Laminated Carbon Fiber Reinforced Polymer (CFRP) composites are very effective in weight saving in aeronautical structural components. For these laminated CFRP structures, it is difficult to detect damage such as delamination, matrix cracks, and local fiber breakages caused by low-velocity impact loading because these damages are difficult to detect from the outside of the structure. This difficulty of inspection of the laminated CFRP structures demands the development of automatic monitoring or damage detection systems.

Figure 1. The specimen of material used in the test is CFRP Prepreg. As the thickness of the specimen is very small (0.5 mm), a GFRP plate of 3-mm thickness is bonded to the specimen.
For the CFRP composites, carbon fibers are adopted as reinforcements, and the carbon fibers are excellent electric conductors. The carbon fiber has been used as a strain sensor for decades. Recently, electrical resistance change measurement has been employed to detect and/or monitor internal damage to CFRP laminates. Since the method adopts the carbon fiber itself as a sensor, it does not cause a reduction in strength, and can be applied to existing CFRP structures. Further, measurement requires no additional research to fabricate composite structures, as it does not require embedded sensors.

An actual impact loading test was performed, and the Time Domain Reflectometry (TDR) method was applied to detect the impact damage. The TDR method uses a pulse signal in a transmission line. The reflected pulse signal from the transmission line is measured and the result is observed in a figure in which the abscissa is time and the ordinate is the voltage. The TDR method requires a wave generator, an oscilloscope, and a target cable. The wave generator produces a pulse wave signal, which is sent in the directional coupler. The signal propagates only into the target cable because of the directional coupler. Part of the signal is reflected at the input end of the cable because of the slight difference of the characteristic impedance. The other signal propagates in the target cable. The signal input in the target cable is divided into the reflection and transmission at the damaged point. The reflected signal returns back and is measured at the oscilloscope. The time difference between the input signal and reflected signal indicates the distance to the damaged point after multiplication by the speed. Using the TDR method, the damage and its location can be measured.

A narrow-copper-mesh strip is adopted as a transmission line. Usually, copper mesh is adopted as an anti-lightning method for aircraft composite structures. On the surface of a target CFRP plate, a thin GFRP plate is stacked as dielectric material, and a narrow copper strip is placed at the middle of the plate on the thin GFRP plate. The narrow copper mesh strip and the CFRP plate with the GFRP dielectric material make a transmission line with the CFRP plate.

Figure 2. The drop weight type Testing System with the impact specimen grip and impactor is shown. The diameter of the impactor is 14 mm, and the impact energy is 14.1 J.
Material used for the experiments is IM600/133 highly toughened CFRP prepreg produced by Toho Tenux Co. Ltd. The long specimen shown in Figure 1 is made from the prepreg. As the thickness of the specimen is very small (0.5mm), GFRP plate of 3-mm thickness is bonded to the specimen. Copper plating was used to make better electric contact at the edge. To make impact damage to the specimen, an impact test of drop weight type was used. The drop weight type testing system is shown in Figure 2. The diameter of the impactor is 14 mm. A wave generator, digital oscilloscope, and directional coupler were connected.

A small dent was observed on the impact surface. The reflected signal from the impact damage is observed at approximately 10 ns. The wave speed can be calculated from the time difference between the input time and the reflected signal from the end. Using the wave speed, the location of the damaged point can be calculated. The calculated damaged point is 0.58 m from the input end terminal. The actual damaged point is 0.6 m. This indicates the method gives good estimation of the impact damage of a composite plate.

Fiber breakages are observed at the dent area because of the fiber microbuckling. Although the fiber breakages have fiber contact, the sudden change of the electric conductance caused the reflection of the impulse signal. This result implies that the TDR method is useful for the detection of actual impact damage of CFRP structures.

This work was done by Akira Todoroki of the Tokyo Institute of Technology for the Air Force’s Asian Office of Aerospace Research and Development. AFRL-0222

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This article first appeared in the August, 2013 issue of Defense Tech Briefs Magazine.

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