Review
Impact resistance of fiber-metal laminates: A review

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Abstract

Combining the suitable properties of metals and fiber reinforced composites, as the idea behind the application of new types of materials, called fiber metal laminates (FMLs), have lead to superior impact properties as well as considerable improvement in fatigue performance. The characteristics of FMLs under impact loading and the ways to improve their properties to withstand this type of loading could be of particular importance in aerospace structures and other applications. This paper reviews relevant literature which deals with experimental evidence of “material related” and “event related” impact resistance parameters as well as the articles related to theoretical and numerical simulation of impact loading of FMLs. Relevant results will be discussed and the recommendations that need to be resolved in the future will be addressed.

Introduction

During the last decades the application of composite materials in various structures has become increasingly popular. Especially in aerospace structures, composites are preferred above conventional materials because of their advantages high specific strength/stiffness and good fatigue resistance. Needs for improving material properties resulted in hybrid materials built up from thin metal sheets and fiber-reinforced adhesives. Fiber-Metal Laminates (FMLs) are composed of alternatively stacked metal and fiber-reinforced composite layers (see Fig. 1), such that the superior fatigue and fracture characteristics associated with fiber-reinforced composite materials may be combined with the plastic behaviour and durability offered by many metals [1]. The development of the family of highly fatigue resistant FMLs, Arall and Glare started in the 80's at the Delft University of Technology [2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16]. These laminates consist of thin high- strength aluminium alloy sheets (typically 0.3–0.5 mm thick) bonded together with alternating unidirectional composite prepregs. The prepregs are aramid or glass fibers in an epoxy resin. Some major advantages of FMLs are: high specific strength, better damage tolerance to fatigue crack growth, fire resistance, blunt notch strength, formability, repairability, etc [5].

For example, high fatigue resistance is achieved by fiber bridging of fatigue cracks [5], [17], [18]. If a crack has initiated in the aluminium alloy layers, some limited delamination will occur at the interfaces between the metal and the fibers. That will accommodate stress re-distribution from the metal to unbroken fibers in the wake of crack. Crack bridging provided by the strong fiber restrains crack opening, and thus reduces the driving force for crack growth in the metal layers [17].

Nowadays, Glare is produced in six different standard grades. These grades are all based on various lay-ups of fiber epoxy prepreg layers composed of unidirectional S2 -glass fiber embedded in a FM94 adhesive. An individual prepreg layer with unidirectional fibers has a nominal fiber volume fraction of 0.59. It is possible to stack prepreg layers with different fiber orientations in between two aluminium layers, resulting in different standard Glare grades [1]. The Glare grades are listed in Table 1.

Impact damage is an important type of failure in aircraft structures. Among different types of damages in an aircraft such as fatigue, corrosion and accidental (impact) damage, and associated repairs, it is reported at least 13% of 688 repairs to 71 Boeing 747 fuselages were related to impact damage [19]. As Vlot [20] reported impact damage is usually located around the doors, on the nose of the aircraft, in the cargo compartments and at the tail (due to tail scrape over the runway).

Impact damage of aircraft is caused by sources such as: runway debris (in order of 60 m/s), hail (on the ground 25–60 m/s and in flight in the order of hundreds of meter per second), maintenance damage or dropped tool (less than 10 m/s), collisions between service cars or cargo and the structure (the velocity is low), bird strike (high velocities), ice from propellers striking the fuselage, engine debris, tire shrapnel from tread separation and tire rupture and ballistic impact (for military aircraft) [20]. However, different impact regimes are possible in aircrafts which should be considered during the design process for reasons of safety. It is also important for economical reasons, because the damage has to be detected and repaired during maintenance. In the present paper, “low-velocity” is regarded to the impacts caused by dropped tool (less than 10 m/s) and “high-velocity” is related to the impacts by devices such as gas gun.

Although the initial attention to Glare was on improvement of fatigue properties of aircraft components, recently the use of Glare has been expanded because of its improved impact properties relative to monolithic aluminium of the same areal density [21]. Similar advantages of considerable impact resistance of Glare have been reported by Hoo Fat [21] for high-velocity impact, such that for an epoxy-based Glare a 15% increase in the ballistic limit was observed compared to bare 2024 aluminium with the same areal density. This improvement is expected to be more for thermoplastic based FMLs at high-velocity impact [22].

In spite of mentioned advantages of FMLs, their impact properties still need more understanding and attention. Although many articles have been published regarding to impact resistance of FMLs, the research on this part of FMLs' performance is still in the early stages. There are several issues to be addressed related to the modeling and experimental investigation of this topic.

The purpose of this paper is to review relevant literature related to impact properties of FMLs in theoretical and experimental studies. During this review, the key technical issues that need to be solved in the future are also addressed. While review articles and even books on impact properties in conventional composites have been published [23], [24], [25], the authors concluded that a detailed review article on impact response of FMLs has not yet been published and that such an article should be of significant value to the FMLs research community and industry.

Section snippets

Experimental studies on impact response of FMLs

The articles that report the experimental studies on impact loading of FMLs are summarized in Table 2.

Simulation of impact performance of FMLs

A systematic and step by step approach to the simulation of impact behavior of the structures is to start from quasi-static indentation followed by low-velocity impact and finally with increasing the strain rate of loading, high-velocity impact should be considered.

Finite element modeling

In spite of considerable efforts to develop efficient numerical models for predicting impact damage in the composite laminates [78], [79], [80], there are a few articles in the literature [81], [82], [83], [84], [85], [86], [87] that report numerical modeling impact resistance of FMLs. Indeed, the difficulties associated with the phenomena of plasticity, crack growth, delamination and perforation caused by impact loading and the nature of loading rate, the modeling of FMLs is a bothersome work.

Conclusions

In the past 20 years, numerous experiments have demonstrated that FMLs show superior impact properties relative to bare aluminium sheets of the same areal densities. Accordingly, this paper summarizes significant results on measurement and simulation of the impact resistance of FMLs as discussed by influences of “material” and “event” related parameters. The effects of some parameters on the impact properties of FMLs are well known; whereas there are still some uncertainties on the exact roles

Recommendations for future works

Considerable attempts to elucidate impact properties of FMLs, have answered many relevant questions, but to achieve a full and detailed understanding of the problem, there is a long way; some of steps are listed below:

  • There are a limited number of articles concerning theoretical modeling of impact response of FMLs. When the amount of work associated with the testing, data recording and analysis and post impact non destructive or destructive inspections are considered, the advantages of

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