Fatigue crack growth from small defects under out-of-phase combined loading

A modified linear elastic fracture mechanics analysis is presented for the evaluation of the crack growth and threshold behavior of small cracks initiated from small defects under combined loading fatigue. For the detailed evaluation of the behavior of small fatigue cracks, the Kitagawa effect, the elastic–plastic behavior of cracks in biaxial stress fields and crack closure effects are taken into account. In-phase and out-of-phase combined tension and torsion fatigue tests were conducted using annealed carbon steel specimens containing small holes. The direction of crack propagation, S–N curves and fatigue limits were found to be in good agreement with the theoretical predictions.     

Fatigue crack growth of a welded tube–flange connection under bending and torsional loading

In this contribution the results of an experimental investigation into the fatigue crack growth of welded tube-to-plate specimens of steel StE 460 under bending, torsion, and combined in-phase and out-of-phase bending/torsion loading are presented. The tests were performed at stress ratios of R = −1 and R = 0. The residual stresses were reduced by thermal stress relief. The fatigue crack development is compared with the prediction on the crack growth rates of Paris. Individual stress intensity factors for the semielliptical surface cracks in the tube-flange specimens are approximated on a weight function analogy using the published solutions of other workers.     

Fatigue crack growth in plate specimens under Mode III loading

Fatigue crack growth in plate specimens of aluminium and an aluminium alloy has been investigated under Mode III loading conditions. It has been found that only when a fully plastic situation exists, does crack extension proceed by a valid Mode III mechanism. When plasticity is restricted to planes of maximum shear there is a strong component of Mode I cracking which results in delamination in the direction of macroscopic growth. Under constant load amplitude cycling the crack growth curve exhibits three stages: a reduction in rate is followed by a period of steady growth prior to acceleration in the final stages. Such a profile is independent of material condition or failure mechanism. Cycling between fixed displacement limits leads to a reducing growth and crack arrest which occurs at a length proportional to the square of the displacement.     

An experimental and numerical investigation of ductile crack growth characteristics in surface cracked specimens under combined loading

Fracture mechanics tests have been performed in the upper shelf region of a steel on large surface cracked plate (SCT) specimens and on small compact tension (CT) specimens. Some of the SCT specimens were subjected to combined tension and bending in such a way that the loading was strongly non-proportional. Crack growth characteristics were compared between the specimens in order to assess possible influences of geometry and nonproportional loading. The differences observed could unambiguously be reduced to and correlated with differences in constraint. Constraint parameters were evaluated by detailed three-dimensional finite element computations and quantities for growing cracks were interpreted on the basis of deformation theory considerations, in analogy with the commonly used J _R-philosophy. It was noted that the initiation of ductile crack growth along a three-dimensional crack front appears to be independent of the degree of local constraint. However, the increase in toughness for a growing crack was markedly affected by the degree of local constraint. Some estimates of the constraints effects regarding stability considerations were also made.     

Procedure for the investigation of crack resistance of materials of specimens with a corner crack under cyclic loading

We propose a computational and experimental procedure for the investigation of the influence of thermomechanical loading on the propagation of a corner fatigue crack under cyclic loading. The taring dependence of the stress intensity factor on the relative radius of the crack is plotted. The limited kinetic diagrams are presented for KhN73MBTYu (éI698VD) high-temperature chromiumnickel alloy for temperatures of 20 and 600°C. Institute for Problems of Strength, National Academy of Sciences of Ukraine, Kiev, Ukraine. Translated from Problemy Prochnosti, No. 4, pp. 116–125, July–August, 1998.     

Effects of crack growth on the load-displacement characteristics of precracked specimens under bending

A critical evaluation of the feasibility of obtaining crack growth parameters from quasi-static bend tests is presented. First derived are the governing differential equations which characterize the time-history of bend test parameters for a given elastic material exhibiting power law crack growth behavior v = v_max(K_I/K_IC)^N. A numerical solution scheme is then developed which is capable of solving the initial value problem, thus quantitatively assessing the influence of crack growth on the load-displacement output. The results of this analysis indicate that in order to calculateK_IC accurately based on the peak load data, the loading rate has to be set sufficiently fast compared to v_max (but below dynamic rates), otherwise, it will generally overestimate its true value whenever slow crack growth occurs during the test; and that for high N materials the flexural test method gives a broad error band inN prediction and hence is not a reliable technique. However, it can be used by a designer to quickly screen the new materials with high Nvalues which are potential candidates for structural application.     

