Fatigue life prediction of a structural steel under service loading

Fatigue life prediction techniques for variable amplitude load histories are reviewed. The fatigue crack growth rate and crack closure responses of BS4360 50B steel are determined for a service load history experienced by a gas storage vessel. Crack propagation rates are found to be independent of specimen thickness. Crack growth is successfully predicted by linear summation using the Paris law; no significant improvement is achieved by incorporating crack closure into the analysis. The particular choice of cycle counting technique is also found to have an insignificant effect on the predicted fatigue life. The load-interaction model proposed by Willenborg et al correctly indicates the absence of retarded growth, whilst the Wheeler and Führing models erroneously predict retarded crack growth.     

Variation of stress intensity factor and crack opening displacement of semi-elliptical surface crack

In this paper a singular integral equation method is applied to calculate the stress intensity factor along crack front of a 3D surface crack. Stress field induced by body force doublet in a semi infinite body is used as a fundamental solution. Then the problem is formulated as an integral equation with a singularity of the form of r ^-3. In solving the integral equations, the unknown functions of body force densities are approximated by the product of a polynomial and a fundamental density function; that is, the exact density distribution to make an elliptical crack in an infinite body. The calculation shows that the present method gives the smooth variation of stress intensity factors along the crack front and crack opening displacement along the crack surface for various aspect ratios and Poisson's ratio. The present method gives rapidly converging numerical results and highly satisfactory boundary conditions throughout the crack boundary.     

Oblique semi-elliptical surface crack in semi-infinite solid subjected to tension

This note is concerned with a semi-elliptical surface crack arbitrarily inclined to the free surface of a semi-infinite solid subjected to tension. The analysis is performed by the body force method, and the stress intensity factor at the maximum depth point on the crack front are given for various shapes and inclination angles of the crack. The numerical results are fitted to a reliable polynomial for convenience in engineering applications.     

Fatigue crack growth and life prediction of a single interference fitted holed plate

To understand the different aspects of fatigue behaviour of complex structural joints it will be much helpful if the effects of different parameters are studied separately. In this article, to study the isolated effect of interference fit on fatigue life a pined hole specimen is investigated. This specimen is a single‐holed plate with an oversized pin which force fitted to the hole. The investigation was carried out both experimentally and numerically. In the experimental part, interference fitted specimens along with open hole specimens were fatigue tested to study the experimental effect of the interference fit. In the numerical part, three‐dimensional finite element (FE) simulations have been performed in order to obtain the created stresses due to interference fit and subsequent applied longitudinal load at the holed plate. The stress distribution obtained from FE simulation around the hole was used to predict crack initiation life using Smith–Watson–Topper method and fatigue crack growth life using the NASGRO equation with applying the AFGROW computer code. The predicted fatigue life obtained from the numerical methods show a good agreement with the experimental fatigue life.     

Fatigue crack growth in CFRP-strengthened steel plates

In this paper fatigue crack growth in steel plates reinforced by using carbon fiber reinforced (CFRP) strips is investigated from the experimental, numerical and analytical point of view. Single edge notched tension (SENT) specimens were strengthened with different reinforcement configurations and tested at a stress ratio R of 0.4. Different initial damage levels were considered and the experimental results showed that the reinforcement application can effectively reduce the crack growth rate and significantly extend the fatigue life. Numerical models (finite elements) were also developed to evaluate the stress intensity factor (SIF) and the crack opening displacement (COD) profile. Based on the numerical results, an analytical model was proposed to predict the fatigue crack growth rate and the fatigue crack growth curves. The analytical results are validated by comparing the fatigue crack growth curves to the experimental ones.     

Fatigue crack growth in thin steel sheets

Surface fatigue crack growth is considered for thin steel sheets composed of two steels widely used in cryogenic engineering: 12Kh18N10T and 07Kh13N4AG20. Specimens with thicknesses of 4, 8, and 12 mm have been tested at 293 and 77 K (in liquid nitrogen). There is a boundary between the zones affected by the rear and front edges of the specimen in the plane of semielliptical crack growth, which affects the stress and strain patterns near the crack front. The boundary between the zones is approximated by a second-order curve. The thickness and the temperature affect the slope of the curve. The kinetic features are seen at points in the crack front at such boundaries.Translated from Problemy Prochnosti, No. 2, pp. 27–34, February, 1992.     

