Effect of Occasional Shear Loading on Fatigue Crack Growth in 7075 Aluminum Alloy M. Makizaki 1, a, H. Matsunaga 2, 4, b, K. Yanase 3, 4, c 3, 4, d

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1 Materials Science Forum Online: ISSN: , Vol. 75, pp doi:1.428/ 213 Trans Tech Publications, Switzerland Effect of Occasional Shear Loading on Fatigue Crack Growth in 775 Aluminum Alloy M. Makizaki 1, a, H. Matsunaga 2, 4, b, K. Yanase 3, 4, c 3, 4, d and M. Endo 1 Graduate School of Fukuoka University, Fukuoka, Japan 2 Department of Mechanical Engineering, Kyushu University, Fukuoka, Japan 3 Department of Mechanical Engineering, Fukuoka University, Fukuoka, Japan 4 Institute of Materials Science & Technology, Fukuoka University a td19@cis.fukuoka-u.ac.jp, b matsunaga.hisao.964@m.kyushu-u.ac.jp, c kyanase@fukuoka-u.ac.jp, d endo@fukuoka-u.ac.jp Keywords: Fatigue crack growth, Occasional shear loading, 775 aluminum alloy Abstract: Effect of occasional mode II loading on subsequent mode I fatigue crack growth behavior was investigated by using a thin-walled tube made of 775-T6511 aluminum alloy. Careful observation of crack growth behavior revealed that the occasional mode II loading has two contradictory effects for crack growth behavior. The first is a retardation effect that is associated with the plastic deformation near crack tip. However, this effect is negligibly small for the crack growth life as a whole. The second is an acceleration effect caused by mode II fatigue crack growth itself. It was found that under relatively high ΔK level, the mode II crack growth was about an order magnitude faster than mode I crack growth. Therefore, to properly evaluate the effect of occasional shear loading in the 775 alloy, the mode II crack growth should be taken into account. Introduction In practical service conditions for various machine components and structures in aircrafts, automobiles and power generators etc., shear loadings are occasionally superimposed on cyclic tensile loading. Accordingly, the effect of occasional mode II loading on mode I fatigue crack growth has been a matter of concern in various materials [1-4]. Nonetheless, the retardation or acceleration of fatigue crack growth due to the occasional mode II loading is still a challenging issue at present. Obviously, the retardation and acceleration are controlled by some dominating factors (e.g. overload ratio, material, stress ratio, etc). Therefore, to understand such a complicated phenomenon correctly, the influence of these factors needs to be examined quantitatively. In this study, our focus was made on 775 aluminum alloy that has been widely used for aircraft structures. Based on a series of observations of fatigue crack growth behaviors, the effect of shear loading and the dominating factors for fatigue crack growth behavior are discussed. Experimental Material. The study was carried out using a commercial grade 775-T6511 aluminum alloy. The chemical composition in mass % was.1 Si,.25 Fe, 1.6 Cu,.6 Mn, 2.6 Mg,.2 Cr, 5.6 Zn,.1 Ti and bal. Al. The.2 % yield and tensile strength of the alloy was found to be 6 and 634 MPa, respectively. The average Vickers hardness, HV, measured with a load of 9.8 N was 184. Specimen. Thin-walled tubular specimens were machined from a 16 mm-diameter round bar. Fig. 1 shows the shape and dimensions of the specimen: 12 mm in outer-diameter and 1 mm in wall-thickness at the test section. The specimen surface was finished by polishing with an emery paper and then by buffing with an alumina paste. In the middle of specimen, a through hole (1 mm in diameter) was introduced as a crack starter. Fatigue tests. Two types of servo-hydraulic fatigue testing machines, the uniaxial tensioncompression type and the combined tension-torsion type, were used to apply the mode I and mode II loadings, respectively. The machines were operated at a test frequency of.1~1 Hz. A stress All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of Trans Tech Publications, (ID: , Pennsylvania State University, University Park, USA-16/5/16,19:43:44)

2 Crack length ( m) Crack length ( m) Crack length ( m) Materials Science Forum Vol ratio R of 1 was selected both for the mode I and mode II loadings. Fatigue crack growth behavior was investigated by applying two series of loading sequences: Sequence A: A single mode II cycle in the middle of mode I fatigue test At first, fatigue crack was initiated and propagated in mode I from the 1-mm drill hole under a constant amplitude tension-compression loading. After the total crack length including hole,, reached 2 mm, a single cyclic torsion was applied. Thereafter, the tensioncompression fatigue test was resumed. Sequence B: I fatigue test after mode I fatigue test Similar to Sequence A, the crack was firstly propagated to the length of 2 mm under a constant amplitude tension-compression loading. Then, the type of loading was changed to cyclic torsion, and the crack was propagated under the mode II loading. The fatigue tests were periodically halted for microscopic observation of crack growth processes. The crack length at specimen surface was measured by using the plastic replica method. Results and Discussion Effect of a single mode II loading on mode I fatigue crack growth (Sequence A). Fig. 2 shows the crack growth curves. In the figures, ΔK I indicates the mode I stress intensity factor range just before and after a single mode II cycle, and ΔK II single indicates the stress intensity factor range for the single mode II cycle. Fig. 3 exhibits the fatigue crack before and after the single mode II loading. The crack path was macroscopically straight even after the shear loading. Fig. 4 shows the crack growth rate, da/dn as a function of ΔK I. In the case of (ΔK I, ΔK II single ) = (2, 2) in MPa m 1/2, the single mode II cycle had no influence on the crack growth rate (cf. Fig. 4(a)). On the other hand, for (ΔK I, ΔK II single ) = (15, 25) and (ΔK I, ΔK II single ) = (1, 25) in MPa m 1/2, a little retardation, if any, was observed just after the mode II loading (cf. Figs. 4(b) and 4(c)). However, the retardation effect was negligibly small for the crack growth life as a whole. In fact, the observed retardation effect is much less than that in AISI 1 steel reported by Dahlin and Olsson [2]. Further, the mode I crack growth acceleration reported by Nayeb-Hashemi and Taslim [1] was not observed in this study. It is noted that, in the present tests, the crack length was measured at specimen surface using the plastic replica method. The crack growth at specimen surface was microstructurally irregular, which resulted in a large scatter in da/dn data, as exhibited by Fig. 4 a = 178 MPa a = 134 MPa a = 89 MPa K II single = 2 MPa m 1/2 K I = 2 MPa m 1/2 K I = 1 MPa m 1/ (a) ( K I, K II single ) = (2, 2) in MPa m 1/2 (b) ( K I, K II single ) = (15, 25) in MPa m 1/2 (c) ( K I, K II single ) = (1, 25) in MPa m 1/2 Fig. 2 Crack growth curves in Sequence A.

