The need for the effective nondestructive testing of composites has recently been emphasized in the aviation industry, following discovery of damage in the reinforced carbon fiber tail fin of a six-year-old Airbus A319 jetliner by United Airlines. According to the Wall Street Journal, this topic is at the center of a debate over the durability and maintainability of composite parts used on commercial jets. This debate was sparked when the tail fin and rudder broke off of American Air Lines Flight 587 less than two minutes after takeoff. The novel techniques described below might allow monitoring and periodic measurements to detect small cracks and defects much the way airlines now keep track of cracks in aluminum structures.

Engineers at ACI have made use of the principle that small geometric changes in a stressed inductor alters its inductance and, as a consequence, shifts the resonant frequency of the LC circuit. As a result, the resonant frequency of the LC circuit will change (shift) upon changes in the coil inductance caused by physical distortion of the coil according to the relationship.

This phenomenon has been used to create a novel and highly useful passive sensor for monitoring strain. The desired characteristics of the sensor are partly influenced by the materials used in its fabrication. Although the sensor is not applicable for metallic materials, it is ideal for non-conductive substrates, including such advanced materials as carbon-fiber reinforced composites.

Of course, the sensor's mechanical properties should be compatible with its composite host. Fortunately, in most circumstances, processing and embedding the ACI detector system does not affect the mechanical or thermal properties of the composite. The sensor itself is a parallel-connected LC tank circuit, which operates by a combination of inductance (L) and capacitance (C). Since the sensor is leadless and passive, it is readily embedded in composite structures, to provide a means of addressing internal composite strain remotely at any subsequent stage of cure, fabrication, and application.

Our remote, non-contact detection method uses an instrument known as a Gate or Grid Dip Meter (GDM). The GDM sweeps frequencies and measures the reflected RF power of a device under study. At the resonant frequency, the GDM exhibits a strong decrease in reflected energy. Using a GDM as a sensitive detector, the change in resonant frequency shift with stress was experimentally confirmed. Preliminary results have shown that there is a consistent relationship between relative strain with shifted resonant frequency independent of what material the sensor was embedded within. This experiment was demonstrated using wire wound inductors and bulk capacitors to form the tank circuit.

Initially, laminated wire spirals or photo-lithography were used to create the inductive sensor. More recently, polymer thick film (PTF) inks have been utilized to fabricate the inductive component of the sensor. These commercially available inks consist of micron-sized particles dispersed within a polymeric matrix. The dispersion can be screen printed onto many different types of substrates such as ceramic, printed wiring boards, polyimide film or Mylar. Screen-printing allows an infinite number of patterns to be formed.

ACI has screen-printed a variety of meander patterns along with circular, square, and rectangular spirals. Each of these inductor geometries has different inductive properties. PTF inks have printed and cured thickness of less than one mil, thus creating minimal interface stresses. In addition, PTF inks are compatible with the molding process and materials. Curing temperature of PTF inks is well below the temperature sensitivity of most composites, usually between 100-150°C. Furthermore, commercial PTF inks can be readily reformulated with a variety of different polymeric matrices for specific requirements. Upon further development, the techniques described here could be directly applied to determine initial strain failures in composites.

Paztor, A., and Michaels, D. “Damage Found in United Airbus Spurs Look at Composites’ Safety.” Wall Street Journal. December 14, 2001.