Tensile tests of composite coupons with prescribed porosity

• Main Author: Tan, Shemual Pinhan
• Other Authors: Chai Gin Boay
• Format: Final Year Project
• Language: English
• Published: 2014
• Subjects: DRNTU::Engineering; DRNTU::Engineering::Aeronautical engineering::Materials of construction
• Online Access: http://hdl.handle.net/10356/60318
• Institution: Nanyang Technological University

Carbon fibre reinforced plastics (CFRP) are widely used to take the place of metals and other materials in the aerospace industry. However, porosity present within composites post a huge problem as they will cause its mechanical properties to deteriorate. Therefore, the purpose of this project is to investigate how different levels of porosity will affect the tensile properties of the carbon fibre composite. The wet lay-up method is used to fabricate the test samples for the purpose of tensile testing in this project. Unidirectional carbon fibre fabric laid in the longitudinal direction is used and different amounts of glass bubbles are added to the resin mixture to induce void content. By using glass bubbles, the author is able to control and calculate the actual volume fraction of glass bubbles in each specimen and thus its void content. The different volume fraction of glass bubbles used in this project is 0%, 5.9%, 12.3% and 18.64%. However, it was observed that the greater the amount of glass bubbles used, the more viscous the resin mixture, thus increasing the difficulty of the fabrication. Tensile tests have yield data of the test coupons such as load, extension, tensile stress and tensile strain. From these data, the ultimate tensile strength and the tensile modulus could be found. Results have shown that as glass bubble volume increased from 0% to 5.9%, the ultimate tensile strength decreases by 35.6% from 1349.6 MPa to 869 MPa, while the tensile modulus decreased by 44% from 112.2 GPa to 62.9 GPa. Further addition of glass bubbles to volume fractions of 12.3% and 18.6% did not significantly affect the tensile properties. This shows that the specimens are affected only by the introduction of porosity, but are mostly insensitive to increments in void content. This may be due to the carbon fibre volume fraction as discussed in the report. In addition, the normalized form of these tensile properties in the terms of the carbon fibre and the matrix could be obtained, as well as the specific tensile properties after taking into account the density of the specimens. The results of these findings were able to shed light on the parts affected by porosity and the showed that specific ultimate tensile strength and modulus increased with increasing glass bubble volume fraction.