3 Crack length ( m) Crack length ( m) Crack length ( m) 266 Advanced Materials Science and Technology Fig. 3 Fatigue cracks before and after a single mode II loading. a = 178 MPa K II single = 2 MPa m 1/2 a = 134 MPa a = 89 MPa Fig. 4 Relationships between da/dn and ΔK I in Sequence A. Fatigue crack growth under cyclic mode II loading (Sequence B). Fig. 5 shows the crack growth curves. Further, Fig. 6 shows the crack growth rate, da/dn as a function of ΔK. In the figures, the open symbols represent the mode I crack growth. The closed symbols represent the mode II crack growth. After changing the loading type from mode I to mode II, the crack growth rate was significantly increased (cf. Figs 5(a) and 5(b)), and this is in strong contrast with Sequence A (cf. Figs. 2 and 4). Fig. 7 exhibits the fatigue cracks before and after the changing of loading type. Under the cyclic mode II loading, no crack branching was observed and the crack grew straight in mode II direction. Fig. 5(c) and Fig. 6(c) show the results of ΔK II -decreasing test from ΔK II = 15 MPa m 1/2 to ΔK II = 7 MPa m 1/2. Fig. 8 summarizes all the da/dn data obtained in this study based on Figs. 4 and 6. At higher ΔK level (e.g., ΔK = 15 ~ 3 MPa m 1/2 ), the mode II crack growth was an order magnitude faster than the mode I crack growth. The two series of da/dn data decrease gradually associated with decrease of ΔK level, and they merged at ΔK 1 MPa m 1/2. According to the present study, it is demonstrated that the mode II loading can significantly affect the crack growth in the 775 alloy. Therefore, when the mode II loading is frequently mixed with the mode I cycles in service loading condition, the effect of mode II loading should be taken into account. a = 134 MPa ( < m) a = 134 MPa ( > m) (a) ( K I, K II single ) = (2, 2) in MPa m 1/2 K II = 15 MPa m 1/ (a) ( K I, K II ) = (15, 15) in MPa m 1/2 1-9 a = 178 MPa ( < m) a = 178 MPa ( > m) (b) ( K I, K II single ) = (15, 25) in MPa m 1/2 K II = 2 MPa m 1/2 K I = 2 MPa m 1/2 Fig. 5 Crack growth curves in Sequence B (b) ( K I, K II ) = (2, 2) in MPa m 1/ (c) ( K I, K II single ) = (1, 25) in MPa m 1/2 5 a = 134 MPa ( < m) a = MPa ( > m) Arrows indicate the K II -change points Numbers show 15 the value of K II in MPa m 1/ (c) ( K I, K II ) = (15, 15 7) in MPa m 1/2

4 Materials Science Forum Vol a = 134 MPa ( < m) a = 134 MPa ( > m) 1-4 a = 178 MPa ( < m) a = 178 MPa ( > m) a = 134 MPa ( < m) a = MPa ( > m) I I 1-9 I (a) ( K I, K II ) = (15, 15) in MPa m 1/ (b) ( K I, K II ) = (2, 2) in MPa m 1/2 Fig. 6 Relationships between da/dn and ΔK in Sequence B (c) ( K I, K II ) = (15, 15 7) in MPa m 1/2 1-4 I 1-9 Fig. 7 Fatigue cracks before and after 1-cycle mode II loading Fig. 8 Summary of da/dn - K relation. Conclusions Effect of occasional mode II loading on subsequent mode I fatigue crack growth was investigated by using a thin-walled tube made of 775-T6511 aluminum alloy. According to the present study, the following conclusions were obtained. (1) After a single mode II loading, fatigue crack closure has little influence for crack growth life. (2) Under relatively high ΔK level, the mode II growth is an order magnitude higher than the mode I growth. Therefore, to evaluate the effect of occasional shear loading on the crack growth behavior, it is important to consider the mode II crack growth itself. Acknowledgements This research was in part sponsored by Japanese Ministry of Education, Science, Sports and Culture, Grant-in-Aid for Scientific Research (C) (Fund number , ) and in part by Shiraisi Research Fund (Fukuoka University). References [1] H. Nayeb-Hashemi and M. E. Taslim, Engineering Fracture Mechanics, 1987, Vol. 26, 789. [2] P. Dahlin and M. Olsson, International Journal of Fatigue, 24, Vol. 26, 183. [3] P. Dahlin and M. Olsson, Engineering Fracture Mechanics, 26, Vol. 73, [4] P. Dahlin and M. Olsson, International Journal of Fatigue, 28, Vol. 3, 931.

5 Advanced Materials Science and Technology 1.428/ Effect of Occasional Shear Loading on Fatigue Crack Growth in 775 Aluminum Alloy 1.428/