a b s t r a c tThe use of polymers in the context of flexible systems such as flexible sensors leads to an incompatibility issue: on the one hand, the flexibility of the polymer must not be to the detriment of the fabrication process, e.g. excessive thermal expansion leading to process failure and on the other hand, certain applications will require high flexibility and also a specific mechanical stiffness, e.g. artificial skin, smart clothes, flexible screen. In other words, a compromise is necessary between rigidy for processing and controlled flexibility for applications. In this context it is crutial to be able to tune the mechanical properties of such polymers. Polydimethylsiloxane (PDMS) is a very versatile and useful soft polymeric material -Elastic modulus typically ≈1 MPa. This paper investigates the stiffness tunability of PDMS by varying the hardening agent to PDMS base ratio over 19:1 to 2:1, and using two extreme curing processes, i.e. 120 min at 100 • C and 2 days at 165 • C. It was observed that the stiffness of PDMS can be accurately controlled from 800 kPa to 10 MPa with a rupture limit higher than 20%. To our knowledge this is the highest reported elastic modulus in PDMS by combining mixing ratio and curing temperature. The impact of such a stiffness variation on potential functional properties such as the rupture limit, Poisson's ratio and material's wetting contact angle is also analysed. We observe that the wetting contact angle depends on the bulk mechanical properties of the PDMS. The observations will be of use to all technological communities who are engaged in using PDMS-type polymers for their specific applications.

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.

Lost Foam Casting (LFC) process is replacing the conventional gravity Die Casting (DC) process in automotive industry for the purpose of geometry optimization, cost reduction and consumption control. However, due to lower cooling rate, LFC produces in a coarser microstructure that reduces fatigue life. In order to study the influence of the casting microstructure of LFC Al-Si alloy on damage micromechanisms under monotonic tensile loading and Low Cycle Fatigue (LCF) at room temperature, an experimental protocol based on the three dimensional (3D) in-situ analysis has been set up and validated. This paper focuses on the influence of pores on crack initiation in monotonic and cyclic tensile loadings. X-ray Computed Tomography (CT) allowed the microstructure of material being characterized in 3D and damage evolution being followed in-situ also in 3D. Experimental and numerical mechanical fields were obtained by using Digital Volume Correlation (DVC) technique and Finite Element Method (FEM) simulation respectively. Pores were shown to have an important influence on strain localization as large pores generate enough strain localization zones for crack initiation both in monotonic tensile and cyclic loadings.

Data-driven computations propose a completely new paradigm to the computational mechanics research community and to experimentalists. Classically, admissible material states can only be obtained experimentally for homogeneous stress/strain configurations or using a parametric optimization of material laws based on heterogeneous tests. Data-driven algorithms aim at circumvent these limitations. However, data-driven algorithms require a large database of admissible material states, otherwise extrapolation is required and some limitations of the classical constitutive equation based approach remain. In this paper, an inverse data-driven approach based on full field measurements is presented. The main idea is to

An innovative identification strategy based on high power ultrasonic loading together with both infrared thermography and ultra-high speed imaging is presented in this article. It was shown to be able to characterize the viscoelastic behaviour of a polymer specimen (PMMA) from a single sample over a range of temperatures and strainrates. The paper focuses on moderate strain-rates, i.e. from 10 to 200 s −1 , and temperatures ranging from room to the material glass transition temperature, i.e. 110 • C. The main originality lies in the fact that contrary to conventional Dynamic Mechanical Thermal Analysis (DMTA), no frequency or temperature sweep is required since the experiment is designed to simultaneously produce both a heterogeneous strain-rate state and a heterogeneous temperature state allowing a local and multi-parametric identification. This article is seminal in nature and the test presented here has good potential to tackle a range of other types of high strain-rate testing situations.

Brittle and ductile dynamic fracture in solids is a complex mechanical phenomenon which attracted much attention from both engineers and scientists due to its technological interests. Modeling cracks in dynamic cases based on a discontinuous description is difficult because it needs additional criteria for branching and widening. Due to the very short time scales and the spatial complexity of the dynamic problem, its experimental analysis is still very difficult. Therefore, researchers still have to rely on numerical simulations to find a deeper explanation for many observed phenomena. This study is set out to investigate the effect of plasticity on dynamic fracture propagation. On the other hand, the diffuse phase-field formulation makes it possible to initiate, propagate, arrest or even branch cracks while satisfying the basic principles of thermodynamics. An implicit, staggered elastoplastic version of the phase-field approach was implemented in the commercial finite element code Abaqus through the UEL option. By means of simple examples we show that localized ductile deformations first increase both resistance and toughness. Then, after a maximum value, the resistance starts to decrease with a significant increment in energy dissipation. By favoring shear deformation over tensile failure the fracture pattern changes. First the branching disappears, then the crack propagation angle changes and becomes a shear band. Finally, we observed the increment of the instantaneous dynamic stress intensity factor during the acceleration stage of the crack without introducing a rate dependent critical fracture energy. We explained this phenomenon with the increasing roughness of the fracture surface.

The aim of this work is to analyse the strain field heterogeneity in an aluminium alloy subjected to low‐cycle fatigue at high temperature. In the cylinder heads produced by a lost‐foam casting process, the microstructure of the studied alloy consists 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 at 250 °C, an experimental protocol using laboratory and synchrotron tomography, finite element simulation and a new digital volume correlation platform have been used. The results showed the role of pores in the crack nucleation and highlighted the importance of hard phases in the crack propagation, thanks to the resolution on the DVC measurement.

International audiencePolycrystalline 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 heterogeneous deformation and 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 microstructure changes. To access the local stored energy, experimental and numerical energy balances are needed at a suitable scale. In this way experiments have already been done in-house on 316L stainless steel to monitor the evolution of temperature and deformation fields at the microstructural scale. The aim of the present study is now to develop a simplified numerical model, based on experimental observations and able to simulate the thermomechanical behavior, in order to provide a first assessment of the stored energy level and heterogeneity. Our model renders well the average thermomechanical behavior of the specimen and reveals a high heterogeneity of the stored energy at the microstructure scale. Better knowledge of the stored energy at this scale should improve our understanding of strain localization mechanisms