Non-contact imaging of subsurface defects using a scanning laser source

The quality of additive manufacturing (AM) components must be guaranteed to ensure wide applicability, and a nondestructive inspection technology is required in this regard. Therefore, this study examined a method for detecting subsurface defects using ultrasonic waves excited by a laser. Fundamental experiments showed that wideband ultrasonic waves can be excited with a suitable signal-to-noise ratio using high-repetition laser pulses. Images of subsurface defects were appropriately obtained using a scanning laser source (SLS) with broadband waves for an aluminum alloy flat plate with artificial defects. The imaging experiments showed that the acquisition condition depends on the local defect resonant (LDR) frequency in the defective part. The imaging technique also enabled to detect subsurface circular defects created by AM with the diameter below 1.0 mm that were undetectably small in our previous study using the SLS. Based on results of these experiments and a finite element analysis, the following guideline is proposed: the LDR frequencies of targeted defects must be included in the tested frequency range.