This study investigated the use of carbon nanotubes (CNTs) as an epoxy adhesive additive for adhesive joints between steel–composite interfaces and composite–composite interfaces. The study also examined the effect of CNT functionalization to improve CNT dispersion and thus improve joint strength. Specimens were constructed by adhesively bonding two parallel coupons, with a starting crack at one end. The specimens were loaded to final failure in three-point bending for Mode II fracture. Critical strain energy release rate was used to compare fracture properties of each set of specimens. It was shown that additions of multi-walled CNTs on the order of 1 wt% with diameters on the order of 30 nm and lengths 5–20 μm enhanced fracture toughness for both steel–composite and composite–composite adhesive joints tested. However, other combinations of CNTs could significantly decrease fracture properties, likely due to agglomeration issues. Functionalization of nanotubes showed some limited promise. Scanning electron microscopy validated the improved dispersion of CNTs using functionalization, but also highlighted the shortening effects due to the harsh chemical treatment. In summary, the study illustrates the importance of various CNT parameters on fracture properties, and encourages further investigation and optimization of these parameters for applications of interest.

Effect of carbon nanotube reinforcement on fracture strength of composite adhesive joints

In order to strengthen the interface of a composite scarf joint, this study investigated the benefits of using locally applied carbon nanotubes to reinforce a carbon fiber composite scarf joint. The effect of carbon nanotubes on enhancing the fracture toughness and interface strength was investigated by performing Mode I and Mode II fracture tests with and without carbon nanotubes applied locally at the joint interface. Furthermore, the effects of seawater absorption and different carbon nanotube concentration values on Mode II fracture were investigated. Finally, a partial application of carbon nanotubes only near the crack tip area was considered. During the study, the image correlation technique was used to examine the fracture mechanisms altered by the introduction of carbon nanotubes. The experimental study showed that an optimal amount of carbon nanotubes could increase the fracture toughness of the composite joint interface significantly, especially for Mode II, including a physical change in the fracture mechanism.

Study of composite joint strength with carbon nanotube reinforcement

Interface fracture of woven fabric composite layers was studied using Mode II fracture testing. Both carbon fiber and E-glass fiber composites were used with a vinyl ester resin. First, the single-step cured (i.e., co-cured) composite interface strength was compared to that of the two-step cured interface as used in the scarf joint technique. The results showed that the two-step cured interface was as strong as the co-cured interface. Carbon nanotubes were then applied to the composite interface using two-step curing, and then followed by Mode II fracture testing. The results indicated a significant improvement of the interface fracture toughness due to the dispersed carbon nanotube layer for both carbon fiber and E-glass fiber composites. The carbon nanotube layer was then evaluated as a means to monitor crack growth along the interface. Because carbon nanotubes have very high electrical conductivity, the electrical resistance was measured through the interface as a crack grew, thus disrupting the carbon nanotube network and increasing the resistance. The results showed a linear relationship between crack length and interface resistance for the carbon fiber composites, and allowed initial detection of failure in the E-glass fiber composites. This study demonstrated that the application of carbon nanotubes along a critical composite interface not only improves fracture properties but can also be used to detect and monitor interfacial damage.

Study of Composite Interface Fracture and Crack Growth Monitoring Using Carbon Nanotubes

Advances in Composite Materials - Ecodesign and Analysis

A comprehensive literature review on the existing analytical models based on different material and geometry behaviors for both single and double-lap joints (as well as others) has been made to ass...

Effects of joint geometry and material on stress distribution, strength and failure of bonded composite joints: an overview

It was investigated whether there was a potentially significant improvement to scarf joint bonding that was achieved through the dispersion of carbon nanotubes (CNTs) along the interface of the composite joint. The study examined various factors that might affect CNT-reinforced joint interface strength. Each composite joint consisted of a vinyl-ester matrix base (DERAKANE 510-A) interlaced with a carbon fiber weave (TORAY T700CF). During the curing process, the research explored several variables concerning the CNT application. The testing included single-walled CNTs (SWCNT), and conventional and bamboo-structure multi-walled CNTs (MWCNT) with varying length, purity, and concentration levels along the surface area of the joint interface. This wide array of data demonstrated the effect of CNTs introduction at the joint interface, and provided the ideal type, size, purity level, and concentration level for composite scarf joint bond reinforcement using CNTs. Furthermore, a computational model was developed to predict the strength of the scarf joints. The predicted model agreed well with the experimental data.

R. Slaff

Papers

Enhancement of composite scarf joint interface strength through carbon nanotube reinforcement