bluuebunny/crossref_metadata_2025 huggingface dataset:
Το φανταστικό στο νεοελληνικό διήγημα (1880-1930)
Authors: Μαρκόπουλος Ιωάννης
Published: November 2023

Η παρούσα εργασία στοχεύει σε μια επαναπροσέγγιση της έννοιας του φανταστικού και μια πρώτη απόπειρα χαρτογράφησής του στη νεοελληνική λογοτεχνία, εστιάζοντας στη διηγηματογραφία της περιόδου 1880-1930. Η εργασία χωρίζεται σε τρία μέρη. Στο πρώτο μέρος επιχειρείται η οριοθέτηση του χώρου της φανταστικής μυθοπλασίας, η διερεύνηση βασικών πτυχών της και η ανίχνευση γενεαλογιών μέσα από την εξέταση τριών καθοριστικών σταθμών στη διαμόρφωση του είδους: 1) το γοτθικό μυθιστόρημα, που αναδύεται μες στην καρδιά του Διαφωτισμού ως αντίδραση στην περιθωριοποίηση οποιουδήποτε στοιχείου παρεκκλίνει από τα προτάγματα του ορθού λόγου και της επιστήμης, 2) το κίνημα του γερμανικού Ρομαντισμού, που σηματοδοτεί τη μετάβαση του είδους σε μία ωριμότερη φάση, εισάγοντας το υπερφυσικό στοιχείο στη σύγχρονη καθημερινότητα και 3) το έργο του Ε. Α. Poe, που παρουσιάζει μια οριακή, απομαγευμένη εκδοχή του φανταστικού, εστιάζοντας στο παράλογο της ίδιας της ανθρώπινης συνθήκης. Στο δεύτερο μέρος προχωρώ σε μια επισκόπηση των επικρατέστερων θεωρητικών προσεγγίσεων του φανταστικού, προκειμένου να καταλήξω στη διατύπωση ενός γενικού ορισμού και μιας νέας τυπολογίας. Πιο συγκεκριμένα, το φανταστικό ορίζεται ως ένα μετα-Διαφωτιστικό λογοτεχνικό είδος που πραγματεύεται την ανεπίλυτη σύγκρουση μεταξύ του πραγματικού και του αδύνατου και βρίσκει το κατεξοχήν μέσο έκφρασης στη φόρμα του διηγήματος. Παράλληλα, προτείνεται μια νέα τυπολογία που ταξινομεί τα κείμενα του είδους σε τρεις κατηγορίες, ανάλογα με το αν αποδέχονται (ο τύπος του φίλτρου), αμφισβητούν (ο τύπος του εκκρεμούς) ή αναιρούν (ο τύπος του καθρέφτη) την υπερβατική διάσταση των γεγονότων της αφήγησης. Με βάση την προτεινόμενη κατηγοριοποίηση, στο τρίτο μέρος επιχειρείται μια χαρτογράφηση του νεοελληνικού φανταστικού, το οποίο αναδύεται, όπως υποστηρίζω, μέσα στο ευρύτερο πλαίσιο της διαμόρφωσης και άνθησης της εγχώριας διηγηματογραφίας (1880-1930). Από την πεντηκονταετία αυτή, επικεντρώνομαι ειδικότερα στο έργο μιας σειράς σημαντικών Νεοελλήνων λογοτεχνών που χωρίζονται, με τη σειρά τους, σε τρεις κατηγορίες, κατ’ αντιστοιχία με τους τύπους του φανταστικού: Ανδρέας Καρκαβίτσας, Ιάκωβος Πολυλάς, Φώτης Κόντογλου (φίλτρο), Κωστής Παλαμάς, Κ. Π. Καβάφης, Γεώργιος Βιζυηνός (εκκρεμές), Κώστας Καρυωτάκης, Αλέξανδρος Παπαδιαμάντης και Δημοσθένης Βουτυράς (καθρέφτης).
Complete blood count, lipid profiles, and inflammatory markers associated with atherosclerotic cardiovascular disease in patients with diabetes
Authors: Amalia Muttia , Saputri Fadlina Chany , Sauriasari Rani , Widyantoro Bambang
Published: November 2023

Background: Complete blood count (CBC) and inflammatory markers derived from hematology parameters, as well as lipid profiles, have emerged as novel biomarkers that aid in predicting the progression of atherosclerotic cardiovascular disease (ASCVD) in people with diabetes. This study aimed to evaluate the alterations in CBC, lipid profiles, and inflammatory markers derived from CBC in Type 2 diabetes mellitus (T2DM)-associated ASCVD and the associations between glycated hemoglobin and hematology, lipid profiles, and inflammatory markers.

Methods: Overall, 75 patients with T2DM ASCVD from the National Cardiovascular Center Harapan Kita were investigated. Patients with diabetes were classified into high-risk (HR), very high-risk (VHR), and acute coronary syndrome (ACS) groups. VHR-ASCVD was defined as having ≥2 major ASCVD events, or one major ASCVD event and ≥2 high-risk conditions. HR-ASCVD were patients with >3 major risk factors, diabetes, chronic kidney disease stage 3B or 4, and a very high LDL-C level. ECG and cardiac biomarker tests ensured an ACS diagnosis. CBC, lipid profiles, and IL-6 were estimated in all groups.

