GB2247315A

GB2247315A - An impacting device for testing structures

Info

Publication number: GB2247315A
Application number: GB9114431A
Authority: GB
Inventors: Anthony Charles Holden
Original assignee: Rolls Royce Mateval Ltd
Current assignee: Rolls Royce Mateval Ltd
Priority date: 1990-08-22
Filing date: 1991-07-03
Publication date: 1992-02-26
Anticipated expiration: 2011-07-03
Also published as: GB9114431D0; GB9018473D0; GB2247315B

Abstract

A device for testing structures comprises an arm 12 which is pivotally mounted to a support structure 14. The arm 12 has a surface contacting member 22 at a first end of the arm 12 and a coil of wire 24 at a second end of the arm 12. The arm 12 has a projection 32 which extends between two stops 34, 36 from the first end of the arm 12. An accelerometer 38 is coupled to the arm 12 to provide an electrical signal indicative of surface stiffness when the surface contacting member 22 strikes a composite structure. The coil 24 is positioned in a magnetic field produced by permanent magnets (Fig 2 not shown 31A, 31B), and a first current supplied to the coil 24 causes the arm 12 to pivot from a position against the stop 34 towards the stop 36 so that the member 22 strikes a composite structure. The coil 24 is isolated after the first current to prevent back e.m.f.s damping the free flight of the arm for uniform impact velocity. A second current causes the arm to return to a position adjacent the stop 34. <IMAGE>

Description

A DEVICE FOR TESTING STRUCTURES The present invention relates to a device for testing structures, particularly for testing metal composite or non metallic composite structures to determine if there are defects present, such as disbonds, delaminations, voids or inclusions.

A known device for testing composite structures comprises an arm pivotally mounted at one end to a support structure and the arm has a surface contacting member at the opposite end. A solenoid is secured to the support structure and is arranged to act on the arm via a movable bar to cause the arm to pivot from a first position to a second position in which the surface contacting member strikes the surface of a composite structure. An accelerometer mounted on the arm produces an electrical signal indicative of the surface stiffness of the composite structure and an analyser is arranged to carry out a Fourier transformation to characterise the surface stiffness of the composite structure. A spring acts on the arm at all times to return the arm from the second position to the first position.The solenoid drives the bar against a stop just before the surface contacting member strikes the surface of the composite structure so that the arm and surface contacting member are in free flight when the contacting member strikes the composite structure.

The solenoid and bar mechanism used to cause the arm to pivot is a relatively crude mechanism which gives variations in the impact velocity of the surface contacting member against a composite structure. The returning spring acts on the arm at all times and therefore the arm and surface contacting member are not travelling in free flight, they are being decelerated by the spring. The variations in the spring force also give variations in the impact velocity.

Also surface irregularities on the composite structure cause variations in the throw of the arm and this gives further variations in the impact velocity. Hence there are significant variations in the electrical signal from the accelerometer. A Fourier transformation is required to obtain the maximum sensitivity from the electrical signal.

The electrical processing required to carry out fast Fourier transformation is relatively complex, expensive and time consuming.

The present invention seeks to provide a novel device for testing composite structures which overcomes the above problems.

Accordingly the present invention provides a device for testing structures comprising an arm pivotally mounted on a support structure, the arm having a surface contacting member, the arm and the support structure having cooperating electric current carrying means and magnetic field generating means, switch means being arranged to allow an electric current to be supplied to the electric current carrying means such that the electric current carrying means and the magnetic field generating means move relatively to cause the arm to pivot from a first position to a second position in which in operation the surface contacting member strikes the surface of a structure with a substantially constant impact velocity, means being arranged to detect the oscillation of the surface of any structure struck by the surface contacting member and being arranged to produce an electrical signal, analysing means being arranged to analyse the electrical signal to measure the surface stiffness of the structure.

Preferably isolating means electrically isolates the electric current carrying means such that back electromotive forces in the electric current carrying means do not damp movement of the arm from the first position to the second position.

Preferably the switch means is arranged to allow a second electric current to be supplied to the electric current carrying means such that the electric current carrying means and the magnetic field generating means move relatively to cause the arm to pivot from the second position to the first position.

Preferably the arm has the electric current carrying means mounted thereon, the magnetic field generating means is statically mounted on the support structure.

Preferably the electric current carrying means is a coil of wire.

Preferably the magnetic field generating means is a permanent magnet.

