Fatigue crack growth retardation under constant amplitude and variable mean stress

Fatigue crack growth retardation under stress spectra with constant amplitude and variable mean stress, respectively, was studied. Flat specimens with a central through crack were tested under tension-tension load. The specimens were made with low alloy steel 4 mm thick, with yield strength of 625 Mpa and ultimate tensile strength of 784 MPa. Overload affected crack length increments Δa¹ were studied. The best correlation was obtained between monotonic plane stress plastic zone size 2r_y and Δa¹. The cyclic plastic zone size 2r_pc correlated with crack length increment of minimum crack growth rate after overload. Forman's equation and Willenborg's model of fatigue crack growth retardation were used for theoretical prediction of fatigue crack propagation life. The best agreement between theoretical and experimental results was also obtained using monotonic plastic zone size instead of monotonic plastic zone radius or cyclic plastic zone size. The agreement is reasonably good, even though in the case of one spectrum, cracks were arrested for several thousand cycles.     

Fatigue crack growth retardation inconel 600

The effect of single-cycle overloads on the subsequent fatigue crack growth behavior of Inconel 600 is studied. Overloads ranging from 10 to 50% are applied to a sample undergoing baseline fatigue crack growth at constant Δ$\text{K}$. In all cases, the crack growth rate increases slightly immediately after the overload and then decreases rapidly to a minimum value before later returning to the pre-overload value. The plastic zone size, affected crack length and the crack growth increment at minimum crack growth rate, $\text{a}\text{?}$, are measured for each overload.The affected crack length is considerably larger than the overload plastic zone size for overloads greater than 20%. Consequently, although the minimum crack growth rate occurs within the plane stress overload plastic zone, the effect of the overload extends well beyond the overload region.Within the overload plastic zone, contact occurs between the crack faces due to the excessive deformation produced during the overload cycle. The size of the contact region agrees very well with the overload plastic zone size. Beyond the overload region, Δ$\text{K}{}_{\mathrm{eff}}$ remains less than the applied Δ$\text{K}$ for some time due to the wedge action of the plastically deformed overload region, delaying recovery of the pre-overload crack growth rate. The crack growth rate recovers only after the crack grows out of the region of influence of the wedge.     

Fatigue crack growth in residual stress fields

A fatigue crack growth (FCG) model for specimens with well-characterized residual stress fields has been studied using experimental analysis and finite element (FE) modeling. The residual stress field was obtained using four point bending tests performed on 7050-T7451 aluminum alloy rectangular specimens and consecutively modeled using the FE method. The experimentally obtained residual stress fields were characterized using a digital image correlation technique and a slitting method, and a good agreement between the experimental residual stress fields and the stress field in the FE model was obtained. The FE FCG models were developed using a linear elastic model, a linear elastic model with crack closure and an elastic–plastic model with crack closure. The crack growth in the FE FCG model was predicted using Paris–Erdogan data obtained from the residual stress free samples, using the Harter T-method for interpolating between different baseline crack growth curves, and using the effective stress intensity factor range and stress ratio. The elastic–plastic model with crack closure effects provides results close to the experimental data for the FCG with positive applied stress ratios reproducing the FCG deceleration in the compressive zone of the residual stress field. However, in the case of a negative stress ratio all models with crack closure effects strongly underestimate the FCG rates, in which case a linear elastic model provides the best fit with the experimental data. The results demonstrate that the negative part of the stress cycle with a fully closed crack contributes to the driving force for the FCG and thus should be accounted for in the fatigue life estimates.     

Fatigue crack growth with retardation under stationary stochastic loading

The crack propagation under Gaussian stationary stochastic loading in presence of crack growth retardation effects is considered. The Wheeler retardation model is applied. The crack growth process is modeled by Markovian diffusion process. Some properties of envelope and clustering effect of the load process are taken into account in calculation of parameters of the Kolmogorov diffusion equation. The mean time to failure when the crack reaches its critical length is determined. The analysis of the mean lifetime equation and results of an example show that the stochastic fluctuations of the stationary load process alone do not affect significantly random variations of the lifetime. A quasi-deterministic relation between the features of the fatigue fracture and some material and load parameters is proposed. It gives a potential to consider effectively the fatigue crack propagation with retardation effects as an additional failure mode in reliability analysis.     

Fatigue crack growth retardation in spot heated mild steel sheet

A fatigue crack can be effectively retarded by heating a spot near the crack tip under nil remote stress condition. The subcritical spot heating at a proper position modifies the crack growth behaviour in a way, more or less, similar to specimen subjected to overload spike. It is observed that the extent of crack growth retardation increases with increasing level of overload as well as with increasing spot temperature. It is also observed that modification in crack growth behaviour is a function of location of heating spot and maximum retardation is observed at + 5 position.     

