@article{Xu2025,
  author = {Xu, Wuke and Li, Liangyu and Shi, Fan and Chen, Qing},
  title = {Ultrasonic spectroscopy for in situ early detection and dynamic monitoring of lithium plating in lithium-ion batteries},
  journal = {Cell Reports Physical Science},
  year = {2025},
  month = apr,
  day = {16},
  publisher = {Elsevier},
  volume = {6},
  number = {4},
  issn = {2666-3864},
  doi = {10.1016/j.xcrp.2025.102507},
  url = {https://doi.org/10.1016/j.xcrp.2025.102507}
}

@article{xu2025sandwich,
  title = {Sandwich Miura-Ori Enabled Large Area, Super Resolution Tactile Skin for Human--Machine Interactions},
  author = {Xu, Qian and Yang, Zhiwei and Wang, Zhengjun and Wang, Ruoqin and Zhang, Boyang and Cheung, YikKin and Jiao, Rui and Shi, Fan and Hong, Wei and Yu, Hongyu},
  journal = {Advanced Science},
  volume = {12},
  number = {18},
  pages = {2414580},
  year = {2025},
  publisher = {Wiley Online Library}
}

@article{jiao2025deep,
  title = {Deep Learning Based Large-Area Contact Sensing for Safe Human--Robot Interaction Using Conformal Kirigami Structure-Enabled Robotic E-Skin},
  author = {Jiao, Rui and Wang, Zhengjun and Wang, Ruoqin and Xu, Qian and Jiang, Jiacheng and Zhang, Boyang and Yang, Simin and Li, Yang and Cheung, Yik Kin and Shi, Fan and others},
  journal = {Advanced Intelligent Systems},
  pages = {2400903},
  year = {2025},
  publisher = {Wiley Online Library}
}

@article{yin2025ultrasonic,
  title = {Ultrasonic Rayleigh wave microscopic shape reconstruction of surface and near-surface defects using geometrical full waveform inversion},
  author = {Yin, Xiao and An, Xuexin and Shi, Fan},
  journal = {Proceedings of the Royal Society A},
  volume = {481},
  number = {2324},
  pages = {20250164},
  year = {2025},
  publisher = {The Royal Society}
}

@article{fei2025broadband,
  title = {Broadband negative refraction based on a pair of anti-parity-time structures},
  author = {Fei, Zhonghan and Gong, Menyang and Shi, Fan and Liu, Xiaozhou},
  journal = {Europhysics Letters},
  volume = {149},
  number = {3},
  pages = {30002},
  year = {2025},
  publisher = {IOP Publishing}
}

@article{Wang2024,
  author = {Wang, Guifeng and Shi, Fan and Chen, Zhenyu and Yu, Yue and Lim, C. W.},
  title = {Controllable flexural wave bandgap in extensible metamaterial beams with embedded multiple resonators},
  journal = {Continuum Mechanics and Thermodynamics},
  year = {2024},
  month = sep,
  day = {01},
  volume = {36},
  number = {5},
  pages = {1109-1127},
  issn = {1432-0959},
  doi = {10.1007/s00161-023-01228-6},
  url = {https://doi.org/10.1007/s00161-023-01228-6}
}

The interest in phononic crystals and acoustic metamaterials has been an intensive subject of research in recent years. Finding a robust way to significantly expand or actively control the bandgap has received extensive attention. In this study, we propose a prestressed metamaterial beam attached with multiply local resonators connected by actively tunable piezoelectric springs. The Euler–Bernoulli beam theory and Timoshenko beam theory are applied in the theoretical analysis of the system. Further, the spectral element method is utilized to analytically compute the dispersion relation and transmission ratio and excellent agreement with reference to the benchmark is reported. The influences of an external axial force on the bandgap range and attenuation behavior are further studied. Subsequently, the effect of resonator number and mass on the local resonance bandgap structure is investigated in two parametric studies. The active control of bandgap range and frequency is then verified. By analyzing frequency response function, the tunable transmission ratio of a supercell can be observed. To conclude, this paper not only provides a guideline for designs of wave attenuation with multiple frequency regimes in a one-dimensional system, but it can also be extended to sub-wavelength wave manipulation designs.

@article{PhysRevApplied.21.024055,
  title = {Far-field superresolution shape reconstruction of objects using level-set-based geometrical full-waveform inversion},
  author = {Yin, Xiao and Shi, Fan},
  journal = {Phys. Rev. Appl.},
  volume = {21},
  issue = {2},
  pages = {024055},
  numpages = {16},
  year = {2024},
  month = feb,
  publisher = {American Physical Society},
  doi = {10.1103/PhysRevApplied.21.024055},
  url = {https://link.aps.org/doi/10.1103/PhysRevApplied.21.024055}
}

@article{WANG2024107345,
  title = {Ultrasonic diffuse bulk wave passive array imaging of internal defects in a complex structure},
  journal = {Ultrasonics},
  volume = {141},
  pages = {107345},
  year = {2024},
  issn = {0041-624X},
  doi = {https://doi.org/10.1016/j.ultras.2024.107345},
  file = {WANG2024107345.pdf},
  author = {Wang, Zhengjun and Shi, Fan},
  keywords = {Ultrasonic damage detection, Non-destructive evaluation, Diffuse field, Complex structure, Array imaging, Laser vibrometer}
}

Ultrasonic bulk wave inspection of defects in safety–critical components with complex external geometries, such as turbine blades is challenging. While ultrasonic phased array imaging can yield high-resolution subsurface images, a commercial phased array probe can hardly be mounted on irregular external boundaries to perform in-situ imaging. In fact, a component with irregular shapes, as a highly reverberant body, is capable of generating elastic random diffuse or coda wavefields. The diffuse wavefields can be utilized to reconstruct Green’s functions between any two passive receiving points. In this paper, an ultrasonic passive array imaging method using the diffuse reverberation resulting from complex boundaries is implemented to image internal defects. The method involves the utilization of active piezoelectric actuators to excite elastic diffuse waves within the component, which are received by a laser vibrometer scanning at multiple points. A passive full matrix capture (FMC) of array signals is extracted for defect imaging using the total focusing method. The proposed method is evaluated by the numerical simulations, and the effects of centre frequency, bandwidth, and source excitation methods on the imaging performance are investigated. An experiment using a turbine blade-like structure is conducted to further evaluate the imaging method.

