Modeling for quantitative non-destructive evaluation

A quantitative approach to non-destructive evaluation (NDE) must be based on models of the measurement processes. A model's purpose is to predict, from first principles, the measurement system's response to material properties and anomalies in a material or structure. For the ultrasonic case a measurement model should include modeling of the generation, propagation and reception of ultrasonic signals, and the ultrasonic interactions that generate the system's response function. A measurement model has many benefits, which are discussed in the paper. Three examples of the productive use of quantitative modeling in conjunction with measured data are presented: the detection and sizing of fatigue cracks which emanate from weep holes in the risers of wing panels in the interior of an aircraft wing by the use of ultrasound generated on the exterior surface of the wing, the determination of the elastic constants of anisotropic thin films deposited on a substrate, and the detection and sizing of surface-breaking cracks by the use of the laser-source scanning technique for laser generated and detected ultrasound.     

Lead-free transducer for non-destructive evaluation

Lead-free piezoelectric ceramics with the formula 0.90(Bi_0.5Na_0.5)TiO₃ - 0.05(Bi_0.5K_0.5)TiO₃ - 0.015(Bi_0.5Li_0.5)TiO₃-0.05BaTiO₃ (abbreviated as BNKLBT-1.5) were prepared by a conventional mixed-oxide method. A disc of this material was fabricated and used to construct an ultrasonic transducer suitable for use in non-destructive evaluation (NDE). Using a laser vibrometer, it was observed that both BNKLBT and PZT exhibited a surface displacement pattern indicative of coupling of the radial mode vibration with the thickness mode vibration. This is consistent with the measurements of electrical impedance vs frequency which showed that for both discs a strong radial mode with many harmonics was clearly observed and many (usually undesired) modes existed near the thickness mode resonance frequency. The discs were mounted in stainless steel housings with appropriate electrical connections to form transducers. Tungsten/epoxy backing was incorporated to provide a very short ring-down time, a characteristic required for many NDE applications. The characteristics of the BNKLBT transducer were quite similar to that of a PZT transducer of similar structure, showing that this lead-free material has the potential to replace PZT in transducers for NDE applications. PACS 43.20.Tb; 43.20.Ye; 43.35.Zc; 43.35.Yb; 43.38.Ar; 43.35.Fx     

A comparison between ultrasonic array beamforming and super resolution imaging algorithms for non-destructive evaluation

In this paper the total focusing method, the so called gold standard in classical beamforming, is compared with the widely used time-reversal MUSIC super resolution technique in terms of its ability to resolve closely spaced scatterers in a solid. The algorithms are tested with simulated and experimental array data, each containing different noise levels. The performance of the algorithms is evaluated in terms of lateral resolution and sensitivity to noise. It is shown that for the weak noise situation (SNR > 20 dB), time-reversal MUSIC provides significantly enhanced lateral resolution when compared to the total focusing method, breaking the diffraction limit. However, for higher noise levels, the total focusing method is shown to be robust, whilst the performance of time-reversal MUSIC is degraded. The influence of multiple scattering on the imaging algorithms is also investigated and shown to be small.     

Terahertz time-domain spectroscopy for explosive imaging

Terahertz (THz) imaging which is a new technology for material classification and nondestructive detection has been extensively investigated in the past decade. The time-domain waveform acquired at each point of the object by using the THz time-domain spectroscopy contains much information about the object. Processing this waveform will present the characters of the object. Several methods are adopted to generate the image of the explosive samples and results are compared and discussed. Experiment results indicate that this new imaging technology can be used for explosive detection.     

Terahertz polarization imaging

We present a new method to measure the polarization state of a terahertz pulse by using a modified electro-optic sampling setup. To illustrate the power of this method, we show two examples in which the knowledge of the polarization of the terahertz pulse is essential for interpreting the results: spectroscopy measurements on polystyrene foam and terahertz images of a plastic coin. Both measurements show a sample-induced rotation of the terahertz electric field vector, which is surprisingly large and is a strong function of frequency. A promising aspect of our setup is the possibility of simultaneously measuring both transversal electric field components.     

Visible and near-infrared light transmission: A hybrid imaging method for non-destructive meat quality evaluation

Visual inspection of the amount of external marbling (intramuscular fat) on the meat surface is the official method used to assign the quality grading level of meat. However, this method is based exclusively on the analysis of the meat surface without any information about the internal content of the meat sample. In this paper, a new method using visible (VIS) and near-infrared (NIR) light transmission is used to evaluate the quality of beef meat based on the marbling detection. It is demonstrated that using NIR light in transmission mode, it is possible to detect the fat not only on the surface, as in traditional methods, but also under the surface. Moreover, in combining the analysis of the two sides of the meat simple, it is possible to estimate the volumetric marbling which is not accessible by visual methods commonly proposed in computer vision. To the best of our knowledge, no similar work or method has been published or developed. The experimental results confirm the expected properties of the proposed method and illustrate the quality of the results obtained.     

