PSI - Issue 66

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8th International Conference on Crack Paths A Comparative Study on Mechanical Properties and Failure Mechanisms in Basalt and Glass Fibre Reinforced Composites M. Totaro * , G. Risitano, G. Di Bella, D. Crisafulli, D. D’Andrea Department of Engineering, University of Messina, Contrada di Dio, 98166 Messina, Italy Abstract In the current scientific context, Basalt Fibres have been proposed as sustainable alternative to synthetic reinforcements in polymeric matrices, thanks to their relatively low cost, good mechanical properties and reduced environmental impact. The aim of this work is to assess the possibility of replacing Glass Fibre with Basalt Fibre Reinforced Composites for structural application. For this purpose, several static tests have been performed in basalt-vinylester and glass-vinylester specimens, and a comparison of the failure behaviour has been conducted. Additionally, the two different failure mechanisms have been analysed with IR Thermography. Results demonstrate that Basalt Fibre composites failure behaviour is advantageous for design purposes, highlighting their potential to replace fibreglass in the near future. © 2025 The Authors. Published by ELSEVIER B.V. This is an open access article under the CC BY-NC-ND license (https://creativecommons.org/licenses/by-nc-nd/4.0) 1. Introduction In the present time, the use of composite materials is becoming increasingly important in order to design high performance structures. In particular, Fibre-Reinforced Polymers (FRP), consisting of a polymer matrix reinforced with fibres have gained interest in the last two decades due to their lightweight, higher specific strength and modulus (Maiti et al., 2022), and good corrosion resistance (Harle, 2024). Therefore, several applications like building and construction (Motavalli et al., 2010), transport (Ilyas et al., 2019), sports and leisure (Sreejith & Rajeev, 2021), and electronics (Pathania & Singh, 2009) have employed these materials. In recent years, researchers have focused on 8th International Conference on Crack Paths A Comparative Study on Mechanical Properties and Failure Mechanisms in Basalt and Glass Fibre Reinforced Composites M. Totaro * , G. Risitano, G. Di Bella, D. Crisafulli, D. D’Andrea Department of Engineering, University of Messina, Contrada di Dio, 98166 Messina, Italy Abstract In the current scientific context, Basalt Fibres have been proposed as sustainable alternative to synthetic reinforcements in polymeric matrices, thanks to their relatively low cost, good mechanical properties and reduced environmental impact. The aim of this work is to assess the possibility of replacing Glass Fibre with Basalt Fibre Reinforced Composites for structural application. For this purpose, several static tests have been performed in basalt-vinylester and glass-vinylester specimens, and a comparison of the failure behaviour has been conducted. Additionally, the two different failure mechanisms have been analysed with IR Thermography. Results demonstrate that Basalt Fibre composites failure behaviour is advantageous for design purposes, highlighting their potential to replace fibreglass in the near future. © 2025 The Authors. Published by ELSEVIER B.V. This is an open access article under the CC BY-NC-ND license (https://creativecommons.org/licenses/by-nc-nd/4.0) Peer-review under responsibility of CP 2024 Organizers Keywords: Composites; Basalt; Glass; Failure; IR Thermography 1. Introduction In the present time, the use of composite materials is becoming increasingly important in order to design high performance structures. In particular, Fibre-Reinforced Polymers (FRP), consisting of a polymer matrix reinforced with fibres have gained interest in the last two decades due to their lightweight, higher specific strength and modulus (Maiti et al., 2022), and good corrosion resistance (Harle, 2024). Therefore, several applications like building and construction (Motavalli et al., 2010), transport (Ilyas et al., 2019), sports and leisure (Sreejith & Rajeev, 2021), and electronics (Pathania & Singh, 2009) have employed these materials. In recent years, researchers have focused on © 2025 The Authors. Published by ELSEVIER B.V. This is an open access article under the CC BY-NC-ND license (https://creativecommons.org/licenses/by-nc-nd/4.0) Peer-review under responsibility of CP 2024 Organizers Peer-review under responsibility of CP 2024 Organizers Keywords: Composites; Basalt; Glass; Failure; IR Thermography

* Corresponding author. Tel.: +393934549812. E-mail address: martina.totaro@studenti.unime.it * Corresponding author. Tel.: +393934549812. E-mail address: martina.totaro@studenti.unime.it

2452-3216 © 2025 The Authors. Published by ELSEVIER B.V. This is an open access article under the CC BY-NC-ND license (https://creativecommons.org/licenses/by-nc-nd/4.0) Peer-review under responsibility of CP 2024 Organizers 2452-3216 © 2025 The Authors. Published by ELSEVIER B.V. This is an open access article under the CC BY-NC-ND license (https://creativecommons.org/licenses/by-nc-nd/4.0) Peer-review under responsibility of CP 2024 Organizers

2452-3216 © 2025 The Authors. Published by ELSEVIER B.V. This is an open access article under the CC BY-NC-ND license (https://creativecommons.org/licenses/by-nc-nd/4.0) Peer-review under responsibility of CP 2024 Organizers 10.1016/j.prostr.2024.11.071

