November AMP_Digital

FEATURE A D V A N C E D M A T E R I A L S & P R O C E S S E S | N O V E M B E R / D E C E M B E R 2 0 1 9 5 9 the backscattered signal when passing the interface. Back- scattered intensity decreases exponentially according to the exponential attenuation law. A huge number of scattered signals from the coherent pulse volume comprise the backscattered signal by interfer- ence. The signal has a typical “peaky” shape that jitters when the probe is moved and it is difficult to read for an accurate assessment of time of flight (Δ t in Fig. 1b). Signal smoothing is accomplished by algorithmic smoothing and local averag- ing (Fig. 2). Signal quality is key to reliable SHD evaluation. Based on the required signal shape, the operator knows whether 9 there is a wrong or poor coupling. Other crucial parameters include sound velocity and wedge angle to determine the SHD computation. These parameters are preset and part of the documentation. The measurable SHD range is limited by the width of the surface signal for small SHD and by wave attenuation for large SHD. For most parts, the SHD range is 1.2 to 40mm. For special applications, SHD can be measured from 0.5 mm on appropriately smooth surfaces using special wedge design. Control for very large SHD depends on part microstructural details requiring the appropriate choice of both frequency and the probe system itself. Fig. 1 — Scanning and signal processing for SHD control. a) Scanning b) Signal processing Fig. 2 — Signal smoothing and averaging. Algorithmic smoothing: 40; averaging: 256; gain: 58 dB. Algorithmic smoothing: 40; averaging: 1; gain: 56 dB. Algorithmic smoothing: 0; averaging: 1; gain: 52 dB. Algorithmic smoothing: 0; averaging: 256; gain: 56 dB.