Dynamic Response and Failure of Composite Materials

Table of Contents

1. Frontmatter

2. Innovative Manufacturing and Inspection Technologies

   1. Frontmatter

   2. Mesh and Homogenization Effects of Simulated High Strain Rate Delamination in CFRP Using VCCT

      Nazanin Pournoori, Jarno Jokinen, Matti Isakov, Mikko Hokka, Mikko Kanerva

      Abstract

      This work uses three-dimensional (3D) continuum models on the explicit finite element (FE) basis to simulate interlaminar delamination of a carbon fiber reinforced plastics (CFRP) laminate at a high strain rate. The high strain rate experiments have been performed using the Split Hopkinson Pressure Bar (SHPB) system. The 3D FE simulations, including both CFRP and the entire experimental setup, were carried out by taking into account the highly dynamic nature of the tests. The delamination propagation was modelled using the Virtual Crack Closure Technique (VCCT). The modelling cases covered I) layer-by-layer models; and II) homogenized orthotropic laminate models. A comparison between the experimental and numerical results indicates a good accuracy of the laminate simulation with homogenization. The simulated data shows that the time step selection significantly affects the prediction of the rapid delamination onset and growth at high strain rates.

   3. Detection of Impact Damages on Full Scale Wing Using Distributed Fiber Optics Sensors Network

      M. Ciminello, L. Pellone, F. Romano

      Abstract

      The aviation industry aims to reduce costs in aircraft design, manufacturing, and maintenance. Structural Health Monitoring (SHM) is a key technique for monitoring the health of aircraft structures, providing valuable data throughout their operational life. This data can enhance design techniques, production control, real-time structural health evaluations, and maintenance plans, improving reliability and safety.

      This study examines the effectiveness of distributed fiber optic sensors in detecting impact damage on a full-scale composite wing. The sensors track changes in strain distribution caused by controlled impacts at predefined locations. A specific algorithm developed by CIRA identifies the position and size of the damage without relying on reference systems of the undamaged structure. Through structural tests under load, the SHM system’s accuracy in detecting damage was experimentally validated.

      Funded by the Clean Sky 2 Joint Undertaking under the EU Horizon 2020 program, this research advances SHM techniques for aeronautical applications and presents an alternative to traditional Non-Destructive Inspection (NDI) methods.

   4. Evolution of Unstable Skin-Stringer Debonding Propagation in Composite Aircraft Structures: Implications on Damage Tolerant Design

      Aniello Riccio, Mauro Zarrelli, Cinzia Toscano, Valeria Vinti, Luigi Trinchillo, Pierluigi Perugini, Rossana Castaldo, Angela Russo

      Abstract

      Interlaminar damages are a key challenge that limits the widespread use of composite materials in aircraft primary structural parts. Although years have already passed since the introduction of these materials, unstable propagation of delamination damage is still the base for evaluating the durability and reliability of structures made of composite laminates. In this paper, the unsteady and sudden propagation of skin-stringer debonding in a typical aircraft composite panel is investigated. Experimental tests and advanced finite element simulations have been performed to assess the evolution of debonding under compressive loading conditions. Fibre-reinforced composite panels, reinforced with a single T-shape stringer and characterized by artificial debonding at the interface between skin and stringer, have been experimentally tested and numerically analysed. The test output has revealed unstable growth of the debonding, with implications for the structural stability of the panels. The experimental results have been then compared with numerical simulations performed by using the Virtual Crack Closure Technique based SMart-time XB numerical procedure and excellent correlation have been found in terms of strain measurements against compressive load.

   5. A Benchmark Between Conventional and Custom Heat Treaments for Inconel 718 Alloy Processed Through Cold Metal Transfer Technology

      Andrea El Hassanin, Alessia Teresa Silvestri, Giuseppe Giaccio, Pino Panariello, Gaetano de Mase, Mario Marchetti, Giovanni Abete, Antonino Squillace

