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The effects of embedding a midplane carbon nanotube (CNT) sheet on the impact damage resistance, tensile strength and propensity for interlaminar shear failure in a carbon fiber reinforced polymer matrix composite is experimentally evaluated. External and internal damage in the impacted composite laminates were characterized via surface profilometry and ultrasonic C-scan, respectively. The external dent areas and dent depths were greater for impacted laminates without the CNT sheet interleave, while the planar internal damage areas were greater for the impacted laminates with the CNT sheet.Destructive evaluation of the impacted laminates showed significant delaminations and matrix cracks in the vicinity of the midplane for the laminates with the CNT sheet, while damage was more distributed through the thickness in the non-CNT sheet ones. While a single CNT sheet interleave did not significantly affect the tensile strength of the laminate, residual strength after impact and the corresponding strain to failure was higher because of the CNT sheet interleave. The propensity for interlaminar shear failure was greater for laminates without the CNT sheet than those with the CNT sheet.

This research presents a new technique using pulse echo ultrasound for sizing foreign objects within carbon fiber laminates. Carbon fiber laminates are becoming increasingly popular in a wide variety of industries for their desirable properties. It is not uncommon for manufacturing defects to occur within a carbon fiber laminates, causing waste, either in the discarding of failed parts or the overdesign of the initial part to account for these anticipated and undetected errors. One such manufacturing defect is the occurrence of a foreign object within the laminate. This defect will lead to a localized weakness within the laminate including, but not limited to, stress risers, delamination, and catastrophic failure. This paper presents a method to analyze high-resolution c-scan full waveform captured data to automatically capture the geometry of the foreign object with minimal user inputs without a-priori knowledge of the shape of the defect. This paper analyzes twelve samples, each a twelve-lamina carbon fiber laminate. Foreign objects are made from polytetrafluoroethylene (PTFE) measuring 0.05 mm (0.002 in.) thick and ranging in diameter from 12.7 mm (0.5 in) to 1.588 mm (0.0625 in), are placed within the laminates during fabrication at varying depths. The samples are analyzed with a custom high-resolution c-scan system and smoothing, and edge detection methods are applied to the collected c-scan data. Results are presented on the sizing of the foreign objects with an average error of 6% of the true area, and an average absolute difference in the estimation of the diameter of 0.1 mm (0.004 in), an improvement over recently presented ultrasonic methods by a factor of three.

Adhesive joints are widely used due to their higher strengths, lower weights, lesser expenses and ease of fabrication than other methods of joining. Hence, they are used extensively in aerospace and automobile industries. High quality bonds require the use of fixtures which are essential to ensure proper curing and to attain uniform thickness. Improper adhesive application and method of fixturing can cause irregularities in the distribution of the adhesive along the overlap which could affect the joint strength. This is especially critical for aerospace components as the replacement of parts can be costly and time consuming. This paper presents a nondestructive test (NDT) methodology to quantify the bonded joints where the adhesive does not completely cover the overlap area. Single lap adhesive joints with carbon fiber adherents were fabricated using a two-part epoxy based adhesive. The adhesive region was fabricated to have various shapes including elliptical and circular of different sizes and a joint with full coverage for comparison. Polytetrafluoroethylene (PTFE) cutouts were used to mask regions of adhesive to achieve the desired coverage pattern. After fabrication all samples were ultrasonically scanned with a 10 MHz spherically focused immersion transducer using pulse-echo ultrasonics to determine the actual as tested adhesive distribution shape. The scans were able to resolve the shape of adhesive distribution across the overlap region of 25.4 × 25.4 mm and were correlated with the actual adhesive distribution at the interface after lap shear tests. Lap shear tests were then performed on the samples fabricated and the samples were loaded to failure. It was found that the shape and the orientation of the adhesive shape relative to the loading direction had an effect on failure strength.