A first adjustable end stop may be positioned on the support structure to define the first position of the arm.

A second adjustable end stop may be positioned on the support structure to define the second position of the arm.

Preferably the permanent magnet is a neodymium/iron/ boron magnet.

Preferably the arm is pivotally mounted on the support structure by a zero clearance bearing.

Preferably the arm has balancing means to balance the weight of the arm.

An accelerometer may be coupled to the arm, the accelerometer is arranged to produce the electrical signal.

The analysing means may analyse the amplitude and frequency spectra of the electrical signal, preferably using Fourier transform techniques.

The electric current carrying means may be the means to produce the electrical signal.

The analysing means may measure the peak force applied by the surface contacting member on the structure.

The analysing means may send a signal to operate an alarm when the surface stiffness of the structure is less than or greater than a predetermined value. The alarm may be audible or visual.

The support structure is preferably mounted in a housing, the housing having an aperture configured to allow the surface contacting member to pass therethrough.

The surface contacting member is preferably arranged at one end of the arm and the electic current carrying means is mounted at the opposite end of the arm.

The housing may comprise a metal or plastic extrusion having two chambers.

The support structure may be mounted in a first one of the chambers.

The analysing means may be mounted in the second one of the chambers.

The extrusion is preferably aluminium.

The extrusion preferably has an integral handle.

The present invention will be more fully described by way of example, with reference to the accompanying drawings, in which: Figure 1 is a longitudinal side elevation of a striking head for a device for testing composite structures according to the present invention.

Figure 2 is a view in the direction of arrow A in Figure 1.

Figure 3 is a block diagram of one embodiment of a device for testing composite structures according to the present invention.

Figure 4 is a block diagram of a second embodiment of a device for testing composite structures according to the present invention.

Figure 5 is a block diagram of a third embodiment of a device for testing composite structures according to the present invention.

Figure 6 is a graph of amplitude versus time for a good and a defective component tested by a device according to the present invention.

Figure 7 is a graph of amplitude versus frequency for a good and a defective component tested by a device according to the present invention.

Figure 8 is a partially cut-away side elevation of a housing for the striking head shown in figures 1 and 2, and Figure 9 is a cross-sectional view along line X-X in figure 8.

A striking head 10 for a device for testing composite structures is shown in figures 1 and 2. The striking head 10 comprises an arm 12 which is pivotally mounted on a support structure 14. The support structure 14 comprises a metal plate 16 to which is secured a bearing support member 18. The arm 12 is pivotally mounted on the bearing support member 18 by a bearing 20. The bearing 20 is preferably a zero clearance bearing, which is preloaded axially so that the balls touch both the inner and outer races. The arm 12 is pivotally mounted at a central region, the arm 12 has a surface contacting member 22 at a first end, and a coil of wire 24 is mounted on the second end of the arm 12. The coil of wire 24 is secured to the arm 12 by a pair of brackets 25 which straddle and are fastened to the arm 12.

The coil of wire 24 is arranged so that a plane B, which is perpendicular to the axis of the coil of wire 24, contains the axis C of the bearing 20. The coil of wire 24 is also arranged such that the plane B is arranged to make an angle C of 45 degrees with a plane D, which extends longitudinally of the arm 12, and also contains the axis of the bearing 20.

The coil of wire 24 is arranged in a magnetic field provided by permanent magnets. A magnet support yoke 26 is fastened to the metal plate 16 and the magnet support yoke 26 carries the permanent magnets 31A and 31B. The magnets 31A and 31B are of opposite polarity. The magnet support yoke 26 comprises a main support member 27 from which extends three parallel spaced limbs 28,29 and 30. The magnet 31A is secured to the first side limb 28 on the side facing the central limb 29, and the magnet 31B is secured to the second side limb 30 on the side facing the central limb 29. The magnet support yoke 26 is arranged such that the magnetic field from the magnets 31A and 31B produce a flow of magnetic flux from the outer poles to the centre pole through the material of the magnet support yoke 26.This produces a flow of magnetic flux in the air gap between the centre pole and the magnets such that the magnetic flux passes through the coil of wire 24. The central limb 29 of the magnet support yoke 26 is positioned within, and is spaced from the coil of wire 24 and the two magnets 31A and 31B are, spaced from and, positioned on opposite sides of the coil of wire 24. The magnets are preferably neodymium/iron/boron magnets although other suitable magnets may be used. The magnet support yoke is preferably mild steel.

