Aerospace Medicine and Biology: A continuing bibliography with indexes (supplement 133)

This special bibliography lists 276 reports, articles, and other documents introduced into the NASA Scientific and Technical Information System in September 1974

Damage detection and identification in fiber reinforced plastic structural members and field bridges using acoustic emission technique

With the increased use of fiber reinforced polymer (FRP) based structural systems for rehabilitation of existing and construction of new bridges there is a requirement for identification of critical components of these structural systems and the determination of critical damage thresholds in them. Of the many available non-destructive techniques (NDT), acoustic emission (AE) monitoring had been identified as one of the most popular techniques applicable for damage discrimination in composites. The current study aimed at using patterns in AE data for the identification of damage modes exhibited by composite structural systems. The extensive experimental program involved testing of two structural systems: (i) Reinforced concrete specimens with CFRP retrofit to study debonding failure mechanism and (ii) GFRP laminates coupon specimens tested under varied load conditions to study critical failure modes such as fiber breakage, matrix cracking, delamination and debonding. Real-time AE monitoring was also conducted for a newly installed FRP deck field bridge subjected to live load tests. The AE data collected from the bridge revealed the overall structural performance of the new bridge and helped establish baseline AE activity for future condition evaluation. The AE data acquired from all the experimental tests conducted in this research were subjected two methods of analysis. The first analysis technique involved subjecting the data to the traditional signal processing techniques and identifying various AE sources by visual observations of trends in correlation plots. Meanwhile the same dataset was analyzed using neural networks to perform pattern recognition. In this work, a methodology based on the use of an unsupervised k-means clustering to generate the learning dataset for the training of the multi-layer perceptron (MLP) classifier was developed. The method adopted here showed good results for the clustering and classification of AE signals from different sources for the specimens studied in this research. But, clustering does not always lead to a unique solution and some failure mode characteristics were more easily identifiable than others. Thus further study for enriching of the training dataset is warranted. The high performance efficiency achieved by the developed neural network model for damage identification in full scale specimens further confirms the potential of the developed methodology in being feasible for damage identification in full-scale structures

Flexural Creep Analysis of Recycled Polymeric Structural Elements.

A structural product made of recycled plastics in standard sections similar to dimensional lumber is referred to as plastic lumber. The recycled plastics used in the manufacture of these members include high density polyethylene, polyethylene terephthalate and polystyrene. Plastic lumber is a viscoelastic material possessing mechanical properties that are time and temperature dependent. This structural product is currently used primarily in non-load-bearing and low-end structural applications because of the non availability of engineering data and concerns about its creep behavior. Creep, which is the time dependent deformation under constant stress, is high for plastic lumber and is temperature sensitive. A good understanding of the creep behavior of plastic lumber is important in the design of structural members since this process is often controlled by serviceability rather than by its strength. This dissertation presents an in-depth study of the flexural creep at different stress and temperature levels for plastic lumber. Different methods for modeling the creep response of plastic lumber are presented. These methods include rheological models and mathematical functions based on statistical analysis. The goal of this study was to develop models for predicting the long term behavior of plastic lumber utilizing relatively short duration creep test results. These predictive models are useful in that they eliminate the need to conduct elaborate and expensive long-term creep tests at different temperature levels to establish long-term response of these members. This investigation shows that the long-term deformation of plastic lumber elements under constant load can be several times the initial elastic deformation. In addition to presenting models to predict the creep behavior of plastic lumber, procedures for determining the strength, based on specified failure criterion, corresponding to different durations of sustained loading are also presented

Aerospace medicine and biology: A continuing bibliography with indexes (supplement 359)

This bibliography lists 164 reports, articles and other documents introduced into the NASA Scientific and Technical Information System during Jan. 1992. Subject coverage includes: aerospace medicine and physiology, life support systems and man/system technology, protective clothing, exobiology and extraterrestrial life, planetary biology, and flight crew behavior and performance

