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Self-healing sandwich panels: Restoration of compressive strength after impact
Composites Science and Technology 68 (2008) 3171–3177
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Self-healing sandwich panels: Restoration of compressive strength after impact H.R. Williams *, R.S. Trask, I.P. Bond ACCIS – Advanced Composites Centre for Innovation and Science, Department of Aerospace Engineering, University of Bristol, Queen’s Building, University Walk, Bristol BS8 1TR, UK
a r t i c l e
i n f o
Article history: Received 12 June 2008 Received in revised form 18 July 2008 Accepted 29 July 2008 Available online 8 August 2008 Keywords: C. Sandwich A. Smart materials B. Impact behaviour B. Strength Vascular self-repair
a b s t r a c t Impact damage can significantly reduce the strength of composite sandwich panels, giving rise to large factors of safety in design. A self-healing sandwich panel was designed and manufactured. Specimens were tested in edgewise compression-after-impact to prove the concept. The self-healing system consists of vascular networks carrying the two liquid components of an epoxy resin system. After damage and autonomous self-healing, the specimens could be restored to their undamaged strength. Key influences on healing efficiency, and the need for further improvements in reliability were identified. A simple specific strength analysis suggested that the mass penalty of self-healing could, in some cases, be offset by the improved design allowables permitted. Published by Elsevier Ltd.
1. Introduction Safety and reliability requires that service impact damage is considered when allowable stresses are defined for structural materials. Modest impact damage can have a detrimental effect on the performance of sandwich structures [1,2]; which are valued in high-performance applications for their high bending and buckling stiffness. A self-healing system could offset its own mass, and cost, by allowing a higher material stress to be used for design. The pattern of impact damage in sandwich structures is a function of the dimensions and properties of the skins and core, and the shape, mass and velocity of the impactor [1,2]. Generally, the primary damage mode for low-velocity blunt impact on a brittlecored sandwich structure is a cohesive disbond in the core located under the impacted face with consequent loss of skin stability. The loss of skin support has been shown to reduce the compressive strength of sandwich panels by over 25% e.g. [3–6]. A damage tolerant design can be used to manage the risk of service impact e.g. [2,7]. In particular, there is a requirement to survive design ultimate load with damage of a specified size. In the aerospace industry, this is termed the allowable damage limit (ADL) and is defined as the minimum size that can be detected during the planned maintenance programme [2,8]. An undamaged structure is, therefore, designed to fail well above design ultimate load; resulting in a structural mass penalty. The nascent field of self-healing materials offers a novel but complementary approach to ‘traditional’ damage tolerant design. * Corresponding author. Tel.: +44 (0)117 331 7499. E-mail address: [email protected] (H.R. Williams). 0266-3538/$ - see front matter Published by Elsevier Ltd. doi:10.1016/j.compscitech.2008.07.016
Self-healing based on the bleeding of a liquid healing agent from a microcapsule or hollow glass fibre into region of damaged material has been reported in pure polymers e.g. [9,10] and polymer composite laminates e.g. [11–15]. The reader is referred to [16,17] for more detailed reviews of these approaches. A key limitation of these techniques is their capacity to fill the larger damage volume that is typical of damage in a sandwich structure. A vascular approach, using a network of vessels to deliver fluid from a remote reservoir, has been shown to recover failure mode and load in sandwich beams subject to flexure-after-impact [18]. A double vascular network was introduced into the foam core of a composite sandwich structure. Each respective network was filled with a liquid epoxy resin and the corresponding amine based hardener. Rupture of the channels by impact damage allowed the two component healing agent to infuse the damage, mix and cure. This study has been used to propose biomimetic design schemes for optimum supply vessel diameter for minimum system mass [19]; tailoring the network density according to critical loading [20] and using the network design to enhance system reliability [21]. A vascular self-healing approach has also been demonstrated on a smaller scale to autonomously repair cracks in a polymer coating where the network delivers a liquid dicyclopentadiene (DCPD) monomer and solid (Grubb’s) catalyst is dispersed in the polymer coating [22]. In this paper, the vascular self-healing approach developed for sandwich beams is applied to the edgewise compression-afterimpact test configuration, a more representative scenario for a practical engineering sandwich structure. The paper describes the design, manufacture and specimen tests of a self-healing system with a network appropriate for the target damage. Experimental
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results are linked to destructive evaluation of the specimens to propose the key mechanisms influencing healing efficiency. These results are used, with simple expressions for panel strength, to support the values of healing efficiency obtained and provide a first comparison of the specific performance of damage tolerant and self-healing sandwich structure. 2. Healing system design In an autonomous self-healing system using a two-part epoxy, cure occurs on intimate contact of the liquid parts. At a simple level, this contact can be considered to occur along a line where the flow fronts meet. Providing both components have sufficiently low viscosity to permit timely infusion of the damage, a system designed to cure with equal parts of resin and hardener (1:1 mix ratio) is desirable. Resintech Ltd. RT310, an epoxy system with a 2:1 mix ratio and listed viscosity of less than 1000 cP was selected as a good compromise. This system requires a 24 h cure at room temperature. The healing network was developed from the simple configuration used in previous flexural studies, with tubing bonded into the midplane of the core and vertical risers supplying the skin-core bond region. The distribution of the vertical risers can be varied by design. Williams et al. [20] report a riser distribution that ensures at least one riser is ruptured by a circular damage event of a critical damage size, x, as shown in Fig. 1. In the present study, several additional factors influence the distribution of risers. The final network design is shown in Fig. 2. This differs from the basic riser distribution previously proposed in several ways: Separate networks are required for an autonomous healing system using two liquid parts, the spacing between channels is therefore x/4 rather than x/2. In this case, the location of damage is known, so it is possible to increase the riser density in this region, some risers have been also placed outside this zone to make the specimen representative of a practical structure. Previous experimental studies [18] and a consideration of system reliability [21] suggest redundant risers are desirable, and the distribution in Fig. 2 provides at least two risers of each component in the nominal damage zone. The number of risers in the damage zone has been used to approximately meter the resin and hardener components to match the 2:1 mix ratio. Four resin risers and two hardener risers are placed within the expected diameter of damage. Although the network of channels shown in Fig. 2 were designed for the separate components of the selected resin system, the infiltration pattern can be better assessed by infusing a pre-
Fig. 2. Network design for self-healing sandwich specimens. (A) Channel designed for resin component and (B) channel designed for hardener component.
mixed resin. By this approach, the extent of infiltration is fixed before destructive assessment. To highlight the zone of healing and distinguish between flow fronts from the A and B networks, pigmented pre-mixed RT310 resin (Resintech Ltd.) would replace the separate resin and hardener components in one specimen group. The viscosity of these systems was compared to ensure they provided a reasonable representation of the different components. The viscosity was measured at 25 °C using a Bohlin Instruments Gemini 200 Advanced Rheometer fitted with a 4°/40 mm cone and plate. Two separate, well-mixed samples of each liquid were tested three times at shear rates between 0.1 and 10 s1. The measured viscosities show little variation with shear rate over the range investigated and are summarised in Table 1. The pre-mixed, pigmented systems have a very similar viscosity to the pure resin. The pure hardener is less viscous, but the difference is modest. The system with blue pigment better represents the hardener for the later tests. These data show the mixed, pigmented systems should provide a reasonable model for the flow behaviour of the pure systems. 3. Panel manufacture and test method 3.1. Manufacture Vascular sandwich cores were manufactured by bonding 1.5 mm bore silicone tubing in channels between two 8 mm thick, 52 kg/m3 polymethacrylimide (Rohacell) closed-cell foam sheets using film adhesive (FM300K, Cytec). Drilling 2 mm holes in the locations shown in Fig. 2 was used to form the vertical risers. Pre-impregnated [0°,90°]S E-glass/913 epoxy (Hexcel) laminates were co-cured onto the vascular core, giving a relatively thinskinned configuration susceptible to core-dominated impact damage. Samples were sectioned into specimens (60 90 mm) using a water-cooled diamond saw and the loading edges ground flat and parallel. Miniature barbed tubing connectors were inserted into the exposed tubes at each end of the samples, allowing external supply tubing to be connected. Specimens were infiltrated in three Table 1 Results of rheometry on selected resin system Component
Mean viscosity (cP)
Standard deviation (cP)
(Total 120 measurements)
Fig. 1. Riser and supply tube distribution which ensures damage of diameter x (large solid circles) occurring at any location can be supplied with healing agent by at least one riser.
