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. 2025 Jun 18;17(12):1689.
doi: 10.3390/polym17121689.

Utilizing Excess Resin in Prepregs to Achieve Good Performance in Joining Hybrid Materials

Nawres J Al-Ramahi  1   2 , Safaa M Hassoni  2 , Janis Varna  1   3 , Roberts Joffe  1
Affiliations

Utilizing Excess Resin in Prepregs to Achieve Good Performance in Joining Hybrid Materials

Nawres J Al-Ramahi et al. Polymers (Basel). .

Abstract

This study investigates the fracture toughness of adhesive joints between carbon fiber-reinforced polymer composites (CFRP) and boron-alloyed high-strength steel under Mode I and II loading, based on linear elastic fracture mechanics (LEFM). Two adhesive types were examined: the excess resin from the prepreg composite, forming a thin layer, and a toughened structural epoxy (Sika Power-533), designed for the automotive industry, forming a thick layer. Modified double cantilever beam (DCB) and end-notched flexure (ENF) specimens were used for testing. The results show that using Sika Power-533 increases the critical energy release rate by up to 30 times compared to the prepreg resin, highlighting the impact of adhesive layer thickness. Joints with the thick Sika adhesive performed similarly regardless of whether uncoated or Al-Si-coated steel was used, indicating the composite/Sika interface as the failure point. In contrast, the thin resin adhesive layer exhibited poor bonding with uncoated steel, which detached during sample preparation. This suggests that, for thin layers, the resin/steel interface is the weakest link. These findings underline the importance of adhesive selection and layer thickness for optimizing joint performance in composite-metal hybrid structures.

Keywords: Al–Si-coated boron steel; adhesive layer thickness; double cantilever beam; epoxy adhesives; metal–composite adhesive joints.

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Conflict of interest statement

The authors declare no conflict of interest.

Figures

Figure 1
Figure 1
XMT image (original and segmented) of bulk adhesive sample showing uniformly distributed small particles within the adhesive as well as that there are no air bubbles.
Figure 2
Figure 2
The (a) DCB and (b) ENF samples using the resin in the prepreg as an adhesive.
Figure 3
Figure 3
The mold that was used to manufacture the DCB and ENF samples.
Figure 4
Figure 4
The (a) DCB and (b) ENF samples using Sika Power-533 MBX as an adhesive.
Figure 5
Figure 5
(a) DCB and (b) ENF test setup.
Figure 6
Figure 6
Load deflection curves in DCB test of a C–cS joint with (a) ET adhesive and (b) Sika adhesive.
Figure 7
Figure 7
Stress–strain curve for Sika Power-533.
Figure 8
Figure 8
Critical energy release rate vs. (∆a=a−a0) for a hybrid joint C–cS, with (a) ET adhesive and (b) Sika adhesive.
Figure 9
Figure 9
A hybrid C–uS joint with Sika adhesive. (a) Load deflection curve in DCB test, (b) GIc vs. (∆a=a−a0), and (c) the critical energy release rate for coated and uncoated steel specimens.
Figure 10
Figure 10
A C–C joint with ET adhesive. (a) Load deflection curve for DCB test, (b) critical energy release rate vs (∆a=a−a0), and (c) GIC for C–C and C–cS adherends of DCB specimens.
Figure 11
Figure 11
A C–C joint with ET adhesive layer. (a) Load deflection curve for ENF test and (b) GIIc vs. delta (∆a=a−a0).
Figure 12
Figure 12
A hybrid C–cS joint with ET adhesive. (a) Load deflection curve for ENF test and (b) GIIc vs. (∆a=a−a0).
Figure 13
Figure 13
Fracture surface of a hybrid C–cS DCB specimen with ET adhesive layer after the test.
Figure 14
Figure 14
Fracture surface of a C–C DCB specimen with ET adhesive layer after the test.
Figure 15
Figure 15
Sketch of a hybrid C–cS DCB specimen with an ET adhesive layer, indicating specific points for the microscopic image: (a) the surface of the composite part, which is in contact with the adhesive layer, contains the remaining ET resin; (b) the outer surface of the steel sheet; and (c) the inner surface of the steel sheet, which is in contact with the adhesive layer, shows only the coating layer on top of the steel, with no resin bonded to it.
Figure 16
Figure 16
Fracture surface of the hybrid C–cS DCB specimen with thick Sika adhesive.
Figure 17
Figure 17
Fracture surface of the hybrid C–uS DCB specimen with Sika adhesive.
Figure 18
Figure 18
Fracture surface of the hybrid C–uS DCB specimen with Sika adhesive showing fibers bridging the crack.
Figure 19
Figure 19
Fracture surface of a hybrid C–cS ENF specimen with ET adhesive.
See this image and copyright information in PMC

References

    1. Li X., Guan Z., Li Z., Liu L. A new stress-based multi-scale failure criterion of composites and its validation in open hole tension tests. Chin. J. Aeronaut. 2014;27:1430–1441. doi: 10.1016/j.cja.2014.10.009. - DOI
    1. Banea M.D., da Silva L.F.M. Adhesively bonded joints in composite materials: An overview. Proc. Inst. Mech. Eng. Part L J. Mater. Des. Appl. 2009;223:1–18. doi: 10.1243/14644207JMDA219. - DOI
    1. Kim K.-S., Yi Y.-M., Cho G.-R., Kim C.-G. Failure prediction and strength improvement of uni-directional composite single lap bonded joints. Compos. Struct. 2008;82:513–520. doi: 10.1016/j.compstruct.2007.02.005. - DOI
    1. Baker A.A., Jones R. Bonded Repair of Aircraft Structures. Martinus Nijhoff Publishers; Dordrecht, The Netherlands: 1988.
    1. Floros I.S., Tserpes K.I., Löbel T. Mode-I, mode-II and mixed-mode I+II fracture behavior of composite bonded joints: Experimental characterization and numerical simulation. Compos. B Eng. 2015;78:459–468. doi: 10.1016/j.compositesb.2015.04.006. - DOI

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