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Glass Fibre Reinforced Composites (GFRC) due to their excellent mechanical properties (Morampudi et al., 2020) and they are currently the predominant material used in shipbuilding (Chairi et al., 2024) and wind turbine blades (Beauson et al., 2014) sectors. However, the main disadvantage of Glass Fibres (GF) is their environmental impact starting from the production, which requires significant amounts of non-renewable energy, thus resulting in substantial pollutant emissions, up to the end of life during incineration where they release black smoke and unpleasant odours that are harmful to melting apparatus (Patti et al., 2021). In addition, the more common recycling method of these fibres (Pyrolysis of shredded fibres) results in deterioration of the mechanical properties (Rani et al., 2021), mainly due to the formation of char on the surface of recycled fibres (Pimenta & Pinho, 2011). Moreover, surface treatments are required for this type of process, further damaging the fibres (Cunliffe & Williams, 2003). In the current scientific global scenario, attention to sustainability is becoming a priority, leading to the need for replacing synthetic fibres with more eco-friendly alternatives for producing sustainable composites. In this context, natural fibres are ideal materials due to their abundance, low cost, good mechanical properties, high specific strength, and non-abrasive nature (Joshi et al., 2004). In addition, their eco-compatibility and biodegradable features make them an ideal solution for creating products that promote a circular economy (Maiti et al., 2022). So Basalt Fibre Reinforced Composites (BFRC) have been proposed as a promising alternative to synthetic reinforced composites. Basalt is produced from a natural resource, volcanic rock, directly suitable for fibres manufacturing. Compared with GF, Basalt Fibres (BF) have competitive properties such as extremely good modulus and strength, higher temperature and chemical resistance and resistance to corrosion (Liu et al., 2022). The present study was conducted in the field of a research activity that aims to compare the mechanical behaviour of Basalt and Glass composites, in order to assess the possibility of replacing GF with BF in composites for marine and wind applications. In a design perspective, understanding the failure mechanisms is essential. With this goal, the two different materials failure behaviours are here analysed by using IR Thermography. It will be demonstrated that, in addition to competitive mechanical and ecological properties, BFRC also offer advantageous failure mechanisms for design necessities.

Nomenclature FRP

Fibre Reinforced Polymers GFRC Glass Fibre Reinforced Composites GF Glass Fibre BFRC Basalt Fibre Reinforced Composites BF Basalt Fibre

2. Materials and Methods 2.1. Specimen Preparation

Two composite laminate panels, each measuring 1 m x 1 m, were supplied by the Intermarine S.P.A. shipyard in Sarzana, Italy. One panel is reinforced with GF, while the other utilizes BF. Each laminate is composed of six layers of woven fabric, with an areal weight of 1100 g/m². GF panel thickness is nominally 6 mm and BF panel thickness is 5.5 mm. For BFRC, the reinforcement used in this study is FILAVATM, a high-performance BF developed by ISOMATEX, which is woven into a double-weave Panama-style fabric. On the other hand, the GF reinforcement employs a conventional woven fabric widely used in marine applications. For the matrix, Atlac® 580 AC 300 Vinylester resin, in combination with a NOROX® catalyst, is used. This thermosetting resin is specifically recommended for fibre-reinforced composite structures in marine environments due to its excellent mechanical properties and resistance to water absorption. The laminates were fabricated using a vacuum infusion technique, as shown in Fig. 1a, which involves drawing resin into the laminate under vacuum pressure to thoroughly wet out the fibres. This process is crucial for ensuring consistent fibre wetting and void-free laminates, both of which are essential for achieving high mechanical performance. The resulting panels, depicted in Fig. 1b and Fig. 1c, demonstrate high structural integrity, making them ideal for marine and wind applications. This manufacturing

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approach also ensures minimal environmental impact due to its low emission of volatile organic compounds. The rectangular specimens were cut by a band saw. Fig. 2a,b show magnified images of Basalt and Glass specimens under the Leica M165 FC optical microscope. It is noted that Basalt Fibres are more robust, and the weave pattern is more distinct, whereas Glass Fibres are thinner and show a less pronounced weaving effect.

Fig. 1 (a) Vacuum Infusion process (b) GF panel (c) BF panel.

Fig. 2 (a) GF specimen, (b) BF specimen

2.2. Experimental test For the evaluation of failure behaviours, static tensile tests were carried out according to ASTM D 3039/D 3039M-00e1 standard. The servo-hydraulic testing machine MTS model 810, equipped with a load cell of 250kN, is used. During the tests, the composite behaviour was monitored from an infrared thermal camera (type FLIR SC640), placed approximately 250 mm in front of the specimen. All static tensile tests were performed at room temperature and under standard humidity conditions. Tests were conducted in displacement control, at 2 mm/min speed of the crosshead, using a set of samples measuring 25 x 250 mm². Experimental setup is shown in Fig. 3.

Fig. 3 Experimental Setup.

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3. Results In this paragraph, results of the experimental tests are first presented from the only mechanical standpoint and then failure mechanisms are analysed with IR Thermography. 3.1. Tensile characterization The stress versus strain curves are shown in Fig. 4 a,b. Multiple tests were conducted, confirming that the results are repeatable. Therefore, only one representative curve for each material is shown. For BFRC, the curve in Fig. 4a does not continuously increase until failure but there is an initial load loss, followed by a clear load drop at 374 MPa, which corresponds to the first delamination. In contrast, as shown in Fig. 4b, that the stress-strain curve for GFRC exhibits a typical brittle behaviour, with an approximate linear region followed by a sudden, catastrophic failure at 295 MPa. Notably, the first load loss for Basalt occurs at around 270 MPa, a lower value compared to the Glass breaking stress. However, in terms of overall strength, BFRC shows better performance with greater elongation, demonstrating more effective energy absorption capacity. In this work, Basalt failure is defined as the stress value that leads to the first delamination, while previous load losses are considered only as initial damage. In the light of this, GFRC and BFRC tensile strengths are reported in Tab.1. This difference is attributed to the two different failure modes, which are analysed using IR Thermography in section 3.2.

Table 1. Results from experimental static tensile tests GFRC

BFRC

Percentage increase (%)

Ultimate tensile strength (MPa)

374 ± 20.2

295 ± 4.7

+26.7%

Fig. 4 Stress/Strain Curve for (a) BFRC (b) GFRC.