Results: Patients with T2DM ACS demonstrated significantly different levels of red blood cell distribution width (RDW), leucocytes, basophils, eosinophils, lymphocytes, monocytes, segmented neutrophils, absolute lymphocytes, absolute monocytes, neutrophil-lymphocyte ratio (NLR), monocyte-lymphocyte ratio (MLR), platelet-lymphocyte ratio (PLR), total cholesterol, LDL, HDL/total cholesterol ratio, hemoglobin A1c (HbA1c), and IL-6. HbA1c was significantly correlated with leucocytes (p<0.05), segmented neutrophils (p<0.001), NLR (p<0.05), PLR (p<0,05), total cholesterol (p<0.05), LDL (p<0.05), total cholesterol/ HDL ratio (p<0.05), and IL-6 (p<0.001), eosinophils (p<0.05), lymphocytes (p<0.05), monocytes (p<0.05), and absolute lymphocytes (p<0.05). Logistic regression analysis showed that monocytes, MLR, leucocytes, eosinophils, and absolute monocytes were found to be valuable predictors for T2DM ACS (p<0.05).

Conclusions: CBC, inflammatory biomarkers derived from CBC, and lipid ratios were inexpensive parameters that could serve as inflammatory biomarkers of increased risks and complications in T2DM ASCVD.
The Role of Problem-Based Learning Approach in Teaching and Learning Physics: A Systematic Literature Review
Authors: Nicholus Gumisirizah , Muwonge Charles Magoba , Joseph Nzabahimana
Published: November 2023

Problem-based learning (PBL) is a learner-centered method in which complex real-world problems are used to stimulate students thinking and problem-solving abilities during the teaching and learning process. This systematic review aimed to reveal the role of the PBL approach in teaching and learning physics. Relevant articles for the review were sourced from Scopus and Web of Science using keywords such as “problem-based learning” and “PBL in physics” education” as search terms. This search yielded 376 results. Thirty-six articles were included in the analysis after passing a crucial condition of empirically investigating the effect of PBL in teaching and learning physics. Only three of these articles did not show a positive effect; others have shown a positive lead of PBL towards improving academic achievement, attitude to learn physics, problem-solving, critical and creative thinking abilities, cooperative learning enhancement, mental model development, and science process skills attainment. Therefore, the review offers important pointers to various classroom environments and activities that ignite learners’ thinking. Thus, it help policymakers to select and maintain the best methodology that promotes high students’ academic achievement levels.
Spatial mobility as a carrier of cultural capital: an analysis based on a longitudinal study of generations
Authors: Szafraniec Krystyna
Published: November 2023

For many years international migrations have been the primary focus of spatial mobility research, whereas internal migrations – occurring within one state – have aroused significantly less interest. However, the latter are not only an important complement to the former, but they also seem to have a much greater impact on the domestic affairs of the state. For example, internal migrations engage the resources (intellectual, cultural, social) of much wider cohorts of people who therefore become, with varying degrees of significance, agents of social changes. This article presents an analysis of the spatial mobility of two generations monitored during a 45-year-long longitudinal research project. Its broad time perspective makes it possible to analyse not only the net changes with regard to the place of residence, a typical focus of migration studies, but also gross changes (migration flows). The aim of this article is to determine the scale and the social embeddedness of the migration flows in both generations.

It is assumed that, due to differences in generational biographies and resources, as well as changes in the quality of life in rural areas compared to urban areas, migrations in both generations, even though occurring at the same stages of life, are different. As a result, the resources contributed by migrants to their new places of residence also carry a different quality.
Disrupted Market Relations in Agriculture in North Macedonia: the COVID-19 Crisis
Authors: Stojcheska Aleksandra Martinovska , Nacka Marina , Tuna Emelj
Published: November 2023

The COVID-19 crisis has stressed the importance of, and need for, a stable and functional food system, able to provide consumers with a continuous supply of quality food at affordable prices. The pandemic has also highlighted the role of agricultural producers in the food supply chain, with small farmers being the most vulnerable category. This global crisis has actually deepened the problems faced by farmers, especially those on the verge of sustainability. The purpose of this analysis was to provide an overview of the disrupted links in agriculture, as a result of the initial COVID-19 crisis. A survey was carried out with 91 farmers in North Macedonia. Moreover, in-depth interviews were conducted with selected traders and processors. In general, farmers assessed that the crisis negatively affected their economic operations. The critical parts of the supply chain derive from the disrupted link between farmers and traders/processors, lack of field technical support resulting from the mobility restrictions, uncertainty in cooperation, lower production quantities, and varying quality of the agricultural products. Availability and cost of seasonal workforce was also pronounced as an issue. The dominant family farms small-scale structure and lack of aggregation in the sector challenged the resilience and effective response to the crisis. From buyers’ and processors’ perspective, the crisis effect was marked through the disrupted communication with farmers, low awareness of the need for change, and drastic decline in the HORECA channel sales. The agricultural and rural policy is designed to address measures adjusted to the needs of the key actors in the sector. The priority set of measures should support the food supply chain, enhancing farmers’ networking and aggregation, and on-farm and off-farm diversification, along with improved communication, information systems and digitalisation. Improving productivity and competitiveness remains an effective strategy for sustainable operations, greater resilience and risk adaptation in crises such as the pandemic.
EXPERIMENTAL STUDY OF THE PURE BYPASS STRUCTURAL RESPONSE OF COMPOSITE STRUCTURE WHEN SUBJECTED TO ELECTRICALLY INDUCED DAMAGE
Authors: CARPENTER BRIAN , MICHAEL LOUISA , JUSTUSSON BRIAN , BOETTCHER PHILIPP , JOHNSTON HUNTER , TUCK DEREK
Published: November 2023

