bluuebunny/crossref_metadata_2025 huggingface dataset:
Βελτιστοποίηση της μικροδομής, μηχανικών ιδιοτήτων και υψηλής διαμορφωσιμότητας μέσω νανοκατακρήμνισης σε προηγμένους χάλυβες υψηλής αντοχής
Authors: Καλδέλλης Άγγελος
Published: November 2023

Η παρούσα διδακτορική διατριβή στοχεύει στην πειραματική διερεύνηση των χαρακτηριστικών της μικροδομής (π.χ. φύτρωση και ανάπτυξη, μορφολογία, κλάσμα όγκου και διασπορά) και την επίδραση τους στις μηχανικές ιδιότητες και στη διαμορφωσιμότητα, χαρακτηριστικά τα οποία είναι άρρηκτα συνδεδεμένα με τον κατάλληλο κραματικό σχεδιασμό και τη βελτιστοποίηση των σύγχρονων κατεργασιών παραγωγής σε μοντέρνους μονοφασικούς χάλυβες σκληρυνόμενους από φαινόμενα νανοκατακρήμνισης σύνθετων καρβιδίων (MxCy), νιτριδίων (MxNy) και καρβονιτριδίων (Mx(Cy,N1-y)), αποτελούμενοι από όλκιμη φερριτική μητρική φάση. Συγκεκριμένα, η έρευνα εστιάζει στις ποικίλες και σύνθετες νανοκατακρημνίσεις, οι οποίες δεν έχουν πλήρως ταυτοποιηθεί και εξηγηθεί από τη διεθνή βιβλιογραφία. Αυτό επιτυγχάνεται μέσω του χαρακτηρισμού και της αξιολόγησης της μικροδομής, της νανοδομής και της μηχανικής απόκρισης των αρχικών δοκιμίων σε κατάσταση ψυχρής έλασης και των τελικών παραγόμενων δοκιμίων έπειτα από θερμικές κατεργασίες ανόπτησης, μέσω κατάλληλων και σύγχρονων πειραματικών μεθόδων και τεχνικών. Ο χαρακτηρισμός της μικροδομής και της νανοδομής πραγματοποιείται μέσω Οπτικής Μικροσκοπίας (LOM), Ηλεκτρονικής Μικροσκοπίας Σάρωσης (SEM), Ηλεκτρονική Μικροσκοπίας Διερχόμενης Δέσμης και Σάρωσης (ΤΕΜ/STEM), σε συνδυασμό με τις μεθόδους Φασματομετρίας Ακτίνων Χ Ενεργειακής Διασποράς (EDS) και Ενεργειακής Απώλειας Ηλεκτρονίων (EELS), όπως επίσης με την Περίθλαση Ακτίνων Χ (XRD). Ο καθορισμός των μηχανικών ιδιοτήτων επιτυγχάνεται μέσω δοκιμών εφελκυσμού και σκληρομετρήσεων. Η διαμορφωσιμότητα διερευνάται μέσω δοκιμών Ικανότητας Διαμόρφωσης Οπής (HET). Η πειραματική προσέγγιση έχει σχεδιαστεί βασισμένη στη εξέταση των μελετώμενων υλικών σε μάκρο, μίκρο και νάνο-κλίμακα, με σκοπό την εξήγηση των φαινομένων της μικροδομής και της νανοδομής τους, όπως επίσης την αξιολόγηση των ιδιοτήτων και της απόδοσής τους. Αξιολογώντας τα αποτελέσματα της διατριβής, τα συμπεράσματα προσπαθούν να ερμηνεύσουν τα φαινόμενα και τους μηχανισμούς των νανοκατακρημνίσεων και την επίδρασή τους στη μικροδομή, όπως επίσης να ορίσουν τη βέλτιστη συνεισφορά των κατάλληλων κραματικών στοιχείων, των διεργασιών παραγωγής και των παραμέτρων τους στους μελετώμενους προηγμένους μονοφασικούς χάλυβες, οι οποίοι αποτελούν σημαντική εξέλιξη στον κλάδο των μοντέρνων βιομηχανικών μεταλλικών υλικών και στην οδό της βιώσιμης ανάπτυξης.
Selected Problems of Assessment in Teaching Philosophy
Authors: Ambrozy Marián
Published: November 2023

The present paper discusses selected issues of evaluation in the teaching of philosophy. It deals with the issue under consideration on a general level since we do not differentiate between high school and college study performance assessment. Critical reflection focuses on the proper mode of evaluation in teaching the history of philosophy as and in the disciplines of systematic philosophy. In doing so, the close interrelation between the history of philosophy and its disciplines is considered. Three basic evaluative approaches are distinguished. These vary depending on whether the assessment is of the history of philosophy, logic, or other philosophical disciplines. The question of whether a philosopher or a philosophy teacher not directly involved in discussing philosophical issues is better as an evaluator is also investigated. Furthermore, the paper also articulates the reasons for preferring the first option.
Is It Possible Not to Teach Philosophy?
Authors: Černín David
Published: November 2023

