Carbon fiber reinforced polymer composites (CFRPs) have been shown to exhibit superior mechanical properties over metallic alloys at a significantly lower density. For these reasons, CFRPs have been extensively integrated into aerospace, automotive, and energy generation sectors. The interphase region between the carbon monofilament and epoxy matrix is immensely vital to researchers. It governs carbon monofilament stress transfer and bulk material properties, including fiber and matrix dominated properties.

To the best of the authors’ knowledge, no previous research has been conducted on the interphase region at subsurface and at-depth levels on hygrothermal aged CFRPs. The samples examined in this study were manufactured by layering stitch-bonded carbon fiber and epoxy resin using a hand layup technique. The samples were aged at 60 C and 90% humidity for one and two years of exposure. Moisture absorptions were measured periodically in two years. Different portions of the CFRP samples were cut and polished up to 0.1-micron grit size. This experimentation employed an advanced Atomic Force Microscopy (AFM) technique to investigate the mechanical properties of the interphase in the aged CFRP samples at subsurface and at-depth levels.

Peak Force Quantitative Nano-Mechanical Mapping (PFQNM) was used to characterize carbon monofilament-epoxy resin interphase in two-phase samples. PFQNM is a highly advanced AFM technique that allows non-destructive and simultaneous capture of imaging and mechanical property data with nanometer resolution. Probes with a nominal tip radius of 5 nm to 8 nm were used. This experiment captured reproducible nanoscale morphological, viscoelastic, and elastic properties of CFRP samples. The interphase was shown to increase in thickness with increased hygrothermal exposure time at subsurface and at-depth levels. The interphase surrounding carbon monofilaments exhibited non-uniform thickness, ranging from \~85 to 95 nm in the subsurface level, and from \~10 to 75 nm on the at-depth level. This depth-dependence interphase is likely due to the variance in the crystallinity of the polymer and the variance of hygrothermal exposure at the different depths of the sample. Aged samples showed a decrease in average surface roughness, likely due to swelling of the epoxy matrix caused by the moisture absorption. The average length of carbon monofilament-epoxy debonding increased substantially in the aged samples with a little deviation between one- and two-year aged samples. Moisture absorption was also monitored throughout the accelerated aging process. The water diffusion of these samples generally followed Fickian pattern, except for a few sharp increases in absorption, providing further evidence of carbon monofilament-epoxy delamination.


Subsurface and At-Depth Interphase Characterization in Hygrothermal Aged Carbon Fiber Reinforced Polymer Matrix Composite

Division / Sponsors: 
Aerospace, Bioengineering, Applied Mechanics, Design Engineering Division, Advanced Energy Systems, Manufacturing Engineering Division, Noise Control and Acoustics, Nondestructive Evaluation, Diagnosis, and Prognosis (NDPD), Nuclear Engineering Division, Process Industries Division, Fluids Engineering, Heat Transfer, Materials, Microelectromechanical Systems, Safety Engineering and Risk Analysis, Computers & Information in Engineering


Paper Proceedings: https://asmedigitalcollection.asme.org/IMECE/IMECE2020/volume/84478

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