Force measurement, defined as the measurement of tensile or compressive loads acting upon an object, is an indispensable method of accessing and ensuring the quality of materials, components and assemblies. Force measurement equipment typically orients a sample axially, to ensure consistency and accuracy. however, a number of further considerations should be made to ensure as accurate a measurement as possible.
Surprisingly, the accuracy of instrument itself- though important, is only one piece of the measurement accuracy puzzle. Many factors contribute to the final accuracy, including proper sample alignment, testing machine crosshead speed, data sampling rate, and other, but one of the most critical factors is grip selection.
A wide range of grips are available to accommodate different sample dimensions. Although many grips are designed specifically for a particular application, sample alignment is not guaranteed. Sample alignment’s role in ensuring accuracy is regularly underestimated. Although it is conventionally supposed that force interactions of a uni axial test are confined to a single linear dimension, these forces actually exit in three dimensions, affording exponentially greater opportunity for misalignment and non-repeatability.
Assuming a force measuring instrument was properly calibrated, it is fair to further assume that its response curve was establised by a series of loads with near perfect alignment. It the instrument’s sensor is then subjected to misaligned loads during testing, the entire basis for measurement tractability is no longer applicable. For valid measurement tractability, the alignment of the measured force, either respect to the sensor, must be the same during testing as it was during calibration.
Proper load alignment during force testing is almost entirely contingent on the grips and fixtures, determining which configuration is best suited for a given application requires much more than just deciding if they are large enough or strong enough.
The goods news is that there are some simple, general rules that underpin the art of grip selection. Putting these rules to work will dramatically improve sample alignment and measurement reliability. Below are a few of the easiest and most effective tips.
SELECT THE SMALLEST APPLICABLE GRIP WIDTH
Often, there will be a basic grip design that is available in several different grip widths. The most common example is grips for peel testing. Since peel strength results are calculated in terms of force per unit width (such as lbF/in or N/cm), films, seals and adhesives need to be tested in a variety of specified widths. Therefore, it is common for a particular peel testing grip design to be available in several widths.
In case like this, it is best to choose the smallest grip width that will still accommodate the sample. If several sample widths require testing, it is advised to use several grips, instead of one larger grip to accommodate all sample widths.
While the widthwise edges of the grip should always extend beyond the sample edge, having a grip width that is only slightly larger than its sample improves the operator’s ability to visually inspect sample alignment. If the edges of a sample run immediately adjacent of those of the grip, it will make sample misalignment more obvious.
ADDING A SWIVEL JOINT FOR TENSILE MEASUREMENTS
The simple swivel joint accessory has one of the largest performance to price ratios in all of force testing-and is also one of the most under utilized.
If a sample exhibits asymmetric elongation when subjected to a tensile force, it will apply an eccentric and/or torsional load to the force sensor when tested. Eccentric and torsional loads can induce extreme measurement errors-to many force sensors.
Inserting a swivel joint into the load chain does not directly address the cause of the off-axis loading but can mitigate some of its negative impacts. This is achieved by better centering the force vector and preventing a force sensor from being rigidly coupled to a bending or twisting load.