Suraj Ravindran

Meso-scale study of non-linear tensile response and fiber trellising mechanisms in woven composites:

The non-linear deformation response of plain woven carbon fiber-reinforced composites is experimentally studied at meso-scales. Stereovision digital image correlation is utilized to capture the full-field strain distribution over a 10 × 10 mm2 area of interest located at the center of the specimens. The evolution of local strains on the fiber bundles and matrix-rich regions as a function of loading is extracted. The effect of fiber orientation angle on fiber bundles stretch ratio and their angle of rotation (fiber trellising) and the related underlying failure mechanisms are analyzed using the measured full-field displacement data. The results indicate that the local load-bearing mechanisms are different in on-axis and off-axis loading conditions, whereas the larger global failure strain noticed in off-axis conditions is attributed to the occurrence of fiber rotation. The fiber trellising is also shown to promote high local shear strain and consequently leads to the protrusion of the matrix material on the deformed specimen surface.

Meso-scale Deformation Mechanisms of Polymer Bonded Energetic Materials Under Dynamic Loading

To understand the plastic deformation mechanism of polymer bonded energetic materials, a meso-scale experiment is conducted under dynamic loading. Energetic simulant material with polymer plasticizer are cold pressed using a mold made of stainless steel. An experimental setup is developed to obtain the local strain field at the meso-scale under dynamic loading conditions. The setup consists of a high speed camera with extension tube and microscope objective lens to obtain magnifications ranging from 1× to 50×. A high intensity halogen light source is used for illumination. The field of view for the experiment is 1700 × 690 μm, with a spatial resolution of 4.427 μm/pixel at 100,000 frames/s. Dynamic loading is performed using a split Hopkinson pressure apparatus to obtain a range of strain rates. The strain fields are obtained using digital image correlation technique. To facilitate for the digital image correlation technique, the specimens are speckled using air brush with average speckle size ranging from 12 to 18 μm. Results are presented for the measured strain fields and the associated deformation mechanisms as a function of loading rate.

On the Mechanical Response of Polymer Fiber Composites Reinforced with Nanoparticles

An experimental study was conducted on the effect of interply nanofiller on the mechanical response of fiber reinforced composite (FRC). Laminate samples were made by hot pressing of woven carbon fiber fabric prepregs. Two batches of samples are prepared, one using five plies of the basic prepreg, the other with silica nanofillers added between the plies during lay-up. Tensile specimen were cut from the laminate under 0, 15, 30, 45, 60, 75 and 90 degrees of fiber orientation are prepared from the laminate. DIC based tensile test is made and the effect of the nano fillers on the mechanical properties are analyzed. Appreciable improvement in strength and Modulus of Elasticity is obtained for fiber orientation of 75° and 60° and reversed response is observed for the fiber angle of 30° and 15°

On the Meso-Macro Scale Deformation of Low Carbon Steel

The multiscale deformation response of low carbon steel is investigated. The meso and macro scale displacement and strain fields for specimen subjected to pure tension are measured using in-situ multiscale digital image correlation technique. The specimen is specked with different scale pattern ranging from 5 to 500 μm size. The smallest scale, 5 μm, speckles are used for local meso-scale deformation measurement. In this case an optical microscope is used to record the local information within 1 mm square field of view. On the other hand, the larger size, 500 μm, speckles are used to measure the continuum level deformation. In this case two digital cameras with 5 megapixel resolution are used in 3D arrangement by considering the entire width of the specimen inside the field of view. Both the optical microscope and the digital camera systems are triggered simultaneously to acquire the deformation at the same time scale. The displacement and strain fields are extracted using digital image correlation. The effect of local deformation on the overall displacement and strain of low carbon steel is presented by comparing with the macro scale deformation and strain fields. Furthermore, microstructure images are obtained by optical microscope and used for the analysis of local strain field coupled with the strain field from digital image correlation.

