The Detection Of Polyethylene Pipe Fusing Joint Defects Based On Terahertz Time Domain Spectroscopy

The polyethylene(PE)materials have become the most preferred material for the gas pipeline owing to its excellent physical performance.However,defects will inevitably occur during the process of using,transporting and storing of the PE pipe.The common testing method have some limitations in the non-destructive testing of PE pipes.Terahertz(THz)wave,electromagnetic radiation between far-infrared and millimeter wave,had attracted attention in areas of non-destructive testing due to its strong penetrability to non-polar materials.In this dissertation,a series of experimental studies were carried out based on the terahertz time-domain spectroscopy.The main content includes:(1)The THz time-domain spectra of PE materials with different degree of aging were obtained.The refractive index and absorption coefficient of samples in the terahertz band were calculated according to the optical parameter model.The changes of the terahertz time-domain data,refractive index and absorption coefficient of each samples was compared and analyzed.The results show that the aging effect do affect the time-domain data,the refractive index of the material do not change significant,but the absorption coefficient show a decrease trend with ongoing of aging time.(2)Transmission and reflection THz imaging experiments were performed on simulated pipe joint defect samples include delamination,inclusion of sand and metal ring.The results show that the THz imaging method has a promising ability to the common defects of PE pipe joints.The images obtained by the reflection THz imaging is better than that of transmission images.The thickness of each sampling point was calculated by using the difference between the time delay of the sample pulse peak and the time difference of the reference signal pulse peak.(3)Image denoising and image restoration method were performed on the terahertz imaging results of metal ring defect.The results show that the quality of images improve after the denoising and restoration method.And the effects of different denoising parameters on the denoising effect were compared.