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Schwarzer N.,Saxonian Institute of Surface Mechanics SIO | Heuer-Schwarzer P.,ESAE and ZumKranich
OPT-i 2014 - 1st International Conference on Engineering and Applied Sciences Optimization, Proceedings | Year: 2014

Recently developed completely analytical tools for the modelling of contact problems on thin film structures are adapted to allow the investigation of arbitrarily mixed purely isotropic and transversally isotropic laminate structures under impact loads. The new tool is applied to model a variety of load problems resulting in the failure of windsurfing boards consisting of a relatively thin laminate shell and a soft polymer foam core. After a thorough failure analysis the new analytical tool can also be inverted and used for fast structural optimization. It is shown that local impact and distributed bending loads due to "bad landing" after high jumps or contact with parts of the sailing gear (the so called rig) especially the front part of the boom are leading to the most critical stress distributions resulting in failure. So, most of the investigated boards were damaged because the rider (windsurfer) landed flat and thus produced a sudden impact force under his feet (impact defect). Other overloading occurred due to overturning of so called loop movements or the landing of the board exactly on respectively between two waves and this way producing high bending moments. Some of those typical loads are analysed in detail and the stresses occurring in the complex structure of the windsurfing boards are evaluated. Finally these worst case situations are then put into structural optimization cycles in order to obtain constructive solutions avoiding such failure mechanisms in future board generations.


Schwarzer N.,Saxonian Institute of Surface Mechanics SIO | Heuer-Schwarzer P.,ESAE and ZumKranich
ECCOMAS Special Interest Conference - SEECCM 2013: 3rd South-East European Conference on Computational Mechanics, Proceedings - An IACM Special Interest Conference | Year: 2013

Recently developed completely analytical tools for the modelling of contact problems on thin film structures are adapted to allow the investigation of arbitrarily mixed purely isotropic and transversally isotropic laminate structures under impact loads. The new tool is applied to model a variety of load problems resulting in the failure of windsurfing boards consisting of a relatively thin laminate shell and a soft polymer foam core. It is shown that local impact and distributed bending loads due to "bad landing" after high jumps or contact with parts of the sailing gear (the so called rig) especially the front part of the boom are leading to the most critical stress distributions resulting in failure. So, most of the investigated boards were damaged because the rider (windsurfer) landed flat and thus produced a sudden impact force under his feet (impact defect). Other overloading occurred due to overturning of so called loop movements or the landing of the board exactly on respectively between two waves and this way producing high bending moments. Some of those typical loads are analysed in detail and the stresses occurring in the complex structure of the windsurfing boards are evaluated.


Gies A.,Balzers Ag | Chudoba T.,ASMEC GmbH | Schwarzer N.,Saxonian Institute of Surface Mechanics SIO | Becker J.,Oerlikon Balzers Coating Germany GmbH
Surface and Coatings Technology | Year: 2013

In this work, homogeneous as well as gradient a-C:H:W coatings were deposited on a steel substrate. The mechanical properties of both coating systems were determined using nanoindentation. In addition, a multiaxial, 3-dimensional nanoindentation test (reciprocating wear test with nanometre resolution) was carried out in order to analyse the wear-performance of the two different coating systems.By comparing the wear-performance of the gradient coating system to the non-graduated one we were able to show that according to a theoretical approach proposed a few years ago the gradient coating systems lead to a better performance in micro-wear tests as well as in macro-scale wear tests.In addition, a new wear model taking into account the changing stress fields during the wear tests is proposed in order to simulate the wear behaviour of both coating systems. © 2013 Elsevier B.V.


