An energy-balance approach to the problem of fatigue-crack growth

An energy-balance approach to the problem of fatigue-crack propagation is considered here, the basic premise being that the energy equivalent of the work input in fatique over a period of time Δt must be greater than or equal to the energy dissipated in the form of heat together with the energy required to produce the plastic zone ahead of the crack tip and to create the necessary the surface for crack propagation over the same time period Δt. A fatigue-crack-growth (FCG) rate equation has been formulated based upon the above energy-balance concept. The fatigue-crack-growth rate is found to be inversely proportional to the plastic work and the energy requirement for the creation of new surfaces and directly proportional to the fatigue input energy. An experimental approach for determining the fatigue-crack-propagation rate using the energy-balance concept has been outlined.     

The effect of D2O on fatigue-crack propagation in a high-strength aluminum alloy

The rates of fatigue-crack propagation for a high-strength aluminum alloy (7075-T651) in an environment of D₂O (99.98% purity) at room temperature were determined and compared with data obtained in high-purity argon and distilled water. The results showed that D₂O caused a ten-fold increase in the rate of fatigue-crack propagation (up to 10^–4 inch per cycle), which is equal to the increase caused by distilled water. These results lend further support to the previous observation that the rate controlling process for fatigue-crack propagation in this alloy (at rates below 10^–4 inch per cycle) is the mechanical process of creating new crack surfaces, instead of either the transport of aggressive environment to the crack-tip or diffusion of hydrogen ions into the material ahead of the crack tip.     

Fatigue-crack propagation in modified 300-grade maraging steel

Fatigue-crack-propagation studies were performed on an 18Ni (300-grade) maraging steel at room temperature in dry and humid argon environments (atmospheric pressure) to examine the effect of moisture on the rate of fatigue-crack propagation and on the fracture path through the microstructure. The results showed that the rate of fatigue-crack propagation was increased by about 30 per cent by moisture in the argon.

Fractographic analyses showed that the crack path in the dry-argon atmosphere was transgranular. In the humid-argon atmosphere, the crack path was predominantly transgranular with respect to prior austenitegrain boundaries, but appeared to be partly intergranular with respect to subboundaries. The fracture surfaces of the specimens were covered with fatigue striations regardless of the testing environment, an indication that the mechanism of crack growth was similar in both environments, even though the fracture surfaces of the specimens fatigued in dry argon were rougher. Therefore, although moisture in the argon environment accelerated crack growth by about 30 per cent, it apparently did not change the mechanism of crack growth. Evidently the effect of moisture in the environment is to promote striation-type crack growth along paths such as subboundaries. A comparison of fractographic results with those of previous work on hydrogen-accelerated fatigue-crack growth in 250-grade maraging steel indicates that in 300-grade maraging steel the increased rate of fatigue-crack growth in the humid environment is not hydrogen-induced.     

Fatigue-crack propagation in high yield-strength steels

Information on fatigue-crack growth in high yield-strength steels is necessary for toe prediction of service lives of structures subjected to fotigue loading. Therefore, as part of a long-range program to establish the mechanical behavior of high yield-strength steels, the fatigue-crack propagation in four high-strength steels-HY-80, HY-130, 10Ni-Cr-Mo-Co, and 12Ni-5Cr-3Mo-was investigated.

A technique was, developed to accurately measure incremental fatigue-crack growth by fofiowing the extension of the crack along uniformly spaced marks on the surface of a wedge-opening-loading (WOL) specimen that was cycled under constant maximum and minimum loads. The primary factor affecting crack growth rates in fatigue was found to be the applied stress-intensity range, ΔK_I. A conservative crack propagation rate for high yield-strength steels fatigue-tested at room temperature in an air environment was determined to be: .

The present results were compared with existing fatigue-crack-growth data. With one exception, all crack-growth data (for a total of 19 high yield-strength steels) could be represented by die above equation. Thus, it appears that fatigue-crack growth rates in high yield-strength steels tested in a room-temperature air environment can be represented by a single equation that is sufficiently accurate for engineering purposes.     

Fatigue-crack advance mechanisms in polymers

The room-temperature fatigue-crack propagation behaviour of poly(butylene terephthalate) is strongly influenced by hysteretic heating near the crack tip since the glass transition temperature is just above room temperature. At low frequencies or stress intensities, the crack tip damage zone consists of several layers of crazes. At high frequencies or stress intensities, hysteretic heating causes a drop in yield stress and a large increase in the depth of the crack tip damage layer. At the same time, the increase in the plane stress plastic zones near the free surfaces produces large shear lips which flank the interior craze zone. This transformation results in a crack growth rate transition which appears as a crack deceleration followed by rapid crack acceleration. This thermal transition can be suppressed or delayed by immersion in water or silicone oil to reduce heat build-up in the sample during testing.     