@article{10679235,
  author = {Wang, Zhengjun and Shi, Fan and Ding, Junhao and Song, Xu},
  journal = {IEEE Transactions on Ultrasonics, Ferroelectrics, and Frequency Control},
  title = {Ultrasonic Rough Crack Characterization Using Time-of-Flight Diffraction With Self-Attention Neural Network},
  year = {2024},
  volume = {71},
  number = {10},
  pages = {1289-1301},
  keywords = {Surface roughness;Rough surfaces;Surface cracks;Acoustics;Deep learning;Fatigue;Diffraction;Defect characterization;non-destructive evaluation (NDE);randomly rough surface;self-attention (SA);time-of-flight diffraction (ToFD);transfer learning (TL)},
  doi = {10.1109/TUFFC.2024.3459619}
}

@article{WEI2024105744,
  title = {Modeling stochastic elastic wave diffraction by the tips of randomly rough defects},
  journal = {Journal of the Mechanics and Physics of Solids},
  volume = {190},
  pages = {105744},
  year = {2024},
  issn = {0022-5096},
  doi = {https://doi.org/10.1016/j.jmps.2024.105744},
  url = {https://www.sciencedirect.com/science/article/pii/S0022509624002102},
  author = {Wei, Zhengyu and Shi, Fan and Wang, Zhengjun},
  keywords = {Randomly rough surface, Tip diffraction, Elastic waves, Stochastic scattering}
}

Elastic wave scattering from a randomly rough surface of a finite length includes surface reflections and diffractions from the tips. Previous research has focused upon reflection waves with applications in ultrasonic defect detection, seismic wave exploration and phonon boundary transport. However, waves diffracted from the tips/edges have been largely neglected so far for rough defects, despite their importance in engineering applications including ultrasonic defect sizing and imaging for assessment of structural integrity. Currently understanding the statistical nature of elastic wave tip diffraction and the role of roughness is limited due to the lack of theoretical studies. In this article, we develop a statistical geometrical tip diffraction (SGTD) theory to rapidly predict the stochastic properties of tip diffraction amplitude as a function of surface roughness and incident angle. By applying a small slope perturbation to the model, a simplified analytical solution of tip diffraction is obtained. It is found that for defects with small to medium roughness, the diffraction amplitude explicitly follows a Gamma distribution, and its mean and the standard deviation are both proportional to the square of the rms slope. High-fidelity Monte Carlo finite element simulations are then run to evaluate the accuracy of the theoretical model. The range of validity of the analytical solution with respect to the level of roughness and the incident angle is obtained. The SGTD method is accurate when the correlation length is approximately equivalent or larger than one wavelength, for a wide range of angles. It is also applicable for a correlation length as short as half wavelength, but only for small rms values and when the beam angle is larger than 45∘. In addition, at large angles, the tip diffraction is almost not affected by roughness, being very similar to that from a smooth crack. This is explained by the significant dependence on the beam angle factor explicitly shown in the theoretical solution.

@article{WEN2024107255,
  title = {Piezoelectric-laser ultrasonic inspection and monitoring of thin-walled structure fabricated by directed energy deposition process based on guided waves},
  journal = {Ultrasonics},
  volume = {138},
  pages = {107255},
  year = {2024},
  issn = {0041-624X},
  doi = {https://doi.org/10.1016/j.ultras.2024.107255},
  url = {https://www.sciencedirect.com/science/article/pii/S0041624X24000179},
  author = {Wen, Fuzhen and Gao, Shiming and Song, Xu and Shi, Fan},
  keywords = {Directed energy deposition, Nondestructive evaluation, Ultrasonic defect detection, Piezoelectric transducer excitation, Laser vibrometer measurement}
}

Thin-walled metallic structures produced by the Directed Energy Deposition (DED) Additive Manufacturing (AM) process are prone to various fabrication defects, which hinder the wider applications of the technique in practice. In-situ inspection and monitoring methodologies are in high demand for improved quality control of printed parts. This paper presents an ultrasonic guided-wave-based method and a prototype that can potentially be used for in-situ inspection of thin-walled structures produced by DED. Lamb waves are excited by a Lead zirconate titanate (PZT) piezoelectric transducer bonded on the DED substrate remotely from the thin wall. The substrate works as a waveguide to transmit the waves which then propagate along the thin wall. A non-contact laser vibrometer is applied to measure the guide wave signals by scanning the surface of the thin wall. The mechanisms of guided wave generation and propagation along the substrate and printed part are theoretically studied. It allows for choosing proper inspection parameters to enhance the measurement sensitivity of guided waves and help interpret the signals for defect detection. Experiments were conducted with DED-produced stainless steel (316L) thin-walled structure. The new method is demonstrated in one example to detect and localize a small defect caused by inconsistent powder delivery of a fabricated thin wall sample, via analysing the B-scan ultrasonic guided wave signals. The new technique provides strong potential for in-situ online monitoring of the DED process.

@article{WANG2024107265,
  title = {Deep learning based ultrasonic reconstruction of rough surface morphology},
  journal = {Ultrasonics},
  volume = {138},
  pages = {107265},
  year = {2024},
  issn = {0041-624X},
  doi = {https://doi.org/10.1016/j.ultras.2024.107265},
  url = {https://www.sciencedirect.com/science/article/pii/S0041624X24000271},
  author = {Wang, Zhengjun and Shi, Fan and Zou, Fangxin},
  keywords = {Non-destructive evaluation, Ultrasonic damage detection, Convolution neural network, Randomly rough surface, Real-time reconstruction, Deep learning interpretability}
}

This paper introduces a methodology to recover the morphology of a complex rough surface from ultrasonic pulse echo measurements with an array of equidistant sensors using the one dimensional convolution neural network (1DCNN). The neural network is trained by the datasets simulated from high-fidelity finite element simulations for surfaces with a range of roughness parameters and is tested on both numerical and real experimental data. To assess the performance of our proposed method, the rough surface reconstruction results from the deep learning approach are compared with those obtained from conventional ultrasonic array imaging methods. Unlike array imaging-based methods that require a large number of sensors (e.g., 128, 64 or 32), the deep learning-based method uses pulse echo signals and can achieve accurate results with much fewer sensors. The developed deep learning approach has the potential to enable low-cost, accurate, and real-time reconstruction of complex surface profiles.

@article{WANG2024107346,
  title = {Ultrasonic diffuse bulk wave passive array imaging of internal defects in a complex structure},
  journal = {Ultrasonics},
  volume = {141},
  pages = {107345},
  year = {2024},
  issn = {0041-624X},
  doi = {https://doi.org/10.1016/j.ultras.2024.107345},
  url = {https://www.sciencedirect.com/science/article/pii/S0041624X24001070},
  author = {Wang, Zhengjun and Shi, Fan},
  keywords = {Ultrasonic damage detection, Non-destructive evaluation, Diffuse field, Complex structure, Array imaging, Laser vibrometer}
}

Ultrasonic bulk wave inspection of defects in safety–critical components with complex external geometries, such as turbine blades is challenging. While ultrasonic phased array imaging can yield high-resolution subsurface images, a commercial phased array probe can hardly be mounted on irregular external boundaries to perform in-situ imaging. In fact, a component with irregular shapes, as a highly reverberant body, is capable of generating elastic random diffuse or coda wavefields. The diffuse wavefields can be utilized to reconstruct Green’s functions between any two passive receiving points. In this paper, an ultrasonic passive array imaging method using the diffuse reverberation resulting from complex boundaries is implemented to image internal defects. The method involves the utilization of active piezoelectric actuators to excite elastic diffuse waves within the component, which are received by a laser vibrometer scanning at multiple points. A passive full matrix capture (FMC) of array signals is extracted for defect imaging using the total focusing method. The proposed method is evaluated by the numerical simulations, and the effects of centre frequency, bandwidth, and source excitation methods on the imaging performance are investigated. An experiment using a turbine blade-like structure is conducted to further evaluate the imaging method.

