Sandeep Medikonda

The effectiveness of studying inter-laminar delamination in laminated composites with the help of thickness-stretch shell elements which utilize a 3-D material model sub-routine as compared to the traditional plane-stress shell elements has been investigated using a non-linear finite element solver (LS-DYNA). A strain-rate-dependent micro-mechanical material model using ply-level progressive failure criteria has been used to simulate the initiation and propagation of delamination. A methodology… Show more

The effectiveness of studying inter-laminar delamination in laminated composites with the help of thickness-stretch shell elements which utilize a 3-D material model sub-routine as compared to the traditional plane-stress shell elements has been investigated using a non-linear finite element solver (LS-DYNA). A strain-rate-dependent micro-mechanical material model using ply-level progressive failure criteria has been used to simulate the initiation and propagation of delamination. A methodology of assigning physical significance to the choice of damage parameters has been presented. The numerical delamination growth has been qualitatively analyzed against the experimental C-scan images for multiple impact events on different composite plates. Show less

A micro-mechanical composite material model is developed to simulate the behavior of uni-directional composites under impact loading conditions in LS-DYNA ® . The non-linear strain-rate and pressure dependency in the composite material model is accounted by the resin, which uses previously developed state-variable viscoplastic equations. These equations have been originally developed for metals, however are modified to account for the significant contributions of hydrostatic stresses typically… Show more

A micro-mechanical composite material model is developed to simulate the behavior of uni-directional composites under impact loading conditions in LS-DYNA ® . The non-linear strain-rate and pressure dependency in the composite material model is accounted by the resin, which uses previously developed state-variable viscoplastic equations. These equations have been originally developed for metals, however are modified to account for the significant contributions of hydrostatic stresses typically observed in polymers. The material model also uses a continuum damage mechanics (CDM) based failure model to incorporate the progressive post-failure behavior. A set of Weibull distribution functions are used to quantify this behavior and a methodology of assigning physical significance to the choice of damage/softening parameters used in these functions is presented. The impact response of composite laminate plates has been simulated and compared to the experiments. In addition, the effect of hydrostatic stresses on impact problems has been further studied in detail. It has been observed that the predicted results compare favorably to the experiments. Show less

Inter-laminar delamination in laminated composites has been studied with the help of thicknessstretch shell elements using a 3-D material model and compared against the traditional plane-stress shell elements. A strain-rate and pressure dependent micro-mechanical material model using ply-level progressive failure criteria has been used to simulate the initiation and propagation of delamination. The material parameters of the non-linear resin have been determined using LS-OPT®. The numerical… Show more

Inter-laminar delamination in laminated composites has been studied with the help of thicknessstretch shell elements using a 3-D material model and compared against the traditional plane-stress shell elements. A strain-rate and pressure dependent micro-mechanical material model using ply-level progressive failure criteria has been used to simulate the initiation and propagation of delamination. The material parameters of the non-linear resin have been determined using LS-OPT®. The numerical delamination growth has been qualitatively analyzed against the experimental C-scan images for multiple impact events on different composite plates. In addition, a non-local model with an isotropic weight function has been implemented to work in conjunction with the composite micro-mechanical material model to alleviate strain softening typically seen in composite materials. Show less

A micromechanical model based on the physically viable sub-cell boundary conditions is developed and implemented for use with uni-directional composite laminates in the explicit finite element method. Stress-strain relations have been presented in a three-dimensional context and hence can be used with solid elements. The objective of this work is to study the effect of boundary conditions in accurately estimating the elastic properties of a uni-directional composite lamina. In order to achieve… Show more

A micromechanical model based on the physically viable sub-cell boundary conditions is developed and implemented for use with uni-directional composite laminates in the explicit finite element method. Stress-strain relations have been presented in a three-dimensional context and hence can be used with solid elements. The objective of this work is to study the effect of boundary conditions in accurately estimating the elastic properties of a uni-directional composite lamina. In order to achieve this, the developed micro-model has been studied alongside 2 other models with different boundary conditions specified in the literature. Numerical results are generated for engineering constants by the considered models and compared against each other for different laminas. In particular, transverse and shear modulus have been analyzed in detail alongside popular analytical methods and verified against available experimental results for various volume fractions. Good agreements have been observed for the presented model in comparison with the experimental results. Show less

A micromechanical model based on the physically viable sub-cell boundary conditions is developed and implemented for use with uni-directional composite laminates in the explicit finite element method. Stress-strain relations have been presented in a three-dimensional context and hence can be used with solid elements. The objective of this work is to study the effect of boundary conditions in accurately estimating the elastic properties of a uni-directional composite lamina. In order to achieve… Show more

