Zoltan Major

Dynamic optical studies in materials testing with spectral-domain polarization-sensitive optical coherence tomography

By combining dynamic mechanical testing with spectral-domain polarization-sensitive optical coherence tomography (SD-PS-OCT) performed at 1550 nm we are able to directly investigate for the first time changes within scattering technical materials during tensile and fracture tests. Spatially and temporally varying polarization patterns, due to defects and material inhomogeneities, were observed within bulk polymer samples and used to finally obtain--with the help of advanced image processing algorithms--quantitative maps of the evolving internal stress distribution. Furthermore, locally increased stress within fiber-reinforced composite materials was identified in situ with SD-PS-OCT to cause depolarizing sites of fiber-matrix debonding prior the onset of complete structural failure.

Cost-Efficient Open Source Desktop Size Radial Stretching System With Force Sensor

The rapid and efficient development of soft active materials requires readily available, compact testing equipment. We propose a desktop-sized, cost-efficient, and open source radial stretching system as an alternative to commercially available biaxial and uniaxial stretching devices. It allows for doubling the diameter of an elastomer membrane while measuring the applied force. Our development enables significant cost reduction (<;300 €) and increase the availability of equibiaxial deformation measurements for scientific material analysis. Construction plans, source code, and electronic circuit diagrams are freely available under a creative commons license.

From Playroom to Lab: Tough Stretchable Electronics Analyzed with a Tabletop Tensile Tester Made from Toy-Bricks

Toy bricks are an ideal platform for the cost‐effective rapid prototyping of a tabletop tensile tester with measurement accuracy on par with expensive, commercially available laboratory equipment. Here, a tester is presented that is not only a versatile demonstration device in mechanics, electronics, and physics education and an eye‐catcher on exhibitions, but also a powerful tool for stretchable electronics research. Following the “open‐source movement” the build‐up of the tester is described and all the details for easy reproduction are disclosed. A a new design of highly conformable all‐elastomer based graded rigid island printed circuit boards is developed. Tough bonded to this elastomer substrate are imperceptible electronic foils bearing conductors and off‐the‐shelf microelectronics, paving the way for next generation smart electronic appliances.

PCB drop test lifetime assessment based on simulations and cyclic bend tests

Abstract The aim of this work was to predict the performance of printed circuit boards (PCBs) in a board level drop test (BLDT). The applied methodology was based on results of a board level cyclic bend test (BLCBT) and an according finite element simulation of the test. A function, describing the relation between a local loading parameter, determined in the simulation model for different deflection amplitudes of the BLCBT, and the according cycles to failure, measured in the experiments, was modelled. The method was evaluated by comparing the predicted results of two additional PCB built-ups with experimentally determined lifetimes. The determined lifetimes agreed very well, although the differences between the analysed PCB types were not very clear. Applying the known correlation between the BLCBT and the BLDT, the predicted results for the BLCBT could be used to estimate the BLDT performance.

Strength of single-lap-joint assemblies of continuous unidirectional carbon fibre-reinforced thermoplastic matrix tapes under tensile loading

Unidirectional tape-placement technologies appeared as a promising alternative due to their potential in large-scale component production. While the optimization strategies used to define the tape lay-out can be of different nature, the utilization of tape-to-tape joints is inevitable. Whereas several studies have focussed their efforts on the process and design stages, no study has yet addressed the influence of the manufacturing process on the mechanics of unidirectional tape joints. In this study, the strength of single-lap-joint assemblies of carbon fibre-reinforced thermoplastic tapes under tensile loading was analysed. The dependence of the strength on the overlap geometry and the manufacturing pressure was of main focus. Single-lap-joint assemblies with rectangular and rounded overlaps of the same overlap area were prepared employing a pre-heating stage at 250℃ and forming pressures from 3 to 100 bar. Failure of the assemblies was not observed on the overlap itself but instead on the zone near the overlap end on the adherend. Traditional determination of strength of single-lap-joint assemblies is not applicable in this case. Consequently, a typical Hashin failure criterion was used to model the failure of the assemblies. The study showed that although cohesive failure is not likely within the analysed pressure range, overlap geometry and forming-pressure affect the strength of single-lap-joint assemblies under tensile loading.