Fatigue life prediction of a stainless steel plate-fin structure using equivalent-homogeneous-solid method

Stainless steel plate-fin heat exchangers are key components in nuclear power stations and hydrogen production systems using High Temperature Gas-cooled Reactors (HTGR). Fatigue is the most failure mode for plate-fin structures because they operate under cyclic high pressures and high temperatures. This paper establishes a life prediction method of fatigue based on equivalent-homogeneous-solid method for a 304 stainless steel plate-fin structure. A finite element analysis (FEA) program of fatigue life has been developed, which has been verified by fatigue experiments. By using this method, both the local stress concentration and the fatigue life for the whole plate-fin structure can be predicted. The results show that the fatigue cracks initiate at the fillet and then propagate to the interface and eventually the base metal of fin. The fatigue fracture in the filler metal shows brittle character, while typical dimple and striation are shown in the base metal.     

Fatigue crack closure in an X70 steel

Fatigue crack closure has been monitored in single-edge notched specimens of X70 line-pipe steel during low frequency fatigue-crack growth tests in 3.5% NaCl solution at free corrosion and cathodic potentials. The closures were measured by an electrical potential technique at stress ratios (R) of 0.05, 0.4 and 0.7, over a wide range of stress intensities (K).The measured closures, expressed in terms of Elber's effective stress ratio, % MathType!MTEF!2!1!+-% feaafiart1ev1aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn% hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr% 4rNCHbGeaGak0Jf9crFfpeea0xh9v8qiW7rqqrFfpeea0xe9Lq-Jc9% vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x% fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaGaamyvaiabg2% da9iaacIcacaWGqbWaaSbaaSqaaiGac2gacaGGHbGaaiiEaaqabaGc% cqGHsislcaWGqbWaaSbaaSqaaiaab+gacaqGWbaabeaakiaacMcaca% GGVaGaamiuamaaBaaaleaacaqGTbGaaeyyaiaabIhaaeqaaOGaeyOe% I0IaamiuamaaBaaaleaaciGGTbGaaiyAaiaac6gaaeqaaaaa!4C41!\[U = (P_{\max } - P_{{\text{op}}} )/P_{{\text{max}}} - P_{\min } \], where P _op is the load at which the crack becomes fully open, are comparable with other published results, as measured by a potential technique in inert environments, or by compliance and acoustic techniques. U depends on both K _max and R. At low K _max the variation of U with K _max appears to result from the pre-cracking stress history. In the upper K _max range, U is almost independent of K _max. The closure is affected by stress ratio only up to R=0.4, while fatigue crack growth rates increase with R through the whole range of stress ratios. This indicates that closure alone cannot explain the crack growth rate dependence on stress ratio.

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Résumé Des éprouvettes simplement entaillées latéralement, en acier pour oléoduc X70, ont été soumises à des essais de croissance de fissure sous fatigue olygocyclique dans une solution à 3,5% NaCl et sous des conditions de corrosion libres ou à potentiel cathodique. On a enregistré la fermeture de la fissure de fatigue et les mesures ont été effectuées par une technique de potentiels électriques au rapport de contrainte 0,05, 0,4 et 0,7 et sur une large gamme d'intensités de contraine K.Si on mesure ces fermetures par le rapport de la contrainte effective de Elber % MathType!MTEF!2!1!+-% feaafiart1ev1aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn% hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr% 4rNCHbGeaGak0Jf9crFfpeea0xh9v8qiW7rqqrFfpeea0xe9Lq-Jc9% vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x% fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaGaamyvaiabg2% da9iaacIcacaWGqbWaaSbaaSqaaiGac2gacaGGHbGaaiiEaaqabaGc% cqGHsislcaWGqbWaaSbaaSqaaiaab+gacaqGWbaabeaakiaacMcaca% GGVaGaamiuamaaBaaaleaacaqGTbGaaeyyaiaabIhaaeqaaOGaeyOe% I0IaamiuamaaBaaaleaaciGGTbGaaiyAaiaac6gaaeqaaaaa!4C41!\[U = (P_{\max } - P_{{\text{op}}} )/P_{{\text{max}}} - P_{\min } \], ou P _op est la charge à laquelle la fissure demeure complètement ouverte, on constate une cohérence entre ce rapport et les résultats publiés par ailleurs dans la littérature, lesquels résultent de techniques de mesure par potentiel dans des environnements inertes ou de techniques de compliance ou de techniques acoustiques. U dépend à la fois de K _max et de R. Pour des valeurs faibles de K _max la variation de U avec K _max apparaît résulter de l'histoire des contraintes de préfissuration. Pour les valeurs supérieures de K _max, U est tout à fait indépendant de K _max. La fermeture n'est affectée par le rapport des contraintes que pour R=0,4 tandis que la vitesse de croissance de la fissure de fatigue augmente en même temps que R sur toute la gamme des ratios de contrainte. Ceci indique que la fermeture seule ne peut expliquer la dépendance du taux de croissance de la fissure avec le ratio de contrainte.

