Overload effects in fatigue crack growth by crack-tip blunting

A basic mechanisms for fatigue crack growth in ductile metals is that depending on crack-tip blunting under tensile loads and re-sharpening of the crack-tip during unloading. In the present paper, the effect of an overload in one of the cycles is studied based on this mechanism. In a standard numerical analysis accounting for finite strain, it is not possible to follow the blunting/re-sharpening process during many cycles, as severe mesh distortion at the crack-tip results from the huge geometry changes developing during the cyclic plastic straining. Here, based on an elastic-perfectly plastic material model, crack growth computations are continued up to 700 full cycles by using remeshing at several stages of the plastic deformation. Crack growth results for purely cyclic loading are compared with predictions for cases where an overload is applied, and it is shown how crack growth slows down after the overload. Different load amplitudes, and an overload at different cycle numbers are considered.     

Toughening mechanisms in elastomer-modified epoxies

Some brittle epoxies can be toughened significantly by the addition of an elastomeric phase. A great deal of controversy still exists on the nature of the toughening mechanisms. In this work tensile dilatometry at constant displacement rates was used to determine whether voiding, crazing or shear banding are the deformation mechanisms. Diglycidyl ether-bisphenol A epoxies toughened by various levels of several types of carboxyl-terminated butadiene nitrile liquid rubber were studied. The results indicate that at low strain rates the rubber particles simply enhance shear deformation. At sufficiently high strain rates the rubber particles cavitate and subsequently promote further shear deformation. No indication of crazing as an important toughening mechanism is found. No significant effect of rubber particle size or type can be ascertained.     

Toughening mechanisms in elastomer-modified epoxies

The role Of matrix ductility on the toughenability and toughening mechanism of elastomer-modified, diglycidyl ether of bisphenol A (DGEBA)-based epoxies is investigated. Matrix ductility is varied by using epoxide resins of varying epoxide monomer molecular weights. These epoxide resins are cured using 4,4 diaminodiphenyl sulphone (DDS) and, in some cases, modified with 10 vol% carboxyl-terminated copolymer of butadiene and acrylonitrile (CTBN). Fracture toughness values for the neat epoxies are found to be almost independent of the monomer molecular weight of the epoxide resin used. However, the fracture toughness of the elastomer-modified epoxies is found to be very dependent upon the epoxide monomer molecular weight. Tensile dilatometry indicates that the toughening mechanism, when present, is similar to the mechanism found for piperidine cured, elastomer-modified epoxies studied previously. Scanning electron microscopy and optical microscopy techniques corroborate this finding.     

Crack blunting mechanisms in polymers

A quantitative model has been developed to account for the degree of blunting that occurs at crack tips in epoxy materials prior to the onset of crack propagation. This mechanism controls the subsequent mode of crack growth and, to a large extent, the toughness as defined by the stress intensity factor for crack initiation. From this model a unique fracture criterion is derived which is applicable over all modes of crack propagation.     

Effects of microvoids on crack blunting and initiation in ductile materials

Effects of microvoid nucleation and growth on the stress and strain fields near a crack tip are studied by employing the Gurson's constitutive model of porous plastic solids and by using a finite element method suitably formulated to admit large geometry change. It is shown that microvoids nucleate and grow rapidly in the region at a distance less than the blunted tip diameter away from the crack tip and have a great influence on the stress fields therein. Effects of a large void near a crack tip on crack initiation are also investigated by taking the shear localization into account. The critical value of the crack tip opening displacement predicted by the present investigation is shown to coincide with the published experimental data.

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Résumé On examine les effets d'une nucléation et d'une croissance de micro-cavités sur les champs de contrainte et de déformation au voisinage de l'extrémité d'une fissure en recourant au modèle constitutif de Gurson pour les solides poreux plastiques, et en utilisant une méthode par éléments finis formulée de manière à admettre des modifications importantes de géométrie. On montre que les microcavités coalescent et croissent rapidement en avant de la fissure sur une distance inférieure au diamètre qu'atteint l'arrondissement de la fissure et qu'elles ont une grande influence sur le champ de contrainte qui règne dans cette région. On étudie également l'influence d'une grande lacune au voisinage de l'extrémité d'une fissure sur l'amorçage de la cassure, en prenant en compte le cisaillement local. On montre que la valeur critique du COD à l'extrémité de la fissure telle que prédite par la présente étude coïncide avec les données expérimentales de la littérature.

