DAMAGE REDIRECTION AND HEALING IN SKIN-STIFFENER
DEBONDING SPECIMENS UNDER FATIGUE CONDITIONS
R. Luterbacher1, I.P. Bond1 and R.S. Trask1
1 Advanced Centre for Composite Innovation and Science (ACCIS), University of Bristol,
Queen's Building, Bristol, BS8 1TR,United Kingdom –e-mail: rafael.luterbachermus@bristol.ac.uk, i.p.bond@bristol.ac.uk, r.s.trask@bristol.ac.uk
Keywords: Fibre reinforced composite, adhesive joint, self-healing, vascular network, interleaves, delamination.
ABSTRACT
Locally stiffened or ‘stringer’ - skin composite structures are extensively used for lightweight applications in the aerospace industry. Due to localised stiffening, stress concentrations arise which can initiate damage e.g. debonding or delamination, within the composite structure. Critically, this damage can propagate under fatigue loading compromising the structural integrity of the component. To mitigate against this risk, significant safety margins are used within the design phase, which limit the potential weight savings offered by the application of advanced fibre reinforced composites.
In this study, the potential to redirect propagating cracks away from critical failure paths and into dedicated self-healing zones has been investigated through the use of thermoplastic interleaves (for redirection) and an embedded vascular network (to mitigate the damage). One of the major challenges in employing a vascular network is to ensure the connectivity between the propagating damage and the vascules. A self-healing agent (low viscosity epoxy resin) was delivered via the vascular network in order to restore the mechanical performance of the deteriorated structural element. A number of different interleave – vascule configurations have been investigated, all successfully redirecting the propagating fatigue damage into the vascules within skin-stiffener debonding specimens. The tensile and fatigue tests performed have shown that these modifications are not detrimental to the mechanical performance in comparison to a baseline configuration. Moreover, the interleave configurations are able to redirect the interfacial damage, between the skin and the stiffener, towards the self-healing functionality embedded within the laminate. During fatigue loading, a notable stiffness loss is observed as the damage propagates. After the healing event, the stiffness in both static and fatigue conditions has been successfully restored.
1. INTRODUCTION
Composite materials are used in lightweight applications, e.g. aerospace industry, for their high specific mechanical properties. One common design concept in the aerospace industry is to use locally stiffened or “stringer”-skin configurations. This design philosophy is efficient as the skin takes the in-plane loads, whereas the stiffening elements provide increased stiffness and a reduction of the effective buckling length. However, due to the localised increment in stiffness, through
thickness stresses arise at the edges of the stiffening element giving rise to delaminations sources, which if untreated, could lead to catastrophic failure in terms of stiffener debonding. The aim of this project is to redirect delaminations within such configurations with the help of interleaves into a vascular network through which a healing agent is injected into the composite.
2. MATERIALS AND METHODS
“Skin/ stringer flange debond specimens” [1] were manufactured using a pre-impregnated E-glass/913 epoxy (Hexcel, UK). The flange was pre-cured, cut to size and co-bonded to the skin. The lay-up for both flange and skin was [-452,02,452,902]S.
Figure 1 summarizes the specimen set-up and the different configurations studied.
A.) B.)
Configuration A – Baseline Configuration B
Configuration C Configuration D
Figure 1: A.) Specimen nominal dimensions (unit: mm) and edge identification. Note vascules only present for ID B, C and D. B.) Co-bonded skin/stringer configuration
with loading
Each specimen of configuration B, C and D contained two vascules manufactured by removing ply sections and embedding steel wires (diameter 0.56mm) between these cut outs [2].The selected interleave material was a nominal 50 µm thickness and 3 mm wide polyethylene-co-methacrylic acid (EMAA) film. A commercially available low viscosity resin (Resintech RT151) was used as a healing agent.