Experimental Study of Residual Plastic Strain and Damages Development in Carbon Fiber Composite

In this work, the correlation between local damage and residual stiffness in carbon fiber composites is investigated experimentally. The study presents the interaction between local damage mechanisms and the global response of the material, with the aim to capture the gradual local failure phenomenon. High resolution digital image correlation technique at micro length scale is used to measure the local deformation in the specimen made of carbon fiber composite, subjected to tension-tension fatigue loading. During testing, image of a speckled surface inside the gage length of the specimen is captured at specified number of loading cycles. Using digital image correlation, the acquired images are processed and the local deformations and strains are extracted. The growth of local plastic deformation as a function of loading cycle is acquired and different damage modes as a function of loading cycle is explored. The study has able to elucidate the event that shows the gradual growth of matrix cracking into inter-ply debonding. Furthermore the degradation of modulus of elasticity as a function of loading cycle is determined and the corresponding type of damage incorporated within the plastic strain distribution around it is presented.

In-Situ Observation of Damage Evolution in Quasi-Isotropic CFRP Laminates

In this work, a Digital Image Correlation (DIC) impaired with high resolution optical system is applied to capture the damage evolution in composites laminates in-situ. The developed method enables to measure the local (micro scale) deformation across the thickness on the free-edge (8-plies laminate) subjected to a quasi-static tension. Three groups of specimens are prepared by arranging plies at different stacking sequence, and the formation of strain localization, initiation of matrix crack, delamination and other damages are acquired. It is obtained that matrix cracking is the primary and dominant form of damage and it usually occurred in the 90°-plies. However, the orientation and quantity of matrix cracks are highly affected by the stacking arrangement of the plies.

Effect of Crystal Density on Dynamic Deformation Behavior of PBX

Polymer bonded explosives (PBX) are heterogeneous materials that contain solid loading varying from 80 to 95 % and bound together by 5–20 % soft binder. An experimental investigation is performed to study the effect of crystal solid loading on the failure process of PBX subjected to dynamic loading at different strain rates. Model materials, with sugar crystals and binder, are fabricated with solid loading varying from 80 to 95 %. Then dynamic compression experiments are performed on each specimens using split Hopkinson pressure bar. During loading, the deformation is captured using the high-speed camera at 1 million frames/s. Digital image correlation technique is used to obtain the local and full field deformation and strain fields at each strain rate. Based on the local deformation field and the load data, the failure process of each sample are investigated, and the effect of solid loading on the strain localization and failure mode of the PBX is discussed.

Impact Response of Density Graded Cellular Polymers

Full-field deformation response of density-graded cellular polymers subjected to high velocity impact is investigated experimentally. Recently developed experimental setup consisting of ultra-high speed imaging in conjunction with digital image correlation is used to measure in-situ full-field deformation on density-graded polymeric foam specimens. Loading of the specimens is performed using a direct impact configuration in a modified Hopkinson bar apparatus. Discretely-layered foam specimens made from three distinct layers each with a different bulk density are subjected to direct impact, while their deformation response is studied via ultra-high speed digital image correlation. Formation and propagation of compaction waves from the impact side to the support end of the specimen are observed and analyzed. Spatial distribution of inertia stress is determined from acceleration fields and density of the layers. Full-field stress distribution in the specimen is later used to estimate the stress gradients within compaction waves. Mechanisms associated with the energy dissipation in graded foam specimens are discussed.

Experimental Investigation of Compaction Wave Propagation in Cellular Polymers

In the present study, an experimental setup has been developed to facilitate the in-situ characterization of wave propagation and full-field deformation response of polymeric foams subjected to high velocity direct impact. Full-field measurements are conducted using ultra high speed digital image correlation (DIC). Impact experiments are carried out over a range of impact velocities, while the formation and propagation of compaction wave across the length of the specimen, from the impact side to the support end of the specimen, is observed and quantified through a comprehensive deformation analysis. Full-field stress distribution obtained from experiment through non-parametric method along with the measured full-field strain are used to identify the energy dissipation mechanisms associated with wave propagation in the material.

Meso-Scale Deformation Behavior of Polymer Bonded Energetic Material Under Quasi-Static Compression

This paper describes the meso scale deformation behavior of polymer bonded energetic materials under quasi-static compression. Polymer bonded energetic material contains 80–90 % of sugar crystals (as simulant of HMX) and 10–20 % of hydroxyl terminated polybutadiene (HTPB) and plasticizer are cold pressed using a mold made of stainless steel. Meso and macro scale experiments are conducted to understand the multi-scale deformation mechanisms under uniaxial compression. Specimen with different amount of solid loading has been tested under compression and the crack nucleation in the case of high solid volume fraction and low volume fractions are studied. The displacement and strain fields of the specimen in all cases are measured using in-situ high resolution optical microscope and digital image correlation.