In modern aircraft, the design of lightning damage tolerant composite structure is an area of continued interest. In this work, a filled hole compression (FHC) experimental data set was created to demonstrate the impact that electrically induced damage has on the pure bypass residual strength of composite laminates. A scaled ARP5412 A-waveform current-pulse is used to induce electrical damage in the coupons. Following the electrical test, the articles underwent a series of inspections and structural testing. Inspection data is shown at various points in the test plan and normalized test data and trends are discussed. Multiphysics models were created and normalized comparisons to test data are made.
PROCESS MODELING OF TORAY 3900 EPOXY RESIN USING MOLECULAR DYNAMICS
Authors: PATIL SAGAR , KEMPPAINEN JOSHUA , MAIARÙ MARIANNA , ODEGARD GREGORY
Published: November 2023

This article details the modeling of Toray epoxy system using Molecular Dynamics to predict the volumetric shrinkage as a function of crosslinking density. The “Reacter” utility in LAMMPS is used to virtually simulate the curing of the epoxy. The results for the shrinkage showed good agreement with other modeling data in the literature. These physical properties and thermo-mechanical properties will be utilized for process modeling of composite laminates of the epoxy system.
THE EFFECT OF FIBER-ANGLE FIDELITY ON THE LINEAR RESPONSE OF TOW-STEERED COMPOSITE PLATES
Authors: KOSZTOWNY CYRUS J. R. , SIMMONS LAUREN M.
Published: November 2023

Composite laminate tailoring is traditionally performed by uniformly changing the in-plane ply orientation to obtain the desired mechanical performance. The emergence of tow-steered plies, where the fibers follow a prescribed curvilinear path, have increased the tailorability of composite laminates. However, characterizing the behavior of tow-steered laminates using finite element analysis is challenging because additional, and often numerous, orientation definitions may be required. The additional orientation definitions detrimentally increase the computational cost and hinder the use of advanced analysis techniques, such as Monte Carlo or uncertainty quantification, in the design process. To reduce computational cost without adversely affecting mechanics-based performance, the results of a parametric study that was used to investigate the effects of fiber-angle fidelity, element size, and tow-steered radius-of-curvature on various loading scenarios for tow-steered composite plates are presented. The three loading scenarios that were analyzed using finite element analysis included an axial tension load, an applied constant through-thickness-direction pressure load, and an axial compression load. Results for mechanics-based metrics of interest are presented and discussed for each loading scenario. Little sensitivity (less than one percent difference) to the effect of fiber-angle fidelity is observed in the mechanics-based metrics until the coarse-end of the considered range. Sensitivity to element size generally dominates the observed results for the mechanics-based metrics. Notable reductions in the preprocessing time are observed for increasingly coarse element size and fiber-rounding parameters. The preprocessing times decreased up to three orders of magnitude from a few thousand seconds to a few seconds without loss of accuracy in the mechanics-based metrics. Such increased computational performance is of particular interest to the structural design and analysis communities that may be conducting large counts of finite-element analyses, such as in other parametric studies, Monte-Carlo analyses, uncertainty quantification, or tow-steered optimization.
EXPERIMENTAL INVESTIGATION OF HELIUM GAS LEAKAGE BEHAVIOR OF CFRP LAMINATES WITH PARTIALLY THIN-PLY LAMINATES UNDER BIAXIAL STRESS AT 20 K
Authors: FUJISHIRO KAZUYOSHI , SATO DAIGO , KUMAZAWA HISASHI , HAMORI HITOSHI , OGASAWARA TOSHIO
Published: November 2023

Carbon fiber reinforced plastic (CFRP) has superior specific strength and stiffness compared to aluminum alloys. Therefore, it is a promising material for cryogenic propellant tanks of rockets and hydrogen aircrafts. One of the technical challenges in realizing a cryogenic tank made of CFRP is the prevention of gas leakage due to microscopic damage evolution. To prevent gas leakage, applying thin-ply CFRP (ply thickness is 0.1 mm or less) is devised. It is reported that thin-ply CFRP is effective in suppressing microscopic damage and reducing gas leakage. However, since the cost of thin-ply CFRP increases due to the increase in the number of layers, methods to apply thin-ply CFRP to only some of the layers (partial thin-ply) have been proposed. Earlier study by Hamori et al. Composite Structures 2019 reported that inserting thin-ply CFRP in the middle of the stacking sequence improved onset strain of leakage and leak rate. They focused only on cross-ply laminates, and the effect of off-axis (±45°) thin-plies on gas leakage behavior has not been investigated. The purpose of this study is to experimentally investigate the leakage behavior of off-axis partial thin-ply under cryogenic environment and biaxial stress. The specimens were [04/904/04/904]s cross-ply laminates with [45/-45]s and [45/0/-45]s offaxis partial thin-ply inserted. The specimens were subjected to biaxial tensile loading using bi-axial loading rig while the center of the specimen was locally cooled to -246 ±5°C by a GM type cryogenic refrigerator. The helium gas leak rate in the thicknessdirection was then measured at room temperature (25°C) with no load. The results showed that there was small difference in the onset strain of leakage and leak rate between the cross-ply and off-axis partial thin-ply. We assumed that the reason for this was the reduction of damage at the crack intersection and the decrease in crack length due to the thinning.
HIGH-ENERGY DYNAMIC IMPACT MODELING OF AN AS4D/PEKK-FC COMPOSITE USING LS-DYNA MAT213
Authors: RICKS TRENTON M. , GOLDBERG ROBERT K. , PEREIRA J. MICHAEL
Published: November 2023

