J. Hartikainen

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.

AN APPLICATION OF INDUCTION HEATING FOR FAST THERMAL NONDESTRUCTIVE EVALUATION

The application of induction heating as a selective heat source in a thermal nondestructive evaluation system is described. The conductive parts of the sample are heated with a scanning induction coil. Defects such as broken fibers or delaminations affect the heat flow in the sample and thus can be detected by monitoring the infrared radiation from the sample surface. In addition to thermal irregularities, the altered electrical characteristics of the flaws also contribute to the signal. Therefore, the detectability of the defects can be enhanced compared to that of defects investigated using the traditional nonselective heating methods. The functionality of the system is demonstrated with carbon fiber reinforced composites.

Inspection of plasma‐sprayed coatings using fast infrared scanning technique

Fast infrared scanning technique (FIRST), in which a laser beam focused to a line is used for heating and the temperature rise is detected with an IR scanner, is applied to the nondestructive inspection of plasma‐sprayed coatings. The surface temperature profiles are computed numerically for faultless and defective coatings, and the functionality of the technique is demonstrated using a sample with artificial defects.

A fast infrared scanning technique for nondestructive testing

A simple and fast thermal NDT measurement system is described and its usefulness is demonstrated using a honeycomb structure as a test sample. The sample is heated with a hot air jet and the surface temperature differences due to subsurface defects are detected with a single HgCdTe detector. An image of the sample is formed by scanning over the sample surface with a deflection mirror in the y direction while moving the sample in the x direction. The measurement time is typically 6 s per image and several images are averaged to improve signal to noise ratio. The main advantages of this system compared to conventional infrared camera techniques are considerably reduced cost and the ease with which the system can be modified to various applications.

Microscopic Thermal Wave Non-destructive Testing

Techniques des essais non destructifs avec echauffement periodique: cellule photoacoustique, radiometrie photothermique, technique deplacement surface, detection piezoelectriques, deflection du faisceau optique et mesure de reflectance. Impulsion photothermique et methode de balayage: technique balayage laser, methode numerique, technique de flash avec camera IR et technique balayage IR

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>>

Quantitative Thermal Wave Characterization of Coating Adhesion Defects

Throughout the development of thermal wave NDE techniques the most common application has been the imaging of subsurface structure of coated and bulk materials [1]. The use of thermal wave imaging for the detection of coating adhesion defects was first proposed by Luukkala and Penttinen [2], and Busse and Ograbek presented the first experimental results on the detection of artificial adhesion defects of a graphite coating on aluminum [3].

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.

Delamination and Crack Detection by the Synchronous Heating Method: Theoretical Aspects

In recent years a variety of NDT techniques utilizing an IR camera have been developed (for example [1–4]). In the synchronous heating method [3] an IR camera is used for detecting the surface temperature rise caused by a scanning laser beam. The movement of the heating beam is synchronized with the deflection mirror of the IR camera so the distance between the object point of the camera and the heating point remains constant. This measurement set-up allows rapid inspection of thin coatings that are otherwise problematic from the NDT point of view. In this paper we have computed numerically the temperature profile produced by a laser beam when it is scanned over the surface of a defective sample.

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.