Impact testing
March 1st 2010

With the aim of standardising the sliotar, the ball used in Hurling, it was necessary to evaluate the impact characteristics of the currently approved brands.Fiachra Collins,Dublin City University performed the dynamic characterisation of high velocity ball impacts using National Instruments LabVIEW

In order for the testing to be representative of the conditions occurring in the sport, the characterisation would require impact velocities of up to 86 mph.

Impact durations in the region of 2 milliseconds necessitated high data acquisition rates for data capture and analysis.

A custom-designed pneumatic system, controlled with millisecond accuracy, was used to project the ball with precise aim and without imparting spin at velocities between 5-38m/s (11-86 mph). Data acquisition, via NI PCIe hardware, was synchronised to record data from a high speed camera running at 5000 fps and a compression loadcell integrated into the impact plate. This data was analysed to evaluate the ball's deformation and energy loss impact characteristics. Various sensors monitoring the status of the system communicate via the LabVIEW control program to allow automated batch testing.

In recent years, variability in behaviour of the sliotar had become evident in championship matches. The resulting controversy amongst players and the media had led the Gaelic Athletic Association, the governing body for the sport, to decide to adopt a single standardised core for the sliotar. In order to produce a core with consistently desirable playing performance, characterisation of the dynamic impact properties of currently approved sliotar cores was necessary.

Properties measured and characterised to this end included the coefficient of restitution (ratio of velocities before and after impact), impact force, deformation characteristics, dynamic stiffness and contact time.

System overview Controlled by LabVIEW via an NI PCIe-6251 multifunction card and SCB- 68 breakout box, the pneumatic system projects the ball vertically upwards with precise aim and zero spin to strike an impact plate. The impact characteristics are measured from data acquired from a compression load-cell via an NI PCIe-6251 and from a high speed camera via an NI PCIe-1429 frame-grabber card. Due to the short exposure times associated with high speed footage, the impact area is illuminated by 400W of halogen light, with a cooling fan installed beside the impact plate to reduce the temperature rise resulting from the bulbs. After impact, the ball rebounds within an enclosed frame and rolls into the water conditioning unit. This allows the testing of balls under wet conditions to investigate the effect of rain on the ball performance. The ball finally rolls into a feeder channel that can accommodate up to 14 balls that can be tested continuously in a cycle.

The entire system is controlled by a userfriendly interface, involving the synchronized use of nine LabVIEW Virtual Instrument (VI) programs. The top-level VI allows the input of the ball identification names and the desired projection velocity, executes the subVIs in the correct sequence and displays the impact characteristics following analysis of the acquired data.

High-speed footage of the impact is acquired from a Mikrotron MC1302 highspeed camera via a base CameraLink connection to the NI PCIe-1429. The framerate, resolution and exposure settings of the camera are specified by serial commands within LabVIEW.

In the interval of time between impact and the ball re-entering the feeder channel, the data acquired at impact is analysed, displayed for the user, and saved in spreadsheet format. The top-level VI passes the path of the AVI to the image processing algorithm, which uses NI IMAQ vision software to perform a variety of functions to threshold, filter and sharpen the image.

The force is plotted against the deformation to produce a hysteresis curve, with the slope corresponding to the dynamic stiffness and the area enclosed within the hysteresis loop corresponding to the dynamic energy loss.

Results and discussion In the calibration of the system, it was shown to have excellent repeatability with accuracies of ± 0.14m/s, ± 0.58mm and ± 0.7N for velocity, deformation and force measurements respectively. From analysis of the data, it is evident that there is a variation in performance between different brands and even within the same brands of sliotar cores.

This variation becomes more pronounced at higher velocities, as is evident from the results becoming increasingly scattered with increasing impact velocity. This can be attributed to different material properties, nonuniform ball construction (thus, non-uniform mass distribution) and imperfect sphericity, all of which are as a result of insufficient quality control in the manufacturing process. This velocitydependent deviation in performance would not be apparent in the current regulation testing, thus validating the need for high velocity testing to comprehensively characterise a ball impact.

In the context of the sport, the coefficient of restitution is regarded as the 'liveliness' of the ball. To a sportsperson, the deformation and contact time are equivalent to the 'feel' of a ball, influencing the control, precision and feedback from the strike to the player. The force would be related to the shock felt in catching or striking the ball. These performance parameters of a ball are dependent upon the viscoelastic behaviour of the material during impact. Viscoelastic characteristics, such as the dynamic stiffness and hysteresis energy loss, are shown to have a non-linear velocity-dependent relationship to the material properties. In order to adjust the performance of the ball, it is necessary to investigate this relationship, in order to produce a ball with desirable playing performance.

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