Effect of fiber bridging on the fatigue crack propagation in carbon fiber-reinforced aluminum laminates

A superior crack propagation resistance was observed on various carbon fiber-reinforced aluminum laminates (CARALL) under tension-tension fatigue. It might be attributed to the restraint on the crack opening imposed by intact fibers in the crack wake. These fibers bridging the crack could reduce the effective stress intensity factor actually experienced by the crack tip. Based on the measurement of crack length and delamination size, the effective stress intensity range, ΔK_eff, of fatigue-damaged CARALL laminate was calculated by using a simplified analytical model. It was shown that the fatigue crack propagation rate in CARALL could be expressed as a unique function of the calculated ΔK_eff, which agree well with the Paris equation for the unreinforced aluminum alloy. This result confirmed the applicability of this simplified analytical model in CARALL laminates.     

Predicting fatigue crack initiation in fibre metal laminates based on metal fatigue test data

A methodology is presented to predict the cycles to crack initiation in a notched fibre metal laminate subjected to cyclic loading. The methodology contains four steps. First, the far-field metal layer stress cycle is obtained using classical laminate theory. Second, the peak stress cycle is estimated from a combination of a handbook solution for the stress concentration factor in a finite isotropic plate, and analytical solutions for the stress concentration for equal situations in infinitely large plates. The third step is to adapt the amplitude of the peak stress cycle to the characteristics of S–N data for monolithic material from the literature to allow for the cycles to initiation to be read from the S–N curve for each metal layer.In contrast to what can be found hitherto in the literature about predicting the cycles to fatigue crack initiation in fibre metal laminates, the authors of this paper leave no obscurities but rather attempt to bring understanding of the complete path from situation to prediction.Test results from the literature for Glare 4B-3/2-0.3 have been replicated using the aforementioned methodology. It is shown that it can accurately predict the number of cycles to crack initiation, although the S–N data that is used for the predictions dictates the obtained accuracy. The closer the stress cycle value of the S–N data is to the value of the case analysed, the higher the accuracy obtained. Such a trend was not observed for the stress concentration factor of the S–N curves used, although a choice for S–N data with a different stress concentration factor can cause a significant change in precision. The method is also shown to work for several other fibre metal laminates.     

An energy approach to predict fatigue crack propagation in metals and alloys

On the basis of the kinetic theory of strength, a new approach to the modeling of material degradation in cyclic loading has been suggested. Assuming that not stress changes, but acting stresses cause the damage growth in materials under fatigue conditions, we applied the kinetic theory of strength to model the material degradation. The damage growth per cycle, the effect of the loading frequency on the lifetime and on the stiffness reduction in composites were determined analytically. It has been shown that the number of cycles to failure increases almost linearly and the damage growth per cycle decreases with increasing the loading frequency.     

Texture effect on fatigue crack propagation in aluminium alloys: an overview

Texture or grain orientation was of crucial importance to fatigue crack propagation (FCP) in aluminium alloys due to boundary character between neighbouring grains and crack closure effect. The current understanding of the relationship among texture, grain size, slipping and crack propagation at fatigue stage I to III was reviewed and discussed. The recommendations for improving FCP resistance were proposed. Intensifying Goss, P and Q textures and moderating these grains were an effective method to improve FCP resistance in Paris regime. However, in stage I, due to the predominated crack closure effect, large grain is beneficial for improving the threshold value of crack propagation. Principally, excellent FCP resistance could be obtained at a balance of crack deflection and crack closure.     

Predicting the influence of temperature on fatigue crack propagation in Fibre Metal Laminates

A robust crack growth prediction tool has been developed for a class of hybrid skin materials known as Fibre Metal Laminates (FMLs) which has been thoroughly validated for fatigue loading cases at room temperature. This paper provides a brief overview of this predictive model and presents an investigation into its predictive capabilities at various temperatures. Amongst the temperature effects investigated are crack growth rate in the metal layers, delamination growth rate along the metal–fibre interfaces, and residual curing stresses within the laminate. Results from this investigation indicate that the present model accounting for these effects can accurately predict crack growth in FMLs at room temperature and elevated temperature, but is overly conservative for predictions at low temperatures.     

