Thickness measurement ultrasonic methods

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. 

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. 

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. 

Tsukahara, Y., Takeuchi, E Hayashi, E., and Tani, Y. (1984). A new method of measuring surface layer-thickness using dips in angular dependence of reflection coefficients. IEEE 1984 Ultrasonics Symposium, pp. 992-6. IEEE, New York. [214] Tsukahara, Y Nakaso, N., Kushibiki, J., and Chubachi, N. (1989a). An acoustic micrometer and its application to layer thickness measurements. IEEE Trans. UFFC 36, 326-31. [213-215]... 

The Dow Freeport in-service inspection procedures are similar to those reported earlier in this chapter. The Dow out-of-service (internal) inspection includes ultrasonic thickness measurements at all benchmark locations. Other test methods include shear wave ultrasonics, eddy current, and radiography. Engineers use ultrasonic thickness readings to project the remaining useful life of the vessel and to determine when the next internal inspection should be scheduled. 

Ultrasonic correlation analysis in frequency (Fourier transformation of frequency) domain analysis was utilized to measure a thickness of the sample and to image the structure of the material. This technique comprises four processes (1) calculation of the spectrum, (2) division by the power spectrum of a pulse or other component, (3) Fourier transformation into the frequency domain, and (4) analysis and imaging in the frequency domain. Here we obtain much higher resolution in the imaging and thickness measurements by applying the echo analysis developed in earthquake theory [12] and the thickness measurements methods for a thin layer [13,14]. 

Ultrasonic methods There exist a variety of methods which are used widely in the aircraft industry. The technique measures changes in acoustic impedance caused by defects in a bonded assembly when an ultrasonic transducer is liquid-coupled to it. The high frequency sound waves are simply scattered by the presence of porosity or voids in the bondline. Interpretation of the data can be difficult, although substrate thickness is not generally a limitation. 

Physical Methods Ultrasonic thickness measurement for Metal Loss Radiography... 

In general, the most widely used field technologies for inspection are ultrasonic thickness measurement, while for online methods they are corrosion test specimens, electrical resistance, and linear polarization probes. Both of the inspection methods and the first two online methods measure metal loss. The last method measures corrosion rate, but only in a sufficiendy conductive process environment, normally water. 

Ultrasonic Measurements—The ultrasonic thickness measurement method is quite popular for in-service corrosion testing. The major advantage of this method is that the equipment is portable and very easy to use. The major disadvantage is that a bare metal surface is required for accurate measurements. The presence of coatings and/or corrosion products can introduce errors into the thickness measurements [69]. Curved surfaces such as piping bends and small tubing diameters require special attention. 

PVDF exhibits pyroelectric as well as piezoelectric properties [44], a feature that has made the material useful for infrared sensing in motion detectors and thermal cameras. This attribute has been used recently in a thermally based method to measure ultrasonic power [45]. A 52 pm thick film of PVDF 60 mm in diameter was used as the sensor, which was backed with a rubber based material that generated heat due to ultrasonic absorption. Powers up to 1 W over a frequency range of 1-3.5 MHz were measured in this preliminary study. 

For the repetitive inspections the required hydrotest can only be performed for a limited number of the small cylinders, and even then the drums have to be removed from the line and the cylinders will be supported in defined distances for the weight of the water and the pressurisation. For the new and long cylinders even this is impossible, because they loose due to the additional weight of the water and the over-pressurisation their roundness and balances. Therefore the law in the most countries within and outside of the EU accept as a replacement of the hydrotest an additional application of different NDT methods, which were often done by an ultrasonic measurement of the wall thickness of the cylindrical part and a MT of the flat covers. 

Ultrasonic techniques are an obvious choice for measuring the wall thickness. In the pulse-echo method times between echoes from the outer and inner surface of the tube can be measured and the wall thickness may be calculated, when the ultrasonic velocity of the material is known. In the prototype a computer should capture the measuring data as well as calculate and pre.sent the results. First some fundamental questions was considered and verified by experiments concerning ultrasonic technique (Table I), equipment, transducers and demands for guidance of the tube. 

The University developed a method of determination of the material residual strength, based on measurement of the change of phase velocity of ultrasonic waves, as well as an ultrasonic flaw detector-tomograph with multi-element transducers of the type of phased acoustic array. It enables control of the internal structure of materials and products of up to 300 mm thickness, with the resolution of up to 0.5 mm. In the same university, work on NDT is also carried out in the welding and electro-acoustic departments. 

Ultrasound can be used to make precise measurements of the thickness of materials [2]. An ultrasonic transducer is pressed against the side of a material and the time taken for a pulse to travel across the material and back is measured (Figure 10). If the velocity of ultrasound in the material is known then the distance can simply be calculated 2d = ct. Ultrasound is particularly useful for measurements on materials which are difficult to access by conventional methods e.g. the determination of the thickness of a pipe when access is only available to the exterior of the pipe. It can also be used to measure the thickness of individual layers in multilayer systems (Figure 10). 

Recent developments have extended the ultrasonic techniques to the characterisation of thin layers of metals and polymers deposited on substances to obtain measurements of the thickness/density product. Using techniques where the film are immersed in a fluid, such as water, measurements have been made, by the low frequency normal incidence double through-transmission method, with film thickness ranging from 20 to 200 pm [112] a range which is of particular relevance to membrane systems. 

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