Peridynamic Approaches for Damage Prediction in Carbon Fiber and Carbon Nanotube Yarn Reinforced Polymer Composites

Aerospace structures are increasingly utilizing advanced composites because of their high specific modulus and specific strength. While the introduction of these material systems can dramatically decrease weight, they pose unique certification challenges, often requiring extensive experimental testing in each stage of the design cycle. The expensive and time-consuming nature of experimental testing necessitates the advancement of simulation methodologies to both aid in the certification process and assist in the exploration of the microstructure design space. Peridynamic (PD) theory, originating from Sandia National Lab’s in the early 2000’s, is a nonlocal continuum-based method that reformulates the equation of motion into an integral equivalent form. The integral form, on which the theory is based, is well suited to explore discontinuity rich phenomena such as damage and material failure. This dissertation develops PD-based simulation approaches to investigate two polymer based composite material systems of different maturity: carbon fiber and carbon nanotube (CNT) yarn. For carbon fiber reinforced composites, simulation approaches were developed to predict damage resulting from low-velocity impact, an important part of the certification process because often damage associated with this loading goes undetected leading to premature structural failure. In contrast to the more established carbon fiber, CNT yarn is a promising constituent material still very much in the developmental process. With this in mind, PD simulation approaches were developed with a different objective, which was to systematically explore microstructure property relationships, providing early feedback in the material design process

Validation Testing of a Peridynamic Impact Damage Model Using NASA's Micro-Particle Gun

Through a collaborative effort between the Virginia Commonwealth University and Raytheon, a peridynamic model for sand impact damage has been developed1-3. Model development has focused on simulating impacts of sand particles on ZnS traveling at velocities consistent with aircraft take-off and landing speeds. The model reproduces common features of impact damage including pit and radial cracks, and, under some conditions, lateral cracks. This study focuses on a preliminary validation exercise in which simulation results from the peridynamic model are compared to a limited experimental data set generated by NASA's recently developed micro-particle gun (MPG). The MPG facility measures the dimensions and incoming and rebound velocities of the impact particles. It also links each particle to a specific impact site and its associated damage. In this validation exercise parameters of the peridynamic model are adjusted to fit the experimentally observed pit diameter, average length of radial cracks and rebound velocities for 4 impacts of 300 m glass beads on ZnS. Results indicate that a reasonable fit of these impact characteristics can be obtained by suitable adjustment of the peridynamic input parameters, demonstrating that the MPG can be used effectively as a validation tool for impact modeling and that the peridynamic sand impact model described herein possesses not only a qualitative but also a quantitative ability to simulate sand impact events

Lower Body Exoskeleton Powered by Epidermal Electronics Systems

The purpose of our design revolves around the concept of enhancing the human body through the use of a lower body exoskeleton. The most applicable demographics for our design consists of paraplegics and non-paraplegics. The various uses we hope to include would allow the user to: walk again, lift heavier loads with the ability to move forward, back and be seated. Although lower body exoskeletons already exist on the market, they still have shortcomings that prevent widespread use among the general public. Our hope is to improve upon the design of existing exoskeletons with the integration of epidermal electronic systems (EES) with a hydraulic systems; allowing more functionality with less human metabolic consumption. We want the system to do most of the work for the user; to further our vision of minimalistic effort. The system will function by utilizing skin surface electromyogram signals (EMG); sent by muscles in the forearms. The signals will be picked up by the epidermal attachments transmitting them wirelessly to a microcontroller, activating the exoskeleton motion. A rigid, yet flexible frame will support the hydraulic systems and electronic components. One to two hydraulic pumps will be needed for three cylinders. One hydraulic cylinder, per leg, will be attached from the hamstring to the calf muscle. The third will be located at the hip, lifting the leg close to a ninety degree angle. The process for completing the lower body exoskeleton is split into three components: the hydraulic system, the electronic components, and the EES “tattoo.” The first step involves creating a CAD design of the frame and hydraulics. Francis Azari will be welding together the frame and attaching the cylinders to these frame at a machine shop with the assistance of Forrest Baber and Karan Patel. Saswat Mishra and Juan Soto will work together to program the Arduino Microcontroller and wire it to calibrate the hydraulic cylinders. Lastly, the EES “tattoo” will be fabricated by Saswat and Karan, using UV-Lithography in the VCU clean room. Our method of achieving our goal consists of splitting up into smaller groups; allowing us to complete work more efficiently. In order to allow ample time to complete the frame of the exoskeleton, the mechanical and electrical work has been split into the fall and spring semesters, respectively. By late December, we want the frame and hydraulic system to be completed so that we may begin coding and fabricating the EES in January. If all minimum goals can be completed early, we hope to include more features that will enhance the functionality of the suit.https://scholarscompass.vcu.edu/capstone/1054/thumbnail.jp

Laser induced dynamic fracture of fused silica: Experiments and simulations

Fused silica samples were subjected to laser induced shock loading. Laser flux was varied in order to obtain different amounts and characteristics of damage in the samples. Three dimensional damage and fracture maps of two identical samples impacted by high and low laser flux values were obtained using both optical microscopy and X-ray computed micro-tomography. Three prevalent fracture and damage patterns were identified. Peridynamic approach was used to simulate the laser impact conditions on the samples in order to explain the causes of the observed fracture and damage morphologies. A proprietary shock physics code, ESTHER, was used to calculate the transient kinetic energy imparted to the samples based on the experimental laser flux values. The kinetic energy values were then integrated over time and provided target values to match for the peridynamic impact conditions. The main fracture patterns were captured by peridynamic simulations with reasonable quantitative accuracy. Explanations for initiation and propagation of each of the fracture patterns were presented based on the peridynamic dynamic fracture simulations. Limitations of the computational approach and recommendations for future work is provided