How can we induce twist in tubular structures without applying a torque?

In nature, such behavior is enabled by material anisotropy. In our new work, we show that isotropic bi-layer tubes with twist incompatible layers can twist upon inflation and extension.
Interestingly, the direction of twist can spontaneously reverse as the load increases!

Check out our new paper at EML:
https://www.sciencedirect.com/science/article/pii/S2352431621000766

Dear colleagues,

We invite you to see the preprint of our new paper "Flexoelectricity in soft elastomers and the molecular mechanisms underpinning the design and emergence of giant flexoelectricity" that will appear in PNAS. Here we present a molecular-to-continuum scale theory for the flexoelectric effect in elastomers. The theory unveils a mechanism for achieving giant flexoelectricity--which finds support in prior experimental results; it is then leveraged for designing elastomers for 1) piezoelectricity, 2) tuning the direction of flexoelectricity, and 3) flexoelectricity which is invariant with respect to spurious deformations (https://doi.org/10.1073/pnas.2102477118).

Flexoelectricity in soft elastomers and the molecular mechanisms underpinning the design and emergence of giant flexoelectricity

Matthew Grasinger, Kosar Mozaffari, and Pradeep Sharma

Abstract

Soft robotics requires materials that are capable of large deformation and amenable to actuation with external stimuli such as electric fields. Energy harvesting, biomedical devices, flexible electronics and sensors are some other applications enabled by electro-active soft materials. The phenomenon of flexoelectricity is an enticing alternative that refers to the development of electric polarization in dielectrics when subjected to strain gradients. In particular, flexoelectricity offers a direct linear coupling between a highly desirable deformation mode (flexure) and electric stimulus. Unfortunately, barring some exceptions, the flexoelectric effect is quite weak and rather substantial bending curvatures are required for an appreciable electro-mechanical response. Most experiments in the literature appear to confirm modest flexoelectricity in polymers although perplexingly, a singular work has measured a "giant" effect in elastomers under some specific conditions. Due to the lack of an understanding of the microscopic underpinnings of flexoelectricity in elastomers and a commensurate theory, it is not currently possible to either explain the contradictory experimental results on elastomers or pursue avenues for possible design of large flexoelectricity. In this work, we present a statistical-mechanics theory for the emergent flexoelectricity of elastomers consisting of polar monomers. The theory is shown to be valid in broad generality and leads to key insights regarding both giant flexoelectricity and material design. In particular, the theory shows that, in standard elastomer networks, combining stretching and bending is a mechanism for obtaining giant flexoelectricity, which also explains the aforementioned, surprising experimental discovery.

We have discovered a peculiar form of fracture that occurs in a highly stretchable silicone elastomer (Smooth-On Ecoflex 00–30). Under certain conditions, cracks propagate in a direction perpendicular to the initial pre-cut and in the direction of the applied load. In other words, the crack deviates from the standard trajectory and instead propagates perpendicular to that trajectory. The crack arrests stably, and thus the material ahead of the crack front continues to sustain load, thereby enabling enormous stretchabilities. We call this phenomenon 'sideways' and stable cracking. To explain this behavior, we first perform finite-element simulations that demonstrate a propensity for sideways cracking, even in an isotropic material. The simulations also highlight the importance of crack-tip blunting on the formation of sideways cracks. Next, we provide a hypothesis on the origin of sideways cracking that relates to microstructural anisotropy (in a nominally isotropic elastomer). To substantiate this hypothesis, we transversely pre-stretch samples to various extents before fracture testing, as to determine the influence of microstructural arrangement (chain alignment and strain-induced crystallization) on fracture energy. We also perform microstructural characterization that indicates that significant chain alignment and strain-induced crystallization indeed occur in this material upon stretching. We conclude by characterizing how a number of loading conditions, such as sample geometry and strain rate, affect this phenomenon. Overall, this paper provides fundamental mechanical insight into basic phenomena associated with fracture of elastomers.

The paper can has been published in PNAS and can be found here: https://esml.tamu.edu/?page_id=12

or here: https://www.pnas.org/content/116/19/9251.short

One Ph.D. candidate position is available in the Fall 2017 in the Elastomer Research Center (Cermel) at the Laboratory of Mechanics and Rheology (LMR) in the Polytechnic School of the University of Tours in France.

