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2023 | OriginalPaper | Chapter

4. Interfacial Fracture Toughness of a Titanium-to-CFRP Adhesive Joint

Author : Panayiotis Tsokanas

Published in: Fracture Analysis of Layered Beams With an Elastically Coupled Behavior and Hygrothermal Stresses

Publisher: Springer International Publishing

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Abstract

In the present chapter, the interfacial fracture behavior of a titanium-to-CFRP adhesive joint is investigated experimentally using the DCB and ENF test configurations. A potential application of this joint is in the wings of future large passenger aircraft. Four distinctive, industry-relevant manufacturing approaches are proposed: co-bonding with and without adhesive; and secondary bonding, using thermoset and thermoplastic CFRP. The VARTM technique is used for all of them. After manufacturing, the panels are cut into test specimens. Because these specimens are too thin (approximately 2.4 mm thick), we stiffen them on both titanium and composite sides with two aluminum backing beams to ensure the titanium will not yield during the subsequent DCB and ENF tests. The DRS proposed in Chap. 2 is used to determine the fracture toughness of the joint. As highlighted in the previous two chapters, this considers effects such as BEC and RTS induced by manufacturing. The load-versus-displacement responses, fracture behaviors during testing, and fracture toughness performances of the four MOs under consideration are presented and compared.

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previous chapter Fracture Toughness of Metal-to-Composite Adhesive Joints with Bending–Extension Coupling and Residual Thermal Stresses

next chapter Energy Release Rate and Mode Partitioning of Moment-Loaded Fracture Tests on Layered Beams with Bending–Extension Coupling and Hygrothermal Stresses

The respective layups of the entire joint for these two cases are as follows:

Thermoplastic CFRP: [0°_Al./0°_CFRP/45°_CFRP/45°_CFRP/0°_CFRP//0°_Ti./0°_Al.]
Thermoset CFRP: [0°_Al./0°_CFRP/45°_CFRP/90°_CFRP/−45°_CFRP/−45°_CFRP/90°_CFRP/45°_CFRP/ 0°_CFRP//0°_Ti./0°_Al.]

In these layups, the single slash, “/,” denotes the interface between two successive layers and the double slash, “//,” denotes the position of the crack plane.

It was a challenge for the TICOAJO project to select appropriate adhesives for the titanium-to-CFRP joint. To be considered suitable, an adhesive must meet a set of specifications: for example, high strength, temperature and moisture resistance, and fatigue resistance. A typical choice for bonding epoxy-based composites is epoxy adhesives; such adhesives can be a suitable option for titanium-to-composite joints.

Throughout the chapter, we refer to the MO 2 joint as an adhesive joint assuming that the matrix material of the CFRP (i.e., the RTM6) plays the role of an adhesive in the interface between the titanium and CFRP.

We shall recall that natural cracks have sharp crack tips, leading to more representative fracture toughness values compared to starter films that introduce blunt crack tips.

More specifically, ΜΟs 1, 3, and 4 were tested by the University of Patras, Greece, whereas MO 2 was tested in another test fixture at the Royal Netherlands Aerospace Centre, the Netherlands.

The same criteria are adopted in Chaps. 3 and 5.

This is a common phenomenon in interfacial fracture tests.

In principle, the desired failure mode is cohesive failure because it implies that the qualities of both the adhesive and bonding process are optimal.

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Tsokanas P, Loutas T (2019) Hygrothermal effect on the strain energy release rates and mode mixity of asymmetric delaminations in generally layered beams. Eng Fract Mech 214:390–409. https://doi.org/10.1016/j.engfracmech.2019.03.006CrossRef

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Tsokanas P, Loutas T, Kotsinis G, Kostopoulos V, van den Brink WM, Martin de la Escalera F (2020) On the fracture toughness of metal-composite adhesive joints with bending–extension coupling and residual thermal stresses effect. Compos B Eng 185:107694. https://doi.org/10.1016/j.compositesb.2019.107694CrossRef

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Tsokanas P, Loutas T, Kotsinis G, van den Brink WM, Nijhuis P (2021) Strain energy release rate and mode partitioning of moment-loaded elastically coupled laminated beams with hygrothermal stresses. Compos Struct 259:113237. https://doi.org/10.1016/j.compstruct.2020.113237CrossRef

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Tsokanas P, Loutas T, Nijhuis P (2020) Interfacial fracture toughness assessment of a new titanium-CFRP adhesive joint: an experimental comparative study. Metals 10(5):699. https://doi.org/10.3390/met10050699CrossRef

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Title: Interfacial Fracture Toughness of a Titanium-to-CFRP Adhesive Joint
Author: Panayiotis Tsokanas
Publisher: Springer International Publishing
Book: Fracture Analysis of Layered Beams With an Elastically Coupled Behavior and Hygrothermal Stresses
Print ISBN: 978-3-031-17620-3

Electronic ISBN: 978-3-031-17621-0

Copyright Year: 2023
DOI: https://doi.org/10.1007/978-3-031-17621-0_4

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