Laurence Bodelot

Interfacial adhesion between the iron fillers and the silicone matrix in magneto-rheological elastomers at high deformations

This work investigates the interfacial adhesion between the iron fillers and the silicone matrix in magneto-rheological elastomers at high deformations. Carbonyl iron powder, composed of mechanically soft spherical particles with a median size of 3.5 μm and a volume concentration of 3.5%, was mixed in a soft silicone matrix (Shore 00-20); the compound was then degassed and cured under temperature. The presence of a homogeneous magnetic field of 0.3 T during the curing process allowed the formation of particle chains. Tensile tests of these samples under scanning electron microscope showed interfacial slipping and debonding between the two phases. To improve interfacial adhesion, a silane primer was applied to the iron particles, following two different procedures, before the mixing and crosslinking process, thus giving two additional types of samples. In tensile testing lengthwise to the particle alignment, with engineering strains up to 150%, the structural responses of the different types of samples were compared. An enhanced adhesion of the iron fillers to the silicone matrix resulting in a reinforced matrix and increased tensile strength during the first loading path could be observed. Furthermore, scanning electron microscope images show that a more elaborated particle-matrix interface was obtained with the primer additive.

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.

Large Strain Mechanical Behavior of HSLA-100 Steel Over a Wide Range of Strain Rates

High-strength low alloy steels (HSLA) have been designed to replace high-yield (HY) strength steels in naval applications involving impact loading as the latter, which contain more carbon, require complicated welding processes. The critical role of HSLA-100 steel requires achieving an accurate understanding of its behavior under dynamic loading. Accordingly, in this paper, we experimentally investigate its behavior, establish a model for its constitutive response at high-strain rates, and discuss its dynamic failure mode. The large strain and high-strain-rate mechanical constitutive behavior of high strength low alloy steel HSLA-100 is experimentally characterized over a wide range of strain rates, ranging from 10^(−3) s^(−1) to 10^4 s^(−1). The ability of HSLA-100 steel to store energy of cold work in adiabatic conditions is assessed through the direct measurement of the fraction of plastic energy converted into heat. The susceptibility of HSLA-100 steel to failure due to the formation and development of adiabatic shear bands (ASB) is investigated from two perspectives, the well-accepted failure strain criterion and the newly suggested plastic energy criterion [1]. Our experimental results show that HSLA-100 steel has apparent strain rate sensitivity at rates exceeding 3000 s^(−1) and has minimal ability to store energy of cold work at high deformation rate. In addition, both strain based and energy based failure criteria are effective in describing the propensity of HSLA-100 steel to dynamic failure (adiabatic shear band). Finally, we use the experimental results to determine constants for a Johnson-Cook model describing the constitutive response of HSLA-100. The implementation of this model in a commercial finite element code gives predictions capturing properly the observed experimental behavior. High-strain rate, thermomechanical processes, constitutive behavior, failure, finite elements, Kolsky bar, HSLA-100.

Magnetorheological Elastomers: Experimental and Modeling Aspects

Magnetorheological elastomers (MREs) are active composite materials that deform under a magnetic field because they are made of a soft elastomer matrix filled with magnetizable micrometric particles. Along with short response times and low magnetic inputs, not only do MREs alter their viscoelastic properties and stiffness in response to external magnetic fields but they can also undergo very high deformation states. While the former effect can be exploited in controllable-stiffness devices, the latter is of interest for haptic devices such as tactile interfaces for the visually impaired. In the perspective of developing a persistent tactile MRE surface exhibiting reversible and large out-of-plane deformations, the first part of this work focuses on the fabrication of MREs that can sustain large deformations. In particular, we determine the critical strain threshold up to which the interfacial adhesion between particles and matrix is ensured. In the second part of this work, an experimental setup is developed in order to characterize MRE composites under coupled magneto-mechanical mechanical loading. The experiments conducted on this setup will eventually serve as an input for a continuum model describing magneto-mechanical coupling.

Experimental determination of a representative texture and insight into the range of significant neighboring grain interactions via orientation and misorientation statistics

Abstract The mechanical response of polycrystalline metallic materials is heavily influenced by the orientations of their grains. To predict polycrystalline behavior more accurately, crystal plasticity models account for grain orientations and also, sometimes, for interactions between neighboring grains. However, these models often lack sound experimental input or validation. Furthermore, experimental studies themselves rarely tackle simply the concept of representativity in terms of texture; neither do they try to analyze up to what range neighbor interactions appear to be significant. In this article, we address both aforementioned issues in a single and easily implementable framework by performing extensive statistical analyses of discrete raw orientation and misorientation data respectively, obtained by means of electron back-scattered diffraction on thousand-grain microstructures. First, we show that the analysis of orientation statistics helps determine whether an experimental dataset can be considered as a microstructurally representative volume element in terms of texture. Second, we explain how the statistical processing of misorientations can shed some light on the range of neighbors that have a significant weight in the misorientation distributions and possibly on the grain interactions.

Controlled, Low-Temperature Nanogap Propagation in Graphene Using Femtosecond Laser Patterning

Graphene nanogap systems are promising research tools for molecular electronics, memories, and nanodevices. Here, a way to control the propagation of nanogaps in monolayer graphene during electroburning is demonstrated. A tightly focused femtosecond laser beam is used to induce defects in graphene according to selected patterns. It is shown that, contrary to the pristine graphene devices where nanogap position and shape are uncontrolled, the nanogaps in prepatterned devices propagate along the defect line created by the femtosecond laser. Using passive voltage contrast combined with atomic force microscopy, the reproducibility of the process with a 92% success rate over 26 devices is confirmed. Coupling in situ infrared thermography and finite element analysis yields a real-time estimation of the device temperature during electrical loading. The controlled nanogap formation occurs well below 50 °C when the defect density is high enough. In the perspective of graphene-based circuit fabrication, the availability of a cold electroburning process is critical to preserve the full circuit from thermal damage.

Wireless nanosensors for embedded measurement in concrete structures

In this work we propose a wireless architecture for embedded monitoring in concrete. The modular structure of the system allows it to be adapted to different types of sensors. We present the application of such architecture for the detection of microcracks in concrete. A carbon nanotube strain sensor recently developed by the group is used to track mechanical deformations. Full temperature compensation is achieved by a specific conditioning circuit.