Experimental analysis of the influence of process parameters on the interlaminar toughness of carbon fiber prepregs for nautical applications

by Marco Arezio

The growing use of carbon fiber composite materials in the marine industry has made it essential to understand the damage mechanisms that can compromise their structural integrity.

Among these, delamination —the separation between the layers that make up a laminate—is one of the most critical fracture modes, capable of drastically reducing the stiffness and residual strength of the structure, especially in environments subjected to cyclic loads or accidental impacts. The causes of these fractures are multiple and often traceable to manufacturing imperfections, design errors, or unexpected service conditions.

In this context, optimizing the crosslinking conditions of epoxy matrix composites with unidirectional carbon fiber reinforcement appears to be a strategic lever for improving their interlaminar toughness.

The aim of this study is to experimentally investigate the influence of three key variables: the thickness of the sheet, the composition of the epoxy matrix and the cooling conditions at the end of the curing cycle.

Mode I (Double Cantilever Beam - DCB) and Mode II (End Loaded Split - ELS) tests were conducted to characterize the material behavior in the presence of different fracture mechanisms.

Materials and Methods

The materials examined are unidirectional prepregs based on carbon fiber and epoxy resin, commonly used in high-performance shipbuilding. The three prepregs used have different characteristics:

- Material A (SE84 - weight 300 g/m²)

- Material B (SE84 - weight 450 g/m²)

- Material C (MTM57 - weight 300 g/m²)

The fiber content by volume was 63% for all materials. The sheets were manually layered to obtain 10- or 12-layer unidirectional laminates, depending on the required test geometry.

The cross-linking process was conducted using a vacuum bagging technique, using peel-ply, variable-density perforated film, a bleeder, and a polyamide bag. Two curing cycles were applied: slow natural cooling (turning off the oven) and rapid cooling using immersion in ice water. The amount of resin in the laminate was adjusted by varying the pressure and the type of perforated film.

Influence of Foil Thickness

An increase in sheet thickness, obtained from 300 to 450 g/m² (materials A vs. B), resulted in a clear increase in delamination resistance during crack propagation. Material B also showed a significant presence of bridging fibers, a phenomenon that contributes to energy dissipation and crack progression retardation. However, the energy release rate at crack initiation was similar for both materials, suggesting that the contribution of thickness occurs mainly during crack propagation.

Effect of Epoxy Matrix

Given the same weight and fiber, the comparison between materials A and C highlighted the critical role of the resin formulation. Material C (MTM57) showed greater dissipated energy, with a final value approximately four times higher than that of material A.

Cooling Speed

Rapid cooling (CR) resulted in an increase in both maximum loads and fracture toughness in both test modes. This behavior is attributable to the increased toughness of the epoxy matrix developed during a faster glass transition, compared to a potentially weaker fiber-matrix interface. In mode II tests, however, propagation occurred instantaneously, preventing the determination of R-curves but confirming the trend through the initiation values.

Resin Content Variation

Increasing the resin percentage (from 27% to 36%) resulted in an improvement in Mode I strength, especially between the 27% and 33% levels, with asymptotic behavior for higher values. This interpretation is related to the greater extension of the interlaminar plastic zone, which allows for more effective energy dissipation at the crack tip.

Interestingly, in mode II, an inverse trend was observed, with a decrease in toughness as the resin content increased. This apparently contradictory behavior suggests that the shear stresses characteristic of mode II penalize excess matrix, which can lead to lower overall cohesion.

Conclusions

Experimental investigations have shown that process parameters in the manufacturing of carbon fiber and epoxy resin laminates have a significant impact on delamination resistance. While crack initiation appears relatively insensitive to most variations, crack propagation is strongly influenced by:

- The thickness of the plate, which promotes bridging fibres and dissipates more energy

- The type of matrix, which determines the ability of the resin to support the development of the fracture zone

- Rapid cooling, which improves the toughness of the matrix at the expense of the interface

- The percentage of resin, which strengthens the response in mode I but can weaken it in mode II

The complexity of the observed mechanisms and the different response between Mode I and Mode II highlight the need for a multiscale and multifactorial approach in the design of high-performance composite materials, especially in critical sectors such as the nautical one.

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Sources and References

ISO 15024:2001 – “Fibre-reinforced plastic composites – Determination of Mode I interlaminar fracture toughness.”

D. R. Moore, A. Pavan, "Fracture Mechanics Testing Methods for Polymers, Adhesives and Composites", ESIS TC4.

K. Hojo et al., “Mode II interlaminar fracture of composite materials: Experimental methods and recent understanding,” Composites Science and Technology.

MJ Hogg, “Matrix effects on interlaminar fracture toughness”, Journal of Composite Materials.