Non-contact method for measurement of strain profile at a location interposed within a soft deformable object with dynamic evolution of the strain under dynamic loading or fracture of the object

Abstract

A non-invasive method for estimation of strain profile and dynamic evolution of the strain at a location interposed inside a block of soft material, includes forming a tracer grid consisting of microscopic lines or regularly spaced microscopic dots on a single plane buried inside the soft block; preparation of a deformable object embedded with the tracer grid in three primary steps: i. preparing a block of crosslinked material by crosslinking a first predetermined quantity of a pre-polymer solution containing a monomer, a crosslinking agent, and an initiator and promoter all mixed in a solvent at a known stoichiometric weight ratio; ii. transferring a grid comprising of lines or dots onto the face by direct writing or transferring from an easy release surface; and iii. crosslinking a second predetermined quantity of the same pre-polymer solution on the gel surface, such that this second crosslinked material gets welded to the first one.

Claims

1. A non-invasive method for estimation of strain profile and dynamic evolution of the strain at a location interposed inside a block of material, comprising: preparing a first block of crosslinked material by crosslinking a first predetermined quantity of a pre-polymer solution containing a monomer, a crosslinking agent, an initiator, and a promoter in a solvent at a known stoichiometric weight ratio, the crosslinking reaction being carried out inside a mold at room temperature or at an elevated temperature, such that the first block of the crosslinked material has one of a flat face and a desired curvature; transferring a tracer grid comprising of tracer particles in a form of lines or dots onto a surface of the first block by direct writing or transferring; preparing a second block of crosslinked material by crosslinking the pre-polymer solution containing a monomer, a crosslinking agent, an initiator, and a promoter in a solvent at a known stoichiometric weight ratio on the surface of the first block to form a composite block comprising the second block of crosslinked material in contact with the surface of the first block and a plane containing the tracer particles is embedded between the first block and the second block; visualizing and recording displacement of the tracer grid when the composite block is subjected to normal shear, torsional loading, or fracture by insertion of an object therein; and estimating a strain field from the displacement of the tracer grid.

2. The method as claimed in claim 1, wherein the block of material is optically transparent or opaque.

3. The method as claimed in claim 1, wherein the tracer grid comprises at least one of an ink, a fluorescent dye, microscopic particles, quantum dots, and single or multi-walled carbon nanotubes.

4. The method as claimed in claim 1, wherein the tracer grid is embedded on a flat or a curved surface.

5. The method as claimed in claim 1, further comprising using an indenter to estimate softness of a fragile material.

6. The method as claimed in claim 1, comprising estimating a dynamic evolution of the strain field.

7. The method as claimed in claim 1, further comprising estimating a change in the strain field using inclusions inside the object.

8. The method as claimed in claim 1, further comprising estimating a Poisson ratio of the block of material.

9. The method as claimed in claim 1, wherein the crosslinked material comprises several layers of material having similar or dissimilar rheological properties.

Description

BRIEF DESCRIPTION OF ACCOMPANYING DRAWINGS

(1) FIG. 1 Shows a process of preparing a block of gel embedded with a plane containing tracer grid.

(2) FIG. 2 Shows the result of uniaxial compression of the embedded gel block.

(3) FIG. 3 Shows a cylindrical block of gel in which the tracer grid is embedded on a curved plane.

(4) FIG. 4 Shows an experimental concept of punching a block of gel with syringe needle.

(5) FIG. 5 Shows a flat punch driven into a cross-linked block of gel.

(6) FIG. 6 Shows a video images depicting the progress of fracture when a block of gel is punctured.

(7) FIG. 7 Shows a graph depicting normal strain as a function of distance from underformed surface of the gel.

(8) FIG. 8 Shows optical micrograph depicting evolution of regions of tensile and compressive strain in the gel.

(9) FIG. 9 Optical image showing profile of punching a block of gel.

(10) FIG. 10 Shows the top and side views of a hemispherical indenter with an embedded plane of tracer grid.

DETAILED DESCRIPTION OF THE INVENTION

(11) Admittedly, acquiring the information about strain field over the surface of a deformable object, using tracer dot or particles is well known. The present invention however provides a process to quantify experimentally using tracer particles, the dynamic evolution of a strain field at a location interposed within the bulk of a soft solid when it is loaded externally or during fracture. Thus, the present invention is developed through combination of the concept of welding two blocks of the same crosslinkable material like hydrogel without use of any second material, and placing a tracer grid on the surface of the crosslinkable material. The hydrogel being optically transparent, it allows visualization and recordal of displacement of the tracer particles accurately with the help of a video camera. The inventive process was validated by creating a fracture on such a block of gel using a blunt object for example, a flat punch, and sharp objects for example, hypodermic syringe needles having either a single tip or closely spaced multiple tips. These experiments have shown that strain profile in the material evolves differently in these different situations, thus signifying that the invented method may be an important tool for carrying out material research in variety of engineering and biomedical applications. In another aspect of the invention, a soft indenter embedded with the tracer gird is produced for estimating the softness or hardness of a substrate. Although, the invention uses an optically transparent material to demonstrate the possible applications, with the use of an infrared camera, it will be possible also to monitor displacement of the tracer particles interposed within a opaque object.