On the development of fatigue failure maps for titanium matrix composites

The purpose of the present article is to demonstrate how fatigue failure maps can be constructed for fiber-reinforced titanium alloys. The maps are constructed using a combination of micromechanical models and experimental measurements of fatigue cracking and fracture. The maps are used to identify the regimes in which various failure mechanisms dominate. Moreover, they provide information about the fatigue life and the critical amount of crack extension at failure. Examples of such maps for a well-characterized titanium matrix composite are presented and used to illustrate the sensitivity of fatigue life and critical crack extension to both the applied stress and the length of pre-existing cracks or notches.     

Effects of secondary phases on the damping behaviour of metals, alloys and metal matrix composites

An understanding of the precise correlation between the presence of secondary phases and material damping has eluded investigators, partly as a result of the fact that often there are various mechanisms involved. As a step towards the clarification of damping phenomena in metals and alloys, this paper provides a systematic review of the studies that have been completed on the damping mechanisms present in metals and alloys, with particular emphasis on precipitation. The damping mechanisms associated with secondary phases in metals and alloys have been subdivided into four categories, namely interface damping theory, thermal mismatch-induced dislocation damping theory, interaction damping theory and the rule of mixtures damping theory. A number of alloy systems are discussed to demonstrate the applicability of the four types of theory and the level of understanding of these complex mechanisms. As an extension of precipitation damping theory, the damping behaviour and mechanisms in particle-reinforced metal matrix composites are extensively discussed.     

Factors leading to grain-boundary fatigue crack propagation in Al-Zn-Mg alloys

Fatigue crack propagation experiments have been carried out at low load amplitudes with a high purity and a corresponding commercial purity Al-Zn-Mg alloy. When the high purity alloy was tested in laboratory air, cracks were often seen to propagate along the grain boundaries. Particularly in the peak aged condition, this alloy is highly susceptible to failure by intercrystalline cracking. However, with dry nitrogen as the test environment, the crack was observed to propagate preferentially along shear bands within individual grains. In the commercial purity alloy, grain-boundary crack propagation was not observed for either laboratory air or dry nitrogen atmospheres. The proportion of intercrystalline cracking in laboratory air could be lowered for the high purity alloy by a thermomechanical treatment.     

Environmental fatigue crack propagation in medium strength titanium sheet alloys

Results are reported for an investigation of environmental fatigue crack propagation resistance in four commercial titanium alloys of medium strength. The materials were IMI 130 (commercially pure titanium with low oxygen content), Ti-70 (commercially pure titanium with high oxygen content), IMI 230 (Ti-2.5 Cu) and Ti-5Al-2.5Sn. The environments were dry argon, normal air, distilled water and 3.5% aqueous NaCl. The conclusions were (1) the ranking of the materials in terms of conventional mechanical properties does not permit a ranking in terms of crack propagation resistance, (2) the material with the highest elastic moduli, Ti-5Al-2.5Sn, also had the best crack propagation resistance in the absence of stress corrosion, (3) there is a correspondence between the degree of isotropy of the static yield strength and the orientation dependence of crack propagation resistance, (4) for all the materials there was a trend of higher crack growth rates at similar ΔK values in the order; dry argon, air, distilled water, 3.5% aqueous NaCl, (5) in the aqueous environments only Ti-5Al-2.5Sn gave evidence of stress corrosion cracking.     

On the statistical characteristics of fatigue crack propagation

In order to investigate the governing factor which causes the statistical fluctuation in the fatigue crack growth process, various experimental and simulated results obtained based on the Paris-Erdogan equation of fatigue crack growth rate were surveyed. Then, the governing factors for the randomness in microscopic fatigue fracture process being reflected on the phenomenological crack growth characteristics were examined. As a result, the distribution of the resisting strength of material to crack propagation with a certain unit size US is considered to be important. Also, the significance of the restriction of crack plane, that it can not rotate remarkably around the crack growth direction axis, is also indicated.     

