R. Lehtiniemi

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.

Thermoelastic Sound Source: Waveforms in a Sensing Application

Photoacoustically generated sound pulses are widely used in various NDT, NDE and sensing applications when a non-touching method is preferred. The generation mechanisms are relatively well known, including types of waves generated, directional patterns, sound pressures and damage thresholds for the laser intensity [1]. The so-called thermoelastic regime is attractive to many applications despite of its low efficiency (usually about sub 0.1%). It is because that the process is nondestructive to samples and the theory is well established [2,3,4]. The current study addresses the prediction of the temporal ultrasound pulse shape of an optimum sound generation scheme using a low power diode pumped high repetition rate Nd:YAG pulse laser [5]. A model is proposed in which the radiation from the thermoelastic sound source is treated as an instantaneous piston source at the solid-fluid interface.

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.

Transferable Measurement System for Fast Non-Destructive Evaluation

Our transferable system consists of a remote unit and a hand held IR measurement head that can be brought to the object under inspection. The object is heated with a laser beam focused on a line and the surface temperature of the sample is monitored with an IR line scanner. The compact size of the measurement head is achieved because of the use of an optical fibre, which allows the placement of the heating laser in the remote unit. The measurement head is moved over the inspected area and the results are displayed as pseudo-color images on the screen of a microcomputer. The equipment is designed for industrial maintenance applications.

A Numerical Study on Temperature Distribution of Line Heated Anisotropic Carbon Fiber Composites

Earlier we have described the various uses of infrared line scanner based thermal nondestructive testing equipment [1]. Time constants of measurements made with these kind of equipment are very suitable for testing carbon fiber composites. Scanning a line heat source over a sample surface causes a nonuniform temperature distribution in the sample. In addition to the heat flow normal to the surface, lateral heat flow exists in the surface plane. In the case of carbon fiber composites with a specific oriented structure, the surface temperature distributions depend on the direction where the line source moves. Generally, this is true of any sample having anisotropic thermal conductivity. In oriented carbon fiber composites the bulk thermal conductivity can be considered anisotropic, because the heat transfer in the composite is different in the direction of the fibers compared to perpendicular directions [2,3]. Varis et al. have discussed these phenomenon briefly with the testing of carbon fiber tubes using numerical methods [4]. Here, we represent a more detailed numerical analysis of the effects of line heating on a sample having anisotropic thermal conductivity.

Photothermal Inspection of Impact Damages on Carbon-Glass Fibre Composites Using a Simple Hand Scanner Measurement System

Composite plates having durable and lightweight structure are most interesting materials considering several applications. Because durability is a major criteria, it is important to develop methods for finding flaws that affect this quality. In the case of carbon-glass fibre composites we have earlier demonstrated the suitability of radio frequency (RF) induction heating combined with IR-detection [1]. Based on the good experiences obtained with the method, a suitable equipment for field experiments was developed. Last year we showed the feasibility of a transportable thermal NDT equipment using laser line heating [2]. Modifying this measurement system for RF induction heating, several disadvantages of laser heating can be overcome.

Induction Heating as a Selective Heat Source in Fast Thermal Non-Destructive Evaluation

The application of radio-frequency induction heating as a selective heat source in fast thermal non-destructive testing system is described. The sample is heated through a scanning induction coil and the increased infrared emission from the surface of the sample is monitored by a line scanner. Because only conductive materials warm up in the induction field, dielectrics and conductors can be clearly distinguished in the thermal images. Both the spatial resolution and the selectivity of the system are demonstrated with test samples.

Photothermal Length Measurement of Vickers Cracks in SI3N4

Ceramic materials are gaining ever increasing popularity in different high—technology applications, especially in those where high temperatures are used. However, the mechanical strength of ceramics has thus far set limitations on their utilizing. The critical size of cracks in ceramic material depends on the force applied, but in typical applications it is less than 100 µm. This small size in addition to the fact that critical cracks are often closed makes the use of conventional NDT methods in crack detection very difficult or even impossible. More nuisance comes from the porosity of the material which takes methods needing immersing in liquids out of consideration.

Detection of Vertical Cracks in Ceramic Materials by the Mirage Effect

Results of numerical calculations of the temperature distribution near the end of a closed, semi-infinite vertical crack are presented with a comparison of optical and thermal length measurements of vertical cracks made by Vickers indentations in Si3N4.

Photoacoustic evaluation of aerogel-glass interfaces

Photoacoustically generated air-ultrasonic pulses were used to determine the ultrasound absorption profiles of aerogel-glass interfaces. The results show the expected lack of homogeneity of such interfaces.