Emmanuel Lafond

Evidence of surface acoustic wave band gaps in the phononic crystals created on thin plates

Phononic structures and acoustic band gaps based on bulk materials have been researched in length in the past decades. However, few investigations have been performed on phononic structures in thin plates to form surface acoustic wave (SAW) band gaps. In this letter, we report a new type of phononic crystals manufactured by patterning periodical air-filled holes in thin plates. We confirmed the existence of SAW band gaps in the created phononic crystals through laser ultrasonics measurements. Wide multiple SAW band gaps and special structures, such as narrow pass bands within a band gap were observed experimentally.

On-machine characterization of moving paper using a photo-emf laser ultrasonics method

Stiffness properties of paper materials can readily be characterized in the laboratory using conventional ultrasonic techniques. For on-line inspection on a paper machine, due to the high translation velocity and the somewhat fragile nature of the moving paper web, contact ultrasonic techniques using piezoelectric transducers are of limited use. To overcome this limitation, non-contact laser- based ultrasonic techniques can be used. Due to the rough surface of the paper, the reflected light is composed of many speckles. For efficient detection, the receiver must be able to process as many speckles as possible. Adaptive receivers using the photorefractive or photo-emf effects are characterized by a large etendue, and thus, are well suited for detection on paper and paperboard. Moreover, the translation velocity of the moving web implies that the detection system must adapt extremely quickly to the changing speckle pattern. In this work, a photo-emf receiver was used to detect Lamb waves excited using a pulsed Nd:YAG laser in moving paper. Experiments were performed using a variable-speed web simulator at speeds much higher than 1 m.s-1. Results corresponding to various translation speeds are shown, demonstrating the feasibility of laser- based ultrasound for on-machine inspection of paper and paperboard during production.

Application of a two-layer semi-analytical model for the improvement of laser-ultrasonic generation

Abstract The new laser-based ultrasound model presented in this paper takes into account the layered structures generally encountered in aeronautic materials (painted metals, composites, lapped joints…) which are subjected to impacts, fatigue and corrosion. The structure is supposed to be a plate made of two layers of different materials. The sample presents a cylindrical orthotropy. Validations of the model are performed by comparing the results it provides with experimental measurements using a Nd:YAG pump laser and an interferometric detection. The symmetry of the model allows fast calculation and prediction of the displacements over a long time period. These advantages are used to study the improvement of the laser-ultrasonic generation, by varying the thickness of the first layer in the case of a NDT strippable paint layer covering a metal layer.

Noncontact determination of elastic moduli by two-dimensional Fourier transformation and laser ultrasonic technique

A laboratory instrument that utilizes broadband laser ultrasonics and two-dimensional Fourier transformation for signal processing has been developed to characterize the properties of various foils and plates. Laser ultrasonics generation is achieved by using a pulsed laser which deposits pulsed laser energy on the surface of the specimen. The displacement of the resulting broadband ultrasonic modes is monitored using a two-wave mixing photorefractive interferometer. By means of the two-dimensional Fourier transformation of the detected spatial and temporal displacement wave forms, the image of density of state (DOS) for the excited ultrasound is obtained, and from it the materials properties are extracted. Results are presented for a 150μm thick paper sample, a 50μm stainless steel foil, and a 1.27mm thick aluminum plate. The DOS image demonstrates the ability to measure the properties of each generated ultrasonic modes and provides a direct, nondestructive, measure of elastic moduli of the tested specimens.

Laser-ultrasonics semi-analytical model for two-layered samples

The early detection of corrosion in aging structures is probably one of the most important challenges of the aeronautic maintenance services. Laser-ultrasonics offers interesting characteristics to become an industrial technique able to solve this problem. Neverless, to become quantitative, this non-destructive method requires a precise description of the laser-ultrasonic generation. This paper presents a new and original model which takes into account the layered structure which is generally encountered in aeronautic materials subjected to impacts, fatigue and corrosion. This model solved the Christoffel equations in an axisymmetrical configuration over an infinite plate of finite thickness presenting a cylindrical orthotropy. The sample is a flat plate made of two layers of different materials and the laser impinges the sample normally to the surface. The method of resolution used allows fast calculation and observation of the displacements over a long time period. This is very useful in NDT, especially in the case of thick samples. Validations were conducted by comparing the results calculated by this model to the ones obtained with a previous model and with experimental measurement using a Nd:YAG pump laser and an interferometric detection.

