Silvio E. Kruger

Elastic modulus measurements via laser-ultrasonic and knoop indentation techniques in thermally sprayed coatings

Nondestructive techniques for evaluating and characterizing coatings were extensively demanded by the thermal spray community; nonetheless, few results have been produced in practice due to difficulties in analyzing the complex structure of thermal spray coatings. Of particular interest is knowledge of the elastic modulus values and Poisson’s ratios, which are very important when seeking to understand and/or model the mechanical behavior or to develop life prediction models of thermal spray coatings used in various applications (e.g., wear, fatigue, and high temperatures). In the current study, two techniques, laser-ultrasonics and Knoop indentation, were used to determine the elastic modulus of thermal spray coatings. Laser-ultrasonics is a noncontact and nondestructive evaluation method that uses lasers to generate and detect ultrasound. Ultrasonic velocities in a material are directly related to its elastic modulus value. The Knoop indentation technique, which has been widely used as a method for determining elastic modulus values, was used to compare and validate the measurements of the laser-ultrasonic technique. The determination of elastic modulus values via the Knoop indentation technique is based on the measurement of elastic recovery of the dimensions of the Knoop indentation impression. The approach used in the current study was to focus on evaluating the elastic modulus of very uniform, dense, and near-isotropic titania and WC-Co thermal spray coatings using these two techniques. Four different coatings were evaluated: two titania coatings produced by air plasma spray (APS) and high-velocity oxyfuel (HVOF) and two types of WC-Co coatings, conventional and multimodal (nanostructured and microsized particles), deposited by HVOF.

On measurement of retained austenite in multiphase TRIP steels - results of blind round robin test involving six different techniques

Abstract The present paper reports the results of a blind round robin test dedicated to the measurement of the retained austenite content of different TRIP assisted multiphase steels. Various surface and volume techniques, i.e. light microscopy, X-ray diffraction, electron backscattered diffraction (EBSD), magnetic saturation, thermal diffusivity and laser ultrasonics, were used by different partners. The compiled results show a quite large variability of the estimated retained austenite content, particularly for well established techniques, such as X-ray diffraction (XRD) and magnetisation. On the other hand, emerging techniques like EBSD, thermal diffusivity and laser ultrasonics warrant further investigations.

Lead-free thick piezoelectric films as miniature high temperature ultrasonic transducers

Thick piezoelectric lead-free bismuth titanate films have been successfully deposited on steel substrates with different shapes by a sol-gel spray technique. The center frequencies of ultrasonic signals generated by the bismuth titanate films on steel samples ranged from 5 to 30 MHz and their bandwidth was broad. The ultrasonic signals generated and received by these ultrasonic transducers had a signal-to-noise ratio of 25 dB at 200/spl deg/C in pulse/echo mode. This transducer can be as small as 1 mm in diameter.

Laser Ultrasonic Thickness Measurements of Very Thick Walls at High Temperatures

Laser‐ultrasonics presents many advantages compared to conventional ultrasonics, but is, generally, considered as less sensitive. As a consequence, laser‐ultrasonics should not be adequate for ultrasonic measurements in coarse microstructure materials or measurements of large thicknesses. However, since the generated waves extend to very low frequencies, measurements in such conditions can be successfully performed if a photorefractive interferometer sensitive also to these low frequencies and properly balanced is used for detection. This is demonstrated by measurements of thicknesses up to 100 mm (4″) for various steel grades and at temperatures up to 1250 °C.

Monitoring microstructure evolution of nickel at high temperature

Laser ultrasonics is used to monitor the microstructure evolution of commercial pure nickel from room temperature up to 1000 °C. The objective is to characterize the response of ultrasonic velocity, attenuation and absorption to recovery/recrystallization and grain growth. These parameters are measured during the applied thermal cycle and for different magnetic fields. Below the Curie temperature, the annealed microstructure shows strong magnetoelastic effects. Above the Curie temperature, the ultrasonic attenuation is dominated by grain scattering, allowing the characterization of grain growth. A relationship between the grain size and attenuation at high temperatures is established.

