Rian Seghir

Controlled mud-crack patterning and self-organized cracking of polydimethylsiloxane elastomer surfaces

Exploiting pattern formation – such as that observed in nature – in the context of micro/nanotechnology could have great benefits if coupled with the traditional top-down lithographic approach. Here, we demonstrate an original and simple method to produce unique, localized and controllable self-organised patterns on elastomeric films. A thin, brittle silica-like crust is formed on the surface of polydimethylsiloxane (PDMS) using oxygen plasma. This crust is subsequently cracked via the deposition of a thin metal film – having residual tensile stress. The density of the mud-crack patterns depends on the plasma dose and on the metal thickness. The mud-crack patterning can be controlled depending on the thickness and shape of the metallization – ultimately leading to regularly spaced cracks and/or metal mesa structures. Such patterning of the cracks indicates a level of self-organization in the structuring and layout of the features – arrived at simply by imposing metallization boundaries in proximity to each other, separated by a distance of the order of the critical dimension of the pattern size apparent in the large surface mud-crack patterns.

Mechanically robust, electrically stable metal arrays on plasma-oxidized polydimethylsiloxane for stretchable technologies

Certain applications of evolving flexible technologies demand that metallic features remain both mechanically robust (crack-free) and electrically stable for large macroscopic mechanical deformation. Examples of this are flexible radio frequency transmission line technologies and soft metamaterials where electromagnetic properties (e.g., functionality and losses), which rely on the integrity of metallic features, are highly sensitive to shape and resistance variation. In this context, we demonstrate here the ability to deposit crack-free chromium/gold metallized mesa structures on polydimethylsiloxane (PDMS) substrates using thermal evaporation. In order to achieve this, the PDMS is exposed to an optimized oxygen plasma prior to the metallization. A shadow mask allowed us to define specific arrays of metallic mesa features having different sizes (100–600 μm) and surface filling factors on plasma-treated and non-treated PDMS. In contrast to non-treated PDMS, we demonstrate for a loading strain >45% that th...

An improved lagrangian thermography procedure for the quantification of the temperature fields within polycrystals

Polycrystalline metallic materials are made of an aggregate of grains more or less well oriented with respect to the loading axis. During mechanical loading, the diversity of grain orientations leads to a heterogeneous deformation at the local scale. It is well known that most of the plastic work generated during the deformation process reappears in the form of heat, whereas a certain proportion remains latent in the material and is associated with microstructural changes. To access the local stored energy during deformation processes, experimental energy balances are needed at a suitable scale. Thus, simultaneous measurements of thermal and kinematic fields were made in-house at the microstructural scale of a 316L stainless steel submitted to a macroscopic monotonic tensile test. The aim of the present study is to propose a complete calibration strategy allowing us to estimate the thermal variations of each material point along its local and complex deformation path. This calibration strategy is a key element for achieving experimental granular energy balances and has to overcome two major experimental problems: the dynamics of each infrared focal plane array sensor that leads to undesired spatial and temporal noise and the complexity of the local loading path that must be captured by simultaneous complementary measurement. The improvement of such a multifield strategy is crucial for performing properly the experimental and local energy balances required to build new energetically based damage criteria.

Influence of the Lost Foam Casting Microstructure on Low Cycle Fatigue Damage of A319 Aluminum Alloy

In cast aluminum alloys used in the automotive industry the microstructure inherited from the foundry process has a strong influence upon the fatigue behavior. In the cylinder heads produced by the Lost Foam Casting process, the microstructure consists of hard intermetallic phases and large gas and microshrinkage pores. In order to study the influence of this complex 3D microstructure on fatigue crack initiation and propagation, an experimental protocol using laboratory and synchrotron tomography, Finite Element simulation and 3D Digital Volume Correlation has been used. Full field measurements at the microstructure scale were performed during a low cycle fatigue test at room temperature performed in situ under synchrotron X-ray tomography (TOMCAT beamline, SLS). Synchrotron tomography allowed characterizing the eutectic Al-Al2Cu, iron based intermetallics phases and above all eutectic Si, which could not be distinguished with laboratory tomography; these constituents were proved a suitable natural speckle for Digital Volume Correlation.