Abstract
The present study evaluates the toughening capability of electrospun PA66 nanofibers for carbon/epoxy composite laminates subjected to mode II loading conditions at elevated temperatures. The Dynamic Mechanical Analysis (DMA) test showed that the glass transition temperature of the produced nanofibers is in a range of ∼60–80 °C. Accordingly, End-Notched Flexure (ENF) carbon/epoxy specimens interleaved by a 50 μm-layer of electrospun PA66 nanofibers were subjected to the quasi-static mode II loading at room temperature (∼25 °C), 100 °C, 125 °C, and 160 °C. At room temperature, the mode II interlaminar fracture toughness (GIIC) of the nano-modified specimen was ∼4 times higher than the virgin specimen (non-modified) (3.12 kJ/m2 vs 0.81 kJ/m2). The results showed that GIIC of the virgin specimen was independent of temperature. However, in the case of the nano-modified specimen, although the GIIC did not change from room temperature to 100 °C (3.12 kJ/m2 vs 3.09 kJ/m2), by further increasing temperature to 125 °C and 160 °C, GIIC dropped by 34% and 43% respectively (2.05 kJ/m2 and 1.77 kJ/m2 respectively). 3D surface scans and Scanning Electron Microscopy (SEM) images of the fracture surface revealed three reasons for decreasing the toughening capability of the PA66 nanofibers at high temperatures: a) the crack crosses the nano-layer less at high temperatures, b) the dominant damage mechanism at low temperature is “cohesive failure”, the damage propagation within the nanolayer, while at higher temperatures “adhesive failure”, the debonding of the nanolayer from carbon fibers, plays a critical role in the fracture, and c) severe plastic deformation of nanofibers at high temperatures.
Highlights
Fiber Reinforced Polymer (FRP) composites are characterized by high specific in-plane mechanical properties
For determining the Tg of Polyamide 66 (PA66) nanofibers, the Dynamic mechanical analysis (DMA) test was per formed on a small strip of electrospun PA66 nanofibers sheet in dimensions of 30 × 9 mm2
The GIIC of the nano-modified spec imen is still 2.3 times the virgin specimen at the highest temperature, the results showed that the mode II toughening capability of PA66 nanofibers significantly depends on the temperature at temperatures higher than 100 ◦C
Summary
Fiber Reinforced Polymer (FRP) composites are characterized by high specific in-plane mechanical properties. Their relatively poor out-of-plane properties, interlaminar fracture tough ness, have limited utilizing the full potential that these materials can offer. Different methods have been proposed in the literature for increasing the interlaminar fracture toughness, such as matrix modifi cation with nanoparticles [1,2,3,4], tufting [5,6,7], z-pinning [8,9,10], stitching [11,12,13], and 3D weaving [14,15]. In the case of other toughening methods, such as z-pinning and 3D weaving, the out-of-plane properties can be improved significantly, they result in the reduction of in-plane mechanical properties of the laminate [17,18]. In the case of 3D woven composites, the increase of yarn crimp reduces in-plane stiffness [20,21]
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