Fatigue crack propagation from a hole in tubular specimens under axial and torsional loading

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Abstract

Fatigue tests of crack propagation from a circular notch in thin-walled tubular specimens made of low-carbon steel were performed under cyclic uniaxial tension-compression, and combined loading of cyclic torsion with and without superposed static and cyclic axial loading. The propagation path of fatigue cracks followed the plane on which the total range of the normal stress including the compressive component of the stress was maximum for the cases of combined mode loading as well as uniaxial loading. For all cases examined, when compared at the same stress intensity range, the propagation rate became faster than that predicted from the crack propagation law obtained for carbon steels under uniaxial loading in small-scale yielding condition. Even when compared at the same value of the effective stress intensity range, the crack propagation rate was faster,because of excessive plasticity ahead of the fatigue crack. The J-integral range determined from the relation between load and displacement was proved to be an appropriate parameter for crack propagation with excessive plasticity. The J-integral range was correlated to the stress amplitude and the crack length for each loading condition.

Introduction

Fatigue fracture of several engineering components such as transmission shafts, pipes and springs occurs under combined torsional and axial loading. Notches are the common sites of crack initiation. For damage tolerance design, the prediction of fatigue crack propagation behavior from notches under combined loading is significant. The fracture mechanics has been used to predict the crack propagation behavior. The crack propagation rate under cyclic torsional loading can be higher than that under uniaxial loading [1], [2]. Current maintenance codes for nuclear power plants (ASME Boiler & Pressue Vessel Code Section XI, JSME S NA1-2000) are based on the crack propagation law obtained under uniaxial loading and small-scale yielding condition. Therefore, this acceleration could be a serious problem for damage tolerance design of torsional components. The limitation of the current code to fatigue crack propagation under torsional loading should be defined and new methodology needs to be developed. The crack acceleration under torsional loading has been ascribed to excessive plastic deformation ahead of the crack tip [1], [2], [3]. Hoshide et al. [2] was first to apply the J-integral range to crack propagation from a pre-crack made by mode I loading under cyclic torsional loading. Tanaka et al. [3] also used the J-integral for crack propagation from a hole in tubular specimens subjected to cyclic torsional loading and found a nice correlation with the uniaxial data. Still the experimental conditions were very limited.

In the present paper, fatigue tests of crack propagation from a circular hole in thin-walled tubular specimens made of low-carbon steel were performed under a wider range of loading condition. The loading condition was cyclic torsion with and without superposed static cyclic axial loading. Fatigue crack propagation tests under uniaxial tension-compression were also conducted as standard tests. The crack propagation behavior was analyzed on the basis of fracture mechanics using stress intensity factor obtained by the body force method and the finite element method. The J-integral range determined from the load–displacement record was used as fracture mechanics parameters for fatigue crack propagation. The J-integral range was correlated to the stress amplitude and the crack length for each loading condition.

Section snippets

Material and specimen

The material in the present experiment was low-carbon steel (JIS SGV410) used for piping in nuclear power plants. The chemical composition of the material is as follows (mass%): C 0.15, Si 0.26, Mn 1.16, P 0.017, S 0.002, Cu 0.01, Ni 0.02, Cr 0.06, Mo 0.14. Fig. 1 shows the shape and dimensions of the test specimen. A tubular specimen has an outer diameter of 16 mm and an inner diameter of 14 mm. The inner surface was finished by honing. After machining, the specimens were annealed at 1173 K for 1 

Crack propagation direction

Fig. 5 shows cracks emanating from a hole for cases A, B, and C, where the specimen axis is vertical. Four cracks are formed from a hole for cases A and B, while two for cases C and D. The cracks were extended almost straight up to the length of about 2 mm and then gradually turned horizontally. The crack path is nearly identical for cases A and B, and is independent of the superposed static tension. For cases A, B, and C, the angle of crack extension to the crack length of 2 mm was measured

Conclusions

Fatigue tests of crack propagation from a circular notch in thin-walled tubular specimens made of low-carbon steel were performed under cyclic uniaxial tension-compression, and combined loading of cyclic torsion with and without superposed static and cyclic axial loading. The results are summarized as follows:

  • (1)

    The propagation path of fatigue cracks followed the plane on which the total range of the normal stress including the compressive component of the stress was maximum for combined mode

Acknowledgements

A part of the present research was conducted as a co-operative work of MF committee in The Japan Welding Engineering Society sponsored by Tohoku Electric Power Co. Ltd, Tokyo Electric Power Co. Ltd, Chubu Electric Power Co. Ltd, Hokuriku Electric Power Co. Ltd, Shikoku Electric Power Co. Ltd, and Japan Atomic Power Co. Ltd.

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    Fig. 15 shows the mapped mesh adopted in the structural volume by setting an element size equal to 0.01 mm, along with the last ring of elements considered for J-integral evaluation, located 0.1 mm away from the crack tip 0. Having the ΔJ range (ΔJ = Jmax − Jmin) from FE analyses (Fig. 18), Fig. 19 shows that all data can be rationalized in a single curve, having a constant slope, according to [79]. The mean curve, the 10%–90% survival probability scatter bands and the scatter index Tda/dN are also reported.

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