3.2. Tensile Failure Analysis Two primary failure events can be identified: the first related to matrix cracking and the second to different fibre failure mechanisms. These events were identified using IR Thermography, which enabled to distinguish them. The colour palette uses ranges from purple to yellow, where the lighter shades correspond to higher temperatures. In the red portions of the curves in Fig. 4a,b , matrix failure is the main damage, appearing as random higher temperature peaks on the specimen surface. The evolution of these random peaks at different time points is shown in Fig. 5a for Basalt and Fig. 5b for Glass, where they appear like lighter violet spots. Since the matrix is the same, similar behaviour is observed for both of them.

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What changes and alters the macroscopic behaviour of the two materials is the fibres response. For BF, the first load loss in the green portion of the curve in Fig. 4a corresponds to the failure of fibre segments in the external regions of the specimen shown in Fig. 5c; while the second load drop, in blue in Fig. 4a, is associated with delamination and debonding shown in Fig. 5d, consisting of separation of plies leading to the material collapse. For GF, in the blue portion of the glass curve in Fig. 4b, a sudden catastrophic failure occurs. Up to this point the specimen appears intact and then experiences a sudden breaking. The failure takes place at the clamping areas (Fig. 5e), either in the lower and upper region. Fig. 6a shows how the lateral fibres breakings appear in Basalt specimens. Then, two types of delamination occur. Two samples are displayed as examples: specimen codified with 3V undergoes a warping of the outer layer; in specimen codified with 2V the first ply separates, followed by the detachment of all plies. (Scalici et al., 2016) also found similar delamination and debonding mechanisms. GF broken specimens are shown in Fig. 6b, with a magnified view of the cross section and side view. In the latter, also a light debonding phenomenon is visible. Also in (Talabari et al., 2019) study, the inter-laminar bonding is high and no layer delamination is observed for vacuum infusion Glass sample. It is possible to affirm that BFRC offer advantages not only from costs and sustainability perspective but also in terms of their failure behaviour, which is highly relevant for design. While showing initial signs of damage, BF components continue to bear the load, allowing the component to survive longer without catastrophic breakage. This suggests that, unlike GF, BF can provide the opportunity to continue using the component after initial damage, enhancing both the safety and durability of the systems in which it is employed.

Fig. 5 Matrix Cracking in (a) BFRC (b) GFRC , (c) BF segments failure, (d) BF delamination, (e) GF breaking

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Fig. 6 Broken specimens (a) BF (b) GF

4. Conclusions In this study, the mechanical properties and failure mechanisms of BFRC and GFRC were analysed and compared under static tensile tests. BF specimens demonstrated higher overall strength compared to GF specimens. For both materials, the use of IR Thermography allowed clear identification of two main failure events: initially, matrix cracking is the primary damage, followed by fibres breakage. Since both composites used a vinylester resin, they exhibited the same matrix failure mode. However, distinct fibre failure mechanisms were observed.  BFRC exhibits a progressive failure, with initial fibres breakage followed by delamination and debonding mechanisms until the specimen fails.  GFRC undergoes sudden and complete failure, with strong inter-laminar bonding and no layer delamination observed. The results of this work indicate that BFRC offer superior performance compared to GFRC. BF laminates not only demonstrate better mechanical properties, but their more gradual failure behaviour provides significant design advantages, enhancing the safety and lifespan of the structure and allowing for continued use after initial damage. Additionally, BF materials offer benefits in terms of cost-efficiency and environmental sustainability, making them a more versatile and durable option for demanding conditions. Acknowledgements The authors would like to thank the financial supports of Ministry of Economic Development on the resources provided by the Decree 5 March 2018 Chapter III, as part of the project “Development of Ahead Systems and Processes for Highly Advanced Technologies for low Magnetic Signature and Highly efficient Electromagnetic shielded ecofriendly vessel – DAS PHANTOMSHIFFE”, grant number F/190001/01/X44.

