Fundamental Study on Nonlinear Ultrasonic Imaging Method for Closed Cracks Using Subtraction of Responses at Different External Loads

To improve the selectivity of closed cracks with re spect to linear scatterers or artifacts, we propose an extension of a novel imaging method, namely, SPACE (subharmonic phased array for crack evaluation) as well as another approach using subtraction of responses at different externa l loads. By applying external static or dynamic load s to closed cracks, the contact state in the cracks varies, res ulting in the change in the responses at the cracks . In contrast, the linear scatterers other than cracks are independent of the external loads. Therefore, only the cracks can be extracted by subtraction of responses at different loads. In thi s study, we performed its fundamental experiments i n a closed fatigue crack formed in an aluminum alloy A7075 specimen. Here we examined static-load dependence of SPACE images and dynamic-load dependence of linear phased array (PA) images by simulating the external loads b y a servohydraulic fatigue testing machine. By subtracti ng the images at different external loads, we show that this method is useful in extracting only the variance in the responses related to closed cracks, with cance ling the responses of other than cracks.