Chalid el Dsoki

Numerical fatigue strength evaluation of inhomogeneous, linear Flow Split Profiles

The innovative sheet metal forming technology “Linear Flow Splitting” offers various new options for designing profile-like components. The forming process leads to severe changes in local material properties, inhomogeneities and residual stresses within the manufactured component. These effects influence the mechanical properties of the manufactured components. If the components are designed to endure cyclic mechanical loads, it is especially important to know the components fatigue properties. This paper focuses on a method to derive the fatigue properties of Linear Flow Split Profiles by nonlinear numerical FE analysis, including durability analysis and forming simulations. This numerical approach offers the possibility to estimate the fatigue properties of components before manufacturing physical prototypes, only based on material parameters derived from tests on smooth samples. The Finite-Element analysis of the Linear Flow Splitting Process provides distributions of local material deformation and residual stresses. These results are mapped by an appropriate interface on FE models, which allow simulating the component behavior under external loads. Thus, the inhomogeneous elastic-plastic material behavior and residual stresses are considered in the computed stresses and strains. Further on, a post-processing tool was implemented to interpret the FE results considering the inhomogeneous distribution of materials fatigue properties, the mean stress distribution and the statistical size effect.Copyright

Effects of LCF Loadings on the HCF Life of Notched Specimens in Ferritic-Bainitic Steel

Abstract Fatigue tests were performed on ferritic bainitic steel notched specimens (Kt = 2.5) under load controlled constant amplitude loading. These tests show that under constant amplitude tension compression loading, periodical overloads application have a detrimental effect on the fatigue crack initiation strength for fully reversed load ratio (Rσ = −1), while they have no influence under pulsating loading (Rσ = 0). A finite element analysis shows that in the fully reversed tension (Rσ = −1), the stabilized cyclic behaviour at the notch root is an elastic-plastic shakedown while elastic shakedown is obtained under pulsated regime (Rσ = 0), so that we can consider that the local cyclic behaviour has an influence on the overload effect. However, the overload application does not imply a remarkable modification of the stress and strain field under the subsequent constant amplitude loading and can not explain such a fatigue strength decrease in fully reversed tension.

ANSLC Artifitial Neural Strain Life Curves

A durable design for linear flow split sheet components requires suitable methods and transferability criteria which are not yet available for dentritic structures. Knowledge of the cyclic material behaviour is essential for this. For this reason, the cyclic material parameters are determined as a function of the product’s properties (level of deformation, microstructure, surface finish, residual stresses) and different loading parameters. However, since the determination of the cyclic parameters is associated with considerable experimental effort and costs, a cost-effective and easy method is sought to determine these parameters. A very promising approach for this is the application of artificial neural networks (ANN) [1, 2, 3, 4, 5] since they have the ability to generate the influences on the fatigue strength from the manufacturing and environmental parameters using sensibly selected input parameters. They offer the possibility to access acquired knowledge and to thus construct a multidimensional map based on a few tests.Copyright