Non-destructive evaluation measurements and fracture effects in carbonepoxy laminates containing porosity

Carbon fiber composites have been increasingly used in aerospace, military, sports, automotive and other fields due to their excellent properties, including high specific strength, high specific modulus, corrosion resistance, fatigue resistance, and low thermal expansion coefficient. Delamination, or interlaminar fracture, is a serious failure mode leading to a loss in composite stiffness and strength. Manufacturing process defects degrade the fatigue life and delamination resistance of the composite. This study investigates the effect of porosity on fatigue interlaminar fracture behavior of carbon fiber composites. Carbon fiber reinforced epoxy samples with different vacuum levels: 0%, 50% and 100% of full vacuum were fabricated using the hand layup vacuum bagging manufacturing process. Porosity was evaluated using eight different NDE methods: X-Ray laminography, X-Ray refraction, ultrasonic testing, high frequency eddy current imaging, pulse thermography, pulse phase thermography and lock-in-thermography, and thermal conductivity measurements. Then, the static interlaminar fracture behavior under Mode I was conducted, as well as Mode I in fatigue. Fractography was implemented on the samples tested by static and fatigue Mode I. Finally, the destructive technique of serial sectioning (SS) was undertaken in two orientations: through-thickness (planar sectioning), and cross-thickness (cross sectioning). The results of X-Ray Refraction and thermal conductivity measurements were the most successful NDE technique for quantifying porosity in the carbon fiber composite materials, because both methods gave specific fixed values with the porosity. The results of X-Ray laminography, pulse thermography, pulse phase thermography and ultrasonic were also successful NDE techniques for evaluating porosity shape, area fractions, size and distribution since they gave images for the porosity. Lock-in-Thermography showed the depth of porosity in the carbon fiber composite material, and eddy current showed how the carbon fibers were oriented in the composite material. Serial sectioning magnified images showed the features of internal microstructure such as porosity type, morphology, distribution and location. Results of static Mode I interlaminar fracture tests showed that porosity could lead to a decrease in interlaminar fracture toughness. In addition, the presence of porosity leads to a decrease in Mode I cyclic strain energy release rate fatigue life. Finally, all approaches conducted were correlated: the resulting NDE percentages and parameters were correlated with the features revealed by the destructive test of serial sectioning and static and fatigue values and fractographic images in order to quantify delamination and porosity