Fischer-Cripps A.C.,Fischer Cripps Laboratories Pty Ltd | Bull S.J.,Northumbria University | Schwarzer N.,Saxonian Institute of Surface Mechanics SIO
Philosophical Magazine | Year: 2012

Claims for ultra-hardness (H≈100GPa) in nanocomposite coatings are critically examined in terms of the experimental evidence first presented in 1999 and theoretical support published over the past 10 years. It is shown that the results of experimental work cannot be validated, and that there are many unresolved issues associated with the supporting theoretical arguments. Using the methods outlined by the authors, whose work is reviewed here, but with more precise application of the equations involved, and reading directly from their reported relationships between Y and H, the best estimate of the hardness for the materials under consideration appears to be of the order of ≈55GPa. This estimate is validated by actual measurements on a diamond sample and super-hard coatings, and finite element computations in comparison with experimental results for ultra-hard coatings. It is shown that the conclusions of the work being reviewed do not stand up to scrutiny and that the hardness of the ultra-hard coatings is most likely over-estimated by a factor of ≈2. © 2012 Taylor & Francis.


Kohl J.G.,University of San Diego | Bierwisch N.,Saxonian Institute of Surface Mechanics SIO | Ngo T.T.,University of San Diego | Favaro G.,CSM Instruments SA | And 2 more authors.
Wear | Year: 2016

The purpose of this study is to characterize the influence of specific micro-indentation test methods and frictional behavior on the viscoelastic properties of polyester fiberglass composite. Previous work shows that viscoelastic parameters for neat thermoset polymers can be successfully determined using a recently developed phenomenological model. In this study, similar depth sensing micro-indentation tests were performed on polyester fiberglass composite surface to determine the effect of fiberglass reinforcement on viscoelastic properties of the composite. Data obtained from micro-indentation tests was used to calculate the viscoelastic properties of the composite using the previously developed model, and compared to those of neat polyester. Results show that fiberglass significantly increased the both viscoelastic indentation modulus parameters, E0* and E1*, of the composite. Scratch tests were also performed on this composite, showing a higher coefficient of friction for a diamond stylus in contact with a polyester fiberglass composite than when in contact with a neat polyester. Moreover, the coefficient of friction decreased as the diamond stylus's sliding speed increased for the composite. A model is then applied to illustrate that the von Mises stress distribution in the material during a scratch test is dependent on scratch velocity. © 2016 Elsevier B.V.


Kohl J.G.,University of San Diego | Randall N.X.,CSM Instruments Inc. | Schwarzer N.,Saxonian Institute of Surface Mechanics SIO | Ngo T.T.,University of San Diego | And 2 more authors.
Journal of Applied Polymer Science | Year: 2012

Silicone elastomer coatings are currently being investigated as foul release coatings on ships hulls. Previous tests on silicone duplex elastomer coatings used a progressive load scratch test. It has been shown that the durability of uniform silicone duplex elastomer coatings is a function of thickness, indentation modulus, and stylus and that the failure mechanism depended on coating thickness and stylus. When applying silicone coatings to a ship's hull, there are regions on the ship where the coating is not uniform. This article investigates the effect of a thickness gradient on the durability of a single layer silicone elastomer coating. In these tests, a constant normal load was used as the stylus moved transversely to the surface. It was found that when the scratch test started in the silicone coating and proceeded in the direction of decreasing coating thickness ("Elastomer to Metal"), there was first a scratch tract followed by the initiation of detachment of the coating, then by gross detachment of the coating. When the scratch started on the exposed aluminum surface and proceeded into the silicone in the direction of increasing coating thickness ("Metal to Elastomer"), there was first gross detachment of the coating, followed by recovery (i.e., silicone coating is intact) and a scratch tract into the silicone. It was also found that the coefficient of friction was much higher in the silicone when the scratch test was going in the direction of decreasing coating thickness as opposed to the scratch test going in the opposite direction. © 2011 Wiley Periodicals, Inc.


