Sabine Seidler

Fabrication and moulding of cellular materials by rapid prototyping

Many biological materials (e.g. wood, cork, bone, etc) are based on cellular designs, since cellular architectures offer the possibility to optimise the properties (stiffness, density, strength, etc) of a structure according to the environmental conditions the structure is exposed to. By using rapid prototyping, it is possible to fabricate cellular materials on a similar size scale as in natural material-structures. By using appropriate moulding techniques, these structures can be fabricated out of a wide variety of materials (polymers, ceramics, composites). In this work, several RP techniques are investigated regarding their suitability for the fabrication of cellular solids. The main focus is on using direct light projection (stereolithography) in combination with gelcasting as moulding technique. Besides using commercial light-sensitive resins, a class of newly developed water-soluble resins has been evaluated regarding its usability as sacrificial mould material.

Application of the instrumented impact test to the toughness characterization of high impact thermoplastics

Abstract The toughness behaviour of some blends and a PA/CF composite under dynamical loading (1 m/s) based on the J -integral was studied. Different methods were used for the tests. It can be shown that the multiple specimen stop-block technique gives the most conservative results. Furthermore the influence of specimen thickness on crack resistance behaviour was studied. The instrumented Charpy impact test enables the determination of valid crack resistance curves on plastics with small specimen geometries. The advantages and disadvantages of using the known standards for metallic materials in comparison to the ESIS TC4 testing protocol were discussed and a physical concept is compared with the empirical description of R-curve behaviour with the standards .

Processing procedures for the realization of fine structured channel arrays and bridging elements by LTCC-Technology

This report deals with technological procedures to provide channel partition walls of minimum width inside of Low Temperature Co-fired Ceramics (LTCC) micro fluidic devices demonstrated by means of the fabrication of parallel closely-spaced channels which may act as a specific functional part of a fluidic heat exchanger. Furthermore, the realization of single layer bridging elements inside of channels is discussed. Such an element may be introduced as a delicate sensor substrate providing adequate thermal insulation and low thermal mass as well. The technological processing steps under consideration start with laser micromachining of green ceramic tapes using Nd-YAG-laser equipment and are followed by a modified low-pressure lamination step comprising the application of appropriate adhesives and the incorporation of polymer sacrificial volume materials (SVMs). Consequently, the increased fraction of involved organics requires an adequate adaptation of the firing process to provide a residue-free burnout. Great attention is paid to the prevention of channel cross-section distortion and to the integrity of structures, verified by optical inspection of microsectioned samples. The optimized processing procedures enable the fabrication of channel arrays with a partition wall thickness as small as 100 μm, while single layer bridging elements may span a channel width of 4 mm.

Crack-resistance behavior of polypropylene copolymers

Two heterophasic reactor-grade propylene-ethylene copolymers (RA-HECO®) were diluted with propylene-ethylene random copolymers to obtain materials with constant EPR/PE-particle diameter but various interparticle distances. According to the results of instrumented impact tests, brittle-to-tough transitions were found at different temperatures. The critical interparticle distances shift linearly over the observed range of temperature. Critical interparticle distances could be determined not only in the region of predominantly unstable crack growth but also in the region of predominantly stable crack growth.

Fracture behaviour and morphology of PC/ABS blends

The toughness behaviour of polycarbonate (PC)/acrylnitrile-butadiene-styrene (ABS) blends under dynamical loading (1 ms−1) based on the J-integral concept was studied. For this the multiple specimen R-curve method was used. A special experimental technique of a stop block method was developed. It was shown that the materials exhibit a very different toughness behaviour depending on temperature and ABS content. The reasons for this material behaviour are discussed with the help of scanning and transmission electron microscopical (SEM and TEM) investigation methods. It can be shown that a combination of fracture mechanics and electron microscopy allows a toughness optimization to be made on the basis of quantitative morphology-toughness correlations.

Determination of the viscoelastic properties of hydrogels based on polyethylene glycol diacrylate (PEG-DA) and human articular cartilage

In this work, a systematic study of the viscoelastic properties of hydrogels based on polyethylene glycol diacrylate (PEG-DA) is presented. In addition to artificial PEG-DA-based hydrogels, natural hydrogels in the form of human articular cartilage were examined. Specimens were (unconfined) compression tested under static and dynamic load. Besides this, instrumented indentation tests with different indenter geometries (cylindrical, spherical) and load ranges (macro- and nano-indentation) were carried out and relaxation tests for the determination of moduli and relaxation time were performed. Tensile tests completed the list of measurement techniques. The measured initial moduli of the evaluated hydrogels range from 104?107 Pa. Spherical indentation was used in testing human articular cartilage in phosphate buffered saline (PBS). Cartilage samples were measured shortly after explantation, being stored at room temperature. The influence of freezing and shock-freezing was evaluated. It turned out that freezing has a massive impact on sample properties, especially on the stress relaxation time and the ratio of initial to equilibrium modulus.

Mechanical properties of micro-injection moulded components

Micro-injection moulded specimens and components were investigated using instrumented indentation testing and tensile testing. Conventional nano indentation testing, from which hardness and modulus can be obtained, is useful for the characterization of thermoplastics and filled thermoplastics. The difficulties of indentation measurements on LCPs can be overcome by a cantilever beam arrangement. Optical strain measurement is suitable for the description of the (local) deformation behaviour of micro scaled polymeric components.

Micromechanical interpretation of fracture toughness of particulate-filled thermoplastics

The toughness behaviour of particulate-filled thermoplastics is determined by different failure mechanisms in the plastic zone and fracture process zone in front of the macrocrack such as particle-matrix debonding, shear processes or crazing and fracture of matrix fibrils. Theoretical expressions describing the critical strain causing microcrack initiation as well as the critical crack opening and the criticalJ integral value for unstable crack initiation are derived on the basis of a micromechanical analysis. Matrix properties, particle diameter, filler content and phase adhesion are taken into account. Critical particle contents and diameters caused by matrix morphology are discussed. Model calculations are compared with experimental results from acoustic emission analysis and dynamic fracture mechanics tests on PS, PVC and HDPE filled with CaCO3 or SiO2 particles.

Crack resistance behavior of polyvinylchloride

The effects of specimen dimension (thickness, width) and specimen configuration (SENB, CT), as well as test conditions (crosshead speed, temperature), over a range of crosshead speeds from 0.01 to 100 mm/min and temperatures from −40 to 60°C, on the crack growth resistance behavior were investigated in a commercial amorphous thermoplastic polyvinylchloride (PVC) using the J-integral and crack opening displacement (COD) concepts. With the combined application of these two different fracture mechanics parameters, more detailed information on fracture processes can be obtained. The relationship between deformation mechanisms and fracture toughness was discussed through a comparative analysis of J versus Δa and COD versus Δa resistance curves.

ESSENTIAL WORK OF FRACTURE AND THE PHASE TRANSFORMATION IN β-iPP

The possibility of phase transformation toughening is demonstrated by the example of the β-modification isotactic polypropylene (β-iPP), which undergoes the β–α transformation (i.e., from hexagonal to monoclinic) during mechanical loading. The β–α recrystallization was examined on essential work of fracture (EWF) specimens with two processing conditions. Differential scanning calorimetry and wide-angle x-ray diffraction demonstrate the occurrence of this β–α transformation. Some structural changes can be identified in EWF specimens by using nano-indentation machine. Toughness of the α- and β-iPP is studied and compared with the EWF concept by using static-loaded deeply double-edge-notched tensile specimens. The main effect of the β–α transformation is a large increase in the specific plastic work consumed in the necked zone.