Digital shearography versus TV-holography for non-destructive evaluation

This paper reviews and compares two major optical methods for non-destructive evaluation: digital shearography and TV-holography. Digital shearography measures surface displacement gradients whereas TV-holography measures surface displacements. Both methods reveal flaws by looking for flaw-induced deformation anomolies. While both methods enjoy the advantages of being full-field and non-contacting, digital shearography has been proven to be more practical than TV-holography because it employs a simpler set-up and does not require special vibration isolation.     

Terahertz quasi-near-field real-time imaging

A terahertz (THz) quasi-near-field real-time imaging system is presented. Not only the consumption of experimental time is dramatically reduced, but also the resolution of the imaging system is improved to the magnitude of sub-wavelength of THz waves. THz images of a razor blade edge are obtained and the spatial resolution of the imaging system is discussed in detail. For checking the imaging capability of this system, three metallic plates with different sub-wavelength air hole arrays are imaged and the microstructure of these samples can be clearly observed in their THz images. It is believed that the THz quasi-near-field real-time imaging system should have tremendous applications in the THz microscopic field.     

Terahertz phonon polariton imaging

In this article, we primarily review the time-resolved imaging of THz phonon polariton, which is generated by femtosecond laser in ferroelectric crystal. We pay more attention to the imaging in thin crystal, which can be used as an integration platform for terahertz-optics or terahertz-electrics. The imaging techniques, which can get quantitatively in-focus time-resolved images, are introduced in more detail. They have made enormous progress in recent years, and are powerful tools for the research of phonon polariton, optics, and THz wave. We also briefly introduce the generation principle and general propagation properties of THz phonon polariton.     

Multispectral imaging of paintings by optical scanning

In the last few years, there has been a growing interest on multi-spectral imaging as non-destructive technique for painting diagnostics. Providing spectral and colorimetric characterization of the whole paint layer, such a technique is suitable to document the conservation state of an artwork. In this work we present a scanning system for multi-spectral imaging of paintings in the 380–800 nm spectral region, specifically developed to overcome most of the problems related to traditional detection systems based on CCD or Vidicon camera. The device is composed of a spectrometer for contact-less single-point spectral measurements, moved by two orthogonally mounted translation stages. It scans an area of 1 m² with 16 dots/mm² spatial resolution and 10 nm spectral resolution. Spectral reflection factor and tristimulus value measurements were carried out on coloured ceramic tiles and the results were compared to the corresponding certified values. We also present the first application to an ancient painting.     

Chemical, morphological and chromatic behavior of mural paintings under Er:YAG laser irradiation

Several pigments (malachite CuCO₃⋅Cu(OH)₂, azurite 2CuCO₃⋅Cu(OH)₂, yellow ochre (goethite α-FeOOH, gypsum CaSO₄⋅2H₂O), St. John’s white CaCO₃ formed from slaked lime) and respective mural paintings specimens were subjected to the free-running Er:YAG laser radiation in order to study their damage thresholds, in a broad range of laser fluences, both in dry and wet conditions. The specimens’ damage thresholds were evaluated by spectroscopic methods, colorimetric measurements and microscopic observation. The pigments containing –OH groups were found to be more sensitive than St. John’s white; hence the most sensitive paint layers in dry conditions are those containing malachite, azurite (both 1.3 J/cm²) and yellow ochre (2.5 J/cm²) as compared to the ones containing St. John’s white (15.2 J/cm²). The presence of wetting agents (w.a.) attenuated the pigments chemical alteration. The damage thresholds of all the paint layers, in presence of w.a., were found to be around 2.5 J/cm². The alteration was caused by thermo-mechanical damage and by binding medium ablation of a fresco and a secco prepared specimens, respectively.     

Non-invasive investigation of art paintings by terahertz imaging

Terahertz electromagnetic waves propose attractive features such as non-invasive and non-destructive analysis, transparency and good penetration depth through various materials, low scattering and broad spectral bandwidth. In this paper, we demonstrate the capability of terahertz imaging for the investigation of art paintings. The imaging system is able to reveal buried layer information such as a graphite handmade sketch covered by several layers of painting. In addition, taking advantage of the pulsed terahertz emission, we show that it is also possible to evaluate the variations of the painting thickness.     