@article{PAIALUNGA2024103089,
  title = {A machine learning surrogate model for time of flight diffraction measurements of rough defects},
  journal = {NDT & E International},
  volume = {144},
  pages = {103089},
  year = {2024},
  issn = {0963-8695},
  doi = {https://doi.org/10.1016/j.ndteint.2024.103089},
  url = {https://www.sciencedirect.com/science/article/pii/S0963869524000549},
  author = {Paialunga, Piero and Shi, Fan and Haslinger, Stewart G. and Corcoran, Joseph}
}

Understanding the uncertainty in nondestructive evaluation measurements of rough defects requires a stochastic analysis due to the random variation between the morphology of different defects. In previous studies, large numbers of finite element models of randomly generated rough defects have been run in order to gain an insight into the statistics of the uncertain results. This approach is limited due to the time taken to run each individual model (typically of the order of minutes per model) and so the total number of models run is limited. In this paper, a surrogate model approach is proposed which produces close approximations to an original finite element model of a time-of-flight diffraction measurement, but in a small fraction of the time (<1 ms). The surrogate model is trained on pairs of input (rough defect) and output (A-scan) data from the finite element model; the surrogate then learns the relationship between inputs and outputs and is then able to generate new results. The architecture employed is a machine learning sequence-to-sequence approach similar to those used in natural language algorithms, based on the idea of ‘translating’ between defect to A-scan. In this study, a surrogate model was trained on 2160 finite element results which spanned across a range of ultrasonic incidence angles, rough defect correlation length and root-mean-square roughness. The surrogate model is shown to produce A-scans with close agreement to those produced by the original finite element model; taking approximately 0.425 ms instead of approximately 8 min. Millions of results are then possible in seconds, rather than the decades that would be required for finite element studies; more complete stochastic analysis are therefore now viable.

@article{ALLISON2025118759,
  title = {The design of two-dimensional elastic diffusers},
  journal = {Journal of Sound and Vibration},
  volume = {596},
  pages = {118759},
  year = {2024},
  issn = {0022-460X},
  doi = {https://doi.org/10.1016/j.jsv.2024.118759},
  url = {https://www.sciencedirect.com/science/article/pii/S0022460X24005212},
  author = {Allison, F.J.P. and Haslinger, S.G. and Selsil, Ö. and Shi, F.},
  keywords = {Stochastic processes, Diffusers, Elastic wave propagation, Randomly rough surfaces, Inverse problem}
}

The ability to design surfaces with specific scattering properties has widespread applicability. In this article, we leverage a stochastic method for designing randomly rough, two-dimensional surfaces with a prescribed mean angular intensity distribution. Analytical and numerical techniques are implemented for a linearly elastic medium-vacuum interface, to generate band-limited diffusers with square and elliptical domains of scattering. Longitudinal incidence is assumed, and the theory allows for bulk wave mode conversions. Multivalued, discontinuous surfaces are formulated, and it is shown that the theoretical model may be satisfied independently of the facet shape utilised, with square and hexagonal discretisations given as examples. In the context of elastic wave scattering, the surfaces presented in this article may have applications in the calibration of scatterometers for wave manipulation as well as for crack detection using ultrasound.

@article{Xu2023,
  author = {Xu, Wuke and Yang, Yuewang and Shi, Fan and Li, Liangyu and Wen, Fuzhen and Chen, Qing},
  title = {Ultrasonic phased array imaging of gas evolution in a lithium-ion battery},
  journal = {Cell Reports Physical Science},
  year = {2023},
  month = sep,
  day = {20},
  publisher = {Elsevier},
  volume = {4},
  number = {9},
  issn = {2666-3864},
  doi = {10.1016/j.xcrp.2023.101579},
  url = {https://doi.org/10.1016/j.xcrp.2023.101579}
}

Non-invasive characterization and monitoring of gas evolution during the operation of commercial Li-ion batteries (LIBs) has been a long-term challenge. This paper presents an in situ subsurface ultrasonic array imaging method to detect, locate, and characterize gases generated inside a LIB. Ultrasonic signals scattered from internal gases are collected by a full matrix capture method. A cross-section image showing gases is produced using the velocity-modified total focusing method. The locations and distribution of the gases in both the lateral and thickness directions can be clearly revealed. The finite element simulations and a controlled experiment are performed to validate the feasibility of the proposed array imaging method. Finally, a realistic long-term cycling experiment is performed on a commercial pouch cell. The appearance, evolution, and accumulation of gases are visualized and characterized by analyzing the images produced at different cycles. The experiment findings are validated against the X-ray CT results.

@article{JIE2023108384,
  title = {Ultrasonic guided wave measurement and modeling analysis of the state of charge for lithium-ion battery},
  journal = {Journal of Energy Storage},
  volume = {72},
  pages = {108384},
  year = {2023},
  issn = {2352-152X},
  doi = {https://doi.org/10.1016/j.est.2023.108384},
  url = {https://www.sciencedirect.com/science/article/pii/S2352152X23017814},
  author = {Jie, Gao and Liangheng, Zhang and Yan, Lyu and Fan, Shi and Bin, Wu and Cunfu, He},
  keywords = {Lithium-ion battery, State of charge, Ultrasonic guided wave, Dispersion curve}
}

This work presents the analytical acoustic model to investigate the interaction mechanism between the state of charge (SOC) of lithium-ion battery and the propagation characteristics of ultrasonic guided waves. Meanwhile, the multi-layered porous structure characteristics of lithium-ion batteries are focused on, the Biot theory and transfer matrix method are introduced to construct the analytical acoustic theory model of lithium-ion batteries. The mechanical performance (modulus and density) of the electrode is dynamically changing during cycling, which will influence the dispersion characteristics of ultrasonic guided waves in lithium-ion battery. Based on this, the intrinsic connection between the SOC and the guided wave dispersion curve of lithium-ion battery is numerically analyzed. Moreover, the frequency domain simulation model with equal structural characteristics is established to verify the validity of the above theoretical results. Theoretical results performed on the customized pouch cell show that the changes in time of flight resulting from shifts in the group velocity of guided wave A0 mode correlate strongly with the SOC. An ultrasonic guided wave detection experiment system is built to extract the variations in time of flight and amplitude of the guided wave signal during cycling, which correctly validates the range of theoretical time of flight. Furthermore, the applicability of the proposed method is verified by battery experiments with different temperature and current rate operating conditions. Furthermore, the ultrasonic guided wave detection approach has also been shown to be a sensitive means of determining the SOC of lithium-ion batteries.

@article{DENG202335,
  title = {A new visual-guided and partition-based multi-setup 3D printing system},
  journal = {Journal of Manufacturing Systems},
  volume = {67},
  pages = {35-56},
  year = {2023},
  issn = {0278-6125},
  doi = {https://doi.org/10.1016/j.jmsy.2022.12.013},
  url = {https://www.sciencedirect.com/science/article/pii/S0278612522002321},
  author = {Deng, Xiaoke and Li, Zhaoyu and Wang, Xiangyu and Shi, Fan and Tang, Kai},
  keywords = {Workpiece setup, Workpiece localization, 3D printing, Visual localization, Process planning}
}

Additive manufacturing, also known as 3D printing, has become a powerful and versatile means of manufacturing for almost any applications in our daily lives. A frequently encountered problem in printing, particular in consumer or lower-end applications, is how to print a workpiece whose dimensions exceed the working space of the printer at hand. Rather than opting for the costly solution of getting a new and larger printer (and hence more expensive and also other induced hassles), a more plausible solution would be an algorithmic one, which is the objective of this paper. We present a partition-based multi-setup printing process planning algorithm that, given the CAD model of an oversized workpiece, will be able to determine a continuous printing path with multiple workpiece setups to correctly print out the workpiece on the same printer. To answer the challenge of workpiece localization during the change of workpiece setups in the middle of printing, we propose a visual-guided workpiece localization system. Both computer simulation and physical printing experiments are carried out, and the results give a positive confirmation on the feasibility and effectiveness of the proposed multi-setup printing methodology and our enabling printing process planning algorithm.

