A novel fiber length measurement technique for discontinuous fiber‐reinforced composites: A comparative study with existing methods

Abstract

The residual fiber length in a molded part is one of the most important microstructural properties of discontinuous fiber‐reinforced composites. While there have been several research studies characterizing the process‐induced fiber length reduction, the measurement procedures vary substantially, calling into question the comparability of reported results. This article introduces a newly developed measurement procedure that aims to provide accurate, repeatable, robust, and time efficient fiber length analyses. A comprehensive study of measurement techniques was performed comparing commercially available systems and the conventional approach of measuring the fiber length manually. The results emphasize the need for a standardized procedure to characterize the fiber length distribution and the risk of generating inadequate results through improper sample preparation. The developed measurement technique was tested and compared for an experimental study of fiber breakage in injection molding. For a simple plaque geometry, the residual fiber length along the flow path was obtained for a long glass fiber‐reinforced polypropylene at 30 and 40%wt for varying process conditions. The new measurement technique showed accurate and repeatable results. The results of the injection molding study showed that screw speed and back pressure are important factors that drive fiber breakage. An increase in back pressure from 13 to 50 bar and screw speed from 27 to 35 rpm reduces the weight‐average fiber length by 37.5%. POLYM. COMPOS., 39:4058–4070, 2018. © 2017 Society of Plastics Engineers

Measuring Fiber Length in the Core and Shell Regions of Injection Molded Long Fiber-Reinforced Thermoplastic Plaques

“…Measuring the fiber length for discontinuous fiber composites is a time-consuming task since even small samples contain millions of fibers [26]. The Polymer Engineering Center, UW-Madison, has developed a fiber length measurement technique adapting features from various measurement methods, aiming to reduce the manual input [26]. The main steps of the technique are depicted in Figure 2.…”

“…Various studies have reported FLD measurements via µCT (Table 1); however, as high resolution is needed to differentiate individual fibers (four voxels per fiber diameter [25]), the size of the evaluated volume is limited to a few millimeters. However, parts molded with LFTs can still have fibers in the 10-15 mm range [14,19,26], far longer than what can be capture with µCT. [33] This work aims to determine FLD for the core and shell regions independently, by expanding on a currently used fiber length measurement technique [26].…”

“…However, parts molded with LFTs can still have fibers in the 10-15 mm range [14,19,26], far longer than what can be capture with µCT. [33] This work aims to determine FLD for the core and shell regions independently, by expanding on a currently used fiber length measurement technique [26]. Mechanical design software can benefit from having through-thickness measurements of FLD, as this additional information means having a more accurate representation of the material.…”

Shredding and sieving thermoplastic composite scrap: Method development and analyses of the fibre length distributions

“…Methods are considered direct when all fibres in a sample are measured, whereas indirect methods only produce an estimated FLD. Direct measurements have been extensively used for short-fibre composites [18]. A typical example is injection moulding, where many fibre-length characterisations are carried out to determine fibre length reduction.…”

“…In a significant share of these direct measurement methods, fibres are reclaimed by burning off the matrix, for instance, after which they are spread out on a piece of paper or scanner to be captured in a photograph. Image analysis is performed to determine the exact FLD [18,19]. This methodology works well for fibres of moderate length (length/diameter (L/D) < 1000).…”