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Beauson, J., Lilholt, H., & Brøndsted, P. (2014). Recycling solid residues recovered from glass fibre-reinforced composites - A review applied to wind turbine blade materials. Journal of Reinforced Plastics and Composites , 33 (16), 1542–1556. https://doi.org/10.1177/0731684414537131 Chairi, M., El Bahaoui, J., Hanafi, I., Favaloro, F., Borsellino, C., Galantini, F., & Di Bella, G. (2024). The effect of span length on the flexural properties of glass and basalt fibre reinforced sandwich structures with balsa wood core for sustainable shipbuilding. Composite Structures , 340 (May), 118187. https://doi.org/10.1016/j.compstruct.2024.118187 Cunliffe, A. M., & Williams, P. T. (2003). Characterisation of products from the recycling of glass fibre reinforced polyester waste by pyrolysis. Fuel , 82 (18), 2223–2230. https://doi.org/10.1016/S0016-2361(03)00129-7 Harle, S. M. (2024). Durability and long-term performance of fibre reinforced polymer (FRP) composites: A review. Structures , 60 (January), 105881. https://doi.org/10.1016/j.istruc.2024.105881 Ilyas, R. A., Sapuan, S. M., Norizan, M. N., Atikah, M. S. N., Huzaifah, M. R. M., Radzi, A. M., Ishak, M. R., Zainudin, E. S., Izwan, S., Noor Azammi, A. M., Jumaidin, R., Ainun, Z. M., Syafik, R., Nazrin, A., & Atiqah, A. (2019). Potential of Natural Fibre Composites for Transport Industry : a Potential of Natural Fibre Composites for Transport Industry : a Review. Prosiding Seminar Enau Kebangsaan 2019 , April , 2–11. Joshi, S. V., Drzal, L. T., Mohanty, A. K., & Arora, S. (2004). Are natural fibre composites environmentally superior to glass fibre reinforced composites? Composites Part A: Applied Science and Manufacturing , 35 (3), 371–376. https://doi.org/10.1016/j.compositesa.2003.09.016 Liu, J., Chen, M., Yang, J., & Wu, Z. (2022). Study on Mechanical Properties of Basalt Fibres Superior to E-glass Fibres. Journal of Natural Fibres , 19 (3), 882–894. https://doi.org/10.1080/15440478.2020.1764438 Maiti, S., Islam, M. R., Uddin, M. A., Afroj, S., Eichhorn, S. J., & Karim, N. (2022). Sustainable Fibre-Reinforced Composites: A Review. Advanced Sustainable Systems , 6 (11). https://doi.org/10.1002/adsu.202200258 Morampudi, P., Namala, K. K., Gajjela, Y. K., Barath, M., & Prudhvi, G. (2020). Review on glass fibre reinforced polymer composites. Materials Today: Proceedings , 43 , 314–319. https://doi.org/10.1016/j.matpr.2020.11.669 Motavalli, M., Czaderski, C., Schumacher, A., & Gsell, D. (2010). Fibre reinforced polymer composite materials for building and construction. In Textiles, Polymers and Composites for Buildings . Woodhead Publishing Limited. https://doi.org/10.1533/9780845699994.1.69 Pathania, D., & Singh, D. (2009). A review on electrical properties of fibre reinforced polymer composites. International Journal of Theoretical & Applied Sciences , 1 (2), 34–37. Patti, A., Nele, L., Zarrelli, M., Graziosi, L., & Acierno, D. (2021). A Comparative Analysis on the Processing Aspects of Basalt and Glass Fibres Reinforced Composites. Fibres and Polymers , 22 (5), 1449–1459. https://doi.org/10.1007/s12221-021-0184-x Pimenta, S., & Pinho, S. T. (2011). Recycling carbon fibre reinforced polymers for structural applications: Technology review and market outlook. Waste Management , 31 (2), 378–392. https://doi.org/10.1016/j.wasman.2010.09.019 Rani, M., Choudhary, P., Krishnan, V., & Zafar, S. (2021). A review on recycling and reuse methods for carbon fibre/glass fibre composites waste from wind turbine blades. Composites Part B: Engineering , 215 (February), 108768. https://doi.org/10.1016/j.compositesb.2021.108768 Scalici, T., Pitarresi, G., Badagliacco, D., Fiore, V., & Valenza, A. (2016). Mechanical properties of basalt fibre reinforced composites manufactured with different vacuum assisted impregnation techniques. Composites Part B: Engineering , 104 , 35–43. https://doi.org/10.1016/j.compositesb.2016.08.021 Sreejith, M., & Rajeev, R. S. (2021). Fibre reinforced composites for aerospace and sports applications. In Fibre Reinforced Composites: Constituents, Compatibility, Perspectives and Applications . Elsevier Ltd. https://doi.org/10.1016/B978-0-12-821090-1.00023-5 Talabari, A. A., Alaei, M. H., & Shalian, H. R. (2019). Experimental investigation of tensile properties in a glass/epoxy sample manufactured by vacuum infusion, vacuum bag and hand layup process. Revue Des Composites et Des Materiaux Avances , 29 (3), 179–182. https://doi.org/10.18280/rcma.290308

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8 th International Conference on Crack Paths th International Conference on Crack Paths

A Comprehensive Review of Forming Methods for Composite Materials and Cracking Mohammad Jameel Ziedan 1 , AbdulKareem F. Hassan 2 , Najim A. Saad 3 , A.M. Al-Mukhtar 4, * 1 Department of Mechanical Engineering, College of Engineering, University of Basrah, Basrah, Iraq 2 College of Engineering, University of Almaaqal, Basrah, Iraq 1 , AbdulKareem F. Hassan 2 , Najim A. Saad 3 , A.M. Al-Mukhtar 4, * 1 Department of Mechanical Engineering, College of Engineering, University of Basrah, Basrah, Iraq 2 College of Engineering, University of Almaaqal, Basrah, Iraq

3 Faculty of Materials Engineering, Babylon University, Babylon Iraq 4 Institute of Structural Mechanics, Bauhaus-Universität Weimar, Germany 4 Al-Hussain University College, Iraq *Corresponding author: almukhtar@structuralintegrity.eu 3 Faculty of Materials Engineering, Babylon University, Babylon Iraq 4 Institute of Structural Mechanics, Bauhaus-Universität Weimar, Germany 4 Al-Hussain University College, Iraq *Corresponding author: almukhtar@structuralintegrity.eu

