Hybrid Nano/Microcomposites for Enhanced Damage Tolerance (Experimental and Applied Mechanics)

ABSTRACT

The objective of this investigation was to develop, process, and test hybrid nano/microcomposites with nano-reinforced matrix and demonstrate an enhancement in thermomechanical properties, with emphasis on damage tolerance measured in terms of fracture toughness, impact damage, residual strength, and fatigue life. The material investigated was carbon fabric/epoxy with the matrix reinforced with multi-walled carbon nanotubes (CNTs). A solvent-based method with a dispersion enhancing block copolymer was used to prepare composites with and without CNTs. It was first shown that CNT reinforced composites have higher matrix dominated properties, such as compressive modulus and strength, in-plane shear modulus and strength, interlaminar shear strength, and interlaminar fracture toughness. The composite with 0.5 wt% of CNTs showed noticeably improved resistance to indentation damage by about 16 % and increased damage tolerance in terms of residual compressive strength by about 35 % over the composite without nanotubes. A significant enhancement was also shown under interlaminar fatigue testing with fatigue lives an order of magnitude longer than those of the reference material. The high increase in fatigue life was related to an increase in static interlaminar shear strength, the logarithmic dependence of the fatigue-life (S-N) curves, and an increase in interlaminar fracture toughness.

Introduction

Recent and ongoing research in nanoparticle-reinforced composites (nanocomposites) has shown significant enhancements in mechanical and other physical properties. In order to maximize the potential for scaled up industrial applications, such as wind turbine blades, it is desirable to incorporate the nanocomposite as a matrix in conventional continuous fiber composites to produce multiscale hybrid nano/microcomposites with enhanced properties, especially fracture toughness. Results obtained to date show enhancements in thermomechanical properties with some added functionalities by incorporating nanoparticles, such as carbon nanotubes, into the polymeric matrix of conventional fiber-reinforced composites [1-8] .

Improvements in damage tolerance are expected from additional energy absorbing mechanisms introduced by the nanoparticles [2]. Addition of carbon nanotubes, for example, provides an additional source of energy absorption through nanotube pullout, stretching and fracture (Fig. 1). This behavior on the nanoscale is reflected in increased fracture toughness on the macroscale, improved impact damage tolerance, higher residual compressive strength, and extended fatigue life.

The objective of this study was to develop, process, and test hybrid multi-scale composite laminates and demonstrate and evaluate the enhancement in thermomechanical properties, with emphasis on fracture toughness, damage tolerance, and fatigue life.

Processing of Materials

The material investigated was carbon fabric/epoxy with the matrix reinforced with multi-walled carbon nanotubes (CNTs). The basic mechanical reinforcement was provided by a 5-harness satin carbon fabric perform (AS4 (AGP370-5H). The matrix was an epoxy (DGEBA) reinforced with multi-walled carbon nanotubes (CNTs). These CNTs were 1 – 2 ^m in length and 20 – 40 nm in outer diameter.

Figure 1. Possible energy absorbing mechanisms in CNTs. (a) initial state, (b) pullout following CNT/matrix debonding, (c) fracture of CNT, (d) telescopic pullout-fracture of outer layer and pullout of inner layer, (e) partial debonding and stretching.

A solvent-based method with a dispersion enhancing block copolymer was used as shown in Fig.2 [10, 11]. First, the block copolymer was dissolved in ethanol. CNTs were added to the solution, stirred and sonicated. A weighed amount of DGEBA was then added to the solution, followed by the hardener. After stirring, the ethanol was removed at an elevated temperature and the mixture was infused into the carbon fiber perform using a wet layup process. The impregnated preforms were placed in a vacuum oven to remove the ethanol. The prepreg layers were stacked and cured in the autoclave.

Figure 2. Process of CNT nanocomposte with dispersion enhancing block copolymer [10, 11].

Characterization

The neat and CNT- modified matrices were characterized by measuring their fracture toughness in both Modes I and II. The Mode I toughness was determined by means of a notched beam specimen under three-point bending (Fig. 3). The measured Mode I stress intensity factors for the neat resin and a nanocomposite with 0.5 wt% of CNTs, were

showing a 20 % increase for the nanocomposite. The calculated strain energy release rates for plane strain conditions, taking into account the increase in Young’s modulus, are showing a 33 % increase for the nanocomposite.

Figure 3. Three-point bending test of notched beam for determination of fracture toughness of neat resin and nanocomposite [13].

Properties relevant to energy absorption and damage tolerance in fiber composites are primarily the matrix dominated ones, such as the compressive modulus and strength, in-plane shear modulus and strength, interlaminar shear strength and fracture toughness. An example of mechanical enhancement in compressive strength as a function of CNT loading, with and without block copolymer dispersant, is shown in Fig. 4. It shows a nearly 40% increase in compressive strength for a 0.5 wt% CNT loading in the matrix with copolymer dispersant. The interlaminar shear strength was measured by means of short beam tests under three-point bending for the hybrid and the reference composite with and without the copolymer dispersant. An increase in interlaminar strength of approximately 15% was noted for the hybrid composite wit 0.5 wt% CNTs and copolymer dispersant (Fig. 4).

