|NDT.net May 2003, Vol. 8 No.05|
Initial studies in the photo-elastic technique seem to have been in 1937 with Heideman and Hoesch but Photo-Elastic visualisation of ultrasound was first practically developed by Drs. Hanstead and Wyatt in the UK, around 1968.
In the 1970s it was popularised by Dr. Ken Hall at British Rail. The image shown is provided by Dr. Hanstead and shows a multiple-exposure of a 4 MegaHertz pulse, refracted at 45°.
|The principles are relatively simple: Pulsed light is passed from the light source at a fixed repetition frequency on the order of 100 Hertz.|
|Unpolarised light from the spark-gap passes through a polarising filter. This changes the light that passes through the polariser to linear polarised light. This is followed by a quarter-wave plate that changes the linear polarised light to circular polarised light.|
A second quarter wave plate and an Analyser are inserted into the light path and the orientation configured to null the light passing through the analyser.
A clear transparent solid such as glass is inserted into the light path between the quarter-wave plates.
When stresses in the glass are set up by a mechanical disturbance such as our ultrasonic pulse, it causes a change in the rotation of the polarised light.
Changes in stress along the light path change the conditions that nulled the light passing through the analyser so a small amount of light can then pass to an observer on the other side of the Analyser. By adding a delay circuit we can adjust the position along the sound path that is illuminated by the pulse and the camera allows the observer to record the image.
A glass block was fabricated with 3 holes drilled in it.
These are nominally 10mm, 6mm and 3mm diameter.
An internally-spherically-focused probe was selected to minimise the off-axis effects. The probe used has a 12.5mm diameter with a nominal frequency of 7.5 MegaHertz. This compression mode element has a 150mm radius of curvature.
A refracting wedge is used to provide a shear-wave beam nominally refracted at about 55 degrees from the normal.
The beam was directed at the 6mm diameter hole and the delay increased to observe the interactions.
At about 3.8 microseconds after the element has been pulsed the mechanical pulse enters the glass from the wedge.
The incident angle is greater than the first critical angle so only a shear mode is present in the glass.
After entering the glass and moving about half way towards the hole the pulse image is approximately 5.3 micro second delay from the entry point.
As the delay is increased we see the pulse maintaining an almost plane wave-front that narrows slightly as the target is approached.
As the as the plane wave front hits the glass/air interface the reflection process begins.
The first point of contact occurs such that the incident shear wave makes a perpendicular incidence at the closest point on the hole surface.
This will result in a maximum reflected pressure of a shear wave moving directly back along the incident path.
As the incident plane-wavefront of the shear wave advances, the angle made with the rounded surface of the hole ensures that no further other part of the plane wave-front can be perpendicular to the hole surface .
Therefore, all other points of the shear wave-front impinge at angles greater than zero. This angle increases until the wave-front is at the halfway point of the hole.
Here a glancing incidence occurs.
The reflected transverse wave that forms as a result of this interaction takes on the cylindrical shape of the hole as it propagates away from the reflecting surface.
But at any angle away from the perpendicular an incident shear wave at a solid to free boundary interface will mode convert, and, in addition to the reflected shear wave, a reflected compression wave occurs.
The compression wave moves off in a similar cylindrical shape but at about twice the shear velocity.
The associated wavelength of the two modes as well as their positions in time and distance from the target can be used to verify the modes.
The wavelength of the shear mode is measured at about 0.5mm and the compression mode is about 0.8mm.
As the shear wave-front progresses, the second critical angle is reached at the curved surface and a Rayleigh wave forms.
The Rayleigh wave is seen linked to the interaction of the shear mode with the free boundary.
The linked wave front can be considered a form of "head wave" and it has the appearance of a spiral.
This headwave can be seen to account for the "Post-cursor" signals that an ultrasonic technician sees as repetitions on the A-scan as a result of several circuits around the cylinder.
|We would like to acknowledge the interest and support provided by Dr. P. D. Hanstead|