Aerospace Medicine and Biology: A continuing bibliography with indexes, supplement 182, July 1978

This bibliography lists 165 reports, articles, and other documents introduced into the NASA scientific and technical information system in June 1978

An investigation into the strength properties of reclaimed timber joists

This research was designed to investigate the mechanical properties of timbers reclaimed from demolition, with the aim of generating a visual grade and model expression to grade these materials for structural reuse. The use of timber, reclaimed from demolition, for new construction or refurbishment has both environmental and economic benefits. The research developed an appropriate, alternate visual grading method which takes account of the unique problems associated with timber reclaimed from demolition. The research also investigated the loading capacity of timbers where previous structural loading may have affected the strength, and how grading without prior knowledge of the timber species can be utilised. Complimentary research suggested that the number of timber growth rings in a specimen has a direct effect on the mechanical properties, and that this can serve as a predictor of elastic modulus, especially when considered in conjunction with the density of the specimen. This thesis presents the findings of the research, which involved developing an alternate visual grading methodology, appropriate to the inherent ‘in service’damage sustained by timber, and quantifying the mechanical properties of reclaimed timber joists and comparing these with small clear tests. The visual grade accounted for the lack of species data available, by becoming independent of timber species. The research analysis considered density, specimen age and tree ring frequency as the variables in generating the model expression. In the final instance the analysis rejected the age of the specimen as a variable; this was found to be an anomalous and inaccurate figure, which could only be estimated and added very little to the accuracy of the model expression. The model expression uses tree ring frequency and specimen density to estimate the modulus of elasticity of the specimen and thus its strength grade. The contribution to knowledge in this research is through the introduction of an alternate, novel method of investigation and an expression to estimate the modulus of elasticity; the method is aimed specifically towards operatives at the demolition site using simple measuring equipment

Aerospace medicine and biology: A continuing bibliography with indexes, supplement 129, June 1974

This special bibliography lists 280 reports, articles, and other documents introduced into the NASA scientific and technical information system in May 1974

Design and analysis of a hybrid timber-steel floating substructure for a 15 MW semisubmersible-type FWT

Wind energy has developed to be among the most promising sources of renewable energy. Furthermore, floating offshore wind turbines have presented the opportunity for higher power production in intermediate (45-150 m) and deep water (> 150 m). However, the manufacturing, installation, and operation of wind turbines in general, and floating wind turbines in particular, can result in significant amounts of greenhouse gas emissions (GHG). This thesis proposes a novel design of a hybrid timber-steel floating substructure for the IEA 15 MW floating wind turbine. The new design presents a modified version of the UMaine VolturnUS-S semisubmersible platform that was initially developed for the same turbine. The main objective of the new design is to reduce the turbine’s overall CO2 footprint. This objective is achieved by replacing structural steel with glued laminated timber, a more sustainable material known for its environmental benefits. Firstly, a robust design methodology is introduced. Secondly, Ansys workbench 2020 R1 is utilized to compare and then select between three preliminary hybrid timber-steel models based on a set of criteria that are extracted from relevant standards for both timber and steel. Compared to the UMaine VolturnUS-S semisubmersible platform, the selected hybrid configuration provides a considerable reduction in the steel mass (around 590 t). Subsequently, fully coupled aero-hydro-servo-elastic dynamic analysis is carried out using OpenFAST to validate the selected model. Only the ultimate limit state design (ULS) for the turbine under extreme and normal operating conditions is considered. The results from the numerical analysis show that the selected model fulfills all design criteria with a utilization factor that varies between 74- 94% for the different design load cases. In the end, the work concludes that the glulam-based supporting structure offers an effective load-bearing solution for the IEA 15 MW turbine, contributing to the development of floating wind energy with minimal cost and CO2 footprint. However, a series of tasks and suggestions are proposed to enhance the process of developing an optimal timber-steel design

High resolution fMRI of hippocampal subfields and medial temporal cortex during working memory