As part of the NASA Hi-Rate Composite Aircraft Manufacturing (HiCAM) Project, multiple thermoplastic composite systems are being considered as candidates for enabling a more rapid production of composite aircraft components. Various analysis methods are being explored to evaluate their capabilities and applicability to simulating the progressive failure of thermoplastic composites. One of these approaches is based on using the MAT213 material model, a tabulated composite material model recently integrated into the commercial release of Ansys LS-DYNA. For this study, a Solvay APC AS4D/PEKK-FC unidirectional tape composite system was analyzed. A MAT213 input deck was generated for this composite utilizing data from standard composite coupon tests. A series of singleelement verification exercises were performed to evaluate the correct performance of MAT213. These model inputs were then implemented into a high energy dynamic impact (HEDI) ply-by-ply model of a flat 32-ply quasi-isotropic specimen. A tie-break contact model, implemented separately from MAT213, was used to simulate delamination between individual plies. Comparisons, including deformations, damage/delamination sizes, and penetration threshold, are made to test data for a variety of impact velocities. The results demonstrate that MAT213 is a viable option for simulating progressive failure in thermoplastic composite systems under HEDI loads.
MEASUREMENT, INTERPRETATION AND APPLICATION OF R-CURVES FOR TRANSLAMINAR FRACTURE TOUGHNESS OF COMPOSITE LAMINATES
Authors: WISNOM MICHAEL R. , XU XIAODONG
Published: November 2023

The reasons for the large variation of reported values of translaminar fracture toughness are considered, both for initiation and propagation. The effects of ply thickness, layup, the development of the fracture process zone and the implications for predicting response of notched structures are discussed.
EQUI-ENERGETIC LOW-VELOCITY IMPACT EFFECTS ON THE COMPRESSION AFTER IMPACT STRENGTH OF CARBON FIBER COMPOSITE TUBE STRUCTURES
Authors: MACK JASON P. , TAN K.T.
Published: November 2023

This study explores the effect of equi-energetic low-velocity impact on the compression after impact (CAI) strength of tubular composite structures. The effects of equi-energetic impact energies are explored, where high-mass-low-velocity (HMLV) and low-mass-high-velocity (LMHV) test configurations are used at three impact energy levels using Instron CEAST 9350 impact test machine. Various tube diameters (76.2 mm and 101.6 mm) are also considered to achieve a better understanding of tube geometries on CAI residual strength. The impact force-time graphs and force-displacement curves are used to analyze the impact responses and damage mechanisms. The results indicate that impact energy, impactor mass, and tube diameter play a significant role in impact response. The larger diameter tubes can withstand higher compressive loads regardless of impact damage, showing significant drops for smaller UT tubes while the larger diameter tube can maintain more of their compressive strength. This study provides insights into the behavior of composite tubes under LVI and highlights the importance of considering impact effects when evaluating the structural integrity and strength of composite materials.
IMPROVEMENT OF DAMPING IN CARBON/EPOXY BEAMS WITH EXTERNALLY BONDED, ALIGNED CARBON NANOTUBES
Authors: BROWN AVERY D. , BAKIS CHARLES E. , SMITH EDWARD C.
Published: November 2023

Adding multiwalled carbon nanotubes (CNTs) to polymer composites such carbon fiber reinforced epoxies (c/ep) has the potential to add beneficial damping in vibration prone structures, such as rotorcraft blades, because of slip at the CNT/epoxy interface and between the multiple walls of the CNTs themselves. Recent research has demonstrated remarkable damping improvement by incorporating CNT-rich interlayers into c/ep laminates, and there is current interest in flight testing this damping approach in rotorcraft blades. In order to avoid the time and cost of developing and flight-certifying an entirely new blade material with CNT interlayers, it has been proposed to bond a CNT-rich patch onto the surface of an existing blade. Thus, this investigation compares and analyzes damping in three types of cantilevered [±45/0]s c/ep beams: without any CNTs; with CNT interlayers; and with CNT layers co-bonded to the surface. The CNT layers were used in 64 g/m2 pairs, covering 20% of the length of the beam near the clamped end, and comprising about 1 vol% of the cantilevered portion of the beam. Damping ratio, , was measured during free vibration in a vacuum chamber. The damping ratio increased from 0.65% in a plain c/ep beam to 0.93 (+43%) and 1.24% (+91%) by adding CNT interlayers and externally bonded CNTs, respectively. Dynamic finite element (FE) analysis of the beams confirmed these results and was further used to parametrically investigate the damping attainable with CNT surface films covering higher lengths of the beam or stacked to higher thicknesses at the root of the beam. The parametric investigation showed that increasing the length of the CNT surface film is the most efficient use of CNTs and that a damping ratio 277% greater than that of the beam without CNTs could be realized. In conclusion, externally bonded CNT layers offer a promising avenue for the rapid, cost-effective flight testing of CNT-augmented damping in composite rotorcraft blades.
INTERLAMINAR R-CURVE EFFECTS IN THERMOPLASTIC COMPOSITES: EXPERIMENTS AND MODEL CORRELATIONS
Authors: RAO PRABHAKAR M. , DÁVILA CARLOS G. , KRUEGER RONALD , JOHNSTON WILLIAM M. , RATCLIFFE JAMES G. , ROSE CHERYL A.
Published: November 2023