Figure 4. In-plane compressive and interlaminar shear strength of hybrid composite as a function of CNT concentration with and without block copolymer dispersant [10].

The Mode I energy release rate of the reference and hybrid composites was measured by means of the double cantilever beam (DCB) test [12]. Eight-ply laminates were prepared of the reference and hybrid composite with an embedded Teflon film strip along one edge. Coupons were machined so that the film strip served as a crack initiator at the loaded end of the beam. Figure 5 shows load-displacement curves obtained from such tests. Values of the interlaminar strain energy release rate were calculated by the area and compliance methods. It was found that that the value for the hybrid composite was more than 100% higher than that of the reference composite.

Similar tests were conducted with end-notched flexure (ENF) beam specimens under three-point bending to determine the Mode II delamination fracture toughness [12]. Load-deflection curves obtained from these tests for the two composite materials tested are shown in Fig. 6. The strain energy release rates measured by these tests were:

a 36.5% increase for the hybrid composite

Figure 5. Load-displacement curves of DCB specimen for the reference and hybrid composites

Figure 6. Load-deflection curves for end-notched flexure specimens of reference and hybrid composites

Resistance to Indentation Damage

Damage resistance of composites to a quasi-static indentation force is considered equivalent to low velocity impact damage. The indentation damage resistance of composites with and without CNTs was evaluated with the experimental set-up shown in Figure 7. A ball indenter of 12.7 mm diameter was used for the test. Composite specimens 25.4 mm wide were supported on two steel rollers over a span length of 25.4 mm. Indentation testing was carried out at a machine crosshead rate of 0.127 mm/min until total indentation failure. Indentation displacement was measured with an extensometer mounted as shown in Figure 7.

Figure 7. Experimental set-up for indentation damage resistance measurement.

Figure 8 shows typical indentation force-displacement curves of two composites with and without CNTs. There are two peaks in the figure. The first peak indicates damage initiation occurring on layers near the indented surface of the composite. The hybrid composites exhibited 13 % higher first peak load than the composite without CNTs. The second peak may be interpreted as the ultimate indentation resistance of the composite specimen. This indentation resistance was quantified by the energy absorbed in generating the damage. This energy was determined by integrating the indentation force-displacement curve. The indentation energy of the composite without CNTs was 4.4 J (average of four tests), and that of the hybrid composite was about 5.2 J, which represents a 16 % higher indentation damage resistance.

One generally accepted measure of impact, and by equivalence, indentation damage is the residual compressive strength following the damage. Following the indentation tests, direct compression tests to failure were conducted to determine the residual strength. These compression tests were conducted at a stroke rate of 0.254 mm/min. Figure 9 shows compressive stress-strain curves. It is seen that the compressive strength of the hybrid composite is about 35 % higher than that of the composite without CNTs, based on four tests. For a 4.4 J of applied indentation energy, the compressive strength of composites with and without CNTs was reduced by 69 % and 73 %, respectively.

Figure 8. Indentation damage resistance; indentation force vs. displacement curves.

Figure 9. Compressive stress-strain curves of composites with/without CNTs after indentation.

Fatigue Behavior

Fatigue tests were conducted with short beams under cyclic three-point bending aimed at producing a cyclic interlaminar shear. Stress-life curves were produced for both the reference composite and the hybrid one containing CNTs. It can be seen that, at a given cyclic load amplitude, there is a significant difference of more than an order of magnitude in lifetimes between the two composite materials (Fig. 10). This can be attributed in part to a vertical shift due to the increased static interlaminar shear strength and in part to an increase in the Mode II interlaminar fracture toughness. The latter decreases the rate of material degradation during cyclic loading.

Guided by the Paris law, the experimental data were fitted by an empirical relation of the form

where <rmax is the maximum cyclic stress, A is the static strength (at the first cycle), N the number of cycles to failure, and m a parameter related to the rate of material degradation. The parameters for the two materials tested were A = 58.8 MPa and m = 20.47 for the reference composite and A = 66.2 MPa and m = 22.17 for the hybrid composite. The difference in the stress parameter, A, is closely tied to the observed increase in static strength of the nanoparticle-enhanced material over the neat one. The difference in the slope parameter, m, is also significant, as it implies a more gradual fatigue life rate, resulting in further separation of fatigue life curves between the neat and nanoparticle-enhanced materials at higher numbers of cycles.

Figure 10. Stress-life curves for reference and hybrid composites under cyclic interlaminar shear

Conclusions

It was confirmed in this study that significant improvements in matrix dominated properties and energy absorption characteristics can be realized in hybrid nano/microcomposites with nano-reinforced matrix. It was demonstrated that such hybrid composites, with 0.5 wt% carbon nanotube reinforcement, have higher compressive and interlaminar shear strengths, higher delamination fracture toughness, increased indentation damage tolerance and residual strength. Composite laminates with 0.5 wt% of MWCNTs showed noticeably improved resistance to indentation damage by about 16 % and increased damage tolerance in terms of residual compressive strength by about 35 % over the composite without nanotubes. Fatigue lives extended by an order of magnitude.