Analytical study of fatigue crack growth in AA7050 notched specimens under spectrum loading

An analytical study of fatigue crack growth in aluminium alloy 7050-T7451 notched specimens under a fighter aircraft wing root bending moment spectrum was conducted. The crack growth data were measured by quantitative fractography for three groups of specimens with different stress concentration geometrical features. Under spectrum loading and for each spectrum peak stress level, a minimum of five specimens were tested. Based on the analysis of the measured spectrum crack growth data using linear elastic fracture mechanics, it was found that the concept of geometry factors formulated in the stress intensity factor could not collapse the crack growth rate data derived from each stress concentration feature, particularly near the small crack growth region. In order to investigate the possible reasons for this, three-dimensional elastic-plastic finite element analysis was used to determine notch plastic zone sizes for each stress concentration geometry. As a consequence, an alternative crack growth driving force by considering both notch elastic-plastic stress field and gross net-section stress field was proposed and used to interpret the fatigue crack growth data under spectrum loading. It was found that the predictions of crack growth under spectrum loading for different stress concentration factors at different peak load levels agree reasonably well with the experimental results.     

Creep crack growth under history-dependent loading

We discuss the influence of loading history on creep crack growth. Our attention is mainly focused on the following three aspects of this problem: (i) principal laws of history-dependent creep strain of materials; (ii) creep behavior of cracks, including the choice of suitable fracture parameters that may help to predict cracking; (iii) the importance of taking the history-dependent response of the material into account. We performed numerical calculations based on the use of an appropriate constitutive model and fracture theory for (1) and (2), respectively, to analyze results of tests for (3).Battelle, Columbus, Ohio. UES Incorporated, Dayton, Ohio. Published in Fiziko-Khimicheskaya Mekhanika Materialov, Vol. 30, No. 4, pp. 37–45, July – Augus, 1994.     

The prediction of fatigue crack growth under flight-by-flight loading

A large number of crack growth computations were made for flight-by-flight loading using several varieties of a fighter spectrum. Various retardation models were applied. With proper adjustment of the retardation model the experimental data could be closely reproduced in a calculation. It is concluded that satisfactory crack growth predictions can be made. The application of safety factors to predictions is discussed.     

The kinetics of subcritical crack growth under sustained loading

The kinetics of subcritical crack growth under sustained loading for an AISI 4340 steel tempered at 400°F in distilled water were determined. Crack growth experiments were carried out over a range of temperatures from 10–75°C, using the crack tip stress intensity factor, K, to characterize the mechanical driving force. Results show that crack growth in distilled water is controlled by a thermally activated process with apparent activation energies that depend on K. Crack growth occurred only above a threshold K level with growth rates showing a strong K dependence at lower values of K and attaining constant, rate limiting values at higher K levels. The rate limiting velocities correspond to an apparent activation energy of 8000±1000 cal/mole, in essential agreement with values reported for crack growth in similar steels. This apparent activation energy corresponds to that for hydrogen permeation in AISI 4340 steel and lends further support to the concept that the rate limiting process for crack growth in high strength steels in water is that of hydrogen permeation into the crack-tip region. Because of the differences in the apparent activation energies for crack growth in water and in gaseous hydrogen, the rate limiting step in the permeation process still needs to be defined.

