**1. Introduction**

With their highly specific mechanical properties, carbon fibre-reinforced plastics (CFRP) are suitable for use in high-performance components such as crash- and impactloaded structures such as composite fan blades [1,2]. In addition, these materials have the advantage that the material properties and especially the failure behaviour can be adjusted. The focus is mostly on the adaption of the fibre orientation to the dominant stress in order to increase the stiffness and strength of the materials [3]. This approach often leads to a brittle failure behaviour of the corresponding structures so that on the one hand, the structural integrity is compromised, and on the other hand, the energy absorption capacity is not optimally utilised [4].

Besides the choice of fibre and matrix material as well as the textile architecture, there are essentially two approaches for increasing the energy absorption of fibre-reinforced plastics. Firstly, the fibre–matrix adhesion is specifically adjusted by influencing the boundary layer during manufacturing. Initial work on this goes back to [5], where the influence of the adhesion properties of an epoxy matrix to boron filaments and their influence on the

**Citation:** Kuhtz, M.; Richter, J.; Wiegand, J.; Langkamp, A.; Hornig, A.; Gude, M. Concepts for Increased Energy Dissipation in CFRP Composites Subjected to Impact Loading Conditions by Optimising Interlaminar Properties. *Aerospace* **2023**, *10*, 248. https://doi.org/ 10.3390/aerospace10030248

Academic Editors: Spiros Pantelakis, Andreas Strohmayer and Jordi Pons-Prats

Received: 30 January 2023 Revised: 16 February 2023 Accepted: 23 February 2023 Published: 3 March 2023

**Copyright:** © 2023 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

fracture toughness of such composites is described. The work in [6] follows this approach and shows that the fracture toughness is further increased by alternating high and low fibre–matrix bonds along the fibres. Secondly, especially in multi-layered composites, suitable interlayers are inserted through the concept of controlled interlaminar bonding. In this methodology, primarily thermoplastic interlayers are inserted into layer-based thermoset composites. A good overview of methods that lead to an improvement of the interlaminar properties by interleaved layers on a thermoplastic basis, but also to a more brittle failure behaviour with lower energy absorption, is given in [7]. In contrast, methods with lower interlaminar properties generally result in significantly higher energy absorption with improved structural integrity and a moderate decrease in structural strength [8–10].

The mentioned studies are exclusively experimental, and their conclusions are based on purely empirical statements. The study presented here, however, introduces a simulation approach that can be used to identify, analyse, and target the phenomenology of the energy absorption mechanisms of impact-loaded composite structures. A methodology is presented that describes the interface modification of CFRP in the framework of finite element analysis (FEA) up to the structural scale. Thus, this approach can be used to extend the experimental database with virtual tests. Finally, it is shown how the interface modification approach can be adapted to an optimisation process [11,12]. Thus, it is possible to derive concepts with which the energy absorption behaviour of impact-loaded CFRP structures can be significantly increased.

#### **2. Materials and Methods**

Deformation, damage, and failure behaviour of interface-modified CFRP beams is investigated numerically based on the experimental data of [10]. An interfacial modification by means of perforated polytetrafluoroethylene (PTFE) foil is used (Figure 1a). Figure 1b shows the geometric dimensions of the perforated PTFE foil.

**Figure 1.** Modification concept. (**a**) Interface modification concept by inter-leaved perforated PTFE foils with quadratic holes; (**b**) geometry parameters of perforated PTFE foil: *a:* edge length of hole, *b*: distance between holes, *d*: edge length of unit cell [13].

As PTFE does not adhere to the composite material, the interface properties are influenced exclusively by the geometric (mesostructural) dimensions and can be adjusted by the so-called interlaminar contact area *κ*:

$$
\kappa = \frac{2}{d^2} \frac{a^2}{}.
\tag{1}
$$

In the experimental study, a constant value of 4 mm was chosen for the edge length of the perforation *a* [13]. Based of targeted interlaminar contact area *κ*, the distances between two perforations *b* and the length of the unit cell *d* are derived. The investigations are carried out using the example of a plain weave fabric composite based on a HexPly M49 200P prepreg semi-finished product from Hexcel. Consolidation is carried out in an autoclave process according to the manufacturer's specifications, resulting in an average fibre volume content of 55% [14]. Further details about the physical and mechanical properties are shown in Table A1 in Appendix A.