RT310 RT310 Mixed Mixed
resin hardener pigmented RT310, Red pigmented RT310, Blue
473 337 469 440
12 23 18 27
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sample groups, each of six specimens, to be compared with undamaged and damaged reference samples. (I) Pre-mixed resin and hardener: the pre-mixed healant supplying the A (resin) channels was pigmented red, and that supplying the B (hardener) channels pigmented blue. These samples assessed the pattern of damage infusion and the healing efficiency using an ideally cured resin. The networks were filled using a syringe, the network clamped downstream of the specimen and the syringe plunger removed to open the reservoir to atmosphere. The network was then lightly pressurised to a static head of 0.5 m, 5.6 103 Pa, before impact. (II) Low-pressure autonomous healing: unmixed resin and hardener were used to fill the corresponding channels. Autonomous healing was therefore initiated by the resin and hardener entering the damage and mixing in-situ. The networks were filled and statically pressurised to 5.6 103 Pa as above. (III) Pressurised autonomous healing: unmixed resin and hardener were used as above, with the network clamped downstream of the specimen and pressurised to 3.0 105 Pa using peristaltic pumps in a bespoke pressure rig. The pressure was intended to improve healing efficiency by enhancing the mixing in the damage zone and restoring skin alignment. The barbed connectors were bonded into these specimens using epoxy structural adhesive (Bondloc B2012) to prevent leakage. The specimen edges were ground flat and parallel after impact and healing was completed.
3.2. Mechanical testing Damage was introduced using an Instron Dynatup 9250HV drop tower (3 J, 5.35 kg dropweight, 20 mm hemispherical head) with the specimen resting on a solid support plate. In the pressurised specimens, the pressure in the networks was observed to fall rapidly after impact as the damage filled. The networks were re-pressurised to 2.0 105 Pa, which was found to remove the residual post-impact indentation. Pre-mixed and low-pressure self-healed samples were allowed to heal for at least 48 h after impact. The third group were left pressurised for at least 48 h. For practical reasons it was not possible to give all the pressurised specimens exactly the same total cure duration, so a short post-cure of 1 h at 60 °C was performed to normalise the cure states. Edgewise compression tests (ASTM C364) were performed using a bespoke fixture on an Instron test frame (250 kN load cell) with the short (60 mm) edge of the specimen clamped (10 mm length each end) and loaded at 0.5 mm/min.
4. Results 4.1. Impact damage The visible damage after impact was limited to a roughly elliptical, inconspicuous residual indentation with no visible damage to the face laminate. The major axis of the dent was aligned with the long axis (0°) of the specimens. The skin has a higher bending stiffness in this direction due to the laminate lay-up, the 0° plies being further from the neutral axis. The dent was measured 1 h after impact, the average dimensions for the damaged specimens and the two groups infiltrated at low pressure (groups I and II) are shown in Table 2. There was no significant difference in indentation dimensions between these three groups. In the pressurised self-healed specimens (group III) the indentation was removed completely.
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4.2. Infiltration assessment The damage zone of the pigmented, pre-mixed specimens (group I) was investigated after compressive testing. The impacted face was removed to reveal the infiltrated damage zone in plan. This allowed inspection of the infusion patterns from the A and B networks. Photographs of the damage zone were interrogated using image analysis software to measure the area of infiltration from each network and the total infiltrated area relative to the residual dent area. One specimen suffered extensive bleeding beyond the damage and towards the sample edge, possibly due to a manufacturing defect, and has been discounted. Results from five specimens are shown in Fig. 3. The ratio of areas infiltrated by the two components is seen to agree reasonably with the target of 2:1, the mean ratio is 2.3:1. Interestingly, the total infiltrated area is significantly less than the area of the residual dent. The number of risers breached by the impact event is seen to be at least equal to the design target in all specimens. It is notable that there is almost no evidence of the components mixing with each other, which is necessary for autonomous healing. When the damage zone of the low-pressure autonomously healed specimens (group II) was inspected in a similar manner, most of the damage zone and the vascular network were filled with uncured liquid. Tactile investigation revealed only small areas of partially cured healing agent. It was not possible to determine the extent of resin infusion reliably because the specimens had been subject to mechanical failure and destructive sectioning. A similar problem was encountered in the pressurised autonomously healed specimens (group III) whose damage zones and vascular network also contained areas of uncured liquid healing agent. However, in these specimens an area with improved cure was located around the centre of the zone of damage, and in the centre of a region with slight discolouration. Fig. 4 shows the damage zone of a representative specimen. 4.3. Edgewise compression results The strength of each specimen is expressed as the ultimate skin compressive stress; ultimate load per unit cross sectional area of both skins. Table 3 summarises the strength and individual failure modes and Fig. 5 shows photographs of the test configuration and key failure modes. Two specimens were discounted from the pressurised self-healing group (III). In one, the action of re-pressurising the network after impact caused the skin to bulge outwards. There was visual and audible evidence of the damage propagating outwards from the original impact, making a fair comparison with the other sample groups impossible. In a second specimen there was no drop in the pressure of the hardener network after impact. Destructive sectioning after compression testing revealed only uncured resin in the fracture plane. The strength of this specimen was 129 MPa – very close to the mean value for the unhealed specimens – giving confidence that there was a failure of either hardener vessel to rupture. There was significant scatter in all specimen groups with the highest standard deviation noted in the pressurised self-healing group. The mean and individual strengths are plotted in Fig. 6 to show all the results. The estimated measurement uncertainty was small compared to the standard deviations of the results, suggesting the scatter is an artefact of the test setup and relatively small specimen group sizes. The effect of the impact damage is to significantly reduce the ultimate strength and change the most common failure mode from wrinkling and skin-core separation to an inward propagation of the residual dent which then causes core shear buckling. The infiltration of pre-mixed resin (group I) restores the failure mode and actually allows an increase in the ultimate strength
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Table 2 Residual dent dimensions of 18 impacted specimens Measurement
Mean dimension (mm)
Standard deviation (mm)
Major axis (0° to specimen) Minor axis (90° to specimen) Maximum dent depth
43 38 0.5
5 3 0.1
calculations. In its factored form it is considered to give useful, conservative predictions [23,24] but Fagerberg and Zenkert [26] argue it is less appropriate for constructions with anisotropic skins. The assumptions of core stress field used by Allen [27] to derive an alternative expression are more refined, but this expression typically gives non-conservative load predictions due to initial imperfections in practical structures. Fagerberg and Zenkert [26] investigate the effect of initial wrinkling imperfections in detail. The Hoff [25] and Allen [27] expressions are:
pffiffiffiffiffiffiffiffiffiffiffiffiffiffi
rHOFF ¼ 0:5 3 Ef Ec Gc rALLEN
3 ¼ qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 3 12ð3 mc Þ2 ð1 þ mc Þ2
qffiffiffiffiffiffiffiffiffiffiffi 3 Efb E2c
ð1Þ ð2Þ
where Ef is the isotropic face sheet modulus, Ec is the core modulus, Gc is the core shear modulus, mc is the core Poisson’s ratio and Efb is the anisotropic face sheet bending modulus in the loading direction. For an isotropic solid:
Ec ¼ 2Gc ð1 þ mc Þ
ð3Þ
The stress in the skins at the critical load for core shear failure [24] can be derived from the effective shear rigidity (AG)eq as:
rSHEAR ¼ Fig. 3. Results of infiltration assessment in specimens with pre-mixed resin (group I).
