Guruswami Ravichandran

John E. Goode, Jr. Professor of Aerospace and Professor of Mechanical Engineering
Director, Graduate Aerospace Laboratories

Graduate Aerospace Laboratories - California Institute of Technology

 

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Mechanics of Peeling and Adhesion

Peeling

The measurement of interface mechanical properties between an adhesive layer and a substrate is significant for optimization of a high-quality interface. This work focuses on the development and investigation of experimental studies in peeling and adhesion. This work is of great interest in the biomechanics community, particularly in the area of gecko adhesion. Our goal is to obtain a better understanding of the fundamental mechanics underlying the problem of adhesives peeled from rigid substrates.

We approach this problem as a combination of fundamental problems in mechanics. An experimental peeling configuration is developed (see figure) in order to conduct investigations to measure key parameters in the peeling process.

The process of peeling an adhesive material from a substrate may be approached as a crack propagating in a medium. Thus, we are currently working on developing analytical models which accurately represent the observed experimental behavior

Validation has been achieved by measuring the peel force through a range of angles for a model adhesive material - 3M Scotch Magic Tape. Current studies include investigation of the stability of the peeling process, rate-dependent effects of the peel angle on adhesion energy, process zone geometry, and crack-front velocity.

Three-Dimensional Traction Force Microscopy

3D Traction Force Microscopy

The mechanical interaction between cells their external surroundings has been shown to affect various cellular activities, including growth, migration, differentiation, and formation of focal adhesions. However, most previous work has studied cultured cells on flat, 2D substrates. We compute the tractions applied by cells to a 3D matrix using 3D traction force microscopy. Digital volume correlation is used to compute the cell-induced matrix displacements. The strain tensor throughout the matrix is computed by differentiating the displacements, and the stress tensor is computed by application of a constitutive law. Finally, the tractions applied by the cell to the matrix are computed from the stress tensor in the material and an image of the cell's shape.

Strength Measurement at High Pressures Using Oblique Shock Waves

Oblique Shock Experiment

Strength of materials at high-pressures and high-strain-rates is relevant to a number of applications including planetary impact and inertial confinement fusion. Understanding how strength depends on pressure allows for the characterization of materials and validation of constitutive models. Traditionally, slotted barrel guns have been used to generate longitudinal and shear waves through an oblique impact such as in the pressure-shear plate impact technique. A new methodology for measuring material strength using oblique shock waves generated by normal impact (1 to 2 km/s) is being researched. In this experimental setup, a composite target with an inclined interface is used rather than an angled impactor and target. By measuring the in-situ longitudinal velocity and particle velocities at two off-normal angles along the interface using VISARs, the shear stress (strength) is inferred as a function of pressure.

High Pressure, High Strain Rate Characterization of Polymers

Polymer failure

Efforts are under way to probe the nonlinear viscoelastic deformation and fracture behaviors of polymers subjected to high pressures and high strain rates through a combination of experiments and simulations. Current work is focused on the use of the Taylor impact test with projectile speeds up to 2 km/s to quantify the constitutive behavior and failure criteria of polyurea, an elastomer commonly used as a coating in blast protection.


 
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