@article{Chen2022,
  author = {Chen, Zhenyu and Wang, Xiangyu and Lim, C. W. and Shi, Fan},
  title = {Robust large-area elastic transverse wave transport in active acoustic metamaterials},
  journal = {Journal of Applied Physics},
  year = {2022},
  month = may,
  day = {12},
  volume = {131},
  number = {18},
  pages = {185112},
  issn = {0021-8979},
  doi = {10.1063/5.0087988},
  url = {https://doi.org/10.1063/5.0087988}
}

We investigate robust large-area elastic transverse wave propagation in an actively tunable membrane-type acoustic metamaterial. The waveguide with multiple degrees of freedom to control the width of the interface mode is realized by designing a heterostructure including three domains. One central domain is constructed by periodic unit cells in an ordinary state, where a Dirac cone can be observed in the band structure. The other two domains consist of periodic unit cells possessing opposite valley Chern numbers, respectively. By employing a finite element model, the topologically protected interface states with tunable degrees of freedom are exhibited. The energy of interface states distributes equally in the large-central region. Although a larger degree of freedom leads to lower amplitudes of interface states, larger total energy is demonstrated by defining a quality factor. Moreover, we design several waveguides with straight lines and sharp corners with different angles and denote three different notations to show clearly that the large-area transverse wave can propagate robustly through sharp corners. Finally, it is found that the large-area transverse wave transport shows immunity to disorders and defects in the propagation path.

@article{XIONG2022104264,
  title = {Smart evaluation of building fire scenario and hazard by attenuation of alarm sound field},
  journal = {Journal of Building Engineering},
  volume = {51},
  pages = {104264},
  year = {2022},
  issn = {2352-7102},
  doi = {https://doi.org/10.1016/j.jobe.2022.104264},
  url = {https://www.sciencedirect.com/science/article/pii/S2352710222002777},
  author = {Xiong, Caiyi and Wang, Zilong and Huang, Yunke and Shi, Fan and Huang, Xinyan},
  keywords = {Building fire, Fire detection, Audible fire alarm, Sound pressure, Fire hazard, Smart firefighting}
}

The audible fire alarm system of the building makes a sharp sound to alert all occupants when fires occur. According to the fire code, the fire alarm should be loud enough to be heard in any corner of the building. Thus, this work explores a smart technology of using alarm attenuation to reveal the fire scene information. Different alarms with frequencies from 500 to 2000 Hz are tested. The propanol pool fires of different sizes and shapes are selected as the detecting targets. Results show that the sound pressure attenuation by the fire plume is positively correlated with the fire scene heat release rate. The sound-pressure attenuation is also greater if the flame thickness is larger along the sound path. Hence, a sound-field fire monitoring model is proposed and verified by experiments using larger wood-crib fire and liquid-pool fire. This work provides a practical sound-based fire monitoring model and helps establish a scientific framework for the smart technology of using the existing audible alarm system to continuously monitor the building fire development.

@article{CHEN2022107292,
  title = {Analytical modeling and numerical analysis for tunable topological phase transition of flexural waves in active sandwiched phononic beam systems},
  journal = {International Journal of Mechanical Sciences},
  volume = {223},
  pages = {107292},
  year = {2022},
  issn = {0020-7403},
  doi = {https://doi.org/10.1016/j.ijmecsci.2022.107292},
  url = {https://www.sciencedirect.com/science/article/pii/S0020740322002077},
  author = {Chen, Zhenyu and Wang, Guifeng and Shi, Fan and Lim, C.W.},
  keywords = {Active control, Interface mode, Phase transition, Sandwiched beam, Topological phononic crystals}
}

Topological phononic crystals (PnCs) have attracted tremendous research attention in recent years. A significant hallmark of these structures is that these crystals can support interface modes that are robust to structural disturbance and protected by topology. In this study, we propose a new type of active sandwiched PnC beam for inducing topological geometric phase transition and topologically protected interface modes (TPIMs) for one-dimensional (1D) systems. The layered system comprises two commonly used active materials, i.e., barium titanate (BaTiO3) and cobalt ferric oxide (CoFe2O4). Analytical modeling for this layered system is derived in the framework of linearly constitutive relations of a magneto-electro-elastic (MEE) material with temperature effects. Two analytical approaches, i.e., the spectral element method (SEM) and the plane wave expansion (PWE) method, are applied to derive the theoretical band structure of the system and excellent agreement is reported. A numerical analysis based on the finite element method (FEM) is adopted for further validation. The influence of outer fields in the bandgap frequency is examined and the size-dependent properties are also analyzed. Moreover, the transmission response is determined via analytical modeling and numerical analysis. It is found that the robust TPIMs are immune to defects and disorders. Conclusively, this study puts forward a new type of beam system for inducing topological phase transition. It can be readily extended to more complex systems and higher-order models.

@article{YIN2022117099,
  title = {Hybrid geometrical full waveform inversion for ultrasonic defect characterisation},
  journal = {Journal of Sound and Vibration},
  volume = {535},
  pages = {117099},
  year = {2022},
  issn = {0022-460X},
  doi = {https://doi.org/10.1016/j.jsv.2022.117099},
  url = {https://www.sciencedirect.com/science/article/pii/S0022460X22003091},
  author = {Yin, Xiao and Shi, Fan},
  keywords = {Geometrical full waveform inversion, Topological imaging, Ultrasonic defect characterisation, Shape reconstruction}
}

A hybrid geometrical full waveform inversion (GFWI) method is presented in this article for accurate ultrasonic defect characterisation using a phased array. The proposed method combines two steps: (1) A newly developed multi-mode topological energy method (TEM) is used to provide a high-resolution image for defect classification and a good initial defect model for the second step. (2) A GFWI procedure is executed to reconstruct the defect shape through iteratively reducing the mismatch in waveforms between the measurements and synthetic data from the defect model. The GFWI procedure enables direct inversion of the defect shape by deforming the elements attached at its boundaries from finite element formulation, controlled by the preconditioned boundary gradient. To suppress the cross-talk among different geometrical parameters with distinct units, the BFGS and the subspace methods are implemented together with GFWI, which enables reconstructing defects of various shapes by taking all the wave physics into the inversion automatically. Numerical examples are shown with a volumetric defect, a straight crack, a curved crack and a surface-breaking notch, giving accuracy of defect sizing within fractions of a wavelength. Two experiments using an ultrasonic phased array system further demonstrate the feasibility of this method.