Fig. 4. Damage zone of a specimen self-healed autonomously under high-pressure, plan view with impacted skin removed. The areas of most complete cure are centred within the area of discolouration.
over the undamaged specimens. Those specimens autonomously self-healed at low pressure (group II) show partial restoration of strength and failure mode, although not sufficient to reliably halt the inward wrinkle growth. With the exception of the reliability issues noted previously, the pressurised self-healing specimens show good recovery of failure mode and strength, with performance close to that of the pre-mixed specimens. 5. Analysis 5.1. Panel strength Common edgewise compressive failure modes for composite sandwich materials are discussed by Zenkert [23] and Fleck and Shridhar [24]. Simple expressions for the specimen strength in various modes are given in these works: panel geometry and the relative thickness and properties of skins and core influence the dominant failure mode and load. Skin wrinkling failure was the dominant failure mode in the work reported herein. Hoff and Mautner [25] derived an analytical expression for the wrinkling load, and factored this in the light of experimental work, to produce an expression which is still in use for basic design
ðAGÞeq bcGc cGc ¼ 2tb 2tb 2t
ð4Þ
where b is the specimen width and t and c are the thicknesses of one skin and the core. Appropriate mechanical properties for the skin and core materials are given in Table 4, and are used with the equations above to construct to Fig. 7. The three properties obtained for the core appear inconsistent with Eq. (3); consistent properties are needed to compare Eqs. (1) and (2). Some degree of anisotropy can be expected in closed-cell foams due to the manufacturing process [31], and experimental variation due to the different test approaches used to obtain each value could also be expected. Therefore, each of the three core properties presented in Table 4 were discounted in turn, and a consistent value calculated from the remaining properties, using Eq. (3), to give the tolerances quoted in Table 4. Fig. 7 is plotted using the consistent dataset that gives the median wrinkling stresses. Error bars are provided on Fig. 7 to show that the dominant failure mode is not sensitive to this uncertainty. The skin laminate compressive strength, rcomp, would be best determined experimentally for each lay-up e.g. [24]. In the absence of a dedicated test programme to obtain the data for this non-critical failure mode, the compressive strength of a unidirectional laminate was factored by 0.5, assuming only the 0° plies carried load. This gives a coarse approximation suitable for a first assessment of the overall trend. Fig. 7 provides good evidence to support the observed failure modes and strengths. The Hoff [25] and Allen [27] predictions for wrinkling load are conservative and optimistic respectively. The next most critical failure mode would be core shear buckling; in the experiments this appeared as a secondary failure in the damaged specimens. This could be accounted for by inward wrinkle growth interacting with the damaged core. Limited evidence of compressive microbuckling is present in one specimen as a secondary mode. The sensitivity of wrinkling stress is explored further in Fig. 8. The observed impact damage and healing agent infusion would be expected to influence the core rather than the skin properties. Fig. 8 is, therefore, plotted from Eqs. (1) and (2) using the core elastic modulus to represent the underlying support. Poisson’s ratio is held constant so the shear modulus will increase with elastic modulus, effectively giving a density scaling or a representation of the extent of infiltration and cure.
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H.R. Williams et al. / Composites Science and Technology 68 (2008) 3171–3177 Table 3 Edgewise compression results Specimen group
Mean ultimate facing stress (MPa) (Note 1)
Standard deviation (MPa)
Relative ultimate stress
Undamaged Damaged Healed, pre-mixed resin (I) Self-healed (II) Pressurised self-healed (III)
184 130 207
22.7 17.6 19.4
1.00 0.70 1.12
151 211
6.55 33.8
0.82 1.14
Failure modes – No. of specimens (Note 2) I
1
II
III
IV
V
3
3 1
4
1
5 1 4
5
Note 1. Estimated measurement uncertainty in ultimate facing stress ±6.53 MPa. Note 2. Failure modes: I – face wrinkle with visible compressive microbuckling damage, II – outward skin wrinkle and abrupt skin–core separation, III – inward wrinkle then abrupt opposite skin separation, IV – Inward skin wrinkling then shear buckling, V – inward skin wrinkle with stable core crush.
5.2. Influences on healing efficiency In the pre-mixed specimen group (I) the damage void is infused with an ideally cured epoxy resin whose elastic and shear moduli would far exceed (approximately a factor of 50) the foam they are ‘replacing’. This allows these repaired specimens to exceed the strength of the undamaged specimens. The experimental strength of this group in fact only shows an increase of 12%. Several factors were identified that could account for this relatively small increase. Only a proportion of the sample width is reinforced in the manner described above because the residual dent only occupies about two thirds of the specimen width – Fig. 3 also shows that residual dent is not entirely infused. It has previously been shown that the wrinkle propagates as a cohesive tensile core rupture near the edge of a residual dent [3], as this region is not fully infused, the healing has not reinforced this crack initiator. The remaining residual dent has the capacity to significantly reduce the wrinkling load by acting as an initial imperfection. The low-pressure, autonomous group (II) shows partial selfhealing. Negligible indication of flow front mixing is observed in the pre-mixed specimens (Fig. 3) so it is reasonable to assume that negligible mixing has occurred between the resin and hardener of this autonomous healing group. There is minimal contact in the narrow damage void and the components flow slowly under low pressure, preventing active mixing at this interface. The extent of cure is compromised, and this accounts for the poor performance relative to the pre-mixed specimen group (I).
Fig. 5. Photographs of main compressive failure modes from Table 3: II – outward skin wrinkle and abrupt skin–core separation, III – inward wrinkle then abrupt opposite skin separation, IV – inward skin wrinkling then shear buckling.
The pressurised, autonomous specimens (III) show strength recovery similar to the pre-mixed specimens. In terms of extent of infusion, the higher pressure restores the skin alignment by removing the residual dent, in the process opening the damage zone into a larger void and, presumably, allowing the edges of the damage to be infused more successfully. The extent of infusion is superior to both the low-pressure specimen groups. In terms of cure extent, this high-pressure autonomous group outperforms the low-pressure autonomous group. Because the resin and hardener flow rapidly into the damage zone, it is suggested that this additional energy facilitates active mixing as flow fronts interact. The opened damage zone also gives a larger contact area between components. However, the extent of cure is inferior to the pre-mixed specimens and variable across the damage zone. The mechanisms of core elastic support, (maximised in the fully cured pre-mixed specimens) and the restoration of skin alignment (maximised with the higher-pressure infusion) thus contribute to healed efficiency. In an autonomous system operating at highpressure, skin alignment can be restored. The clear driver for further improving healing efficiency is in maximising the restoration of core elastic support by enhancing the degree of cure in the damage zone. If this is achieved, the undamaged strength of the panel can be exceeded. Given that the aim is only to restore undamaged strength, a useful margin exists. The reliability of the cure process, infusion of the edge of the damage and the influence of the network on the baseline properties are key areas for future study. The influence of the risers on baseline properties will be from two sources; a change of effective core density and an influence as a crack initiator. The aerial density of the risers shown in Fig. 2 is around 2.5%, so should have a relatively minor influence. The influence on skin–core disbond toughness requires further experiment and analysis. A comparison of the
Fig. 6. Edgewise compressive strength.