      Abstract

      Inconel 718, one of the most employed nickel-based superalloys, proved to be efficiently processable through Wire-Arc Additive Manufacturing (WAAM), including Cold Metal Transfer Technology (CMT). Nevertheless, a multi-step heat treatment is still necessary to control the microstructure and the mechanical properties of this complex alloy. This work aimed to compare the effects on microstructure, tensile properties, Vickers microhardness and chemical composition of two heat treatments performed on Inconel 718 parts produced through CMT, namely: i) the heat treatment used for conventionally processed Inconel 718 parts, defined by the AMS5662 and AMS5663 standards; ii) a custom heat treatment that reduces the number of steps included in the previous case. The experiments were performed on prismatic CMT-produced parts, subsequently machined to extract the test specimens, considering both the aforementioned heat treatment conditions as well as the as-deposited material. The results suggested that comparable results were achieved for the selected heat treatments, however, a complete recrystallization of the fine grains never occurred for all the investigated conditions, thus retaining the typical microstructural anisotropy deriving from the CMT process. Moreover, the heat-treated parts showed, in any case, comparable properties to those of the conventionally cast alloy, opening a new scenario in the context of industrial production of semi-finished parts with huge costs and time savings.

   6. Real-Time Shearography Measurements of 3D Printed Auxetic Specimens During Flexural Tests

      V. Pagliarulo, C. Saltarelli, M. Paturzo, I. Papa, F. Napolitano, P. Russo

      Abstract

      Auxetic structures with negative values of Poisson’s ratio attract attention due to their surprising deformation response, durability under long-term loading and energy absorption properties. In this work, different 3D-printed specimens with the internal auxetic structure were investigated in real time during flexural tests by means of shearography.

      The 3D printed specimens have been mechanically characterized by performing flexural tests according to the ASTM D790 standard in order to investigate the effect of the new manufacturing process on the flexural strength and modulus. The lateral face of the specimens was monitored in real time during the test. The preliminary results showed the points of greatest stress where the specimens starts to fail. This information can be useful to understand the behaviour of the investigated structures in order to improve geometries and printing processes.

   7. Analysis of the Crushing Behavior of Flat Composite Plates Produced by Sheet Molding Compound

      Maria Pia Falaschetti, Johan Birnie Hernández, Francesco Semprucci, Luca Raimondi, Davide Serradimigni, Enrico Troiani, Lorenzo Donati

      Abstract

      SMC composites, comprising chopped fiber bundles within a matrix material, offer the potential to create lightweight structures with high strength and stiffness, serving as viable substitutes for heavier metal components. The complex task of designing SMC composites with optimized performance characteristics is compounded by their inherently localized anisotropic behavior and the presence of various interacting forms of damage. Consequently, simulating the performance of SMC in crash-worthy applications necessitates a comprehensive characterization of the material’s mechanical properties through tailored experimental tests. In this study, an experimental study has been conducted on a commercial SMC material, with the primary objective of comprehensively comprehending its behavior and deriving specific parameters essential for numerical simulations. Employing ESI-VPS, the crashworthiness of flat components was simulated and successfully achieved a favorable correlation between the obtained results and the experimental data.

   8. Effect of Tightening Torque on Bearing Performance of Kevlar Fiber Composite Bolted Joints Produced by Additive Manufacturing

      M. Ravelli, L. Giorleo, I. Papa, A. Silvestri, R. Mascolo, A. Squillace

      Abstract

      Composite materials are common in industries like aerospace, automotive, construction, and wind energy. Understanding the behavior of composite bolted joints is crucial but challenging due to fiber anisotropy and stress concentration. Factors influencing joint efficiency include washer size, bolt torque, and hole-to-edge distance. Studies suggest that higher clamping pressure improves bearing strength and affects failure mechanisms in composites.

      Additive Manufacturing (AM), particularly using continuous fibers, has enabled new customized composites with enhanced mechanical properties. Fused Filament Fabrication (FFF) is a key method, but there is limited research on the bearing behavior of FFF composites and the role of geometric parameters.

      While joint failure modes are well studied, data on the effect of torque on bolted joints in additively manufactured composites is scarce. This study investigates the mechanical behavior of these joints under various torques, focusing on bearing behavior, strain fields, and deformation.

