Comparison of methods for the characterization of voids in glass fiber composites

Abstract

Voids are a concern in composite materials, as they may have a negative effect on the mechanical properties of the laminates. Voids may develop especially in low cost or off-optimum process conditions. In this study, samples of glass reinforced epoxy laminates with void volume fractions in the 0.5–7% range were successfully obtained by varying the vacuum in the hand layup vacuum bagging manufacturing process. Void content was experimentally characterized using four different methods: ultrasonic scanning, epoxy burn off, serial sectioning, and X-ray computed tomography. The goal of this paper was to determine how the methods compared with respect to each other at quantifying void content. The specimens were taken from nearby locations in the same panels, so a true comparison of the methods could be obtained. The results showed, for the specific material and manufacturing conditions used, that the four different techniques can quantify voids content but with a large variation in the accuracy. X-ray computed tomography was the most successful technique to characterize voids, followed by serial sectioning. Ultrasonic scanning and epoxy burn off were not recommended techniques to characterize voids for laminates manufactured with these materials and process conditions. However, epoxy burn off was a successful technique to calculate fiber and resin weight fraction.

Detailed characterization of voids in multidirectional carbon fiber/epoxy composite laminates using X-ray micro-computed tomography

“…This may be because the latter excludes the voids connected to the specimen's outer surface. The difference between void content measured via micro-CT and that measured via other techniques was also reported in [23][24][25][26].…”

Voids in fiber-reinforced polymer composites: A review on their formation, characteristics, and effects on mechanical performance

“…With the same reasoning, the number of detected small voids increased with improving the resolution of micro-CT. However, for an NCF glass/epoxy composite, Abdelal and Donaldson 187 concluded that micro-CT could detect the high increase in void content with decrease in manufacturing vacuum, while the increase detected by “manual” tomography (explained in Void shape (morphology) section) and density determination techniques were less significant. They found the US technique incapable of identifying the voids in their case.…”

“…More recently, Straumit et al 187 used the structure tensor method to build a voxel model with the fiber orientation data extracted from micro-CT images (VoxTex software). Using the k-means clustering algorithm, they could segment the voxel models into yarns, matrix, and voids, which can be used in creating realistic models of the composite.…”

Volume imaging NDE and serial sectioning of carbon fiber composites

“…High resolution X-Ray Computed Tomography (CT-scan) visualizes the interior features within a solid object by obtaining digital images of the object geometry and the properties [1]. The CT technique is a non-destructive slicing technique depicting what the object being scanned would look like if it were sliced open along a plane.…”

“…Advanced composites can be divided into laminates and sandwiches. The laminates are layers or plies of composite materials bonded together, whereas, the sandwich is a multiple-layer composite structure consisting of a low-density core between thin laminate faces (skins) of composite materials [1].…”