Knowledge of philosophy is commonly imparted in various philosophy courses, and the teaching process may take multiple forms, ranging from a historical overview to interdisciplinary approaches. However, the theoreticians of education across different subjects and disciplines often turn to philosophy, and they adopt some philosophical assumptions about their field to set up warranted goals and methods of teaching, delineate subject matter, and identify skills they hope to impart. Due to this process involving lengthy theoretical discussions and intellectual exchanges, teachers pass on specific philosophical attitudes even if they are not explicitly teaching philosophy. This paper examines an intersection between history education and philosophy in particular. The focus is on philosophical theories of history that history educators directly address. The paper navigates a specific roadmap portraying the development of the philosophical approaches to the knowledge of the past, discusses diverging traditions, and shows their potential contribution to reaching educational goals.
OVERALL PROPERTIES OF POLYMER MATRIX COMPOSITES AT HIGH TEMPERATURES: MODELING AND APPLICATION TO THE LIGHTNING STRIKE PROBLEM
Authors: KONDURI TEJA G. K. , ZHUPANSKA OLESYA I.
Published: November 2023

In this work, a micromechanics-based model for determining high-temperature thermophysical properties of carbon fiber reinforced polymer (CFRP) composites undergoing pyrolysis is presented. The model accounts for the mass loss and material phase changes. Material phase changes include emergence of the secondary char and gas phases in the polymer matrix. First-order Arrhenius kinetics is used to model polymer pyrolysis. The model also accounts for the temperatureand heating rate-dependent volume fractions of the polymer, char, and pyrolysis gas phases. Temperature- and heating rate-dependent representative volume elements (RVEs) of evolving microstructures were generated and numerical homogenization was performed to determine overall thermophysical properties of the CFRP composites. The micromechanics-based models were embedded into the finite element analysis (FEA)-based multiphysics modeling of lightning strike damage in CFRP composites. Computational studies were performed to analyze lightning strike induced thermal damage in a CFRP composite.
STRESS ANALYSIS OF FGM-MATRIX MEDIUM
Authors: KRISHNA NILABH , NOMURA SEIICHI
Published: November 2023

This study presents the derivation of the three-dimensional elastic stress field is derived for an elastic medium that contains a spherical inclusion composed of a functionally graded material (FGM). The material properties for the matrix phase are uniform, while those for the FGM inclusion are assumed to vary linearly. The FGM-matrix medium is subject to constant strain at the far fields. First, the elastic field subject to uniform hydrostatic strains at the far-field is analytically solved. Subsequently, the elastic field subject to uniform deviatoric strains at the far-field is sought and is solved semi-analytically by converting the Navier equation for the displacement into a set of two ordinary differential equations which can be solved using the method of weighted residuals. Numerical examples show that the stress distribution is continuous at the FGM-matrix interface.
PROGRESSIVE FAILURE ANALYSIS OF FIBER STEERING COMPOSITE STRUCTURES WITH A HOLE
Authors: MIZUMOTO MASATAKA , ASAKAWA KENJI , HIRANO YOSHIYASU , OGASAWARA TOSHIO
Published: November 2023

The application of fiber steering to composite structures is expected to reduce the stress concentration around a hole and to improve overall strength. The objective of this study is to establish an optimal design method for fiber steering composite structures with an open hole, with the consideration of damage progression. In this study, fiber steering is applied to 0° layers of a panel with an open hole under uniaxial tensile load by aligning the fiber steering direction to the maximum principal stress direction. Parametric studies of 1500 panel models with different hole geometries are performed by finite element analysis for both quasi-isotropic models and fiber steered models. The effect of the geometry change of the hole on failure load of the composite structure is investigated with progressive failure analysis based on Hashin’s failure criteria. In addition, the effect of hole geometry on performance of the fiber steering is also investigated. In most models, two stages of damage progression are observed. Result of the parametric study shows that failure load has increased with decreasing curvature of the hole. The smaller curvature of the hole around the point of origin of the failure reduces stress concentration and increases the maximum load. When fiber steering is applied, models with high strength are less sensitive to fiber steering. This is because the curvature of the hole around the stress concentration area is small, and the fiber orientation is straight. It is suggested that the curvature of the hole may influence the strength enhancement effect of fiber steering.
A STUDY ON THERMAL CHARACTERIZATION OF HONEYCOMB SANDWICH PANELS WITH GFRP FACESHEETS
Authors: RAVICHANDRAN SRINATH , HOJJATI MEHDI
Published: November 2023

The thermal characterization of honeycomb and foam core sandwich panels is studied by both experimental (guarded hot plate apparatus, and transient laser flash technique), and theoretical (Swann Pittman model, and composite theory) means to analyze and modify their thermophysical properties. Then, the fabric’s structural attributes and their effects on the thermal conductivity of the GFRP facesheets were studied by metallographic techniques, polynomial curve fitting of the longitudinal fibers, Procrustes analysis and subsequent modelling in TexGen (open-source software) and ABAQUS/CAE 2022, Dassault Syst??̀mes Simulia Corp. Initially, the sandwich panels were dismantled into their constituent parts i.e., facesheets and cores and then the graphite & gold coated samples were subjected to transient laser flash thermal analysis technique and guarded hot plate apparatus per the ASTM E1461, ASTM C177 and ASTM C1058 standards. Though thermal diffusivity is not applicable to heterogeneous GFRPs, good accordance between analytical solution for homogeneous material and experimental results for composites allowed us to calculate the “effective thermal diffusivity”. These results were then utilized in the composite theory to compute the overall thermal conductivity of the sandwich panels. Moreover, with regards to other theoretical approaches, the thermal emissivity values of the facesheet and core were measured by emissivity compensation in FLIR thermal camera. Later, the epoxy mounted metallographic samples were subjected to dark field, bright field, and polarized microscopy to extract various attributes of the fibers that are obscured in all the techniques but one. For instance, to enhance the contrast between the glass fibers and the polypropylene matrix dark field microscopy was utilized but this technique is not of much help in studying the fibers from an angle orthogonal to the weave plane of the fabric; polarized light technique had to be used.
DYNAMIC CRUSH PERFORMANCE OF THERMOPLASTIC AND RESIN INFUSED THERMOSET TRIAXIALLY BRAIDED COMPOSITE STRUCTURES
Authors: PEREIR J. MICHAEL , STROHMINGER BRANDON , PINI NICCOLO , REVILOCK DUANE , RUGGERI CHARLES , MILLER SANDI
Published: November 2023