The interlaminar fracture behavior of thermoplastic composites under Mode-I and Mode-II static loading is investigated in this research. The material system under consideration is a carbon fiber reinforced low melt semi-crystalline resin TC1225 LMPAEK T700G (T700/LMPAEK)†. Characterization experiments were conducted to measure the load-displacement response, peak load, and propagation of delamination from a pre-implanted Kapton® insert. The Mode-I response was characterized using a double cantilever beam (DCB) test according to the ASTM D5528-22 standard. It was observed that the propagation exhibited significant resistance to delamination growth, i.e., an R-Curve effect. The test data indicated that the steady-state fracture toughness, ? ?? ? ????, was approximately twice the mean initiation fracture toughness, ??????. The Mode-II response of an end notch flexure (ENF) specimen tested according to the ASTM D7905- 19 standard exhibited an unexpected stable behavior upon attaining peak load. Prior to attaining peak load, the Mode-II response exhibited some non-linearity, which may be attributed to nonlinear bulk material response, e.g., hyper-elasticity, viscoelasticity, plasticity, etc., and potentially friction at the load/support rollers and R-Curve effects. Finite element models based on the cohesive zone method (CZM) and the virtual crack closure technique (VCCT) were used to predict the DCB and ENF specimen responses. DCB analysis results obtained using ?????? predicted propagation consistent with experimental data. However, results obtained with the as characterized ?????? were inaccurate in predicting delamination growth onset and peak load. ENF analysis results obtained with the as-characterized Mode-II fracture toughness, ????????, were inaccurate in predicting the peak load and propagation. Hence, R-Curve effects were included in both DCB and ENF simulations, which enabled improved predictions. The experimentally determined Mode-I R-Curve was used for the DCB simulations, whereas an R-Curve was derived for Mode-II assuming that the change in compliance of the specimen was associated with crack growth only.
HYPERVELOCITY IMPACT INVESTIGATIONS OF COMPOSITE TRUSS TUBES FOR IN ORBIT ASSEMBLY RISK ASSESSMENT
Authors: MORTON JOSEPH , HODGES GREYSON , PANKOW MARK , LAMBERSON LESLIE
Published: November 2023

The number of threats to space structures due to micrometeoroid and orbital debris (MMOD) impact continues to increase as the density of structures in orbit increases. Upon impact from (MMOD), the structure will have significantly reduced strength, however additionally; the debris shed post-impact causes a compounding risk of damage to the in-orbit assembly field [1]. MMOD impacts have an average velocity envelope between 9 and 10 km/s in the orbit of the International Space Station (ISS). Still, they can range between 3 and 20 km/s depending on the position of the space vehicle at the time of impact [1]. Many space structures, like that of the James Webb space telescope (JWST), and the in-space assembled telescope (ISAT), utilize fibermatrix composites consisting of various shapes and sizes [2]. In particular, the ISAT is designed to use composite truss members to connect instruments to the rest of the telescope and structurally serve as the telescope's backbone [3]. Hypervelocity impact (HVI) damage to flat plate composites has been widely discussed. However little to no research has been published on the effect of HVI on composite tubes. Understanding the effect of the localized damage caused by MMOD impact on the overall strength of the individual truss member and structure as a whole is crucial to providing insight into the state of current structures and aiding in the design of future structures. To simulate MMOD impact, a unique two-stage light-gas gas gun, designed for impact velocities between 2 and 7 km/s is used [4]. Investigating the postmortem inlet and exit diameters, residual compression strength, and in situ debris cloud tracking of characteristic carbon fiber reinforced polymer (CFRP) tubes will provide data used to reinforce current computational models of the event. Combining the results from the Lagrangian finite element (FE) model with the Eulerian hydrodynamic model will allow the verification of damage effects only seen due to hypervelocity impact. These results will be used to aid in understanding the risk of MMOD impact on current structures and assist in developing systems more resilient to HVI.
EFFECT OF POLYMER/FIBER AFFINITY ON THE PERFORMANCE OF SILK COMPOSITE LAMINATES
Authors: HAMIDI YOUSSEF K.
Published: November 2023

Aerospace industry is intensely shifting towards the usage of composite materials, as the new Airbus and Boeing aircrafts, among others, comprise increasing percentages of these materials. Beyond their higher specific mechanical properties, fiber reinforced polymer composites are non-magnetic, and thus do not interfere with the magnetic compass of the aircraft, making them particularly suitable for the aircraft’s skin. Recent sustainability concerns and environmental awareness has led to the popularity of natural fibers as eco-friendly reinforcements in polymer composites. Among natural fibers with established potential applications, silk fibers have recently received considerable attention from researchers due to their remarkable properties. Strong, lightweight and flexible, silk is a viable natural reinforcement that can be repurposed for composite industrial use. However, silk composite laminates are hardly used in aerospace products due, in part, to a lack of affinity between the silk fibers and epoxy, the polymeric resin often used to manufacture silk composites. This paper investigates the effect of polymer/fiber affinity on the performance of silk composite laminates. To this effect, the silk/resin contact angle is measured for different resins, and correlated with the mechanical properties of the silk composite laminates fabricated by vacuum assisted resin transfer molding (VARTM). The ability of the resin to impregnate the woven silk fiber preform is observed to affect the mechanical performance of these laminates as well as their overall quality.
HOMOGENIZATION METHODS FOR MULTISCALE PROCESS MODELING THROUGH FULL FACTORIAL DESIGN
Authors: OLAYA MICHAEL N. , MAIARÙ MARIANNA
Published: November 2023