Noncontact characterization of static paper materials using a photorefractive interferometer

Laser-Based Ultrasound (LBU) systems are now entering their maturity years by penetrating the factory in both the areas of non-destructive testing and process control. A LBU system can be used for the on-line characterization of a paper web in a paper mill. Compared to contact techniques, LBU is able to generate and detect on a paper web both symmetric and antisymmetric waves with a non-contact tool which is the spot of the generation laser. This provides all at once a rich amount of data about the stiffness properties of the sheet. To demonstrate this concept we made some experiments on static paper first, our ultimate goal being to monitor the stiffness properties of a paper web, on-line, at industrial speeds. A photorefractive interferometer using the two-wave mixing method with a continuous electric field applied to a BSO crystal has been built for ultrasound detection. Results are presented on different paper grades, using a Nd:YAG laser for generation. Both A0 (anti- symmetric) and S0 (symmetric) modes of Lamb waves have been detected with acceptable signal to noise ratio in single laser shot. The dispersive nature of A0 wave is clearly visible as well as the higher frequency content of S0 wave.

Laser ultrasonic in‐process inspection of paper for elastic properties

A laser‐ultrasonic (LUS) sensor has been developed that allows measurement of the bending stiffness (BS) and shear rigidity (SR) of paper and paperboard as it is being made on the papermaking machine. A prototype system was recently tested in a paper mill at web speeds up to 5000 ft/min with excellent precision and accuracy. The LUS technique performs well on paper and board with basis weights up to 130 g/m2. Several laboratory methods exist for measuring the bending stiffness in small samples of paper and board. Currently, no commercial method exists for nondestructively measuring this property on the papermaking machine at production speeds. Commercial instruments using contact transducers measure ‘‘tensile strength orientation’’ (TSO) on heavier boards, where marking of the sheet by the contact transducers is not of concern. Unlike contact ultrasonic techniques, LUS does not visibly mark even the lightest grade papers. Contact ultrasonic measurements correlate approximately to the tensile strength of t...

Laser Ultrasonic Measurement of Elastic Properties of Moving Paper: Mill Demonstration

An automated sensor has been developed for use during paper manufacture that can measure flexural rigidity (bending stiffness). Based on laser ultrasonic technology, this sensor provides continuous noncontact on‐machine measurements on paper having area densities from 35 to 205 g/m2, moving at commercial manufacturing speeds, at any angle in the plane of the sheet. It was demonstrated on a high speed printing paper grade machine during commercial production. For that demonstration, the sensor was integrated into an existing scanning sensor system. Cross‐direction profiles of flexural rigidity had the expected shape, and compared well with traditional bending stiffness measurements on samples collected for that comparison.

A Laboratory Laser‐Ultrasonic Instrument for Measuring the Mechanical Properties of Paper Webs

For the paper industry, stiffness properties are an important parameter for producing more efficiently a fibrous material like paper. Some stiffness properties of paper webs can be obtained in a non‐contact fashion using two lasers. The authors have developed an automated laboratory laser‐ultrasonics instrument for paper, described here. The results of non‐contact laser generation and detection of ultrasound are also presented. The paper grades investigated were heavy grades like linerboard, as well as copy paper.

An automated instrument for the measurement of mechanical properties of thin materials

Flexural rigidity and shear rigidity of sub-millimeter thin webs can be determined by analysis of the zero order antisymmetric (A0) Lamb wave in the MHz range, even when the web thickness is unknown. Using Laser Ultrasonics to excite and detect Lamb waves in webs is advantageous because it is a non- contact technique suitable to both on-line and off-line measurements. The authors have developed an interferometer using an undoped Gallium Arsenide photorefractive crystal. They employed a two-wave mixing technique that is sensitive to displacement amplitudes in the nanometer range. The interferometer is part of a laboratory instrument in which both the generation and the detection laser beams are carried through fiber optics. The instrument uses several motorized translation stages to manipulate the position and orientation of the web sample. The instrument is computer-controlled and fully automated using LabVIEW for routine testing of foil samples. The results of measurements demonstrating the wide bandwidth of the instrument and the dispersion of the waves are presented. The probed materials were as different as paper, paperboard, non-woven and brass foils.