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Expanded hole method for arresting crack propagation: residual stress determination using neutron diffraction
ARTICLE IN PRESS
Physica B 350 (2004) e503–e505
Expanded hole method for arresting crack propagation: residual stress determination using neutron diffraction I. Carona, F. Fiorib,c,*, G. Mesmacquea, T. Pirlingd, M. Sua b
a Laboratoire de M!ecanique de Lille, Universit!e de Lille 1, Villeneuve d’Ascq 59650, France Dipartimento di Scienze Applicate ai Sistemi Complessi - Sez. Fisica, Universita" Politecnica delle Marche, Via P.Ranieri 65, Ancona I-60131, Italy c Istituto Nazionale per la Fisica della Materia (INFM), U.d.R. Ancona, Italy d Institute Laue-Langevin (ILL), Grenoble F-38042, Cedex 9, France
Abstract Fracture of components and structures are usually produced by the propagation of fatigue cracks. A fatigue crack can be stopped by a decrease of the notch sharpness and by residual compressive stresses at the crack tip. Many methods can be used to arrest the crack propagation, but one of the simplest ones is to drill a hole at the crack tip or at a convenient distance ahead of the expected crack growth path. In the expanded hole method, the crack arrest is obtained by expanding a hole at the crack tip, so as to produce a compressive residual stress field, reducing the effective stress around the crack tip. In the present work the results of neutron diffraction experiments, carried out at the D1A diffractometer of ILL-Grenoble, for the evaluation of the residual stress field in precracked steel submitted to the hole expansion method are presented. The results are compared to calculations and discussed. r 2004 Elsevier B.V. All rights reserved. PACS: 61.12.q; 07.10.Lw; 02.70.Dc Keywords: Expanded hole; Residual stress; Neutron diffraction
1. Introduction The simplest way for arresting crack propagation in metal alloys is the use of cold expansion method at the crack tip to stop the propagation. The decrease of the notch sharpness is generated by a drilling at the crack tip and the residual compressive stresses by a cold expansion. However, the effect is limited by the hole size, *Corresponding author. Tel.: +39-071-2204603; fax: +39071-2204605. E-mail address: f.fi[email protected] (F. Fiori).
influencing the structure strength. By practicing a cold expansion, the lifetime is strongly improved and the minimum crack propagation rate is lower. More effective results can be obtained by expanding a hole at the crack tip so as to produce residual compressive stresses that can reduce the effective stresses around the crack tip [1,2]. The knowledge of the residual stress field in the neighbourhood of the expanded hole is fundamental for the verification of the effectiveness of the method, and also for the lifetime prediction of components submitted to it. The residual compressive stresses and the plastic zone size can be determined from
0921-4526/$ - see front matter r 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.physb.2004.03.132
ARTICLE IN PRESS e504
I. Caron et al. / Physica B 350 (2004) e503–e505
calculation codes, but these numerical models need to be experimentally validated. In this work the results of neutron diffraction (ND) experiments for the determination of residual stresses in a specimen of A42 ferritic steel, and the results are compared with finite element (FEM) calculations.
2. Experimental conditions The geometry of the investigated specimen is shown in Fig. 1. The specimen, obtained by lamination, was precracked with a 27.5 mm long crack. A hole (f ¼ 5:9 mm) was drilled at the crack tip, and subsequently cold expanded using a 6 mm diameter steel ball. The specimen was then submitted to fatigue cycling with the following characteristics: frequency 30 Hz, stress amplitude 3.75 kN, maximum cyclic stress 44 MPa. The ND experiments were carried out at the D1A diffractometer of ILL, Grenoble (F). The (1 1 0) Bragg peak of Fe was considered, using a neutron wavelength l ¼ 0:299 nm. A gauge volume of 0.5 0.5 3 mm3 was used, centred in several points located along a radial line at different distances from the hole center, in the midthickness plane (Fig. 1).
3. Results The unstrained interplanar distance d0 was calculated imposing the plane-stress condition (sx ¼ 0) in the region faraway from the hole, which is likely to be fullfilled due to the geometry of the specimen. In this way, a different d0 value is
Fig. 1. Geometry of the investigated specimen and definition of the principal axes.