@article{陈振宇2022地震超材料,
  title = {地震超材料: 从自然结构到新型人工结构},
  author = {陈振宇 and 林志华 and 施帆},
  journal = {科学通报= Chinese Science Bulletin},
  year = {2022}
}

@article{https://doi.org/10.1002/stc.3103,
  author = {Lin, Shibin and Ashlock, Jeramy and Shams, Sadegh and Shi, Fan and Wang, Yujin},
  title = {Analytical computation of the dominant dispersion trend of Lamb waves in plate-like structures with an improved dynamic stiffness matrix method},
  journal = {Structural Control and Health Monitoring},
  volume = {29},
  number = {11},
  pages = {e3103},
  keywords = {dispersion, forward modeling, lamb waves, nondestructive testing, plate-like structures},
  doi = {https://doi.org/10.1002/stc.3103},
  url = {https://onlinelibrary.wiley.com/doi/abs/10.1002/stc.3103},
  eprint = {https://onlinelibrary.wiley.com/doi/pdf/10.1002/stc.3103},
  year = {2022}
}

Summary Lamb waves have infinite number dispersion modes; however, no every mode is excitable and detectable. Traditional matrix methods can calculate the dispersion curve of each mode over the full range of possible frequencies. However, the resulting numerically calculated multimodal dispersion curves do not fully represent the dispersion curves measured in real experiments, which are most often dominated by energy from specific modes. An improved dynamic stiffness matrix method is proposed herein to overcome such challenges of the traditional matrix methods. The first step of the improved method is to calculate the displacement response of a plate-like structure under a vertical dynamic loading using the global stiffness matrix of the structure, then the dominant dispersion trend is extracted from the displacement using the phase-velocity scanning scheme. The improved method is verified with three case studies representing typical plate-like structures in structural engineering. The results demonstrate that dispersion trends calculated with the improved method have good agreement with those obtained from experimental measurements.

@article{SHI2021104550,
  title = {Variance of elastic wave scattering from randomly rough surfaces},
  journal = {Journal of the Mechanics and Physics of Solids},
  volume = {155},
  pages = {104550},
  year = {2021},
  issn = {0022-5096},
  doi = {https://doi.org/10.1016/j.jmps.2021.104550},
  url = {https://www.sciencedirect.com/science/article/pii/S0022509621002040},
  author = {Shi, F.},
  keywords = {Random rough surface, Elastic waves scattering, Variance, Statistical distribution}
}

Elastic waves scattered by randomly rough surfaces in solid media play an important role across research topics including ultrasonic wave detection and imaging, seismic wave exploration and phonon boundary interaction. Previous attention has focused upon the mean/expected scattering intensity for both compressional (P) and shear (S) waves. In this article, the variance or the standard deviation (sd) of elastic wave scattering from randomly rough surfaces is studied, which so far has been neglected despite their practical importance in elastic wave applications, via an analytical approach. Theoretical formulae are derived with the aid of the elastodynamic Kirchhoff approximation (KA), to rapidly predict the variance of the scattering amplitude and the intensity, once the statistical parameters of the roughness are known. Theoretical formulae are then successfully validated against high-fidelity Monte Carlo finite element (FE) simulations at different scattering angles across a range of roughness. With the analytical approach the effects of rms roughness, the correlation length and the surface length on the standard deviation of the scattering amplitude are analysed. The significance for applications is illustrated in one important example taken from the field of ultrasonic wave detection of planar rough defects. The theoretical formulae accurately predict the lower bound of the scattering amplitude, which helps set an amplitude threshold confidently, for detecting any possible rough crack in a single inspection while minimising the risk of false alarm. The significantly improved accuracy and confidence of detection enable reliable decisions to be made about whether it is safe to continue using an engineering component.

@article{Haslinger:2021:1354-2575:28,
  title = {Prediction of reflection amplitudes for ultrasonic inspection of rough planar defects},
  journal = {Insight - Non-Destructive Testing and Condition Monitoring},
  year = {2021},
  volume = {63},
  number = {1},
  pages = {28-36},
  issn = {1354-2575},
  url = {https://www.ingentaconnect.com/content/bindt/insight/2021/00000063/00000001/art00007},
  doi = {doi:10.1784/insi.2021.63.1.28},
  author = {Haslinger, S G and Lowe, M J S and Craster, R V and Huthwaite, P and Shi, F}
}

The characteristics of planar defects (no loss of material volume) that arise during industrial plant operation are difficult to predict in detail, yet these can affect the performance of non-destructive testing (NDT) used to manage plant structural integrity. Inspection modelling is
increasingly used to design and assess ultrasonic inspections of such plant items. While modelling of smooth planar defects is relatively mature and validated, issues have remained in the treatment of rough planar defect species. The qualification of ultrasonic inspections for such defects
is presently very conservative, owing to the uncertainty of the amplitudes of rough surface reflections. Pragmatic solutions include the addition of large sensitivity thresholds and more frequent inspection intervals, which require more plant downtime. In this article, an alternative approach
has been developed by the authors to predict the expected surface reflection from a rough defect using a theoretical statistical model. Given only the frequency, angle of incidence and two statistical parameter values used to characterise the defects, the expected reflection amplitude is obtained
rapidly for any scattering angle and size of defect, for both compression and shear waves. The method is applicable for inspections of isotropic media that feature surface reflections such as pulse-echo or pitch-catch, rather than for tip signal-dependent techniques such as time-of-flight
diffraction. The potential impact for inspection qualification is significant, with the new model predicting increases of up to 20 dB in signal amplitude in comparison with models presently used in industry. All mode conversions are included and rigorous validations using numerical and experimental
methods have been performed. The model has been instrumental in obtaining new statistically significant results related to the effect of tilt; the expected pulse-echo backscattered amplitude for very rough planar defects is independent of tilt angle, with convergence obtained for a range of
frequencies.

@article{HASLINGER2021102521,
  title = {Time of flight diffraction for rough planar defects},
  journal = {NDT & E International},
  volume = {124},
  pages = {102521},
  year = {2021},
  issn = {0963-8695},
  doi = {https://doi.org/10.1016/j.ndteint.2021.102521},
  url = {https://www.sciencedirect.com/science/article/pii/S0963869521001201},
  author = {Haslinger, Stewart G. and Lowe, Michael.J.S. and Wang, Zhengjun and Shi, Fan},
  keywords = {Non-destructive evaluation, Time of flight diffraction, Randomly rough surface, Elastic wave scattering, Nuclear power}
}

Ultrasonic non-destructive evaluation techniques, such as time-of-flight diffraction (ToFD) for which the arrival times of waves diffracted from crack tips are analysed to locate and size defects, are well understood for smooth defects. In environments where extreme changes in temperature and pressure occur, the damage that may arise is often non-uniform and more difficult to characterise when designing and qualifying an inspection. This article investigates the implementation of ToFD methods for sizing rough defects using a purely theoretical approach. High-fidelity finite element modelling and stochastic Monte Carlo methods are used to provide physical and statistical insights for the dependence on both incident beam angle and degree of roughness for the case of planar defects. Optimal incident angles for ultrasonic ToFD techniques were determined in the 1980s but largely based on theoretical and experimental investigations for smooth defects. However, rough defects produce tip-diffracted signatures that are more complicated than for their smooth counterparts, largely due to multiple scattering effects related to mode conversion and propagation of surface waves along the rough surface. It is shown that roughness may cause larger diffraction amplitude values at different angles, which leads to increased uncertainty when sizing, with illustrative examples and physical interpretations provided. Comparisons of amplitudes for smooth and rough defects of the same size are also demonstrated. The ToFD method, using envelope peak detection and autocorrelation approaches, is implemented to estimate the size of rough cracks, and the effects of roughness on the accuracy of this sizing are investigated with statistical analysis.