     

An FEM study on crack tip blunting in ductile fracture initiation

Ductile fracture is initiated by void nucleation at a characteristic distance (I_c) from the crack tip and propagated by void growth followed by coalescence with the tip. The earlier concepts expressed I_c in terms of grain size or inter-particle distance because grain and particle boundaries form potential sites for void nucleation. However, Srinivas et al. (1994) observed nucleation of such voids even inside the crack tip grains in a nominally particle free Armco iron. In an attempt to achieve a unified understanding of these observations, typical crack-tip blunting prior to ductile fracture in a standard C(T) specimen (Mode I) was studied using a finite element method (FEM) supporting large elasto-plastic deformation and material rotation. Using a set of experimental data on Armco iron specimens of different grain sizes, it is shown that none of the locations of the maxima of the parameters stress, strain and strain energy density correspond to I_c. Nevertheless, the size of the zone of intense plastic deformation, as calculated from the strain energy density distribution ahead of the crack tip in the crack plane, compares well with the experimentally measured I_c. The integral of the strain energy density variation from the crack tip to the location of void nucleation is found to be linearly proportional to J_IC. Using this result, an expression is arrived at relating I_c to J_IC and further extended to CTOD_c.     

On the thermal blunting of crack tips in polymers

The analysis of crack tip blunting in the impact test is reviewed and the results for polymethylmethacrylate at room temperature are presented, together with more recent data for which loading times have been measured. A successful comparison is drawn between the results of impact tests at various strain rates and those of static single edge notched tests where the crack tip region is heated. Such a correlation confirms the suspected suppression of crack tip crazing owing to thermal effects.     

Orientation-dependent crack-tip blunting and crack propagation in a single crystal BCC iron

Atomistic simulations of cracks with four different orientations in body-centered cubic single crystal iron are presented using molecular dynamics. Crystal orientation has considerable effect on the activation and evolution of crack propagation mechanisms. The results reveal that (a) crack-tip blunting depends on the crystallographic orientation, (b) continuous generation of dislocations form crack tip occurs for large crack-tip blunting, and (c) absence of deformation activities like dislocation generation, twin formation, etc. at the crack tip results in crack propagation in a brittle manner.     

Hold-time effects on high temperature fatigue crack growth in Udimet 700

Crack growth behaviour under creep-fatigue conditions in Udimet 700 has been studied, and the crack growth data were analysed in terms of the stress intensity factor as well as theJ-integral parameter. Crack growth behaviour is shown to depend on the initial stress intensity level and the duration of hold-time at the peak load. For stress intensities that are lower than the threshold stress intensity for creep crack growth, the crack growth rate decreases with increase in hold time even on a cycle basis, da/dN, to the extent that complete crack arrest could occur at prolonged hold times. This beneficial creep-fatigue interaction is attributed to the stress relaxation due to creep. For stress intensities greater than the threshold stress intensity for creep crack growth, the growth rate on a cycle basis increases with increase in hold time. For the conditions where there is no crack arrest, the crack growth appears to be essentially cycle-dependent in the low stress intensity range and time-dependent in the high stress intensity range. Both the stress intensity factor and theJ-integral are shown to be valid only in a limited range of loads and hold-times where crack growth rate increases continuously.     

Influence of particle size and particle size distribution on toughening mechanisms in rubber-modified epoxies

The principal toughening mechanism of a substantially toughened, rubber-modified epoxy has again been shown to involve internal cavitation of the rubber particles and the subsequent formation of shear bands. Additional evidence supporting this sequence of events which provides a significant amount of toughness enhancement, is presented. However, in addition to this well-known mechanism, more subtle toughening mechanisms have been found in this work. Evidence for such mechanisms as crack deflection and particle bridging is shown under certain circumstances in rubber-modified epoxies. The occurrence of these toughening mechanisms appears to have a particle size dependence. Relatively large particles provide only a modest increase in fracture toughness by a particle bridging/crack deflection mechanism. In contrast, smaller particles provide a significant increase in toughness by cavitation-induced shear banding. A critical, minimum diameter for particles which act as bridging particles exists and this critical diameter appears to scale with the properties of the neat epoxy. Bimodal mixtures of epoxies containing small and large particles are also examined and no synergistic effects are observed.     