Abstract This review examines three primary forming techniques for composite materials: single-point incremental forming (SPIF), vacuum forming, and compression molding. SPIF involves applying localized pressure to thermoplastic sheets, enabling the formation of complex geometrical shapes. Vacuum forming uses vacuum pressure to mold heated sheets over forms, making it well-suited for large-scale production of simpler components. Compression molding, including techniques like Sheet Molding Compound (SMC) and Injection Molding, entails compressing heated composite materials in molds under high pressure to produce strong, durable parts. The discussion of the SPIF method will cover process parameters and optimization, SPIF of hole flanges, workpiece geometry, software tools, biocompatible polymers, and heating techniques. Additionally, the review will explore different formation types in vacuum forming and compression molding. Understanding these methods is crucial for various industries utilizing composite materials, particularly with regard to their cracking behavior. These techniques offer flexibility, efficiency, and precision in producing composite components with diverse geometries and properties, addressing contemporary engineering and design challenges. © 2024 The Authors. Published by ELSEVIER B.V. This is an open access article under the CC BY-NC-ND license (https://creativecommons.org/licenses/by-nc-nd/4.0) Peer-review under responsibility of CP 2024 Organizers Abstract This review examines three primary forming techniques for composite materials: single-point incremental forming (SPIF), vacuum forming, and compression molding. SPIF involves applying localized pressure to thermoplastic sheets, enabling the formation of complex geometrical shapes. Vacuum forming uses vacuum pressure to mold heated sheets over forms, making it well-suited for large-scale production of simpler components. Compression molding, including techniques like Sheet Molding Compound (SMC) and Injection Molding, entails compressing heated composite materials in molds under high pressure to produce strong, durable parts. The discussion of the SPIF method will cover process parameters and optimization, SPIF of hole flanges, workpiece geometry, software tools, biocompatible polymers, and heating techniques. Additionally, the review will explore different formation types in vacuum forming and compression molding. Understanding these methods is crucial for various industries utilizing composite materials, particularly with regard to their cracking behavior. These techniques offer flexibility, efficiency, and precision in producing composite components with diverse geometries and properties, addressing contemporary engineering and design challenges. © 2024 The Authors. Published by ELSEVIER B.V. This is an open access article under the CC BY-NC-ND license (https://creativecommons.org/licenses/by-nc-nd/4.0) Peer-review under responsibility of CP 2024 Organizers © 2025 The Authors. Published by ELSEVIER B.V. This is an open access article under the CC BY-NC-ND license (https://creativecommons.org/licenses/by-nc-nd/4.0) Peer-review under responsibility of CP 2024 Organizers Keywords: Composite materials; Compression molding; Thermoforming; Thermoplastic; Single point incremental forming; Vacuum thermoforming. Keywords: Composite materials; Compression molding; Thermoforming; Thermoplastic; Single point incremental forming; Vacuum thermoforming. 1. Introduction Composite materials have been a subject of extensive research and development across various fields for several decades. Composite materials are engineered materials made from two or more constituent materials with different 1. Introduction Composite materials have been a subject of extensive research and development across various fields for several decades. Composite materials are engineered materials made from two or more constituent materials with different

* Corresponding author.. E-mail address: almukhtar@structuralintegrity.eu * Corresponding author.. E-mail address: almukhtar@structuralintegrity.eu

2452-3216 © 2025 The Authors. Published by ELSEVIER B.V. This is an open access article under the CC BY-NC-ND license (https://creativecommons.org/licenses/by-nc-nd/4.0) Peer-review under responsibility of CP 2024 Organizers 10.1016/j.prostr.2024.11.074 2452-3216 © 2024 The Authors. Published by ELSEVIER B.V. This is an open access article under the CC BY-NC-ND license (https://creativecommons.org/licenses/by-nc-nd/4.0) Peer-review under responsibility of the IGF chairpersons 2452-3216 © 2024 The Authors. Published by ELSEVIER B.V. Peer-review under responsibility of the IGF chairpersons

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physical or chemical properties, which when combined, produce a material with characteristics different from the individual components (AbdulRazaq et al., 2023; Hussein et al., 2020; Talla et al., 2022). Therefore, composite materials aim to create new aspects of mechanical properties such as toughness and stiffness for automobile, biomedical, and aeronautical applications (Al-Mukhtar, 2012, 2019, 2020). However, the formation of defects during the manufacturing process may influence the desired design. Voids and cracks are material discontinuities with distinct characteristics and effects. Voids in composites are empty spaces at the interfaces or within the matrix, which reduce the material's strength and stiffness. Key areas driving extensive research on composites include advancements in manufacturing techniques, testing methods for composite materials, exploration of novel material options, and related research fields. This work aims to highlight recent developments in metal forming, focusing on associated properties and cracking phenomena. A review of various forming methods and their related aspects is provided, with particular emphasis on Single Point Incremental Forming (SPIF), vacuum forming, and compression molding. 1.1. Single Point Incremental Forming (SPIF) Single point incremental forming, also known as mechanical forming, is a thermoforming technique where a single point (usually a plug or mold) is used to apply force to a heated thermoplastic sheet, causing it to conform to the shape of the mold (Bagudanch, Vives-Mestres, et al., 2017; Buffa et al., 2013); see Fig. 1. The composite sheet is heated until it becomes pliable, then placed over the mold. A plug or other mechanical device is then used to apply pressure at a single point, forcing the sheet to stretch and take the shape of the mold (Centeno et al., 2014).

Fig. 1. Single point forming process (Centeno et al., 2014)

1.2. Process Parameters and Optimization Method of SPIF Many studies have been carried out to determine the effective and optimal parameters for the process of SPIF of composite materials (Lozano-Sánchez et al., 2018). Previous studies have explored various optimization methods and process parameters to enhance the formability, mechanical properties, and surface quality of formed parts. Since polymethylmethacrylate (PMMA) is typically brittle, it was modified by adding a plasticizer (triacetin) to relax the polymer chains, along with a nanoclay reinforcement (Cloisite 30B) to maintain mechanical strength suitable for the

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SPIF process at room temperature (Clavijo-Chaparro et al., 2018a; Le et al., 2008). Using a factorial design of experiments (DoE), the concentrations of these additives were optimized, see Table 1. The researchers found that the plasticizer and nanoclay worked together, with the nanoclay enhancing the plasticizing effect and improving stress transfer within the material. This allowed them to create PMMA composite sheets with improved formability, making them suitable for manufacturing customized cranial implants by SPIF. Table 1. The compositions of the PMMA-based samples added with plasticizer and C-30B, and the corresponding codified matrix for the DoE (Clavijo-Chaparro et al., 2018a).

Samples

Weight percent (wt%)

Codified matrix

PMMA

Triacetine

Cloisite

Triacetine

Cloisite

T15-C1 T15-C3 T25-C1 T25-C3 T20-C2

84 82 74 72 78

15 15 25 25 20

1 3 1 3 2

-1 -1

-1

1 1 1 0

1 1 0

Table 2. Taguchi DoE (orthogonal array selected is L16 ) (Karthik et al., 2019)

Exp. No.

