J. Rantala

Photoacoustic evaluation of elasticity and integrity of pharmaceutical tablets

A nondestructive method based on pulse photoacoustics was applied for evaluation of elasticity and integrity of pharmaceutical tablets. Variations in porosity, density and sodium chloride content of microcrystalline cellulose tablets were found to be related to parameters extracted from the through-transmitted ultrasonic wave forms. By using the amplitudes and ultrasonic velocities of these wave forms, it was possible to obtain values of a transverse to longitudinal amplitude ratio, and also elastic parameters, such as Youngs and shear moduli, for the tablets. Poissons ratio was calculated from the elastic moduli as well as from the amplitudes. An exponential relationship between tablet porosity and the attenuation of longitudinal wave form was noticed. The transverse to longitudinal amplitude ratio and the amplitudinal Poissons ratio were indicative of structural variations, e.g., changes in the porosity and the sodium chloride content of tablets. Youngs and shear moduli of microcrystalline cellulose tablets were found to follow similar porosity trends to those in previously published beam bending and twisting studies, although the absolute values and the values extrapolated to zero porosity were slightly smaller. The Poissons ratio calculated from the experimental Youngs and shear modulus values was also in agreement with earlier studies, but the values extrapolated to zero porosity differed significantly. The method is a promising tool for evaluating the elastic properties of tableting materials and the structural variations in tablets.

Photothermal determination of vertical crack lengths in silicon nitride

In this paper we describe the use of thermal microscopes in measuring the lengths of vertical cracks in silicon nitride. Results of theoretical calculations for temperature profiles in samples near the end of the cracks are demonstrated. These calculations showed that the effect of the crack decreases to a non-measurable level at distances of about 10 μm from the end of the crack. Experimental measurements showed that thermally obtained crack lengths were at least 34% longer than those that were optically measured. The obtained crack length was independent of the heat modulation frequency used.

The effects of experimental parameters in the thermal diffusivity measurements of oriented polymer films using mirage effect: Computer simulation

Computer simulations have been carried out to study the effects of the experimental parameters when the mirage method has been applied to thermal diffusivity measurements of oriented polymer films. The parameters under study are the thermal diffusivity of the fluid surrounding the sample, the modulation frequency and the radius of the heating beam, the height and the radius of the probe beam, and the sample thickness and thermal diffusivity. Proposals for the optimum parameter values to maximize the measurement sensitivity for the sample diffusivity are made and the difficulties arising from the low diffusivity of the samples are described. It is also concluded that because the thermal properties of the fluid surrounding the sample have a strong contribution to the mirage signals, the signals do not include any simple feature corresponding to the sample diffusivity. Therefore it should be determined from the entire measurement data using regression methods.

A measurement method for the determination of the anisotropy ratio of thermal conductivity of plastic foils

An experimental system to determine the anisotropy ratio of thermal conductivity of polymer foils parallel to the sample surface is presented. This kind of measurement is important because the drawing of the foil orients the molecular chains making thermal conductivity parallel to the orientation higher than perpendicular to it. The measurement method is based on heating the foil with a dc‐supplied resistor and detecting the temperature by an infrared detector. The temperature of the foil is measured as a function of distance from the heat source in two separate measurements perpendicular to each other, i.e., rotating the sample 90° between the measurements, and the anisotropy ratio of thermal conductivities in these directions is obtained by using curve fitting. In this article the method has been applied on several plastic foils showing anisotropies ranging from 1 to 2.

An infrared line scanning technique for detecting delaminations in carbon fibre tubes

A thermal nondestructive testing method based on an infrared line scanner for cylindrical specimens is presented. A seven-layered carbon-glass fibre tube with an artificial delamination was used as a test sample. A finite difference computational model of the heating of the tube was constructed in order to simulate the measurement and evaluate the capability of detecting delaminations. The numerical results show the temperature difference caused by the delamination to be well above the practical detection limit, which was verified by the actual measurements.

Photoacoustic inspection of matching layers of ultrasonic air-coupled transducers

The quality of matching layers suitable for air-coupled transducers was studied with an air-coupled photoacoustic microscope. The matching layers studied were silica aerogel and balsa wood. Ultrasonic pulses were used to determine the ultrasound absorption profiles of aerogel-glass and balsa-wood-glass interfaces. The results show a need for developing bonding techniques. The main advantages of the method considered are its noncontacting character and the good control of the bandwidth, the power, and the geometry of the sound beam. The drawback of the method is a fairly long measurement time, because of which the main use of the measurement method would be the development of transducer-manufacturing techniques.<<ETX>>

Observations on Image Formation in the Line Scanning Thermal Imaging Method

Crack detection is one of the major areas of interest in thermal non-destructive testing. Cracks provide a very difficult target for thermal NDT due to several reasons. In thermal methods a heat pulse is usually applied to the surface of an inspected sample and that pulse propagates into the sample in a direction perpendicular to the surface plane. This means that the cracks opening to the surface are not necessarily detected since very little heat is trapped in the surface area around the crack and the infrared detectors are not sensitive enough to detect the difference in the surface temperature. Naturally, not all the cracks reach the surface, making the detection process even more cumbersome. Finally, the cracks are often extremely narrow, contributing only a very small distortion to the thermal characteristics of the sample.

Determination of thermal diffusivity of polymers by means of multivariate calibration of mirage data

In this paper we describe how multivariate calibration techniques including experimental design and partial least‐squares regression, PLS, can be used to determine thermal diffusivity from mirage data. In order to illustrate the utility of the theory a case study is presented where photothermal mirage data were simulated for a series of polymeric film samples. A PLS model was calculated that predicts thermal diffusivity with high precision.

Fast Photothermal Inspection of Plasma-Sprayed Coatings of Primary Circulation Seal Rings of a Nuclear Reactor

A need for NDE of plasma-sprayed coatings has been a serious problem in the industry for a long time. Traditional methods like ultrasonics or x-ray absorption usually cannot be used because of the high attenuation and the heterogeneity of the coating. On the other hand, sub-surface flaws cannot be detected with liquid penetration technique and electromagnetic methods are not suitable for dielectric coating. However, photothermal techniques have been successfully applied [1,2], but first the introduction of fast infrared scanning systems has lead to reasonable speed of inspection from the practical point of view.

Fast Photothermal Inspection of Plasma-Sprayed Coatings of Primary Circulation Seal Rings of a Nuclear Reactor. Part Two: After the Trial Run

In the 1991 QNDE conference [1] we described a photothermal inspection of the plasma-sprayed coatings of two seal rings used in the main pump of the primary circulation in the PWR-type nuclear reactor. The measurements concentrated on detecting the most critical flaw type, adhesion defects at the interface between the coating and the substrate. The samples were tested immediately after they were coated and lapped, and already then two thermal anomalies could be found.