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Table 4 Assumed mechanical properties for analysis Component
Property
Value
Notes
Skin Hexcel UD E-glass/913 epoxy [0 90]s
Ef Efb
25.1 GPa 37.9 GPa 555 MPa
Ply data from [28]. Laminate data calculated using classical laminate analysis
68.6 MPa (52.1–68.6) 18.6 MPa (18.6–24.5) 0.4 (0.40–0.84)
[29]. See Section 5.1 for discussion on tolerance [29]. See Section 5.1 for discussion on tolerance [30]. See Section 5.1. mc = 0.84 used to plot Fig. 7
rcomp Core Rohacell 51IG
Ec Gc
mc
Ply data from [28]. Approximation assuming zero contribution from 90° plies
Note: Tolerances on core properties are only those tolerances required to generate a consistent set of material properties according to Eq. (3). They do not include the normal experimental variation which would be expected in determining any material properties.
performance of the autonomous heal with a conventional manual repair could be instructive, as could a consideration of the performance under fatigue loading conditions. 5.3. Specific strength and application The mass of all except the pressurised specimens was measured before filling with healant, and then after healing was complete. The pressurised samples were measured after they were selfhealed and ground to test dimensions. Using measured or calculated masses of the various components and after normalising for minor dimensional variation, the specimen mass was estimated in various configurations. The results are given in Table 5. The damage in this experimental study is inconspicuous, but detectable, so it is reasonable to use it as an approximation for an allowable damage limit (ADL). In this case the ultimate stress available for design is reasonably estimated by the mean strength of the damaged specimens. This assumes the post-impact strength is dominated by the damage and the risers have negligible influence on impact response. The midplane bond is retained in this comparison as it influences performance under impact [32,33]. Autonomous self-healing under pressure has been shown to restore the undamaged performance, so the mean strength of the undamaged specimens is taken as a design allowable for the self-healing panel. The final two columns of Table 6 show that the specific performance of the self-healing panel is competitive. It is important to note that the configuration considered was selected for a laboratory specimen-scale study confined to coredominated damage. A thin-skinned sandwich with low-density core and large size of damage relative to the specimens make the configuration highly susceptible to disbonding and wrinkling failure. A detailed understanding of the effect of the network on the
Fig. 7. Predicted failure modes of test specimens calculated using data in Table 4.
basic properties of the panel would also be required for a rigorous engineering trade-off. Set against this is the potential for significant improvements in healing efficiency and mass of a self-healing system as the technology matures. It is useful to consider qualitatively the implication of a selfhealing system over a product lifecycle. For self-healing to be a competitive design option, the upfront manufacturing costs, periodic system maintenance and final disposal costs must be outweighed by a overall improvement in material specific performance, plus the potential savings from removing the demand for inspections capable of detecting damage close to the ADL. Over a product lifecycle, a few percent improvement in structural weight can produce significant cost savings (especially in an environment of increasing fuel costs) and a reduction in lifecycle environmental impact. The conclusion to this analysis is, therefore, balanced; a selfhealing system certainly has the potential to be a competitive design option in advanced structures, although many details of the design will need to be carefully optimised. 6. Conclusions This work has drawn together several studies on vascular selfhealing to design and test an autonomous self-healing system for foam-cored sandwich panels. Three interacting components were considered: a conventional sandwich panel subject to a nominal design damage event, a vascular network primarily tailored for that event and an appropriate healing agent. The residual strength of an impact damaged sandwich specimen with a residual dent and underlying core damage has been successfully restored to the undamaged strength using a vascular self-healing system which releases a liquid healing agent into the damage zone created by an impact event. An ideally cured resin infused at low pressure produces comparable performance to a fully
Fig. 8. Effect of underlying core properties on wrinkling strength.
H.R. Williams et al. / Composites Science and Technology 68 (2008) 3171–3177 Table 5 Estimated specimen mass in various configurations Configuration
Mass (g)
Relative mass
Reference specimen: midplane bond, no network Self-healing specimen with network, filled but no damage Self-healing specimen, healed, low-pressure case Self-healing specimen, healed, high-pressure case
18.8 22.8
1.00 1.21
23.8 24.7
1.27 1.31
Table 6 Specific strength assessment Configuration
Mass (g)
Strength (MPa)
Specific strength (MPa/ g)
Relative specific strength
No self-healing system, impacted Self-healed specimen, impacted, 2–3 bar heal
18.8
130
6.91
1.00
24.7
184
7.45
1.08
autonomous self-healing system operating at higher pressures. In the latter, separate epoxy resin and hardener systems are infused under significant pressure to facilitate active mixing and removal of the residual dent. Competing mechanisms determine the healing efficiency; the restoration of core shear continuity and support is necessary to restore resistance to skin wrinkling. A residual dent acts as an imperfection that reduces wrinkling strength, and pressurised infusion can be applied to offer significant remediation. A simple specific strength analysis has been applied to show that this system has the potential to be competitive with more traditional damage tolerant designs. For rigorous cost and environmental impact these should be performed over the whole lifecycle. Further work is required to improve the system design and healing agent selection to account for the variability and reliability issues highlighted in the study. Acknowledgements The research reported in the article was supported by the UK Ministry of Defence under Grant FATS/RAOWPE/02 and the University of Bristol via a Convocation Scholarship for H.R. Williams. The authors are grateful to Mr. Andy Limmack for his assistance with resin rheometry and the improvements suggested by the reviewers. References [1] Abrate S. Localised impact on sandwich structures with laminated facings. Appl Mech Rev 1997;50(2):69–82. [2] Tomblin J, Lacy T, Smith B, Hooper S, Vizzini A, Lee S. Review of damage tolerance for composite sandwich airframe structures. DOT/FAA/AR-99/ 49. USA: Federal Aviation Authority; 1999.
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[3] Shipsha A, Zenkert D. Compression-after-impact strength of sandwich panels with core crushing damage. Appl Compos Mater 2005;12:149–64. [4] Aviles F, Carlsson LA. Experimental study of debonded sandwich panels under compressive loading. J Sandwich Struct Mater 2006;8(1):7–31. [5] Edgren F, Asp LF, Bull PH. Compressive failure of impacted NCF composite sandwich panels – characterisation of the failure process. J Compos Mater 2004;38(6):495–514. [6] Bull PH, Edgren F. Compressive strength after impact of CFRP-foam core sandwich panels in marine applications. Composites B 2004;35(6– 8):535–41. [7] Zenkert D, Shipsha A, Bull PH, Hayman B. Damage tolerance assessment of composite sandwich panels with localised damage. Compos Sci Technol 2005;65:2597–611. [8] Razi H, Ward S. Principles for achieving damage tolerant primary composite aircraft structures. In: 11th DOD/FAA/NASA conference on fibrous composites in structural design, Fort Worth, USA; 1996. [9] White SR, Sottos NR, Geubelle PH, Moore JS, Kessler MR, Sriram SR, et al. Autonomic healing of polymer composites. Nature 2001;409:794–7. [10] Brown EN, White SR, Sottos NR. Retardation and repair of fatigue cracks in a microcapsule toughened epoxy composite – part II: in situ self-healing. Compos Sci Technol 2005;65:2474–80. [11] Motuku M, Vaidya UK, Janowski GM. Parametric studies on self-repairing approaches for resin infused composites subjected to low velocity impact. Smart Mater Struct 1999;8(5):623–38. [12] Bleay SM, Loader CB, Hawyes VJ, Humberstone L, Curtis PT. A smart repair system for polymer matrix composites. Composites A 2001;32(12): 1767–76. [13] Kessler MR, White SR, Sottos NR. Self-healing structural composite materials. Composites A 2003;34:43–753. [14] Pang JWC, Bond IP. A hollow fibre reinforced polymer composite encompassing self-healing and enhanced damage visibility. Compos Sci Technol 2005;65:1791–9. [15] Trask RS, Williams GJ, Bond IP. Bioinspired self-healing of advanced composite structures using hollow glass fibres. J R Soc Interface 2007;4:363–71. [16] Trask RS, Williams HR, Bond IP. Self-healing polymer composites: mimicking nature to enhance performance. Bioinspiration Biomimetics 2007;2(1):P1–9. [17] Kessler MR. Self-healing: a new paradigm in materials design. P I Mech Eng G-J Aer 2007;221(G4):479–95. [18] Williams HR, Trask RS, Bond IP. Self-healing composite sandwich structures. Smart Mater Struct 2007;16:1198–207. [19] Williams HR, Trask RS, Weaver PM, Bond IP. Minimum mass vascular networks in multifunctional materials. J R Soc Interface 2008;5:55–65. [20] Williams HR, Trask RS, Bond IP. Design of vascular networks for self-healing sandwich structures. In: 1stInternational conference on self-healing materials, Noordwijk ann Zee, The Netherlands; 2007. [21] Williams HR, Trask RS, Knights AC, Williams ER, Bond IP. Biomimetic reliability strategies for self-healing vascular networks in engineering materials. J R Soc Interface 2008;5:735–47. [22] Toohey KS, Sottos NR, Lewis JA, Moore JS, White SR. Self-healing materials with microvascular networks. Nat Mater 2007;6:581–5. [23] Zenkert D. An introduction to sandwich construction. Warley, UK: EMAS; 1995. [24] Fleck NA, Shridhar I. End compression of sandwich columns. Composites A 2002;33:353–9. [25] Hoff NJ, Mautner SE. The buckling of sandwich-type panels. J Aeronautical Sci 1945;12:285–97. [26] Fagerberg L, Zenkert D. Imperfection-induced wrinkling material failure in sandwich panels. J Sandwich Struct Mater 2005;7:195–219. [27] Allen HG. Analysis and design of structural sandwich panels. Oxford, UK: Pergamon; 1969. [28] QinetiQ. Matdat polymer composite materials database [accessed 20.03.08]. [29] EMKay plastics Ltd. Rohacell foam technical product manual; 2006. [30] Personal Communication. B Wakefield, Evonik Degussa AG 27/3/2008. [31] Gibson LJ, Ashby MF. Cellular solids: structure and properties. Cambridge, UK: Cambridge University Press; 1997. [32] Ishai O, Hiel C. Damage tolerance of a composite sandwich with interleaved foam core. J Compos Technol Res 1992;14:155–68. [33] Weeks CA, Sun CT. Multi-core composite laminates. J Adv Mater 1994;25: 28–37.
