切欠きを有する溶接継手部の片振圧縮疲れ強度

This study presents the effects of the geometry of the notches on the fatigue life, crack initiation, and growth in mixed mode and first mode of the fracture. For this purpose, some samples were tested with U-shaped notches with different radii and depths and various orientations with respect to the loading directions under fatigue loads. Using the fatigue crack growth rates, the Paris law coefficients were obtained for the used material in different conditions. It was shown that these coefficients are independent of the geometry of the samples. Fatigue crack growth behaviors in the mixed mode tests were also in good agreement with the result of the numerical simulations. Predictions of the maximum tangential stress criterion and numerical simulation for the position of the crack initiation on the notch root were compared with the tested observations, which showed good agreement. The fatigue life of the test samples was compared with the analytical results provided by the Manson-Coffin law. It was shown that a 25% decrease in notch depth increases the fatigue life by 4 to 6 times, also, an increase of 0.5 mm in the notch radius increases the fatigue life by 40%. Finally, the fracture surface of the samples was checked using an optical microscope. This study showed that the fracture surfaces have one or two lateral shear lips and plane stress conditions were established.

Fatigue crack initaition and propagation from hole defects

In this paper, fatigue crack initiation and propagation properties of three structural steels with different static strength and cyclic softening behavior were evaluated and compared with the estimation results based on the local strain response. Steel C had the highest cyclic softening rate, which was 10 times that of steel A and twice that of steel B. The relationship between strain range and fatigue life in the fatigue life region shorter than 105 cycles was almost the same regardless of the test steels. The fatigue crack initiation life from the notch bottom of the SENT specimen was almost the same independent of static strength and cyclic softening rate. The crack initiation life estimated from the Strain range versus fatigue life equation using the local strain response measured by DIC was roughly in agreement with the experimental results. Steel C had the highest crack opening load and the slowest fatigue crack propagation rate compared to the other two steels. The local strain range at the fatigue crack tip showed a good correlation with the fatigue crack propagation rate irrespective of the steel grade. In addition, the estimates of fatigue crack propagation rate based on the local strain response were in almost agreement with the experimental results.

The initiation and propagation of fatigue cracks from notches in 10 mm thick sections of two engineering steels O80M15 and O80A47 has been studied in three point bending fatigue. A range of notch root radii varying from 0.13 mm to 3.17 mm provided different absolute distances within which the fatigue crack propagation is influenced by the notch stress field. The fatigue crack propagation data from the regions affected by the notch stress field were compared with that not influenced by the notch stress field, to obtain the modified stress intensity factor coefficient for cracks emanating from notches. It was found that the modified stress intensity factor coefficient does not fall to zero at the tip of a blunt notch. The effective stress intensity factor coefficient for blunt notches could be described as a function of notch geometry and microstructural features of steels, viz. grain size. The initiation and propagation of fatigue cracks from notches in 10 mm thick sections of gas carburised, gas carbonitrided and nitrocarburised O80M15 and induction hardened 080A47 were tested in three point bending fatigue. The number of cycles to produce a detectable fatigue crack, N., correlated well with the effective stress intensity factor range. The microstructural feature in case hardened steel was the prior austenitic grain size. The fracture surface examination did not indicate subsurface crack initiation generally associated with the case hardened steels subjected to fatigue. The results of fatigue crack initiation in carburised and carbonitrided notched geometries indicated that fatigue crack initiation is not affected by the case depth. The fatigue crack propagation data for cracks emanating from case hardened notches in O80M15 could be correlated to the effective stress intensity factor range. The results provided an adequate engineering method for design against fatigue failure from a range of stress concentration for the surface hardened steels examined.

Structural failure of a bulk carrier was caused in gale sea under service condition of carrying vehicles enroute from Japan to Canada, where cracked damage was found at upper deck plate and slant plate of topside tank near the after corner of No. 3 cargo hatchway port side. The cracks propagated from hatch side almost up to the stringer angle and total length of the cracks was about 14 m.Investigation on the fractured surface disclosed a fact that fast fracture started near the intersection of the sidecoaming and endcoaming of the hatch. It was observed that low cycle fatigue crack had been initiated there prior to the fast fracture, of which the crack length was estimated about 75 mm as a result of the observation by means of microfractographs.In this paper, the failure analysis is made on the phenomenon such as the fast fracture of upper deck plating and the fatigue crack initiation in upper deck as well as fatigue crack propagation and initiation in hatch endcoaming bracket.Fracture toughness of the failured material as obtained by deep notch test is directly compared with the maximum working stress intensity factor which has been evaluated from hull girder analysis by means of three dimensional FEM analysis on the failured deck structures. Fatigue crack initiation and propagation are analysed by using the cumulative fatigue damage and fatigue crack growth law, respectively. It is assumed that the maximum expectation value of working stress range in her service life corresponds to the value when the fast fracture occurs and that the long term stress range distribution is logarithmic one.From the above estimations, it is considered that the maximum wave height which the ship encountered varies from 10 m to 12 m. The life of fatigue crack propagation on the endcoaming bracket without taking into account of corrosion is long enough as compared with the inspection period after her delivery, but the effect of corrosion on the propagation life becomes of practical significance.It is found from the study that the stress intensity at the end of hatch sidecoaming is increased when the height of hatch coaming is higher than usual and that the fracture mechanics analysis is applicable to predict reliability on the failure life of ships at the stage of design on their new type of structures.