Pump-probe imaging of historical pigments used in paintings

A recently developed nonlinear optical pump-probe microscopy technique uses modulation transfer to sensitively extract excited-state dynamics of endogenous biological pigments, such as eumelanin and pheomelanin. In this work, we use this method to image and characterize several inorganic and organic pigments used in historical art. We show substantial differences in the near-IR pump-probe signatures from nominally similar pigments and suggest extensions to art restoration.     

复合材料的超声检测与评价

由于复合材料在工业上得到了广泛的应用，使工业界对复合材料的超声无损检测与评价技术提出了更高的要求，在复合材料超声检测与评价中遇到了一些新问题，因而也就有新发现，本文介绍了纤维增强复合材料超声检测与评价技术的某些进展。     

Terahertz interferometric synthetic aperture tomography for confocal imaging systems

Terahertz (THz) interferometric synthetic aperture tomography (TISAT) for confocal imaging within extended objects is demonstrated by combining attributes of synthetic aperture radar and optical coherence tomography. Algorithms recently devised for interferometric synthetic aperture microscopy are adapted to account for the diffraction-and defocusing-induced spatially varying THz beam width characteristic of narrow depth of focus, high-resolution confocal imaging. A frequency-swept two-dimensional TISAT confocal imaging instrument rapidly achieves in-focus, diffraction-limited resolution over a depth 12 times larger than the instrument's depth of focus in a manner that may be easily extended to three dimensions and greater depths.     

Degradation of azurite in mural paintings: distribution of copper carbonate, chlorides and oxalates by SRFTIR

This article illustrates the analysis by synchrotron micro-analytical techniques of an azurite painting presenting greenish chromatic degradation. The challenge of the experiments was to obtain the spatial distribution of the degradation products of azurite. Copper hydroxychlorides, carbonates and copper oxalates have been mapped by SR FTIR imaging of cross sections in transmission mode. To complement the information, Py/GC/MS and GC/MS techniques were applied in order to characterize the binding media and organic materials present as well as their degradation products. Results contribute to a better understanding of the decay of blue areas in ancient paintings not only from the particular point of view of azurite weathering, but also by adding information regarding the oxalates’ formation and their distribution in painting samples. Synchrotron radiation demonstrates its capability for the mapping in painting cross sections.     

Terahertz polarization imaging with birefringent materials

We present a terahertz (THz) imaging method with a birefringent material which is sensitive to THz wave polarization. The subtle THz polarization modulations introduced by the target are extracted by the amplitudes of the two separated THz peaks passing through the birefringent material. The anisotropy of the industrial sprayed-on-foam-insulation (SOFI) is characterized by measuring its azimuthal angle dependent THz polarization response. This work demonstrated that this polarization sensitive THz imaging technique can be used for industrial inspection and biological related characterization.     

Terahertz reflection imaging using Kirchhoff migration

We describe a new imaging method that uses single-cycle pulses of terahertz (THz) radiation. This technique emulates data-collection and image-processing procedures developed for geophysical prospecting and is made possible by the availability of fiber-coupled THz receiver antennas. We use a simple migration procedure to solve the inverse problem; this permits us to reconstruct the location and shape of targets. These results demonstrate the feasibility of the THz system as a test-bed for the exploration of new seismic processing methods involving complex model systems.     

Near-Infrared spectroscopy and hyperspectral imaging for non-destructive quality assessment of cereal grains

Hyperspectral imaging (HSI) combines spectroscopy and imaging, providing information about the chemical properties of a material and their spatial distribution. It represents an advance of traditional Near-Infrared (NIR) spectroscopy. The present work reviews the most recent applications of NIR spectroscopy for cereal grain evaluation, then focuses on the use of HSI in this field. The progress of research from ground material to whole grains and single kernels is detailed. The potential of NIR-based methods to predict protein content, sprout damage and α-amylase activity in wheat and barley is shown, in addition to assessment of quality parameters in other cereals such as rice, maize and oats, and the estimation of fungal infection. This analytical technique also offers the possibility to rapidly classify grains based on properties such as variety, geographical origin, kernel hardness, etc. Further applications of HSI are expected in the near future, for its potential for rapid single-kernel analysis.     

Research on high-T-c SQUID based non-destructive evaluation

A non-destructive evaluation system based on high-T_c dc-SQUID (superconducting quantum interference device) incorporating a gradient field excitation has been built. By using this system a 1mm-diameter hole at a depth of 2mm inside an aluminium plate at room temperature can be easily detected and imaged in an unshielded environment. The relation between the spatial resolution, or the smallest detectable flaw size, and experimental parameters is briefly analysed in terms of a simple metal ring model. The result shows that the spatial resolution depends strongly on the sensor-sample separation as well as on some other parameters, such as signal-to-noise ratio of excitation, excitation frequency and material conductivity.