Kristof Denys

Identification of post-necking strain hardening behavior of pure titanium sheet

This paper deals with the identification of the post-necking strain hardening behavior of pure titanium sheet. Biaxial tensile tests using a servo-controlled multi-axial tube expansion testing machine revealed that commercial pure titanium sheet exhibits significant differential work hardening (DWH). The latter phenomenon implies that the shapes of the work contours significantly change during plastic deformation which is accurately measured in the first quadrant of the stress space up to an equivalent plastic strain of approximately 0.3. In this paper we focus on the plastic material behavior beyond the point of maximum uniform strain in a quasi-static tensile test. To this purpose, the material is subjected to a post-necking tensile experiment during which the strain field in the diffuse necking zone is measured using a dedicated Digital Image Correlation (DIC) system. The key point in the identification of the post-necking strain hardening is the minimization of the discrepancy between the external work and internal work in the necking zone. In this study, we scrutinize the influence of DWH in the pre-necking regime on the identification of the post-necking strain hardening behavior of pure titanium sheet. Finally, a strain hardening model which enables disentangling pre- and post-necking hardening behavior is presented.

Reproducing the experimental torque-to-turn resistance of blind rivet nuts using FEA

A blind rivet nut is a mechanical fastener enabling a fast installation from one side. The setting process relies on plastic deformation due to axial compression of the rivet nut in such a way that a bulge is formed on the underside of the work piece. The mechanical performance of such a connection partly depends on the final geometry of the rivet nut after forming. In this contribution, the torque-to-turn resistance of an internally threaded round tubular rivet is studied with the aid of finite element techniques. A strategy is presented to simulate the setting process involving large plastic strains and contact pressures. An FE-based inverse method was used to identify the local plastic material properties of the blind rivet nut. The forming simulation was validated in terms of predicted shape of the rivet nut and the evolution of the setting force. A quasi-static FE model using the shape and solution variables of the deformed rivet nut was used to reproduce the torque-to-turn resistance as a function of the setting force. Finally, the aim is to apply the presented modelling strategy to explore innovative possibilities to increase the torque-to-turn resistance of a blind rivet nut.

Identification of a 3D Anisotropic Yield Surface Using a Multi-DIC Setup

Two synchronised stereo-DIC setups, referred to as a multi-DIC setup, have been used to acquire the displacement and strain fields over the front and thickness surface of a 10 mm thick high strength steel (S690QL). In a first step, using Finite Element Model Updating, the strain hardening behaviour has been identified beyond the point of maximum uniform elongation up to strains of 70%. As a result of the multi-DIC setup, full-field surface strain information is available in all three material directions: the lateral, longitudinal and through thickness direction. Consequently, a 3D anisotropic yield surface could be identified. Since the parameters of a yield surface are only identifiable if the sensitivity to these parameters is substantial, a new type of specimen has been designed having a perforation over the front and thickness surface. This double perforated specimen has then successfully been used to identify a 3D anisotropic yield surface.

Error assessment in post-necking strain hardening behaviour identification of mild steel sheet

The information hidden in the diffuse neck of a tensile test on a thin metal sheet can be extracted using a special case of the non-linear virtual fields method yielding the so-called post-necking strain hardening behaviour. The method, however, requires a number of assumptions which are scrutinized in this paper. To eliminate experimental errors which could potentially hamper the assessment, virtual test data (i.e. strain fields at different load steps) is generated using a FE model of the tensile test. The identification strategy is then used to retrieve the reference strain hardening behaviour used in the FE simulation. This approach is used to study the necessity of incorporating rate-dependent plasticity in the identification procedure. Additionally, the necessary plane stress condition in the diffuse neck is studied.