Carbon nanotube (CNT) fibres, especially if perfect in terms of their purity and alignment,\ud are extremely anisotropic. With their high axial strength but ready slippage between the\ud CNTs, there is utmost difficulty in transferring uniformly any applied force. Finite element\ud analysis is used to predict the stress distribution in CNT fibres loaded by grips attached to\ud their surface, along with the resulting tensile stress–strain curves. This study demonstrates\ud that, in accordance with St Venant’s principle, very considerable length-to-diameter ratios\ud (103) are required before the stress becomes uniform across the fibre, even at low strains.\ud It is proposed that lack of perfect orientation and presence of carbonaceous material\ud between bundles greatly enhances the stress transfer, thus increasing the load the fibre\ud can carry before failing by shear. It is suggested that a very high strength batch of fibres previously\ud observed experimentally had an unusually high concentration of internal particles,\ud meaning that the pressure exerted by the grips would assist stress transfer between the\ud layers. We conclude that the strength of CNT fibres depends on the specific testing geometries\ud and that imperfections, whether by virtue of less-than-perfect orientation or of\ud embedded impurities, can act as major positive contributors to the observed strength

Adhesively bonded multi-layer structures are frequently used, mostly in the aerospace industry, for their structural efficiency. Nondestructive evaluation of bond integrity in these types of structures, both after manufacturing and for periodic inspection during service, is extremely important. A laser-based ultrasonic technique has been evaluated for non-contact detection of disbonds in aluminum multi-layer structures. Two configurations have been used to detect disbonded areas: pitch-catch with unidirectional guided wave scan and through-transmission with bidirectional scan. Guided wave scanning was done with a laser line source and air-coupled transducer sensing at 500 kHz, 1 MHz, and 2 MHz. Signals showed attenuation of the main frequency component and frequency shift on disbonded areas, whereas, a regular and standard waveform is seen outside disbonds. In through-transmission the longitudinal wave at normal incidence was monitored with a 1 MHz probe. One sample showed, besides the introduced inserts, other disbonded areas. After the ultrasonic measurements the sample was cut to visually check adhesive and interfaces. The guided wave pitch-catch scan allowed fast inspection and quick indication of disbonded zones, while the through-transmission C-Scan provided better definition of defects but was slower and required access from both sides of the test part.