Numerical Simulation Research on Response Characteristics of Grouting Defects of Ground Penetrating Radar for Detection of Grouting Quality behind Tunnel Wall

The effect of grouting behind tunnel wall directly affects the surrounding ground settlement and the stability of tunnel structure, so the grouting quality detection is very necessary. As an efficient and convenient shallow geophysical exploration method, ground-penetrating radar can meet the high-resolution and non-destructive requirements of grouting quality detection behind the tunnel wall, so it is widely used in engineering in recent years. Most of the existing studies have obvious regional pertinence and special geological conditions, and there are few universal studies on the characteristics of the ground penetrating radar reflection image of the grouting defect behind the tunnel wall. In view of this, this paper uses the finite difference time domain method to simulate several grouting defects behind the wall, such as voids, water-bearing anomaly, cracks, and other grouting defects. The simulation results show that the reflection image of the direct wave is characterized by a white band with strong amplitude; the interface between primary support and second lining, primary support, and surrounding rock is also banded; the circular cavity and water anomaly characteristics are all hyperbolic, the difference is that the phase of the lower part of the radar image of the cavity anomaly is 0, and there are only hyperbolic tails on both sides, and the water-bearing anomaly also has obvious hyperbolic characteristics at each interface; the reflected wave characteristics of the rectangular crack are striped and watery and the reflected wave characteristic of rectangular cracks is striped, and the abnormal range of water-bearing cracks on the radar image is larger than that of air. The research results can provide an effective theoretical reference for the engineering application of ground penetrating radar detection of grouting defects behind the tunnel wall.

Laser Re-Melt Technique for Laser Additive Repair of Narrow Rectangular Cracks in Grade 5 Titanium Alloy (Ti-6Al-4V)

Groove inaccessibility, top groove powder impedance, irregular sidewall powder delivery and lack of sidewall vertical irradiation have been reported as major limitations for the use of Laser Additive Technology (LAT) for narrow rectangular crack repair applications. As a result, most reported repair attempts were concluded unsuccessful. In the present work, a multi-track laser re-melt technique was developed for the repair of narrow rectangular cracks of sizes 2 and 3 mm, both 5 mm deep on 7 mm thick Ti-6Al-4V plates. The laser re-melt technique was carried out at controlled laser power, focal length, spot size, powder feed rate, gas flow rate and scanning speed. The repaired substrates were evaluated for defects through optical microscopy (OM) and scanning electron microscopy (SEM). The obtained results showed densely fused defect-free repaired substrates with good evolving microstructure.