     

Tension and bending of finite thickness plates with a semi-elliptical surface crack

This paper deals with the tension and bending of finite thickness plates with a semi-elliptical surface crack. The analysis is performed by means of a modified body force method, in which the accuracy and the validity are greatly improved by using reasonable patterns of the body force densities and the resultant force boundary conditions. Numerical calculations are done for various combinations of the shape and the size of the crack, and practically exact values of the stress intensity factors are determined by extrapolating the numerical results for several mesh patterns. Empirical formulae in polynomial forms are also presented for convenience of practical use.

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Résumé L'étude traite de la mise sous traction et flexion d'une plaque d'épaisseur finie comportant une fissure de surface semi-elliptique. On recourt pour l'analyse à une méthode modifiée d'évaluation des champs de force, où la précision et la validité ont été améliorées en utilisant des configurations pondérées des densités des forces, et des conditions aux limites qui en résultent.Des calculs numériques sont effectués pour diverses combinaisons de forme et de taille d'une fissure, et l'on détermine des valeurs pratiquement exactes des facteurs d'intensité de contraintes en extrapolant les résultats numériques ä diverses configurations de maillage.Des formules empiriques sous forme polynominale sont également présentées en vue de rendre aisée leur utilisation pratique.

     

Variation of mixed modes stress intensity factors of an inclined semi-elliptical surface crack

In this paper, a singular integral equation method is applied to calculate the stress intensity factor along crack front of a 3D inclined semi-elliptical surface crack in a semi-infinite body under tension. The stress field induced by displacement discontinuities in a semi-infinite body is used as the fundamental solution. Then, the problem is formulated as a system of integral equations with singularities of the form r ^–3. In the numerical calculation, the unknown body force doublets are approximated by the product of fundamental density functions and polynomials. The results show that the present method yields smooth variations of mixed modes stress intensity factors along the crack front accurately for various geometrical conditions. The effects of inclination angle, elliptical shape, and Poisson's ratio are considered in the analysis. Crack mouth opening displacements are shown in figures to predict the crack depth and inclination angle. When the inclination angle is 60 degree, the mode I stress intensity factor F _I has negative value in the limited region near free surface. Therefore, the actual crack surface seems to contact each other near the surface.     

桥梁用钢板14MnNbq的试制开发

介绍了桥梁用钢板14MnNbq的试制开发情况,进行了控轧及热处理试验,并分析了钢的化学成分、轧制及冷却工艺对钢板性能的影响作用,最终得出14MnNbq钢板的生产工艺.     

Fatigue crack initiation and crystallographic crack growth in an austenitic stainless steel

The lifetime of a specimen under cyclic loading is usually limited by the initiation and growth of microcracks. Experimental results for the austenitic stainless steel X6 CrNiNb 18-10 are given and a model for crack initiation and crack growth in the first grains of a polycrystalline aggregate is proposed.     