Fig. 2. Residual stresses obtained from neutron diffraction experiments.
obtained for each of the gauge points where the plane-stress condition is imposed. In our data analysis, the mean value of them was taken for the calculation of strains and stresses. The residual stresses obtained in this way are reported in Fig. 2. The compression region near the hole reaches a minimum of about 300 MPa (z component), very close to the hole edge, well below the yielding stress of steel. Then this component becomes tensile, reaching a maximum (about 150 MPa) at 5 mm from the hole edge. In the most faraway region, it maintains itself compressive, probably due to already existing stresses caused by the lamination process from which the specimen is obtained. The other components follow approximately the same behaviour, but with lower stress values. These results are in a satifactory agreement with FEM simulation results (Fig. 3), carried out using the ANSYS software for non-linear calculations [3]. In particular, the measured compressive minimum at about 300 MPa of the z component near the hole is confirmed by FEM calculations, and so is the general behaviour in the x direction, at least in the hole neighbourhood. Higher compressive stresses are obtained from experiments with respect to FEM calculations in the y direction. This can be reasonably ascribed to already existing stress states, probably due to the lamination process, which are not taken into
ARTICLE IN PRESS I. Caron et al. / Physica B 350 (2004) e503–e505
e505
4. Conclusion The effectiveness of the hole expansion method in producing compressive residual stresses to increase the fatigue resistance of the component is confirmed by neutron diffraction experiments. The experimental results are in a satisfactory agreement with FEM calculations, but the comparison put also into evidence the presence of preexisting stresses, probably due to the lamination process, which are not taken into account in the FEM model. Fig. 3. Residual stresses obtained from FEM simulations.
account in the FEM calculations. This is confirmed also by compressive measured stresses in the z direction, faraway from the hole.
References [1] W.H. Cathey, A.F. Grandt, J. Eng. Mater. Technol. 102 (1980) 85. [2] K.J. Kang, J.H. Song, Y.Y. Earmme, J. Strain Anal. Eng. Des. 24 (1989) 23. [3] R. Ghfiri, H.J. Shi, R. Guo, G. Mesmacque, Mater. Sci. Eng. A 286 (2000) 244.
Physica B 350 (2004) e503–e505
Expanded hole method for arresting crack propagation: residual stress determination using neutron diffraction I. Carona, F. Fiorib,c,*, G. Mesmacquea, T. Pirlingd, M. Sua b
a Laboratoire de M!ecanique de Lille, Universit!e de Lille 1, Villeneuve d’Ascq 59650, France Dipartimento di Scienze Applicate ai Sistemi Complessi - Sez. Fisica, Universita" Politecnica delle Marche, Via P.Ranieri 65, Ancona I-60131, Italy c Istituto Nazionale per la Fisica della Materia (INFM), U.d.R. Ancona, Italy d Institute Laue-Langevin (ILL), Grenoble F-38042, Cedex 9, France
Abstract Fracture of components and structures are usually produced by the propagation of fatigue cracks. A fatigue crack can be stopped by a decrease of the notch sharpness and by residual compressive stresses at the crack tip. Many methods can be used to arrest the crack propagation, but one of the simplest ones is to drill a hole at the crack tip or at a convenient distance ahead of the expected crack growth path. In the expanded hole method, the crack arrest is obtained by expanding a hole at the crack tip, so as to produce a compressive residual stress field, reducing the effective stress around the crack tip. In the present work the results of neutron diffraction experiments, carried out at the D1A diffractometer of ILL-Grenoble, for the evaluation of the residual stress field in precracked steel submitted to the hole expansion method are presented. The results are compared to calculations and discussed. r 2004 Elsevier B.V. All rights reserved. PACS: 61.12.q; 07.10.Lw; 02.70.Dc Keywords: Expanded hole; Residual stress; Neutron diffraction
1. Introduction The simplest way for arresting crack propagation in metal alloys is the use of cold expansion method at the crack tip to stop the propagation. The decrease of the notch sharpness is generated by a drilling at the crack tip and the residual compressive stresses by a cold expansion. However, the effect is limited by the hole size, *Corresponding author. Tel.: +39-071-2204603; fax: +39071-2204605. E-mail address: f.fi[email protected] (F. Fiori).
