Attenuating materials and TGC function

Carbon-fibres like other composite materials are quickly replacing many metallic-based materials in industry due to their mechanical properties, strength-to-weight ratio, and resistance to fracture. These materials behaviour greatly differs from that of conventional isotropic structural materials due to interfacial characteristics of plies and matrix material, inclusive the presence of porosity inside. These complex material properties can cause interferences by structural noise and/or attenuation on ultrasonic signals, which in both cases, reduces the image quality.

On the other hand, a frequency of 5 MHz is commonly used for inspections of monolithic solid laminates, providing a good enough resolution. But in the case of carbon-fibre which has attenuating properties, for higher frequencies, the penetration of the ultrasound signal is not as good as lower frequencies and detectability becomes reduced with increasing depth. Therefore, in order to keep using a high frequency of inspection, the gain applied to captured signals must be modified in function of time (or distance) to receive similar echo amplitudes from different distances.

This post shows how to improve ultrasonic imaging quality in inspection of attenuating materials, such as carbon-fibre, using the Time-Gain Compensation (TGC) function which is used to counteract absorption effects.

As an example, a piece of carbon-fibre is inspected by performing a linear scan with a 128-elements phased-array transducer of 5 MHz and a SITAU-111 Phased Array System.

Fig. 1. Experimental test arrangement.

The test block has three flat-bottom holes of 3.5 mm diameter which have depths of 12, 8 and 4 mm from the bottom side (Fig. 2). The phase array probe was located on the top side over the holes position without interfacing wedge (Fig. 1) .

Fig. 2. Carbon-fibre specimen scheme.

The control of the system and data visualization were carried out on MATLAB. The acquisition range was configured to visualize the total test material depth. The linear scan (B-scan) was performed with a sub-aperture of 32 elements without deflection which is displaced to the adjacent element after each trigger over the probe. 

When the B-scan is taken using a constant gain value (25 dB), echoes coming from deeper distances are extremely attenuated due to the material absorption features which is frequency dependent. Thus for increasing depths, resolution and detectability are reduced. Furthermore, a subsequent gain increase can make that echo signals coming from a close region to the transducer become saturated.

Fig. 3. B-scan using a constant gain value.

Now, a simple curve of 4 gain points increasing with depth is configured and enabled into the TGC function. Note that all holes indications are viewed on the ultrasonic image. This functionality allows keeping a higher inspection frequency with good enough resolution. Moreover, the gain curve can be modified with a great flexibility according to material attenuation features.

Fig. 4. Acquired B-scan with a TGC function.

However, it is important to take into account that the TGC function can be applied down to reasonable depths, depending on the frequency. At some point, the signal-to-noise ratio (SNR) is so low that applying any TGC only gives noise amplification. Under this situations, others methods should be applied, i.e. emission of coded pulses, etc.