Fatigue tests and life prediction of 16 Mn steel butt welds without crack-like defect

Fatigue test results of 16 Mn steel butt welds without crack-like defect under both constant and variable amplitude loads are reported and new procedures are used to predict fatigue crack initiation (FCI) life, fatigue crack propagation (FCP) life and total life of the butt welds. The results indicate that the FCI life and FCP life should be calculated separately and the total life is the sum of the FCI life and FCP life. For the butt welds investigated, stress cycles to initiate a crack of engineering size may occupy more than 70 percent of the total life of the butt welds and it is more suitable to express the total life as a power function of the equivalent stress amplitude {ie275-1}. In predicting the FCI life, the expression of FCI life obtained from the test results of notched specimens is used but the effects of microstructure, surface condition, macro- and micro-geometrical discontinuities at weld toe should be taken into account. In predicting the FCP life, the formula developed by Zheng and Hirt is used and the stress ratio is taken as 0.6 to account for the residual stresses effect on the FCP rate. Because overload produced by the maximum load in a load spectrum has no effect on the FCI life of 16 Mn steel and weldment of the steel, according to the procedures outlined in the paper, one can use the FCI life expression mentioned and the linear damage accumulation rule proposed by Miner to predict the FCI life of 16 Mn steel butt welds under variable amplitude loads. A good agreement is achieved between the predicted results and the test data.     

Stress intensity factors for a semi-elliptical crack in the surface of a semi-infinite solid

The stress intensity factors are presented for a vertical semi-elliptical surface crack in an elastic semi-infinite body which is subjected to a constant pressure on the crack surface. The approach utilizes a singular integral equation which is defined over the crack area only, where the weakness of the stress singularity is taken into account near the corner points at which the crack periphery intersects the surface of the semi-infinite body.This pertinent approach provides the proper assessment of the stress intensity factors in the vicinity of the corner points, and reveals that the stress intensity factor reaches a maximum value near the corner point and then decreases to zero as the point is approached.

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Résumé On présente les facteurs d'intensité de contrainte pour une fissure de surface verticale semi-elliptique dans un corps élastique semi-infini soumis à une pression constante sur la surface de la fissure. L'approche utilisée recourt à une intégrale singulière définie uniquement sur la superficie de la fissure, où l'on prend en considération l'affaiblissement de la singularité des contraintes au voisinage des points où la périphérie de la fissure est en intersection avec la surface du corps semi-infini.Cette approche permet d'obtenir les facteurs d'intensité de contrainte au voisinage des points considérés et révèle que le facteur d'intensité des contraintes passe par un maximum en ceux-ci et décroît vers zéro lorsqu'on s'en éloigne.

     

Fatigue crack tip plastic zones in low carbon steel

Microscopic plastic zone parameters at the tips of fatigue cracks in low carbon steel, as derived by the X-ray microbeam technique, have been correlated with measurement of some of the same parameters by the electron channeling contrast technique. Good correlation has been obtained for the parameters common to both techniques. The results for low carbon steel are found to correlate well with fatigue crack plasticity in other metals.

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Résumé On met en corrélation les paramètres de la microdéformation plastique à l'extrémité des fissures de fatigue dans l'acier doux, tels qu'ils se déduisent d'une technique de microbombardement par des rayons X, et la mesure de certains de ces paramètres par une technique d'émission électronique à contraste renforcé.Une bonne corrélation a été obtenue pour les paramètres qui sont communs aux deux techniques et les résultats pour l'acier doux sont en corrélation satisfaisante avec les caractéristiques de plasticité des fissures de fatigue dans d'autres métaux.

     

Fatigue crack growth in TRIP steel under positive R-ratios

Load-controlled fatigue tests are conducted for four positive R values on a low-alloy TRIP steel for two different heat treatments: an optimal treatment leading to a multiphase microstructure containing retained austenite, ferrite, bainite and martensite, and a non-optimal treatment leading to a ferritic–martensitic dual-phase microstructure. A significantly increased resistance to fatigue crack growth is found for the optimal case with respect to the non-optimal case. The amount of crack closure is found to be larger in case of the non-optimally treated (ferritic–martensitic) steel. Close to the crack tip, an increased hardness suggests martensite formation. An EBSD technique is used to quantify the volume of retained austenite ahead of the crack tip, within the plastic zone. It is found that martensite formation only occurs within the monotonic plastic zone during fatigue. By evaluation of the retained austenite fraction during straining in static tensile tests, the plastic strain levels within the plastic zone are assessed. Additionally, the effect of martensite formation on fracture toughness is estimated.