Numerical stress-life curves for the AISI 4340 steel using two sets of materials properties and different bi-axial stress ratios

Abstract

Fatigue assessments by the more robust strain-based approach demand the determination of the local strain history from nominal stresses. For notched members, a cyclic constitutive relation, the stress concentration factor SCF and a strain concentration rule are used with this aim in some approximate solutions. The plastic part of the cyclic constitutive relations for many materials is well adjusted by a Ramberg-Osgood RO type equation. The parameters in the RO equation are the cyclic strength coefficient and exponent H' and n' respectively. These parameters can be experimentally determined or estimated from the condition of strain compatibility between the RO and the Coffin-Manson-Basquin CMB equations. The present paper discusses the influence that the use of both types of parameters, independent or experimentally determined and compatible or estimated , has on the numerical stress-life curves of the AISI 4340 Aircraft Quality steel. By numerical stress-life curves we mean the stress amplitudes and the fatigue-life that result from the numerical solution of both, the strain-life CMB and the stress-strain RO relations, for the same strain amplitude. This would be equivalent to using a linear strain concentration rule notched members with two RO equations, one with independent parameters and the other with compatible parameters, for stress and life calculations. The effects of the stress state are also accounted for in the present investigation since both, stress-life and stressstrain equations are modified in accordance with the total deformation theory of plasticity and through the introduction of a plane stress biaxial ratio. The principal finding of the present paper is that, for the studied material, the numerical stress-life curves that result from the use of compatible and independent parameters are indistinguishable for the same stress state. Consequently, there are no important implications on life time calculations when the cyclic stress-strain curve is estimated in such a way that compatibility conditions for the AISI 4340 aircraft quality steel are ensured.

Automotive suspension component behaviors driven on flat and rough road surfaces

“…The fatigue life assessments were conducted based on the strain-life approach [ 25 ] such as the Coffin-Manson model [ 26 , 27 ] which can be expressed by: where is the strain amplitude, is the fatigue strength coefficient, E is the material modulus of elasticity, N f is the number of cycles to failure for a particular stress range and mean, b is the fatigue strength exponent, is the fatigue ductility coefficient and c is the fatigue ductility exponent.…”

“…Small cycles are extracted at the beginning of the process, leaving larger cycles to be extracted at the end. The fatigue life assessments were conducted based on the strain-life approach [25] such as the Coffin-Manson model [26,27] which can be expressed by:…”

On fatigue life predictions for notched members by the nominal stress-based and the local strain-based methods

“…It is also worth remembering that, in this regime, stresses can be considered as an ill-conditioned parameter for damage characterization, mainly for low hardening materials (e.g., see the non-linear term in the second line of the system of equations ( 8)). The equation (2) remains valid for multiaxial proportional loading as long as the effective strain and stress quantities are used instead [8].…”

Preface to the special issue of the International Symposium on Solid Mechanics (MecSol 2017)

“…• Structural Reliability Methods and Reliability-Based Design Optimization: Multi-objective optimization (Passos and Luersen, 2018) and experimental crack identification (Oliveira Filho et al, 2018). • Multiaxial and Fretting Fatigue: identification of fatigue limits and continuum damage mechanics (Castro and Bemfica, 2018), stress-life curves for alloy steels (Duran et al, 2018) and assessment of fatigue limits (Bandeira et al, 2018).…”