@article{haslinger2020elastic,
  title = {Elastic shear wave scattering by randomly rough surfaces},
  author = {Haslinger, Stewart G and Lowe, Michael JS and Huthwaite, Peter and Craster, Richard V and Shi, Fan},
  journal = {Journal of the Mechanics and Physics of Solids},
  volume = {137},
  pages = {103852},
  year = {2020},
  publisher = {Elsevier}
}

@article{HASLINGER2020103852,
  title = {Elastic shear wave scattering by randomly rough surfaces},
  journal = {Journal of the Mechanics and Physics of Solids},
  volume = {137},
  pages = {103852},
  year = {2020},
  issn = {0022-5096},
  doi = {https://doi.org/10.1016/j.jmps.2019.103852},
  url = {https://www.sciencedirect.com/science/article/pii/S0022509619307835},
  author = {Haslinger, Stewart G. and Lowe, Michael J.S. and Huthwaite, Peter and Craster, Richard V. and Shi, Fan},
  keywords = {Elastic shear waves, Randomly rough surface, Kirchhoff approximation, Stationary phase, Non-destructive evaluation, Correlation length}
}

Characterizing cracks within elastic media forms an important aspect of ultrasonic non-destructive evaluation (NDE) where techniques such as time-of-flight diffraction and pulse-echo are often used with the presumption of scattering from smooth, straight cracks. However, cracks are rarely straight, or smooth, and recent attention has focussed upon rough surface scattering primarily by longitudinal wave excitations. We provide a comprehensive study of scattering by incident shear waves, thus far neglected in models of rough surface scattering despite their practical importance in the detection of surface-breaking defects, using modelling, simulation and supporting experiments. The scattering of incident shear waves introduces challenges, largely absent in the longitudinal case, related to surface wave mode-conversion, the reduced range of validity of the Kirchhoff approximation (KA) as compared with longitudinal incidence, and an increased importance of correlation length. The expected reflection from a rough defect is predicted using a statistical model from which, given the angle of incidence and two statistical parameters, the expected reflection amplitude is obtained instantaneously for any scattering angle and length of defect. If the ratio of correlation length to defect length exceeds a critical value, which we determine, there is an explicit dependence of the scattering results on correlation length, and we modify the modelling to find this dependence. The modelling is cross-correlated against Monte Carlo simulations of many different surface profiles, sharing the same statistical parameter values, using numerical simulation via ray models (KA) and finite element (FE) methods accelerated with a GPU implementation. Additionally we provide experimental validations that demonstrate the accuracy of our predictions.

@article{haslinger2019appraising,
  title = {Appraising Kirchhoff approximation theory for the scattering of elastic shear waves by randomly rough defects},
  author = {Haslinger, Stewart G and Lowe, Michael JS and Huthwaite, Peter and Craster, Richard V and Shi, Fan},
  journal = {Journal of Sound and Vibration},
  volume = {460},
  pages = {114872},
  year = {2019},
  publisher = {Elsevier}
}

@article{shi2019waveform,
  title = {Waveform-based geometrical inversion of obstacles},
  author = {Shi, Fan and Huthwaite, Peter},
  journal = {Physical Review Applied},
  volume = {12},
  number = {6},
  pages = {064008},
  year = {2019},
  publisher = {APS}
}

@article{shi2019expected,
  title = {Expected amplitudes of ultrasonic elastic waves scattered from rough defects},
  author = {Shi, Fan},
  year = {2019}
}

@article{wise2019improving,
  title = {Improving ultrasonic imaging in complex components through topological imaging with cycle-skipping eliminated},
  author = {Wise, Elliott and Huthwaite, Peter and Shi, Fan},
  journal = {Review of Progress in Quantitative Nondestructive Evaluation},
  year = {2019},
  publisher = {Iowa State University Digital Press}
}

@article{shi2019geometrical,
  title = {Geometrical full waveform inversion of defects},
  author = {Shi, Fan and Huthwaite, Peter},
  journal = {Review of Progress in Quantitative Nondestructive Evaluation},
  year = {2019},
  publisher = {Iowa State University Digital Press}
}

@inproceedings{lowe2019prediction,
  title = {Prediction of the amplitude of ultrasound reflection from rough defects (Conference Presentation)},
  author = {Lowe, Michael and Shi, Fan and Haslinger, Stewart and Huthwaite, Peter and Craster, Richard},
  booktitle = {Health Monitoring of Structural and Biological Systems XIII},
  volume = {10972},
  pages = {109720K},
  year = {2019},
  organization = {SPIE}
}

@article{shi2018time,
  title = {A time-domain finite element boundary integral approach for elastic wave scattering},
  author = {Shi, Fan and Lowe, MJS and Skelton, EA and Craster, RV3799102},
  journal = {Computational Mechanics},
  volume = {61},
  pages = {471--483},
  year = {2018},
  publisher = {Springer}
}

@article{shi2018ultrasonic,
  title = {Ultrasonic wave-speed diffraction tomography with undersampled data using virtual transducers},
  author = {Shi, Fan and Huthwaite, Peter},
  journal = {IEEE Transactions on Ultrasonics, Ferroelectrics, and Frequency Control},
  volume = {65},
  number = {7},
  pages = {1226--1238},
  year = {2018},
  publisher = {IEEE}
}

@inproceedings{shi2018elastid,
  title = {Elastic Wave Propagation in Geological Media Embedded With Discrete Fractures Following Power Law Length Scaling},
  author = {Shi, Fan and Lei, Qinghua},
  booktitle = {ARMA International Discrete Fracture Network Engineering Conference},
  pages = {D023S012R001},
  year = {2018},
  organization = {ARMA}
}

@article{choi2018rough,
  title = {Rough surface reconstruction of real surfaces for numerical simulations of ultrasonic wave scattering},
  author = {Choi, Wonjae and Shi, Fan and Lowe, Michael JS and Skelton, Elizabeth A and Craster, Richard V and Daniels, William L},
  journal = {NDT \& E International},
  volume = {98},
  pages = {27--36},
  year = {2018},
  publisher = {Elsevier}
}

@inproceedings{shi2018elastic,
  title = {Elastic wave propagation in fractured geological media: effects of connectivity and stiffness of fractures},
  author = {Shi, Fan and Lei, Qinghua},
  booktitle = {Proceedings of the 2nd International Discrete Fracture Network Engineering Conference. Seattle, USA},
  volume = {527},
  year = {2018}
}

@article{PhysRevB.95.214305,
  title = {Diffusely scattered and transmitted elastic waves by random rough solid-solid interfaces using an elastodynamic Kirchhoff approximation},
  author = {Shi, Fan and Lowe, Mike and Craster, Richard},
  journal = {Phys. Rev. B},
  volume = {95},
  issue = {21},
  pages = {214305},
  numpages = {13},
  year = {2017},
  month = jun,
  publisher = {American Physical Society},
  doi = {10.1103/PhysRevB.95.214305},
  url = {https://link.aps.org/doi/10.1103/PhysRevB.95.214305}
}