Dynamic crush tests were conducted on triaxially braided composite tubes and Cchannel structural elements to measure their relative energy absorbing characteristics. The elements were manufactured from triaxially braided [0/+60/-60] T700 fiber preforms with two different resin systems, a PR520 thermoset resin and a LM-PAEK thermoplastic resin. T700S/PR520 composite tubes and C-channel elements were manufactured using a resin infusion process. T700GC/LMPAEK composite tubes were manufactured by braiding slit tape prepreg onto cylindrical mandrels. Consolidation pressure was applied through thermally activated shrink tape in an oven. The energy absorbing characteristics of the composite elements were measured using a pneumatically actuated sled with a flat-faced impacting mass, which provided several independent measurements of force, displacement, and energy absorption. The parameter primarily used for comparison between the systems was the specific energy absorption (SEA), a measure of energy absorbed per unit crushed mass. The T700GC/LMPAEK tubes performed slightly better than T700S/PR520 tubes, with an approximately 8% higher SEA. T700S/PR520 tubes had over 20% higher SEA than Cchannels made from the same material. Additional tests were conducted on tubes using a tapered-conical impactor to force the tubes to fail in circumferential tension. This type of impact significantly reduced the energy absorption.
NANO-POROUS NETWORKS ENABLED OUT-OF-AUTOCLAVE CURING OF CARBON FIBER REINFORCED PLASTIC AEROSPACE COMPOSITES
Authors: FURTADO CAROLINA , KALFON-COHEN ESTELLE , LEE JEONYOON , HANK TRAVIS J. , LI CARINA XIAOCHEN , . KESSLER SETH S. , WARDLE BRIAN L.
Published: November 2023

Large-scale aerospace composite parts utilizing autoclave-required carbon fiber reinforced plastic (CFRP) epoxy prepreg were manufactured with a novel Nano- Porous Network (NPN) enabled and CNT-heater based out-of-oven (OoO) for a combined out-of-autoclave (OoA) approach. While prior work has established open porosity carbon nanotube films as effective NPN materials, here we utilize electrospun polymer nanofiber veils to provide capillary pressure in the inerlaminar region during curing, thus enabling void elimination without requiring an autoclave. A CNT-based surface heater was used to cure a 0.6 m  0.6 m plate. Utilizing the fast rate of the CNT heaters, an accelerated cure cycle was developed to shorten the cure cycle by 35% compared to the manufacturer-recommended cure cycle (MRCC). X-ray microcomputed tomography (μCT) inspections showed that the cured parts (autoclave baseline and combined OoA approach) were void-free. The mechanical tests (shortbeam shear, tension, and compression) revealed that the accelerated-cure combined OoA approach composites were equivalent to the autoclave cured parts. Having attributes of reducing capital cost and increasing manufacturing efficiency intrinsic to OoA, this scaled fabrication method is a facile alternative to conventional autoclave fabrication, and future work will study a broader array of mechanical testing.
RAPID MANUFACTURE OF THIN COMPOSITES VIA INFRARED-ASSISTED FRONTAL POLYMERIZATION
Authors: NASERI IMAN , MASOUMIPOUR ALIREZA , YOURDKHANI MOSTAFA
Published: November 2023

State-of-the-art technologies for manufacture of fiber-reinforced polymer composites (FRPCs) rely on bulk polymerization of matrix thermoset resins at elevated temperatures for several hours (up to 24 h) in an autoclave or oven. This process is energy- and time-intensive, generates a significant carbon footprint, and requires expensive equipment. In recent years, frontal polymerization (FP) has emerged as an alternative approach for rapid and energy-efficient curing of FRPCs. However, widespread and practical adoption of this curing technique at industrial scale is limited by the low front velocities (<10 cm min-1), the need for using highly thermally insulating tooling materials (e.g., polymer foams), and low degree of cure of resin in produced composites. In addition, frontal curing of thin composite laminates is quite challenging even on highly thermally insulating tooling materials due to the high rate of heat loss through boundaries compared to the generated heat, resulting in quenching of reaction and incomplete cure. Here, we present a new approach for rapid curing (~50 cm min-1) of FRPC panels via FP by supporting the underlying thermal curing reaction using a moving infrared heating source. In our approach, following the infusion of a carbon fiber composite layup by an FP resin system, an infrared heating source is linearly moved along the length of the layup to not only initiate the FP reaction via photothermal energy conversion effect but also locally pre-heat the resin and increase its frontal reactivity and front velocity. We successfully cure a 5 Å~ 15 cm2 composite panel on a glass tool plate within 24 s with a total energy consumption of 8.3 kJ as a proof of concept. Differential scanning calorimetry measurements are also conducted on produced laminates to determine the degree of cure of produced FRPCs. Finally, a thin laminate, consisting of a single ply of carbon fabric is fabricated as a proof-of-concept to further demonstrate the capability of our technique for rapid manufacturing of thin composites, which is promising for use in space and aerospace applications.
A PERFORMANCE COMPARISON OF LOW-COST NIR NANO TO NIR MICROPHAZIR FOR POLYMER COMPOSITE CHARACTERIZATION
Authors: OLUWAJIRE OLUWATIMILEHIN , BERKOWITZ KATHERINE , GRACE LANDON
Published: November 2023

