Pulsed mode ultrasonic measurement

General ultrasonic spectrometry relies on direct measurements of the physical changes caused by the US-sample interaction, namely velocity changes and attenuation of the radiation. The different modes of this technique arise from factors such as (1) the way US is applied e.g. as a single frequency, broad-band pulses, scanning frequency), after which modes are named (2) the way US impinges on the sample (normal, parallel, oblique), after which the waves produced in the material (longitudinal, shear, oblique) are named (3) the way the experimental data provided are used viz. amplitude or phase spectra) or processed viz. frequency or time domain). 

The two major techniques in ultrasonic testing arc the pulse-echo mode and the through-transmission mode. In pulse-echo mode, the same transducer is used to emit and receive ultrasonic waves and requires access to only one side of the structure inspected (Fig. 12). The pulse-echo technique is effective on the near side skin laminate only, and the sensitivity decreases as a function of depth [29]. With the through-transmission mode, access to both sides of a structure is required as an emitter probe is placed on one side and a receiver on the other (Fig. 13). This technique measures the signal amplitude of ultrasonic waves transmitted through the material tested and is more sensitive to small defects than the pulse-echo mode. The detection of flaws throughout the whole depth of a... 

A wide-field pulse scanning acoustic microscope (WFPAM) was used in the reflection mode at the driving frequencies of / = 25 — 50 — 100 MHz to measure the local values of ultrasonic velocities and elastic moduli (the microacoustic technique) and to visualize the bulk microstructure of a specimen (scanning acoustic microscopy). The method makes it possible to measure the elastic characteristics of small specimens and inclusions [26,27]. 

---