WO1997039345A1

WO1997039345A1 - Method of inspecting a workpiece

Info

Publication number: WO1997039345A1
Application number: PCT/GB1997/001012
Authority: WO
Inventors: Peter Julian Mudge; Alan Marcel Lank
Original assignee: The Welding Institute
Priority date: 1996-04-12
Filing date: 1997-04-11
Publication date: 1997-10-23
Also published as: GB9607628D0; AU2516797A

Abstract

A method of inspecting a workpiece (1) comprises: a) injecting a pulse of acoustic energy into the workpiece, the pulse having a finite bandwidth with a centre frequency; and b) monitoring over time the receipt of signals resulting from transmission of the pulse in the workpiece. Steps a) and b) are repeated at least once, the centre frequency of the pulse being different in each step a); and the information obtained in the step b) is compared to identify one or both of those types of signals which are received at substantially the same time at each frequency, and those types whose times vary with frequency. Information about the workpiece (1) is determined from one or more of the signals identified in step d).

Description

METHOD OF INSPECTING A WORKPIECE

The invention relates to a method of inspecting a workpiece in which the response of the workpiece to the passage of acoustic energy is monitored.

The non-destructive testing of workpieces using acoustic signals, particularly ultrasound, is well known and involves injecting an acoustic pulse into a structure and monitoring the received echoes or modified σnward- transmitted signals caused by discontinuities in the structure. These discontinuities will be caused by geometric changes and other features in the workpiece but also by defects. Thus, defects may be detected. If this process is repeated at periodic intervals and the received signals compared, the presence of a new defect or development of an existing defect can be detected.

One means of examining large volumes of material rapidly is to use guided acoustic waves which can propagate over large distances in solid materials with minimal attenuation. A characteristic of such guided waves is that, at any given frequency, a number of propagating modes exist. In certain frequency ranges, each mode exhibits dispersive behaviour, i.e. the phase velocity and hence the group velocity vary with frequency. The frequencies at which this behaviour occurs are characteristic for each wave mode, for a constant material thickness.

For practical testing, it is necessary to distinguish between signals arising from dispersive and non-dispersive wave modes at the frequencies of interest. Both types of mode are important for the following reasons. i) Owing to the constancy of velocity with frequency for non-dispersive modes, their presence in a signal allows calibration of the time base. Hence, it is possible to determine the location of defects relative to the transmitting and receiving transducers. ii) Dispersive signals may occur as a result of mode conversion at defects, and hence may provide additional evidence of the presence and nature of any defects.

In accordance with the present invention, a method of inspecting a workpiece comprises: a) injecting a pulse of acoustic energy into the workpiece, the pulse having a finite bandwidth with a centre frequency; b) monitoring over time the receipt of signals resulting from transmission of the pulse in the workpiece; c) repeating steps a) and b) at least once, the centre frequency of the pulse being different in each step a); d) comparing the information obtained in the steps b) to identify one or both of those type of signals which are received at substantially the same time at each frequency, and those type whose times vary with frequency; and, e) determining information about the workpiece from one or more of the signals identified in step d) .

We have realised that it is possible to identify signals corresponding to the non-dispersive modes by injecting pulses of acoustic energy into the workpiece at different frequencies and comparing the received signals at each frequency. For non-dispersive modes, signals from a particular discontinuity in the workpiece will always be received at the same time relative to the time of injection of an acoustic pulse into the workpiece. However, for dispersive modes signals will be received at different times. Furthermore, the apparatus needed can be kept relatively simple and, for example, a simple ring array of transducers acting as both transmitter and receiver, or a pair of ring array transducers, one acting as transmitter and the other as receiver, is sufficient for use with pipes. Interpretation of signals may be improved further by the use of multiple rings of transducers, the excitation signals to which are arranged so that a particular wave mode passes through the structure in one direction only, due to appropriate destructive and non-destructive interference between signals emanating from the multiple rings. The term "workpiece" refers to any type of component, construction, structure or fabrication which can be acoustically inspected.

Typically, the pulse will be a finite duration, modulated sinusoidal wave. In general, the received signals will be echoes of the injected pulse but they could also constitute onward transmitted (unreflected) forms of the injected pulse.

The invention can be used for automatic detection of signals due to discontinuities but is particularly suited to providing a display for visual inspection allowing discontinuities easily to be identified.

In one example, step d) comprises successively displaying plots of amplitude v time of receipt of signals obtained in the steps b) , and determining those type of signals which remain substantially stationary and/or those type of signals which change with respect to time in successive plots. In this case, if the successive plots are displayed relatively quickly then the observer will see certain signals remaining in the same position but other signals appearing to move. He can then identify the fixed signals as being due to non-dispersive modes, and the moving signals as being due to dispersive modes.