The arm 12 has a projection 32 which extends away from the pivot mounting, at the first end of the arm 12. The metal plate 16 has two spaced apart stops 34 and 36, which are positioned either side of the projection 32 to limit the pivotal movement of the arm 12. The stops 34 and 36 are adjustable in position so that the amount of pivotal movement of the arm 12 may be adjusted. The stops 34 and 36 are provided with rubber bushes.

An accelerometer 38 is coupled to the arm 12, at the same end as the surface contacting member 22.

The arm 12 is provided with a bolt 40 which is threaded into an aperture at the second end of the arm 12. The arm 12 is also provided with a bolt 42 which is threaded in an aperture in the central region of the arm 12, and the axes of the bolts 40 and 42 are arranged perpendicularly. The positions of the bolts 40 and 42 are adjustable so that the arm assembly is balanced. The coil of wire 24 and the accelerometer 38 are connected via loops of flexible cable attached close to the pivot to prevent damping of the motion of the arm 12.

The striking head is arranged in a housing 80, shown in figures 8 and 9, and the housing 80 is formed from a metal or plastic extrusion having two chambers 82 and 84 defined by a single integral wall 86. The housing is preferably formed from an aluminium extrusion. The striking head is arranged in the chamber 82 and the support structure 14 is secured to the wall 86 by suitable fastenings. The wall 86 has an aperture 92 through which the surface contacting member 22 passes to strike the surface of a composite structure. The housing has an integral handle 88, which is an extension of that part of the wall 86 defining the chamber 84. The analysing/ processing equipment , on printed circuit boards, is arranged in the chamber 84, and the printed circuit boards are held in cooperating slots 92 on the inner surface of that portion of the wall 86 defining the chamber 84.End plates 94,96 and 98 are secured to the open ends of the housing 80 to close the chambers 82 and 84.

Alternatively part, or the whole, of the analysing/processing equipment may be arranged in a separate housing together with a visual display unit.

In operation a first current is supplied through the coil 24 such that the interaction between the magnetic field, produced by the permanent magnets 31A and 31B, and the first current flowing through the coil 24 causes the coil 24 to be moved in a first direction, from a position adjacent the stop 34, to pivot the arm 12 towards the the stop 36. The first current is a precisely reproducable pulse. Immediately after the first current is supplied to the coil 24, the coil 24 is electrically isolated so that any back electromotive forces produced do not damp the movement of the arm 12.Thus the first current supplied to the coil 24 causes the arm 12 to pivot with a precise rotational velocity, and because there is no damping by any back electromotive forces in the coil 24, the arm 12 is in free flight and the surface contacting member 22 strikes any composite structure with a substantially constant impact velocity. During the time of impact of the surface contacting member 22 with a composite structure, the surface of the composite structure in theory goes through half a cycle of a harmonic oscillator, and the stiffer the surface of the composite structure the higher the frequency of the oscillation of the surface of the composite structure.

During the time of impact the accelerometer 38 on the arm 12 detects the oscillation of the surface of the composite structure and produces an electrical signal indicative of the surface stiffness of the composite structure. The stop 36 limits the movement of the arm 12 in the first direction, when the projection 32 abuts against the stop 36.

Immediately after the impact of the surface contacting member 22 with the surface of the composite structure, a second current, of opposite sense to the first current, is then supplied through the coil 24 such that the interaction between the magnetic field and the second current flowing through the coil 24 causes the coil 24 to be moved in a second, opposite, direction to pivot the arm 12 until the projection 32 abuts against the stop 34. The arm 12 is then back in its first position, abutting the stop 34, and the procedure may start again.

The zero clearance bearing allows the reaction, to the impact, to be passed from the arm 12 to the bearing support member 18 immediately the impact occurs, this is necessary because the impact duration is so short, to ensure that the effective impact mass is consistent over a wide range of surface stiffness.

The first precise electrical current pulse supplied to the moving coil of wire gives a repeatability of impact velocity of + 2%.

The balanced arm assembly together with the use of the coil allows free flight of the arm to be achieved before impact on a surface of a composite structure in any orientation with respect to gravity.

The use of the moving coil to return the arm to its starting position after the arm has struck the composite structure enables higher rates of impact against the composite structure.

In Figure 3 is shown one arrangement for a device for testing composite structures. A trigger button 44 is pressed to operate moving coil timing and control device 46.

