Ultrasonic wave velocity in double-braided nylon rope

Abstract

Ultrasonic wave speeds are measured in new Samson double-braided 2-in-1 nylon ropes of diameters ranging from 0.535 cm ( 1 4 in) to 1.588 cm ( 5 8 in). Two resonant frequency transducers with resonant frequencies at 30 k Hz are mounted on the loaded rope and spaced at various distances and relative circumferential locations. An impulse voltage is introduced into the input transducer, and the wave speed is evaluated from the response of the output transducer. The output signal appears as a complicated wave packet, with the maximum amplitude peak arriving much later than the first detectable peak. The wave speed corresponding to the arrival of the first peak increases with increasing rope tension, ranging from 1700 m s −1 to 4500 m s −1. Wave speed at a given rope tension increases with cyclic loading, whereas wave speed at a given rope strain decreases with cyclic loading. This loading history dependence becomes less pronounced both at higher rope tensions and also after three loading cycles on new rope. At any given strain, the wave speed corresponding to the arrival of the first peak is nearly independent of rope diameter. The wave speed corresponding to the arrival of the first peak is also a function of the path angle of propagation through the rope interior. The wave speed measured along a generator on the rope surface is 2.33 times as large as the wave speed measured across the rope diameter. Values of longitudinal wave speed are predicted by measuring the modulus in the rope, and assuming the rope to act as a uniform elastic rod. Predicted wave speeds have an average percentage deviation from wave speeds corresponding to the arrival of the first peak of 13.2%, with the percentage deviation decreasing with increasing rope tensions to 10% or less. The wave speed corresponding to the arrival of the first peak is much faster than the apparent wave speed corresponding to the arrival of the maximum amplitude peak, with an average ratio of the two speeds of 4.1. The wave speeds predicted by assuming the rope to act as a uniform taut string in transverse vibration have an average percentage deviation from the apparent wave speeds corresponding to the arrival of the maximum amplitude peak of 47.9%, with the percentage deviation decreasing at the higher rope tensions to about 30%. Thus, ultrasonic wave speeds in double-braided nylon ropes are significantly affected by tension, strain and loading history. These effects should be taken into account when conducting nondestructive evaluation using ultrasonics and acoustic emission. These effects also suggest the possibility of monitoring the tension and/or strain in a rope via wave speed measurements.