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Research Papers

The Effect of Cold Expansion on the Fatigue Life of the Chamfered Holes

[+] Author and Article Information
Jae-Soon Jang

School of Engineering and Computer Science, Washington State University, Vancouver, WA 98686jangjsoon@gmail.com

Dave Kim1

School of Engineering and Computer Science, Washington State University, Vancouver, WA 98686kimd@vancouver.wsu.edu

Myoung-Rae Cho

Computer Aided Mechanical Design, Induk Institute of technology, Seoul 139-749, Koreamrcho@induk.ac.kr

1

Corresponding author.

J. Eng. Mater. Technol 130(3), 031014 (Jun 11, 2008) (7 pages) doi:10.1115/1.2931156 History: Received September 08, 2007; Revised March 24, 2008; Published June 11, 2008

The cold expansion method is one of the most popular techniques in the fatigue enhancement processes, and it has been widely used as a means of improving the fatigue resistance for aircraft structures with holes. Cold expanded holes have lower compressive residual stresses on the entry surface rather than the middle and exit surfaces. Due to the nonuniform residual stress distribution, fatigue crack initiation often occurs on the entry surface. This study proposes a new approach to increase the compressive residual stress magnitude at the entry of the hole. The new method is to apply chamfers into holes before the cold expansion process. Split mandrel process was used to cold work the hole with and without chamfers. Both numerical and experimental studies were done to verify the effects of hole chamfers on the residual stress distribution of the cold expanded holes. Finite element analysis (FEA) was conducted in order to see the effects of the chamfer geometries on the residual stress distributions. The FEA results showed an improvement of compressive residual stress magnitudes at the entry position of the cold expanded hole. The numerical results were compared with X-ray diffraction measurements. Fatigue tests were done to compare the fatigue life of the holes with various chamfer sizes and angles. The cold expansion chamfered holes showed a clear improvement in fatigue life over cold expanded holes without chamfers.

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Copyright © 2008 by American Society of Mechanical Engineers
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Figures

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Figure 1

Schematics of the cold expansion method for the chamfered hole

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Figure 2

Residual stress distributions for a cold expanded hole without chamfers (9); (a) residual stress contours; (b) residual stresses for three locations

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Figure 3

True strain-stress curve of Al6061-T6

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Figure 5

Geometries of the expanded hole (schematic not to scale); (a) with chamfer; (b) without chamfer

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Figure 6

Residual stress distributions for the chamfered fastener hole (d=0.1mm and θ=45deg); (a) residual stress contours; (b) residual stresses

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Figure 7

Spatial displacement magnitude at the node. Dotted lines denote the samples’ original shapes before the cold expansion process (schematic not to scale). (a) without chamfer (Unit: mm); (b) with chamfer (Unit: mm).

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Figure 8

Effect of the chamfer size on compressive residual stress distributions at the hole entry (θ=45deg)

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Figure 9

Effect of the chamfer angle on compressive residual stress distribution at the hole entry (d=0.1mm)

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Figure 10

Configuration of the fatigue test specimen (unit: mm)

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Figure 11

XRD test setup and measurement positions; (a) test setup picture; (b) measurement positions on a test sample (unit: mm)

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Figure 12

Residual stress comparison of FEA results and experimental results

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Figure 13

Fatigue life according to chamfer size at the constant chamfer angle of 45deg. The dotted line indicates the fatigue life, 4.1×104cycles, of the untreated sample; error bars show data maximum and minimum values.

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Figure 14

Fatigue life according to chamfer angle at the constant chamfer size of 0.5mm. The dotted line indicates the fatigue life, 4.1×104cycles, of the untreated sample; error bars show data maximum and minimum values.

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