Job description

The aim of this Ph.D. is to extend the comprehension of the local mechanisms involved in elastomer materials, in order to faithfully reproduce their behavior in finite element models. The Ph.D. applicant is expected to carry out both theoretical and experimental work.

The global mechanical behavior of an elastomer is quite complex. It has a non linear response in large strains while dissipating energy with a strong dependency to strain rate and environment. This makes it difficult to model due to a large number of involved parameters. The proposed subject will aim to study the local behavior of one or more elastomers, assessing the impact of specimen elaboration (formulation, vulcanization rate…) and the nanoindentation parameters (type of indents, velocity…). The main characteristics of the materials will first be identified considering general models dedicated to polymers and taken from the literature. A new model dedicated to elastomers of their local behavior will then be established. One of the final objectives is to obtain a multi-scale modeling of this type of materials. The interest is to be able to numerically predict the response (both static and fatigue) of real parts (e.g. human artificial organs) based on experimental local measurements.

Requirements

The candidate will join a dynamic research team with a research focus on elastomers. Good teamwork and communication skills are essential. S/he will have to continue the current research efforts on the characterization of elastomers, following another thesis in progress “NanoRubb” which is held by Elastopole (French Competitiveness Cluster in Rubbers and Polymers).

The position is for 3 years. The applicant should be fluent in English (spoken and written), familiar with giving presentations and lectures in English and also interested in learning French. The expected start date is 1st October 2017.

Motivated and ambitious students with excellent grades and the following backgrounds or experiences are encouraged to apply:
• Bachelor of Science in engineering, math or physics;
• Model reduction, optimization, inverse methods;
• Computer programming, finite elements, applied mechanics;
• Instrumented indentation, AFM, SEM, EBSD.


Contact

For further details or application, please contact Guenhael Le Quilliec and Florian Lacroix, e-mail:
elastomer.phd2017@univ-tours.fr

Applications with a cover letter, CV, any publications and other scientific works, certified copies of transcripts and reference letters should be submitted electronically by 30 April 2017. Preselected candidates will then pass an interview organized by the doctoral school the 18 or 19 May 2017.

Additionally, a list of reference persons would be appreciated. This could be a previous project leader, university professor or any other supervisor you have worked with. Be aware that a reference check will be conducted. Any other document you consider relevant may also be submitted.

Two-dimensional (2D) systems have great promise as next generation electronic materials but require intimate knowledge of their interactions with their neighbors for device fabrication and mechanical manipulation. Although adhesion between 2D materials and stiff substrates such as silicon and copper has been measured, adhesion between 2D materials and soft polymer substrates remains difficult to characterize due to the large deformability of the polymer substrates. In this work, a buckling-based metrology for measuring the adhesion energy between few layer molybdenum disulfide (MoS2) and soft elastomeric substrates is proposed and demonstrated. Due to large elastic mismatch, few layer MoS2 flakes can form spontaneous wrinkles and buckle-delaminations on elastomer substrates during exfoliation. MoS2-elastomer interface toughness can therefore be calculated from the buckle delamination profile measured by atomic force microscopy (AFM). The thickness of the MoS2 flake is obtained by analyzing co-existing wrinkles on the same flake. Using this approach, adhesion of few layer MoS2 to 10:1 Sylgard 184 polydimethylsiloxane (PDMS) is measured to be 18 ± 2 mJ m-2, which is about an order of magnitude below graphene-to-stiff-substrate adhesion. Finally, this simple methodology can be generalized to obtain adhesion energies between various combinations of 2D materials and deformable substrates.

The rapid and efficient development of soft active materials requires readily available, compact testing equipment. We propose a desktop-sized, cost-efficient, and open source radial stretching system as an alternative to commercially available biaxial and uniaxial stretching devices. It allows for doubling the diameter of an elastomer membrane while measuring the applied force. Our development enables significant cost reduction (<300 €) and increase the availability of equibiaxial deformation measurements for scientific material analysis. Construction plans, source code, and electronic circuit diagrams are freely available under a creative commons license.

Full Publication (OpenAccess):
http://ieeexplore.ieee.org/xpl/articleDetails.jsp?arnumber=7107973

Supplementary files:
https://www.dropbox.com/s/qz365z9ikogsst3/Supplementary_RSS.zip

Updates and community plattform:
http://www.somap.jku.at/rss

Greetings,

I'm conducting a simple shear test (single lap joint) between aluminum bonded with 3M adhesive.