Effect of underloads or overloads in fatigue crack growth by crack-tip blunting

Crack-tip blunting under tensile loads and re-sharpening of the crack-tip during unloading is one of the basic mechanisms for fatigue crack growth in ductile metals. Here, based on an elastic-perfectly plastic material model, crack growth computations are continued up to 700 full cycles by using re-meshing at several stages of the plastic deformation. A compressive underload in one of the cycles tends to increase the rate of cyclic crack growth, and this effect is studied in detail for a single underload, based on the blunting re-sharpening mechanism. Subsequently, the increased rate of crack growth due to periodically occurring underloads is analysed. A single overload has the opposite effect of giving a significant delay in the subsequent fatigue crack growth. An analysis is carried out to compare the effect of a small overload to that of a larger overload.     

Effect of hydrogen on crack tip blunting characteristics of 304L stainless steel

Abstract

J integral measurements have been conducted on various samples of 304L stainless steel having different levels of austenite stability as a result of variations in composition. In certain materials, significant deviations from the theoretical blunting line (?a=J/2σ_f, where ?a is the effective crack length for a particular J integral and σ_f is the mean of the 0·2% proof stress and the ultimate tensile strength) were shown in the experimental results. These deviations are discussed and a method is proposed for rationalising the results to allow a more meaningful assessment to be made of the effects of hydrogen charging on the various materials. In this way, a significant reduction in the value of J_Ic is clearly revealed in materials in which martensite may be produced by deformation. Hydrogen charging also produces a significant decrease in the slope of the experimental blunting line for these materials that is not consistent with the observed increase in proof stress. It is suggested that this decrease is associated with the localisation of plastic deformation by the presence of hydrogen.

MST/775     

A crack tip blunting analysis in the ductile-to-brittle transition of a structural steel

The J -integral versus crack growth resistance curves ( J – R curves) of an AE-460 structural steel were evaluated at different temperatures in the ductile-to-brittle transition region. This curve as well as the critical J _I value at the initiation of crack growth was found to be independent of temperature, being dependent only on the stress triaxiality state of the specimen.
The blunting expressions described in the ASTM and ESIS standards were evaluated using fractographic techniques. The ESIS blunting line fits the linear regression of the experimental results very well, while the blunting line predicted by the ASTM standard is clearly located below the experimental points.     

The effects of low temperature fatigue on the RRR and strength of pure aluminum

Low temperature fatigue effects on residual resistivity ratio (RRR = rho_{273 K}/rho_{4.2K}) and strength of 300 and 1000 RRR aluminum are reported. The objective of this investigation is to select the best initial purity for the stabilizer aluminum used in energy storage magnets. Monolythic centimeter diameter specimens were fatigued at 4.2 K to strains (ε) reaching 0.3 percent. The resistivity ratio rapidly decreases during the first 100 cycles and approaches saturation (RRRf) after about 1000 cycles for all strains tested. The RRRfvalues are different for different initial resistivity ratio (RRRi) values, but all tend to come together at 0.3% strain independent of RRRi. The maximum specimen stress (sigma_{max}) is reached after about 1000 cycles also, and approaches a common value (sigma_{max} = εE/2, where ε is the strain range and E the elastic modulus) independent of RRRi. Thus high purity aluminum becomes "fully hard" at equilibrium and behaves elastically. The impact of fatigue damage on conductor design and choice of stabilizer purity is considered.     

A study of frequency and temperature effects on fatigue crack growth resistance of CPVC

Excellent corrosion resistance of chlorinated polyvinyl chloride (CPVC) makes it an attractive material for piping systems carrying corrosive materials. The relatively high glass transition temperature of CPVC has increased its use in hot water distribution. Establishing a relationship that describes the effect of test frequency on fatigue crack propagation (FCP) rate of polymers is an interesting challenge. FCP rates can decrease increase or remain constant with increasing test frequency. Moreover, FCP sensitivity to frequency of some polymers is known to be dependent on test temperature. In this study, fatigue crack propagation in a commercial grade chlorinated vinyl chloride (CPVC) over the frequency and temperature ranges of 0.1-10 Hz and −10 °C to 70 °C, respectively, was investigated. FCP tests were conducted on single edge notch (SEN) specimens prepared from 100-mm injection molded CPVC pipefittings. The crack growth rate (da/dN) was correlated with the stress intensity range ΔK. The FCP rate was found to be insensitive to frequency at sub room temperatures. The fatigue crack propagation resistance of CPVC was enhanced with increasing cyclic frequency at 50 and 70 °C. Frequency effect on FCP rate was found to be higher in the low frequency range.Macro-fractographic analysis of fracture surface showed that stepwise crack propagation existed at 0.1 and 1 Hz for all temperatures of interest.     

