Electromagnetic Inductive Coupling Analysis (EMICA): A New Tool for Imaging Internal Defects in Carbon Fiber Composites

Abstract

Carbon fiber laminates enjoy a wide range of applications from innovative architectural design to aerospace and the safety overwrap for pressure vessels. In the case of carbon fiber overwrapped pressure vessels (COPVs), the overwrap thickness can vary from 6 mm (∼ 0.25 inch) for thin-walled COPV up to 25 mm (∼ 1”) or more for thick walled COPV, depending on the vessel type. The failure mechanisms for carbon fiber are more complex than for metals and monitoring COPVs for defects or fatigue over their lifetime is further complicated by the thickness of the carbon fiber used. Traditional electromagnetic NDE methods, such as eddy current testing (ECT) for imaging defects in these structures has been severely limited, achieving accurate identification to about 4 mm in depth. In this paper, a new technique is introduced to address these shortcomings, Electro-Magnetic-Inductive-Coupling-Analysis, or EMICA, can be used to detect damage inside thick carbon fiber laminate pieces. EMICA is based on the interaction of the repeating three-dimensional structure of carbon fiber and low-frequency electromagnetic waves that are allowed to actively spread through the conductive bulk composite material highlighting defects such as delamination and fiber disruptions, well below the laminate surface. In this paper, EMICA is demonstrated in flat carbon fiber laminates up to ∼ 12 mm (0.5”) thick, made in-house, with known defects hidden through the thickness of the piece that cannot be detected via visual inspection. Delaminations, cuts/cracks, and the underlying ply layup structure can all be identified in the EMICA images. It is shown that three imbedded PTFE delaminations at varying depths (3 mm, 6 mm, 9 mm) are simultaneously imaged using EMICA in a ½” thick CF laminate [0°/90°] panel with an excitation frequency of 40 kHz. Furthermore, the electromagnetic focal point can be chosen within the depth of CF composites by intelligently selecting the excitation frequency for the ply layup being probed, while the traditional penetration depth equation does not hold true in these complex structures.