SELF-HEALING ADHESIVE FILM FOR COMPOSITE LAMINATE REPAIRS ON METALLIC STRUCTURES

Transcription

1 SELF-HEALING ADHESIVE FILM FOR COMPOSITE LAMINATE REPAIRS ON METALL STRUCTURES Gina M. Miller, Jason M. Kamphaus, Scott R. White and Nancy R. Sottos Beckman Institute, University of Illinois at Urbana-Champaign, 45 N. Mathews Ave., Urbana, IL, 6181, USA Deparment of Aerospace Engineering, University of Illinois at Urbana-Champaign, 36 Talbot Lab, 14 S. Wright St., Urbana, IL, 6181, USA Department of Materials Science and Engineering, University of Illinois at Urbana- Champaign, 134 W. Green St., Urbana, IL, 6181, USA A novel application for self-healing materials is thin epoxy structural adhesive films. These films are commonly used in the aerospace industry for composite laminate repairs of metal structures. In this work a self-healing epoxy adhesive film is developed using an unsupported commercially available adhesive film, FM 73 (Cytec Engineered Materials) as the polymer matrix. The mechanism for healing is consistent with previously demonstrated self-healing polymers [1]. Embedded microcapsules rupture during crack propagation and deliver the healing agent endo-dicyclpentadiene (DPCD) to the crack plane via capillary action. Upon contact with embedded Grubbs catalyst, ring-opening-metathesis-polymerization (ROMP) occurs effectively healing the material [1]. Fracture toughness is assessed using width tapered double cantilever beam (WTDCB) specimens loaded in mode I. The energy release rate is measured for manual injection and self-activated specimens. Healing efficiencies from 25-5% based on fracture toughness recovery were observed for both specimen types. Keywords: Adhesives, self-healing, composite laminate repairs Abbreviations: DCPD, dicyclopentadine; ROMP, ring-opening metathesis-polymerization; WTDCB, width tapered double cantilever beam 1 Introduction In the aviation industry composite doubler systems have been developed to repair damaged aluminum structure. When a fatigue crack occurs in the parent metal structure, a composite patch is attached to the metal surface using an epoxy adhesive film to locally reinforce the parent structure. Failure of composite doublers often occurs in the adhesive film and can be difficult to detect using nondestructive testing. This research aims to develop a self-healing adhesive film for use in composite laminate repairs. 1 Springer 27

2 The development and selection of the appropriate adhesive polymer matrix, catalyst and healing agents is reviewed. Healing efficiency is assessed through fracture testing of width tapered double cantilever beam specimens loaded in mode I. Healing efficiency is evaluated for specimens in which repair is accomplished manually and for the case when the catalyst phase is embedded, but the healing agent is delivered manually to the crack plane. 2 Materials and methods 2.1 Adhesive film system FM 73 adhesive film (Cytec Engineered Materials) was chosen as the matrix for the selfhealing adhesive film system. This adhesive is a general purpose aerospace epoxy adhesive film that is suitable for adhesion to both metal and structural composite substrates. FM 73 adhesive film is available as an unsupported film or supported by either a polyester knit or mat. The unsupported form, FM 73U was chosen for this study. 2.2 Healing system The mechanism for healing is consistent with previously demonstrated self-healing polymers [1]. For manually healed specimens, healing occurred via a precatalyzed mixture of DCPD and as-received 1 st Generation Grubbs catalyst (Aldrich) injected into the crack face. For self-activated specimens, 1 st Generation Grubbs catalyst was embedded in the adhesive film, and DCPD injected into the crack face. 2.3 Specimen manufacture and preparation Prior to manufacture, steel adherends underwent both mechanical and chemical surface preparation. To mechanically prepare the adherend surface, mechanical sanding using 8 grit sandpaper was carried out followed by manual sanding with a circular motion using 8 grit sandpaper. To chemically prepare the steel surface, a 1 vol% solution of the silane coupling agent (3- Glycidyloxypropyl)trimethoxysilane in deionized water was prepared. Using a squeeze bottle, the mechanically prepared surface was rinsed several times with the silane solution. After drying for 3 minutes the adherends were placed in a 6 ºC oven for 1 hour. Upon returning to room temperature, the adherend surface was coated with a primer, BR (Cytec Engineered Materials), using a bristle brush. To complete the chemical surface preparation the adherends were placed in an oven at 6 ºC for 15-2 minutes, removed and cooled to room temperature. For manually injected specimens, a layer of FM 73 adhesive film was placed on each prepared adherend. A 25 μm thick fluoropolymer release ply was placed between the mating films and served as a precrack. Once lay-up was complete, samples were autoclaved according to the cure cycle shown in Figure 1. 2 Springer 27