A micromechanical model based on the physically viable sub-cell boundary conditions is developed and implemented for use with uni-directional composite laminates in the explicit finite element method. Stress-strain relations have been presented in a three-dimensional context and hence can be used with solid elements. The objective of this work is to study the effect of boundary conditions in accurately estimating the elastic properties of a uni-directional composite lamina. In order to achieve this, the developed micro-model has been studied alongside 2 other models with different boundary conditions specified in the literature. Numerical results are generated for engineering constants by the considered models and compared against each other for different laminas. In particular, transverse and shear modulus have been analyzed in detail alongside popular analytical methods and verified against available experimental results for various volume fractions. Good agreements have been observed for the presented model in comparison with the experimental results. Show less

Monitoring the structural health of composite materials is critical to ensure their reliability. Damage in composite materials is difficult to detect because delamination and fiber breakage occur inside the material and are thus not obvious at the surface. Existing non-destructive evaluation techniques are expensive, labor intensive, and require that the structure, component or vehicle be taken out of service for inspection. A practical way to monitor a composite material for damage at its… Show more

Monitoring the structural health of composite materials is critical to ensure their reliability. Damage in composite materials is difficult to detect because delamination and fiber breakage occur inside the material and are thus not obvious at the surface. Existing non-destructive evaluation techniques are expensive, labor intensive, and require that the structure, component or vehicle be taken out of service for inspection. A practical way to monitor a composite material for damage at its microstructure level is to integrate micro or nano-scale continuous materials with sensing capabilities into the material. Recently, carbon nanotube forests were spun into a thread that is tough and electrically conductive. The thread was integrated into composite materials and used for the first time to detect damage including delamination and debonding through electrochemical impedance spectroscopy measurements. As the thread is strained, its piezoimpedance changes, which can be used to sense damage, strain or chemical changes in the composite material. This sensor thread is integrated throughout the laminated material creating a self-sensing composite. It is revealed that the sensor thread detects mode I or mode II-dominated delamination in self-sensing composites of different fiber architectures. The sensor thread can also detect other damage and failure modes that may occur in the surface or inside the composite material and monitor its state of strain. This simple sensor thread provides a new integrated and distributed sensor technology that does not alter the integrity or reliability of the structure and adds negligible weight. The self-sensing composites will help to revolutionize the maintenance of composite structures that will now be based on their condition and not their amount of use. Show less

Laminated composite materials can reach high mechanical properties at low weight. Composite materials, however, are susceptible to damage due to their low interlaminar mechanical properties and poor heat and charge transport in the transverse direction to the laminate. Moreover, methods to inspect and ensure the reliability of composites are expensive and labor intensive. Recently carbon nanotube forests were spun into thread that is tough and electrically conductive. The thread was integrated… Show more

Laminated composite materials can reach high mechanical properties at low weight. Composite materials, however, are susceptible to damage due to their low interlaminar mechanical properties and poor heat and charge transport in the transverse direction to the laminate. Moreover, methods to inspect and ensure the reliability of composites are expensive and labor intensive. Recently carbon nanotube forests were spun into thread that is tough and electrically conductive. The thread was integrated into composite materials and used for the first time as a sensor to monitor strains and detect damage including delamination in the material. These self-sensing composites were found to be very sensitive to damage and will help to revolutionize the maintenance of composite structures, which will now be based on their condition and not their amount of use.
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Monitoring the state of strain throughout an entire structure is essential to determine its state of stress, detect potential residual stresses after fabrication, and also to help to establish its integrity. Several sensing technologies are presently available to determine the strain in the surface or inside a structure. Large sensor dimensions, complex signal conditioning equipment, and difficulty in achieving a widely distributed system have however hindered their development into robust… Show more

Monitoring the state of strain throughout an entire structure is essential to determine its state of stress, detect potential residual stresses after fabrication, and also to help to establish its integrity. Several sensing technologies are presently available to determine the strain in the surface or inside a structure. Large sensor dimensions, complex signal conditioning equipment, and difficulty in achieving a widely distributed system have however hindered their development into robust structural health monitoring techniques. Recently, carbon nanotube forests were spun into a microscale thread that is electrically conductive, tough, and easily tailorable. The thread was integrated into polymeric materials and used for the first time as a piezoresistive sensor to monitor strain and also to detect damage in the material. It is revealed that the created self-sensing polymeric materials are sensitive to normal strains above 0.07% and that the sensor thread exhibits a perfectly linear delta resistance–strain response above 0.3%. The longitudinal gauge factors were determined to be in the 2–5 range. This low cost and simple built-in sensor thread may provide a new integrated and distributed sensor technology that enables robust real-time health monitoring of structures. Show less