A combined experimental and simulation approach for modelling the mechanical behaviour of heterogeneous materials using rapid prototyped microcells

Microcells or representative volume elements (RVEs) are frequently used in many numerical micromechanics simulations of heterogeneous polymer compounds and can be created in convenient computer tools. The virtual deformation behaviour of microcells revealing different microstructures (e.g. filler volume, size, shape, orientation and arrangement) and different material properties (reversible elastic, time- and temperature-dependent viscoelastic, large-scale deformation hyperelastic or irreversible elastic plastic) can be calculated based on various mechanical approaches. However, the experimental validation of these numerical calculations is a challenging task. To support this effort, in addition to the conventional macroscopic product prototyping, a novel two-component rapid prototyping system was used for the material microstructure prototyping of heterogeneous polymer compounds. Furthermore, various microcell models were manufactured in the present study. To facilitate the experimental investigations, these microcells were periodically integrated into a test specimen configuration. The deformation behaviour of the fabricated microcells and test specimen was subsequently characterised and both, nominal stress-strain curves and the local strain distribution around the particles on the specimen surface were determined with the digital image correlation (DIC) technique. The virtual deformation behaviour of these microcells was characterised in the DigiMat-FE tool using elastic and hyperelastic material models and randomly oriented particles (quasi-isotropy) as well as 80% oriented particles (anisotropic behaviour). The results of the microstructure simulations and the experiments were compared both on a global (nominal stress-strain curves) as well as on a local scale (strain distribution around the particles in the matrix). The comparison of the results may contribute to the optimisation of the structure and the improvement of the accuracy of the material model used in micromechanics simulation.

An Inverse Finite Element Approach to Calculate Full-Field Forming Strains

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.

Cyclic Loading of Polyetheretherketone at High Tensile Stress Levels

In a recent study of the corresponding author, it was found that PEEK bearing elements revealed high (irreversible) surface strains if they were loaded between steel sheets. Since this reflects the conditions in the practical application and because the rolling properties are dominated by the surface material, a more detailed analysis of highly strained PEEK was required. Hence, fatigue tests in the high stress tensile regime were conducted. The experiments were carried out on servo-hydraulic testing machines and during the tests the mechanical response of the specimens was recorded. Two material modifications of PEEK were investigated in the research: untreated PEEK (without heat treatment) and annealed PEEK which was modified using defined thermal conditions. The analysis of the recorded test data aimed on the distinction between cumulative material response (creep deformation, material hardening / softening) and spontaneous material response (material hardening / softening). At the highest stress levels, the cumulative response pretended material softening with increasing number of cycles. However, by examining the spontaneous material response which became stiffer with increasing number of cycles, it was shown that the cumulative softening was caused by time-dependent deformation processes.

Experimental Determination of Fatigue Crack Growth Behavior and Surface Strain Distribution of ‘Faint-Waist Pure Shear’ Specimens with Different Crack Tip Radii

Abstract The influence of the initial crack-tip radius on the fatigue behavior and the strain distribution in the vicinity of the blunted crack tip was determined experimentally using a servo-hydraulic testing machine and an optical full-field strain analysis method. Two different elastomer grades (SBR, EPDM) were selected for the experimental work. The strain analysis method used, based on the image correlation technique, was found to be an effective tool to determine strains, strain distributions and gradients near the crack tip for elastomeric materials. Different material behavior was observed in the two rubber types investigated. While the crack tip was regularly blunted (half circle shape) for EPDM and the strain gradient was low (less steep), the crack tip was sharp (less blunted) with a higher strain gradient for SBR. Furthermore the crack tip radius was found to be an important influencing factor on the initiation of crack growth, but not on the crack growth behavior after initiation. Based on th...

SEMI-ACTIVE DAMPING PERFORMANCE OF IRON PARTICLE FILLED SILICONE RUBBER

The aim of this work was to design, produce and evaluate a demonstrator to visualize the magneto-induced damping behaviour of materials. In contrast to standard materials, the damping coefficient of iron particle filled silicone rubbers can be controlled by a semi-active magnetic field. This field effect should be characterized in order to evaluate the suitability of these magnetorheological silicone elastomers for the use in different configurations and applications.