Investigations into the fatigue crack initiation and propagation behaviour in austenitic stainless steel X5 CrNi 18 9 (1.4301)

Controlled load fatigue-crack growth rate tests were conducted using three point bend type specimens. The austenitic stainless steel X5 CrNi 18 9 (material number 1.4301, similar to AISI 304) was tested. For this high toughness material, the crack initiation and crack propagation is discussed in terms of linear-elastic analysis even for the highest plasticity regime. The curve for the onset of stable crack growth during fatigue starting from a notch is discussed as well as the stable crack growth up to large plasticity. From the standpoint of load controlled fatigue it is indicated that the time between crack initiation and crack length at the ?end of life”? is large.     

Estimation of the effects of plasticity and resulting crack closure during small fatigue crack growth

On the basis of a plastic-strip model and the method of singular integral equations, a closed-form analytical solution of the problem of an elastic-plastic plate containing a rectilinear fatigue crack is considered. The solution is used for the prediction of fatigue growth of `mechanically-small' crack by accounting for reverse plastic yielding and plasticity-induced crack closure in the material. The main effects of these factors on the crack-growth rate are analyzed, and the predicted results are compared with experimental data on small fatigue-crack growth in a aluminum-lithium alloy 2091-T351 and Fe-3% Si alloy.     

Fatigue-crack propagation characteristics of aluminum alloys in thick sections

Fatigue-crack propagation data have been obtained for a variety of aluminum alloys, tempers, and products in relatively thick sections of interest for large aircraft shapes. For the higher stress-intensity ranges, the alloys rate in about the same order with regard to resistance to fatigue crack propagation as with regard to plane-strain fractare toughness. However, for low stress intensity ranges (i.e. short cracks or low load ranges) the rate of fatigue-crack propagation was lowest for two alloys, 2020-T651 and 2014-T652. which have low plane-strain fracture toughness. The relative order for different specimen orientations was generally consistent with that based upon plane-strain fracture toughness. High humidity results in higher rates of fatigue-crack propagation, with the effect indicated to be largest for those alloys which are susceptible to stress corrosion cracking.     

EFFECT OF TEMPERATURE ON THE FATIGUE-CRACK PROPAGATION BEHAVIOUR OF INCONEL X-750

Abstract— Linear-elastic fracture mechanics techniques were used to characterize the effect of temperature on the fatigue-crack propagation behaviour of precipitation heat-treated. Inconel X-750 in an air environment over the temperature range 24 to 649°C. In general, crack growth rates were found to increase with increasing temperature, particularly at the highest test temperature (649°C). The effect of stress ratio on the fatigue-crack growth behaviour of Inconel X-750 was examined at 538°C, and results indicated that the elevated temperature fatigue response of this nickel-base superalloy was relatively insensitive to stress ratio level at the growth rate levels studied. Metallographic and electron fractographic examination of Inconel X-750 fatigue fracture surfaces revealed operative crack growth mechanisms to be a function of temperature and prevailing stress intensity factor. Under room temperature and intermediate temperature conditions (up to 538°C), all fatigue fracture surfaces exhibited a faceted crystallographic morphology at low crack growth rates followed by striations in the higher growth rate regime. At the highest test temperature (649°C), the fatigue crack was found to propagate by an intergranular mechanism.     

Microstructural control of fatigue-crack growth in a brittle,two-phase polymer

Fatigue-crack propagation in a brittle, two-phase polymer important in bioengineering applications has been studied. It was found that the polymeric microstructure, although based on PMMA, enhances the fatigue-crack growth resistance of the composite polymer, in comparison with pure PMMA. This enhanced behaviour derives from several micromechanisms, i.e. (1) crack front pinning by constituent particles, (2) a reduction of the intrinsic PMMA crack growth rate within the partially polymerized MMA matrix, and (3) crack-tip wandering and energy absorption through microcrack nucleation at matrix voids lying within the path of the crack.     

Modelling of short crack propagation – Transition from stage I to stage II

The propagation behaviour of short cracks under cyclic loading is simulated using a mechanism-based model for two-dimensional crack propagation in stage I. Experimental investigations on a duplex steel have been performed to characterise the different barrier effects of grain and phase boundaries, so that the crack propagation can be simulated in a real microstructure. The model allows the activation of additional slip systems resulting in a crack propagation on multiple slip bands, which is the preliminary step to stage II crack growth. By use of virtual microstructures based on Voronoi diagrams, it is possible to simulate the overall fatigue-crack propagation process starting from a microstructurally short crack in a single grain until the crack has crossed several (10–20) grains with just one model.     

Micromechanisms of fatigue-crack advance in PVC

The fatigue-crack propagation characteristics in poly(vinyl chloride) (PVC) are examined in terms of fracture mechanics concepts where the crack growth rate is related to the applied stress intensity factor range. The microscopic details of fatigue crack extension are examined with the aid of light optical, scanning and transmission electron microscopes. The mechanism of crack advance is found to be that of void coalescence through craze material generated in advance of the crack tip. While the craze is shown to grow continuously with cyclic loading, the crack is found to grow discontinuously in several hundred cycle increments.     

