Recovery of Mode I self-healing interlaminar fracture toughness of fiber metal laminate by modified double cantilever beam test

Highlights

• Modified DCB method is proposed to obtain G_IC between metal and composite interface for fiber metal laminate (FML) samples.

• Bending effect of backing beam and influence of ductility of Al in determining the fracture toughness is explained.

• Self-healing thermoplastic adhesive film is used to improve the fracture toughness at metal and composite interface.

• This work provides an alternative method to estimate the self-healing efficiency at MCI for FML structures.

Abstract

Fiber metal laminate (FML) is a sandwich combination of thin metal and a composite layer which possess high fatigue, creep, and corrosion resistance. The performance of FML depends on the bonding behavior between the metal and composite interface (MCI). Introducing Redux 335k polymer interleaf can improve MCI, but cannot retain the mechanical properties after its first delamination. Toughened self-healing co-polymer film EMAA (Ethylene-methacrylic acid) can be used as an alternative due to its self-healing property which can recover the mechanical property after each delamination. Double-cantilever-beam (DCB) test is carried out by supporting a thick CFRP composite as a backing beam to avoid the yielding of thin metal in FML to estimate the interlaminar fracture toughness (G_1C) at the MCI. Results from the DCB test shows that the incorporation of EMAA thin film improves G_1C at the MCI. However, G_1C gradually decreases despite recovering its fracture toughness after each delamination when the self-healing reaction is thermally activated. The Mode I fracture toughness of EMAA before the first heal is 1.95 kJ/m², which is higher due to its thermoplastic nature. This article emphasizes only on the recovery of Mode I self-healing efficiency of EMAA at MCI. However, the article also shows the fracture toughness at MCI by using non-self-healing film adhesive at MCI. This comparison is to understand that the standard structural adhesive (Redux 335K) cannot retain the mechanical properties after its first delamination, and also to understand the failure modes of both thermosetting and thermoplastic interleaf at MCI.

Introduction

Sandwich combinations of thin metal layers with fiber-reinforced plastic (FRP) layer forming a hybrid composite structure are termed as FML. Examples include aluminium, magnesium or titanium which are currently used as a metal layer, and either using glass-reinforced, carbon-reinforced or kevlar-reinforced composite as the fiber-reinforced layer. Combinations of such ductile, isotropic metal and fiber-reinforced layer possess excellent impact characteristics, superior fatigue resistance, and excellent corrosion resistance property [1]. Firm bonding between the metal layer with FRP provides excellent mechanical properties. Introducing a thin film adhesive as a polymer interleaf during the co-curing fabrication process at the MCI is an ideal solution for improved bonding. Redux 335K which is an epoxy-based thin film structural adhesive can be used, however since it is not a self-healing material; the mechanical properties deteriorated immediately after initial delamination. A high degree of adhesion can be assured by introducing thermoplastic interleaf between metal and composite interface [2]. Introducing a highly toughened self-healing thermoplastic material can be adopted to provide superior mechanical properties prior to its first delamination, then mechanical properties can be recovered by thermal activation after delamination [3]. Elium® is a new liquid thermoplastic resin which can be used as thermoplastic matrix and can be processed at room temperature [4]. However, Elium resin is liquid at room temperature and cannot be formed as a thin adhesive film for MCI as an adhesive intleraf. The mendable thermoplastic self-healing EMAA (Nucrel® 2940) can be used as a polymer interleaf to ensure a high degree of adhesion between metal and composite to improve the performance of FML.

Wang et al. [5] used EMAA by placing a small patch between the composite laminate interface and characterized its healing mechanism. A thermally reversible healing mechanism is used to heal the EMAA after its delamination and has been found to heal repeatably via thermal activation, after being subjected to multiple damage events. The mechanism has been shown to consist of a sequence of chemical and physical processes, but fundamental to any healing occurring is the requirement for esterification to take place between the carboxylic acid of the EMAA and residual hydroxyl groups from the cured epoxy resin [6]. Significant binding between EMAA and cured resin is carried out by pressure delivery mechanism when post-cured at 150 °C for 30mins. The process involves by forming a hydrogen bond between acid groups in the EMAA and hydroxyl, ether group and tertiary amine group within the polymer network. This show hydrogen bonding between EMAA and epoxy plays a vital role in bonding mechanism during the EMAA healing process [7].

Pingkarawat et al. [8] added small pellets of EMAA in the resin and found an increment up to 190% of fracture toughness. EMAA filament is stitched in through-thickness of the composites to increase the interlaminar fracture toughness by forming large scale crack bridging transition zone along with the delamination and found the acid-hydroxyl reactivity is essential to activate the healing reaction in epoxy based composite [9,10].

The healing efficiency of EMAA by tensile load experiment confirms, EMAA is good toughening and healing agent compared to PEGMA (poly (ethylene glycol) methacrylate), and EVA (Ethylene-vinyl acetate) self-healing material [11]. The healing fracture toughness is around 4 kJ/m² by placing three layers of a thin film of the same thickness EMAA placed in-between the composite [12]. Khor et al. [13] found the fracture energy can be expressed in terms of elastic-plastic properties of the constrained layer, with parameters of (t_h) thickness of adhesive and (ε_f) failure–strain of the constrained layer (EMAA self-healing co-polymer). From the tensile stress-strain data [14], the fracture energy can be determined for EMAA constrained layer between the monolithic composite interface. The calculated fracture energy G_1c for a thickness of 150 μm is 2.25 kJ/m² [5].

The modified double cantilever beam (DCB) method was adopted to study the Mode I delamination behavior. In order to avoid yielding of thin metal in FML, and to determine the fracture toughness, thick aluminium is used as a backing beam [15]. Secondary bonding of FML to thick aluminium adherend is a tedious process, and the repeatability of the result is not consistent. The debonding may happen between the backing beam and the FML when the bonding agent is weaker than adhesive at the MCI. Perfect alignment of FML to the backing beam is necessary. If not, it may be lead to inconsistent result for all the samples. To avoid the yielding of metal and to have consistent results, the composite backing beam can be replaced to thick aluminium backing beam. Attaching the backing beam can be done as one step process and can be carried as a co-curing process during the FML fabrication. Mode I interlaminar fracture toughness is calculated by the modified area method, which has the correction factor for the bending of the adherends (backing beam effects) [15]. This report highlights the self-healing recovery of EMAA after the delamination at MCI by double cantilever beam testing, and the result is compared to its counterpart the thermosetting Redux 335 K polymer interleaf by thick composite replacing aluminium alloy backing beam.