Finite element visualization of fatigue crack closure in plane stress and plane strain

Elastic-plastic finite element simulations of growing fatigue cracks in both plane stress and plane strain are used as an aid to visualization and analysis of the crack closure phenomenon. Residual stress and strain fields near the crack tip are depicted by both color fringe plots and x-y graphs. Development of the residual plastic stretch in the wake of a growing plane stress fatigue crack is shown to be associated with the transfer of material from the thickness direction to the axial direction. Finite element analyses indicate that crack closure does occur under pure plane strain conditions. The development of the residual plastic stretch in plane strain is shown to be associated with the transfer of material from the in-plane transverse direction to the axial direction. This in-plane contraction also leads to the generation of complex residual stress fields. The total length of closed crack at minimum load in plane strain is shown to be a small fraction of the total crack length, especially for positive stress ratios. This suggests that experimental measurement of plane strain closure would be extremely difficult, and may explain why some investigators have concluded that closure does not occur in plane strain.     

Fatigue crack retardation and closure in polymethylmethacrylate

The presented results are from an investigation of crack surface profiles and the influence of intermittent overloads on fatigue crack growth in polymethylmethacrylate, a transparent polymer. Fatigue cracks were grown in compact tension specimens under conditions of constant range in stress intensity factor. Tensile overloads were found to perturb subsequent crack growth in polymethylmethacrylate under certain conditions.

Examination of monochromatic light interference fringe patterns emanating from the fatigue cracks indicates that the crack perimeter was closed at zero load. The crack surfaces were displaced, however, in the interior of the specimen at zero load. Measurements of the crack opening displacements during loading revealed that a significant tensile load was required to displace the portions of the crack surfaces which Were initially closed. These observations are discussed in light of the Elber concept of crack closure.     

Numerical investigation on stable crack growth in plane stress

Large deformation finite element analysis has been carried out to investigate the stress-strain fields ahead of a growing crack for compact tension (a/W=0.5) and three-point bend (a/W=0.1 and 0.5) specimens under plane stress condition. The crack growth is controlled by the experimental J-integral resistance curves measured by Sun et al. The results indicate that the distributions of opening stress, equivalent stress and equivalent strain ahead of a growing crack are not sensitive to specimen geometry. For both stationary and growing cracks, similar distributions of opening stress and triaxiality can be found along the ligament. During stable crack growth, the crack- tip opening displacement (CTOD) resistance curve and the cohesive fracture energy in the fracture process zone are independent of specimen geometry and may be suitable criteria for characterizing stable crack growth in plane stress. This revised version was published online in August 2006 with corrections to the Cover Date.     

Fatigue crack growth retardation following load reductions in a plain C-Mn steel

Transient fatigue crack growth effects following perturbations in applied loading conditions have been studied using a low strength plain C-Mn steel. Reductions in both the alternating and maximum components of the fatigue loading spectrum have been systematically investigated and evaluated using a linear elastic fracture mechanics approach. Results are discussed in terms of the residual stress concept, and a model based on an effective stress intensity concept is propossed to rationalise growth rates within the retardation affected zone. Using constant amplitude fatigue threshold and crack growth data obtained under similar conditions, the model is shown to be in good agreement with experimental data.     

Fatigue crack growth retardation due to the application of impact overloads

As part of an on-going study to investigate crack growth in impact fatigue, fatigue crack growth retardation due to the application of an impact overload has been determined using centre-cracked plate specimens of a dual-phase steel and the features with results obtained by applying a static overload. The results suggest that the size of the deformed zone formed by the impact overload is relatively small but that the intensity of this zone is relatively high; ie highly dense slip deformations concentrate in this small zone.     

Fatigue crack growth in PMMA and rigid PVC under biaxial stress

Fatigue crack growth rate results for PMMA and rigid PVC are presented on a fracture mechanics basis. Centre-notched specimens were used to measure the effects of the stress biaxiality factor $\text{B}$, defined as the ratio of load acting in the crack line to that acting on a line normal to it. Variations in $\text{B}$ from 0 (uniaxial) to 2 have little effect on the fatigue crack growth rate in PVC, but reduce that in PMMA by a factor of 2 or 3. If the variation in $\text{B}$ is a stepwise one, this growth rate reduction may be accompanied by a temporary crack arrest period. The observations can be explained for both materials by postulating a direct effect of $\text{B}$ on the stress intensity factor, for which a possible mechanism-using crack closure concepts is suggested. It is shown that the regime of true fatigue crack growth in PMMA, which is investigated here, is limited at its upper end by a sharp transition to a slow-growth dominated crack extension mode.     

