1,720,959 research outputs found
Special session honoring the contributions of Dr. T. Kevin O'Brien, NASA Langley Research Center
No full textAeronautical composite stiffened structures have the capability to carry loads deep into postbuckling, yet they are typically designed to operate below the buckling load to avoid potential issues with durability and structural integrity. Large out-of-plane postbuckling deformation of the skin can result in the opening of the skin-stringer interfaces, especially in the presence of defects, such as impact damage. To ensure that skin-stringer separation does not propagate in an unstable mode that can cause a complete collapse of the structure, a deeper understanding of the interaction between the postbuckling deformation and the development of damage is required. The present study represents a first step towards a methodology based on analysis and experiments to assess and improve the strength, life, and damage tolerance of stiffened composite structures subjected to postbuckling deformations. Two regions were identified in a four-stringer panel in which skin-stringer separation can occur, namely the region of maximum deflection and the region of maximum twisting. Both regions have been studied using a finite element model of a representative single-stringer specimen. For the region of maximum deflection, a seven-point bending configuration was used, in which five supports and two loading points induce buckling waves to the specimen. The region of maximum twisting was studied using an edge crack torsion configuration, with two supports and two loading points. These two configurations were studied by changing the positions of the supports and the loading points. An optimization procedure was carried out to minimize the error between the out-of-plane deformation of the representative single-stringer specimen and the corresponding region of the four-stringer panel
Effect of Composite Stiffened Panel Design on Skin-Stringer Separation in Postbuckling
No full textTo design aeronautical composite multi-stringer panels that can safely operate in a postbuckled state, it is important to identify the parameters that can influence the different modes in which skin-stringer separation might occur. A methodology is under development to study the interaction between the skin-stringer separation and the postbuckling deformation using the building block approach and single-stringer specimens. In particular, the methodology can identify whether the skin-stringer separation occurs due to bending or twisting, so that these two possible modes can be studied separately. For bending, a simple criterion that can predict the location of initiation is presented. This procedure has the potential to reduce the overall development cost and allows the investigation of the design parameters that influence the skin-stringer separation
Experimental and Computational Studies of the Low Velocity Impact and Compression After Impact of Carbon Fiber Reinforced Polymer Composites
No full textCarbon fiber reinforced polymer (CFRP) composites have been widely used in many industrial sectors because of their high strength-to-weight ratio, cost advantages, and the possibility of tailoring the design. During the lifetime of a composite structure, low velocity impact (LVI) tends to happen in many scenarios, including manufacturing, service, and maintenance operations. A small event such as a tool drop may cause barely visible impact damage (BVID) with extensive internal cracking while without any noticeable marks on the outer surface of a composite structure. LVI-induced damage can lead to a significant reduction in post-impact compressive strength. Therefore, the LVI and compression after impact (CAI) problems have received continued attention for decades. Considerable effort on experimental and numerical investigations persists. Generations of accurate, efficient, versatile, and robust numerical tools have been developed to tackle the LVI and CAI problems virtually to save experimental costs and accelerate the verification and validation (V&V) process. Experimental and computational studies on the LVI and CAI damage of CFRP laminated composites are presented in this dissertation. Effects of laminate stacking sequence, impact energy, and panel size on the LVI and CAI are investigated.
In the experimental part, LVI test results are reported. Non-destructive inspection (NDI) techniques, including ultrasound C-scanning and micro computed tomography (µCT), are conducted to characterize the impact damage. With the high-resolution µCT scanning, damage mechanisms are analyzed. Following the LVI tests and NDI characterization, CAI tests are done to relate the impact energy to the degradation of compressive strength. CAI fixtures for standard-size composite panels are modified to customize to panels with increased sizes. Post-buckling responses during the CAI of panels with increased sizes are presented.
Parallel with the experimental results, computational results are obtained with the enhanced Schapery Theory (EST) model, which is a seamless combination of the Schapery theory (ST) and the crack band (CB) method. EST based on 2D plane stress is extended to a 3D stress state. A novel mixed-mode cohesive law is integrated to model the degradation of stress components progressively and simultaneously. The capability of EST to capture matrix inelasticity is implemented. 2D and 3D EST applied to LVI and CAI modeling are verified against the experimental results. A high-fidelity and high-efficiency LVI-CAI computational framework is established based on 2D EST to accurately predict the LVI damage and CAI strength with significantly improved efficiency. The computational results agree well with the experimental results in terms of the load responses, damage morphology, and CAI strength.
This dissertation provides detailed LVI and CAI results concerning the effects of stacking sequence, impact energy, and panel size. The EST model developed is validated and will be useful in the V&V process of aeronautical composite structures. Furthermore, the high-fidelity and high-efficiency LVI-CAI computational framework enables obtaining the CAI strength with significantly reduced computational time, which will help enlarge the design space of CFRP laminated composites with increased impact resistance.PhDAerospace EngineeringUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/171345/1/sylinae_1.pd
Study of Skin-Stringer Separation in Postbuckled Composite Aeronautical Structures
No full textAeronautical composite stiffened structures have the capability to carry loads deep into postbuckling, yet they are typically designed to operate below the buckling load to avoid potential issues with durability and structural integrity. Large out-of-plane postbuckling deformation of the skin can result in the opening of the skin-stringer interfaces, especially in the presence of defects, such as impact damage. To ensure that skin-stringer separation does not propagate in an unstable mode that can cause a complete collapse of the structure, a deeper understanding of the interaction between the postbuckling deformation and the development of damage is required. The present study represents a first step towards a methodology based on analysis and experiments to assess and improve the strength, life, and damage tolerance of stiffened composite structures subjected to postbuckling deformations. Two regions were identified in a four-stringer panel in which skin-stringer separation can occur, namely the region of maximum deflection and the region of maximum twisting. Both regions have been studied using a finite element model of a representative single-stringer specimen. For the region of maximum deflection, a seven-point bending configuration was used, in which five supports and two loading points induce buckling waves to the specimen. The region of maximum twisting was studied using an edge crack torsion configuration, with two supports and two loading points. These two configurations were studied by changing the positions of the supports and the loading points. An optimization procedure was carried out to minimize the error between the out-of-plane deformation of the representative single-stringer specimen and the corresponding region of the fourstringer panel.Aerospace Structures & Computational Mechanic
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Get PDF“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Get PDFWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Dispelling the Myths Behind First-author Citation Counts
Get PDFWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
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