(12) FIG. 1 shows the sequence of steps depicting the method of preparing a block of gel embedded with a plane containing tracer grid. FIG. 2 establishes that an uniaxial compression of the gel block embedded with tracer grid can result in Poisson ratio and Young's modulus of the material of the gel. FIG. 3 shows that a tracer grid is embedded on a curved plane at the bulk of a cylindrical block of gel. Torsion of the cylinder about its axis can yield torsional modulus of the gel material.

(13) FIG. 4 shows a schematic of a typical experiment. (a) An indenter, e.g. a flat bottom punch or a syringe needle is driven vertically down at a uniform speed into the gel block bonded to a rigid substrate, such that the indenter drives through the plane of tracer grid and it remains oriented symmetric to this plane. The optical micrographs of typical single and double needles are shown in (b)-(d). (e) Schematic of a typical sample of gel embedded with the plane of dot pattern. The needle is driven into the gel such that it remains symmetric to this plane.

(14) FIG. 5 shows that a flat punch is driven into a crosslinked block of polyacrylamide gel through the plane in which the tracer grid remains embedded. Images A and B show the displacement of the grid because of progress in the fracture.

(15) FIG. 6 depicts a sequence of video micrograph which interalia shows the progress of the fracture when a block of gel of modules =30 kPa is punctured with a single tip needle of diameter d=1.2 mm. Optical image A to D represent time 0, 2, 4 and 6.5 sec respectively. The regions represent respectively the zones of tensile and compressive vertical strains, e.sub.yy. The dotted line represents the un-deformed gel surface. The scale bar represents 1.2 mm.

(16) FIG. 7 shows Plots a-d representing normal strain e.sub.yy as a function of distance x from the central axis of the needle, also with respect to vertical distance from underformed surface of the gel. Symbols: ,,,, and .diamond-solid. represent respectively 4.07, 5.08, 5.82, 7.41, 8.97 and 10.54 mm from the underformed gel surface. Scale bar represents 1.2 mm.

(17) FIG. 8 is a sequence of the optical micrograph showing evolution of regions of tensile and compressive strain e.sub.yy in the gel, when a double tip needle with constituent tip diameter d=1.2 mm and inter-tip gap =1.2 mm is used for puncturing a block of gel of modulus =30 kPa. Green and yellow color signify, respectively, zones of tensile and compressive vertical strain. Optical micrographs A to C represent the strain profile at time 0.0, 5.0 and 8.25 sec respectively.

(18) Optical image of FIG. 9 shows e.sub.yy profile of puncturing of a block of gel (modulus =30 kPa) by a double tip needle with systematically varied gap. A-d denotes =1.2, 0.92, 0.45 and 0.0 mm respectively.

(19) Optical image of FIG. 10 shows the top and side views of a hemispherical indenter with an embedded plane of tracer grid.

ADVANTAGES OF THE INVENTION

(20) 1. In contrast to all conventional methods of obtaining strain of a body, the idea of obtaining deformation and strain profile at a location buried inside a deformable object is a novel idea.

(21) 2. The idea of obtaining the dynamic evolution of different components of strain at the vicinity of the crack tip inside the body is a novel idea.

(22) 3. The idea of embedding an array of tracer particles in the form of a grid or uniformly spaced dots on a plane which can be embedded inside the body is a novel idea.

(23) 4. The idea of welding of two separate bodies of the same crosslinkable material without use of any second material as glue is a novel idea.

(24) 5. The ideal of welding of two separate bodies of the same crosslinkable material for embedding a plane containing the grid of tracer dots or lines is a novel idea.

(25) 6. The idea of creating gel layer consisting of multiple layers of different modulus via successive welding of layers is a novel idea.

(26) 7. The idea of carrying out fracture of the soft gel through the said plane in order to visualize and measure the displacement of the tracer particles is a novel idea.

(27) 8. The idea of developing tests by using the said gel blocks embedded with a plane of tracer particles as model material for studying fracture of similar soft objects is a novel idea.

(28) 9. The idea of developing tests for examining the effect of closely spaced cracks driven by multiple objects on the strain field inside a crosslinkable material is a novel idea.

FUTURE APPLICATIONS

(29) 1. For designing get based optical load sensor for measuring very small loads

(30) 2. For designing indenters embedded with a tracer grid for variety of applications, e.g. measuring local modulus of a substrate

(31) 3. Safety valves and optical switches

(32) 4. For gel labeling useful for gel electrophoresis purposes

(33) 5. For preparing gel block with discontinuously varying pore sizes and other properties for gel electrophoresis purposes.