Electrical resistivity of an incrementally fatigued lead alloy: Lawson, L. and Meshii, M. Scr. Metall. Mater. July 1991 25, (7), 1713–1717

On some basic problems of fatigue research in engineering

Fatigue crack initiation in notched members is controlled by local strains at the notch root. A number of approaches have been developed for calculating local notch-tip stresses and strains from nominal stress and notch geometry considerations. One such approach uses the parameter ΔK/ρ1/2, where ΔK is the fracture mechanics stress intensity range and ρ is the notch root radius. The parameter ΔK/ρ1/2 has been shown to correlate with local notch-tip strain and provide a means of normalizing cycles-to-initiation, Ni, data for various notch-tip geometries. Fatigue crack growth rate specimens described in ASTM Test Method for Plane-Strain Fracture Toughness of Metallic Materials (E 399), ASTM Test Method for Measurement of Fatigue Crack Growth Rates (E 647) and elsewhere can be used for fatigue crack initiation testing if they have blunt notches. Data in the form of parameter ΔK/ρ1/2 versus Ni has the same units as the traditional S-N curves. The advantage of having data in the form parameter parameter ΔK/ρ1/2 versus Ni is that information is available on ΔK and ρ . As in all fracture mechanics testing, data obtained on one specimen geometry can be applied to a wide variety of structural geometries. This approach can be used in conjunction with commercially available software for fatigue crack growth rate testing, servohydraulic testing equipment, and modified fracture mechanics specimens to automate fatigue crack initiation testing. The combination of all of these elements represents a new test method. Results are presented for aluminum alloy 7075, where the effects of corrosion pits were studied and for titanium alloy Ti-6A1-4V, where the effects of heat treatment on initiation were studied.

In order to investigate the effect of notch shapes on fatigue crack initiation, the axial load controlled fatigue tests were carried out employing the double edge notched plate specimens. Materials used in this study were SM 50 B, SM 58, HT 80 steel plates and A 5083 aluminium alloy.SM 50 B butt welded joints were also tested in order to investigate the relation with the notched plates and were discussed from a viewpoint of the fatigue crack propagation and initiation.It was found from these test results that Nc depended on mainly the local maximum cyclic stress range of notch root as same as that proposed by Topper et. al.. Also, in the case of the notched plates, a good correlation of the normalized stress intensity parameter, ΔK0/σyc√πρ, with Nc was obtained in the high cycle region and, from that, the crack initiation threshold level, ΔKth, ρ, proposed by Barsom et. al. was obtained. After that, the correlation of ΔKth, ρ with the crack propagation threshold level, ΔKth, was investigated and was discussed in relation with the notch shapes. And it was found that in the region of ρ that was larger than ρth, ΔKth, ρ had larger values than ΔKth and so that the parameter, ΔKth, ρ, was a useful for the design of structures.Moreover, the definition for the fatigue crack initiation was discussed by the way of investigating the fatigue crack propagation from notch root and was applied for the case of the test results of this study. And the difference of the crack initiation life was discussed for several crack lengths that had physically a meaning from a viewpoint of the fatigue crack propagation.

Recent developments in polymer nanocomposites have led to improvements in conventional short‐term, but the long‐term mechanical properties have received little attention. The objective of the present study was to characterize the effect of nanoparticles on the fatigue crack initiation and propagation mechanisms and on the fatigue properties of polyamide‐6 (PA6) nanocomposite (PA6NC) prepared by in situ polymerization with montmorillonite clay. Two approaches were employed: fatigue life measurements and crack growth monitoring. Compared with non‐filled PA6 at the same stress amplitude, the number of cycles to fracture was higher for the nanocomposite, which suggests an increase in the intrinsic resistance of the material to crack initiation. However, the crack growth rate results indicated that nanoparticles decreased the resistance to crack propagation. Post‐fatigue fractographic observations indicated a change in the fatigue crack propagation mechanism resulting from the addition of nanoparticles, primarily attributed to the increase in yield stress, which favors the development of a fibrillated deformation zone. The fibrillation process in the relatively high crack propagation rate regime appeared to be preceded by plastic deformation at approximately constant volume. Most of the effect of nanoparticles on the fatigue behavior and properties results probably from the mechanical reinforcement on the microstructure and its effect on the yield stress and Young's modulus rather than from the effect of the inorganic filler to act as a stress concentrator. Polym. Compos. 25:433–441, 2004. © 2004 Society of Plastics Engineers.

