Fiber reinforced plastic (FRP) composites are extensively used in aerospace, automobile, marine, civil construction and wind turbine industries, where they are competitive for some properties, such as in-plane strength-to-weight ratio and fatigue resistance, with conventional metals. However, mechanical properties in FRP are controlled by their fiber direction [1
]. As a consequence, composite laminates are usually weak when subjected to out-of-plane loading, due to the lack of fiber reinforcement in the through-thickness direction [2
]. Hence, interlaminar failure, usually referred to as delamination, occurs as key failure mechanism within the interplay–resin domain, which is activated by out-of-plane stresses [3
]. Several techniques, such as through-thickness stitching (TTS), Z-pinning, 3D weaving, were employed to enhance the strength of the interlaminar region. However, these techniques may on the other side cause a reduction in in-plane mechanical properties, fiber volume loss, and also cause increased production cost [5
An alternative approach to improve the out-of-plane properties of FRP composites is to modify the resin interlayer domain [8
]. The fracture toughness of epoxy resin employed in FRP composites can be enhanced by adding microsized fillers [9
]. Similarly, the use of carbon nanofibers (CNFs) and carbon nanotubes (CNTs) has attracted a considerable attention as a possible strategy for improving the interlaminar fracture toughness of FRP composites, because of their high stiffness, strength, specific surface area and aspect ratio [13
]. The presence of CNTs promotes percolation at low volume fraction, enhances the contact area and adhesion with the epoxy, introducing energy dissipation mechanisms, such as fiber debonding, pullout, crack bifurcation, crack arresting and/or nanocrack toughening [12
]. However, in practice, the modulus and fracture toughness of FRP composites is not always increased by adding these nanofillers, since it is difficult to control their uniform dispersion in FRP composites [18
]. For this reason, the controlled introduction of defects in the interface has been proposed, together with methods, such as ion implantation, able to modify the characteristics of the filler [22
]. However, it is an expensive and challenging process to implement the introduction of these nanofillers in FRP composites for large-scale structural applications.
On the other side, the waste materials generated by the FRP industries usually end up in landfill due to the difficulties in recycling thermosetting polymer matrices [23
]. In the particular case of fiberglass, as reported by Pickering et al. [24
], mechanical recycling of glass fibers offers significant environmental and economic advantages and the recovered fibrous/powdered products can be reincorporated either as reinforcement or filler in new composite materials. This represents an opportunity, provided that the structural modifications involved for practical use of these fillers do not entail excessive costs, an aspect very critical for the recycle of fiberglass, as it is discussed in [25
]. If this is the case, therefore they are employed as close as possible to the as-received state after grinding, recycled milled glass fibers can be employed to reinforce the polymeric materials, as an alternative to commercial particles/fillers, therefore enhancing their mechanical properties.
This paper focuses on improvement of mode I and mode II delamination resistance by employing milled glass fiber fillers as filler particles in glass/epoxy composite laminates, discussing the effect of interface modification by appropriate microscopic observation and linking it to the flexural behavior of laminates. This is a challenging task, especially when it comes to determining empirically toughness, since it requires an accurate control over the experimental dimensions and the measurements, which brought also to innovative ways to measure toughness on nanocomposites [26
]. This has been reached after fine-tuning the experiment over a number of previous investigations. In this work, Mode I (DCB) and Mode II (ENF) tests have been performed to evaluate the fracture toughness of glass/epoxy composites. The specific objective of this work lies in the idea of giving insight on the possibility to improve these properties of glass/epoxy composites by using definite amounts of the filler, suggesting that the potential of this introduction is strictly linked with the fracture mechanism, as it is demonstrated also by scanning electron microscope (SEM) micrographs.
Experimental investigation of Mode I and Mode II interlaminar fracture toughness test on glass/epoxy samples were performed with various loading of recycled milled glass fiber filler (2.5, 5, 7.5 and 10% by weight of epoxy matrix). This work was accomplished with insight of implementing low cost, economic, and recycled filler to have a sustainable substitute over the commercially available expensive fillers.
The main conclusions can be drawn as follows:
The addition of 5 wt.% milled glass fibers have improved the mode I (GIC) and mode II fracture toughness (GIIC) significantly by 102% and 175%, respectively. This observation is attributed to the good filler/matrix interfacial strength and high energy dissipation mechanism through crack deflection, filler/matrix debonding and interlocking.
The inclusion of milled glass fibers in the interlaminar domain has significantly improved the flexural strength which was attributed to enhanced load transfer at lower filler loading. In contrast, higher filler loading leads to local agglomeration of excess fillers in the inter/intralaminar resin channel and poor adhesion (insufficient resin), which result in reduction of strength and fracture toughness.
Therefore, inclusion of milled glass fibers has substantially improved the interlaminar fracture toughness of the glass/epoxy laminates without affecting the flexural properties. The conclusion is that milled fibers, despite their simplicity of recovery from end-of-life composites, can represent alternative fillers for composites laminates to be efficiently used in limited amounts—not exceeding a few percentage points—before agglomeration issues arise.