Gerald Pinter

Tough and degradable photopolymers derived from alkyne monomers for 3D printing of biomedical materials

This contribution deals with the synthesis and exploration of alkyne carbonate derivatives as biocompatible building blocks in the thiol–yne photopolymerisation reaction with the aim to facilitate the fabrication of tailor made medical devices by UV based additive manufacturing technologies. It turned out that the investigated alkyne carbonates offer curing rates similar to comparable acrylates, while providing much higher conversion and lower monomer cytotoxicity. Curing the synthesized building blocks in combination with the commercially available thiol pentaerythritol tetra(3-mercaptopropionate) (PETMP) leads to networks that degrade in aqueous alkaline and acidic media in a surface erosion manner. Additionally, a selective adjustment of the degradability is feasible by the choice and content of thiol monomers. Notably, monomers containing a tricyclo[5.2.1.02,6]decane-4,8-dimethanol backbone provide decent thermo-mechanical properties and appropriate impact strengths similar to polylactic acid (PLA). Most importantly, selected thiol–yne formulations were printed successfully with an accuracy of 40 × 40 μm, which seems to be sufficiently high to prints medical devices in appropriate resolution.

Determination of the orthotropic material properties of individual layers of printed circuit boards

Abstract To improve the reliability of printed circuit boards (PCBs), occurring failure modes have to be studied and analysed. In order to reduce the expenses of experimental testing, finite element analysis (FEA) is used to describe the failure behaviour. Therefore, the availability of the proper material data is crucial. As the materials used in printed circuit boards, glass fibre reinforced epoxy resin and structured copper, show anisotropic material behaviour, it is necessary to determine the direction-dependent material properties. However, as only very thin layers were available of these materials, no experimental mechanical out of plane characterisation could be performed. Thus, a combination of experiments, mean-field calculations and FEA simulations was used to generate the material data for both, the insulating and the conducting layers of the PCB. The generated material models were tested in board level simulations and compared to the results of simulations based on isotropic material behaviour. Significant differences were observed, approving the importance of proper material data, especially when the local stress and deformation field are important.

Exploring Network Formation of Tough and Biocompatible Thiol-yne Based Photopolymers.

This work deals with the in-depth investigation of thiol-yne based network formation and its effect on thermomechanical properties and impact strength. The results show that the bifunctional alkyne monomer di(but-1-yne-4-yl)carbonate (DBC) provides significantly lower cytotoxicity than the comparable acrylate, 1,4-butanediol diacrylate (BDA). Real-time near infrared photorheology measurements reveal that gel formation is shifted to higher conversions for DBC/thiol resins leading to lower shrinkage stress and higher overall monomer conversion than BDA. Glass transition temperature (Tg ), shrinkage stress, as well as network density determined by double quantum solid state NMR, increase proportionally with the thiol functionality. Most importantly, highly cross-linked DBC/dipentaerythritol hexa(3-mercaptopropionate) networks (Tg ≈ 61 °C) provide a 5.3 times higher impact strength than BDA, which is explained by the unique network homogeneity of thiol-yne photopolymers.

A correlation of creep and fatigue crack growth in high density poly(ethylene) at various temperatures

The creep crack growth (CCG) and fatigue crack growth (FCG) behaviour of two PE-HD pipe grades was studied based on a linear elastic fracture mechanics (LEFM) methodology. The FCG-tests were performed under a sinusoidal load at a frequency of 1 Hz and R-ratios (Fmin/Fmax) of 0.1, 0.3 and 0.5; the test temperatures were 23 (only FCG), 60 and 80 °C. The results showed that FCG rates in PE-HD are caused by a combination of cyclic-induced and creep-induced damage, depending on the mean stress level. While for given values of KImax (FCG tests) and KI (CCG tests), respectively, at low test temperatures the cyclic component of the applied stress dominates crack growth rates with CCG rates (R = 1) being lower than the FCG rates (R < 1), at high test temperatures the creep component becomes increasingly important in affecting crack growth rates so that CCG rates even exceed FCG rates. The point of inversion from fatigue to creep dominated failure on the temperature scale apparently depends on molecular and morphological characteristics of the PE-HD type and occurs at around 80 °C for PE-HD 1 and around 60 °C for PE-HD 2 in this investigation.

