GB2139756A - Component inspection by acoustic emission detection - Google Patents

Abstract

A method of inspecting a component 10 in which the component is continuously vibrated at such a frequency or frequencies that any cracks or fractures within the component 10 generate acoustic emissions. The acoustic emissions are detected to provide evidence of the existence of any cracks or fractures. A variable frequency may be used, and the spectrum of the exciting vibrations at which acoustic emissions are produced may be compared with a standard. The method may be applied to a mould 10 for costing cooled turbine aerofoil blades in order to detect cracks in quartz rods 11 forming part of the mould. <IMAGE>

Description

SPECIFICATION Component inspection by acoustic emission detection This invention relates to the inspection of components by acoustic emission detection.

In the non-destructive testing of components it is difficult to detect internal abutting surfaces such as cracks and fractures which cannot be revealed by a visual examination of the component. Moreover, if in the case of the abutting surfaces which are cracks and fractures, they have been caused by vibration, it is diffucult to isolate the particular mode of vibration which initiated them. It is known to detect such cracks and fractures by the techniques of X-ray photography and ultrasonic scanning but each sometimes provides difficulties in the interpretation of the results which it provides. Moreover potential health hazards are involved with the use of X-ray photography.

It is an object of the present invention to provide an improved method of inspection of a component which method is particularly useful in the detection of hidden cracks and fractures within the component and in their failure analysis if such cracks or fractures were caused by vibration.

According to the present invention, a method of inspecting a component comprises continuously vibrating said component at such a frequency and amplitude or over a range of frequencies and amplitudes as to cause fretting between any abutting surfaces within said component, so that said fretting generates acoustic emissions within said component and detecting said acoustic emissions at frequencies other than that of those at which said component is continuously vibrated to provide evidence of the existence of any such abutting surfaces.

Acoustic emission is generally understood to be transient high frequency broad band stress more activity which travels through a material and can be detected by a suitable transducer positioned on or adjacent to the material. Acoustic emission are generated by mechanisms, such as straining of the material, which give rise to a iocal rapid change in stress within the material. This is a phenomenom which is commonly associated with such things as crack growth, plastic deformation or phase transformations in materials. Acoustic emissions are generally detected in frequency ranges which are beyond the normal audible spectrum and are thus less prone to interference by extreneous noise. Consequently acoustic emissions at low levels may be readily detected without severe ambient noise problems arising.

The component is preferably vibrated over a range of frequencies and amplitudes in order to ensure that it is vibrated at the appropriate frequency or frequencies and amplitudes which cause any such abutting surfaces within it to generate said acoustic emissions.

The frequency or frequencies which give rise to any said acoustic emissions are preferably determined to provide an indication of the vibrational frequency or frequencies which initiated any such cracks or fractures.

The invention will now be described, by way of example, with reference to the accompanying drawings in which: Figure lisa sectioned side view of a mould incorporating a number of quartz rods which mould is suitable for inspection by the method of the present invention.

Figure 2 is a graph indicating the acoustic emissions of the mould shown in Figure 1 when vibrated and in which none of the quartz rods are cracked or fractured.

Figure 3 is a graph similar to that shown in Figure 3, but indicating the acoustic emissions of the mould shown in Figure 1 when one of the quartz rods is broken.

With reference to Figure 1, a mould generally indicated at 10 is intended for the casting of a cooled turbine aerofoil blade. Cooled turbine aerofoil blades contain a number of passages through which cooling air flows. These passages are defined in the mould 10 buy a plurality of quartz rods 11,the ends of which are embedded in the ceramic shell 12 of the mould 10.

The mode of construction of the mould 10 is such that when assembled, the quartz rods 11 are contained wholly within it and therefore impossible to inspect visually. This being so, if any of the quartz rods 11 is cracked or fractured, the problem will not be detected until the mould is actually used to cast a blade. By this time, of course, the faulty rod 11 will have given rise to a correspondingly faulty blade.

In order to determine whether the mould 10 contains a cracked orfractured quartz rod 11, it is mounted on interconnected upper and lower supports 13 and 14 respectively which facilitate the vibration of the mould 10. The upper and lower supports 14 and 13 are attached byway of beam 15 to a source of vibrational energy (not shown). The upper support 14 also has a hole 16 passing through it which permits the entry of a microphone 17 into the mould interior 18.

In operation, the source of vibrational energy continuously vibrates the mould 10 via the beam 15 and supports 14 and 13 over a range offrequencies up to 800 Hz. The microphone 17 detects the response of the mould 10 to this vibration at frequencies above 50KHz and provides an output to conventional measurement equipment (not shown).

