Herringbone-Bouligand CFRP structures: A new tailorable damage-tolerant solution for damage containment and reduced delaminations

Highlights

• First high-performance Herringbone-Bouligand microstructure (with CFRP).

• Novel micro-moulding technique to design tailorable composite solutions.

• The Herringbone-Bouligand laminates achieved highly reduced (71%) delamination damage.

• Damage successfully contained within the tailored Herringbone-Bouligand region.

• Herringbone-Bouligand shown to be more damage-tolerant than classical Bouligand.

Abstract

In this work, we design, prototype, test and analyse the first high-performance Herringbone-Bouligand microstructure (with Carbon Fibre Reinforced Plastic (CFRP)) inspired to the high-impact-resistant mantis shrimp's dactyl club. To this end, we devised the first prototyping procedure to manufacture point-by-point tailorable Herringbone-Bouligand CFRP microstructures; this was based on the micro-moulding of uncured CFRP prepreg, and led to mimicking features of the club microstructure never achieved before with CFRPs. We investigated the damage tolerance of the prototyped Herringbone-Bouligand CFRP laminates, compared against ‘classical’ Bouligand CFRP laminates, using quasi-static indentation tests. Our test results show that the Herringbone-Bouligand microstructure resulted in delayed onset of delaminations, reduced in-plane spreading of damage, increased energy dissipation capability, and in the containment of damage within the tailored Herringbone-Bouligand region. We conclude that Herringbone-Bouligand CFRP microstructures offer an excellent tailorable damage-tolerant solution with great potential for composite applications where resistance to through-the-thickness loads is paramount.

Introduction

Helicoidal lamination sequences such as Bouligand-inspired architectures (Fig. 1a) have been widely investigated in the past years, leading to successful attempts to enhance the damage tolerance to through-the-thickness loads of fibre reinforced composite materials, including glass fibre reinforced composites [1] and, more frequently, Carbon Fibre Reinforced Plastics (CFRPs) [[2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13]]. Motivated by the enhanced damage tolerance under dynamic and quasi-static through-the-thickness loads of tailored CFRP Bouligand solutions with respect to traditional CFRP laminates, there have been recent developments in our knowledge of the mechanics of Bouligand CFRPs [[2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13]]; these, together with recent developments in automated tow placement, have greatly increased the relevance of Bouligand and Bouligand-inspired structures in Engineering practice.

Bouligand architectures can be found in a wide variety of biological microstructures such as the one of the mantis shrimp's dactyl club (Odontodactylus scyllarus in Fig. 2a and 2b). While the high damage tolerance of the club has been largely attributed to the features of the so-called periodic region of the microstructure [14,15], a recent investigation of the mantis shrimp [15] has revealed that the microstructural traits of the club's impact region greatly contribute to the overall damage tolerance of the club. These recently-identified features have never been explored with CFRPs.

By observing the cross-section of the mantis shrimp's dactyl club (Fig. 2c), we can distinguish three main areas: (i) the periodic region, (ii) the impact region and (iii) the impact surface [15] (Fig. 2c). The periodic region is characterised by a through-the-thickness stack-up of several sublaminates, where each sublaminate represents a Bouligand unit, i.e. an helicoidal layup of planar chitin-fibre layers, parallel to each other, with small mismatch (pitch) angles ranging from 6.2° to 1.6° [3] for a rotation of 180° within each unit (Fig. 1a). The impact region consists of Bouligand units, repeated through the thickness of the club and featuring a periodic out-of-plane (z-thickness direction) fibre component, resulting in several wavy plies stacked in a helicoidal fashion (see Fig. 1b and the ‘zig-zag’ (Herringbone) pattern in Fig. 2c). The wavy architecture defines bi-sinusoidal interfaces between neighbouring plies that can be expressed as

$$z\left(x,y\right)=\frac{A}{2}\text{sin}\left(\frac{2\pi }{\lambda }x\right)\text{sin}\left(\frac{2\pi }{\lambda }y\right),$$

where A is the amplitude and λ the wavelength of the Herringbone pattern [15]. While the value of λ is uniform across the impact region ($\lambda \approx 45\mu \text{m}$), the amplitude A progressively decreases from 100 $\mathrm{\mu m}$ at the impact region/impact surface interface to 70 $\mathrm{\mu m}$ at the impact region/periodic region interface, thus avoiding the presence of a weak interface between the two regions [15]. Finally, the impact surface consists of a highly-oriented crystalline apatite mineral phase layer [15].