Crucial to developing the next generation of high-performance composites are multiscale process models which have the capacity to predict in-service performance affecting residual stresses and deformations generated during manufacturing. Length scale and geometrical complexity of composites increase in tandem; thus, process models must rely upon methods for homogenization of the curing process. This work proposes a novel, high-fidelity computational approach which leverages finite element (FE) analysis at the microscale to predict effective composite properties at arbitrary cure and temperature states. The approach is applied to a high-fidelity representative volume element (RVE) and low-fidelity repeating unit cell (RUC) that models a 75% fiber volume fraction AS4-carbon fiber/epoxy composite microstructure. Elastic properties of the composite are characterized across several temperature and cure states for each FE model. Two classical closed-form composite homogenization approaches are also used to predict effective properties at each cure state. Predictions obtained by each method are compared against those of the RVE on a state-by-state and property-to-property basis.
MECHANICAL CHARACTERIZATION OF EPOXY RESIN MANUFACTURED USING FRONTAL POLYMERIZATION
Authors: TARAFDAR AMIRREZA , WOODBURY CAMERON , NADERI ALI , WANG XINLU , LIN WENHUA , HOSEIN IAN , WANG YEQING
Published: November 2023

for thermoset-based fiber-reinforced polymer composites (FRP) in comparison with the traditional autoclave/oven-curing method, due to its rapid curing process, low energy consumption, and low cost. Optimizing the weight contents of initiators relative to the resin’s mass is needed to adjust the mechanical properties of FRPs in industrial applications. This study investigates the effect of varying the photoinitiator (PI) weight content on tensile properties and the frontal polymerization characteristics, including the front velocity, front temperature, and degree of cure, in the FP process of the epoxy resin. Specifically, a dual-initiator system, including PI and thermal-initiator (TI), is used to initiate the polymerization process by ultraviolent (UV) light. The weight content of the TI is fixed at 1 w%, and the relative PI concentration is varied from 0.2 w% to 0.5 wt%. Results show that increasing the PI amount from 0.2 wt% to 0.3 wt% significantly improves the front velocity and the degree of cure by about two times. Increasing the PI content from 0.3 wt% to 0.4 wt% results in 15% and 26% higher degree of cure and front velocity, respectively. Moreover, due to the different front velocity in the top and bottom regions of the specimen, the specimens with 0.4 wt% PI exhibited a curved shape. The specimen with 0.5 wt% PI is thermally degraded and foamed. By comparing tensile properties, it is found that increasing the PI concentration from 0.2 wt% to 0.3 wt% improves the tensile strength and Young’s modulus by 3.91% and 7%, respectively, while the tensile strength and the Young’s modulus of frontal polymerized specimens are on average 8% and 14% higher than traditionally ovencured ones, respectively.
EFFECT OF FABRIC STABILIZATION MEDIUM ON THE INTERLAMINAR SHEAR STRENGTH OF UNIDIRECTIONAL CARBON/EPOXY LAMINATES
Authors: SCHWARZ JACKSON A. , ROESSLER ARYA K. , BAKIS CHARLES E. , BRANCO RICARDO BRAGA NOGUEIRA , YAMAMOTO NAMIKO
Published: November 2023

Dry unidirectional fiber preforms need to be stabilized by stitched or adhesively bonded materials to maintain a fixed fiber position and alignment during infiltration, compaction, and cure processes for composite laminates. However, these stitching and binding materials can have deleterious effects on the quality of manufactured laminates, such as in-plane-wavy fibers, resin rich areas, and a weak interface between the stabilization material and the matrix of the composite. Little information is available on the interlaminar shear strength (ILSS) of unidirectional carbon/epoxy composites with various types of stabilizing materials within individual plies. The objective of the current investigation is to experimentally evaluate the effect of plywise stitching and thermoplastic adhesive binding on the ILSS of unidirectional carbon/epoxy composites. One stitched fabric and two thermoplastic-bound fabrics were evaluated. Four laminates of two different sizes were manufactured using resin film infusion (RFI). Laminate quality was evaluated based on void and fiber volume fractions and ILSS from short beam shear (SBS) testing. Laminates with stitches demonstrated a lower ILSS (34.3 MPa) than those with binder (49.6 MPa). Manual removal of the binder before processing provided the highest ILSS (52.7 MPa). Laminates manufactured with stitched and bound fabrics had interlaminar failure modes connected to the stabilization materials. Laminates fabricated with the stitched fabric exhibited lower fiber volume fractions (55-56%) than those fabricated with binder (62%) and removed binder (66%). This result is attributed to the greater resistance of the stitched fabrics to compaction, in comparison to the binder and no binder fabrics. Void volume content was nearly the same in all four laminates— between 1% and 2%.
GENERATING WOVEN TEXTILE MODELS WITH INDUCED FIBER ENTANGLEMENT FOR CONTROLLING COMPOSITE THICKNESS
Authors: BARLOW GEORGE , STAPLETON SCOTT E.
Published: November 2023

Generating realistic tow geometries is important for making effective models for textile composite materials. The geometry of the tows is controlled by the compaction response of the textile during manufacturing. Attaining a realistic compaction response from a textile model has been difficult due to the representation of fibers as unentangled. These tows containing idealized, parallel fibers without fiber entanglement or meandering compact with less resistance than tows with the entanglement found in manufactured textile composites. In this study, a method of introducing entanglement to the tows within a textile was created which selectively swapped fiber positions at different cross sections along the length of the tow. This method introduces entanglement and meandering into a textile while ensuring the fibers do not cross into the bounds of the surrounding tows. The entanglement was controlled by varying the frequency (probability of swapping) and distance (swapping radius standard deviation) of fiber path pairs being swapped within each tow at cross sections separated by the swapping plane spacing along the length of the tow. When applied to a plain weave textile, it was found that the greater the entanglement, the greater the compaction pressure vs composite thickness of the plain weave textile. While examining the ability for entanglement to control the compaction response of the textile, it was found that the compaction pressure-composite thickness response of a manufactured specimen was able to be replicated.
INTEGRATED OPTIMIZATION FRAMEWORK FOR COMPOSITE MANUFACTURING: MINIMIZING SPRING-IN EFFECTS AND IMPROVING CURE CYCLES
Authors: ENOS RYAN , ZHANG DIANYUN , WEI QINGXUAN
Published: November 2023

