A technique has been developed, using optical microscopy, for the characterization of localized fiber waviness in composite laminates. Since most of the process-induced waviness which develops in unidirectional thermoplastic laminates is clearly discretized into little packets or fiber-wrinkled regions, the spatial distribution of fiber waviness can be estimated by surface inspection of the laminates. The waviness in these fiber-wrinkled regions is approximately sinusoidal so the waviness parameters chosen were amplitude and wavelength. The waviness in each of the fiber-wrinkled regions is approximately coherent; thus, one measurement of the local fiber geometry is enough to characterize the wave packet. Another issue this technique attempts to address is the thru-thickness waviness. To investigate the presence of waviness through the thickness a two pronged approach is presented: 1) a general survey in which an entire laminate was carefully sectioned into small pieces and inspected in cross-section for the existence of fiber waviness below the part surface and 2) a rigorous three-dimensional serial reconstruction of a "typical" fiber-wrinkled region to illustrate the nature of the fiber waviness in these zones. The laminates were surveyed using a statistical sampling routine and the fiber-wrinkled regions were carefully measured using microscopy and image analysis both on the surface and through the thickness of the plates. Results from a series of plates are included to demonstrate the application of this technique.
A detailed survey was conducted of the localized fiber waviness which develops in unidirectional thermoplastic laminates (T300/P1700) in order to determine how part length affects the distribution of fiber waviness. Eleven laminates of varying length were manufactured using identical processing histories. Each plate exhibited discrete wrinkle regions concentrated in the lengthwise center of the part. The amplitude, wavelength, and distribution of these regions were characterized for each plate processed, while penetration depths were studied for sample plates of each length. The results indicate that the waviness severity increases slightly with increasing part length. Furthermore, the major component is in-plane and the waviness frequently penetrates several plys below the surface.
A nanocarbon-infused aluminum-matrix composite, termed "covetic," has been developed by Third Millennium Metals, LLC, and we have evaluated the enhanced performance prospects for strength and electrical conductivity. This paper examines the effects of the nanoscale carbon on the physical, electrical and mechanical properties of the metal-matrix composite based on microscopy, hardness, quasi-static tensile strength, high strain-rate compression strength and electrical conductivity measurements. In the as-extruded condition (warm worked at 400°F) the results show that the nanocarbon provides approximately a 30% improvement in yield strength compared to baseline 6061-T0. High strain rate, Split Hopkinson Pressure Bar (SHPB) tests revealed an opposite trend-the as-extruded covetic exhibited lower stresses at equivalent strains. In the T6 condition, the strength and ductility of 6061 with and without nanocarbon are approximately equal at all strain rates. The nanoscale carbon increased the electrical conductivity of 6061 by 43% in the as-extruded condition, but by only about 1% in the T6 condition. Electron microscopy showed that the covetic 6061 was more resistant to grain growth and coarsening during extrusion. The carbon/aluminum composite displays potential as an improved strength aluminum alloy with much higher electrical conductivity than is typical for other aluminum alloys and aluminum matrix composites.
Fiber Bragg grating (FBG) temperature sensors are embedded in composites to detect localized temperature gradients resulting from high energy infrared laser radiation. The goal is to detect the presence of radiation on a composite structure as rapidly as possible and to identify its location, much the same way human skin senses heat. A secondary goal is to determine how a network of sensors can be optimized to detect thermal damage in laser-irradiated composite materials or structures. Initial tests are conducted on polymer matrix composites reinforced with either carbon or glass fiber with a single optical fiber embedded into each specimen. As many as three sensors in each optical fiber measure the temporal and spatial thermal response of the composite to high energy radiation incident on the surface. Additional tests use a 2 × 2 × 3 array of 12 sensors embedded in a carbon fiber/epoxy composite to simultaneously measure temperature variations at locations on the composite surface and through the thickness. Results indicate that FBGs can be used to rapidly detect temperature gradients in a composite and their location, even for a direct strike of laser radiation on a sensor, when high temperatures can cause a non-uniform thermal response and FBG decay.
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