Polymer composites are increasingly adopted to complement or replace their metal counterparts used in aerospace, wind turbine, automotive, and other safety-critical structures. However, these structures are susceptible to the development of undetected sub-surface damage, such as low velocity impact damage (LVI), that limits their performance and utility in certain cases. The ability to detect and monitor the progression of this type of damage from the micron level prior to growth to the macroscale is critical to ensuring the long-term safety and reliability of these structures. Near Infrared Spectroscopy (NIRS) is a promising technique for detecting LVI damage because it can probe the internal structure of the material and identify subtle changes in its properties. This study compared the performance of a low-cost NIR Nano (a Texas Instruments NIR Nano evaluation model) to a commercial NIR Microphazir instrument for the detection of LVI damage in glass fiber reinforced polymer composites. We evaluated the two instruments for their ability to detect and measure different levels of impact damage at increasing moisture absorbed by weight. Forty-eight fiberglass laminates consisting of e-glass and epoxy were subjected to either 0J (no damage), 1J, 1.5J, and 2J impact energy using a drop tower outfitted with a 9mm radius hemispherical striker tip impactor. Spectral scans were collected between wavelengths of 900-1700 nm (NIR Nano) and 1600-2400 nm (Microphazir) for all samples prior to moisture absorption. Following moisture contamination, spectral scans were taken at regular intervals of gravimetric moisture gain from 0.05% to 0.15% by weight. Multivariate data analysis methods were used to assess the spatial variation of the absorbance parameter at various amounts of absorbed moisture. The results and discussion emphasize the importance of rigorous calibration and technique selection for reliable and accurate NIR spectroscopy investigation in polymer composites, especially in situations where mobility,
PROGRESSIVE DAMAGE AND FAILURE ANALYSIS OF THERMOPLASTIC COMPOSITES IN LOW VELOCITY IMPACT USING MAT299
Authors: CUTTING REBECCA , JACKSON WADE , RAO PRABHAKAR , JUSTUSSON BRIAN
Published: November 2023

As part of NASA’s Hi-Rate Composite Aircraft Manufacturing (HiCAM) Project, state-of-the-art progressive damage and failure analysis (PDFA) tools developed for use with thermosets are being evaluated for use in modeling alternative material systems, like thermoplastics. This work presents experimental low-velocity impact data of a thermoplastic material system, AS4D/PEKK-FC, including characterization of the impact damage mechanisms as well as associated load and displacement data from testing. Following the presentation of experimental results, two simulation approaches using the PDFA tool MAT299 in the commercial off-the-shelf finite element software LS-DYNA are employed to predict damage area and forcedisplacement responses of thermoplastic panels subjected to various impact energies. The first modeling method uses solid elements with a high-density mesh and a ply-byply modeling approach similar to previously published work for thermosets. This method has the capability of capturing individual crack development, progression, and delamination on a per ply basis. The second modeling method uses TSHELL elements that have in-plane dimensions that are an order of magnitude larger than the elements used in the solid element approach and reduces the number of elements through the thickness of the laminate. This produces a lower-fidelity model with reduced run times that is incapable of monitoring every delamination plane and damage within each ply. Results from both simulation methods are compared to experiment, and limitations of the methods are discussed.
FASTDM4C: A FAST AND EFFICIENT DISCRETE MODEL FOR COMPOSITES
Authors: DELEO MANTONIO A. , PHENISEE EAN E. , PELESSONE DANIELE , FLORES MARK , SALVIATO MARCO
Published: November 2023

The adoption of composite materials in the aerospace industry has enabled the achievement of structural performance levels and weight savings unimaginable even just twenty years ago. Yet only a fraction of the true potential of these materials has been expressed to date due to lack of high-fidelity models which has resulted in the adoption of extremely conservative designs compared to metallic counterparts. In part this is because, in contrast to high performance metallic alloys, the damaging and fracturing behaviors of composites are way more complex and difficult to simulate. Fiber reinforced composites feature many interacting mechanisms spanning several material and structural length scales, from the fiber scale of a few microns to the structural scale of a composite wing with a span of several meters. This makes the development of computational models for the design and optimization of composite structures extremely challenging due to the conflicting need of being able to capture microdamage events at the fiber scale while still being efficient enough to simulate structures that are at least six orders of magnitude larger. This work attempts to address this challenging problem by formulating a novel discrete, sub-lamina-scale model aimed at providing an effective description of damage at the microscale while maintaining computational costs comparable to continuum, homogenized formulations. In this new approach, composites are simulated as an assembly of Representative Unit Cells of roughly the same dimensions of the average distance between splitting cracks and their arrangements are designed to replicate the orthotropic behavior of the lamina. One distinct regularized strain-softening constitutive law is utilized to describe the behavior of the fibers using a new element called Discrete Fiber Model (DFM).
EFFECT OF CRACK LENGTH ON FRACTURE TOUGHNESS IN EPOXY POLYMERS USING ATOMISTIC-CONTINUUM COUPLING OF MD WITH FEM
Authors: ADITYA SANKHA SUBHRA , SOHAIL TANVIR , ROY SAMIT
Published: November 2023

