Research Papers

A Numerical and Experimental Investigation on the Fatigue Behavior of a Steel Nitrided Crankshaft for High Power IC Engines

[+] Author and Article Information
Sergio Baragetti

Dipartimento di Progettazione e Tecnologie, Università degli Studi di Bergamo, Viale Marconi 5, Dalmine (BG) 24044, Italysergio.baragetti@unibg.it

Stefano Cavalleri

Dipartimento di Progettazione e Tecnologie, Università degli Studi di Bergamo, Viale Marconi 5, Dalmine (BG) 24044, Italystefano.cavalleri@unibg.it

Angelo Terranova

Dipartimento di Meccanica, Politecnico di Milano, Via La Masa 34, Milano 20156, Italyangelo.terranova@polimi.it

J. Eng. Mater. Technol 132(3), 031014 (Jun 24, 2010) (11 pages) doi:10.1115/1.4001834 History: Received July 14, 2009; Revised April 16, 2010; Published June 24, 2010; Online June 24, 2010

Nitriding is usually applied to increase the surface properties of mechanical components and can also enhance the fatigue resistance. The aim of this paper is to investigate, by means of numerical models and experimental tests, the effects of residual stresses induced by nitriding on the fatigue behavior of a marine diesel engine crankshaft. The residual stress gradient induced by the thermochemical treatment was taken into account by means of finite element models. Experimental tests were carried out with an axial testing machine in order to validate the numerical models and assess the crankshaft mechanical parameters such as the yield strength and the fatigue limit. An experimental innovative method applied to evaluate the crankshaft residual stresses by means of strain measurements under bending was also developed. This methodology proved to be useful to determine the magnitude of the residual stresses induced by the thermal treatment into the crankshaft, and it could be applied for the evaluation of the residual stress field in several cases.

Copyright © 2010 by American Society of Mechanical Engineers
Topics: Stress , Fatigue
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Figure 1

(a) crankshaft section geometry; (b) image of the crankshaft section; and (c) specimen geometry

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Figure 2

(a) Three dimensional model of the specimen with the visualization of the applied loads, the boundary conditions, and the mesh; (b) cylindrical coordinate system used for the residual stress function definition; (c) trend of the residual stresses in a specimen section; and (d) fitting constants used in Eqs. 2,3

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Figure 3

(a) crankshaft geometry, loads, and boundary conditions; (b) three-dimensional model of a quarter of the crankshaft with the visualization of the mesh

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Figure 4

von Mises ellipse for the residual stress evaluation (a) on the specimen and (b) on the crankshaft

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Figure 5

(a) Image of the crankshaft installed on the testing machine; (b) image of the load application device and the UPM 100 acquisition system

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Figure 6

(a) Strain gauge disposition on the left crankpin fillet, (b) on the symmetry plane, (c) on the right crankpin fillet, and ((d) and (e)) on the main journal fillet; (f) image of the strain gages mounted on the fillet between the crankpin journal and the counterweight.

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Figure 7

Stress trends in the specimen section with and without residual stresses for an applied bending load equal to (a) 987 N m and (b) 1410 N m

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Figure 8

(a) von Mises stress map in the crankshaft model; (b) image of the critical 45 deg plane joining the fillet centers; (c) plot of the von Mises stresses in the critical plane (45 deg)

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Figure 9

(a) trend of the von Mises stresses on the crankpin and main journals; (b) two-dimensional polar coordinate system; (c) plot of the bending residual and composed stresses in the critical section

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Figure 10

Example of the load-strain diagram (a) of strain gauge 4 for an applied load of ±60 kN and (b) of strain gauge 9 for an applied load of ±90 kN

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Figure 11

von Mises ellipse for 700 MPa yield strength

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Figure 12

(a) Example of the initial and final load-strain curves extrapolated from the strain measurements (strain gauge 4) during the fatigue test at the beginning of the first and second step-loading cycles, respectively; (b) fracture surface on the 45 deg plane after 2×104 cycles of the second step-loading cycle




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