Riccardo Fincato

Fatigue life assessment of a non-load carrying fillet joint considering the effects of a cyclic plasticity and weld bead shape

Fatigue life depends strongly on irreversible contributions that accumulate during cyclic loading and unloading of structures. However, the correct identification of the loading path in terms of uniaxial or multi-axial stress states, proportional or non-proportional loading is essential because these factors can significantly alter the material response. In this study, finite element analysis was conducted to assess the fatigue crack initiation life of a non-load carrying fillet joint by considering weld bead shape and a cyclic plasticity accumulation during fatigue loading, which is a main cause of crack initiation. Cyclic plasticity behaviour including cyclic hardening and softening together was investigated with an unconventional plasticity model called the subloading surface model and extended to include both elastic boundary and cyclic damage concepts. The cyclic plasticity model can capture realistic plastic strain accumulation during high cycle fatigue under macroscopically elastic stressing conditions. KEYWORDS. Unconventional plasticity; Fatigue; Loading path; Crack initiation.

Numerical modelling of ductile damage mechanics coupled with an unconventional plasticity model

Ductility in metals includes the material’s capability to tolerate plastic deformations before partial or total degradation of its mechanical properties. Modelling this parameter is important in structure and component design because it can be used to estimate material failure under a generic multi-axial stress state. Previous work has attempted to provide accurate descriptions of the mechanical property degradation resulting from the formation, growth, and coalescence of microvoids in the medium. Experimentally, ductile damage is inherently linked with the accumulation of plastic strain; therefore, coupling damage and elastoplasticity is necessary for describing this phenomenon accurately. In this paper, we combine the approach proposed by Lemaitre with the features of an unconventional plasticity model, the extended subloading surface model, to predict material fatigue even for loading conditions below the yield stress. KEYWORDS. Unconventional plasticity; Ductile damage; Subloading surface; Cyclic loading.

A numerical study of the return mapping application for the subloading surface model

Purpose Many practical problems in engineering require fast, accurate numerical results. In particular, in cyclic plasticity or fatigue simulations, the high number of loading cycles increases the computation effort and time. In this work, it is demonstrated that the return mapping technique in the framework of unconventional plasticity theories is a good compromise between efficiency and accuracy in finite element analyses. Design/methodology/approach The accuracy of the closest point projection method and the cutting plane method implementations for the subloading surface model are discussed under different loading conditions by analysing the error as a function of the input step size and the efficiency of the algorithms. Findings Monotonic tests show that the two different implicit integration schemes have the same accuracy and they are in good agreement with the solution carried out with an explicit forward Euler scheme, even for large input steps. However, the closest point projection method seems to...

Return mapping considerations for tangential inelastic effect on the subloading surface model

The assumption of an associative flow rule in the classical elastoplastic models generates a plastic stretch which is not influenced by the tangential component of the stress rate. As a consequence, a significant underestimation of the inelastic strains is predicted whenever a non-proportional loading is take into account. The work presented in this paper aims to correct the excessive stiffness estimated by the traditional elastoplastic theories using the formulation proposed by [1] and at the same time it combines the subloading surface model constitutive equations with the return mapping technique ([2], [3]) for a faster but more accurate computation.