For the periodic region, the ability of the club in nesting helicoidal sub-critical matrix cracks inside each Bouligand unit results in a highly-dissipative failure mechanism which delays fibre failure and enhances energy dissipation [14]. For the impact region, Yaraghi et al. [15] have shown, using 3D-printed Herringbone-Bouligand structures compared against Bouligand architectures under uniaxial compression (a photo-polymer was used as fibre reinforcement while a rubber-like elastomer as matrix), that the Herringbone-Bouligand pattern provides a mechanism for stress redistribution, leading to a more uniform through-the-thickness stress state and a higher compressive stiffness. This was expected to promote damage diffusion under compression. Additionally, a combined experimental/numerical analysis of the impact surface of the club, showed that the nano-particles within the impact surface are able to redistribute contact stresses via plastic deformation through particle motions, particle pile-up and crack redirection [15]. This was expected to prevent stresses from localising on the peaks of the underlying Herringbone-Bouligand pattern.

Recently, Han et al. [17] created a basalt fibre-reinforced sandwich composite attempting to mimic the Herringbone pattern with a 2D woven fabric and the periodic region with layers of discontinuous basalt fibres. Their results showed that, compared to a traditional basalt discontinuous fibre composite, the club-inspired samples achieved higher tensile and compressive moduli, higher tensile strength and higher impact resistance.

While the highly-dissipative failure mechanisms typical of the periodic region have been successfully exploited with CFRPs [7,9,12,13], the recently uncovered features of the impact region of the club, fundamental to the resistance of the mantis shrimp's dactyl club to through-the-thickness loads, have never been explored with CFRPs.

In view of this, we devised an original study to investigate high-performance Herringbone-Bouligand microstructures with CFRP materials, aiming at creating a point-by-point tailorable CFRP bio-inspired composite which comprises the main features of the impact surface, impact region and periodic region of the mantis shrimp dactyl club (Fig. 2e). To this end, we devised a manufacturing technique, based on the micro-moulding of uncured prepreg CFRPs and used it to prototype the first CFRP Herringbone-Bouligand microstructure. We used quasi-static indentation (QSI) tests to compare the damage tolerance to through-the-thickness loads of the prototyped Herringbone-Bouligand microstructure against classical Bouligand laminates. We demonstrate that, by mimicking the main features of the microstructure of mantis shrimp's dactyl club, we can achieve similar toughening mechanisms to the ones observed in nature, hence greatly enhancing the damage tolerance of CFRP structures.

This paper contains the following original contributions:

• we successfully design, prototype, test and analyse the first high-performance Herringbone-Bouligand microstructures (with CFRP);

• we devise the first prototyping technique, based on the moulding of uncured prepreg, suitable for designing point-by-point tailored Herringbone-Bouligand microstructural solutions;

• we show that, with a tailored Herringbone-Bouligand design, it is possible to achieve highly reduced in-plane spreading of damage, and contain this damage to the tailored Herringbone-Bouligand region;

• we demonstrate the capability of bi-sinusoidal CFRP ply interfaces in activating sub-critical dissipative mechanisms of failure such as discontinuous (i.e. non-linked) delamination areas, deflected delaminations, and diffused matrix cracks; and

• we show that a tailored Herringbone-Bouligand microstructural design can greatly enhance the damage tolerance of tailored classical Bouligand CFRP microstructures which have been assessed, in the literature, as capable of greatly outperforming ‘conventional’ CFRP structures [12,13].