Computational models combined with electrophysiological studies have informed our understanding about the role of hippocampal subfields (dentate gyrus, DG; CA subfields, subiculum) and Medial Temporal Lobe (MTL) cortex (entorhinal, perirhinal, parahippocampal cortices) during working memory (WM) tasks. Only recently have functional neuroimaging studies begun to examine under which conditions the MTL are recruited for WM processing in humans, but subfield contributions have not been examined in the WM context. High-resolution fMRI is well suited to test hypotheses regarding the recruitment of MTL subregions and hippocampal subfields. This dissertation describes three experiments using high-resolution fMRI to examine the role of hippocampal subfields and MTL structures in humans during WM. Experiment 1 investigated MTL activity when participants performed a task that required encoding and maintaining overlapping and non-overlapping stimulus pairs during WM. During encoding, activity in CA3/DG and CA1 was greater for stimulus pairs with overlapping features. During delay, activity in CA1 and entorhinal cortex was greater for overlapping stimuli. These results indicate that CA3/DG and CA1 support disambiguating overlapping representations while CA1 and entorhinal cortex maintain these overlapping items. Experiment 2 investigated MTL activity when participants performed a WM task that required encoding and maintaining either low or high WM loads. The results show a load effect in entorhinal and perirhinal cortex during the delay period and suggest that these regions act as a buffer for WM by actively maintaining novel information in a capacity-dependent manner. Experiment 3 investigated MTL activity when participants performed a WM task that required maintaining similar and dissimilar items at different loads. Analysis of a load by similarity interaction effect revealed areas of activity localized to the CA1 subfield. CA1 showed greater activity for higher WM loads for dissimilar, but not similar stimuli. Our findings help identify hippocampal and MTL regions that contribute to disambiguation in a WM context and regions that are active in a capacity-dependent manner which may support long-term memory formation. These results help inform our understanding of the contributions of hippocampal subfields and MTL subregions during WM and help translate findings from animal work to the cognitive domain of WM in humans

Ultrasonic characterization of engineering performance of oriented strandboard

Direct-contact (DC) and non-contact (NC) ultrasonic transmission (UT) methods were developed to characterize the structural performance of oriented strandboard (OSB). The UT variable velocity was shown to be sensitive to the physical impediments caused by flake interfacial boundaries and embedded voids. Both attenuation and root mean square (RMS) voltage were good indicators of the “zero void” densification level for OSB, a point of the greatest transmissivity of the stress wave energy. For both DC and NC methods, the predicted densities of the model were validated for spatial distribution over each OSB type. Based on the EN300 standard for panel manufacturing, the control limits were ±10% of the panel average density. The density prediction was found to improve with higher resin content (RC) and higher nominal density (ND) levels. From the out-of-limits plots, the predicted in-situ densities produced a reasonably spatial coherence to the measured values. All panels made with ND 0.60 g/cm3 or greater conformed well within the limits, with declining conformity towards lower RC panels. For each composite type made of different particle sizes, the equilibrium moisture content showed a decreasing trend toward smaller particle panels. The attenuation and RMS were good indicators for moisture change and densification level for each composite type. The velocity, sensitive to physical resistance of particle sizes, increased with increasing IB strength and sample density, manifesting the positive influence of layering, resin content, and the negative effect of bark as a constituent. The results of the creep rupture tests on commercial OSB using an acoustic emission (AE) technique indicated that the cumulative AE event count parameter was highly correlated with deflection parameter and appropriately represented the accumulation of incipient damage. Under high stress levels, specimens with high moisture content (MC) sustained the worse damages having the shortest creep rupture time followed by specimens with dynamically rising MC. Defects on the compression-side of the bending specimen were found critical to creep rupture than those on the tension-side. The in-plane fracture patterns tended to follow the defect trenches of low-density valleys, and worsened with greater variability of the horizontal density, indicating the need to measure and control the horizontal density variation within reasonable limits