Given the importance of continuous fiber reinforced polymer matrix composites (PMCs) for the development of light-weight, fuel efficient, tough, durable and corrosion-resistant structures, the National Institute of Standards and Technology (NIST) has maintained a research program focused on solving problems that impede the widespread use of PMCs. A critical factor controlling composite performance is the fiber-matrix (F-M) interphase region. NIST, in partnership with the composites community, is working to develop the following:

  • A standardized test that quantifies the fiber-matrix interfacial shear strength (F-M IFSS)
  • A fundamental understanding of how the F-M IFSS obtained from the single fiber fragmentation test (SFFT) correlates to the fracture of fibers in a real composite
  • How composite interphase chemistry and morphology differs from that of the bulk resin and fiber.

Acquiring this knowledge will lead to appropriate modifications to composites that will ultimately result in enhanced toughness and durability.

NIST has made significant advancements in each of these areas. Specifically, the SFFT was expanded to include multiple embedded fibers by redesigning a multi-fiber rotation device to prepare new types of multi-fiber fragmentation test (MFFT) specimen. As an example, see the type 1 MFFT specimen in Figure 1. A critical feature is the precise placement of an array of closely spaced fibers between two single fibers. With an inter-fiber spacing (IFS) of approximately one fiber diameter, d_f, this MFFT specimen enables the study of the impact of fiber-fiber interactions on composite failure. This improved device was also used to precisely embed three to five well-spaced fibers in a single specimen. Since the test time for a SFFT specimen is six to eight hours, this new specimen can reduce the effective testing time to one to two hours for each fiber and relies on the fact that fibers do not interact when IFS>8 d_f.

To handle the volume of data from these new MFFT specimen and minimize manpower requirements, an automated machine has been built that takes archival images of each fiber or fiber array. Current efforts focus on developing image analysis methods that will obtain the outputs needed to automatically extract the parameters required to predict full-scale composites performance.

Since matrix crack formation during composite use is known to reduce toughness, NIST has also studied the effects of variations in interphase chemistry on matrix crack behavior – in particular those variations that form during the manufacturing process. Sizing typically deposited on fibers can be very complex, therefore it is essential that the interphase deposition process be controlled on a laboratory scale to understand what factors are essential for enhancing composite performance.