The moving coil timing and control device 46 sends operating signals to a moving coil drive circuit 48 which in turn sends the first and second electrical currents to the coil 24. The moving coil timing and control device 46 has a timing device and a switch which initiate the supply of the first electric current to the coil 24, the isolating of the coil and the supply of the second electric current to the coil.

The accelerometer 38 detects the oscillation of the surface of a composite structure, when struck by the surface contacting member 22, and produces an electrical signal which is amplified by a preamplifier 50. The amplified electrical signal is supplied in series to a zero crossing detector and analogue switch 52 and to an inverter 54. The electrical signal from the inverter 54 is an inverted peak and this is supplied to a fast Fourier transformer 56, which analyses the amplitude and frequency spectra of the electrical signal. The amplitude and frequency content of the electrical signal are directly related to the surface stiffness of the composite structure. The fast Fourier transformer 56 may display the electrical signal as an amplitude versus time curve, shown in figure 6, or an amplitude versus frequency curve, shown in figure 7, on a display 58.Curves E, in figures 6 and 7, indicate a good component and curves F indicate a defective component. A defective component has a different ampliude and frequency spectra compared to that for a good component, it is noticeable that there is a marked shift towards lower frequencies in the spectra for a defective components. The fast Fourier transformer 56 may supply the electrical signal to a threshold detector 60, which determines if the surface stiffness is less than or greater than a predetermined value. The threshold detector 60 is arranged to send a signal to operate a visual alarm 62, or an audible alarm 64, if the surface stiffness is less than or greater than the predetermined value.

In figure 4 a second arrangement for a device for testing composite structures is shown, this is similar to that shown in figure 3, but the amplified electrical signal from the preamplifier 50 is supplied to a peak force detector 66. A threshold detector 68 determines if the peak force is greater than or less than a predetermined value and sends a signal to operate a visual alarm 70 or an audible alarm 72 if the peak force is greater than or less than the predetermined value. Alternatively the peak force detector 66 may display the actual force as a bar graph on a display.

In Figure 5 a third embodiment for a device for testing composite structure is shown, this is similar to figure 4, but the back electromotive force of the coil of wire 24 is utilised rather than the electrical signal produced by the accelerometer.

The back electromotive force of the coil of wire is differentiated and then amplified by the preamplifier 50 and is supplied to a peak force detector 66. A threshold detector 68 determines if the peak force is greater than or less than a predetermined value and sends a signal to operate a visual alarm 70 or an audible alarm 72 if the peak force is greater than or less than the predetermined force.

The back electromotive force of the coil of wire may be utilised rather than the electrical signal produced by the accelerometer in Figure 3.

In Figures 4 and 5 the device for testing composite structures works entirely using peak force to determine the surface stiffness of the composite structure, this enables much simpler signal processing compared to the fast Fourier transform used in Figure 3.

The striking/tapping head is designed to cope with surface variations of upto t lmm about the centre point and can cope with convex or concave profiles.

The striking head can be easily scaled to different sizes to cater for different applications.

A device according to the present invention is suitable for testing metallic composite structures, non-metallic composite structures, structures having honeycomb cores particularly of aluminium or nomex (Trade Mark).

The device is suitable for detecting disbonds, delaminations, voids, or inclusions, and may be used for production line inspection or an in-service inspection of components.

Although the present invention has been described as having a moving coil of wire mounted on the pivotted arm and permanent magnets fixed to the support structure, it may be possible to mount the permanent magnets on the arm and to fix the coil of wire on the support structure. In this arrangement the magnet support structure may be made integral with or secured to the arm. This arrangement is not preferred because the continued shocks on the magnets caused by the impacts is detrimental to the magnets. Also the weight of the arm assembly is increased and greater balancing weights may be required.

The description has referred to the use of a coil of wire, it may be possible to use other suitable electric current carrying arrangements, such as a coil of ribbon.

The description has also referred to the use of a permanent magnet, however it may be possible to use an electromagnet.

Although the description has referred principally to the testing of composite structures the device may be used to test other structures.

