grain boundaries and reduce fatigue lifetime. The effect of relative humidity on this process is to increase the observed mean crack length, and decrease the number of crack initiations apparently due to weakening of the matrix-intermetallic interface at potential initiation sites. The overall result is that no significant dependence of fatigue life on relative humidity is found.