Developing fundamental methodologies for predicting material properties, such as strength and toughness, is one of the core objectives of Integrated Computational Materials Engineering (ICME). For this purpose, a generalized framework for anchor point based concurrent coupling of FEM and MD domains, encompassing previous related methods, is presented in this paper. The motivation for this work comes from the fact that the crack length and fracture process zone size required to simulate brittle fracture in certain materials, such as epoxy polymers, often exceed the computational size limit needed for pure MD simulation. The proposed concurrent coupling framework is robust and is agnostic of material crystallinity and atomistic description. Two distinct forms of the method are discussed in detail, both of which are able to make use of specialized MD solvers such as LAMMPS with little or no modification. One of the two forms has the added advantage of being able to couple to specialized FEM solvers as well, such as ABAQUS. The two methods are tested against each other in a uniaxial tension test using a graphene monolayer at 300 K temperature and a block of thermosetting polymer EPON862 at 1 K temperature, showing comparable results. Convergence behavior of the two coupling methods are studied. The methods are then applied to study the fracture of a center-cracked graphene monolayer and compared with results from an identical pure MD simulation, using the atomistic Jintegral. Their good agreement corroborates the effectiveness of the developed method and potential use as a plug-and-play tool to couple commercial FEM and MD solvers. Future work will entail applying these methods to simulate elevated-temperature amorphous polymer models and their brittle fracture, and the effect of crack length and process zone size.
EFFECT OF LAYER MATERIAL PROPERTIES ON THE RADIAL STRESS AT THE INTERFACE IN HYBRID TUBES MADE OF MANY LAYERS
Authors: RASTAK MOHAMMADALI , HOA SUONG V.
Published: November 2023

Composite cylinders possess characteristics not seen in other types of structures. First they are light weight as compared to metallic cylinders due to the light weight of composite materials. Secondly they do not have free edges (along the circumferential direction) which prevents delamination along free edges like that in composite plates or panels. Thirdly, they can be manufactured using automated fiber placement which provides good repeatability and high speed of production. Fourthly, they are corrosion resistant which makes them applicable for making pipes, or undersea transmission cables for electrical lines. Due to these characteristics, composite cylinders have found applications in pipes, and potentially as cross piece for landing gears for helicopters. Nonetheless, when subjected to bending loads, composite cylinders exhibit failure on the compression side. The failure is due to buckling of the material. The buckling seems to be caused first by delamination, which in turn depends on the radial stress at the interface between the layers. This paper examines cylinders made of two different isotropic layers. The effect of different elastic moduli and Poisson ratios on the development of the radial stresses throughout the thickness, particularly at the interface of the layers, is examined.
MULTISCALE PREDICTION OF YARN PULLOUT FAILURE MODE IN UNREINFORCED TEXTILE FABRICS
Authors: HEARLEY BRANDON L. , PINEDA EVAN J. , BEDNARCYK, BRETT A. , MURMAN SCOTT M. , PANKOW MARK
Published: November 2023

Unreinforced woven fabrics have been implemented in a variety of high performance applications, including body armor, deployable structures, and as the reinforcement material in composites. Multiscale modeling techniques have significantly improved the capabilities of simulation-based tools to capture fabric mechanics efficiently and accurately, but often lack in their prediction of failure and require pairing with finite element analysis (FEA) software, limiting their application to the design of ‘fit-for-purpose’ materials. NASA’s Multiscale Analysis Tool (NASMAT) is a standalone multiscale program that has been traditionally used in the analysis of reinforced composites materials. More recently, it has been amended to simulate unreinforced fabric behavior by allowing the geometric state of the tows to change with applied loading due to the lack of a reinforcement material, such as the matrix seen in composites. Previous work has shown the ability of NASMAT to capture nonlinear macroscale behavior by predicting geometric changes in the state of each subcell as a function of the applied loading and allowing each subcell in the analysis to rotate according to these predicted changes, as well as predict nonlinear behavior due to the fiber breakage failure mode. In this work, the capability of predicting the onset and propagation of failure in plain woven fabrics in NASMAT is presented for the yarn pullout failure mode, which occurs when a fabric is loaded at an off-axis angle relative to the warp of weft tow direction. Yarn pullout behavior is initiated by determining the applied load in which the shear resistance of the contact area between yarn families is overcome. When failure is initiated, yarn pullout is determined to have occurred when the applied displacement, calculated from global strain, exceeds the deformed position of a given contact points between yarn families, determined from pin-joint kinematics. Contact points where pullout has occurred contribute to a global damage parameter used to modify the homogenized stiffness of the fabric, resulting in nonlinear behavior observed at the macroscale. The off-axis loading behavior and yarn pullout failure theory have been developed and implemented into NASMAT such that users can simulate off-axis tensile behavior of fabrics in a single, standalone multiscale tool. Simulations are compared to uniaxial tensile tests at various off-axis angles to demonstrate the capability of the tool in its prediction of both the onset of failure at each off-axis angles and the stress-strain behavior as failure progresses.
EFFICIENT RESIDUAL STRESS MODELING FOR LARGE- FORMAT POLYMER COMPOSITE EXTRUSION-BASED ADDITIVE MANUFACTURING
Authors: BHANDARI SUNIL , LOPEZ-ANIDO ROBERTO A.
Published: November 2023