Effect of hydride precipitation on the fatigue cracking behavior in a zirconium alloy cladding tube

The coupled systems of fatigue crack initiation problem undergoing large range damage is defined. The zero different work principle, coupled potential energy principle, coupled complementary energy principle in the coupled system is established. By using of coupled potential energy principle, Closed form solution about predicting fatigue crack initiation life of three-dimensional component which leads to large range damage is derived. Compared with reference [1], the close form solution derived from this some. It is proved that the method is correct. The method adopted in this paper is definite in mechanical concept,it can be widely used in analysis of predicting fatigue crack initiation life of various component which leads to large range damage.

An increasing number of the offshore structures in the North Sea are being operated in a life extension phase, which means that their original design life has been exceeded. As the structures age, they are deteriorating, mainly due to fatigue and corrosion. In order to ensure that the structural integrity is maintained during the life extension phase, the remaining fatigue life of the structures needs to be assessed. While simple and conservative approximations are used in fatigue design, it will often be necessary to use more advanced and accurate models when assessing the remaining fatigue life of an existing structure. Among the different effects that influence the fatigue life, load sequence effects and mixed-mode conditions are currently not explicitly included in relevant fatigue life assessment standards for offshore structures. Therefore, this thesis addresses how these effects can be accounted for in fatigue life evaluation. The following three topics are investigated: – Load sequence effects in variable amplitude fatigue crack initiation – Mixed-mode fatigue crack propagation – Load sequence effects in variable amplitude mixed-mode fatigue crack propagation A number of experimental studies have previously demonstrated the influence of load sequence effects in variable amplitude fatigue crack initiation. In the present work, a model is proposed for estimating the fatigue life, in which the load sequence effects are taken into account. The model contains one material parameter, but a single value for this parameter is found to give reasonable agreement between predicted and experimental fatigue lives for four different steels from three different experimental studies. This indicates that the proposed model is able to predict the observed load sequence effects quite well and easily. Mixed-mode fatigue crack propagation usually needs to be modelled using a computational method. Currently, the finite element method seems to be the most easily applicable method for practicing engineers. One of the main tasks when modelling mixed-mode fatigue crack propagation is the determination of the stress intensity factor, which is a measure of how severely a crack is loaded. Here, seven different techniques for obtaining the stress intensity factor from a finite element model are compared. It is found that the domain integral method is the most reliable technique. If this method is not readily available, the displacement extrapolation technique can be used as an alternative for plane cracks. Models for estimating the fatigue crack propagation rate and crack path of mixed-mode cracks are also evaluated. It is shown that the Richard effective stress intensity factor is more conservative than the Tanaka effective stress intensity factor for estimating the fatigue crack propagation rate in most practical cases. Furthermore, most of the existing criteria for predicting the crack path work reasonably well, but the criterion of maximum tangential stress is the easiest one to apply. Very few experimental studies have previously considered variable amplitude mixed-mode fatigue crack propagation. In the present work, fatigue crack propagation tests have been carried out, in which the specimens were subjected to a mixed-mode overload, followed by mixed-mode constant amplitude cyclic loading. It is found that the fatigue crack growth retardation caused by the overload lasted longer than predicted by the models proposed in the literature. This demonstrates the need for new models, and the experimental results presented here may be used in the development of such a model.

Modeling of I + II mixed mode crack initiation and growth from the notch

Fatigue life estimation accuracy of mechanical parts and assemblies has always been the source of concern in different industries. The main contribution of this article lies in a study on the accuracy of different multiaxial fatigue criteria, proposing and investigating the accuracy of four optimized fatigue crack initiation life estimation methods—volume, weighted volume, surface and point, thereby improving the multiaxial fatigue life estimation accuracy. In order to achieve the goal, the fatigue lives of bolt clamped specimens, previously tested under defined experimental conditions, were estimated during fatigue crack initiation and fatigue crack growth and then summed together. In the fatigue crack initiation part, a code was written and used in the MATLAB software environment based on critical plane approach and the different multiaxial fatigue criteria. Besides the AFGROW software was utilized to estimate the crack growth share of fatigue life. Experimental and numerical results showed to be in agreement. Furthermore, detailed study and comparison of the results with the available experimental data showed that a combination of Smith–Watson–Topper approach and volume method results in lower error values, while a combination of Fatemi–Socie criterion and surface or point method presents estimated lives with lower error values. In addition, the numerical proposed procedure resulted in a good prediction of the location of fatigue crack initiation.