Fatigue-crack propagation in vinyl urethane polymers

The fatigue properties of a series of vinyl urethane polymers have been examined by determining the crack propagation rates as a function of the applied stress intensity factor. Measurements were also made of the craze lengths at the tips of the fatigue cracks, and the craze stress and crack opening displacement calculated. A model for fatigue-crack propagation based on cumulative damage within the crazed material has been proposed which is consistent with the observed results.     

Fatigue crack propagation in microcapsule-toughened epoxy

The addition of liquid-filled urea-formaldehyde (UF) microcapsules to an epoxy matrix leads to significant reduction in fatigue crack growth rate and corresponding increase in fatigue life. Mode-I fatigue crack propagation is measured using a tapered double-cantilever beam (TDCB) specimen for a range of microcapsule concentrations and sizes: 0, 5, 10, and 20% by weight and 50, 180, and 460 μm diameter. Cyclic crack growth in both the neat epoxy and epoxy filled with microcapsules obeys the Paris power law. Above a transition value of the applied stress intensity factor ΔK _T, which corresponds to loading conditions where the size of the plastic zone approaches the size of the embedded microcapsules, the Paris law exponent decreases with increasing content of microcapsules, ranging from 9.7 for neat epoxy to approximately 4.5 for concentrations above 10 wt% microcapsules. Improved resistance to fatigue crack propagation, indicated by both the decreased crack growth rates and increased cyclic stress intensity for the onset of unstable fatigue-crack growth, is attributed to toughening mechanisms induced by the embedded microcapsules as well as crack shielding due to the release of fluid as the capsules are ruptured. In addition to increasing the inherent fatigue life of epoxy, embedded microcapsules filled with an appropriate healing agent provide a potential mechanism for self-healing of fatigue damage.     

Fatigue crack propagation after overloading and underloading at negative stress ratios

This paper intends to evaluate the influence of the intrinsic properties of the materials, namely plastic and cyclic plastic properties, on the overloading/underloading effect on crack propagation rate, at baseline negative stress ratios, under plane strain conditions. The importance of the negative loading part of the fatigue cycle on crack propagation rate has been shown by previous works of this same author. In those works has also been shown that under baseline negative stress ratios there exists negative open loads and crack propagation rate does not correlate properly with the crack closure concept. These features were shown to be strongly related to plastic properties and cyclic plastic properties of the materials. It has been concluded that the Bauschinger effect may be the explanation for the different sensitivity to negative loads. Thus, some materials may be very sensitive to negative loads and some others may not be so sensitive. Tensile overload and compression underload tests, at positive and negative baseline stress ratios were made in different materials, with different plastic properties, in order to predict their influence on crack propagation rate. The main emphasis in this paper is the importance of the compressive part of the loading cycle under negative baseline R ratios on overloads/underloads effect on crack propagation rate. Results will show that the effect of overloads and underloads on crack propagation rate, at baseline negative stress ratios, are not fully accounted for by crack propagation models and that the generalized accepted behaviour of OL/UL may not be the same at baseline negative stress ratios. It will be shown that Overloads may produce acceleration instead of the accepted retardation effect. A physical understanding on the effects of OL/UL is also provided in the paper.     

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.     

FATIGUE AT LOW GROWTH RATES IN A MARAGING STEEL

This paper describes an experimental investigation of fatigue-crack growth in an ultra-high strength maraging steel, with particular attention to the relationship between crack growth rate and microstructural element size. The occurrence of intergranular fracture is shown to be unrelated to any grain size-plastic zone size equivalence, while inflections in the growth rate vs ΔK relationship are discussed in terms of the interaction between the plastic zone and other microstructural dimensions. The incidence of intergranular fracture and crack surface oxide is shown to decrease with decreasing test frequency.     

LOW CYCLE FATIGUE CRACK PROPAGATION AT A NOTCH UNDER PULSATING LOAD

Abstract Fatigue crack propagation tests were carried out under pulsating stress, partly alternating stress and alternating plastic fatigue for Cr- Mo-V steel and mild steel. Although stress and strain intensity factors do not effectively correlate the fatigue crack growth rate over a wide range of stress and strain conditions, a normalized fatigue crack growth rate [(d a /d N )/ a ] is related to the strain range. The fatigue crack propagation behavior at a notch under pulsating load is analyzed with the above relation by considering the cyclic elastic- plastic condition at the notch.     

Role of plastic work in fatigue crack propagation in metals

The plastic work required for a unit area of fatigue crack propagation U was measured by cementing tiny foil strain gages ahead of propagating fatigue cracks and recording the stress-strain curves as the crack approached. Measurements of U and plastic zone size in aluminum alloys 2024-T4, 2219-T861, 2219 overaged, and A1-6.3 wt% Cu-T4, and a binary Ni-base alloy with 7.2 wt% A1 are herein reported. The results are discussed along with previously reported measurements of U in three steels and 7050 aluminum alloy. When U is compared to the fatigue crack propagation rate at constant ΔK along with strength and modulus, the conclusion is drawn that U is one of the parameters which determines the rate of fatigue crack propagation. The relation of U to microstructure is also discussed. “Homogeneous” plastic deformation in the plastic zone ahead of the crack seems desirable.