Residual stresses are generated in thermoplastic composite parts produced using extrusion-based additive manufacturing (AM). The layer-by-layer deposition of molten thermoplastic composite material and the subsequent cooldown results in the differential thermal contraction of the deposited layers, giving rise to residual stress in the additively manufactured parts. These residual stresses depend on the material properties of the polymer composite and the processing parameters used during the AM process. The residual stresses affect the mechanical performance and the final printed shape of the manufactured part. Efficient numerical modeling methods can predict residual stresses for AM parts and hence help design geometrically accurate and mechanically reliable parts. However, large-scale AM introduces computational challenges for evaluating residual stresses due to the large number of degrees of freedom in the numerical models. This research work explores the use of an explicit thermal model and a mesh merging technique to expedite the numerical analysis of thermal history and residual stresses in large-scale additively manufactured thermoplastic composite parts. The goal of the research is to develop an efficient modeling method for aiding the design of AM polymer composite parts.
TUNING LOAD BEARING CAPACITIES OF FLEXIBLE HONEYCOMB WITH POISSON’S RATIO MODULATION
Authors: UDDIN KAZI ZAHIR , ANNI IBNAJ ANAMIKA , YOUSSEF GEORGE , KOOHBOR BEHRAD
Published: November 2023

Polymeric honeycomb structures are used in impact mitigation applications due to their high specific strain energy dissipation capacity at extended strains. The load bearing and energy absorption capacities of honeycomb lattices can be concurrently improved by tuning their Poisson's ratio. For instance, the inward-facing cavities in reentrant honeycomb geometries result in an auxetic behavior, effectively increasing the impact energy absorption performance. This study investigates the interrelationship between cell wall thickness, the Poisson effect, and load-bearing capacities of hexagonal and reentrant honeycomb lattices fabricated from thermoplastic polyurethane (TPU) by fused filament fabrication (FFF) additive manufacturing technique. The wall thickness of the samples is varied to investigate its effect on the mechanical performance of the honeycomb structures. The fabricated samples are characterized under quasi-static compression. The results indicate that the load-bearing responses of conventional and reentrant honeycomb structures improve with increasing wall thicknesses, while the energy absorption efficiency is inversely related to the cell wall thickness. Hexagonal honeycomb lattices exhibit superior stiffness and strength compared with their reentrant counterparts, whereas reentrant lattices are lighter and show a more compliant nature due to their more open geometry and inward-facing cavities. We observe that the reentrant lattices tend to fail due to shear deformation while honeycomb lattices fail by the bending and stretching of the cell walls. The findings suggest that the honeycomb lattice structure can be optimized by tuning the wall thickness based on the desired mechanical properties informed by a particular application.
ACCELERATED PYROLYSIS ANALYSIS OF THICK HIGH-TEMPERATURE COMPOSITE PARTS USING THEORY-GUIDED PROBABILISTIC MACHINE LEARNING AND FINITE ELEMENT ANALYSIS
Authors: PICAZO PAULINA PORTALES , ONG DERRICK , GRAY ALEXANDER , ZOBEIRY NAVID
Published: November 2023

Fabrication of advanced composites for high-temperature applications typically involves a complex, multi-step process. This process includes an initial lay-up step, curing, high-temperature pyrolysis to transform the cured resin into a carbonized amorphous structure, several resin-backfilling steps to fill voids and cracks formed during pyrolysis, additional pyrolysis-densification cycles to further increase the carbon content, and a final graphitization step to achieve the desired crystalline structure of carbon atoms. The processing parameters in each step, as well as the layup and overall thickness of the part, directly impact the kinetics of reactions, phase transformations, and the resulting end-part properties. During pyrolysis, the resin undergoes a complex system of degradation reactions affected by heat and mass transfer through the part thickness. To achieve optimal performance of hightemperature composites, it is crucial to accurately establish the relationship between processing conditions, part geometry, and end-part properties. Current process optimization relies heavily on time-consuming and resource-intensive testing campaigns to characterize pyrolysis kinetics, followed by trial-and-error efforts. This is further complicated by complex temperature cycles, gradients of pyrolysis rate across thick composite parts, uncertainties, and variabilities of the material and process. This paper introduces a novel framework combining theory-guided probabilistic machine learning (ML) and finite element (FE) process simulation to address these challenges. Limited targeted experiments are used to characterize pyrolysis kinetics using an in-house developed ML code while quantifying uncertainty at each testing step. The kinetics model is then used by an FE model to simulate heat transfer and rate of pyrolysis, allowing the gradient of degree of pyrolysis across the thickness of a laminate to be simulated. For validation, laminates of varying thicknesses were fabricated and pyrolyzed using different temperature cycles. Imaging techniques including light microscopy and scanning electron microscopy (SEM) were utilized to capture the resulting microstructures.
DAMAGE ASSESSMENT AND CLASSIFICATION OF GLASS FIBER-REINFORCED COMPOSITE MATERIALS DURING TENSILE TESTS BASED ON ACOUSTIC EMISSION AND UNSUPERVISED LEARNING APPROACHES
Authors: MORENO JORGE PALACIOS , NAZARIPOOR HADI , MERTINY PIERRE
Published: November 2023

