Superimposed imaging of acoustic wave reflections for the detection of underground nonmetallic pipelines

Due to the non-conductive and non-magnetic properties, nonmetallic pipelines are difficult to be detected by traditional pipe location technologies. This paper presents a superimposed imaging method of acoustic wave reflections for pipe location, which operates on the propagation of elastic waves. The propagation and attenuation model of elastic waves in soil are constructed according to the geometric relationship between the acoustic source, geophone array and pipelines. The two-dimensional and three-dimensional acoustic field diagrams of underground pipeline are generated by superimposed imaging of the cross-correlation coefficients between the signals from the sound source and geophone array, with the attenuation caused by hysteretic damping and geometric dissipation considered in the imaging process. In order to suppress clutter interference on the imaging results, the strategy of ‘multi-point transmit, multi-point receive and cross-correlation coefficient superposition’ is adopted. In the simulation, comparison is made to demonstrate the influences of different excitation sources on the detection imaging results, including the single frequency signal, multi-frequency signal, Gaussian pulse signal and sweep signal. It is found that the excitation signals with rich frequency components are more conducive to improving the resolution of detection images. The effectiveness of the proposed imaging method is further verified in the experimental work, which may be beneficial for the visualization and determination of the location, depth and orientation of underground non-metallic pipelines

Structures and formation mechanism of borides with varied morphologies in cast γ-TiAl alloys

Borides with varied morphologies and structures obtained by changing B content in as-cast Ti-45Al-2Mn-2Nb alloys are investigated. With the increase of B content, morphologies of borides change from curved to straight, and flake shape, ribbon shape, needle/rod shape, and blocky appear in turn. Curved boride particles have a lamellar structure composed of TiB2 and B2 layers, accompanied by the random appearance and distribution of Ti3B4 and Bf-TiB layers. Except for the absence of the B2 layer, straight needle-like and blocky boride particles have roughly the same structure as curved boride particles, revealing that the formation of curved boride particles is accompanied by divorced eutectic with the matrix and internal irregular eutectic, while no eutectics occurs in straight boride particles. The lack of B atoms during solidification in alloys with lower B content results in a less regular structure and higher stacking fault density of curved boride particles compared to straight boride particles in alloys with higher B content. In addition, some needle-like boride particles are observed to have pure B27-TiB structure without intergrowth. The difference in morphologies and structures can be attributed to the difference in formation mechanism and crystallographic structures of the borides