Frequency spectrum spatially resolved acoustic spectroscopy for microstructure imaging

The microstructure of a material influences the characteristics of a component such as its strength and stiffness. A previously described laser ultrasonic technique known as spatially resolved acoustic spectroscopy (SRAS) can image surface microstructure, using the local surface acoustic wave (SAW) velocity as a contrast mechanism. The technique is robust and tolerant of acoustic aberrations. Compared to other existing methods such as electron backscatter diffraction, SRAS is completely noncontact, nondestructive (as samples do not need to be polished and sectioned), fast, and is capable of inspecting very large components. The SAW velocity, propagating in multiple directions, can be used to determine the crystallographic orientation of grains. Previously, the method used a fixed frequency laser and variable grating period (k-vector) to determine the most efficiently generated surface waves, and hence the velocity. However, SRAS can also be implemented by using a fixed grating period with a broadband laser excitation source; the velocity is determined by analyzing the measured frequency spectrum. In this paper, experimental results acquired using this ¿frequency spectrum SRAS¿ (f-SRAS) method are presented for the first time. The results are illustrated as velocity maps of material microstructure in two orthogonal directions. The two different ways of performing SRAS measurements - f-SRAS and k-SRAS - are compared, and excellent agreement is observed. Furthermore, f-SRAS is much simpler, and is potentially much more rapid than k-SRAS because it can determine the velocity at each sample point in one single shot from the laser rather than scanning the grating period.