Claims

Claims:

1. A device for testing structures comprising an arm pivotally mounted on a support structure, the arm having a surface contacting member, the arm and the support structure having cooperating electric current carrying means and magnetic field generating means, switch means being arranged to allow an electric current to be supplied to the electric current carrying means such that the electic current carrying means and the magnetic field generating means move relatively to cause the arm to pivot from a first position to a second position in which in operation the surface contacting member strikes the surface of a structure with a substantially constant impact velocity, means being arranged to detect the oscillation of the surface of any structure struck by the surface contacting member and being arranged to produce an electrical signal, analysing means being arranged to analyse the electrical signal to measure the surface stiffness of the structure.

2. A device as claimed in claim 1 in which isolating means electrically isolates the electric current carrying means such that back electromotive forces in the electric current carrying means do not damp movement of the arm from the first position to the second position.

3. A device as claimed in claim 1 or claim 2 in which the switch means is arranged to allow a second electric current to be supplied to the electric current carrying means such that the electric current carrying means and the magnetic field generating means move relatively to cause the arm to pivot from the second position to the first position.

4. A device as claimed in any of claims 1 to 3 in which the arm has the electric current carrying means mounted thereon, the magnetic field generating means is statically mounted on the support structure.

5. A device as claimed in any of claims 1 to 4 in which the electric current carrying means is a coil of wire.

6. A device as claimed in any-of claims 1 to 5 in which the the magnetic field generating means comprises permanent magnets.

7. A device as claimed in claim 6 in which the permanent magnet is a neodymium/iron/boron magnet.

8. A device as claimed in any of claims 1 to 7 in which a first adjustable end stop is positioned on the support structure to define the first position of the arm.

9. A device as claimed in any of claims 1 to 8 in which a second adjustable end stop is positioned on the support structure to define the second position of the arm.

10. A device as claimed in any of claims 1 to 9 in which the arm is pivotally mounted on the support structure by a bearing.

11. A device as claimed in claim 10 in which the arm is pivotally mounted on the support structure by a zero clearance bearing.

12. A device as claimed in any of claims 1 to 11 in which the arm has balancing means to balance the weight of the arm.

13. A device as claimed in any of claims 1 to 12 in which an accelerometer is coupled to the arm, the accelerometer is arranged to produce the electrical signal.

14. A device as claimed in claim 13 in which the analysing means analyses the amplitude and frequency spectra of the electrical signal.

15. A device as claimed in claim 14 in which the analysing means analyses the amplitude and frequency spectra of the electrical signal using Fourier transform techniques.

16. A device as claimed in any of claims 1 to 12 in which the electric current carrying mean is the means to produce the electrical signal.

17. A device as claimed in claim 13 or claim 16 in which the analysing means measures the peak force applied by the surface contacting member on the structure.

18. A device as claimed in any of claims 1 to 17 in which the analysing means sends a signal to operate an alarm when the surface stiffness of the structure is less than or greater than a predetermined value.

19. A device as claimed in claim 18 in which the alarm is audible or visual.

20. A device as claimed in any of claims 1 to 19 in which the surface contacting member is arranged at one end of the arm and the electric current carrying means is mounted at the opposite end of the arm.

21. A device as claimed in any of claims 1 to 20 in which the support structure is mounted in a housing, the housing having an aperture configured to allow the surface contacting member to pass therethrough.

22. A device as claimed in claim 21 in which the housing comprises a metal or plastic extrusion having two chambers.

23. A device as claimed in claim 22 in which the support structure is mounted in a first one of the chambers.

24. A device as claimed in claim 23 in which the analysing means are mounted in the second one of the chambers.

25. A device as claimed in claims 22, claim 23, or claim 24 in which the extrusion is aluminium.

26. A device as claimed in any of claims 22 to 25 in which the extrusion has an integral handle.

27. A device for testing structures substantially as hereinbefore described with reference to and as shown in Figures 1 to 3 of the accompanying drawings.

28. A device for testing structures substantially as hereinbefore described with reference to and as shown in Figures 1,2 and 4 of the accompanying drawings.

29. A device for testing structures substantially as hereinbefore described with reference to and as shown in Figures 1,2 and 5 of the accompanying drawings.

30. A striking head for a device for testing structures comprising an arm pivotally mounted on a support structure, the arm having a surface contacting member, the arm and the support structure having cooperating electric current carrying means and magnetic field generating means arranged such that in operation electric currents supplied to the electric current carrying means cause the arm to move between a first position and a second position in which the surface contacting member strikes the surface of a structure.

31. A stiking head for a device for testing structures substantially as hereinbefore described with reference to and as shown in figures 1 and 2 of the accompanying drawings.