Vorhersage von Spanbruch bei der Zerspanung mit geometrisch bestimmter Schneide mit Hilfe schädigungsmechanischer Ansätze

摘要

The chip shape – besides the cutting forces, tool wear and surface roughness – is one of the most commonly used criteria for the evaluation of the machinability. The importance of the chip shape can be explained by its strong influence on process reliability. In consequence an automation of cutting operations is only possible for fa-vourable chip shapes. For longitudinal turning as well as for other processes with con-tinuous cutting edge contact, periodic chip breakage is necessary to avoid unfavourable ribbon or thread chips. Chip breakage is caused by a variety of mutually dependant in-fluencing factors related to workpiece, tool or process parameters. A possibility to predict chip breakage is to model these influences. The three-dimensional Finite Element Method (3D-FEM) offers the possibility to take the complex chip breaker geometries of the cutting tool into account. For FE-modelling of chip breakage, a criterion is needed, capable of calculating material failure at the position of chip breakage. However, the definition of such a criterion requires research into the thermo-mechanical loads that cause chip breakage. Accordingly, this thesis aims to predict chip breakage using a damage model, which is based on the mechanical and thermal loads in the chip. It contributes to a more funda-mental understanding of cutting processes with geometrically defined cutting edges in general and contributes the prediction of chip breakage in longitudinal turning of AISI 1045. A model-based approach was chosen in order to calculate thermo-mechanical loads. Based on these loads, an adequate damage model could be defined. The localisation of chip breakage and the dominant failure mechanisms in the chip breakage zone were determined in empirical investigations of the cutting process. By applying a sensitivity analysis, the strongest tool- and process-related influences that cause chip breakage were identified and different process conditions for finishing and roughing tool geome-tries were determined. The process conditions included parameter sets with controlled chip breakage as well as sets with unfavourable chip shapes. For the parameter sets with controlled chip breakage, the time and position of material failure were identified using a high speed filming device. The analysis of the chip fracture faces revealed duc-tile material failure as the dominant failure mechanism. Different damage models for this type of material failure were presented and discussed regarding their potential and ap-plicability to predict chip breakage. The decision between the use of existing damage models and the definition of a new damage model was made on the basis of the thermo-mechanical loads in the chip breakage position. An analytic model was then de-veloped to transform the distribution of stress in a blocked chip to a status of plane stress. The maximum tensile stresses depending on the cutting conditions and tool ge-ometry were calculated based on this model. The model correlated with the real borders of controlled chip breakage in a wide range of cuttings depths and feed rates. Only for small cutting depths and large feed rates a significant difference compared to real chip breakage was identified. This deviation is caused by the complex three-dimensional chip flow under these process conditions. It is assumed that these cutting conditions require three-dimensional modelling of the chip flow. Consequently the chip formation, chip flow and expansion of the chip were modelled three-dimensionally using the Finite Element Method (FEM). In this model the current loads as well as their time-dependent developments could be determined. The evalua-tion of the existing damage criteria showed that none of the presented criteria offer suf-ficient reliability for the prediction of chip breakage. Therefore an independent damage criterion for chip breakage prediction was developed based on a damage model of Johnson and Cook. This criterion calculated the residual deformability of the material depending on the tolerated temperatures, strain rates and stress conditions. The calcu-lated damage value lead to a reduction of the material strength and to a local softening in the chip. The implementation of this damage criterion in the FEM-model enabled three-dimensional simulation of chip breakage. The model correlated well with the chip flow and breakage as well as with the empirically determined cutting forces and tem-peratures on the lower surface of the chip. The FEM-model developed enables system-atic investigation of the influence of tool geometries and cutting conditions on chip breakage for turning operations of AISI 1045.