In another, particularly preferred arrangement, step d) comprises representing the amplitude of the received signals using corresponding grey scale values; and displaying simultaneously the grey scale values on a plot of frequency v time. This provides a static display which may be more easily viewed than the dynamic display mentioned above and is also suitable for hard copy output. Signals due to dispersive modes will appear as curved regions on the display while signals due to non-dispersive modes will appear as vertical regions. An example of^" a method according to the invention will now be described with reference to the accompanying drawings, in which: -

Figure 1 is a schematic diagram of the apparatus; Figures 2-6 illustrate amplitude v time plots at respective different frequencies; and,

Figure 7 illustrates a grey scale frequency v time plot.

Figure 1 illustrates a pipe 1 having a T-joint 2 and an area of wall loss caused by corrosion 3. A transduction system 4 is placed on the pipe 1, connected by a cable 5 to enable transmission of excitation signals to the transduction system 4 from a control and monitoring system 6 and also to enable transmission of received signals from the transduction system 4 to the control and monitoring system 6. The transduction system 4 may have any appropriate form for transmitting sound in the sub 100 kHz frequency range into the walls of pipe 1 for propagation along the walls of pipe 1, but preferably will consist of a set of piezoelectric transducers arranged in a number of rings around the pipe l.

By suitably energising the transduction system 4, it is possible to cause appropriate destructive and non¬ destructive interference between signals such that an acoustic pulse of the desired wave mode can be transmitted in one direction 7 along the pipe 1. The frequency of the pulse is chosen to lie in the range 30-100 kHz and in this example, successive pulses are generated having frequencies in the range 40 kHz to 80 kHz. Thus, in a first step, a sine wave pulse having a frequency of 40 kHz is generated and will pass along the pipe 1 being partially reflected at each discontinuity. In the Figure 1 example, the first discontinuity will be the T-joint 2 and the next discontinuity the defect 3. The control and monitoring system 6 is capable of displaying received signals on a visual display 8, in a form in which time is displayed on the horizontal axis and amplitude is displayed on the vertical axis. In practice, there will be many discontinuities and Figure 2 illustrates a typical example of the reflected signal as detected by the transduction system 4. The region 10 in Figure 2 could correspond to a major distortion such as a T-joint while other regions of significant amplitude, for example 11,12, could correspond to defects or further fixed discontinuities. The problem with a single received echo trace such as shown in Figure 2 is that no distinction is made between echoes caused by the propagation of non- dispersive modes and those caused by dispersive modes. To achieve this distinction, the process is therefore repeated by injecting further short pulses of acoustic energy at successively higher centre frequencies and monitoring the received echoes in each case. Figures 3-6 illustrate similar amplitude v time plots for injected pulses at 50 kHz, 60 kHz, 70 kHz and 80 kHz.

It is then necessary to compare the plots shown in Figures 2-6 so as to identify which echoes correspond to echoes of non-dispersive modes and which to dispersive modes. This could be done by rapidly displaying each plot in sequence. If the display is sufficiently rapid, the eye will appear to observe certain echoes remaining stationary and others moving. Alternatively, this comparison could be carried out electronically.

In the preferred arrangement, however, each amplitude v time plot is converted to a grey scale v time representation according to a predetermined amplitude-grey scale conversion calibration and then the grey scale v time versions are displayed simultaneously on a frequency/time plot on the visual display 9 in the control and monitoring system 6, and shown in detail in Figure 7. As can be seen in Figure 7, this display reveals generally vertically extending regions 13,14,15 etc. which correspond to received echoes. Where the echo is received at substantially the same time relative to the injection of the pulse at each frequency (corresponding to the passage of a non-dispersive mode) then the region on the plot of Figure 7 will appear with a precisely vertical leading edge as shown, for example, at 13. Note that the trailing edge of this and similar regions will exhibit a slight curve owing to the shortening of the signal wavelength with increasing frequency and hence shortening of the time occupied by the pulse. Where, however, the received echo corresponds to a dispersive mode then the leading edge of the corresponding region on the plot appears to slope as shown, for example, at 14. This therefore provides a very easy way for the operator to identify the modes present as an aid to diagnosis. It is also possible, of course, to print the plot shown in Figure 7 in order to obtain a permanent record.

Claims

1. A method of inspecting a workpiece comprising: a) injecting a pulse of acoustic energy into the workpiece, the pulse having a finite bandwidth with a centre frequency; b) monitoring over time the receipt of signals resulting from transmission of the pulse in the workpiece; c) repeating steps a) and b) at least once, the centre frequency of the pulse being different in each step a); d) comparing the information obtained in the steps b) to identify one or both of those type of signals which are received at substantially the same time at each frequency, and those type whose times vary with frequency; and, e) determining information about the workpiece from one or more of the signals identified in step d) .

2. A method according to claim 1, wherein step d) comprises successively displaying plots of amplitude v time of receipt of signals obtained in the steps b) , and determining those type of signals which remain substantially constant with respect to time in successive plots and/or those type of signals which change with respect to time in successive plots.

3. A method according to claim 1, wherein step d) comprises representing the amplitude of the received signals using corresponding grey scale values; and displaying simultaneously the grey scale values on a plot of frequency v time.

4. A method according to any of the preceding claims, wherein the signals constitute echoes of the injected pulse.

5. A method according to any of the preceding claims, wherein the pulse is injected in one direction into the workpiece.