influencing the structure strength. By practicing a cold expansion, the lifetime is strongly improved and the minimum crack propagation rate is lower. More effective results can be obtained by expanding a hole at the crack tip so as to produce residual compressive stresses that can reduce the effective stresses around the crack tip [1,2]. The knowledge of the residual stress field in the neighbourhood of the expanded hole is fundamental for the verification of the effectiveness of the method, and also for the lifetime prediction of components submitted to it. The residual compressive stresses and the plastic zone size can be determined from
0921-4526/$ - see front matter r 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.physb.2004.03.132
ARTICLE IN PRESS e504
I. Caron et al. / Physica B 350 (2004) e503–e505
calculation codes, but these numerical models need to be experimentally validated. In this work the results of neutron diffraction (ND) experiments for the determination of residual stresses in a specimen of A42 ferritic steel, and the results are compared with finite element (FEM) calculations.
2. Experimental conditions The geometry of the investigated specimen is shown in Fig. 1. The specimen, obtained by lamination, was precracked with a 27.5 mm long crack. A hole (f ¼ 5:9 mm) was drilled at the crack tip, and subsequently cold expanded using a 6 mm diameter steel ball. The specimen was then submitted to fatigue cycling with the following characteristics: frequency 30 Hz, stress amplitude 3.75 kN, maximum cyclic stress 44 MPa. The ND experiments were carried out at the D1A diffractometer of ILL, Grenoble (F). The (1 1 0) Bragg peak of Fe was considered, using a neutron wavelength l ¼ 0:299 nm. A gauge volume of 0.5 0.5 3 mm3 was used, centred in several points located along a radial line at different distances from the hole center, in the midthickness plane (Fig. 1).
3. Results The unstrained interplanar distance d0 was calculated imposing the plane-stress condition (sx ¼ 0) in the region faraway from the hole, which is likely to be fullfilled due to the geometry of the specimen. In this way, a different d0 value is
Fig. 1. Geometry of the investigated specimen and definition of the principal axes.
Fig. 2. Residual stresses obtained from neutron diffraction experiments.
obtained for each of the gauge points where the plane-stress condition is imposed. In our data analysis, the mean value of them was taken for the calculation of strains and stresses. The residual stresses obtained in this way are reported in Fig. 2. The compression region near the hole reaches a minimum of about 300 MPa (z component), very close to the hole edge, well below the yielding stress of steel. Then this component becomes tensile, reaching a maximum (about 150 MPa) at 5 mm from the hole edge. In the most faraway region, it maintains itself compressive, probably due to already existing stresses caused by the lamination process from which the specimen is obtained. The other components follow approximately the same behaviour, but with lower stress values. These results are in a satifactory agreement with FEM simulation results (Fig. 3), carried out using the ANSYS software for non-linear calculations [3]. In particular, the measured compressive minimum at about 300 MPa of the z component near the hole is confirmed by FEM calculations, and so is the general behaviour in the x direction, at least in the hole neighbourhood. Higher compressive stresses are obtained from experiments with respect to FEM calculations in the y direction. This can be reasonably ascribed to already existing stress states, probably due to the lamination process, which are not taken into
ARTICLE IN PRESS I. Caron et al. / Physica B 350 (2004) e503–e505
e505
4. Conclusion The effectiveness of the hole expansion method in producing compressive residual stresses to increase the fatigue resistance of the component is confirmed by neutron diffraction experiments. The experimental results are in a satisfactory agreement with FEM calculations, but the comparison put also into evidence the presence of preexisting stresses, probably due to the lamination process, which are not taken into account in the FEM model. Fig. 3. Residual stresses obtained from FEM simulations.
account in the FEM calculations. This is confirmed also by compressive measured stresses in the z direction, faraway from the hole.
References [1] W.H. Cathey, A.F. Grandt, J. Eng. Mater. Technol. 102 (1980) 85. [2] K.J. Kang, J.H. Song, Y.Y. Earmme, J. Strain Anal. Eng. Des. 24 (1989) 23. [3] R. Ghfiri, H.J. Shi, R. Guo, G. Mesmacque, Mater. Sci. Eng. A 286 (2000) 244.