On the crushing behavior and microstructure of closed-cell polymeric foams : experiments and phenomenological investigation

This thesis investigates the compressive behavior and crushing of Divinycell H100 under combined axial compression and external pressure. Divinycell H100 is a closed-cell polymeric foam with a relative density of 0.077 that is used extensively in sandwich composites for marine, transportation, energy, and infrastructure applications. The study starts with characterization of the microstructure using micro-computed tomography. It has a nearly monodisperse polyhedral microstructure with mean cell size and wall thickness of 0.50 mm and 0.0348 mm respectively. A custom triaxial apparatus is used to compress cylindrical specimens at different levels of external pressure. A typical axial stress-displacement response exhibits a stiff elastic branch that terminates into a maximum beyond which deformation localizes into a horizontal axisymmetric, radially contracted band of crushed cells. The band then propagates axially with the stress remaining essentially constant. Both the initial stress maximum and the plateau stress decrease linearly as the pressure increases. X-ray imaging of the microstructure of a specimen crushed axially at zero pressure confirmed that during the stress plateau, a highly crushed zone of cells with an average strain of about 50% coexists with zones of essentially undeformed cells. Postmortem images of specimens tested under triaxial loading reveal a similar evolution of crushing. Above a critical pressure, the mode of instability switches to predominantly lateral contraction that evolves into a neck. This is also the mechanism of failure under pure pressure. The localized crushing behavior observed is similar to that of low-density open-cell foams and the results should inform and guide further development of homogenized models for this class of materials.Aerospace Engineerin

First observations with a GNSS antenna to radio telescope interferometer

We describe the design of a radio interferometer composed of a Global Navigation Satellite Systems (GNSS) antenna and a Very Long Baseline Interferometry (VLBI) radio telescope. Our eventual goal is to use this interferometer for geodetic applications including local tie measurements. The GNSS element of the interferometer uses a unique software-defined receiving system and modified commercial geodetic-quality GNSS antenna. We ran three observing sessions in 2022 between a 25 m radio telescope in Fort Davis, Texas (FD-VLBA), a transportable GNSS antenna placed within 100 meters, and a GNSS antenna placed at a distance of about 9 km. We have detected a strong interferometric response with a Signal-to-Noise Ratio (SNR) of over 1000 from Global Positioning System (GPS) and Galileo satellites. We also observed natural radio sources including Galactic supernova remnants and Active Galactic Nuclei (AGN) located as far as one gigaparsec, thus extending the range of sources that can be referenced to a GNSS antenna by 18 orders of magnitude. These detections represent the first observations made with a GNSS antenna to radio telescope interferometer. We have developed a novel technique based on a Precise Point Positioning (PPP) solution of the recorded GNSS signal that allows us to extend integration time at 1.5 GHz to at least 20 minutes without any noticeable SNR degradation when a rubidium frequency standard is used.Comment: 33 pages, 19 figure

Perspectives of Childhood Cancer Symptom-Related Distress: Results of the State of the Science Survey

Management of symptom-related distress is an important area of pediatric oncology nursing. Participants who attended the Children’s Oncology Group (COG) State of the Science Symposium on symptom distress completed an anonymous survey. The purpose was to explore participant perceptions of symptom distress experienced by children receiving cancer treatment on clinical trials, determine how symptom distress is currently assessed at COG institutions, and to identify what interventions are used to reduce symptom distress for these children. Among the 90 symposium attendees, 72% completed the survey, the majority (92%) of whom were nurses. The five most distressing symptoms in children with cancer enrolled on clinical trials identified by survey respondents were nausea/vomiting, fatigue, pain, anxiety, and sleep disturbances. Results from our survey also suggest that symptom distress may differ by disease type. For example, symptoms associated with leukemia/lymphoma included steroid side effects, procedural pain, and neuropathy. The majority of respondents (90%) also reported that symptoms go unrecognized by health care providers. The most commonly described unrecognized symptoms were behavioral (i.e., sadness, anxiety, fear, depression, and emotional needs; 45%) and fatigue (19%). Key focus areas reported by respondents included informal and inconsistent symptom assessment, the need for uniform measurement tools, and improved documentation of symptom-related distress. Management of symptom-related distress is an important aspect of pediatric oncology nursing. Further exploration of symptom distress experienced by children with specific types of cancers, and the development of standardized symptom assessment processes, will provide a foundation for developing future interventions aimed at alleviating symptom-related distress