Tool diameter (mm), Φ

Step size (mm)

Sheet thick (mm)

Spindle speed, ω (rpm)

Table feed (mm/min), υ

1 2 3 4 5 6 7 8 9

10 10 10 10 12 12 12 12 14 14 14 14 16 16 16 16

0.25

1

1000 1500 2000 2500 1000 1500 2000 2500 1000 1500 2000 2500 1000 1500 2000 2500

600

0.5

1.5

1000 1400 1800 1800 1400 1000

0.75

2

1

2.5

0.25

1

0.5

1.5

0.75

2

1

2.5

600

0.25

1

1000

10 11 12 13 14 15 16

0.5

1.5

600

0.75

2

1800 1400 1400 1800

1

2.5

0.25

1

0.5

1.5

0.75

2

600

1

2.5

1000

SPIF for thermoplastic sheets has been explored (Michael Rabinovich et al., 2008). The cone-shaped part was used with varying wall angles to assess formability of the thermoplastic sheets, focusing on the maximum wall angle reached without tearing and/or failure. To optimize the process, a (2 4-1 ) fractional factorial design of experiments were employed, with four parameters; step size, tool size, feed rate, and spindle speed. The results indicate that these factors, and their interactions, have an important impact on formability. Specifically, smaller tool sizes and higher spindle speeds resulted in better outcomes. The Taguchi design method was employed to optimize and analyze the impact of process parameters such as tool diameter, step size, spindle speed, and sheet thickness on formability, surface roughness, and depth of failure. The depth of failure is influenced by the sheet thickness and the thinning of the

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material's side wall. The findings indicated that spindle speed and sheet thickness significantly impacted formability and surface roughness. Additionally, a smaller tool diameter resulted in better surface quality; see Table 2. The increasing of the step size and applying heat after forming led to reduction in spring back. The types of failure in polymers differ from those in metals. While polymers more likely to resemble a tear, unlike metals, which primarily crack. Therefore, it is clear that the methods used in forming composite materials differ from those in metals. The results indicated that υ and ( ω × Φ ) significantly influence the forming performance of PVC sheet material, υ is the main influencing factor on SPIF performance of PVC sheet. Excessive ω can lead to material wear-out. (Bagudanch, Centeno, et al., 2017) revisited the formability and failure of different polymers PVC, polycarbonate (PC), polypropylene (PP), polycaprolactone (PCL), and ultrahigh molecular weight polyethylene (UHMWPE) during deformation using SPIF, see Table 3. Traditionally, spindle speed is a crucial parameter affecting forming temperature and material formability limits as well as failure mode. Table 3. The results of maximum depth, maximum angle and failure occurrence and mode of failure for the different SPIF tests (Bagudanch, Centeno, et al., 2017). Test Materials Spindle speed, S (rpm) Maximum depth (mm) Maximum angle (º) Failure? 1 PVC Free 42.5 89.5 YES 2 2000 43 90 Twisting without fracture 3 PC Free 29 79.8 YES 4 2000 32.5 82.5 YES 5 PCL Free 43 90 Small cracks and twisting 6 2000 43 90 Small cracks 7 PP Free 43 90 NO 8 2000 43 90 NO 9 UHMWPE Free 43 90 NO 10 2000 43 90 NO 1.3. SPIF of Hole-Flanges SPIF of hole-flanges requires careful consideration of process parameters such as tool size, step size, feed rate, spindle speed, and material properties to achieve the desired formability, dimensional accuracy, and surface finish. But the formability is limited by fracture, unlike conventional press working where necking precedes fracture, see Table 4. SPIF also resulted in cracks forming around the circumference of the flange wall, while press working led to cracks along the hole edge, see Fig. 2. Table 4. Summary of the results obtained in hole-flanging of conventional PW and SPIF. The dark grey cells correspond to failure fracture (Cristino et al., 2015). D o (mm) Conventional press working SPIF Drawing angle of the intermediate stages � � º � 65 70 75 80 85 90

127 121 115 102

95 73 52

The feasibility of using SPIF to create hole-flanges in polymer sheets was investigated (Centeno et al., 2014). Therefore, it was found that the polyethylene terephthalate (PET) is suited for this process due to its ability to withstand significant deformation at room temperature. The research shown that PET sheets could be formed into hole-flanges

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with nearly vertical walls using a multi-stage SPIF approach, where the wall angle is gradually increased in steps. SPIF involves a mode of deformation and failure not seen in conventional SPIF. The new deformation mode combines in-plane stretching and bending, limiting strain path growth and causing failure through meridional fracture without necking. Strain measurements support this by highlighting fracture forming lines (FFLs) as the loci of failure (Silva et al., 2013b). However, the new methodology was used in Ref. (Alkas Yonan et al., 2014).

Fig. 2. The hole-flanging produced by conventional press working (left) and by SPIF (right) (Cristino et al., 2015)

1.4. Workpiece geometry SPIF is utilized for producing common geometries, including simple shapes like cylindrical and conical forms, complex contours such as curved surfaces, and functional parts like ribs and channels used in various industries (Xu et al., 2013). The pyramid frusta, due to their sharp edges and corners, exhibited greater strain concentration and were more susceptible to premature failure compared to cones. The research shown that adjusting the slope angle during the forming process resulted in an overestimation of the sheet's formability. (Hernández-Ávila et al., 2019) used a bilayer sheets made of polypropylene (PP) and Santoprene (ST) in SPIF. It was found that the arrangement of the layers significantly influenced the mechanical and thermal behavior during forming. When PP was placed on the outer wall of the cone-shaped work piece, it provided reinforcement and reduced softening, resulting in higher forming forces. The dual properties of bilayer sheets provide advantages over single-material sheets and blends, enabling the fabrication of complex geometries with enhanced performance. In SPIF, tool rotation and the associated heat generation significantly influence the fracture behavior of ST. The specific heat capacity of the materials plays a key role in determining their heating rate, which in turn affects their fracture response. Under the tested SPIF conditions, PP demonstrates superior formability compared to ST, primarily due to its higher ductility and softening behavior at elevated temperatures (see Fig. 3).