Adhesive material is 3M VHB 4930 adhesive (acrylic)

Result obtains =

Load vs Displacement Curve

True Stress vs Stretch Ratio

From the literature, mostly they used Yeoh model for 3M adhesive (Modeling and simulation of dielectric elastomer actuators,M.Wissler 2005).

The material constant for C10, C20, C30 I get from the curve fitting equation from the True Stress vs Stretch Ratio.

Then, from the material constant I input on Abaqus software to run the FEM analysis.

C10 = 1.774 MPa

C20 = - 4.669 Mpa

C30 = 39.24 MPa

D1 =?

D2 =?

D3 =?

My question, how can i get material constant for D1, D2, D3? And D1, D2, D3 represents what?

Could you please give me an explanation.

Electroactive soft elastomers require huge electric field for a meaningful actuation. We demonstrate, by means of numerical simulation, that this can be dramatically reduced and large deformations can be achieved with suitably designed heterogeneous actuators. The mechanism by which the enhancement is attained is illustrated with the aid of both idealized and periodic models.

Elastomers are outstanding in their ability to repeatedly endure large deformations, and they are often applied where fatigue performance is a critical consideration. Because the macromolecular structure of elastomers gives rise to a number of unique behaviors, appropriately specialized methods are needed to characterize, analyze, and design for durability. This 2-day course provides the know-how for engineering durable elastomeric components and systems. The course is taught at Axel Products, and includes live demos of typical behavior.
Instructor: Dr. Will Mars, President of Endurica LLC

For more info, see: http://www.endurica.com/PDF/EngineeringDurableElastomerStructuresv8.pdf

Cost: $2000

I am working on a model in which i have a soft layer of material bonded between to rigid layers of material via cohesive elements following a linear separation-traction law based on a critical fracture energy value I determined from experimental and numerical work. I have the interlayer and cohesive elements as one part with a partitioned face and connected to the two adherends via surface-to-surface ties. I have found that my model runs successfully only if the soft interlayer material is made very stiff. I have tried many ways of defining the material, from elastic to different methods of hyperelastic, and I have tried different ways of defining the elements. From building simpler models without the cohesive elements, it appears the problem is the soft confined material. Does anyone of suggestions on working with soft confined materials in Abaqus? All I found in the manuel is that soft confined materials are difficult.

Why the elastomers are giving large actuation,what is the reason behind it

Amit Acharya and Kaushik Dayal

(To appear in Quarterly of Applied Mathematics)

This paper presents a generalization of traditional continuum approaches to liquid crystals and
liquid crystal elastomers to allow for dynamically evolving line defect distributions. In analogy with
recent mesoscale models of dislocations, we introduce fields that represent defects in orientational
and positional order through the incompatibility of the director and deformation ‘gradient’ fields.

These fields have several practical implications: first, they enable a clear separation between
energetics and kinetics; second, they bypass the need to explicitly track defect motion; third, they
allow easy prescription of complex defect kinetics in contrast to usual regularization approaches;
and finally, the conservation form of the dynamics of the defect fields has advantages for numerical
schemes.

We present a dynamics of the defect fields, motivating the choice physically and geometrically.
This dynamics is shown to satisfy the constraints, in this case quite restrictive, imposed by materialframe
indifference. The phenomenon of permeation appears as a natural consequence of our kinematic
approach. We outline the specialization of the theory to specific material classes such as nematics,
cholesterics, smectics and liquid crystal elastomers. We use our aproach to derive new, non-singular,
finite-energy planar solutions for a family of axial wedge disclinations.

Internship Announcement

Title : FEA of Elastomers

Category: Internship in Industry

Employer: Schlumberger Technology Corporation

Location: Sugar Land, TX, USA

Starting Date: 01/01/2010

Schlumberger is the leading supplier of technology, project management, and information solutions, trusted to deliver superior results and improved E&P performance for oil and gas companies around the world. Through our well site operations and in our research and engineering facilities, we are working to develop products, services and solutions that optimize customer performance in a safe and environmentally sound manner. Reflecting our belief that diversity spurs creativity, collaboration, and understanding of customer's needs, we employ over 70,000 people of more than 140 nationalities working in 80 countries. With 25 research and engineering facilities worldwide, we place strong emphasis on developing innovative technology that adds value for our customers. In 2007, we invested $720 million in R&D. For more information about Schlumberger, please refer to our web site http://www.slb.com . Schlumberger is an Equal Opportunity Employer.