   9. General Damage Detections on Composite Panels Using Computer Vision Algorithms

      Salvatore Merola, Michele Guida, Francesco Marulo

      Abstract

      This study aims to present a novel approach for composite panel inspection to identify Barely Visible Impact Damage (BVID) and Visible Impact Damage (VID), using computer vision algorithms. A new methodology for visual inspections was developed using computer vision algorithms is based on the YOLO - You Only Look Once architecture about the aircraft composite components, that use digital optics and deep learning techniques to identify and classify damages. Computer vision algorithms are used as instruments for automating the process of surface inspection and damage detection, increasing productivity and safety for maintenance operators (in the case of inspection of remote areas of aircraft). An overview of the problems, methods, and recent developments in deep learning algorithms used for general damage detection is provided. Data for Convolutional Neural Network (CNN) training were collected using a high-quality acquisition system. The database was populated by collecting images with damages from many composite panels, previously damaged by different impactors. Each defect has been photographed under different lighting conditions (bright or dark), resolution, and lighting angles to accurately simulate the environmental conditions of an aircraft maintenance hangar.

3. Structural Integrity Assessment of Composite Structures

   1. Frontmatter

   2. Crashworthiness of C-Shaped CFRP Composites: A Numerical and Experimental Study

      Monica Capretti, Maria Rosaria Ricciardi, Ilaria Papa, Valentina Lopresto, Simonetta Boria, Vincenza Antonucci

      Abstract

      Carbon fiber-reinforced polymers (CFRPs) are widely used in transportation applications for structural components due to their outstanding lightweight and crashworthiness properties. Unlike metals, these composites fail through a combination of different failure mechanisms that contribute to the energy absorption capability of the final structure. In this study, the crash absorption capability of flat and C-shaped CFRPs, specifically carbon/epoxy composites, under dynamic conditions was investigated both experimentally and numerically. Initially, Charpy impact tests were conducted at different impact velocities using a three-point bending procedure. Subsequently, a finite element model was developed and simulated using LS-DYNA software. The numerical results closely replicated the outcomes of the physical experiments and exhibited a strong correlation with the experimental data, thereby validating the effectiveness of the designed model. Furthermore, the model successfully reproduced the observed damage mechanisms occurring during the physical tests, demonstrating its capability to accurately capture the composite behavior under the prescribed impact loading conditions.

   3. A Numerical-Experimental Study on the Impact Behaviour of Additive Manufactured bcc Lattice Structures

      Giovanni Maisto, Valerio Acanfora, Antonio Garofano, Aniello Riccio, Mauro Zarrelli, Andrea Alaimo, Davide Tumino, Giuseppe Catalanotti

      Abstract

      Several industries make an extensive use of sandwich structures in structural design, especially in the aerospace and automotive Engineering fields.

      Sandwich structures are characterised by two thin layers (skins) with a thick, but light, core in the middle, for increased stiffness to bending. The use of additive manufacturing allows us to adopt very light and geometrically complex cores based on the lattice structures concepts. Lattice structures performances are strongly affected by geometrical factors such as the array configuration, the unit cell shape or topology and the relative density.

      The research presented in this paper is aimed at adopting hybrid sandwich structures with lattice cores to develop structural engineering solutions characterised by high energy absorption properties and low weight for occupant protection in transportation.

      Actually, these kind of hybrid shock absorbers have been already demonstrated to be a good compromise between crashworthiness performances and weight efficiency.

      To improve the shock absorbers designs presented in [1], the additive manufacturing technology has been used to produce three different sandwich panels based on a Body Centred Cubic (BCC) unit cell type, such as a Waved Body Centred Cubic (WBCC) unit cell. The Direct Metal Laser Sintering (DMLS) technology has been adopted by using the EOS M290 3D Printer for metals.

      The shock absorbers’ configurations have been numerically designed by modifying the core unit cell topology starting from a linear shape to a curved shape with two different orientation.

   4. Development of a Numerical Methodology Able to Simulate the Unstable Mixed Mode Delamination Growth in Stiffened Composite Panels Under Cyclic Loading Conditions

      Aniello Riccio, Rossana Castaldo, Angela Russo

      Abstract

      Aeronautical structures, often, experience impacts with foreign objects in service and during maintenance operations. Foreign Objects Impacts (FOI) can lead to critical damages and can compromise the overall performances of structural components. Indeed, among the others, composite components can exhibit various interacting post impacts damage mechanisms, including fiber breakage, matrix fracture, and interlaminar damages, such as delamination between different layers of the laminates. Delamination represents the most critical failure mechanisms, as it is, often, undetectable by visual inspections and it may, unstably and silently, develop within the component. This Phenomenon may be amplified under cyclic loading conditions, as the residual strength and stiffness can decrease rapidly after a certain number of cycles, potentially leading to structural collapse [1]. Unstable propagation of delaminations is particularly critical, since it, actually, can take place without the need of increasing the load acting on the structure. This phenomenon, which is very dangerous for the structural integrity of components, can be very challenging to be predicted, under fatigue loading conditions, by the standard geometrically non-linear Finite Elements Methodologies (FEM) which use a sequence of simulations under force control to mimic the fatigue behaviour of composite materials. Actually, FEM simulations under controlled force levels lead to convergence issues when predicting the highly dynamic behaviour of the unstable growth of delaminations under fatigue loading conditions.