Fig. 3. The different sheets fabricated and shaped by SPIF (Hernández-Ávila et al., 2019)

1.5. Software Programs Several software programs like CAD/CAM, CATIA V5, Master CAM, and SolidWorks are used to simulate and design SPIF processes. ABAQUS, a powerful FEA software suite, is frequently employed to simulate composite forming processes such as SPIF to forecast material behavior and enhance process parameters and tooling design (Centeno et al., 2017; Silva et al., 2013b). The shape memory polymer (SMP) foam in SPIF and a corrected STL (Standard Tessellation Language) file imported into the CAD/CAM have been used (Mohammadi et al., 2015). The material's properties and formability at different temperatures have been investigated. it was found that heating the SMP foam improved its formability by 28%, allowing for the creation of complex shapes. Accuracy was assessed using a laser line scanner, revealing the need for toolpath compensation to achieve higher precision. The two approaches have been developed for predicting forming forces in metal cutting; a semi-analytical model based on forming energy and deformed volume, requiring calibration but offering a simple prediction method, and a numerical

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model using ABAQUS software with a hyperelastic material model, considering temperature dependence for accurate predictions of forces, thickness reduction, and temperature distribution. (Franzen et al., 2009) explored the use of SPIF for shaping commercial PVC sheets at room temperature. CATIA V5 software was employed for toolpath generation, geometry analysis, and evaluation of deviations between the formed parts and the original CAD models. Three failure modes were identified; cracking due to stretching, wrinkling from twisting, and cracking from redundant straining. Surface finish quality was significantly influenced by the tool diameter. Experiments with various polymers (polyethylene terephthalate (PET), polyamide (PA), PVC and PC revealed that PET exhibits exceptional formability, while PC maintains transparency after forming. A theoretical framework based on membrane analysis was developed to explain the mechanics of deformation and the influence of process parameters. The process involves common equipment such as a blank holding fixture, heat gun, CNC machine, and software for toolpath planning. Three software tools are used; SolidWorks for generating CAD models of the desired shapes, Slic3r for generating G-code toolpaths from the CAD models, and Universal G-code sender for controlling the CNC machine and executing the toolpaths. 1.6. Biocompatible Polymers SPIF is used for many applications, including those requiring biocompatible materials like Polylactic Acid (PLA), Polylacticglycolicacid (PLGA), Polyethylene glycol (PEG), Polycaprolactone (PCL), Polyethylene (PE), Polypropylene (PP), Polylacticaprolactone (PLCL), ultrahigh molecular weight polyethylene (UHMWPE), and poly methyl methacrylate (PMMA). These biocompatible polymers offer properties such as biocompatibility, mechanical strength, processability, and controllable degradation, making them suitable for SPIF applications in medical and healthcare industries (Centeno et al., 2017; Clavijo-Chaparro et al., 2018a; Hernández-Ávila et al., 2019; Raheem & Al-Mukhtar, 2020; Raheem & Al ‐ Mukhtar, 2021). The implants using a polymer known for its biocompatibility, light weight, and bone-like mechanical properties have been manufactured (Raheem & Al-Mukhtar, 2020; Raheem & Al ‐ Mukhtar, 2021). The SPIF method resulted in lower accuracy, the TPIF variant with a negative die achieved better geometric precision (Table 5). These results are the most important in the context of the bilayer approach. This method offers potential for creating parts with combined properties, such as strength and elasticity, which could be valuable for biocompatible applications like medical devices, see Fig. 4. (Bagudanch et al., 2019) explored the use of (ISF) to manufacture cranial implants from biocompatible polymers and the formability of (PCL) have been inveiagted. But were found it exhibited significant springback, leading to geometric inaccuracies. Work with (UHMWPE) shows promising results. By using (TPIF), the suitable geometric accuracy has achieved and demonstrated the possible of ISF for creating low-cost, customized cranial implants with properties similar to bone; see Table 3. The impact tests on the samples for various alloys and thicknesses, anchored to supports made of (PMMA) have been conducted (Bagudanch et al., 2019). It was found that the prostheses effectively absorbed impact energy without fracturing, with energy absorption exceeding 70% in most cases. Table 5. Comparison of nonabsorbable polymers for cranial implants (Bagudanch et al., 2018). Polymer Advantages Disadvantages PMMA - Biocompatible and biostable - No resorption - Low cost - Elastic modulus similar to the skull bone (between 3.0 and 3.4 GPa) - Large amount of heat is required to mold the part, which could damage surrounding tissues - Fracture due to impact trauma - Tissue ingrowth is not possible - Migration

PEEK

- Strong, inert and biocompatible - Density, mechanical strength and elastic modulus (3.24 GPa), comparable to cortical bone

- Lack of osteointegration - High cost

- Risk of infection

PE

- Bone ingrowth for porous PE enhancing biocompatibility - Elastic modulus between 0.3 and 1.0 GPa - Easy to shape

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Fig. 4. Left) Cranial fracture; right) prosthesis (Bagudanch et al., 2018).