@article{SHI2017483,
  title = {Recovery of correlation function of internal random rough surfaces from diffusely scattered elastic waves},
  journal = {Journal of the Mechanics and Physics of Solids},
  volume = {99},
  pages = {483-494},
  year = {2017},
  issn = {0022-5096},
  doi = {https://doi.org/10.1016/j.jmps.2016.11.003},
  url = {https://www.sciencedirect.com/science/article/pii/S0022509616305695},
  author = {Shi, F. and Lowe, M.J.S. and Craster, R.V.},
  keywords = {Random rough surface, Inverse problem, Diffuse elastic waves, Correlation function}
}

We propose an ultrasonic methodology to reconstruct the height correlation function of remotely inaccessible random rough surfaces in solids. The inverse method is based on the Kirchhoff approximation(KA), and it requires measuring the angular distribution of diffuse scattering intensities by sending in a narrow band incident pulse. Near field scattering effects are also included by considering the Fresnel assumption. The proposed approach is successfully verified by simulating the scattering from multiple realizations of rough surfaces whose correlation function is known, calculating the mean scattering intensities from these received signals, and then deploying the inverse method on these to reconstruct the original correlation function. Very good agreement between the reconstructed correlation function and the original is found, for a wide range of roughness parameters. In addition, the effect of reducing the number of realizations to approximate the mean intensity are investigated, providing confidence bounds for the experiment. An experiment on a corrugated rough surface is performed with a limited number of scans using a phased array, which further validates the proposed inversion algorithm.

@article{shi2016solution,
  title = {Solution to Reconstruct the Height Correlation Function of a Randomly Rough Surface Using the Diffuse Scattered Elastic Waves},
  author = {Shi, Fan and Lowe, Michael and Craster, Richard},
  year = {2016}
}

@article{SHI2016260,
  title = {Diffuse scattered field of elastic waves from randomly rough surfaces using an analytical Kirchhoff theory},
  journal = {Journal of the Mechanics and Physics of Solids},
  volume = {92},
  pages = {260-277},
  year = {2016},
  issn = {0022-5096},
  doi = {https://doi.org/10.1016/j.jmps.2016.04.003},
  url = {https://www.sciencedirect.com/science/article/pii/S0022509615302866},
  author = {Shi, F. and Lowe, M.J.S. and Xi, X. and Craster, R.V.},
  keywords = {Diffuse scattered waves, Elastic waves, Randomly rough surface, Kirchhoff approximation}
}

We develop an elastodynamic theory to predict the diffuse scattered field of elastic waves by randomly rough surfaces, for the first time, with the aid of the Kirchhoff approximation (KA). Analytical expressions are derived incorporating surface statistics, to represent the expectation of the angular distribution of the diffuse intensity for different modes. The analytical solutions are successfully verified with numerical Monte Carlo simulations, and also validated by comparison with experiments. We then apply the theory to quantitatively investigate the effects of the roughness and the shear-to-compressional wave speed ratio on the mode conversion and the scattering intensity, from low to high roughness within the valid region of KA. Both the direct and the mode converted intensities are significantly affected by the roughness, which leads to distinct scattering patterns for different wave modes. The mode conversion effect is very strong around the specular angle and it is found to increase as the surface appears to be more rough. In addition, the 3D roughness induced coupling between the out-of-plane shear horizontal (SH) mode and the in-plane modes is studied. The intensity of the SH mode is shown to be very sensitive to the out-of-plane correlation length, being influenced more by this than by the RMS value of the roughness. However, it is found that the depolarization pattern for the diffuse field is independent of the actual value of the roughness.

@article{10.1063/1.4914806,
  author = {Huthwaite, P. and Shi, F. and Van Pamel, A. and Lowe, M. J. S.},
  title = {High-speed GPU-based finite element simulations for NDT},
  journal = {AIP Conference Proceedings},
  volume = {1650},
  number = {1},
  pages = {1815-1819},
  year = {2015},
  month = mar,
  issn = {0094-243X},
  doi = {10.1063/1.4914806},
  url = {https://doi.org/10.1063/1.4914806},
  eprint = {https://pubs.aip.org/aip/acp/article-pdf/1650/1/1815/7543855/1815\_1\_online.pdf}
}

The finite element method solved with explicit time increments is a general approach which can be applied to many ultrasound problems. It is widely used as a powerful tool within NDE for developing and testing inspection techniques, and can also be used in inversion processes. However, the solution technique is computationally intensive, requiring many calculations to be performed for each simulation, so traditionally speed has been an issue. For maximum speed, an implementation of the method, called Pogo [Huthwaite, J. Comp. Phys. 2014, doi: 10.1016/j.jcp.2013.10.017], has been developed to run on graphics cards, exploiting the highly parallelisable nature of the algorithm. Pogo typically demonstrates speed improvements of 60-90x over commercial CPU alternatives. Pogo is applied to three NDE examples, where the speed improvements are important: guided wave tomography, where a full 3D simulation must be run for each source transducer and every different defect size; scattering from rough cracks, where many simulations need to be run to build up a statistical model of the behaviour; and ultrasound propagation within coarse-grained materials where the mesh must be highly refined and many different cases run.

@article{10.1063/1.4914794,
  author = {Shi, Fan and Choi, Wonjae and Skelton, Elizabeth and Lowe, Mike and Craster, Richard},
  title = {Investigation of the validity of the elastic Kirchhoff approximation for rough cracks using a finite element approach},
  journal = {AIP Conference Proceedings},
  volume = {1650},
  number = {1},
  pages = {1722-1729},
  year = {2015},
  month = mar,
  issn = {0094-243X},
  doi = {10.1063/1.4914794},
  url = {https://doi.org/10.1063/1.4914794},
  eprint = {https://pubs.aip.org/aip/acp/article-pdf/1650/1/1722/7543757/1722\_1\_online.pdf}
}

The Kirchhoff approximation to calculate the elastic wave scattering from 2D and 3D rough cracks is examined by a comparison with a finite element (FE) approach. This approach couples a time domain finite element solver and a hybrid method to compute the scattering signals from rough cracks. Random rough surfaces with Gaussian profiles are used in this paper to study the validity of the Kirchhoff approximation. Simulations are run as a function of incident/scattering angle and roughness. Both the shape and the peak amplitude of the received signal are compared using the two different numerical approaches. Certain restricted ranges for the Kirchhoff approximation are found through the comparison with the FE method, and these are found to be different between 2D and 3D.

