Stefan Weihe

Implicit integration of elastoplastic constitutive models for frictional materials with highly non-linear hardening functions

Constitutive relations in elastoplasticity may be formulated in a variety of ways, and different update algorithms may be employed to solve the resulting equations. Several implicit integration schemes, although some not widely used, have been suggested in the last years. Among them, the closest point projection method (CPPM) has proven to be an effective and robust integration scheme. In order to gain maximum control of the stress projection, a two-level CPPM iteration scheme is proposed. The hardening variables are fixed during the stress projection onto consequently fixed yield surfaces, and after the stress projection, new values of the hardening variables are calculated defining new yield surfaces. The update of the hardening parameters which, in general, may be highly nonlinear functions, may be determined by a combination of a Picard Iteration (PI) on the hardening variables and an adaptative order inverse interpolation (AOII) on the difference of subsequent iterations of the hardening variables. The integration scheme has been implemented in a general constitutive driver which has been formulated independent of the selected constitutive model and easily linked to finite element codes. A third stress invariant dependent, cone–cap elastoplastic constitutive model, referred to as the MRS–Lade, with a highly non-linear hardening function has been used to show the applicability of the proposed iteration scheme. Error analyses and accuracy assessment are presented along with some representative test results.

Experimental investigation of the friction stir welding dynamics of 6000 series aluminum alloys

Friction stir welding (FSW) is a resource-efficient and environmental-friendly solid state joining process that allows to combine especially aluminum alloys with superior joint quality. Therefore, FSW is perfectly suitable for light-weight applications. The interaction of material, welding tool and machine during welding results in process forces which show characteristic periodic variations. The reasons for this periodicity, however, are not completely understood yet. Since the welding force feedback data can presumably be used for online process monitoring, a deeper understanding of the processes leading to the friction stir welding dynamics is necessary. To reach this goal, an approach for a systematic investigation of the friction stir welding dynamics using postulated hypotheses is presented in this work. The hypotheses combine insights from literature as well as results from own welding experiments. In the experiments two aluminum alloys, EN AW 6016 and EN AW 6111, in tempers T4 and T6 each, were friction stir welded. The welding machine, the tool as well as the welding parameters were held constant for each material. The process forces, accelerations and spindle deflection were measured for each weld and additionally the joints were inspected visually for flaws. It was shown that the height of the process forces correlates loosely with the yield strength of the materials. The frequencies occurring during welding are identified to mainly consist of the spindle rotating speed and multiples thereof. The acceleration measurements are found to be a suitable way to identify welds with irregular surfaces, i.e. they provide a method for online process monitoring regarding the weld surface quality. Combining the findings from literature and insights from the experiments, five hypotheses are developed that allow a systematic investigation of the dynamics of the friction stir welding process. Each hypothesis covers a phenomenon that can lead to dynamic effects. The hypotheses consider not only the process but the whole system of process and machine. In addition to the hypotheses, a method to prove or disprove them, where the specific effects are triggered intentionally, is presented.