The ability to test the tensile strength and other mechanical properties of single fibers is of great importance for researchers, engineers, and manufacturers. Single fiber testing can be used to isolate trends and modes of failure for materials, such as for fundamental materials investigations or product-line quality control testing. During development of new fiber materials, the new fibers are often available in very small quantities due to complex processing or exotic materials. In these cases, multi-filament yarn-level testing is not practical. Instead, it is desired to generate mechanical properties of individual filaments.
Developments in capacitive load cells, electromagnetic load cells, and stepper motor technology have led to the commercialization of compact load frames with extremely sensitive force (for example, nanoNewton (nN) sensitivity) and displacement (for example, nanometer (nm) sensitivity) measurement capabilities. These miniature load frames are ideally suited for testing single fibers.
Standard testing procedures for single fibers[1] call for directly gripping single fibers in clamping action grips, or gluing the fibers onto cardboard tabs that are then gripped in clamping action grips. The first technique, direct clamping, concentrates stresses at the grip point and often leads to failure at the fiber grips[2]. Grip failure is undesirable during fiber characterization, and these results are generally not accurate, and, thus, are not reportable. The second technique, bonding fibers onto cardboard tabs, relies on strong bonding between the fiber and a dollop of adhesive. For fibers that are difficult to bond to, such as aramid, polyethylene, or ultrahigh molecular weight polyethylene, it is common to observe fiber slippage from the adhesive. These results are also not accurate and, thus, are not reportable, and generally prevent loading the fiber to failure. In addition, the time and complication associated with utilizing curable adhesives adds considerable inconvenience, especially considering that many test repetitions are required to establish statistically significant data. Accordingly, there is a need for a method, a device and/or an assembly for clamping or otherwise securing a fiber in a tensile testing device.
For testing yarns, it is common to use capstan grips (also called roller grips) or contour/horn grips (also called half-capstan grips)[2, 3]. In these designs, from the primary loading gage section of the yarn the yarn is passed over a series of “snubbing” surfaces which gradually reduce the tensile load on the yarn due to frictional interactions of the yarn with the snubbing surfaces. The snubbing surfaces are curved gently to induce normal forces on the yarn without creating stress concentrations that can fail the yarn. After passing over these snubbing surfaces, the yarn is gripped mechanically, typically using mechanical or pneumatic clamping action. Because the tensile loading on the yarn has been gradually reduced, the total stress on the yarn at the grip point is significantly less likely to lead to failure at the grip.
The present invention improves upon previous single fiber techniques by providing a single fiber clamping device that can reliably grip single fibers and decrease slippage and/or failure at the grips. In addition, no adhesive is required, so that gripping is fast and uncomplicated. Furthermore, the device uses magnetic clamping action to grip the fibers, so that the total mass and volume of the clamping device are minimized. Maintaining low clamping device mass is desirable for operation in sensitive load frames, since the total load limits (including clamp weight) are typically very low for these miniature load frames (for example, 1 N maximum force capacity). For these applications, traditional clamping approaches such as mechanical action or pneumatics are undesirable because they are either impractical or cumbersome to implement.