1.7. Heating Methods Heating methods in SPIF can enhance material formability and reduce springback. heating the workpiece uniformly reduces yield strength, increasing ductility. Heating parameters like temperature, heating rate, and duration must be controlled for effective heating and to avoid thermal damage (Clavijo-Chaparro et al., 2018a; Davarpanah et al., 2015). (Conte et al., 2017) investigated the use of SPIF for shaping PMMA sheets. As PMMA has a high glass transition temperature, external heating was necessary for successful forming. It was found that a combination of moderate initial heating (controlled by an electric heater) and low spindle speed (which affects frictional heating) yielded the best results, preventing cracking and spring back while maintaining good surface finish. The "downward heating" strategy, where the temperature is gradually decreased as the forming depth increases, resulted in more homogeneous heating and reduced defects compared to a constant heating approach and formed GFRP cones with wall angles up to 55 degrees. Internal cracks and voids started to appear when reaching the wall angle of 50°. The trajectory, created by interpolating neighbouring contour lines, effectively decreased twist and wrinkle formation. A thermal-assisted incremental forming setup was devised to improve polymer sheet formability and decrease fracture possibility; see Fig. 5.

Fig. 5. Thermal assisted single point incremental forming (Yang & Chen, 2020).

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2. Vacuum Forming Vacuum forming of composite materials is a manufacturing process in which a composite material is shaped by applying vacuum pressure with heat assistance. In this process, the composite material is placed over a mold, and a vacuum is created between the mold and the material. The atmospheric pressure then presses the material against the mold surface, causing it to take the shape of the mold. This method is commonly used to produce lightweight and strong components with complex shapes, making it popular in industries such as aerospace, automotive, and marine engineering (Afshariantorghabeh et al., 2023; Grankäll et al., 2021; Jones et al., 1995). 2.1. Vacuum-Assisted Resin Infusion Molding (VARIM) VARIM is a manufacturing process for creating composite materials by infusing resin into dry reinforcement materials like fiberglass or carbon fiber. The process involves mold preparation, laying up reinforcement materials, creating resin channels and vacuum lines, vacuum bagging, resin infusion, curing, and finishing. VARIM offers advantages such as improved resin distribution control, reduced cycle times, and high-strength composite parts (Kedari et al., 2011, 2011); see Fig. 6. Divided into zones with independent temperature control, the system employs a Programmable Logic Controller (PLC) to adjust temperatures and curing cycle intervals. Ensuring even temperature distribution is crucial for composite part production, ensuring consistent curing conditions across all zones. The PLC allows for adaptable control to adjust curing conditions, enhancing system flexibility. Monitoring real-time temperature values from all zones and logging them for data analysis optimizes the properties of the composite samples.

Fig. 6. Schematic illustration of the vacuum assisted resin infusion molding (VARIM) process (Goren & Atas, 2008).

Fig. 7. Schematic explains the difference between the typical VARTM process and the modified VARTM process (Menta et al., 2014).

Three cooling methods (air cooling, air-water cooling, water cooling) were examined to evaluate their effects on processing time, residual strains, and mechanical properties. By monitoring residual strains with embedded strain gauges, it was proposed gradual cooling pre-glass transition temperature and swift cooling post-transition for effective

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processing and superior mechanical properties in composite materials. Scanning electron microscope (SEM) was carried out to examine the fracture surfaces of composite samples after flexural tests. The model assesses the impact of process parameters on pressure distribution during resin infusion, taking into account factors such as reinforcement, permeability, and resin viscosity. The main problems are the void formation in the matrix and the micro- cracking in the interlaminar regions. The research focuses on the resin film infusion process for autoclave processing of composite structures with high fiber volume content, aiming to mitigate issues like micro-cracking and void formation that can compromise the composite material (see Fig. 7). Flexural and tensile tests were conducted, showing good results.

Fig. 8. Void removal mechanisms (Zhang et al., 2017).

2.2. Vacuum Bag Processing Vacuum bag processing is a common technique in composite material forming. The process involves mold preparation, reinforcement material layup, resin application, vacuum bag assembly, sealing, evacuation, curing, demolding, and finishing. This method offers benefits such as enhanced resin infusion, reduced void content, improved fiber wet-out, and the capability to produce high-strength, lightweight complex shapes (Kazmi et al., 2014; Levy & Hubert, 2019). (Zhang et al., 2017) investigated void reduction mechanisms in oven vacuum bag processing of high performance carbon fiber thermoplastic composites. Key mechanisms include through-thickness air diffusion and in plane flow to laminate edges through interlayer permeability. Experimental results suggest that effective void reduction is achievable with open edges to vacuum, allowing air removal through a single layer and interlayer flow. The importance of interlayer permeability and edge conditions in reducing void content in thick thermoplastic composite laminates through economical oven vacuum bag processing was highlighted, see Fig. 8. It was found that air removal happens through two mechanisms: air diffusing through the prepreg layers and air flowing out through the edges of the part. Their model showed that for thick parts, simply relying on diffusion isn't enough and venting the edges is important to achieve low void content. The DVB process is adaptable to various composite manufacturing methods and equipment, providing a cost-effective solution for producing high-quality, low-void composite parts. It is particularly effective for resin matrices with viscosities below 250 Poises when used in an oven, while higher viscosity matrices may require additional pressure from an autoclave or press for consolidation, see Table 6. Findings indicate that external pressure notably boosts surface quality by minimizing superficial pores and enhancing mechanical properties like flexural and tensile strength. The enhanced fiber-to-resin ratio and decreased void content from compression molding aid in improving the composites' overall quality; see Figs. 11, 12. (Levy & Hubert, 2019) examine the thickness variation in composite laminates produced through vacuum-bag processes, particularly in intricate shapes such as corners. Two key factors influence this variation: pressure differences between corners and flanges impacting fiber bed compaction, and ply friction impeding accurate mold conformation. An analytical model is proposed to elucidate thickness variations in composite parts, corroborated by experimental results. The comprehensive model exhibits a 5% standard deviation error, offering a predictive instrument for manufacturing engineers to anticipate thickness fluctuations in composite elements. (Alshahrani & Hojjati, 2016) investigated the in plane shear behavior of carbon/epoxy prepress under diaphragm compaction conditions, focusing on the influence of vacuum sealing and compaction on shear properties, see Fig. 13. Findings revealed a significant decrease in shear