THERMOPLASTIC FIBRE STITCHING: A NEW SELF-HEALING
METHOD FOR CARBON-EPOXY COMPOSITES
K. Pingkarawat1, C.H. Wang1, R.J. Varley2 and A.P. Mouritz1
1
Sir Lawrence Wackett Aerospace Research Centre, School of Aerospace, Mechanical and Manufacturing Engineering, RMIT University, GPO Box 2476, Melbourne, Victoria 3001,
Australia - e-mail: s3249363@student.rmit.edu.au; adrian.mouritz@rmit.edu.au;
chun.wang@rmit.edu.au 2
CSIRO Materials Science and Engineering, Private Bag 33, Clayton South MDC, Victoria 3169, Australia – e-mail : Russell.Varley@csiro. au
Keywords: Delamination, Stitching, Self-healing, Mendable polymer, Fracture toughness
ABSTRACT
This paper presents an investigation into the delamination toughening and self-healing properties of carbon-epoxy laminates using through-thickness stitches of mendable thermoplastic (poly[ethylene-co-(methacrylic acid)]). The effect of increasing stitch density on the improvement to the interlaminar fracture toughness and healing of mode I delamination cracks generated under static and fatigue interlaminar loads is determined. The self-healing and delamination toughening mechanisms of the mendable stitches are identified.
1. INTRODUCTION
A new type of self-healing polymer composite material was recently developed that synergistically combines high interlaminar fracture toughness (to resist delamination crack growth) with high healing efficacy (for in-situ repair and toughening of delaminations). Yang et al. [1] developed this self-healing composite by reinforcing conventional carbon-epoxy laminates in the through-thickness direction with mendable polymer stitches. The stitches consist of poly(ethylene-co-(methacrylic acid)) (EMAA) filament, which can heal cracks via a condensation reaction with the epoxy matrix phase which generates high pressure microbubbles that force the mendable polymer into open cracks [2].
The present study extends the work by Yang et al. [1] by investigating the influence of EMAA stitch density on the synergistic combination of delamination toughening and healing properties. The focus is to quantify the effect of increasing stitch density on the healing efficiency of delamination cracks generated by mode I static or fatigue cracks. The efficacy of the stitched composites to restore the mode I interlaminar fracture toughness and delamination fatigue resistance is determined for multiple healing operations.
2. MATERIALS AND METHODS
Mode I static and fatigue delamination tests were performed on unstitched and stitched carbon fibre-epoxy composites. The uncured prepreg stack was manually stitched through-the-thickness with EMAA filament. The stitch areal density along the
laminatemid-plane was 0.25, 1, 1.6 and 4 stitches/cm2. The architecture of the EMAA
filaments in the laminate material was the same for each stitch density (Figure 1). The unstitched and stitched composites were cured and consolidated inside an autoclave at 120°C and 620 kPa for one hour. The interlaminar fracture toughness properties were measured using the double cantilever beam (DCB) test under mode I static and fatigue loading. Static DCB tests were performed at a crack opening displacement rate of 2 mm/min. Fatigue DCB tests were performed at a cyclic loading frequency of 10 Hz and R ratio of 0.1. After static and fatigue testing, the stitched specimens were healed by heating at 150oC for one hour followed by compaction at a pressure of 20 kPa for ten minutes. Further details are given by Pingkarawat et al. [3,4].
Figure 1 : Architecture of the EMAA stitches in the self-healing carbon fibre-epoxy composite [1].
3. RESULTS AND DISCUSSION
3.1 Static Delamination Fracture Properties of Mendable Stitched Composites Figure 2 shows the effects of stitch density and number of healing operations on the mode I interlaminar fracture toughness of the carbon-epoxy composite. Stitching increased the delamination resistance of the laminate before healing. The fracture toughness was increased by the stitches bridging the delamination crack, The bridging stitches generated traction loads which resisted crack opening, thereby increasing the interlaminar fracture toughness. The bridging traction load generated by the stitches increases with their volume content, and this accounts for the interlaminar fracture toughness increasing rapidly with stitch content.
0 1 2 3 4 5 0 1 2 3 4 St ea dy-St ate Interlaminar Toug hn es s (k J/m 2 )
Number of Healing Operations
4 stitches/cm2
1.6 stitches/cm2
1.0 stitch/cm2
0.25 stitches/cm2
no stitches
Figure 2 : Effects of EMAA stitch density and number of healing operations on the mode I interlaminar fracture toughness.
