Research Papers

Fatigue Life Improvement of Holed Plates Made of an Innovative Medium C Micro-Alloyed Steel by Local Plastic Deformation

[+] Author and Article Information
Dario Croccolo, Giorgio Olmi, Lorella Ceschini, Alessandro Morri

DIN, Department of Industrial Engineering,
University of Bologna,
Bologna 40136, Italy

Massimiliano De Agostinis

DIN, Department of Industrial Engineering,
University of Bologna,
Bologna 40136, Italy
e-mail: m.deagostinis@unibo.it

1Corresponding author.

Contributed by the Manufacturing Engineering Division of ASME for publication in the JOURNAL OF MANUFACTURING SCIENCE AND ENGINEERING. Manuscript received December 1, 2014; final manuscript received March 31, 2015; published online September 9, 2015. Assoc. Editor: Donggang Yao.

J. Manuf. Sci. Eng 138(2), 021005 (Sep 09, 2015) (11 pages) Paper No: MANU-14-1646; doi: 10.1115/1.4030378 History: Received December 01, 2014

This paper deals with the influence of local plastic deformation on the fatigue strength of holed plates manufactured with an innovative medium-carbon micro-alloyed steel with high silicon content (hi-Si MCM). Local deformation around the hole is achieved by means of an interference fitted pin. The effect was investigated both experimentally and numerically. Microstructural characterization, hardness, and tensile tests were carried out first. Tension–tension fatigue tests were performed under two different conditions: open-hole (OH) specimens and specimens with a press fitted pin with 0.6% nominal specific interference. A 2D elastic–plastic finite element analyses (FEAs) investigation was done as well, in order to analyze the stress field in the vicinity of the hole. The stress history and distribution in the neighborhood of the hole indicate a significant reduction of the stress amplitude produced by the external loading (remote stress) when a residual stress field is generated by the pin insertion. In fact, experimental stress-life (SN) curves pointed out increased fatigue strength of the interference fit specimens, compared with the OH ones. Finally, scanning electron microscope (SEM) analyses of the fractured fatigue specimens were carried out, in order to investigate the mechanisms of failure and to relate them to the peculiar microstructural features that characterize this innovative steel.

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Fig. 1

Examples of chain joints or rivet connections

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Fig. 2

Shape and dimensions (mm) of the fatigue specimens. Kt = 3.31 [35].

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Fig. 3

Fatigue specimen with interference fitted pin: (a) coupling phase and (b) clamping by wedge grips

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Fig. 4

Optical micrographs of the hi-Si MCM steel at low (a) and high (b) magnification; CCC of the steel (c) [37]

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Fig. 5

S–N curves for OH and I06 specimens

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Fig. 6

Low magnification fatigue fracture surfaces for OH (a) and I06 (b) specimens. Fatigue crack nucleation sites on both (c) or one side (d) of the hole.

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Fig. 7

Fracture surfaces of the OH specimen tested with a RSOH = 400 MPa failed after N = 40,094 cycles. (a) and (b) multiple crack nucleation sites on both sides of the hole, (c) crack nucleation site at machining defect, and (d) fatigue nucleation site.

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Fig. 8

Fracture surfaces of the I06 specimen tested with a RSOH = 300 MPa failed after N = 370,015 cycles. (a) Multiple crack nucleation sites; (b) overload failure; (c) crack nucleation site; (d) ductile dimples and secondary cracks parallel to the loading direction.

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Fig. 9

EDS analysis of Mn–Si particles at the bottom of the dimples

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Fig. 10

Reference system for stresses

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Fig. 11

Example of FEA model

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Fig. 12

Δσy stress amplitude comparison for OH and I06 specimens at 150,000 cycles

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Fig. 13

Δσy_av average stress amplitude comparison for OH (a) and I06 (b) specimens at 150,000 cycles

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Fig. 14

Average of the stress amplitude, histograms for OH and I06 over a range of life cycles

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Fig. 15

S–N curve for OH and I06 specimens based on the average of the stress amplitude

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Fig. 16

Fatigue behavior of the hi-Si MCM micro-alloyed steel compared with that of the traditional QT steel 35KB2 S–N curve for OH and I06 specimens [29]

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Fig. 17

Maximum RS for given interference level as a function of maximum RS for OH




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