Various components of the mould 10 provide acoustic emissions as a result of this vibration and it is these acoustic emissions which are detected by the microphone 16. The relationship between the acoustic emissions and the frequency of mould 10 vibration is shown in the graph of Figure 3. In the graph of Figure 3 the axes are calibrated in frequency Hz and the root mean square of the acoustic emission level at frequencies above 50KHz.

It is clearfrom Figure 3 that there are acoustic emissions at around 250Hz and above 600 Hz. These are the bow string frequencies of the quartz rods 11.

The mould 10 tested in this particular instance was subsequently found to have no broken quartz rods 11. However in Figure 4 there is shown a graph similar to that shown in Figure 3 but indicating the response obtained with a mould 10 which was subsequently found to have a fractured rod 11. It is clear from Figure 4thatthe defective mould 10, as well as providing acoustic emissions resulting from thefundemantal bow string modes of vibration of the quartz rods 11, also provided a variety of other acoustic emissions emanating from the fractured rod 11.

Moreover it will be seen that the acoustic emissions are detected at frequencies above 50KHzwhich is above those at which the mould is vibrated. Thus generally speaking when an object is continuously vibrated at a given frequency and amplitude or over a range of frequencies and amplitudes, acoustic emissions generated by abutting surfaces within the object are at frequencies which are additional to those at which the object was vibrated.

Thus it is possible, by exposing the mould 10 to a range of vibrational frequencies and monitoring the resultant acoustic emissions therefrom to distinguish a mould 10 with afractured rod 11 from a mould 10which has non-fractured rods 11.

If it is desired to inspect a large number of similar moulds 10 in the manner described above, it will be appreciated that it would not be necessary to expose each mould 10 to such a large range of vibrational frequencies. Thus each mould could be vibrated at a single frequency which is known to not provide acoustic emissions in good moulds 10 but which does provide acoustic emissions from moulds 10 which have fractured quartz rods 11.

The method of the present invention is not restricted to the inspection of moulds 10 but can be utilised for other purposes. Thus the method is useful in the detection of internal cracks in components. For instance, the method of the present has been used to detect an internal crack in a gas turbine engine combustion liner. Other examples of components which can be inspected by the method of the present invention include composites which can be inspected for delamination and bonded components which can be inspected to determine the integrity of the bonding. Microphones are not usually utilised in the inspection of such components. It is preferred that a component surface contacting transducer is used instead. It will be appreciated however that other types of transducers could be used in appropriate circumstances.

The method of the present invention, as well as being useful in the inspection of components, is also useful in the failure analysis of components. Such failure analysis depends upon the fact that if a component is cracked as a result of being exposed to vibration, then the crack will generally provide an acoustic emission if the component is exposed to vibration at the same frequency as that which originally caused the cracking. Consequently if a vibration-cracked component is exposed to a range of frequencies and a similar, but uncracked component is exposed to the same range, the determination of the differences in their acoustic emissions provides evidence of the vibrational frequencies which initiated the cracking. This technique has been used successfully in the determination of the vibrational frequency of a source of fatigue cracking in the combustion liner of a gas turbine engine.

Although the present invention has been described with reference to the inspection and failure analysis of gas turbine components, it will be appreciated that it is also applicable to the inspection and failure analysis of other components which may have abutting surfaces. Moreover it is particularly useful in the inspection of components in situ. For instance it could be used in the in situ inspection of hollow gas turbine engine fan blades. Such hollow fan blades contain a honeycomb material which is brazed to the internal surfaces thereof. The method of the present invention is particularly useful in the testing of the integrity of the brazed joints.

Claims (6)

1. A method of inspecting a component comprising continuously vibrating said component at such a frequency and amplitude or over a range of frequencies and amplitudes as to cause fretting between any abutting surfaces within said component so that said fretting generates acoustic emissions within said component and detecting said acoustic emissions at frequencies other than that or those at which said component is continuously vibrated to provide evidence of the existence of any such abutting surfaces.

2. A method of inspecting a component as claimed in claim 1 or wherein the frequency or frequencies which give rise to any said acoustic emissions are determined to provide in the event that said abutting surfaces are defined by vibration induced cracks or fractures an indication of the vibrational frequency or frequencies which initiated any such cracks or fractures.

3. A method of inspecting a component or components as claimed in claim 1 or claim 2 wherein said any acoustic emissions are detected by means of a micorphone.

4. A method of inspecting a component as claimed in claim 1 or claim 2 wherein said any acoustic emissions are detected by a component surface contacting transducer.

5. A method of inspecting a component as claimed in any one preceding claim wherein said component is continuously vibrated at a frequency or over a range of frequencies of up to 800 Hz.

6. A method of inspecting a component substantially as hereinbefore described with reference to the accompanying drawings.