Predicting corner crack fatigue propagation from cold worked holes

We predict the fatigue propagation of corner cracks from cold worked holes using three dimensional finite element models. The models account for the through thickness variation in residual stress left after cold working. The predictions are compared to experimental results in aluminum 2024-T351 and 7075-T651. The models show the evolution of P-shaped crack fronts similar to those observed in experiments. Predictions based on the initial residual stress field left after cold working were nonconservative, predicting either slower than experimental crack growth or crack growth that arrests. Predictions based on an estimate of the stable relaxed residual stress field near the hole were conservative, and predicted 5-10 times greater life than the current Department of Defense reduced initial flaw size approach.     

Probabilistic models of fatigue crack propagation and their experimental verification

To study the fatigue crack growth problems and to emphasize the variability of the growth curves in addition to their average growth trend, three stochastic fatigue crack growth models are presented. These models include the Markov chain model, Yang's power law model and a polynomial model. Experimental work is carried out to produce the required fatigue crack growth data, which are then used for verification of the models. Two sets of statistically meaningful data, one under constant-amplitude loading and the other under random loading, are produced. Numerical study shows that all three models can be used to depict the experimentally obtained fatigue crack growth data with certain degrees of accuracy. However, out of the three models, one is superior to the others concerning a certain data set while the other two models are better for other data sets. Comparison and comments on employing individual model are made at the end of the paper based on experience gained in the present study.     

An improved model for predicting the crack size and plasticity dependence of fatigue crack propagation

The angled crack problem has been given special attention in the recent years by fracture mechanics investigators due to its close proximity to realistic conditions in engineering structures. In this paper, an investigation of fatigue crack propagation in rectangular steel plates containing an inclined surface crack is presented. The inclined angle of the crack with respect to the axis of loading varied between 0° and 90°. During the fatigue tests, the growth of the fatigue crack was monitored using the AC potential drop technique. A series of modification factors, which allow accurate sizing of such defects, is recommended. Paris power law is normalized and adopted for data analysis. Subsequently, this concept is applied to predict crack growth due to fatigue loads. The results obtained are compared with those obtained using the commonly employed fracture criterion and the experimental data.     

Cyclic fatigue crack propagation of nanoparticle modified epoxy

An experimental study on the fatigue performance of nanoparticle modified epoxy was conducted. Seven material systems were examined which were: neat epoxy (E), 6 and 12 weight percent (wt.%) silica nanoparticle modified epoxy (S6, S12), 6 and 12 wt.% rubber nanoparticle modified epoxy (R6, R12), 3 wt.% each of silica and rubber nanoparticle modified epoxy (S3R3) and 6 wt.% each of silica and rubber nanoparticle modified epoxy (S6R6). Effects of those nanoparticles on the fatigue threshold (ΔG_th and ΔK_th) and fatigue crack propagation rates (da/dN) were studied. It was found that, compared to neat epoxy (E), nanosilica (S6, S12) increased ΔG_th (and ΔK_th) but nanorubber (R6 and R12) did not. However, a synergistic effect was observed on the fatigue threshold when both silica and rubber nanoparticles were added into epoxy. All these nanoparticles, individually or conjointly, decreased da/dN with silica the most effective. Morphology of the fracture surface was examined to understand the role of nanoparticles on toughening mechanisms under cyclic loading, which depended on the applied ΔG levels.     

Microstructure dependent fatigue crack growth in aged hardened aluminium alloys

Fatigue crack propagation tests in constant amplitude loading, as well as with single peak overloads, have been performed in AlMgSi1-T6 aluminium alloys with different Mn and Cr contents. Crack closure was monitored in all tests by the compliance technique using a pin microgauge. A moderate stress ratio and a strong material dependence effects on the fatigue crack growth were observed. These effects are discussed in terms of the different dominant closure mechanism (plasticity-induced closure or roughness-induced closure). Roughness-induced closure dominates crack closure in the alloys with higher contents of Mn and Cr elements. In the alloy with a lower content of these elements, plasticity-induced closure is dominant. When roughness-induced closure is the prime pre-overload closure mechanism, the retardation effect is decreased in comparison to when plasticity-induced closure is dominant.     

