Capstan grips for yarn testing - A432

 

Capstan grips for yarn testing
Foundational knowledge article
Document Type	Article
Document Identifier	A432
Prerequisites	Materials science Material properties Reinforcement properties Foundational method documents Tensile Testing

"A" can not be assigned to a declared number type with value 432.

Introduction

Capstan grips are essential for yarn tensile testing because they prevent premature breakage at the grip points by distributing tension via friction along multiple wraps, rather than crushing the specimen like tabbed or clamped methods. This is critical for fine, delicate, or high-strength yarns that might slip or snap under direct clamping. Tabbed methods (where yarn is glued or clamped between tabs) work well for composites or fabrics but can introduce stress concentrations in single yarns. Capstan grips ensure failures occur in the test region, not near the grips, providing more accurate strength and elongation measurements for textile fibers.

Significance

Capstan grips enable accurate tensile characterization of reinforcing fibers (e.g., carbon, glass, or aramid tows) without introducing grip-induced defects that could compromise data integrity. Unlike conventional clamps, capstan grips prevent premature fiber breakage at clamping points, ensuring failures occur within the gauge length. While alternative grips (e.g., tabbed methods) suit laminated composites, capstan grips are preferred for testing individual fibers or tows and the information obtained may be used to inform performance predictions and perform quality control in composites.

Scope

This document describes capstan grips used in yarn tensile testing, detailing their design and function, and presenting their advantages, limitations, and applications in testing. An example is presented illustrating grip geometry, wrap configuration, and failure of specimen with a final mention of alternative grips available.

Design and Function

A capstan grip is a friction-based gripping mechanism used in tensile testing machines to hold yarns, filaments, or fibers without causing premature breakage at the clamping point. Instead of crushing the specimen like mechanical or pneumatic grips, the capstan grip distributes the tension along a wrapped section of the yarn, reducing stress concentrations.

The yarn is wrapped multiple times around a rotating cylindrical drum (capstan) relying on the friction between the yarn and the capstan’s surface to prevent slippage as tension increases. The efficiency of the grip is affected by the number of turns and surface texture (smooth or grooved) of the drum and rely on Euler’s law of belt friction where increased tension enhances the grip force.

Advantages and limitations

Unlike direct clamping, which can crush delicate yarns, the capstan grip avoids localized stress which minimizes specimen damage; furthermore, because multiple wraps increase friction and holding force, they provide an effective means to mount fine and slippery yarns.

The capstan grip requires sufficient length for multiple wraps making it problematic for specimens where supply may be limited. Additionally, variability might arise as friction depends on the yarn-capstan interface, which is subject to coatings and texture, plus misalignment or wrapping patterns can cause uneven tension distribution.

Yarn Testing Applications

Capstan grips can be used to perform tensile strength testing to measure breaking force and elongation. Because they maintain consistent tension over time, they are useful for creep and stress relaxation tests and in fatigue testing where repeated loading could cause slippage in conventional grips.

In addition to capstan grips, other common gripping methods for yarn tensile testing include mechanical clamps (serrated or rubber-faced jaws), which apply direct pressure but risk crushing delicate fibers; pneumatic grips, which use air pressure for uniform clamping but may still cause stress concentrations; and adhesive or tabbed grips, where yarns are bonded between stiff tabs to distribute load, though this method is more suited to composites or fabrics. Additionally, wrap-and-pin grips secure yarns by winding them around pins, offering a balance between friction and minimal damage. Each method has trade-offs in precision, specimen protection, and suitability for different fiber types.

Case Study

The following example outlines the standard procedure for preparing test samples and conducting tensile tests using a universal testing machine in accordance with ASTM D2256. Due to the machine's weight, the installation of the grips requires safety belts; the faces are correctly aligned and levels to prevent introducing twisting motion.

There are a few critical points to consider when mounting the fiber to the grips. The tows must be mounted first onto the top grip carefully, avoiding any twist and their ends fixed securely where a small piece of 180 grit sandpaper. After locking the tow ends, wrap the tow onto the grips without twisting at least two times being careful to avoid overlapping and pin the capstan. Repeat the same steps for the bottom grips again, ensuring sure there are no twists or overlaps in the tow. Refer to Video 1 to see the process.

Video 1. Showing the mounting of a carbon fiber tow onto a set of capstan grips: mounting on upper grip (0:00-1:05), mounting on lower grip (1:06-1:55), adjusting length and position (1:56-3:20), and measuring the gauge length (3:21-3:27)

Once the sample is mounted on the capstan grips, the gauge length of the fiber tow must be adjusted (see Figure 1) and a small preload (between 5 to 10 N) maintained before proceeding with testing. An example of typical data output showing the breaking load and extension at break is displayed in Figure 2.

Figure 1. Testing of Resin-Coated Tows Using Capstan Grips: (1) Pre-test, (2) Gauge length, (3) Post-failure

Figure 2. Example of Typical Data Output Showing the Breaking Load (453 N) and Extension at Break (6.30mm) for a Commercial Glass-Fiber Yarn.

Capstan grips application

Capstan grips utility in evaluating the mechanical degradation of fibers under humidity-exposure and hydrothermal aging, has been demonstrated in studies by Sunny et al. In these works, carbon, basalt, and E-glass fibers were subjected to accelerated aging conditions to assess their durability and predict service life. While conventional clamps could compromise results precluding precise characterization of aging effects on fiber properties (by introducing additional damage to already degraded samples displaying surface damage or increased brittleness), capstan grips supported the validation of Arrhenius-based lifetime prediction models. These studies highlight the capstan grip’s importance in advancing the understanding of fiber-reinforced composite longevity under environmental stressors.

References

About	Help

With regards to manufacturing, risk is the combination of the probability and consequences of undesirable manufacturing outcomes. Manufacturing risk can lead to technical issues, program/schedule delays, and cost overruns.

A tow is a bundle or yarn of individual fibres. The tow size is inherent to the fibre manufacturing process (i.e. a tow is manufactured in one process, rather than each fibre individually then bundled together after).

Typically, smaller tows are better because they result in a more homogeneous material.

The larger the tow:

• The faster it is to deposit material
• The easier is it for resin to flow between tows
• Harder for resin to saturate

Typical tow sizes:

• 1k (thousand)
• 3k
• 6k
• 12k
• 24k
• 50k

With regards to modelling & simulation, validation demonstrates the accuracy of a computational model — in the context of its intended use — to check that it represents the physics of the problem and the ‘reality of interest’ (needs are satisfied).

Engineered materials (designed to have specific properties) made from two or more constituent materials with different physical or chemical properties. The constituents remain separate and distinct on a macroscopic level within the finished structure.