Papers by Author: Zoltan Major

Abstract: Squeeze-flow testing is a commonly used experimental method for characterising the flow behaviour of high-performance C-SMCs under typical compression moulding conditions. A European benchmark exercise involving 14 research institutes is currently being conducted to identify the sources of variability in squeeze flow testing results and to support the development of a standardised testing methodology. Experimental testing of five C-SMC materials has been completed using a well-defined testing procedure.This paper focuses on the data processing stage of the benchmark exercise, in which experimental data collected from all participants are processed and analysed to extract information on raw material variability, force–gap height relationships, and flow-front profiles. Qualitative assessment of these results is used to identify the critical sources of variability, which subsequently informs a more detailed statistical analysis.

Abstract: Hail impact is a critical loading situation for unidirectional fiber reinforced composites in automotive and aerospace applications. Therefore, it was already analyzed frequently in literature, focusing on thin-walled structural components and panels. Discontinuous fiber reinforced materials, such as sheet molding compounds (SMCs), and their behavior under local, high velocity impact of hail, was not considered in detail so far. The current study is aimed to investigate the fracture behavior of 2 mm and 4 mm thick SMCs sheets under a high velocity ice ball impact (80 m/s) from a numerical and experimental point of view. The strain-rate dependent material modeling of ice balls is based on an elasto-plastic material model and utilizes smooth particle hydrodynamic (SPH) modeling. Each unidirectional fiber bundle of the SMC plate was modeled individually and the space between these discrete patches was filled by elements representing the matrix material. A micromechanical analysis using representative volume elements (RVEs) was conducted to determine the homogenised strain rate dependent response of the SMC fiber bundle. In addition, a 3D Maximum Stress and Hashin damage model was calibrated to simulate the fracture of the SMC sheets. The accuracy of the applied models was evaluated by comparing the numerical model to results gathered from experimental hail impact trials. Hereby, a high-speed camera system was used to record the experiment and to gain insight into the fracture behavior of the composite structures. In general, it was observed that a 2 mm thick SMC plate was not able to withstand the impact of an ice ball whereas the 4 mm thick sheet was not visually affected. Computer tomography measurements of the impacted region revealed significant damage within the 4 mm thick SMC plate.

Abstract: The characterization of the loading rate dependence of the fracture behavior of polymers is of prime theoretical and practical interest for supporting demanding engineering applications. To gain more insight into the high rate fracture behavior of polymers, fracture tests were performed under tensile loading conditions up to 12 m/s loading rate using a neat model polymer (PVC grey) in this study. A conventional single actuator test set-up for compact tension C(T) specimens was developed based on the previous experience of the authors and implemented on a new high rate servohydraulic testing machine. In addition, a novel double action test set-up was developed by applying two twin actuators and implemented in a rigid horizontal test frame. The conventional load and force measurement was extended by instrumented test specimens and by a high speed optical strain analysis system for both set-ups. Force based fracture toughness values using the peak load values, K_Ic^PL and displacement based values using the critical crack opening displacement (CTOD) K_IC^CTOD were determined up to a loading rate of 10 m/s. While the K_Ic^PL values decreased up to a loading rate 10³ MPam^1/2s^-1 an increase with a high data scatter was observed above them. Corresponding to the CTOD values the calculated K_IC^CTOD values revealed a slight decrease and moderate data scatter up to the maximal loading rate.

Abstract: A methodology to calculate surface strains from a rectangular grid placed on a forming blank is introduced. This method consists of treating the grid points as nodes of a finite element (FE) model and assigning elements to the grid. The strains are then computed following FE analysis. If higher order elements are used, also more information within the element can be obtained which allows a coarser grid without loss of accuracy. This is the major advantage of the approach presented herein.

Abstract: As particle filled and fiber reinforced polymer matrix composites are frequently used in many demanding industrial applications, the proper prediction of the deformation behavior of these materials is of high practical importance for a reliable product design. To predict the thermo-mechanical behavior, micromechanics based simulations were performed using both the mean field homogenization methods (MFH) and full-scale finite element (FS-FE simulations on a material specific representative volume. The applicability and limitations of both methods are introduced based on five different practical examples. Both thermoplastic polymers and elastomers were used as matrix materials with combination of fillers made from different materials having different aspect ratio and revealing a wide variation of alignments and arrangements. While conventionally the behavior of composites revealing processing induced microstructure is predicted for practical engineering applications, novel artificial micro-structures revealing special functionalities might also be designed and their behavior predicted for supporting material development efforts.

Abstract: Medical images were generated in computed tomography devices at the medical partner. The medical image format was transformed into engineering data format by the authors and these data were used for printing the macroscopic model of the spine and the spinal disc and subsequently they were assembled. In addition to this macroscopic prototyping process, a novel microstructure based prototyping process was used for generating structure details of the spinal disc. The fiber reinforced and layered structure of the annulus fibrosus was modeled and in a prototype realized using soft and stiff polymeric materials. This method is based on the material microstructure prototyping methodology developed by the research group of the authors.This basic model could be used for medical education, for patient counselling and to support prosthesis development efforts.

Abstract: Injection molded discontinuous fiber reinforced components are widely used in many demanding engineering applications and are exposed to a complex combination of thermo-mechanical loads. Mean field homogenization approach was successfully applied for predicting the global stiffness behavior over wide part geometry complexity, fiber orientation distribution (FOD) and loading situations including loading rate and temperature dependence. The prediction of the component strength, however, is significantly more complicated and requires additional and theoretical considerations as well as the application of various numerical tools and sophisticated experiments. To overcome above difficulties the MFH technique was extended with the first pseudo grain failure or damage (FPGF or FPGD) approach proposed by the research group of Doghri [1] elaborated in detail using short glass fiber reinforced PP-GF in the PhD Thesis of Reiter [2] and shortly described in this study.

Abstract: The fracture behavior of engineering polymers is usually characterized at high loading rates using Charpy specimens. However, due to the presence of dynamic effects the conventional force based analysis for determining fracture toughness values is applicable only up to 1 m/s using tree point bending test configurations. This difficulty can be overcome in principle, by applying dynamic analysis methods (e.g. dynamic key curve (DKC) analysis) or by applying tensile loading fracture configurations. The applicability of pre-cracked Charpy specimens for determining fracture toughness values for polymeric materials over a wide loading rate range is investigated in this study.

Abstract: The current work is aimed at development of a numerical model able to describe complex damage phenomena that occur during an impact event in a sandwich structure having a honeycomb core. The complex material models employed within the research include linear-elastic and elasto-plastic material models having transverse isotropy as well as damage evolution models for both brittle failure and plastic failure. Within this paper remarks concerning the failure of the skins and core components as well as dissipated impact energy and affected regions are done.

Abstract: Different grades of several thermoplastic elastomer types were selected and are investigated over a wide frequency/time, temperature and loading range in a research project of the authors. Relevant material models are selected for different loading situations and based on these experimental data the material model parameters were determined either directly or by applying fitting procedures. These models along with the proper data were used for modeling the deformation and the failure behavior of typical engineering thermoplastic elastomer components. Furthermore, based on the modeling of various elastomers under different service relevant loading situation several design proposals were formulated.