We are looking for extremely high-energy, self-motivated Ph.D. candidates with exceptional problem-solving, communication, leadership and interpersonal skills who are seeking challenges to solve real technical problems. If you have any of the academic qualifications listed below, submit an application to join our team of international experts in the Simulation & Modeling Group located in Sugar Land, TX. Please send your cover letter and resume via email to Dr. Muralidhar Seshadri, MSeshadri@slb.com.

Job Description

The objective of the internship is to analyze and optimize the design of a downhole tool used for oil and gas drilling operations. The objective is to study the mechanical response of elastomers to shock and vibration loading, while considering the effect of heat transfer on the temperature dependent material properties. Fatigue and ageing of elastomeric components will be considered to optimize the design of the elastomeric components to maximize life and performance.

Requirements

1. Ph.D. degree in Mechanical Engineering, Aerospace Engineering, Civil Engineering, or Engineering Mechanics

2. In-depth understanding of continuum mechanics, elastomer behavior, finite element method, engineering design concepts, and materials engineering. Sound knowledge of hyperelasticity, viscoelasticity and temperature effects on these.

3. Experience in modeling and simulation of elastomers subjected to large mechanical and thermal deformations.

4. Solid knowledge of ABAQUS is essential. Experience in Ansys Classic/Workbench is a plus.

5. Experience in developing user subroutines for ABAQUS

Hi,

Is anyone aware of a pressure-overcosure function for (nearly) incompressible materials.

I am aware of functions for PE and articular cartilage but they do not work with v=0.5. I have been trying to find similar functions for elastomers, but can't seem to find anything.

I am grateful for any input.

Andreas

This study deals with the identification of constitutive parameters for hyperelastic materials. Classically, these parameters are deduced from several homogeneous tests. Here, these parameters are obtained from only one multiaxial mechanical test that gives rise to heterogeneous stress / strain fields. Since no analytical relationship is available between measurements and unknown parameters, a suitable tool, namely the virtual fields method, is developed in case of large deformations and used to identify these unknowns. Several results obtained with numerical simulations and experiments performed on rubber specimens illustrate the approach.

Dear colleagues, this paper deals with the determination of the thermal response of elastomeric materials subjected to cyclic loading. In this case, the material undergoes large deformations, so a suitable motion compensation technique has been developed to track the material points and their temperature during the test. Special attention is paid to the Narcissus effect and to the detector matrix of the infrared camera used in the study. Heat sources are then derived from the temperature maps. The thermoelastic inversion phenomenon has been experimentally evidenced during a cyclic test performed on an elastomeric notched specimen. The heat source distribution close to the crack tip has also been deduced from the temperature maps, thus highlighting the relevance of the approach. This article is accepted for publication in Experimental Mechanics and is available online at http://dx.doi.org/10.1007/s11340-008-9138-0

An internship position is available in the area of Finite Element Analysis of Elastomers in the Simulation and Modeling Group at Schlumberger, Sugar Land, TX.

The objective of the internship is to analyze and optimize the design of a downhole tool used for oil and gas drilling operations. The objective of the first phase is to develop a coupled thermomechanical model for elastomers in ABAQUS or COMSOL to capture the temperature increase in the elastomer components of the tool due to the viscoelastic and frictional heat dissipated. In the second phase of the project, the thermomechanical model will participate in a coupled Fluid-Solid Interaction (FSI) model, providing appropriate boundary conditions to the flow. In parallel, the candidate is expected to optimize the design of the elastomeric components to maximize life and performance.

We are looking for extremely high-energy, self-motivated Ph.D. candidates with exceptional problem-solving, communication, leadership and interpersonal skills who are seeking challenges to solve technical problems. If you have any of the academic qualifications listed below, submit an application to join our team of international experts in the Simulation & Modeling Group located in Sugar Land, TX. Please send your cover letter and resume via email to me at MSeshadri@slb.com