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Composites Science and Technology journal homepage: www.elsevier.com/locate/compscitech
Self-healing sandwich panels: Restoration of compressive strength after impact H.R. Williams *, R.S. Trask, I.P. Bond ACCIS – Advanced Composites Centre for Innovation and Science, Department of Aerospace Engineering, University of Bristol, Queen’s Building, University Walk, Bristol BS8 1TR, UK
a r t i c l e
i n f o
Article history: Received 12 June 2008 Received in revised form 18 July 2008 Accepted 29 July 2008 Available online 8 August 2008 Keywords: C. Sandwich A. Smart materials B. Impact behaviour B. Strength Vascular self-repair
a b s t r a c t Impact damage can significantly reduce the strength of composite sandwich panels, giving rise to large factors of safety in design. A self-healing sandwich panel was designed and manufactured. Specimens were tested in edgewise compression-after-impact to prove the concept. The self-healing system consists of vascular networks carrying the two liquid components of an epoxy resin system. After damage and autonomous self-healing, the specimens could be restored to their undamaged strength. Key influences on healing efficiency, and the need for further improvements in reliability were identified. A simple specific strength analysis suggested that the mass penalty of self-healing could, in some cases, be offset by the improved design allowables permitted. Published by Elsevier Ltd.
1. Introduction Safety and reliability requires that service impact damage is considered when allowable stresses are defined for structural materials. Modest impact damage can have a detrimental effect on the performance of sandwich structures [1,2]; which are valued in high-performance applications for their high bending and buckling stiffness. A self-healing system could offset its own mass, and cost, by allowing a higher material stress to be used for design. The pattern of impact damage in sandwich structures is a function of the dimensions and properties of the skins and core, and the shape, mass and velocity of the impactor [1,2]. Generally, the primary damage mode for low-velocity blunt impact on a brittlecored sandwich structure is a cohesive disbond in the core located under the impacted face with consequent loss of skin stability. The loss of skin support has been shown to reduce the compressive strength of sandwich panels by over 25% e.g. [3–6]. A damage tolerant design can be used to manage the risk of service impact e.g. [2,7]. In particular, there is a requirement to survive design ultimate load with damage of a specified size. In the aerospace industry, this is termed the allowable damage limit (ADL) and is defined as the minimum size that can be detected during the planned maintenance programme [2,8]. An undamaged structure is, therefore, designed to fail well above design ultimate load; resulting in a structural mass penalty. The nascent field of self-healing materials offers a novel but complementary approach to ‘traditional’ damage tolerant design. * Corresponding author. Tel.: +44 (0)117 331 7499. E-mail address: [email protected] (H.R. Williams). 0266-3538/$ - see front matter Published by Elsevier Ltd. doi:10.1016/j.compscitech.2008.07.016
Self-healing based on the bleeding of a liquid healing agent from a microcapsule or hollow glass fibre into region of damaged material has been reported in pure polymers e.g. [9,10] and polymer composite laminates e.g. [11–15]. The reader is referred to [16,17] for more detailed reviews of these approaches. A key limitation of these techniques is their capacity to fill the larger damage volume that is typical of damage in a sandwich structure. A vascular approach, using a network of vessels to deliver fluid from a remote reservoir, has been shown to recover failure mode and load in sandwich beams subject to flexure-after-impact [18]. A double vascular network was introduced into the foam core of a composite sandwich structure. Each respective network was filled with a liquid epoxy resin and the corresponding amine based hardener. Rupture of the channels by impact damage allowed the two component healing agent to infuse the damage, mix and cure. This study has been used to propose biomimetic design schemes for optimum supply vessel diameter for minimum system mass [19]; tailoring the network density according to critical loading [20] and using the network design to enhance system reliability [21]. A vascular self-healing approach has also been demonstrated on a smaller scale to autonomously repair cracks in a polymer coating where the network delivers a liquid dicyclopentadiene (DCPD) monomer and solid (Grubb’s) catalyst is dispersed in the polymer coating [22]. In this paper, the vascular self-healing approach developed for sandwich beams is applied to the edgewise compression-afterimpact test configuration, a more representative scenario for a practical engineering sandwich structure. The paper describes the design, manufacture and specimen tests of a self-healing system with a network appropriate for the target damage. Experimental
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results are linked to destructive evaluation of the specimens to propose the key mechanisms influencing healing efficiency. These results are used, with simple expressions for panel strength, to support the values of healing efficiency obtained and provide a first comparison of the specific performance of damage tolerant and self-healing sandwich structure. 2. Healing system design In an autonomous self-healing system using a two-part epoxy, cure occurs on intimate contact of the liquid parts. At a simple level, this contact can be considered to occur along a line where the flow fronts meet. Providing both components have sufficiently low viscosity to permit timely infusion of the damage, a system designed to cure with equal parts of resin and hardener (1:1 mix ratio) is desirable. Resintech Ltd. RT310, an epoxy system with a 2:1 mix ratio and listed viscosity of less than 1000 cP was selected as a good compromise. This system requires a 24 h cure at room temperature. The healing network was developed from the simple configuration used in previous flexural studies, with tubing bonded into the midplane of the core and vertical risers supplying the skin-core bond region. The distribution of the vertical risers can be varied by design. Williams et al. [20] report a riser distribution that ensures at least one riser is ruptured by a circular damage event of a critical damage size, x, as shown in Fig. 1. In the present study, several additional factors influence the distribution of risers. The final network design is shown in Fig. 2. This differs from the basic riser distribution previously proposed in several ways: Separate networks are required for an autonomous healing system using two liquid parts, the spacing between channels is therefore x/4 rather than x/2. In this case, the location of damage is known, so it is possible to increase the riser density in this region, some risers have been also placed outside this zone to make the specimen representative of a practical structure. Previous experimental studies [18] and a consideration of system reliability [21] suggest redundant risers are desirable, and the distribution in Fig. 2 provides at least two risers of each component in the nominal damage zone. The number of risers in the damage zone has been used to approximately meter the resin and hardener components to match the 2:1 mix ratio. Four resin risers and two hardener risers are placed within the expected diameter of damage. Although the network of channels shown in Fig. 2 were designed for the separate components of the selected resin system, the infiltration pattern can be better assessed by infusing a pre-
Fig. 2. Network design for self-healing sandwich specimens. (A) Channel designed for resin component and (B) channel designed for hardener component.