3 Temperature ( o C) Vacuum Pressure ( " Hg) Vacuum Pressure ( " Hg) 2 Temperature ( o C) Time (min) Figure 1: Cure cycle of adhesive film in autoclave For self-activated specimens, a layer of FM 73 adhesive film was placed on each prepared adherend. Grubbs catalyst at 2.5 wt% was deposited on the adhesive film adhesive film surface and distributed as evenly as possible using a razor blade. As with the manually injected specimens, a 25 micron thick fluoropolymer release ply was placed between the mating films and served as a precrack. Once lay-up was complete, the samples were autoclaved according the cure cycle shown in Figure Specimen geometry The width tapered-double cantilevered beam (WTDCB) specimen geometry was chosen for evaluation of healing efficiency since it provides a crack length independent measure of fracture toughness [2]. This geometry, shown in Figure 2, allows for the determination of energy release rate, by, P k G I =, 3 Eh where P is the load, k is the width taper ratio of the specimen, E is the modulus of elasticity, and h is the half thickness of the specimen [2]. The healing efficiency is defined as the ratio of healed fracture toughness to virgin fracture toughness. For the WTDCB, this is equal to the ratio of critical loads of the healed to virgin material [2], G K P η = = =. G K P C C 3 Springer 27

4 Figure 2: Geometry of width-tapered double cantilevered beam (WTDCB) specimen which consists of a thin layer of adhesive film between two steel adherends 2.5 Test procedure Specimens were loaded in displacement control at a rate of 5 mm/min. The specimens were loaded until the crack opening displacement was 7 mm. For manually healed specimens,.5 ml of DCPD was mixed with 2.5 mg of Grubbs catalyst and manually injected into the crack plane using a syringe. For self-activated samples,.5 ml DCPD was manually injected into the crack plane. After injection, the specimens were unloaded quickly at a rate of 15 mm/min. After approximately 24 hours of healing at room temperature the specimens were reloaded to a crack opening displacement of 7 mm at a rate of 5 mm/min and then unloaded at 15 mm/min. 3 Results and discussion Besides functioning as an experimental control, manually healed specimens indicate the maximum amount of healing one would expect for self-activated and in situ specimens. By delivering premixed catalyst and monomer, the variables of complete catalyst and monomer mixing and catalyst survival are removed. Figure 3 shows a typical load displacement curve for both virgin and healed tests of a manually healed WTDCB specimen. In the virgin curve, the specimen is loaded and the crack propagates until the maximum crack opening displacement has been reached (δ = 7 mm). The specimen is then injected with precatalyzed DCPD and Grubbs catalyst and then unloaded. In the healed test, the specimen is loaded and the crack propagates through the healed region before reaching virgin material and continuing to load. At the same crack opening displacement (δ = 7 mm) the specimen is then unloaded. 4 Springer 27

5 Load (N) Displacement (mm) Figure 3: Typical Load Displacement Curve for Reference Specimen Self-activated control specimens provide evidence of catalyst survival and healing without the variable of monomer delivery. Comparison of self-activated specimens with reference specimens indicates how undergoing the cure cycle and embedding the catalyst affects heal. A summary of manually injected and self-activated specimens is shown in Table 1. Selfactivated specimens display similar healing efficiency to manually injected specimens. This indicates the cure cycle does not adversely affect the catalyst reactivity. Table 1: Summary of Manually Injected and Self-Activated Data Specimen Type No. of samples P C virgin avg. (N) P C healed avg. (N) η avg. (%) Manually Injected Self-Activated ACKNOWLEDGEMENTS The authors gratefully acknowledge the support of Sandia National Laboratories. The authors would also like to thank Prof. P.H. Geubelle and graduate student Gerald Wilson for technical support and helpful discussions. Machining was completed by the Aerospace Engineering Machine shop at the University of Illinois. REFERENCES [1] S. R. White, N. R. Sottos, P. H. Geubelle, J. S. Moore, M. R. Kessler, S. R. Sriram, E. N. Brown, and S. Viswanathan, "Autonomic healing of polymer composites," Nature, vol. 49, pp , 21. [2] M. K. Kessler, N. R. Sottos, S. R. White, Self-healing structural composite material, Composites Part A: Applied Science and Manufacturing, vol. 34:8, pp , Springer 27