Thickness, measurements

In order to reply to some particular cases of NDT such that the detection two close defects and the fine thickness measure, we have developed techniques adapted to this type of problem. 

The apparatuses working on the TCMC prineiple can find the application in thickness measuring, tomography and other areas of the NDT. 

The corrosion inspection and wall thickness measurement of pipes was performed in the classical way both on film and on the monitor using simple software measuring tools. Additionally algorithms were developed for an interactive, computer supported evaluation. 

Projection radiography is widely used for pipe inspection and corrosion monitoring. Film digitisation allows a direct access to the local density variations by computer software. Following to a calibration step an interactive estimation of local wall thickness change based on the obtained density variation is possible. The theoretical model is discussed, the limitations of the application range are shown and examples of the practical use are given. The accuracy of this method is compared to results from wall thickness measurements with ultrasonic devices. 

Applicability of Computed Radiography (CR) Corrosion and Wall Thickness Measurements, P. Willems, B. Vaessen, W. Hueck, U. Ewert, U. Zscherpel, Y. Onel, Poster at the 7 ECNDT Conference, Copenhagen 1998... 

The second example concerns a thickness measurement problem. Normally this is an easy task for ultrasonic testing However,... 

However, this probe design is dedicated to a special turbine blade type and a special inspection task on this blade. But similar probe types and probe holders can be provided also for other engine types and for other inspection tasks like wall thickness measurement and crack detection in other zones of a blade. 

In general a thickness measurement using ultrasound is done by measuring the time of flight of the ultrasonic pulse and calculating the thickness of the objeet multiplying the time and the known constant sound velocity in the material. 

The figures 9 and 10 show the A-Scans of these two steps. Figure 9 presents the velocity measurement of the longitudinal wave parallel to the surface in the first step and figure 10 presents the A-Scan of the thickness measurement in the second step. 

This automatic velocity measurement method (AUTO-V) has also been suecessfijlly applied to other materials sueh as non-ferrous metals and even eeramics and plasties. The only requirement for this type of sequential velocity and thickness measurement is a certain isotropy of the material materials having anisotropie properties will lead to incorrect thickness readings due to the velocity difference in the two orthogonal directions. 

Based upon a piezoelectric 1-3-composite material, air-bome ultrasonic probes for frequencies up to 2 MHz were developped. These probes are characterized by a bandwidth larger than 50 % as well as a signal-to-noise ratio higher than 100 dB. Applications are the thickness measurement of thin powder layers, the inspection of sandwich structures, the detection of surface near cracks in metals or ceramics by generation/reception of Rayleigh waves and the inspection of plates by Lamb waves. 

In wide sectors of industry there is a growing need of inspection methods which go without liquid coupling media. The excitation of bulk and surface waves by means of air-coupled ultrasonic probes is therefore an attractive tool for NDE. This is tme e.g. for the rapid scanning of large composite structures in the aerospace industry [1]. In other cases, the use of liquid couplants is prohibitive like the thickness measurement of powder layers. 

Of our special interest is the thickness measurement of powder coatings. While the sound velocity of the electrostatic applied powder/air mixture is estimated to be two times the velocity in air it is also an estimation that thickness powder / air layer is reduced by a factor of 5 by smelting (burning in process, hardening). 

The required resolution for the powder coating thickness measurement problem is about 5 microns. Therefore a processing time unit of 0.08 ps (12.5 MHz) seemed to be ok (compare line 2 in table 1). First results based on real measurement are documented in Fig. 7. and are very promising result. Today corresponding patent is pending [7]. 

S. Whiteley, Gr. Hayward and D. Rilly. Thickness measurement using air-coupled ultrasound... 

Table 1. Three different ultrasonic measuring techniques for thickness measurement.

Electrical noise (from the electronic control of the motors in the drawing bench) influenced the wall thickness measurements. 

Now days the devices operating in the radiowave range are designed and they used for oil film thickness measurements and for the oil spills volume evaluation. The device operating on the frequencies from 37,5 to 10,7 begHz provides the measurements of the film thickness in the range from 100 to 6 — 7 pm. It means that all accident happening on the seas surface may be estimated. 

In addition, NDT plays an important part in industrial maintenance. During plant shutdowns for instance, many thousands of ultrasonic wall thickness measurements are taken on piping, vessels, furnace tubes etc. All these thickness readings have to go into extensive data bases, and this process is, thanks to modem computers and data loggers, ever more automated. 

Studying modem approaches for such schemes, one can see that knowledge of operational conditions and potential degradation mechanisms play a prominent role. Surprisingly, the role of NDT is often limited to tlie use of conventional methods such as ultrasonic wall thickness measurements, ultrasonic inspection, radiography, and last but not least visual inspection. 

INCOTEST for instance caimot only measure through insulation, but also in direct contact. This makes it a method for wall thickness measurements to... 

MFL is currently being used for the inspection of hundreds of kilometres of piping in the Oman desert, see figure 5., whereby an inspection rate of one kilometre per day is easily achieved. Try to realise what effort would have been needed if this would have been done with conventional ultrasonic wall thickness measurements. 

In order to maintain Class and to comply with international legislation a ship must undergo surveys at prescribed intervals. Ships such as tankers and bulk carriers must comply with International Maritime Organisation resolutions which require a Class Enhanced Survey Programme. This programme includes Special Surveys at five yearly intervals with close-up visual inspection and thickness measurements to be made at specified locations, the number of which increases with the age of the ship. 

Under this programme 25% of the stiffeners and adjacent plate material in bulk carriers will be examined with thickness measurements being taken at upper, lower and mid points in both face and web plates of the stiffeners and associated end brackets. The extent of thickness measurement applied to transverse bulkheads has also been increased. In addition. Classification has revised the scantling requirements for new buildings to nominate thicker material in the bottom structure and at the transverse watertight bulkheads of the No 1 cargo holds. 

Zinc coatings are covered by an ASTM specification (143). Because of the varying purity of 2inc deposits from chloride and 2incate baths, some thickness measuring methods can vary considerably (39). Methods based on physical measurement of magnetic methods (144,145) are used for best accuracy. 

Defect imaging and characterization atomic arrangements in amorphous materials and multilayers concentration profiles with depth film thickness measurements... 

---