Void and crack detection of polymethacrylimide foams based on terahertz time-domain spectroscopic imaging

Terahertz reflection imaging is considered as a potential diagnostic tool for the investigation of polymethacrylimide foam defects. Using terahertz time-domain spectroscopy (TDS) and detection methods based on terahertz spectroscopic analysis, the reflection imaging results of different thicknesses of polymethacrylimide foam with two kinds of detections (cracks and voids) are studied. The samples (Degussa Rohacell WF71) are planar slabs of polymethacrylimide foams with thicknesses of 35 mm, 60.5 mm and 10 mm. It is found that the same kinds of polymethacrylimide foam defects with different foam thicknesses have similar spectral characteristics, with marked differences only in the amplitude and phase of the reflected wave. In view of this, we focus our study on the defective spectral characteristics of one of the foams (35 mm thickness). The characteristics of void and crack defects are analyzed in the time domain, which is based mainly on the variation of the reflected waveform. In particular, the imaging and clear identification of voids of less than 2.4 mm in diameter, fine cracks (0.3 mm wide), and the quantification of defects can be readily achieved using the terahertz non-destructive testing technique described here.