# Anisotropy Corrected FMC/TFM Based Phased Array Ultrasonic Imaging in an Austenitic Buttering Layer
## 1 Introduction
These include (i) the ray-tracing method [19; 20; 21], (ii) the time reversal method [13; 22], (iii) the velocity correction method or time delay correction method [22; 23], and (iv) controlling beam characteristics [24]. The first three methods are fundamentally similar, as the aim of these methods is to determine the exact time delays, ensuring 100% constructive interference in the ROI. In an anisotropic material, the actual time delay could be predicted by the ray tracing method. [PERSON] et al. [19] used the ray tracing method to predict the ultrasound propagation path in an austenitic weld. The propagation time predicted by the ray tracing was used to construct the TFM image using the data acquired by the FMC method experimentally. The path-corrected FMC images show minimum error in locating flaws. [PERSON] et al. [22] presented a finite element method (FEM) based on time delay estimation in the dissimilar weld. The estimated time delay was used to compare different imaging methods. The time reversal method outperforms the general and adaptive focusing methods. In line with controlling beam characteristics for enhanced flaw detection, [PERSON] and [PERSON] [24] employed the FMC+TFM-based angle beam virtual source (ABVS) method in thick (-200 mm) coarse grain austenitic material (alloy 617) to study the effect of beam directivity in ABVSFMC+TFM technique for detection of planar defects. In the ABVS method, a beam is focused near the inspection surface during transmission using a set of array elements. The performance of the ABVSFMC+TFM without a wedge was superior to the ABFMC+TFM with a wedge. Further, the ABVSFMC+TFM was able to detect top-tip and bottom-tip diffracted signals clearly. [PERSON] et al. [25] used the Markov Chain Monte Carlo (rj-MCMC) method to map the microstructure. This map is then used in conjunction with the TFM to correct for discrepancies in the expected arrival times caused by the anisotropic nature of the material. [PERSON] et al. [26] performed a time reversal acoustic (TRA) method based on FEM for the reconstruction of an FMC+TFM image in an austenitic weld. In this method, the time delay is determined using the arrival time of the reflected signal. The TRA method could effectively image a flaw in austenitic material like in an isotropic material. In line with enhanced flaw detection and characterization using FMC/TFM, this paper presents a method using PAUT FMC based on through transmission technique (TT) to estimate the angle-dependent ultrasonic longitudinal wave velocity in an austenitic alloy 617 buttering material. The angle-dependent ultrasonic velocity has been utilized to generate FMC-TFM images with improved sensitivity and resolution. Further, most of the approaches discussed above for circumventing the effects of anisotropy require post-processing and cannot be implemented for online inspection using commercial PAUT equipment. In this paper, we propose an alternate approach also for obtaining enhanced FMC-TFM imaging in the anisotropic buttering material by using a suitable isotropic velocity much higher than those observed in the material in any direction. This can be directly implemented in any commercial equipment for improved inspection of an anisotropic material with a symmetrical anisotropy in the inspection plane about the vertical axis.