| Damage resistance and tolerance are major issues in composite laminates because of their poor interlaminar strength and toughness, which causes delamination that can lead to premature structural failure. Several toughening methods were developed over the years to reduce the onset delamination; those resulted in increased cost, weight and loss in-plane properties. Polymer nanofibers interleaving technique had shown to improve toughness with no loss of in-plane mechanical properties, glass transition temperature and nearly no change of weight in composite laminates. The objective of this study is to assess the impact damage resistance and tolerance of Nylon-66 nanofibers interleaved carbon/epoxy composite laminates and determine a limit of nanofiber required to enhance the impact energy threshold. In the past, basic concept of interleaving was studied with a small amount of nanofibers (0.25% weight). Focus of this research is to expand the amount of Nylon-66 nanofibers from zero to 1.15% of the ply weight per ply. An in-house electrospinning equipment was used to prepare the electrospun nanofibers. Aerospace grade AS4/3501-6 carbon/epoxy prepreg was used to fabricate the 32-ply quasi isotropic base and interleaved laminates by autoclave process. A 12% concentration Nylon-66 polymer dissolved in formic acid and chloroform mixture was used to electrospun nanofibers for 2, 6, and10 hours to produce mats of areal density 0.5, 1.5 and 2.5 g/m 2. These mats translate to 0.23, 0.69 and 1.15% increase in ply weight.;The base and interleaved laminates were impact tested as per ASTM D7136/D7136-07M at impact heights ranging from 50 to 260 mm, which translate to impact energy ranging from 2.82 to 14.67 J and impact velocity range from 1.00 to 2.26 m/s. All impact tested specimen were inspected for impact surface indentation, back surface damage, and through the thickness damage by through-transmission C Scan and pulse echo scan. The specimens were then compression tested as per ASTM D7137/D7137-07M to measure compression residual strength and failure modes. Results of the study revealed that: Nylon-66 nanofiber interleaving did enhance the threshold impact force and energy. The threshold is the minimum impact force or the energy that cause damage in the laminate. The threshold impact force increased by 8, 42, and 45% over the base laminate for 0.23, 0.69, and 1.15% interleaved laminates. The corresponding increase in threshold impact energy, respectively, was 16, 89, and 97%. These threshold values correspond to the back surface visible damage. The damage growth beyond the threshold impact energy decreased by 11, 32, and 48%, for 0.23, 0.69, and 1.15% weight of nanofiber interleaf, respectively. Specimens impacted below the threshold energy failed by edge compression and specimen at and above the threshold energy failed by middle section failure at the impacted site. Rate of residual compression strength reduction were increased by 0.3, 7, and 34% for 0.23, 0.69, and 1.15% interleaved laminate. Indeed, nylon-66 nanofiber interleaved composite laminate had higher impact damage resistance, threshold force and energies, and residual compression strength while reducing the damage growth rate. The performance improvement reached limit between 0.69 and 1.15% weight of nanofibers and value between 0.75 and 1.00% would be a good choice for structural application. |
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