Effect of CNT on the Mechanical Properties of Composite Materials and Structures

Abstract

Numerous papers have shown improvements in mechanical properties of epoxy and other resins and their composites, by introducing small amounts of carbon nanotubes (CNT). Most reports deal with lab scale and/or standard specimens. Lesser work has been done on full-scale composite parts, especially large parts for the aerospace industry. This chapter reviews what has been done in this area and also reveals an effort to demonstrate the effect of introducing multi-walled carbon nanotubes (MWCNT) to the resin in a series of full-scale carbon/epoxy filament wound pressure vessels. The study covers processing, physical properties, mechanical behavior, and failure modes during burst tests. A dispersion method was developed to achieve excellent dispersion of the CNT in the resin. The CNT-modified carbon/epoxy composite exhibits higher interlaminar shear strengths, compared to the neat composite. Following the improvements in standard samples, full-size pressure vessels with metallic bosses and rubber shear plies were produced and tested by a routine testing protocol. The vessels were designed to burst in the domes, where interlaminar stresses may lead to failure. Process parameters were adjusted for minimum resin viscosity and best processibility. However, CNT led to some increase in resin viscosity, thus slightly affecting resin content in the vessels, especially at the domes. All the vessels burst at the domes as expected. Failure occurred at very high tensile strains, and the interlaminar shear failure mode did not develop, as evidenced by high-speed video-camera frame-by-frame analysis. Tensile strains and burst pressures of the vessels containing CNT were very similar to those of the neat vessels. CNT in the epoxy resin interacted with the rubber surface and led to enhanced cohesive failure of the rubber/composite interface. Acoustic emission from the matrix supports a mechanism of crack arrest and energy dissipation, in agreement with previous results. Prediction of failure is apparent at pressures below proof.

Appendix 1: Terminology

AE parameters used in this study follow ASTM E1316 Terminology for nondestructive examinations [19]:

• 1. P significant type II activity  – pressure level at which significant type II activity starts. Lower P significant type II activity indicates lower strength pressure vessels.

• 2. FR (felicity ratio ) – ratio of pressure at initiation of significant damage development during the second cycle to the proof test pressure. Lower FR value indicates more significant damage accumulation in pressure vessels.

• 3. AE amplitude  – the peak voltage of the largest excursion attained by the signal waveform from an emission event. AE amplitude is normally reported in dBAE – a logarithmic measure of acoustic emission signal amplitude, referenced to 1 μV at the sensor, before amplification. Signal peak amplitude (dBAE) = 20 log10(A1/A0), where A0 = 1 μV at the sensor (before amplification) and A1 = peak voltage of the measured acoustic emission signal (also before amplification).

• 4. Energy and acoustic emission signal  – the energy contained in an acoustic emission signal, which is evaluated as the integral of the volt-squared function over time.

• 5. AE signal duration  – the time between AE signal start and AE signal end.

• 6. AE signal start  – the beginning of an AE signal as recognized by the system processor, usually defined by an amplitude excursion exceeding threshold.

• 7. AE signal end  – the recognized termination of an AE signal, usually defined as the last crossing of the threshold by that signal.