1,721,044 research outputs found
Ultrasonic detection and visualization of sub-surface cracks below the wheelrail interface
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Ultrasonic assessment of wheel-rail contact evolution exposed to artificially induced wear
No full textOne of the most important effects that wear originates on the wheel-rail system is represented by the significant changes introduced in contact area features and contact pressure distribution. In fact, in most cases, even regular operation gradually modifies the profile of the contacting elements so as to transform the original Hertzian counterformal conditions into a conformal contact characterized by lower average pressures and relevant alterations in the size and shape of the contact patch.
On the basis of the aforementioned considerations, this study presents the results of a series of experimental tests carried out with an ultrasonic non-invasive technique, with the final purpose of assessing to what extent wear induces modifications in contact parameters. To this end, in a regular flanged wheel, increasing degrees of wear were artificially produced at the centre of the tread. The interface of the wheel-rail systems thus obtained was investigated with high-frequency ultrasonic waves and the observed coefficient of reflection (which is known to be related to contact conditions) was graphically processed to build contact maps from which to extract information about the size and shape of the contact area and pressure distribution.
The results allowed the monitoring of the transition from counterformal to conformal contact, and helped to establish an empirical relationship between the amount of wear and both the contact area value and average pressure. Moreover, contact pressure distribution (obtained with a dedicate calibration procedure) showed that conformal contacts lose the original 'one-peak' pressure trend and exhibit a multi-peak configuration. Thus, the ultrasonic method confirmed its capabilities to faithfully monitor contact conditions even in the case of randomly altered geometries of the contacting elements and, in the specific case, may represent an effective tool in validating the wear simulation models
Impiego di una tecnica ultrasonica per l’identificazione di anomalie nell’interfaccia di contatto ruota-rotaia
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Alterations in the plantar pressure patterns of overweight and obese schoolchildren due to backpack carriage
No full textBackground: Among other adverse consequences, childhood obesity is known to influence foot structure and functionality. Yet little information is available regarding how the physiologic foot-ground interaction is altered when a localized load is carried, as occurs in the case of schoolbags. We investigated plantar contact area and pressure modifications induced by backpack carriage under actual conditions. We hypothesized that a localized load acting on the body would further increase the already excessive plantar pressure that exists with overweight and obese status. Methods: Seventy overweight and obese schoolchildren aged 6 to 11 years underwent two 30-sec trials on a pressure platform during a regular school day, with and without a backpack. Total and subregion contact areas along with peak plantar pressures were obtained, and results were compared with those of an equal-numbered group of normalweight schoolchildren. Results: Overweight and obese children generally had larger contact areas and higher peak plantar pressures compared with their normal-weight peers. In overweight and normal-weight participants, the backpack induced a similar generalized increase in contact area and pressures. However, the largest changes were observed in the forefoot, suggesting that load action tends to modify the physiologic pressure patterns. Conclusions: Backpack carriage raises the already elevated peak plantar pressures in overweight children during upright stance and modifies the physiologic pressure patterns. Further investigations are needed to clarify the features of such phenomenon when dynamic activities are performed and to verify the existence of fatigue and overexertion on the foot as well as other possible negative long-term effects
Ultrasonic assessment of wear-induced modifications in engineering contacts
No full textWear in engineering contacts, which occurs as an unavoidable phenomenon linked to the relative motion
between the interacting bodies, may originate significant changes in contact features such as size and
shape of the nominal contact area, contact pressure distribution and real contact area (RCA). While in
some cases such alterations have little (or no) practical effects, there are situations in which the deviation
of contact parameters from the original design is a primary source of malfunctioning and unexpected
early failures; thus, the analysis of contact modifications may represent an indirect yet effective method
for estimating wear effects. On the basis of these considerations, this study proposes the application of
an ultrasonic-based technique to investigate the way wear alters contact conditions on three couples
of relevant engineering interest (wheel-rail, tire-ground and gear-rack systems). Basically this method
analyzes the reflection of high-frequency ultrasonic waves from the stressed interface, a parameter that
is known to be related to contact conditions. The size and shape of the nominal contact area are obtained
practically in real time by graphically processing the reflection data while further post-processing pro-
cedures allow the assessment of contact pressure distribution. A series of experiments was performed
on artificially altered surfaces (to simulate both random and controlled wear patterns) by scanning the
contact interface with 10 and 15 MHz frequency focussed transducers and the results, mainly expressed
in terms of ‘contact maps’ allow us to state that the ultrasonic method may represent a very effective tool in investigating wear-induced modifications of contact conditions
Effect of fatigue on postural sway in sport-specific positions of young rhythmic gymnasts
No full textBackground In rhythmic gymnastics, balance represents a key element in achieving optimal performance. However, there is a lack of data on the modalities with which postural control is impaired by fatigue originated by actual competitive performance.
Aims Thisstudyaimstoanalyzetheeffectoffatigueconsequenttoacompetitiveperformanceonstaticbalanceofrhythmic gymnasts (quantitatively assessed by means of postural sway analysis) for several sport-specific positions.
Methods Fifteen rhythmic gymnasts (mean age 12.7 years) who compete at national and regional levels participated in the study. Their balance was assessed before and after a 90 s performance for 4 unipedal positions (i.e., “ring with hand support”, “back split without hand support”, “arabesque”, and “split with hand support”) using a pressure platform. Center-of-pressure (COP) time series acquired for 10 s was processed to calculate sway area, COP path length, and COP displacements in the antero-posterior and medio-lateral directions.
Results After the performance, a generalized increase in postural sway was detected for all the positions tested, with the exception of “arabesque”. Sway area doubled for most positions after the performance, while fatigue appears to have a reduced effect on COP displacements in the ML direction, which were found significantly increased only for the “split with hand support” position.
Conclusions Thephysicaleffortassociatedwiththeperformanceinfluencestheeffectivenessoftheposturalcontrolsystem. Coaches could include specific balance exercises even when gymnasts are fatigued to improve their adaptations to competi- tive conditions and reduce the risk of injuries
Ultrasonic measurements of contact area and pressure distribution of a pneumatic tire on a rigid surface
No full textContact phenomena which occur at the tire-ground interface play a crucial role in most issues related to optimal performances of the vehicle, safety, comfort, and energy consumption. Thus, it is essential to have available experimental tools capable of supplying detailed information about the main contact parameters (size and shape of nominal contact area and contact pressure distribution), especially when unknown or unpredictable external conditions make it difficult to use numerical tools in assessing them. Although a number of laboratory techniques have been devised to address this problem, here we propose a novel approach that exploits the property of ultrasonic waves to be differently reflected by a contact interface depending on its stress state. This noninvasive method is capable of supplying in real-time detailed maps of contact conditions as well as quantitative information with regard to geometric features of the contact area and contact pressure distribution values after suitable postprocessing procedures.
This study reports the results of the application of the ultrasonic method in the case of contact of a motor-bicycle tire on a rigid surface. A number of tests were carried out under different conditions with regard to inflation pressure and applied load. In each case, the raw reflection data were converted into graphic maps that display the contact area features and contain information about contact pressure. Moreover, to assess the quantitative reliability of the technique, ultrasonic data were compared with those obtained by means of a commercial pressure-sensitive film. The results are discussed to evaluate the capability of the ultrasonic method to correctly capture contact patch features
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