Kohl J.G.,University of San Diego | Schwarzer N.,Saxonian Institute of Surface Mechanics SIO | Ngo T.T.,University of San Diego | Favaro G.,CSM Instruments SA | And 2 more authors.
Materials Research Express | Year: 2015

Epoxy and polyester thermosets are currently used as the polymer matrix material for many fiberreinforced composite systems. The viscoelastic properties of these materials were investigated in this study by use of depth sensing microindentation. The microindentation tests performed had the load being applied at a rapid loading rate to a maximum load. This maximum load was held constant while the stylus continued to sink. It was then unloaded at a nominal rate. A new phenomenological model developed by Schwarzer in 2014 was used in this work to determine the viscoelastic properties. Two approaches or methods were used. The first method involved determining the three parameters by using hold time data combined with unloading data (this method is referred to in this work as 'hold time method'). The second method used only the unloading data (this method is referred to in this work as 'unloading method'). It was shown that the properties determined by the two methods are not the same but are dependent upon the strain rate behavior during the indentation test. Low load scratch tests were also performed. It was seen that as the sliding speed increased the depth of penetration decreased resulting in the coefficient of friction also decreasing. An understanding of the viscoelastic properties at the surface and how they affect friction are important in studying wear of these materials. © 2015 IOP Publishing Ltd.


Schwarzer N.,Saxonian Institute of Surface Mechanics SIO
OPT-i 2014 - 1st International Conference on Engineering and Applied Sciences Optimization, Proceedings | Year: 2014

In order to result in optimization procedures also considering non-linear mechanical behavior this study resorts to first principles respectively effective first principles. It will be shown how relatively simple models simulating bond interactions in solids using effective potentials like Lennard-Jones and Morse can be used to investigate the effect of time dependent stress-induced stiffening or enhancement in these solids. The usefulness of the current study is the possibility of deriving relatively simple dependencies of the mechanical material properties (like Young? modulus or yield strength) on time and the invariants of the stress and strain tensor. However, in case of time dependent (viscose) material behavior the parameters are no constants anymore. They themselves depend on time and the actual stress field, especially the shear field. The influence of the time dependent pressure-induced Young's modulus change is discussed especially with respect to mechanical contact experiments and their analysis in the case of viscose thin films and substrates. Applying techniques known from exact solutions of the Einstein field equations one can still obtain practicable models allowing structural mechanical surface optimization also for such complex and non-linear cases.


Schwarzer N.,Saxonian Institute of Surface Mechanics SIO
Philosophical Magazine | Year: 2012

It will be shown how relatively simple models simulating bond interactions in solids using effective potentials, such as Lennard-Jones and Morse, can be used to investigate the effect of pressure-induced stiffening or enhancement in these solids. The value of the current study is the possibility of deriving relatively simple dependencies of the bulk-modulus B on the pressure P in a way that is completely free of microscopic material parameters wherever the solid bond interaction can be described, or at least partially described, by Lennard-Jones potential approaches. Instead of bond energies and length, only specific integral constants, such as Young's modulus and Poisson's ratio, are required. The influence of the pressure-induced Young's modulus change is discussed, especially with respect to mechanical contact experiments. © 2012 Taylor & Francis.


Schwarzer N.,Saxonian Institute of Surface Mechanics SIO
ECCOMAS Special Interest Conference - SEECCM 2013: 3rd South-East European Conference on Computational Mechanics, Proceedings - An IACM Special Interest Conference | Year: 2013

To obtain tribological parameters like Archard's wear depth parameter kd usually requires some severe effort in performing and analyzing complex tribological experiments. The paper features an approach where such parameters are extracted from effective interaction potentials [1], which themselves are built up and fed from more physical-oriented measurements like Nanoindentation and PHYSICAL scratch. By using such effective material potentials one can derive critical loading situations leading to failure (decomposition strength) for any contact situation. A subsequent connection of these decomposition or failure states with the corresponding stress or strain distributions allows the development of rather comprehensive tribological parameter models applicable in wear and fatigue simulations as demonstrated in this work. From this a new sophisticated wear model has been developed on the basis of the effective indenter concept [1, 2] by using the extended Hertzian approach [3, 4]. The models do not only allow to analyze certain tribological experiments like the well known pin-on disk test or the more recently developed nano-fretting tests, but also to forward simulate such tests and even give hints for better component life-time predictions. The work will show how the procedure in principle is to be applied and a few examples will be presented in the corresponding talk.

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