Analysis of damage and fracture formulations in cold extrusion

In forming processes, components generally undergo large deformations. This induces the evolution of damage, which can influence material and product properties. To capture these effects, a continuum damage mechanics (CDM) model, based on the work of Lemaitre [8] and Soyarslan [13, 14] as well as different fracture criteria according to Cockcroft and Latham [2], Freudenthal [4] and Oyane [10] are implemented and in- vestigated. While the CDM theory considers the evolution of damage and the associated softening, fracture criteria do not affect the results of the mechanical finite element (FE) analysis. However, a coupling is generally possible via element deletion, but material softening cannot be depicted in the simulation. Tensile tests with notched specimens are performed in order to obtain the material parameters associated with these models by inverse parameter identification processes. The optimized set of parameters is finally ap- plied to the damage and fracture models used for the FE simulations of a cold extrusion process, which are investigated in terms of damage evolution and material failure. It is demonstrated that the CDM model predicts the evolution of damage observed for differ- ent process parameters in cold extrusion quantitatively. The prediction of the failure by the fracture criteria does not agree well with the experiments

Comparison of gurson and lemaitre model in the context of blanking simulation of a high strength steel

The process of blanking takes place in a short band with high accumulated strain undergoing various stress triaxialities. Enhanced implementations for shear and compressive loads of Gurson’s and Lemaitre’s model are directly compared for the same blanking setup. For a dual phase steel DP600 the Lemaitre parameters are identified completely by an inverse strategy, while the parameters of the Gurson’s porous plasticity model are predominantly gained from analysis with a scanning electron microscopy (SEM). The models are validated by comparison of force-displacement curves, time point and location of crack initiation. Advantages and disadvantages of both approaches are discussed with respect to prediction accuracy and costs of parameter identification. Both of the models deliver an exact prediction for the location of the crack and a good prediction of the punch displacement at the onset of cracking

Testing of Formed Gear Wheels at Quasi-Static and Elevated Strain Rates

Geared components can be manufactured from sheet metals by sheet-bulk metal forming. One relevant load case in service are overload events, which might induce elevated strain rates. To determine the characteristic hardening and fracture behavior, specimens manufactured from the deep-drawing steel DC04 were tested with strain rates ranging from 0.0001 to 5 s−1. The gear wheels manufactured by sheet-bulk metal forming are tested at crosshead velocities of 0.08 mm/s and 175 mm/s. The tests are analyzed by measuring deformed geometry and hardness. While the tensile tests results show obvious strain-rate dependency, the hardness measurements show no strain-rate depended effect. The analyses are complemented by finite-element-simulations, which assess the homogeneity of deformation and point out the mechanisms of failure. Both coupled and uncoupled ductile damage models are able to predict the critical areas for crack initiation. The coupled damage model has slight advantages regarding deformed shape prediction

Phenomenological modeling of anisotropy induced by evolution of the dislocation structure on the macroscopic and microscopic scale \ud

This work focuses on the modeling of the evolution of anisotropy induced by the development of the dislocation microstructure. A model formulated at the engineering scale in the context of classical metal plasticity and a model formulated in the context of crystal plasticity are presented. Images obtained by transmission-electron microscopy (TEM) show the influence of the strain path on the evolution of anisotropy for the case of two common materials used in sheet metal forming, DC06 and AA6016-T4. Both models are capable of accounting for the transient behavior observed after changes in loading path for fcc and bcc metals. The evolution of the internal variables of the models is correlated with the evolution of the dislocation structure observed by TEM investigations

Advanced material model for shear cutting of metal sheets

A finite-element simulation of the shear cutting process is used to predict thegeometry of the cutting surface. A fully-coupled Lemaitre-type model is used in the process model for the description of the material behaviour. The extended Lemaitre model considers the influence of shear and compression-dominated stress states on the propagation of damage. Tensile tests with and without notches are used for the identification of material parameters. These methods are advantageous for the analysis of different blanking processes. Since damage parameters have a strong influence on the cutting surface quality, a numerical study isconducted to analyse their influence. The results of the simulations are compared with experimental data

Investigations of ductile damage in DP600 and DC04 deep drawing steel sheets during punching

The paper presents numerical and microstructural investigations on a punching process of 2 mm thick steel sheets. The dual phase steel DP600 and the mild steel DC04 exhibit different damage and fracture characteristics. To distinguish the void development and crack initiation for both materials, interrupted tests at varied punch displacements are analyzed. The void volume fractions in the shearing zone are identified by scanning electron microscopy (SEM). The Gurson model family, which is recently extended for shear fracture, is utilized to model the elastoplastic behavior with ductile damage. The effect of the shear governing void growth parameter, introduced by Nahshon and Hutchinson (2008), is discussed

Evaluation of micro-damage by acoustic methods

Several methods for the determination of integral damage in sheet metal are reviewed and investigated at the example of the ferritic steel DC04. A novel method to determine damage in cylindrical rings is proposed. Such a method is useful for the measurement of damage in components manufactured by sheet-bulk forming. In sheet-bulk-forming or precision forging sheets with a thickness in the range of 1-5 mm are processed by making use of an intended three-dimensional material flow to form toothed components such as gears. Damage leads to the modification of physical properties, such as Young’s modulus and the related resonance frequency. Young’s moduli were determined by various methods such as tensile tests and the frequency of natural oscillations of rectangular samples as well as cylindrical rings. Additionally, the change in the propagation velocity of ultrasonic waves in rectangular bars was examined as a damage criterion and reference measurements of damage by electron microscopy were carried out. The damage values obtained by electron microscopy are consistent with the results of the other investigated methods

Shifting value stream patterns along the product lifecycle with digital twins

The concept of digital twins promises high potentials for product design, manufacturing, user experience and recycling. Thus, digital twins have received increasing interest in academia and industry. However, the actual benefits of digital twins remain in many cases unclear. This article aims to summarize selected recent developments in this field and demonstrate use cases from different phases of the product lifecycle. For that purpose, examples from the design, manufacturing, use and recycling phase are presented. In a subsequent discussion, ideas for new value stream patterns using digital twins are envisioned and research questions are derive