Improvements and effects of porosity on interlaminar tensile strength of curved beam carbon fiber composites

The orthotropic nature of composites provides excellent performance in the fiber direction, but it is susceptible to failure in the orthogonal direction. Failure due to the delamination is limited to composites, and has an adverse consequence on its mechanical properties. The interlaminar tensile (ILT) strength of a composite part is compromised by delamination failure caused mainly by the applied loads and environmental factors. This phenomenon creates a challenge in design and manufacturing of composite parts for the aerospace industry. This study is focused on improving the ILT strength of a curved composite laminate by suppressing failure modes associated with free edge effects and to evaluate the ILT strength of a curved section with various porosities. Composite panels were built on an angle bend tool using the fiber reinforced carbon/epoxy tape pre-pregs. The manufactured panels were machined using a water jet to produce test coupons as required by ASTM 6415. The curved sections of the test coupons were reinforced with metal clamps manufactured from aluminum 7075-O alloy and another pre-preg material (glass fiber reinforced pre-preg). The observed ILT strength of the reinforced coupons was found to be much higher than the baseline because the reinforcement reduces the effects of the stresses induced by the free edge of the composite specimen. There was a 25% increase in the interlaminar tensile strength for the coupons reinforced by the metal clamps and a 21% increase in strength for the coupons reinforced with the glass pre-preg material. We also observed that with various percentages of porosity, the ILT strength varied significantly, which needs to be addressed in the future studies. The failure of the coupons occurred due to the delaminations caused by the interlaminar stresses under the applied loads. This study may open up new possibilities to reinforce the various fiber reinforced composites used in many manufacturing industries in the field.

Thesis (M.S.)--Wichita State University, College of Engineering, Dept. of Mechanical Engineering