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The effect of manufacturing conditions on discontinuity population and fatigue fracture behavior in carbon/epoxy composites

: Hakim, I.; Laquai, R.; Walter, D.; Mueller, B.; Graja, P.; Meyendorf, N.; Donaldson, S.


Chimenti, D.E. ; American Society for Nondestructive Testing -ASNT-, Columbus/Ohio:
43rd Annual Review of Progress in Quantitative Nondestructive Evluation 2016 : Atlanta, Georgia, USA, 17-22 July 2016
Woodbury, N.Y.: AIP, 2017 (AIP Conference Proceedings 1806)
ISBN: 978-0-7354-1474-7
Art. 090017, 12 pp.
Annual Review of Progress in Quantitative Nondestructive Evaluation <43, 2016, Atlanta/Ga.>
Conference Paper
Fraunhofer IKTS ()

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. Interlaminar fracture is a serious failure mode leading to a loss in composite stiffness and strength. Discontinuities formed during manufacturing process degrade the fatigue life and interlaminar fracture resistance of the composite. In his study, three approaches were implemented and their results were correlated to quantify discontinuities effecting static and fatigue interlaminar fracture behavior of carbon fiber composites. Samples were fabricated by hand layup vacuum bagging manufacturing process under three different vacuum levels, indicated High (-686 mmHg), Moderate (-330 mmHg) and Poor (0 mmHg). Discontinuity content was quantified through-thickness by destructive and nondestructive techniques. Eight different NDE methods were conducted including imaging NDE methods: X-Ray laminography, ultrasonic, high frequency eddy current, pulse thermography, pulse phase thermography and lock-in-thermography, and averaging NDE techniques: X-Ray refraction and thermal conductivity measurements. Samples were subsequently destructively serial sectioned through-thickness into several layers. Both static and fatigue interlaminar fracture behavior under Mode I were conducted. The results of several imaging NDE methods revealed the trend in percentages of discontinuity. However, the results of averaging NDE methods showed a clear correlation since they gave specific values of discontinuity through-thickness. Serial sectioning exposed the composite's internal structure and provided a very clear idea about the type, shape, size, distribution and location of most discontinuities included. The results of mechanical testing showed that discontinuities lead to a decrease in Mode I static interlaminar fracture toughness and a decrease in Mode I cyclic strain energy release rates fatigue life. Finally, all approaches were correlated: the resulted NDE percentages and parameters were correlated with the features revealed by the destructive test of serial sectioning and static and fatigue values in order to quantify discontinuities such as delamination and voids.