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.

Method development for the cyclic characterization of thin copper layers for PCB applications

Purpose – The overall aim of this research work was the improvement of the failure behavior of printed circuit boards (PCBs). In order to describe the mechanical behavior of PCBs under cyclic thermal loads, thin copper layers were characterized. The mechanical properties of these copper layers were determined in cyclic four-point bend tests and in cyclic tensile-compression tests, as their behavior under changing tensile and compression loads needed to be evaluated. Design/methodology/approach – Specimens for the four-point bend tests were manufactured by bonding 18-μm-thick copper layers on both sides of 10-mm-thick silicone plates. The silicone was characterized in tensile, shear and blow-up tests to provide input data for a hyperelastic material model. Specimens for the cyclic tensile-compression tests were produced in a compression molding process. Four layers of glass fiber-reinforced epoxy resin (thickness 90 μm) and five layers of copper (thickness 60 μm) were applied. Findings – The results showed...

Stiffness Based Fatigue Characterisation of CFRP

This paper investigates the possibility of determining damage under fatigue loading in carbon fibre reinforced plastics (CFRP) by using mechanical stiffness. Therefore, stress-strain-hysteresis recorded in fatigue tests under sinusoidal loads are used for moduli calculation. Additionally, a new method for stiffness evaluation called cyclic tensile tests is presented. Its results are compared to results from hysteresis analysis and to actual damage mechanisms monitored non-destructively with thermography analysis.

Local damage simulations of printed circuit boards based on in‐plane cohesive zone parameters

Purpose – The purpose of this paper is to analyse, in a finite element simulation, the failure of a multilayer printed circuit board (PCB), exposed to an impact load, to better evaluate the reliability and lifetime. Thereby the focus was set on failures in the outermost epoxy layer.Design/methodology/approach – The fracture behaviour of the affected material was characterized. The parameters of a cohesive zone law were determined by performing a double cantilever beam test and a corresponding simulation. The cohesive zone law was used in an enriched finite element local simulation model to predict the crack initiation and crack propagation. Using the determined location of the initial crack, the energy release rate at the crack tip was calculated, allowing an evaluation of the local loading situation.Findings – A good concurrence between the simulated and the experimentally observed failure pattern was observed. Calculating the energy release rate of two example PCBs, the significant influence of the chos...

Characterisation of the thermo‐oxidative degradation of polyethylene pipes by chromatographical, rheological and thermo‐analytical methods

In this work the influence of three different stabiliser systems on the stress rupture behaviour of high density polyethylene (PE-HD) pipes under constant internal pressure was examined at 60 and 80 °C with special consideration of the quasi-brittle failure by growth of a single crack in the failure regime preceding the global chemical degradation of the pipes. It could be proven that the pipes of the three formulations did not show global molecular and morphological differences in this failure regime and that the stabilisation was still intact. Therefore, the differences in failure times observed for the three formulations are believed to be a result of local ageing around the crack tip related to the combined influence of time, the elevated temperature, the presence of oxygen and water, and the high mechanical stresses in the immediate crack tip region.

Numerical lifetime prediction of polymer pipes taking into account residual stress

In this paper a methodology for assessment of residual stress effects on crack behaviour in the polymer pipe is developed. For simplicitys sake, a linear distribution of residual stresses across the pipe wall is assumed. Linear elastic fracture mechanics is used for the fracture mechanics analysis of the cracked pipe. An approximate relation for the stress intensity factor estimation for a crack in a polymer pipe, with residual stress taken into account is suggested and discussed. The methodology presented can be helpful for a rapid lifetime estimation of polyolefin pipelines.