Identification of fatigue crack growth mechanisms in IN100 superalloy as a function of temperature and frequency

A study is undertaken to investigate the fatigue crack growth rate properties of polycrystalline IN100 through the identification of crack growth mechanisms as a function of temperature, frequency and ΔK. An additional goal is to determine the stress free activation energy of IN100. Constant amplitude, load controlled tests are performed at room temperature (22 °C), 316 °C, 482 °C and 649 °C under two different loading frequencies of 20 and 0.33 Hz. These specimens are then analysed via scanning electron microscopy (SEM) to determine failure mechanisms. SEM shows that, as temperature increased from room temperature to 649 °C, the fracture mechanism transitions from transgranular to intergranular. The fracture mechanism is shown to transition from intergranular to transgranular at elevated temperatures as da/dN increases as a result of growing ΔK. Scanning electron microscopy shows that, as frequency decreases from 20 to 0.33 Hz at 649 °C, the fracture mechanism transitions from transgranular to intergranular.     

Microstructure variation effects on room temperature fatigue threshold and crack propagation in Udimet 720Li Ni-base superalloy

An assessment of the effects of microstructure on room temperature fatigue threshold and crack propagation behaviour has been carried out on microstructural variants of U720Li, i.e. as‐received U720Li, U720Li‐LG (large grain variant) and U720Li‐LP (large intragranular coherent γ′ variant). Fatigue tests were carried out at room temperature using a 20 Hz sinusoidal cycling waveform at an R‐ratio = 0.1 on 12.5 mm × 12.5 mm square cross‐section SENB specimens with a 60° starter notch. U720Li‐LG showed the highest threshold ΔK (ΔK_th), whilst U720Li‐LP showed the lowest ΔK_th value. U720Li‐LP also showed higher crack growth rates in the near‐threshold regime and at high ΔK (although at higher ΔK levels the difference was less marked). Crack growth rates of U720Li and U720Li‐LG were relatively similar both in the near‐threshold regime and at high ΔK. The materials showed crystallographic stage I type crack growth in the near‐threshold regime, with U720Li showing distinct crystallographic facets on the fracture surface, while U720Li‐LG and U720Li‐LP showed mostly microfacets and a lower proportion of large facets. At high ΔK, crack growth in the materials becomes flat and featureless indicative of stage II type crack growth. The observed fatigue behaviour, which is an effect of the combined contributions of intrinsic and extrinsic crack growth resistances, is rationalized in terms of the microstructural characteristics of the materials. Enhanced room temperature fatigue threshold and near‐threshold long crack growth resistance are seen for materials with larger grain size and higher degree of planar slip which may be related to increased extrinsic crack growth resistance contributions from crack tip shielding and roughness‐induced crack closure. Differences in the deformation behaviour, either homogeneous or heterogeneous due to microstructural variation in this set of materials may provide approximately equivalent intrinsic crack growth resistance contributions at room temperature.     

Mixed mode interface crack in a pure power-hardening bimaterial

Analytical and numerical analysis of the dominant singularity solutions of the stress and strain field near an interface crack in a pure power-hardening bimaterial indicates that the crack stress singularity is –1/(n _II+1) for hardening power of n _I and n _II(n _I<n _II). This result is obtained by solving the non-linear eigenvalue equations of the stress field near a plane crack while observing the conservation properties of the J-integral and the continuity conditions of the interface. Numerical results are presented for the distribution of stress, strain and displacement field of various mixed mode interface cracks when n _I=1 and n _II=5. Possible further destruction of the bimaterial with interface cracks is also discussed.

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Résumé On procède à una analyse théorique et numérique des solutions de singularité dominantes dans les champs de contraintes et de dilatation voisins d'une fissure d'interface dans un complexe bimétallique répondant à une loi simple de durcissement parabolique. On montre que la singularité de la contrainte vaut –1/n _II+1 pour des modules d'écrouissage n _I et n _II(n _I<n _II).On obtient ce résultat en résolvant les équations non linéaires d'eigenvalues du champ de contraintes au voisinage d'une fissure plane, tout en respectant les propriétés de conservation de l'intégrale J, et les conditions de continuité numérique pour la distribution des champs de contraintes, de dilatations et de déplacements correspondant à diverses fissures d'interfaces sollicitées selon des modes mixtes, avec n _I=1 et n _II=5.On discute également d'une destruction ultérieure possible du bimatériau en présence de fissures d'interface.