Vibration, Volume 5, Issue 1 (March 2022) – 11 articles

Late detection of cracks can lead to serious failures and damages of drilling components, especially drill pipes and drill bits. Currently, the widely used method of repairing rotary drilling systems after a failure is corrective maintenance. Although this strategy has shown its effectiveness in many cases, waiting for a failure to occur and then performing a repair can be an expensive and time-consuming operation. Thus, the use of preventive maintenance under the aspect of periodic inspections can solve this problem and help engineers detect cracks before they reach critical sizes. In this study, modal analysis and finite element analysis (FEA) combined with artificial neural networks (ANN) were used to dynamically estimate the depth and location of a circular arc crack in the drill pipes of rotary drilling systems. To achieve this goal, a detailed analytical approach based on Euler–Bernoulli beam theory was adopted to validate the first four natural frequencies found by FEA for an undamaged pipe. Afterwards, an arc crack was assigned to the pipe already created using Abaqus, and the first four natural frequencies were obtained for each depth and location of the crack. Simulations with FEA led to the generation of a dataset with two inputs—depth and location of cracks—and four outputs: natural frequencies. Moreover, a multilayer perceptron (MLP) was designed and trained by the data collected from simulations. Finally, a comparison between the results obtained by FEA and ANN was performed, where both approaches showed a good agreement in predicting the depth and location of cracks. Full article

This study aimed to investigate the acute effect of grip and feed exertions on the vascular system at the fingers during exposure to hand-arm vibration (HAV), and to identify which active hand force situation would have the most effect on finger vascular function. A total of 12 individuals attended the test, and each of them were subjected to eight sets of force-and-vibration situations: four with combinations of forces and vibration, and four control ones with only hand forces applied. The vibration stimulus was applied on the right hand at 2.75 m/s² with a frequency of 125 Hz for three minutes, during which the application of grip and feed forces were set at either 10 N or 50 N. The weakening of the finger vascular function was reflected by a reduction in the finger blood flow (FBF) and finger skin temperature (FST). They were tested on both hands at fixed intervals before, during and after the exposure for in-time measurement. Hand forces resulted in clear reductions in FBF and FST in exposed right fingers whether the force was exerted solely or combined with vibration. The greater the hand force (especially grip force), the stronger the vascular response, while the additional reductions in FBF and FST from vibration were not significant. In the non-exposed left fingers, no significant changes in finger circulation occurred in response to force or vibration. Generally, vibration-induced acute finger vasoconstriction was affected by the hand forces, in which hand force seemed to play a more important part than vibration. A larger grip force would lead to a greater loss in the digital circulation than feed force. Thus, the level of hand force exerted on the tool handle should be limited to reduce the risk of harm from HAV. Full article

In this study, the vibration total value of the acceleration transmitted to the wrist and elbow was measured in the laboratory with a group of 13 male subjects holding a cylindrical handle while modifying the coupling force under varying levels of vibration. The results were used to establish the relationship between hand-transmitted vibration and coupling forces and to compare with the relations proposed as part of an ISO Technical Specification, ISO/TS 15230-2. This was done to determine the suitability of the proposed relationships when variations are introduced on the level of vibration on the handle. While tracing back the origins of the relations proposed in ISO/TS 15230-2, this paper further brings in evidence of the importance of considering the role of coupling forces when evaluating the exposure to hand-transmitted vibration and provides additional evidence to support the relationships which are proposed as part of the ISO Technical Specification. Irrespective of the level of broadband random vibration excitation considered, the agreement with these relationships was found to be best when setting the reference coupling force at 50, 75, 100 and 125 N and whenever the coupling forces applied on the handle were maintained below 150 N. Full article

Many studies in the literature have already evidenced that pedestrians are able to change the dynamic properties of slender structures (e.g., footbridges and staircases). The aim of this paper is to analyse which pedestrians’ features mostly affect the structure behaviour, in order to properly account for them in a human–structure interaction problem, while disregarding the less relevant ones. This is accomplished by measuring the apparent mass (i.e., the frequency response function between the vibration of the structure at the contact point and the consequent force exerted by the pedestrian to the structure itself) curves of human bodies and coupling them to the dynamics of a slender structure. In more detail, this paper aims at analysing which factors must be accounted for among intra-subject variability (i.e., the dynamic behaviour of the same subject can change because it is characterised by a natural dispersion), inter-subject variability (i.e., different subjects have different dynamic behaviours) and the posture (i.e., the same subject changes posture during motion and this causes a change of his/her dynamic features). The influence of the apparent mass properties on the modal parameters of the hosting structure is addressed by means of a modal approach. Full article

Before the coronavirus pandemic, there were 4.5 billion passenger movements by aircraft annually; this is expected to recover after the pandemic. Despite the large numbers of flights per year, there are few reports of whole-body vibration in fixed-wing aircraft. This paper reports a review of literature intended to collate reported data related to exposure to whole-body vibration. Following a filtering process to select relevant articles, a literature search elicited 26 papers reporting measurements of vibration. These included measurements made in the cockpit and cabin, and for pilots, crew and passengers. Aircraft included military, commercial and passenger aircraft, turboprops, jets and piston prop aircraft. There was a lack of consistency on measurement method and analysis, and few met the full requirements of ISO 2631-1. However, measurements showed significant components of vibration at frequencies largely attenuated by the ISO frequency weighting filters, but have been shown to be important in terms of human vibration perception. Propeller aircraft showed strong tonal components in vibration frequency spectra. There was also a significant effect of the flight phase in the vibration exposure. It is recommended that the body of literature related to human response to whole-body vibration on aircraft is augmented with further studies in order to understand in-flight experiences and to optimize human health, wellbeing, comfort and performance. Full article

