Tensile Properties of Carbon Fiber: Single Filament Vs Tow Based Testing

Abstract

In the work presented here, single filament PAN carbon fibers (~7 micron diameter) are tested using a highly precise MTS Nano UTM tensile test machine. Current practice of ISO or ASTM methods involve testing tensile properties of carbon fibers using an infused tow of fibers. It is a current norm that carbon fiber companies report tensile strength and modulus (normal, intermediate, and high) based on measurements using full tow of fiber impregnated with some resin type, usually an epoxy system. For example, ASTM D4018 standard allows for the fiber tow to be collimated, impregnated with resin, and tensioned while keeping a fiber volume fraction in the range of 35% to 60%. This process can be completed two different ways: manually or automated. The manual method allows for the strand of a certain length and weight to be saturated in a resin bath and pulled through a die and attached to a tensioning rack. An automated setup for large volume applications typically involves passing the fiber tow through a resin bath, through multiple rollers (to help align the fiber) and wound on a tensioned spindle. After the strands are cured, they are tabbed using a standard gage length of ~150 mm. The present study considered how the mechanical properties measured using single carbon fibers translate to those obtained using a 24k tow based testing. Since tow based testing is largely dependent on morphology of the tow, interfacial shear strength between carbon fiber and resin, and the specific type of resin used, and relative tension of filaments within a tow, the motivation of this paper is to develop a fundamental understanding of translation of single carbon fiber mechanical properties to a resin reinforced composite tow. We are also developing a simple and repeatable test procedure for tow based properties without resin infusion. Weibull statistics of single filament based results are considered in the discussion.