mixed resin. By this approach, the extent of infiltration is fixed before destructive assessment. To highlight the zone of healing and distinguish between flow fronts from the A and B networks, pigmented pre-mixed RT310 resin (Resintech Ltd.) would replace the separate resin and hardener components in one specimen group. The viscosity of these systems was compared to ensure they provided a reasonable representation of the different components. The viscosity was measured at 25 °C using a Bohlin Instruments Gemini 200 Advanced Rheometer fitted with a 4°/40 mm cone and plate. Two separate, well-mixed samples of each liquid were tested three times at shear rates between 0.1 and 10 s1. The measured viscosities show little variation with shear rate over the range investigated and are summarised in Table 1. The pre-mixed, pigmented systems have a very similar viscosity to the pure resin. The pure hardener is less viscous, but the difference is modest. The system with blue pigment better represents the hardener for the later tests. These data show the mixed, pigmented systems should provide a reasonable model for the flow behaviour of the pure systems. 3. Panel manufacture and test method 3.1. Manufacture Vascular sandwich cores were manufactured by bonding 1.5 mm bore silicone tubing in channels between two 8 mm thick, 52 kg/m3 polymethacrylimide (Rohacell) closed-cell foam sheets using film adhesive (FM300K, Cytec). Drilling 2 mm holes in the locations shown in Fig. 2 was used to form the vertical risers. Pre-impregnated [0°,90°]S E-glass/913 epoxy (Hexcel) laminates were co-cured onto the vascular core, giving a relatively thinskinned configuration susceptible to core-dominated impact damage. Samples were sectioned into specimens (60 90 mm) using a water-cooled diamond saw and the loading edges ground flat and parallel. Miniature barbed tubing connectors were inserted into the exposed tubes at each end of the samples, allowing external supply tubing to be connected. Specimens were infiltrated in three Table 1 Results of rheometry on selected resin system Component
Mean viscosity (cP)
Standard deviation (cP)
(Total 120 measurements)
Fig. 1. Riser and supply tube distribution which ensures damage of diameter x (large solid circles) occurring at any location can be supplied with healing agent by at least one riser.
RT310 RT310 Mixed Mixed
resin hardener pigmented RT310, Red pigmented RT310, Blue
473 337 469 440
12 23 18 27
H.R. Williams et al. / Composites Science and Technology 68 (2008) 3171–3177
sample groups, each of six specimens, to be compared with undamaged and damaged reference samples. (I) Pre-mixed resin and hardener: the pre-mixed healant supplying the A (resin) channels was pigmented red, and that supplying the B (hardener) channels pigmented blue. These samples assessed the pattern of damage infusion and the healing efficiency using an ideally cured resin. The networks were filled using a syringe, the network clamped downstream of the specimen and the syringe plunger removed to open the reservoir to atmosphere. The network was then lightly pressurised to a static head of 0.5 m, 5.6 103 Pa, before impact. (II) Low-pressure autonomous healing: unmixed resin and hardener were used to fill the corresponding channels. Autonomous healing was therefore initiated by the resin and hardener entering the damage and mixing in-situ. The networks were filled and statically pressurised to 5.6 103 Pa as above. (III) Pressurised autonomous healing: unmixed resin and hardener were used as above, with the network clamped downstream of the specimen and pressurised to 3.0 105 Pa using peristaltic pumps in a bespoke pressure rig. The pressure was intended to improve healing efficiency by enhancing the mixing in the damage zone and restoring skin alignment. The barbed connectors were bonded into these specimens using epoxy structural adhesive (Bondloc B2012) to prevent leakage. The specimen edges were ground flat and parallel after impact and healing was completed.
3.2. Mechanical testing Damage was introduced using an Instron Dynatup 9250HV drop tower (3 J, 5.35 kg dropweight, 20 mm hemispherical head) with the specimen resting on a solid support plate. In the pressurised specimens, the pressure in the networks was observed to fall rapidly after impact as the damage filled. The networks were re-pressurised to 2.0 105 Pa, which was found to remove the residual post-impact indentation. Pre-mixed and low-pressure self-healed samples were allowed to heal for at least 48 h after impact. The third group were left pressurised for at least 48 h. For practical reasons it was not possible to give all the pressurised specimens exactly the same total cure duration, so a short post-cure of 1 h at 60 °C was performed to normalise the cure states. Edgewise compression tests (ASTM C364) were performed using a bespoke fixture on an Instron test frame (250 kN load cell) with the short (60 mm) edge of the specimen clamped (10 mm length each end) and loaded at 0.5 mm/min.
4. Results 4.1. Impact damage The visible damage after impact was limited to a roughly elliptical, inconspicuous residual indentation with no visible damage to the face laminate. The major axis of the dent was aligned with the long axis (0°) of the specimens. The skin has a higher bending stiffness in this direction due to the laminate lay-up, the 0° plies being further from the neutral axis. The dent was measured 1 h after impact, the average dimensions for the damaged specimens and the two groups infiltrated at low pressure (groups I and II) are shown in Table 2. There was no significant difference in indentation dimensions between these three groups. In the pressurised self-healed specimens (group III) the indentation was removed completely.