Wheelchair users are exposed to whole-body vibration (WBV) when driving on sidewalks and in urban environments; however, there is limited literature on WBV exposure to power wheelchair users when driving during daily activities. Further, surface transitions (i.e., curb-ramps) provide wheelchair accessibility from street intersections to sidewalks; but these require a threshold for water drainage. This threshold may induce high WBV (i.e., root-mean-square and vibration-daily-value accelerations) when accessibility guidelines are not met. This study analyzed the WBV effects on power wheelchairs with passive suspension when driving over surfaces with different thresholds. Additionally, this study introduced a novel power wheelchair with active suspension to reduce WBV levels on surface transitions. Three trials were performed with a commercial power wheelchair with passive suspension, a novel power wheelchair with active suspension, and the novel power wheelchair without active suspension driving on surfaces with five different thresholds. Results show no WBV difference among EPWs across all surfaces. However, the vibration-dose-value increased with higher surface thresholds when using the passive suspension while the active suspension remained constant. Overall, the power wheelchair with active suspension offered similar WBV effects as the passive suspension. While significant vibration-dose-value differences were observed between surface thresholds, all EPWs maintained WBV values below the ISO 2631-1 health caution zone. Full article

The automatization of construction activities, which aims to reduce the time and cost of constructions, makes impact pile driving an interesting technique. However, these activities in urban areas can generate excessive vibrations and interfere with people and structures in the vicinity. With that in mind, predicting the expected vibration levels during the project design stage is essential. Different methodologies can be employed in this task, from empirical approaches to detailed and complex numerical formulations. This paper intends to present an overview of the empirical methods and the main physics of the problem from a numerical point of view. The results obtained are then compared with experimental vibration data reported in the literature in order to discuss the adequacy of empirical and numerical methodologies in predicting ground-borne vibrations induced by impact pile driving. Full article

Health surveillance in industries is an important prospect to ensure safety and prevent sudden collapses. Vibration Based Structure Health Monitoring (VBSHM) is being used continuously for structures and machine diagnostics in industry. Changes in natural frequencies are frequently used as an input parameter for VBSHM. In this paper, the frequency shift coefficient (FSC) is used for the assessment of various numerical damaged cases. An FSC-based algorithm is employed in order to estimate the positions and severity of damages using only the natural frequencies of healthy and unknown (damaged) structures. The study focuses on cantilever beams. By considering the minimization of FSC, damage positions and severity are obtained. Artificially damaged cases are assessed by changes in its positions, the number of damages and the size of damages along with the various parts of the cantilever beam. The study is further investigated by considering the effect of uncertainty on natural frequencies (0.1%, 0.2% and 0.3%) in damaged cases, and the algorithm is used to estimate the position and severity of the damage. The outcomes and efficiency of the proposed FSC based method are evaluated in order to locate and quantify damages. The efficiency of the algorithm is demonstrated by locating and quantifying double damages in a real cantilever steel beam using vibration measurements. Full article

The focus of this investigation is on reduced order models (ROMs) of the nonlinear geometric response of structures that are built nonintrusively, i.e., from standard outputs of commercial finite element codes. Several structures with atypical loading, boundary conditions, or geometry are considered to not only support the broad applicability of these ROMs but also to exemplify the different steps involved in determining an appropriate basis for the response. This basis is formed here as a combination of linear vibration modes and dual modes, and some of the steps involved follow prior work; others are novel aspects, all of which are covered in significant detail to minimize the expertise needed to develop these ROMs. The comparisons of the static and dynamic responses of these structures predicted by the ROMs and by the underlying finite element models demonstrate the high accuracy that can be achieved with the ROMs, even in the presence of significant nonlinearity. Full article

Modelling errors and robust stabilization/tracking problems under parameter and model uncertainties complicate the control of the flexible underactuated systems. Chattering-free sliding-mode-based input-output control law realizes robustness against the structured and unstructured uncertainties in the system dynamics and avoids the excitation of unmodeled dynamics. The main purpose of this paper was to propose a robust adaptive solution for stabilizing and tracking direct-drive (DD) flexible robot arms under parameter and model uncertainties, as well as external disturbances. A lightweight robot arm subject to external and internal dynamic effects was taken into consideration. The challenges were compensating actuator dynamics with the inverter switching effects and torque ripples, stabilizing the zero dynamics under parameter/model uncertainties and disturbances while precisely tracking the predefined reference position. The precise control of this kind of system demands an accurate system model and knowledge of all sources that excite unmodeled dynamics. For this purpose, equations of motion for a flexible robot arm were derived and formulated for the large motion via Lagrange’s method. The goals were determined to achieve high-speed, precise position control, and satisfied accuracy by compensating the unwanted torque ripple and friction that degrades performance through an adaptive robust control approach. The actuator dynamics and their effect on the torque output were investigated due to the transmitted torque to the load side. The high-performance goals, precision and robustness issues, and stability concerns were satisfied by using robust-adaptive input-output linearization-based control law combining chattering-free sliding mode control (SMC) while avoiding the excitation of unmodeled dynamics. The following highlights are covered: A 2-DOF flexible robot arm considering actuator dynamics was modelled; the theoretical implication of the chattering-free sliding mode-adaptive linearizing algorithm, which ensures robust stabilization and precise tracking control, was designed based on the full system model including actuator dynamics with computer simulations. Stability analysis of the zero dynamics originated from the Lyapunov theorem was performed. The conceptual design necessity of nonlinear observers for the estimation of immeasurable variables and parameters required for the control algorithms was emphasized. Full article