Corner 2 Corner 1 Corner 3 Corner 4 A B 50 50 50 50 50 A B 3 5 1 2 .5 1 2 .5 1 0 V as cu le s 0 ° 9 0 ° 45202 -452 -452 02 452 904 -45202 452 904 Resin Pocket 452 02 -452 -452 02 452 904 -45202 452 904 Resin Pocket Vascules 452 02 -452 45202 -452 Interleave A Interleave B -452 02 452 904 -45202 452 904 Resin Pocket Vascules 452 02 -452 45202 -452 Interleave A Interleave B 902 902 -452 02 452 -45202 452 904 Resin Pocket Vascules 452 02 -452 45202 -452 Interleave A Interleave B
In order to assess the influence of the interleaves and the vascular network on the mechanical properties, specimens of the different configurations were tested in static and tensile fatigue loading.
For assessment of the healing performance, the healing agent was injected after 10 000 cycles and then cured at 65ºC and then cycled again.
3. RESULTS AND DISCUSSION
Static tensile test results (Figure 2 A) and tensile fatigue test (Figure 2 B) performed at 40% of the ultimate tensile strength (Fmax=10kN, R=0.1) show that the introduction
of vascules and interleaves do not alter the global mechanical properties.
A.) B.)
Figure 2: A.) Representative tensile test results for the different tested configurations B.)Stiffness loss as function of cycles for the different tested configurations After fatigue testing, the damage patterns on the specimen edges were analysed using optical microscopy. Specimens of configuration A (baseline) showed delamination growth along several ply interfaces. However, specimens containing interleaves developed “ damage free zones” located inwards from interleave B (refer to
Figure 1). In the observed configurations (total 14 specimens 4 of B, 5 of C, 5 of D), in 96% of all the cases, the delaminations migrate into a lower ply interface when exiting the interleave. In the remaining 4% the delaminations are arrested in the interleave. Our observations suggest that the interleaves enhance the “inherent” tendency of delaminations to migrate into lower plies.
The following mechanism is suggested: as the delaminations are slowed down in the interleave due to the higher toughness [3], [4], the mechanism of ply splitting (migration) overtakes that of propagation and induces the delamination to migrate into the next ply interface within the interleave. In addition, the exit of the delamination accelerates damage growth due to the decrease of fracture toughness [3]. Therefore, the migration along the brittle fibre matrix interface into a lower ply interface is enhanced.
Through this approach, the delaminations have been successfully steered into the interfaces where the vascules are located. In this way, delaminations propagating along these interfaces will intersect with the vascules and thus connectivity is assured.
Connectivity between the vascules and the damage site has been observed by fatigue testing two specimens (one of configuration C and one of configuration D) with red dye penetrant within the vascules (Ardrox 996PA) (Figure 3 A). From approximately 10 000 cycles onward, connectivity between the damage sites at the corners and the vascules has been observed in both cases by “bleeding” on the corners. In addition, damage has been visualised in the skin area.
A.) B.)
Figure 3: A.) Damage visualisation B.) Stiffness recovery after healing
Figure 3 B shows an example for the healing performance: One specimen of configuration B was cycled for 10 000 cycles resulting in a stiffness loss of 40% as compared to the initial stiffness. The healing agent was injected successfully through both vascules and cured at 65°C for 1h. A stiffness recovery to 87% of the initial modulus was achieved. After additional cycling of the specimen for 10 000 cycles the stiffness dropped to 72% of the initial stiffness.
4. CONCLUSION
The delamination paths have been steered successfully by the interleaves into the interfaces with vascules thereby giving the possibility of (1) damage detection and (2) healing. Healing provided partial recovery of the mechanical properties under fatigue conditions.
ACKNOWLEDGEMENTS
The authors would like to acknowledge the Spanish “Fundació la Caixa” and the UK Engineering and Physical Sciences Research Council (EPSRC) for funding for the project.
REFERENCES
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carbon fibre reinforced plastics at multidirectional ply interfaces under static and cyclic loading,” Plastics, rubber and composites processing and applications, vol. 27, no. 5, pp. 220–226, 1998.
