This paper discusses the dynamic interaction between a monoatomic chain of solid particles and a thin-walled spherical pressure vessel. The objective is to find a relationship between the highly nonlinear solitary waves (HNSWs) propagating within the chain and the internal pressure of the vessel. The paper introduces first a general finite element model to predict the abovementioned interaction, and then a specific application to tennis balls. The scope is to demonstrate a new nondestructive testing (NDT) method to infer the internal pressure of the balls. The overarching idea is that a mechanically induced solitary pulse propagating within the chain interacts with the thin-walled ball to be probed. At the chain–ball interface, the acoustic pulse is partially reflected back to the chain and partially deforms the rubber giving rise to secondary pulses. The research hypothesis is that one or more features of the reflected waves are monotonically dependent on the internal pressure. Both numerical and experimental results demonstrate a monotonic relationship between the time of flight (TOF) of the solitary waves and the internal pressure of the tennis balls. In addition, the pressure inferred nondestructively with the HNSWs matches very well the pressure measured destructively with an ad hoc pressure gauge needle. In the future, the results presented in this study could be used to develop a portable device to infer anytime anywhere the internal pressure of deformable systems (including biological systems) for which conventional pressure gages cannot be used noninvasively.

Sometimes, nondestructive evaluation (NDE) or structural health monitoring methods commonly used in engineering structures are used for the betterment of consumer goods. A classic example is the use of sensor systems to monitor the pressure and the quality of car tires. In this paper, we present a nondestructive method to characterize tennis balls. The International Tennis Federation (ITF) specifies which characteristics a tennis ball must have in order to be commercialized. One of these characteristics is bounciness and the standardized method to measure it is the rebound test, where a ball is released from 2.54 m onto a smooth rigid surface and, in order to be approved, the ball must bounce within a certain range. This test can be staged by manufacturers and testing authorities but the equipment necessary to perform it is not readily available to the average consumer. In the study presented in this paper, an empirical method based on the propagation of highly nonlinear solitary waves (HNSWs) is proposed to establish whether a given ball conforms the specifications set by the ITF in terms of bounciness and allowed deformation. The experiments conducted in this study aim to discover a correlation between some features of the waves and the values obtained with the rebound test and the compression test in which the deformation of the ball under a known load is measured. The presence of such correlations could represent a viable alternative to establish the conformity of tennis balls. Based on the empirical evidences collected in this study, a possible new standard is suggested.

The in-situ measurement of thermal stress in beams or continuous welded rails may prevent structural anomalies such as buckling. This study proposed a non-contact monitoring/inspection approach based on the use of a smartphone and a computer vision algorithm to estimate the vibrating characteristics of beams subjected to thermal stress. It is hypothesized that the vibration of a beam can be captured using a smartphone operating at frame rates higher than conventional 30 Hz, and the first few natural frequencies of the beam can be extracted using a computer vision algorithm. In this study, the first mode of vibration was considered and compared to the information obtained with a conventional accelerometer attached to the two structures investigated, namely a thin beam and a thick beam. The results show excellent agreement between the conventional contact method and the non-contact sensing approach proposed here. In the future, these findings may be used to develop a monitoring/inspection smartphone application to assess the axial stress of slender structures, to predict the neutral temperature of continuous welded rails, or to prevent thermal buckling.

Abstract. The study presented in this manuscript deals with a non-contact structural health monitoring approach based on the use of smartphone and computer vision algorithm to estimate the vibrating characteristics of a cantilever slender beam. We hypothesize that the vibration of the beam can be captured using a smartphone in slow-motion modality and the natural frequency of the beam can be extracted using a computer vision algorithm. The results show an excellent agreement between the conventional contact method and the non-contact novel approach proposed here.