      The research activity, presented in this paper, is aimed to develop an alternative efficient methodology able to mimic the unstable delamination propagation under cyclic loading conditions in composite structures by non-linear static analyses. This new methodology has been demonstrated to be able to correctly consider the fast variation of delamination size associated to decrease in loading during the unstable growth phenomenon under cyclic loading conditions. To achieve this objective, the Paris Law [2, 3] approach has been implemented in the ANSYS FEM code together with an enhanced Virtual Crack Closure Technique (VCCT) based method.

   5. Numerical Investigation on the Static and Dynamic Behaviour of an Additive Manufactured UAV’s Horizontal Tail

      M. Battaglia, A. Riccio

      Abstract

      In recent years, the advancement of new manufacturing technologies has greatly influenced engineering applications. Among these, the additive manufacturing technology has been progressively applied to aircraft structures design and production, as it allows the effective optimization of the strength and the outstanding reduction of the weight in structural components. The research activity presented in this paper is focused on the static and dynamic numerical verification of the horizontal tail of an UAV, entirely Designed for Additive Manufacturing (DfAM) with Technopolymers. A proper selection of a variable internal infill has been introduced to reduce the overall weight of the structural component, while static and dynamic numerical analyses, simulating low-speed impacts, have been adopted to evaluate the structural strength and the ability of the proposed configuration to withstand damage onset and evolution due to in-service loads and impact events. The obtained numerical results indicate that the structural components Designed for Additive Manufacturing can be considered effective in terms of weight reduction and they are characterised by stresses significantly below the failure limits of the proposed techno Polymer. They also demonstrate good resistance of the proposed structural components to low-speed impacts without compromising the overall structural performance of components selves. This study provides solid bases for increasing the use of Additive Manufacturing technologies in the aviation industry.

   6. Low Velocity Impact (LVI) Simulation of a CFRP Ply-Drop Laminates: A Numerical Analysis

      Mario Russo, Evanthia Pappa, A. Baroni, S. Corvaglia, Valentina Lopresto, Ilaria Papa

      Abstract

      Carbon Fiber Reinforced Polymer (CFRP) composites are well established in aircraft, aerospace and automotive industry due to their lightweight structure and robustness. Generally, in uni-directional (UD) composite laminates the mechanical properties depend of fiber orientation, the number and the thickness of the plies and the stacking sequence [1], Fig. 1. In continuous fiber laminates the prediction of failure is dependent on the fiber orientation and the failure criteria that are adopted.

   7. Structural Sandwich Panels for Roofing Systems in Civil Applications

      Andrea Ferrarese, Mauro Corrado, Danilo Acquesta, Alessandro Scattina, Andrea Strino, Giovanniluca De Vita, Maurizio Italiano

      Abstract

      The use of composite and sandwich materials in civil engineering applications can be an efficient and sustainable alternative to the traditional materials. The design of a composite sandwich panel for applications in roofing systems such as the canopies on the platforms of train stations, made up of glass fiber reinforced polymer (GFRP) skins and a PET foam core, is herein presented, as a case study. The main advantages compared to traditional steel/concrete systems are: a high strength-to-weight ratio, durability, lightness, limited interference on railway traffic thanks to reduced installation times, factory production which reduces dangerous activities carried out on construction sites and the sustainability provided by the possibility of using recycled materials.

      The design approach, developed on the recommendations provided by UNI CEN/TS 19101, an extensive experimental activity carried out on specimens and full-scale elements to characterize the mechanical and physical properties and the durability of the materials, and numerical models having different level of details developed to assess internal stressed and deformations, are critically presented and discussed. From the results, it emerges that the geometrical characteristics, i.e. the thickness of the panel, are mainly defined by the fulfilment of the limitations imposed to deformation in operational conditions, while the selection of the materials and, therefore, their mechanical and physical properties, are mainly determined by durability and fire reaction requirements.