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4.2. Infiltration assessment The damage zone of the pigmented, pre-mixed specimens (group I) was investigated after compressive testing. The impacted face was removed to reveal the infiltrated damage zone in plan. This allowed inspection of the infusion patterns from the A and B networks. Photographs of the damage zone were interrogated using image analysis software to measure the area of infiltration from each network and the total infiltrated area relative to the residual dent area. One specimen suffered extensive bleeding beyond the damage and towards the sample edge, possibly due to a manufacturing defect, and has been discounted. Results from five specimens are shown in Fig. 3. The ratio of areas infiltrated by the two components is seen to agree reasonably with the target of 2:1, the mean ratio is 2.3:1. Interestingly, the total infiltrated area is significantly less than the area of the residual dent. The number of risers breached by the impact event is seen to be at least equal to the design target in all specimens. It is notable that there is almost no evidence of the components mixing with each other, which is necessary for autonomous healing. When the damage zone of the low-pressure autonomously healed specimens (group II) was inspected in a similar manner, most of the damage zone and the vascular network were filled with uncured liquid. Tactile investigation revealed only small areas of partially cured healing agent. It was not possible to determine the extent of resin infusion reliably because the specimens had been subject to mechanical failure and destructive sectioning. A similar problem was encountered in the pressurised autonomously healed specimens (group III) whose damage zones and vascular network also contained areas of uncured liquid healing agent. However, in these specimens an area with improved cure was located around the centre of the zone of damage, and in the centre of a region with slight discolouration. Fig. 4 shows the damage zone of a representative specimen. 4.3. Edgewise compression results The strength of each specimen is expressed as the ultimate skin compressive stress; ultimate load per unit cross sectional area of both skins. Table 3 summarises the strength and individual failure modes and Fig. 5 shows photographs of the test configuration and key failure modes. Two specimens were discounted from the pressurised self-healing group (III). In one, the action of re-pressurising the network after impact caused the skin to bulge outwards. There was visual and audible evidence of the damage propagating outwards from the original impact, making a fair comparison with the other sample groups impossible. In a second specimen there was no drop in the pressure of the hardener network after impact. Destructive sectioning after compression testing revealed only uncured resin in the fracture plane. The strength of this specimen was 129 MPa – very close to the mean value for the unhealed specimens – giving confidence that there was a failure of either hardener vessel to rupture. There was significant scatter in all specimen groups with the highest standard deviation noted in the pressurised self-healing group. The mean and individual strengths are plotted in Fig. 6 to show all the results. The estimated measurement uncertainty was small compared to the standard deviations of the results, suggesting the scatter is an artefact of the test setup and relatively small specimen group sizes. The effect of the impact damage is to significantly reduce the ultimate strength and change the most common failure mode from wrinkling and skin-core separation to an inward propagation of the residual dent which then causes core shear buckling. The infiltration of pre-mixed resin (group I) restores the failure mode and actually allows an increase in the ultimate strength
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Table 2 Residual dent dimensions of 18 impacted specimens Measurement
Mean dimension (mm)
Standard deviation (mm)
Major axis (0° to specimen) Minor axis (90° to specimen) Maximum dent depth
43 38 0.5
5 3 0.1
calculations. In its factored form it is considered to give useful, conservative predictions [23,24] but Fagerberg and Zenkert [26] argue it is less appropriate for constructions with anisotropic skins. The assumptions of core stress field used by Allen [27] to derive an alternative expression are more refined, but this expression typically gives non-conservative load predictions due to initial imperfections in practical structures. Fagerberg and Zenkert [26] investigate the effect of initial wrinkling imperfections in detail. The Hoff [25] and Allen [27] expressions are:
pffiffiffiffiffiffiffiffiffiffiffiffiffiffi
rHOFF ¼ 0:5 3 Ef Ec Gc rALLEN
3 ¼ qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 3 12ð3 mc Þ2 ð1 þ mc Þ2
qffiffiffiffiffiffiffiffiffiffiffi 3 Efb E2c
ð1Þ ð2Þ
where Ef is the isotropic face sheet modulus, Ec is the core modulus, Gc is the core shear modulus, mc is the core Poisson’s ratio and Efb is the anisotropic face sheet bending modulus in the loading direction. For an isotropic solid:
Ec ¼ 2Gc ð1 þ mc Þ
ð3Þ
The stress in the skins at the critical load for core shear failure [24] can be derived from the effective shear rigidity (AG)eq as:
rSHEAR ¼ Fig. 3. Results of infiltration assessment in specimens with pre-mixed resin (group I).
Fig. 4. Damage zone of a specimen self-healed autonomously under high-pressure, plan view with impacted skin removed. The areas of most complete cure are centred within the area of discolouration.
over the undamaged specimens. Those specimens autonomously self-healed at low pressure (group II) show partial restoration of strength and failure mode, although not sufficient to reliably halt the inward wrinkle growth. With the exception of the reliability issues noted previously, the pressurised self-healing specimens show good recovery of failure mode and strength, with performance close to that of the pre-mixed specimens. 5. Analysis 5.1. Panel strength Common edgewise compressive failure modes for composite sandwich materials are discussed by Zenkert [23] and Fleck and Shridhar [24]. Simple expressions for the specimen strength in various modes are given in these works: panel geometry and the relative thickness and properties of skins and core influence the dominant failure mode and load. Skin wrinkling failure was the dominant failure mode in the work reported herein. Hoff and Mautner [25] derived an analytical expression for the wrinkling load, and factored this in the light of experimental work, to produce an expression which is still in use for basic design
ðAGÞeq bcGc cGc ¼ 2tb 2tb 2t
ð4Þ
where b is the specimen width and t and c are the thicknesses of one skin and the core. Appropriate mechanical properties for the skin and core materials are given in Table 4, and are used with the equations above to construct to Fig. 7. The three properties obtained for the core appear inconsistent with Eq. (3); consistent properties are needed to compare Eqs. (1) and (2). Some degree of anisotropy can be expected in closed-cell foams due to the manufacturing process [31], and experimental variation due to the different test approaches used to obtain each value could also be expected. Therefore, each of the three core properties presented in Table 4 were discounted in turn, and a consistent value calculated from the remaining properties, using Eq. (3), to give the tolerances quoted in Table 4. Fig. 7 is plotted using the consistent dataset that gives the median wrinkling stresses. Error bars are provided on Fig. 7 to show that the dominant failure mode is not sensitive to this uncertainty. The skin laminate compressive strength, rcomp, would be best determined experimentally for each lay-up e.g. [24]. In the absence of a dedicated test programme to obtain the data for this non-critical failure mode, the compressive strength of a unidirectional laminate was factored by 0.5, assuming only the 0° plies carried load. This gives a coarse approximation suitable for a first assessment of the overall trend. Fig. 7 provides good evidence to support the observed failure modes and strengths. The Hoff [25] and Allen [27] predictions for wrinkling load are conservative and optimistic respectively. The next most critical failure mode would be core shear buckling; in the experiments this appeared as a secondary failure in the damaged specimens. This could be accounted for by inward wrinkle growth interacting with the damaged core. Limited evidence of compressive microbuckling is present in one specimen as a secondary mode. The sensitivity of wrinkling stress is explored further in Fig. 8. The observed impact damage and healing agent infusion would be expected to influence the core rather than the skin properties. Fig. 8 is, therefore, plotted from Eqs. (1) and (2) using the core elastic modulus to represent the underlying support. Poisson’s ratio is held constant so the shear modulus will increase with elastic modulus, effectively giving a density scaling or a representation of the extent of infiltration and cure.
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H.R. Williams et al. / Composites Science and Technology 68 (2008) 3171–3177 Table 3 Edgewise compression results Specimen group
Mean ultimate facing stress (MPa) (Note 1)
Standard deviation (MPa)
Relative ultimate stress
Undamaged Damaged Healed, pre-mixed resin (I) Self-healed (II) Pressurised self-healed (III)
184 130 207
22.7 17.6 19.4
1.00 0.70 1.12
151 211
6.55 33.8
0.82 1.14
Failure modes – No. of specimens (Note 2) I
1
II
III
IV
V
3
3 1
4
1
5 1 4
5
Note 1. Estimated measurement uncertainty in ultimate facing stress ±6.53 MPa. Note 2. Failure modes: I – face wrinkle with visible compressive microbuckling damage, II – outward skin wrinkle and abrupt skin–core separation, III – inward wrinkle then abrupt opposite skin separation, IV – Inward skin wrinkling then shear buckling, V – inward skin wrinkle with stable core crush.
5.2. Influences on healing efficiency In the pre-mixed specimen group (I) the damage void is infused with an ideally cured epoxy resin whose elastic and shear moduli would far exceed (approximately a factor of 50) the foam they are ‘replacing’. This allows these repaired specimens to exceed the strength of the undamaged specimens. The experimental strength of this group in fact only shows an increase of 12%. Several factors were identified that could account for this relatively small increase. Only a proportion of the sample width is reinforced in the manner described above because the residual dent only occupies about two thirds of the specimen width – Fig. 3 also shows that residual dent is not entirely infused. It has previously been shown that the wrinkle propagates as a cohesive tensile core rupture near the edge of a residual dent [3], as this region is not fully infused, the healing has not reinforced this crack initiator. The remaining residual dent has the capacity to significantly reduce the wrinkling load by acting as an initial imperfection. The low-pressure, autonomous group (II) shows partial selfhealing. Negligible indication of flow front mixing is observed in the pre-mixed specimens (Fig. 3) so it is reasonable to assume that negligible mixing has occurred between the resin and hardener of this autonomous healing group. There is minimal contact in the narrow damage void and the components flow slowly under low pressure, preventing active mixing at this interface. The extent of cure is compromised, and this accounts for the poor performance relative to the pre-mixed specimen group (I).