@article{doi:10.1098/rspa.2014.0977,
  author = {Shi, F. and Choi, W. and Lowe, M. J. S. and Skelton, E. A. and Craster, R. V.},
  title = {The validity of Kirchhoff theory for scattering of elastic waves from rough surfaces},
  journal = {Proceedings of the Royal Society A: Mathematical, Physical and Engineering Sciences},
  volume = {471},
  number = {2178},
  pages = {20140977},
  year = {2015},
  doi = {10.1098/rspa.2014.0977},
  url = {https://royalsocietypublishing.org/doi/abs/10.1098/rspa.2014.0977},
  eprint = {https://royalsocietypublishing.org/doi/pdf/10.1098/rspa.2014.0977}
}

 The Kirchhoff approximation (KA) for elastic wave scattering from two-dimensional (2D) and three-dimensional (3D) rough surfaces is critically examined using finite-element (FE) simulations capable of extracting highly accurate data while retaining a fine-scale rough surface. The FE approach efficiently couples a time domain FE solver with a boundary integration method to compute the scattered signals from specific realizations of rough surfaces. Multiple random rough surfaces whose profiles have Gaussian statistics are studied by both Kirchhoff and FE models and the results are compared; Monte Carlo simulations are used to assess the comparison statistically. The comparison focuses on the averaged peak amplitude of the scattered signals, as it is an important characteristic measured in experiments. Comparisons, in both two dimensions and three dimensions, determine the accuracy of Kirchhoff theory in terms of an empirically estimated parameter   2/  0 (   is the RMS value, and   0 is the correlation length, of the roughness), being considered accurate when this is less than some upper bound c, (  2/  0<c). The incidence and scattering angles also play important roles in the validity of the Kirchhoff theory and it is found that for modest incidence angles of less than 30  , the accuracy of the KA is improved even when   2/  0>c. In addition, the evaluation results are compared using 3D isotropic rough surfaces and 2D surfaces with the same surface parameters. 

@phdthesis{shi2015elastic,
  title = {Elastic wave scattering from randomly rough surfaces},
  author = {Shi, Fan},
  year = {2015}
}

@article{Choi2014,
  author = {Choi, Wonjae and Skelton, Elizabeth A. and Shi, Fan and Lowe, Michael J. S. and Craster, Richard V.},
  title = {Rough surface reconstruction for ultrasonic NDE simulation},
  journal = {AIP Conference Proceedings},
  year = {2014},
  month = feb,
  day = {18},
  volume = {1581},
  number = {1},
  pages = {587-594},
  issn = {0094-243X},
  doi = {10.1063/1.4864873},
  url = {https://doi.org/10.1063/1.4864873}
}

The reflection of ultrasound from rough surfaces is an important topic for the NDE of safety-critical components, such as pressure-containing components in power stations. The specular reflection from a rough surface of a defect is normally lower than it would be from a flat surface, so it is typical to apply a safety factor in order that justification cases for inspection planning are conservative. The study of the statistics of the rough surfaces that might be expected in candidate defects according to materials and loading, and the reflections from them, can be useful to develop arguments for realistic safety factors. This paper presents a study of real rough crack surfaces that are representative of the potential defects in pressure-containing power plant. Two-dimensional (area) values of the height of the roughness have been measured and their statistics analysed. Then a means to reconstruct model cases with similar statistics, so as to enable the creation of multiple realistic realizations of the surfaces, has been investigated, using random field theory. Rough surfaces are reconstructed, based on a real surface, and results for these two-dimensional descriptions of the original surface have been compared with those from the conventional model based on a one-dimensional correlation coefficient function. In addition, ultrasonic reflections from them are simulated using a finite element method.

@article{6968699,
  author = {Shi, Fan and Choi, Wonjae and Skelton, Elizabeth A. and Lowe, Michael J.S. and Craster, Richard V.},
  journal = {IEEE Transactions on Ultrasonics, Ferroelectrics, and Frequency Control},
  title = {A time-domain finite element boundary integration method for ultrasonic nondestructive evaluation},
  year = {2014},
  volume = {61},
  number = {12},
  pages = {2054-2066},
  keywords = {Scattering;Time-domain analysis;Monitoring;Numerical models;Acoustics;Surface cracks;Computational modeling},
  doi = {10.1109/TUFFC.2014.006507}
}

@article{Shi2013,
  author = {Shi, Fan and Michaels, Jennifer E. and Lee, Sang Jun},
  title = {In situ estimation of applied biaxial loads with Lamb waves},
  journal = {The Journal of the Acoustical Society of America},
  year = {2013},
  month = jan,
  day = {30},
  volume = {133},
  number = {2},
  pages = {677-687},
  issn = {0001-4966},
  doi = {10.1121/1.4773867},
  url = {https://doi.org/10.1121/1.4773867}
}

Spatially distributed arrays of piezoelectric disks are being applied to monitor structural integrity using Lamb waves. Applied loads directly affect waves propagating between array elements because of dimensional changes and the acoustoelastic effect. Resulting changes in phase velocity depend upon the propagation direction as well as the Lamb wave mode and frequency. This paper shows from numerical solutions of the acoustoelastic wave equation for an isotropic plate that it is possible to decouple the effects of a homogeneous biaxial stress into its two principal components. As a consequence of both this decoupling and material isotropy, the acoustoelastic response of a specific mode and frequency is described by only two constants, which can be determined from a uniaxial loading experiment. Using this formulation, a method is developed and verified via simulations to estimate an arbitrary biaxial load from phase velocity changes measured along multiple directions of propagation. Results from uniaxial loading experiments on two different plates further demonstrate the efficacy of the method. It is also shown that opening fatigue cracks may significantly degrade results by interfering with Lamb wave direct arrivals, but that this degradation can be mitigated by using a reduced set of data from unaffected paths of propagation.

@article{10.1063/1.4716401,
  author = {Shi, Fan and Michaels, Jennifer E. and Lee, Sang Jun},
  title = {An ultrasonic guided wave method to estimate applied biaxial loads},
  journal = {AIP Conference Proceedings},
  volume = {1430},
  number = {1},
  pages = {1567-1574},
  year = {2012},
  month = may,
  issn = {0094-243X},
  doi = {10.1063/1.4716401},
  url = {https://doi.org/10.1063/1.4716401},
  eprint = {https://pubs.aip.org/aip/acp/article-pdf/1430/1/1567/12076458/1567\_1\_online.pdf}
}

Guided waves propagating in a homogeneous plate are known to be sensitive to both temperature changes and applied stress variations. Here we consider the inverse problem of recovering homogeneous biaxial stresses from measured changes in phase velocity at multiple propagation directions using a single mode at a specific frequency. Although there is no closed form solution relating phase velocity changes to applied stresses, prior results indicate that phase velocity changes can be closely approximated by a sinusoidal function with respect to angle of propagation. Here it is shown that all sinusoidal coefficients can be estimated from a single uniaxial loading experiment. The general biaxial inverse problem can thus be solved by fitting an appropriate sinusoid to measured phase velocity changes versus propagation angle, and relating the coefficients to the unknown stresses. The phase velocity data are obtained from direct arrivals between guided wave transducers whose direct paths of propagation are oriented at different angles. This method is applied and verified using sparse array data recorded during a fatigue test. The additional complication of the resulting fatigue cracks interfering with some of the direct arrivals is addressed via proper selection of transducer pairs. Results show that applied stresses can be successfully recovered from the measured changes in guided wave signals.

@techreport{michaels2012understanding,
  title = {Understanding and exploiting the effects of loading on ultrasonic sensing systems for structural health monitoring},
  author = {Michaels, JE and Michaels, TE and Lee, SJ and Chen, X and Gandhi, N and Shi, F},
  year = {2012},
  institution = {Final Report No. AFRL-RX-WP-TR-2012}
}