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Journal of Materials Engineering and Performance

, Volume 26, Issue 8, pp 3784–3793 | Cite as

High Load Ratio Fatigue Strength and Mean Stress Evolution of Quenched and Tempered 42CrMo4 Steel

  • Leonardo Bertini
  • Luca Le Bone
  • Ciro Santus
  • Francesco Chiesi
  • Leonardo Tognarelli
Article

Abstract

The fatigue strength at a high number of cycles with initial elastic–plastic behavior was experimentally investigated on quenched and tempered 42CrMo4 steel. Fatigue tests on unnotched specimens were performed both under load and strain controls, by imposing various levels of amplitude and with several high load ratios. Different ratcheting and relaxation trends, with significant effects on fatigue, are observed and discussed, and then reported in the Haigh diagram, highlighting a clear correlation with the Smith–Watson–Topper model. High load ratio tests were also conducted on notched specimens with C (blunt) and V (sharp) geometries. A Chaboche model with three parameter couples was proposed by fitting plain specimen cyclic and relaxation tests, and then finite element analyses were performed to simulate the notched specimen test results. A significant stress relaxation at the notch root became clearly evident by reporting the numerical results in the Haigh diagram, thus explaining the low mean stress sensitivity of the notched specimens.

Keywords

42CrMo4+QT steel Chaboche kinematic hardening model Haigh diagram high load ratio fatigue mean stress relaxation strain ratcheting 

Nomenclature

FE

Finite element

SWT

Smith–Watson–Topper (model)

R

Load (or stress) ratio

Kt

Theoretical stress concentration factor

Sy

Yield strength

Sut

Ultimate tensile strength

Sf

Fracture strength

Sn

Fatigue limit

\(S_{\text{f}}^{\text{t}}\)

True fracture strength

σm

Mean stress

σa

Alternating stress

εm

Mean strain during ratcheting tests

Δε

Range of alternating strain

Δσ

Range of alternating stress

σmax

Maximum stress

σmin

Minimum stress

K, n

Ramberg–Osgood parameters

\(\sigma_{ \hbox{max} }^{\text{e}}\)

Maximum elastic stress

\(\sigma_{ \hbox{min} }^{\text{e}}\)

Minimum elastic stress

\(\sigma_{\text{m}}^{\text{e}}\)

Mean elastic stress

σ

Stress tensor

σ′

Deviatoric stress tensor

X

Backstress tensor

X′

Deviatoric backstress tensor

Sy

Size of the yield surface

f

Yield function

εp

Plastic strain tensor

εp

Uniaxial plastic strain component

p

Accumulated plastic strain

σ

Uniaxial stress component

X

Uniaxial backstress component

Ci, γi

Chaboche model parameters

\(\sigma^{\bmod } (\varepsilon_{i} )\)

Model cyclic stress for the Chaboche parameter optimization

\(\sigma^{\exp } (\varepsilon_{i} )\)

Experimental cyclic stress for the Chaboche parameter optimization

\(\sigma_{\text{m}}^{\bmod } (i)\)

Model relaxation mean stress for the Chaboche parameter optimization

\(\sigma_{\text{m}}^{\exp } (i)\)

Experimental relaxation mean stress for the Chaboche parameter optimization

M, N

Number of sample points for the objective functions

w1, w2

Relative weights for the Chaboche parameter optimization

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Copyright information

© ASM International 2017

Authors and Affiliations

  1. 1.DICI - Department of Civil and Industrial EngineeringUniversity of PisaPisaItaly
  2. 2.General Electric Oil & Gas, Nuovo PignoneFlorenceItaly

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