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A new sensor based on complementary split-ring resonators is presented to detect sub-millimeter surface cracks. The sensing mechanism is based on perturbing the electromagnetic field around an electrically small resonator, thus initiating a shift in the resonance frequency. The sensor is simple to fabricate and inexpensive as it is etched out in the ground plane of a microstrip-line using printed circuit board technology. The sensor exhibits a frequency shift of more than 240 MHz for a 100 μm crack.