Advancements in composite materials design have rendered glass fiber-reinforced polymer composites (GFRPC) an effective candidate for various engineering and industrial applications. Some attractive characteristics of GFRPC are low specific mass and high specific mechanical stiffness and strength. Identifying and classifying damage mechanisms is difficult for these types of materials. Several scientific works investigated the failure mechanisms of GFRPC considering different approaches, such as non-destructive techniques and various learning algorithms. However, studies to reliably identify mechanical failures in GFRPC are ongoing. Therefore, the present work investigates non-destructive testing (NDT) combined with unsupervised learning algorithms to identify damage mechanisms in GFRPC during tensile tests. The NDT used for this study is the acoustic emission (AE) technique that detects the waveforms of the different failure mechanisms during testing. Therefore, these waveforms are classified based on unsupervised learning algorithms such as principal component analysis (PCA) and self-organizing maps. The PCA method selects the most suitable AE descriptors to identify the different damage mechanisms. Thus, the self-organizing maps algorithm performs the clustering analysis and classification of the failure mechanisms. The obtained information from the AE descriptors is compared to AE data from the technical literature associated with specific damage mechanisms, matrix cracking, matrix/fiber debonding, delamination, and fiber breaking. Electron microscope images of the failures are presented to validate the results.
MOLECULAR DYNAMICS MODELING OF PEEK RESIN PROPERTIES FOR PROCESSING MODELING
Authors: KASHMARI KHATEREH , MAHMUD HASHIM AL , . PATIL SAGAR U. , PISANI, WILLIAM A. , DESHPANDE PRATHAMESH , MAIARU MARIANNA , ODEGARD GREGORY M.
Published: November 2023

thermoplastic composites. This study implemented molecular dynamics (MD) modeling to predict the thermo-mechanical properties and volumetric shrinkage of PolyEther Ether Ketone (PEEK) resin, a thermoplastic material, as a function of temperature and crystallinity content. The thermo-mechanical properties are linked to processing time and temperature via crystallization-kinetics models.The remarkable properties of Thermoplastic Matrix Composites (TMCs) including their ability to tolerate damage, low weight, high resistance to impact, and mechanical toughness, make them highly desirable for use in aerospace structures. TMCs undergo various cooling and heating cycles at elevated temperatures during the processing, which triggers the formation of crystalline regions resulting in matrix shrinkage and the generation of residual stresses in the final composite. To minimize the development cycle time and cost of future applications of TMCs, it is crucial to optimize materials manufacturing parameters, and component design. Manufacturing process of TMCs has a significant impact on their microstructure, which plays a critical role in controlling their in-service performance. Given that the thermo-mechanical properties of composite materials are influenced by phenomena at various length scales, a multiscale computational approach is necessary to predict manufacturing parameter-property relationships accurately and efficiently. Also, thermomechanical properties in semi-crystalline thermoplastics are closely linked to their crystallinity content, which is precisely regulated by both material properties such as melting and crystallization temperatures, and processing parameters like crystallization temperature and temperature holding time. Therefore, molecular-level material properties should be determined across a broad range of crystallinity levels to facilitate the multi-scale process modeling of
CROSS SCALE SIMULATION OF FIBER-REINFORCED COMPOSITES WITH UNCERTAINTY IN MACHINE LEARNING
Authors: YAO ZHENGTAO , HAWI PHILIPPE , AITHARAJU VENKAT , MAHISHI JAY , GHANEM ROGER
Published: November 2023

Cross scale simulation aims to develop computational models that can capture the behavior of a system at multiple scales, from microscopic to macroscopic level. To explore the constitutive relationship of materials, a widely used conceptual and numerical tool called Representative Volume Element (RVE) is introduced to solicit information from their subscales in finite element analysis. It characterizes the behavior at one quadrature point of a physical device by homogenizing the local behavior over finer scales. By replicating the RVE throughout the analysis domain, a homogeneous finite element model can be synthesized, simplifying the analysis of a heterogeneous material in the presence of uncertainty, such as variations in loading, material properties of micro-constituents, and modeling physical behavior. Moreover, the generation of many similar data at subscale introduced by homogenization in RVE is usually tackled by statistical methods. In this work, we explore the mechanical behavior of a hybrid composite material, consisting of glass and carbon fibers embedded in an epoxy resin for use as a battery enclosure, using a novel energy density informed neural network architecture. We model the material properties of fiber tows and resin as random variables and use input strain tensors from a macro-scale three-point bend test simulation. History of stress tensors and strain energy density are extracted as outputs for supervise learning. In the neural network architecture, Physical constraints are implemented by introducing a novel energy density layer for the prediction of stress along the direction of tows. A long-short-term-memory (LSTM) neural network training on the same dataset is also conducted for comparison. Elimination of spurious discontinuities is obtained by PCA (Principal Components Analysis) in the stress-strain curve when predicting stresses in directions transverse to the tows, YY and ZZ directions. We develop a user-defined subroutine in LS-Dyna and conduct a mesoscale simulation to verify its accuracy.
ADAPTATION OF THE MICROPLANE CONSTITUTIVE MODEL FOR BRITTLE-PLASTIC GLASSY POLYMERS
Authors: WARDHEKAR SANKET VINOD , SMITH GARRET , KIRANE KEDAR
Published: November 2023