This study presents a structured methodology for the optimization of composite manufacturing processes, focusing specifically on autoclave techniques to mitigate issues such as the spring-in effect. The initial phase employs Sequential Quadratic Programming (SQP) in a weighted-sum approach to optimize the cure cycle, using Radford's equation for spring-in angle estimation and a multi-physics, multi-scale MATLAB model to investigate the cure and temperature-dependent laminate response. This phase underscores the efficacy of the selected optimization algorithm, demonstrating a significant reduction in spring-in while ensuring a high degree-of-cure. Subsequently, the study incorporates an integrated Finite Element Analysis (FEA) optimization framework linking ABAQUS and MATLAB. This framework utilizes the Non-dominated Sorting Genetic Algorithm II (NSGA-II) for multi-objective optimization with an integrated composite manufacturing processing model. This second phase illustrates the framework's robust capabilities in composite cure cycle optimization, providing a well-distributed set of optimal solutions in an efficient timeframe. The study highlights the potential of the approaches and frameworks investigated to improve the efficiency, performance, and quality of composite parts.
FATIGUE PERFORMANCE OF HIGHLY ALIGNED SHORT FIBER IM7/LM-PAEK THERMOPLASTIC COMPOSITES
Authors: PARAMBIL NITHIN K. , CHEN BRANNDON R. , GILLESPIE, JR. JOHN W.
Published: November 2023

(R=0.1, 3Hz). In comparison, this significant level of applied max strain (0.9 %) with no micro-cracking is ~50% higher than the initiation strain level for a typical carbon/epoxy having a much lower intralaminar fracture toughness (initiation strain is 0.6% and exhibits crack saturation (24 cracks/cm). The high degree of fatigue damage resistance observed for the 3mm IM7/LM-PAEK TuFF is attributed to the significant level of interface adhesion between the fiber and the resin (Interfacial Shear Strength of 120 MPa) and high matrix ductility (>40% strain to failure) and high intralaminar fracture toughness of the 90˚ layers.Tailorable universal Feedstock for Forming (TuFF) is a highly aligned short fiber composite material (fiber aspect ratio of 600) that can achieve a high fiber volume fraction (50-60%). Previous studies on unidirectional (UD) 3mm IM7/polyetherimide (PEI) have demonstrated quasi-static and tension-tension fatigue properties comparable to continuous fiber composites (57% fiber volume fraction). In this work, UD 3mm IM7/Low-Melt Polyaryletherketone (LM-PAEK) TuFF panels (Vf = 54%) were tested in the fiber direction using a tensile fatigue methodology to generate an S/N curve (R = 0.1, 3 Hz) with periodic checks for modulus loss over a minimum of 1 million cycles. The results indicate 3mm IM7/LM-PAEK is superior to other discontinuous fiber composites and equivalent to other continuous fiber composites in the literature. The comparison is based on the slope of normalized strength versus cycles on a semi-log plot with LM-PAEK exhibiting the lowest slope/strength reduction. No reduction in axial fiber tension modulus was measured. This study further looks at the tension-tension fatigue performance of [0/90/0] IM7/LM-PAEK TuFF cross-ply laminate with a thick ((240 gsm) 90˚ ply subjected to a maximum tensile strain of 0.9% (R = 0.1, 3 Hz). Typically, cross-ply composite layups exhibit transverse cracking in the transverse plies (90˚ layer) that extend across the entire thickness. Models and experimental studies show that the initiation and saturation of transverse cracking during static loading depend on several factors including mechanical loading, residual stress, and ply thickness. The effect of fatigue loading has been shown to reduce the initiation strain. In general, initiation strain for transverse cracks decreases with increases in residual stress and ply thickness (the typical range of initiation strain is 0.1 to 0.6% tensile strain). The key material property at the continuum length scale governing transverse cracking is the intralaminar fracture toughness of the 90˚ layers. In the present study, the thickest 90˚ layers (240 gsm) in the cross-ply 3mm IM7/LM-PAEK TuFF laminates exhibited no transverse micro-cracking up to ~1.5 M fatigue cycles with a 0.9% maximum strain
INTERFACIAL CHARACTERIZATION OF S2 GLASS-EPOXY RESIN USING SINGLE FIBER PULLOUT TESTS AT VARYING TEMPERATURE AND MOISTURE LEVEL
Authors: DOSHI SAGAR M. , SCHNEIDER ALEX , DEITZEL JOSEPH M. , GILLESPIE, JR JOHN W.
Published: November 2023

