3-D ultrasonic imaging of bolt thread cracks using a linear array probe

The bolt is a widely-used standard component, and the cracking of its thread often leads to final fractures. Traditional 1-D (one-dimensional) A-scan detection and 2-D (two-dimensional) imaging methods only display the position and depth of bolt cracks, without providing information about the crack's circumferential characteristics. While 1-D array linear scanning and 2-D array can achieve 3-D (three-dimensional) imaging in simple planar components, they have limitations when it comes to bolts. The former has a narrow scanning area, and the latter requires a long post-processing time. To overcome these limitations, the three-dimensional reconstruction algorithm (3 -DR) and the half-matrix sparse total focus method (HMS-TFM) are proposed. Our approach involves adopting 360 circumferential scanning based on a 1-D linear array to achieve 3-D imaging of bolt cracks. At each scanning position, the 2-D image data are captured and are then fused to obtain the 3-D image. Different from the simple stacking of 2-D image data in linear scanning, our 3 -DR algorithm reconstructs 3-D images based on discrete sampled 2-D images. To reduce the time required for 2-D imaging, the proposed HMS-TFM reduces the required data volume while maintaining acceptable image quality. Experiment results demonstrate that circumferential scanning based on the linear array can achieve 3-D imaging of bolt defects with lower time cost compared to using a matrix array for 3-D imaging. We obtained accurate circumferential quantification for radial-direction cracks with depths of 1 mm, 1.5 mm, and 2 mm with a relative error better than 6.23 %.