Fig. 5. Photographs of main compressive failure modes from Table 3: II – outward skin wrinkle and abrupt skin–core separation, III – inward wrinkle then abrupt opposite skin separation, IV – inward skin wrinkling then shear buckling.
The pressurised, autonomous specimens (III) show strength recovery similar to the pre-mixed specimens. In terms of extent of infusion, the higher pressure restores the skin alignment by removing the residual dent, in the process opening the damage zone into a larger void and, presumably, allowing the edges of the damage to be infused more successfully. The extent of infusion is superior to both the low-pressure specimen groups. In terms of cure extent, this high-pressure autonomous group outperforms the low-pressure autonomous group. Because the resin and hardener flow rapidly into the damage zone, it is suggested that this additional energy facilitates active mixing as flow fronts interact. The opened damage zone also gives a larger contact area between components. However, the extent of cure is inferior to the pre-mixed specimens and variable across the damage zone. The mechanisms of core elastic support, (maximised in the fully cured pre-mixed specimens) and the restoration of skin alignment (maximised with the higher-pressure infusion) thus contribute to healed efficiency. In an autonomous system operating at highpressure, skin alignment can be restored. The clear driver for further improving healing efficiency is in maximising the restoration of core elastic support by enhancing the degree of cure in the damage zone. If this is achieved, the undamaged strength of the panel can be exceeded. Given that the aim is only to restore undamaged strength, a useful margin exists. The reliability of the cure process, infusion of the edge of the damage and the influence of the network on the baseline properties are key areas for future study. The influence of the risers on baseline properties will be from two sources; a change of effective core density and an influence as a crack initiator. The aerial density of the risers shown in Fig. 2 is around 2.5%, so should have a relatively minor influence. The influence on skin–core disbond toughness requires further experiment and analysis. A comparison of the
Fig. 6. Edgewise compressive strength.
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Table 4 Assumed mechanical properties for analysis Component
Property
Value
Notes
Skin Hexcel UD E-glass/913 epoxy [0 90]s
Ef Efb
25.1 GPa 37.9 GPa 555 MPa
Ply data from [28]. Laminate data calculated using classical laminate analysis
68.6 MPa (52.1–68.6) 18.6 MPa (18.6–24.5) 0.4 (0.40–0.84)
[29]. See Section 5.1 for discussion on tolerance [29]. See Section 5.1 for discussion on tolerance [30]. See Section 5.1. mc = 0.84 used to plot Fig. 7
rcomp Core Rohacell 51IG
Ec Gc
mc
Ply data from [28]. Approximation assuming zero contribution from 90° plies
Note: Tolerances on core properties are only those tolerances required to generate a consistent set of material properties according to Eq. (3). They do not include the normal experimental variation which would be expected in determining any material properties.
performance of the autonomous heal with a conventional manual repair could be instructive, as could a consideration of the performance under fatigue loading conditions. 5.3. Specific strength and application The mass of all except the pressurised specimens was measured before filling with healant, and then after healing was complete. The pressurised samples were measured after they were selfhealed and ground to test dimensions. Using measured or calculated masses of the various components and after normalising for minor dimensional variation, the specimen mass was estimated in various configurations. The results are given in Table 5. The damage in this experimental study is inconspicuous, but detectable, so it is reasonable to use it as an approximation for an allowable damage limit (ADL). In this case the ultimate stress available for design is reasonably estimated by the mean strength of the damaged specimens. This assumes the post-impact strength is dominated by the damage and the risers have negligible influence on impact response. The midplane bond is retained in this comparison as it influences performance under impact [32,33]. Autonomous self-healing under pressure has been shown to restore the undamaged performance, so the mean strength of the undamaged specimens is taken as a design allowable for the self-healing panel. The final two columns of Table 6 show that the specific performance of the self-healing panel is competitive. It is important to note that the configuration considered was selected for a laboratory specimen-scale study confined to coredominated damage. A thin-skinned sandwich with low-density core and large size of damage relative to the specimens make the configuration highly susceptible to disbonding and wrinkling failure. A detailed understanding of the effect of the network on the
Fig. 7. Predicted failure modes of test specimens calculated using data in Table 4.
basic properties of the panel would also be required for a rigorous engineering trade-off. Set against this is the potential for significant improvements in healing efficiency and mass of a self-healing system as the technology matures. It is useful to consider qualitatively the implication of a selfhealing system over a product lifecycle. For self-healing to be a competitive design option, the upfront manufacturing costs, periodic system maintenance and final disposal costs must be outweighed by a overall improvement in material specific performance, plus the potential savings from removing the demand for inspections capable of detecting damage close to the ADL. Over a product lifecycle, a few percent improvement in structural weight can produce significant cost savings (especially in an environment of increasing fuel costs) and a reduction in lifecycle environmental impact. The conclusion to this analysis is, therefore, balanced; a selfhealing system certainly has the potential to be a competitive design option in advanced structures, although many details of the design will need to be carefully optimised. 6. Conclusions This work has drawn together several studies on vascular selfhealing to design and test an autonomous self-healing system for foam-cored sandwich panels. Three interacting components were considered: a conventional sandwich panel subject to a nominal design damage event, a vascular network primarily tailored for that event and an appropriate healing agent. The residual strength of an impact damaged sandwich specimen with a residual dent and underlying core damage has been successfully restored to the undamaged strength using a vascular self-healing system which releases a liquid healing agent into the damage zone created by an impact event. An ideally cured resin infused at low pressure produces comparable performance to a fully
Fig. 8. Effect of underlying core properties on wrinkling strength.
H.R. Williams et al. / Composites Science and Technology 68 (2008) 3171–3177 Table 5 Estimated specimen mass in various configurations Configuration
Mass (g)
Relative mass
Reference specimen: midplane bond, no network Self-healing specimen with network, filled but no damage Self-healing specimen, healed, low-pressure case Self-healing specimen, healed, high-pressure case
18.8 22.8
1.00 1.21
23.8 24.7
1.27 1.31
Table 6 Specific strength assessment Configuration
Mass (g)
Strength (MPa)
Specific strength (MPa/ g)
Relative specific strength
No self-healing system, impacted Self-healed specimen, impacted, 2–3 bar heal
18.8
130
6.91
1.00
24.7
184
7.45
1.08
autonomous self-healing system operating at higher pressures. In the latter, separate epoxy resin and hardener systems are infused under significant pressure to facilitate active mixing and removal of the residual dent. Competing mechanisms determine the healing efficiency; the restoration of core shear continuity and support is necessary to restore resistance to skin wrinkling. A residual dent acts as an imperfection that reduces wrinkling strength, and pressurised infusion can be applied to offer significant remediation. A simple specific strength analysis has been applied to show that this system has the potential to be competitive with more traditional damage tolerant designs. For rigorous cost and environmental impact these should be performed over the whole lifecycle. Further work is required to improve the system design and healing agent selection to account for the variability and reliability issues highlighted in the study. Acknowledgements The research reported in the article was supported by the UK Ministry of Defence under Grant FATS/RAOWPE/02 and the University of Bristol via a Convocation Scholarship for H.R. Williams. The authors are grateful to Mr. Andy Limmack for his assistance with resin rheometry and the improvements suggested by the reviewers. References [1] Abrate S. Localised impact on sandwich structures with laminated facings. Appl Mech Rev 1997;50(2):69–82. [2] Tomblin J, Lacy T, Smith B, Hooper S, Vizzini A, Lee S. Review of damage tolerance for composite sandwich airframe structures. DOT/FAA/AR-99/ 49. USA: Federal Aviation Authority; 1999.
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