A BIT MORE ABOUT OUR WORK
Soft Solids
Soft Solids in Electric fields
Soft materials with very low stiffness (not more than a few kPa) but high dielectric permittivity (relative permittivity above ~4) and high breakdown strength (above about 40 MV/m), can deform, often by large amounts, when placed in a strong electric field.
We are performing both sophisticated Finite Element simulations as well as some simple experiments to characterise the performance of devices made with such soft solids. There are several computational challenges involved:
- The soft materials are often such that they preserve volume when they deform. This additional constraint of `incompressibility’, though well known, makes the Finite Element simulations prone to some peculiar problems, most commonly `locking’ where a computational model just refuses to deform when subjected to forces. The problem is well known and ready solutions are available. We have implemented some of them in our codes.
- The problem is a coupled one where the large deformation of the solid alters the electric field around it while the altered electric fields, in turn, change the stresses generated in the solid. This means that both the electrostatic and mechanical problems have to be solved at each step. Computational schemes for coupled problems with different physical origins often lead to ill-conditioning of numerical schemes. Smart techniques are needed to couple the multi-physical problem effectively.
- The computational complexity is further enhanced by the fact that such soft solids often undergo sudden changes in deformation patterns akin to buckling in columns. For example, a flat plate can suddenly develop wrinkles, or undergo rapid localised thinning. Dealing with such instabilities is an important challenge that the computational schemes have to deal with.
Natural Materials: Plant stems/tall grasses
Plants have evolutionally perfected a microstructure that performs specific structural functions very effectively. For example, tall grasses like bamboo can withstand very high fluctuating wind loads over many many cycles without uprooting or breaking.
The microstructure that most of these tall grasses have adopted can be described as a functionally graded composite with a foamy matrix reinforced by short stiff fibres. No synthetic material has a microstructure that fits this description.
Is there a good reason to try and emulate this microstructure in engineering materials? This is an unanswered question. To answer this, we first need to figure out how this microstructure helps a plant withstand cyclic bending loads. We are trying to do this through a series of experiments, mechanical characterisation exercises and simulations.
Surface effects in soft solids
Ultra soft solids are a bit like fluids. Surface energies in soft solids are high and effects of strain on surface tension become very important in small sized structures made of these soft materials. For example, a short, soft cylinder may form a meniscus even when it is not loaded. Very small voids in a matrix filled with a soft solid may lead to an overall stiffening. We are computationally exploring a number of problems that involve surface elasticity in soft solids.
Polymers
Polymers are probably the most suitable candidate for ‘designer materials’. Polymer chemists have immense control over architectures of the macromolecules. We are, through Molecular Dynamics and non-linear Finite Element simulations, trying to figure out if molecular architectures can be connected to mechanical properties. The ultimate goal will be to reverse design the architectures that leads to a material with a given combination of mechanical properties.
We work with amorphous thermoplastic and thermoset polymers. The work is primarily classified into
- devising simulation strategies for different classes of polymers and polymer interfaces.
- understanding connections between the deformation and fracture behaviour of polymers to their molecular architecture.
- understanding the mechanics of propagating cracks in polymeric materials.
Multifunctional Materials
Demands on modern materials are often diverse and contradictory. For instance, aerospace materials need to be stiff, tough, light, thermally conducting, able to withstand lightning strikes and reflect incoming electromagnetic waves. Windshields of cars must be tough, transparent and light.
We are looking at a nano-reinforced composites which offer a combination of diverse properties. In fact, combining nano reinforcements like nanoparticles, nanotubes and graphene flakes in an optimal manner can indeed lead to composite materials than can perform a number of functions effectively.
We are looking at two specific possibilities:
- A composite that has reasonably high thermal conductivity, can absorb or reflect electromagentic waves and has good mechanical properties.
- A thin coating that is elastic and offers low thermal resistance.
Failure in metals
Accurate prediction of failure in ductile metals is a long standing technological problem, in spite of the fact that ductile fracture is probably the most studied and understood mode of fracture. We are working on developing computational techniques for predicting yield and failure surfaces of ductile materials under monotonic as well as non-monotonic loading.