Nonlinear phased array imaging of flaws a modulation technique

This work proposes a nonlinear dual frequency (modulated) phased array technique for fatigue crack evaluation in aluminium and metallic structures, and near field enhancement in composite materials. A standard ultrasound system with multiple transmitting and receiving elements was used to excite an aluminium fatigue sample and impacted composite plate with two frequencies. The method relied on the evaluation of harmonic sidebands which can be correlated to defects/damage in materials, with nonlinear methods having shown increased sensitivity versus standard techniques. An initial pump frequency (f₂) is used to initialize a ‘breathing/ringing’ crack after which a second frequency (f₁) is used to further excite the cracked region and generate sidebands from the modulated frequencies. This method relies on both an amplitude and frequency subtraction techniques to filter out linear ultrasound effects. One of the benefits of modulation techniques in general are reduction of equipment based nonlinearities, commonly produced when conducting single frequency tests. An aluminium coupon with a closed fatigue crack was evaluated using single and dual frequency methods. The results showed that the dual frequency excitation method was able to more accurately define the extent of the crack and improved accuracy versus standard phased array techniques.