Kinetics of plastic strain zone formation near crack tip in fracture of construction materials under plane stress and plane strain

X-ray-diffraction analysis is used to determine the depth of the plastic deformation zones in steel. 15Kh2MFA and alloy D16 beneath the surface of fractures that are obtained under plane stress (PS) and plane deformation (PD) conditions. We propose a scheme illustrating the kinetics of the formation of two plastic deformation zones near the crack tip under one-time loading in conditions of the plane stress state: a strongly deformed microzone and a weakly deformed macrozone.Translated from Problemy Prochnosti, No. 4, pp. 57–63, April, 1993.     

Fatigue life and crack closure in specimens subjected to variable amplitude loads under plane strain conditions

The propagation of fatigue cracks in specimens subjected to variable amplitude loading under plane strain conditions was investigated experimentally and numerically, to find the influence of the variable amplitude loading on the stabilised crack closure level. Experiments on four-point-bend specimens with a Gurney block load scheme, showed that the crack closure level depends on the crack length but not on the stress range of the fluctuations. Numerical simulations performed in the fatigue crack growth program FASTRAN-II showed good agreement with the experimental results. In addition, statistical uncertainty analyses performed on the fatigue life show that, for technical applications, the uncertainties in initial crack length and load levels have a greater influence on the uncertainty in fatigue life, than the fluctuation level of the load.     

Fatigue crack growth in polymers

A number of fatigue crack propagation laws applied in the study of polymers is described. Consideration of the stress field distribution at the crack tip leads to the application of fracture mechanics. It is shown that a simplified relationship of the form da/dN =F , where is a function ofK _IC,K _max,K _min andK _TH appears to be a convenient expression for cyclic crack growth. The effect of mean stress is more complicated than that in the field of metals, the compressive component of cyclic stress may delay the crack growth. Cyclic tests in tension performed on PMMA and PVC are dependent on K and its mean value,K _m . The threshold value,K _TH, is also influenced byK _m but a more complicated behaviour due to strain rate effects may be observed. Other differences, such as the position of upper and lower transition points and growth rate changes with frequence, are noted. The effect of biaxial cyclic loading of PMMA and PVC plates is compared and some differences highlighted. The results available so far indicate little effect of the crack curving on its growth. However, it is shown that, while the increasing biaxiality can substantially retard the crack growth in PMMA, no such effect was recorded in PVC. Finally, it is shown that at very high stress levels (region III), the cyclic crack growth consists of two propagation modes, namely, a pure cyclic propagation, together with slow growth. At lower stress levels, slow growth disappears and the crack propagates in pure fatigue (region II). In region I, the propagation is very slow, without the usual correspondence between cycles and striations. The results recently obtained on glass reinforced plastics (GRP) are also presented and differences highlighted.

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Résumé On décrit les diverses lois de propagation des fissures de fatigue appliquées a l'étude des polymères. En considérant la distribution de champs de contrainte a 1'extremité de la fissure, on est conduit á appliquer la mécanique de la rupture. On montre qu'une relation simplifiée de la forme da/dN =F ^a, où est une fonction deK _IC,K _max,K _min etKTH apparait être une expression convenable pour la croissance cyclique d'une fissure. L'effet de la contrainte moyenne est plus complexe que dans le domaine des métaux et la composante de compression du cycle de contraintes pent différer la croissance de la fissure. Des essais cycliques en traction exécutés sur du PMMA et du PVC dépendent de K et de la valeur moyenneK _m . La valeur de seuilK _TH est également influencée parK _m mais un comportement plus complexe associé aux effets de vitesses de déformation peut être observé aux effets de vitesees de déformation peut être observé. D'autres differénces, telles que la position des points de transition supérieurs et inférieurs ainsi que les changements de vitesse de croissance avec la fréquence ont été notées. L'effet d'une mise en charge cyclique biaxiale d'un PMMA ou d'un PVC sous forme de plaque est comparé et on met en avant certaines des différences observées. Les résultats disponibles jusqu'ici indiquent un effet modéré de la courbure de la fissure sur sa propagation. Cependant, on montre que si une biaxialité croissante pent retarder d'une manière substancielle la croissance d'une fissure dans du PMMA, aucun effet de ce genre n a été enregistré dans le cas d'un PVC. Enfin, on montre que pour des niveaux de contrainte très élevés (région III) la croissance cyclique d'une fissure consiste en deux modes de propagation, à savoir une propagation purement cyclique accompagnée d'une croissance lente. A des niveaux de contrainte plus faible, la phase de croissance lente disparait et la propagation de la fissure s'effectue en fatigue pure (région II). Dans la région I, la propagation est très lente sans que se présente la correspondance usuelle entre les cycles et les striures. Les résultats récemment obtenus sur des plastiques renforcés de verre (GRP) sont également présentés et les differences en sont mises en évidence.