Mechanisms of overload effect on fatigue crack propagation in aluminium alloys

Fatigue crack growth rate is retarded after application of one or several tensile overloads. This effect has been studied m 2024 and 2618 aluminum alloys. The main parameters which control the retardation are the rate of overload and its intensity, but other factors such as the number of overloads and the frequency of the test are to be considered. The delay effect is notable in aluminum alloys and might reach millions of cycles. A mechanism of retardation is proposed, deriving from metallographic observations.     

The importance of compressive stresses on fatigue crack propagation rate

This paper is concerned with the importance of compressive stresses on crack propagation rate. In a previous paper, namely ‘Crack Closure Inadequacy at Negative Stress Ratios’, Int. Journal of Fatigue, 26, 2004, pp. 241–252, was demonstrated the inadequacy of the crack closure concept and ΔK_eff, at a negative stress ratio, R=−1, to predict crack propagation rate. It that paper was verified that, at negative stress ratios, crack closure changes with P_max, for the same R ratio. The main conclusion was about plastic properties and mainly cyclic plastic properties, the Bauschinger effect included, on crack propagation when compressive stresses exist. It was then suggested that in the place of the crack closure concept, another concept based on plasticity should be used to explain fatigue crack propagation.In this paper, instead of working with the same negative R ratio (R=−1), a study on the behavior of crack propagation rate as a function of R ratio, from negative to positive stress ratios, is made. Both the effect of P_max and of R ratio is taken into consideration. Measurements of roughness and of crack opening loads are made, in order to verify their influence on crack propagation rate. Different materials, in order to cover different cyclic plastic properties and different sensitivities to roughness are studied (Ck45-cyclic hardening; Ti6Al4V-cyclic softening, and aluminum, Al 7175-cyclically neutral) are studied. Aluminium alloys and titanium alloys are considered to be sensitive to roughness induced crack closure (RICC) while steels are more dependent on plastic properties (PICC).In this study it is emphasized the importance of the compressive part of the cycle, and of cyclic plastic properties, on crack propagation rate. It is reassessed the inadequacy of crack closure concept and ΔK_eff to describe crack propagation rate, at negative stress ratios. It is also verified that models based solely on extrinsic properties of materials, like da/dN−ΔK or da/dN−ΔK (K_max) should also incorporate intrinsic properties of the materials in order to properly correlate fatigue crack growth.     

On statistical moments of fatigue crack propagation

A new statistical theory is proposed for the analysis of fatigue crack propagation, based on the concepts of fracture mechanics and random processes. Focus is centered on conceivably more useful information of the random time at which the crack size grows to any specific value. Given an initial crack size, recursive relationship is obtained for the statistical moments of this random time for a rather general class of material behaviors, and examples are given for the case where the crack propagation rate is governed by a power law. A procedure to estimate the parameters in the power-law model is also illustrated, using the experimental data of some 7475-T7351 aluminum fastener hole specimens subjected to the excitation of a certain bomber load spectrum.     

Modelling fatigue crack propagation of a cracked metallic member reinforced by composite patches

The concept of fracture for material elements at front of a crack for fatigue crack propagation was extended to the fatigue crack propagation of a cracked metallic member reinforced with a composite patch in this paper. From static mechanics and linear elastic fracture mechanics, force transfer on a cracked member through a composite patch was analyzed and a formula connecting the stress intensity factor with crack length was obtained. Thereafter, a fracture model for fatigue crack propagation of a repaired cracked metallic member was proposed. A new expression for the fatigue crack propagation rate has thus been derived. The expression was verified objectively by the test data. It is in good agreement with the test results.