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Zusammenfassung Die Kinetik der subkritischen Rißausbreitung bei konstanter Belastung in distilliertem Wasser wird für ein Stahl AISI 4340, der auf 400°F Wärme behandelt wurde, aufgestellt.Die Rißausbreitungsversuche wurden, unter Verwendung des Spannungsintensitätfaktors K an der Spitze des Risses zur Charakterisierung der mechanischen Fortbewegungskraft, in einem Temperaturbereich von 10 bis 75°C ausgeführt.Die Ergebnisse zeigen daß die Ausbreitung des Risses im distillierten Wasser durch ein thermisches Beschleunigungsverfahren regiert wird, wo die Beschleunigungsenergie von K abhängt. Rißausbreitung kann nur über einem gewissen Niveau der Höhe von K zu stande kommen mit Rißausbreitungsgeschwindigkeiten die stark von K abhängen für die Werte die nahe über diesem Niveau liegen, die aber einen Konstanten Oberwert anstreben für höhere Werte von K.Diesem Oberwert der Geschwindigkeit gehört eine Beschleunigungsenergie von 8000±1000 Kal/mole an was mit den Werten die man für ähnliche Stähle gefunden hat übereinstimmt.Diese Beschleunigungsenergie ist die gleiche wie die, die zur Diffusion von Wasserstoff in Stahl AISI 4340 nötig ist. Dies ist ein zusätzliches Argument zur Annahme daß die Rißausbreitungsgeschwindigkeit für Stähle, mit hoher Bruchlast, im Wasser durch ein Diffusionsverfahren vom Wasserstoff in der Gegend der Spitze des Risses begrenzt wird.

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Résumé On a détermine la cinétique de la croissance subcritique des fissures sous charge constante dans l'eau distillée pour un acier AISI 4340 revenu à 400°F.Les essais de propagation de fissure ont été exécutés dans une gamme de températures comprise entre 10 et 75°C, les efforts étant caractérisés par le facteur K d'intensité des contraintes à l'extrémité de la fissure.Les résultats démontrent que la croissance de la fissure dans l'eau distillée est régie par un processus d'activation thermique, dont les énergies d'activation apparente dépendent de K. La propagation de la fissure ne se produit que lorsque K dépasse un certain seuil; les vitesses de propagation dépendent fortement de K à ses niveaux les moins elevés au-dessus de ce seuil, mais tendent vers une valeur limite constante lorsque K est important. Cette vitesse limite correspond à une énergie d'activation apparente de 8000±1000 cal/môle, ce qui est en accord avec les valeurs que l'on a trouvées pour la propagation des fissures dans des aciers similaires.Cette énergie d'activation correspond à celle qui est nécessaire pour la diffusion de l'hydrogène dans l'acier AISI 4340. Ceci constitue un argument supplémentaire en faveur du concept suivant lequel la vitesse de croissance des fissures dans les aciers à haute résistance en présence d'eau est limitée par un processus de diffusion de l'hydrogène dans la région de l'extrémité de la fissure.

     

Elastic–plastic fatigue crack growth in 18G2A steel under proportional bending with torsion loading

The paper presents the results of fatigue crack growth on low‐alloy 18G2A steel under proportional bending with torsion loading. Specimens with square sections and a stress concentration in the form of external one‐sided sharp notch were used. The tests were performed under the stress ratios R=?1, ?0.5 and 0. The test results were described by the ΔJ‐integral range and compared with the ΔK stress intensity factor range. It has been found that there is a good agreement between the test results and the model of crack growth rate, which includes the ΔJ‐integral range.     

矩形截面高层建筑气动基底扭矩系数均方根值研究

利用高频天平技术,在4种不同风场中对71种不同几何参数的矩形截面高层建筑刚性模型进行了风洞试验,得到了典型风向角下气动基底扭矩系数均方根值,考察了建筑厚宽比、长细比及建筑所处风场类型对气动基底扭矩系数均方根值的影响。不同厚宽比的矩形建筑的气动扭矩的作用不同,试验结果表明厚宽比对气动基底扭矩系数均方根值的影响很大;长细比不同导致的基底扭矩系数均方根值的差异主要是由模型顶部三维流的比重变化引起的,试验结果显示长细比的影响可以忽略不计;风场类型对基底扭矩系数均方根值的影响强弱随厚宽比的变化而变化。基于试验数据,拟合得到了矩形截面高层建筑的气动基底扭矩系数均方根值的经验公式,并对其进行了误差分析。通过与国内外相关研究成果的比较,验证了该拟合公式的精度与合理性,可以为荷载规范的修订和补充提供很好的参考。     

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.