   8. Comparison of Various Processing Treatments on Uniaxial Compression Performance of Large Scale Additively Manufactured Thermoplastic ABS Chopped Fiber Composite Structures

      Alfonso A. Perez, Kenan H. Sehnawi, Arjun S. Chandar, David E. Hardt

      Abstract

      The structures presented in this paper demonstrate that large (8′ × 2′ × 1′), lightweight, and strong structures are indeed manufacturable through industrial large scale polymer additive manufacturing. The production experiments done to manufacture these structures resulted in process defects that were identified and documented. Processing failures during manufacturing prompted the modification of several process control parameters (layer height, material choice, bed temperature) and post-processing techniques (annealing). This yielded five specimens, each with a unique treatment modification combination.

      This paper describes an experimental setup for analyzing the effect of these various processing treatments on large-scale additively manufactured thermoplastic structures. Specifically, it presents and compares the uniaxial compression performance results of ABS chopped fiber composite structures made with different processing treatments.

      Force-displacement experiments were conducted to determine and compare the effects of such processing modifications on the uniaxial compression performance of the different specimens. The data are also analyzed for transient behavior related to plastic deformation and creep. These data indicate that the large additively manufactured polymer composite structural specimens are stiff and strong in bulk. However, the authors do not draw significant conclusions about the effects of the individual processing treatments themselves. The experimental setup and process of identifying and characterizing significant performance effects of manufacturing modifications are generally useful and applicable to other large scale additively manufactured structural products.

   9. Layer Height Treatments on Uniaxial Compression Performance of Industrial Large-Scale Additively Manufactured Polymer Composite Structures

      Alfonso A. Perez, Pablo Arroyo, Kenan H. Sehnawi, Arjun S. Chandar, David E. Hardt

      Abstract

      The prefabricated pile footing structures presented in this paper demonstrate that large (3’ × 2’ × 1’), mostly hollow structures are indeed manufacturable through industrial large scale polymer additive manufacturing (ILSPAM). Additionally, this work identifies manufacturing process defects which resulted from the production of such structures. Processing failures during manufacturing prompted the modification of process control parameters (layer height, material choice). This yielded four specimens, each with a unique treatment modification combination.

      This paper utilizes an experimental setup [1] for analyzing the effect of various processing treatments on large-scale additively manufactured thermoplastic structures. Specifically, it presents and compares the uniaxial compression results of layer height and material treatments on pile footings made with virgin and recycled glass fiber reinforced PETg (PETg-GF) using cyclic loading experiments.

      Force-displacement experiments were conducted to determine and compare the effects of such modifications on the uniaxial compression performance of the different specimens. The authors note that the data supports the hypothesis that increasing layer height will decrease the stiffness of additively manufactured structures. Furthermore, the cyclic uniaxial compression experiments conducted on the pile footings validate the applicability of the related experimental setup for multiple large-scale additively manufactured structural products. Finally, the authors recommend further experiments and analysis to determine the effect of material choice, as well as experiments with smaller specimens to address the limitations of measuring large-scale structures.

   10. Homogenization Techniques for Nanocomposites: A Comprehensive Review

       Davide Angelini, Enrico Cestino, Paolo Piana, Fabio Mallamo

       Abstract

       The study analyzes the development of analytical models for the reinforcement of structures using nanomaterials. Initially, it introduces the “Surface stress” phenomenon within the context of composite materials. In fact, nanocomposites show distinct characteristics compared to traditional composites due to the high surface-to-volume ratio at the nanoscale, which significantly influences the mechanical properties. Secondly, the research explores the Eshelby Tensor for a homogeneous solid with inclusions. Thirdly, various homogenization methods are compared, such as the Mori-Tanaka method, the self-consistent method with Eshelby Tensor and the refined Mori-Tanaka method for determining the effective stiffness tensor of nanocomposites. Fourthly, the introduction of “interface stress” between matrix and nanomaterials is analyzed. All the models are compared based on values of experimental results; moreover, a comparison with traditional composite techniques is performed. Overall, this work provides a detailed analytical framework for understanding and predicting the behavior of nanocomposites, integrating advanced mathematical models to account for the unique effects observed at the nanoscale.