After self-healing by heating the delaminated DCB specimens to 150oC, the interlaminar fracture toughness of the stitched composites was fully and/or more than fully recovered and this was retained for multiple healing cycles (Figure 2). Recovery to the interlaminar fracture toughness was achieved by the EMAA flowing from the stitches into the delamination crack during the healing process. Healing was activated by heating the stitched composites, which caused acid functional groups along the EMAA polymer chains to react with hydroxyl groups in the epoxy matrix phase of the composite with tertiary amine groups acted as the catalyst. Meure et al. [2] found that the acid-hydroxyl reaction generates volatiles (mostly water) that phase separate into high-pressure microbubbles within the EMAA, which is a molten insoluble phase at the healing temperature of 150oC. Due to this reaction process, the molten EMAA stitches had a high internal gas pressure which forced them into the delamination and thereby healed the crack. The interlaminar fracture toughness was restored by the formation of a crack bridging traction zone generated by the EMAA stitches (which refused during healing) as well as the presence of a thin film of EMMA healing agent along the crack plane.
3.2 Fatigue Delamination Properties of Mendable Stitched Composites
Figure 3 presents Paris curves of delamination crack growth rate plotted against cyclic stress intensity range for the composites with the lowest and highest stitch contents. Curves are shown for laminates in their original condition and after one or multiple healing operations. The figure shows that the EMAA stitches were able to restore the delamination fatigue resistance of the laminate after healing, and the efficacy of the healing process did not change significantly with number of healing operations. The stitches restored the interlaminar fatigue resistance by flowing into the delamination crack during healing via the same healing process as mode I static loading, and then forming a bridging traction zone during repeated fatigue crack growth.
10-2 10-1 100 101 10-8 10-6 1x10-4 10-2 100 D e la m in a tio n G ro w th R a te , d a /d N ( m m /c yc le )
Cyclic Stress Intensity Range, GI (kJ/m2) original condition one healing operation two healing operations three healing operations four healing operations five healing operations
10-2 10-1 100 101 10-8 10-6 1x10-4 10-2 100 D e la m in a tio n G ro w th R a te , d a /d N ( m m /c yc le )
Cyclic Stress Intensity Range, G
I (kJ/m
2) original condition one healing operation two healing operations three healing operations four healing operations five healing operations
(a) (b)
Figure 3: Effect of number of healing operations on the fatigue crack growth rate of the (a) lightly stitched (0.25 stitches/cm2)and (b) heavily stitched (4 stitches/cm2)
laminates. 4. CONCLUSION
This study has shown that EMAA stitching is an effective interlaminar toughening method for restricting mode I delamination crack growth under mode I static and fatigue loading in mendable composites, which thereby reduces the amount of damage that must be repaired via the healing process. The fracture toughness and fatigue resistance increased with the stitch density due the increase in traction load arising from a greater number of stitches within the crack bridging zone. In-situ repair of static and fatigue delaminations occurred by the healing agent flowing from the stitches into open cracks under a pressure delivery mechanism unique to EMAA. This healing process fully restored the mode I interlaminar fracture toughness and fatigue resistance of the stitched composite, and these properties were maintained for multiple healing cycles.
REFERENCES
[1] T. Yang, C.H. Wang, J. Zhang, S. He, A.P. Mouritz, Toughening and self-healing of epoxy matrix laminates using mendable polymer stitching, Composites Science and Technology, 72, (2012), 1396-1401.
[2] S. Meure, D.Y. Wu, S. Furman, Polyethylene-co-methacrylic acid healing agents for mendable epoxy resins, Acta Materialia, 57, (2009), 4312-4320.
[3] K. Pingkarawat, C.H. Wang, R.J. Varley, A.P. Mouritz, Self-healing of delamination cracks in mendable epoxy matrix laminates using poly(ethylene-co-(methacrylic acid)) thermoplastic, Composites Part A: Applied Science and Manufacturing, 43, (2012), 1301-1307.
[4] K. Pingkarawat, C.H. Wang, R.J. Varley, A.P. Mouritz, Self-healing of delamination fatigue cracks in carbon fibre-epoxy laminate using mendable thermoplastic, Journal of Materials Science, 47, (2012), 4449-4456.