This paper presents a microplane constitutive model for the tensile and compressive damaging behavior of brittle-plastic glassy polymers. The model is formulated and validated using experimental data on such polymers and is an adaptation of the previous microplane model formulation developed for concrete. The model considers a material point as a Gaussian approximation of a unit sphere, discretized into several “microplanes”. The macroscopic strain tensor is projected onto these microplanes yielding various micro-strain vectors. The various damage mechanisms such as tensile microcracking, shear microplasticity, and micro-crack friction are formulated in terms of the microplane level strain vectors, yielding corresponding stress vectors. These are then homogenized via the principle of virtual work yielding the macroscopic stress tensor. One of the salient features of the present adaptation includes a volumetric-deviatoric (V-D) split in the microplane level stresses and strains. This enables properly capturing a Poisson’s ratio greater than 0.25, which happens to be the case with most polymers. On the other hand, a normal stress comparison algorithm is implemented to alleviate the overestimation of stresses which can happen due to the V-D split. The proposed adaptation is validated using experimental data from uniaxial tension and compression tests on two different polymers, which includes post-peak inelastic behaviors such as hardening and softening or both. The model shows excellent agreement with the experimental data, demonstrating its ability and applicability to accurately capture the various damage mechanisms in brittle-plastic glassy polymers. The results presented here are at the material point level, and the application to structure level problems via a user material subroutine in a finite element software is in progress.
CHARACTERIZING THERMAL DEGRADATION IN SEMI-CRYSTALLINE THERMOPLASTIC COMPOSITES
Authors: WYNN MATHEW , CHEN KUAN-TING , ZOBEIRY NAVID
Published: November 2023

Semi-crystalline thermoplastic composites, such as carbon fiber-reinforced polyetheretherketone (PEEK) and polyetherketoneketone (PEKK), are processed and consolidated while melted at high temperatures. During cool-down, polymer chains fold into lamellar structures at the nanoscale to form crystalline morphology. These lamellar structures radiate from a nucleus, creating spherulitic structures in bulk polymers, and transcrystallinity in fiber reinforced polymers. A certain amount of thermal degradation occurs when the thermoplastic matrix is melted, and the amount of degradation is a factor of several parameters such as melting temperature, time at melt, and whether the material is processed in an inert environment such as nitrogen. One form of degradation that occurs in the matrix is cross-linking and oxidation. In this case, the polymer chain breaks and new bonds form between chains or within the chain. Moreover, thermal degradation affects crystallization, the lamellar thickness and spacing, and overall degree of crystallinity. The spacing between lamellar structures can be measured through Small Angle X-ray Scattering (SAXS) at nanoscale, while the degree of crystallinity can be found using Wide Angle X-ray Scattering (WAXS). To study thermal degradation, the effect of melting temperature, environmental condition, and reprocessing was investigated on samples of neat PEEK, as well as carbon-fiber PEKK prepreg. Additionally, these samples were thermally cycled multiple times, and repeats were performed for each condition. Degree of crystallinity, spacing, and lamellar thickness were measured using an x-ray scattering system. To study underlying physics and correlations, a probabilistic Machine Learning (ML) framework was used for regression. Using this approach, different thermal degradation mechanisms for neat resin and prepreg samples were demonstrated at the nanoscale. The differences were explained in terms of crystallinity and nucleation around fibers in prepreg. This framework provides a unique holistic understanding of crystal formation and degradation, which affects the reprocessability and end-part properties of semicrystalline thermoplastic composites.
OPTIMIZED CORE GEOMETRY FOR ENERGY ABSORPTION IN 3D-PRINTING SANDWICH PANELS: A COMPARATIVE STUDY
Authors: IRANMANESH NAZANIN , SARVESTANI DR. HAMIDREZA YAZDANI , ASHRAFI DR. BEHNAM , HOJJATI DR. MEHDI
Published: November 2023

The demand for lightweight and durable materials is continuously increasing, leading to the escalated utilization of sandwich structures across various engineering applications, including biomedical, automotive, renewable energy, aircraft, and transportation. This preference is driven by the unique mechanical properties of sandwich materials including superior energy absorption, reduced density, higher stiffness-to-weight, and strength-to-weight ratios, Consequently, this demand has driven the development of advanced composite materials with multifunctional performance. Sandwich panels encompass a wide range of designs, including Triply Periodic Minimal Surfaces (TPMS), commonly observed in diverse natural systems. This study aimed to assess the mechanical properties of sandwich structures with different core geometries through a combination of numerical simulation and experimental impact testing. Specifically, the investigation focused on a type of TPMS sheet structure and its comparison with a typical honeycomb structure to identify the most suitable core geometry for 3D-printed lightweight sandwich panels with high energy absorption capacity. The architected sandwich panels firstly were fabricated using the 3D printing technique and PLA filaments. A numerical simulation was performed to assess the response of sandwich structures when subjected to impact testing. Furthermore, the experimental impact test was carried out to estimate the mechanical behavior. Both experimental and numerical results clearly demonstrated the significant impact of meta-sandwich structure topology and geometrical parameters on the energy absorption capabilities of the panels.
IMPROVING UNIAXIAL DYNAMIC TENSION TESTING OF COMPOSITE SPECIMENS
Authors: GOMEZ R. , WHISLER D.
Published: November 2023

Validation of composite specimens on the order of a few millimeters in length in tension can be examined dynamically via uniaxial test rigs and split Hopkinson pressure bars. For compression testing, hydraulic/gravity drop towers and the Hopkinson bar are permissible for larger specimens (on the order of centimeters in length) and up to 1,000 1/s. Uniaxial tension test methods for composite specimens enabling both configurations, i.e., several centimeters in length and at strain rates up to 1,000 1/s, is much more limited. The goal of this research investigation is to test precisely in this regime by optimizing the composite coupon specimen for dynamic material characterization. For this project, a tension setup using a single Hopkins on bar was adapted to enable up to 6 cm long specimens tested at the requisite strain rates. Coupon specimens were then optimized using machine learning to promote stress in the gage region thereby minimizing testing failures if the specimen were to break near the grips. Using these two systems, we may have a proof of concept for high strain rate tension testing of composites that is optimized for validation studies using machine learning based geometries.