Due to their high specific strength, stiffness, and energy-absorbing properties, glass fiber epoxy composites are commonly used in automotive, aerospace, and other consumer applications. The Army uses these composites in ground vehicles for various structures, and they are often subjected to a wide range of environmental conditions, with varying temperature and moisture conditions. They may also be subjected to high strain rate loading. While the influence of environmental conditions on the properties of the reinforcing fibers is minimal, a fundamental understanding of its effect on the resin and interfacial adhesion is critical in evaluating the behavior of these composites. This research focuses on characterizing the effect of temperature and moisture on the interfacial bonding between S2 glass fibers (with 463 sizing) and epoxy resin using single fiber pullout testing. Epoxy resin SC-15, commonly used in these applications, is tested at room temperature, -55ºC and 76ºC. The range is chosen based on the operating temperature defined by MIL 810 standards. The embedded length is comparable for all tests, of the order of 55-60 μm. The embedded length and average diameter are measured using a scanning electron microscope image of the failed specimen to calculate the apparent IFSS. The IFSS shows a strong correlation with temperature; an increase in IFSS at -55ºC and a decrease at 76ºC. An 89% increase was measured at -55ºC and a 45% decrease at 76ºC when compared to room temperature tests. The IFSS measured is dependent on various factors such as the surface roughness of the fiber, chemical bonds between the resin and the fiber (sizing), and the residual compression forces that induce friction within debond regions due to the difference in CTE (coefficient of thermal expansion) between the fiber and the resin. Due to competing mechanisms of resin failure or interfacial failure, in some cases, a residue of the resin on the fiber may be present. A discussion of these mechanisms and a comparison with the resin properties at different temperatures is discussed.
STIFFNESS AND FATIGUE LIFE ESTIMATOR FOR POLYMER COMPOSITE LAMINATES USING MACHINE LEARNING
Authors: ARNOLD STEVEN M. , MITAL SUBODH K. , HEARLEY BRANDON L.
Published: November 2023

Machine learning (ML) models are increasingly being used in many engineering fields due to the advancements in ML algorithms and availability of high-speed computing power. One of the most popular ML class of models is artificial neural networks (ANN). ML is increasingly being used in the design and analysis of composite materials and structures, specifically in the constitutive modeling of composite materials with the focus on greatly accelerating multiscale analyses of composite materials and structures through development of surrogate models. Towards that end, Python-based neural nets have been developed to predict initial stiffness and fatigue life of an eight-ply symmetric polymer matrix composite laminate. Two types of neural networks, a Multilayer Perceptron (MLP) and a Recurrent Neural Network (RNN), have been established. Results show that both neural net type algorithms can provide an excellent estimate of initial laminate stiffness as well as fatigue life of eight-ply symmetric polymer matrix composite laminates (PMCs). RNNs are better able to capture the shape of the fatigue curve of a laminate. The resulting tool and GUI can be very useful for system level studies to obtain an estimate of desired properties and life of PMC composite laminates. Further, the associated surrogate models can also be used in composite multiscale analyses to replace the actual physics-based calculations at lower scales and thereby significantly increase the computational efficiency of such analyses and thus make micromechanics-based multiscale analyses a viable industrial tool for large scale structural problems.
CURE SHRINKAGE CHARACTERIZATION OF A THERMOSETTING RESIN WITH THREE-DIMENSIONAL DIGITAL IMAGE CORRELATION (3D-DIC)
Authors: BUKENYA KALIMA , SABATO ALESSANDRO , MAIARÙ MARIANNA
Published: November 2023

This work presents the application of three-dimensional digital image correlation (3D-DIC) to capture pre- and post-gelation volumetric chemical shrinkage of EPON 862 thermosetting resin paired with hardener triethylenetetramine (TETA) and cured at room temperature. The manufacturer-recommended 100:14 resin-hardener mixing ratio is used for DIC experimentation. A stress-free boundary condition is employed with a thin Teflon sheet placed between the resin mixture and the specimen container. For the first time, a spray-painted speckle pattern is applied directly to the resin to record full-field strain measurements during curing. This speckle pattern is employed immediately after the resin mixture is poured on the Teflon-covered specimen container for the pre-gelation case, and after gelation has been visibly observed for the post-gelation case. Strain capture with 3D-DIC is initiated directly after speckle pattern application with stereo-images captured every 30 seconds for one hour and then every 60 seconds for two hours. Differential scanning calorimetry (DSC) is used to determine resin cure kinetics to correlate chemical shrinkage with degree of cure. The presented technique provides an innovative methodology to characterize the chemical shrinkage of thermosets during curing.
EFFECT OF SILICA AND MIXING TIME ON MICROSTRUCTURES OF POROUS POLYMER COMPOSITE BY EMULSION TEMPLATING
Authors: RHULE DAVIN , MONDAL ANIRBAN , SAHA MRINAL C. , CUMMINGS LAURA , ROBISON THOMAS
Published: November 2023

Porous polymer composite with tailored porosity is applied in the myriads of areas such as energy storage, oil/water absorption, bioengineering, and advanced areas of material science. The emulsion templating technology is one of the most popular methods for synthesizing porous polymer composite. It involves solidifying a two-phase mixture of porogen and polymer, then removing porogen to create pores within the continuous emulsion phase by polymerization or curing. The surfactant plays a pivotal role in accomplishing a stable emulsion, a key factor in designing the internal porous structure. This study highlights the effect of silica filler and mixing time on pore morphology, i.e., shape, size, and distribution. on polydimethylsiloxane (PDMS) porous structure utilizing the water-in-oil emulsion templating method. Span® 80 is used as a surfactant to reduce the surface tension between water, silica, and PDMS and simultaneously create a strong foaming effect. Different weight concentrations of silica (1-10 wt%) were chosen while keeping the internal phase, i.e., water (50 wt%) constant. The designed porous structures were further characterized through scanning electron microscopy (SEM). Porous composite specimens fabricated with higher silica content and mixing time consistently exhibit smaller pore sizes than specimens fabricated with lower mixing time and silica content. A breakthrough of pore morphology is seen at silica content higher than 5wt% at 1 min mixing, however, pore morphology drastically changes when mixing time increases from 1 min to 6 min. Variation of finer mixing time beyond 1 min shows stepwise changes in pore morphology from a large single-phase porous structure to a bi-modal porous structure which eventually become a smaller single-mode porous structure. Thus, the emulsion templating technique, in combination with different filler content and mixing time, will effectively aid in designing engineered porous polymer composite with varying stiffness and pore morphology.