Multi-element, high-temperature integrated ultrasonic transducers for structural health monitoring

This paper reports recent developments on high-temperature, multi-element integrated ultrasonic transducers (IUTs). The multi-element IUTs are fabricated from a sol-gel route, where piezoelectric films are deposited, poled and machined into an array of 16 elements. Electrical wiring and insulation are also integrated into a practical, simple high-temperature assembly. These multi-element IUTs show a high potential for structural health monitoring at high temperatures (in the 200-500°C range): they can withstand thermal cycling and shocks, they can be integrated to complex geometries, and they have broadband and suitable operating frequency characteristics with a minimal footprint (no backing needed). The specifics of multi-element transducers, including the phased array approach, for structural health monitoring are discussed.

Flexible ultrasonic transducers for structural health monitoring of metals and composites

Flexible ultrasonic transducers (FUTs) which have the on-site installation capability are presented for the non-destructive evaluation (NDE) and structural health monitoring (SHM) purposes. These FUTs consist of 75 μm thick titanium membrane, thick (> 70 μm) thick piezoelectric lead-zirconate-titanate (PZT) composite (PZT-c) films and thin (< 5 μm) thick top electrodes. The PZT-c films are made by a sol-gel spray technique. Such FUT has been glued onto a steel pipe of 101 mm in diameter and 4.5 mm in wall thickness and operated up to 200°C. The glue served as high temperature ultrasonic couplant between the FUT and the external surface of the pipe. The estimated pipe thickness measurement accuracy at 200°C is 34 μm. FUTs also were glued onto the end edge of 2 mm thick aluminum (Al) plates to generate and receive predominantly symmetrical and shear-horizontal (SH) plate acoustic waves (PAWs) to detect simulated line defects at temperature up to 100°C. FUTs glued onto a graphite/epoxy (Gr/Ep) composite are also used for the detection of artificial disbonds. An induction type non-contact method for the evaluation of Al plates and Gr/Ep composites using FUTs is also demonstrated.

Laser‐Ultrasonic Characterization of Thermal Spray Coatings

The laser‐ultrasonic technique uses lasers to generate and detect ultrasound and present interesting possibilities for characterizing thermal spray coatings in a non‐destructive manner. In this study, the thickness, density and elastic moduli of very uniform, dense and nearly isotropic TiO2 and WC‐Co thermal spray coatings are evaluated by using laser generated surface Rayleigh waves and surface skimming longitudinal waves. The elastic properties of the layer are obtained via an inverse procedure based on the minimization of the difference between the measured and calculated Rayleigh wave dispersion curves. The results are compared to the values obtained with an optical microscope for the thickness and the Knoop indentation technique for the elastic modulus. The agreement between the techniques is found to be excellent and laser‐ultrasonics appears to be very promising for characterizing such materials. In particular, coatings deposited at high temperature on parts of complex shape could be characterized ...

Laser‐Ultrasonic Characterization of the Annealing Process of Low‐Carbon Steel

Laser‐ultrasonic measurements are performed during the annealing of cold‐worked low‐carbon steel at temperatures between 590°C and 610°C. During annealing, the ultrasound velocity behaves in a peculiar way, first decreasing and then increasing to a final value. The recrystallized fraction evaluated by metallography indicates that recrystallization occurs during both the decrease and the increase in longitudinal velocity. Texture evaluated using laser ultrasonics suggests that the peculiar behavior of the velocity might be due to two different regimes of texture evolution. Internal friction, investigated through ultrasound absorption measurements, is also revealed to be very sensitive to the annealing process. This internal friction is attributed to magnetomechanical effects that are also known to decrease the ultrasonic longitudinal velocity and probably contribute to the observed peculiar behavior of the velocity. From this work, we conclude that, although there remains much work to do to explain our obs...

Laser ultrasonic measurements of scattered waves in steel

A laser ultrasonic system has been used to measure backscattered waves with the intention of evaluating microstructures of steels. The frequency spectrum of the backscattered signal from samples with different grain sizes is evaluated and compared with a simple model that accounts for single scattering and a first order correction to multiple scattering. This model predicts a maximum in the spectrum of the backscattered signal at the frequency near the transition between the single and multiple scattering. Accordingly, the experimental results show that for larger grain sizes, the scattered signal has a lower frequency content. A good quantitative correlation between the grain size and a spectrum parameter is presented.