The probe is the most critical part of the ultrasonic test system. Its abilities and limitations define all aspects of UT, from instrument design to test specifications.
The main types probes are
-1. NORMAL BEAM PROBE or STRAIGHT BEAM PROBE :
The beam is parallel to the normal to the surface on which it is incident. (In other words the beam is perpendicular to the in surface on which it is incident ). Refer Fig. 14
2. ANGLE BEAM PROBE
The beam is at some angle, say 45 degrees, to the normal to the surface. See Fig. 15
3. DUAL PROBE
A normal probe but with seperate crystals for transmitting and receiving; main uses of this type of probes is detection of flaws close to the surface and thickness testing of thin sections.
4. IMMERSION PROBE
The probe dips into water in which the job immersed. There is no probe-to-job contact and the water acts as the couplant. Curved lenses can be fitted to the probe to get the beam focussed.
5. SPECIAL TYPE OF PROBES
Mosaic, array or matrix probes; wheel probes for plate testing etc.
Construction of normal probe :
The figure 14 & 15 show the cross-section of a typical normal probe. The piezo-electric element ( or the crystal) is coated on both sides with thin conducting layers. The contact surface conducting layer is internally connected to the probe case; this in turn gets 'earthed' or 'grounded' through the cable and the instrument. The metallic conducting layer on the other side or top side, is connected to the central contact of the probe connector (either directly or through an electronic network). The voltage pulse is applied from the instrument through the cable to this point.
The crystal and its metallization is rather delicate and has to be protected against
1. Hard Wearface : Probes of this type are called hardfaced probes. A thin layer of very hard material such as ruby or alumina is bonded to the metallized crystal (at the manufacturing stage). Such probes give a long life on smooth surfaces; these probes can yet stand quite some amount of abrasion from moderately rough surfaces.
The wear face may be somewhat brittle.
2. Wear Membrane : Wear membrane probe are designed to be used with a thin replaceable membrane which is stretched over the probe contact surface, with some oil in between. Such probes should never be used without the membrane except for an occasional direct light contact and even in such case it should not be slid over the surface; otherwise, the metallizing or even the crystal will be permanently damaged.
Wear membrane protection is more suited for low frequency probes and improves contact on curved and rough surfaces. The membrane, made of a special grade of plastic, can be replaced in the field quite easily.
3. Wear Caps and Delay blocks : Occasionally a rather thick (2 to 3 mm approx) plastic wear cap is used. This would adversely affect near surface resolution but can be used on very rough surfaces. A very long wear cap would actually be called a 'delay block' and would give quite good-near-surface resolution (i.e. less dead zone), but test range in say, steel would be limited to a maximum of approximately twice the plastic wear cap length.
Also note that some old quartz probes were used without any protective layer, quartz being quite hard and wear resistant. The ground electrode for such probes is the test piece itself, contact being maintained by a spring loaded ring around the probe front.
As ultrasonic probes have to be damped to achieve better resolution, a damping body is bonded to the back face of the crystal. This damping body, called 'backing member', should ideally absorb the vibrations of the back surface of the piezoelectric element. Also the energy received by the damping body should be totally absorbed, as otherwise they will be reflected back to the crystal. To meet these critical requirements, the damping body is usually made of plastic or rubber containing powdered heavy metals.
Finally the probe must be reasonably water-tight to avoid damage from the various liquids used as couplants. The case is usually made of stainless steel or aluminium to avoid corrosion from couplant as well as for mechanical strength.
Probes are usually specified by size (diameter or sides of rectangle or square) and frequency, e.g. 2MHz, 24 mm dia probe. Note that the diameter refers to crystal diameter and not case diameter; the case obviously will be larger. These two parameters are invariably written on modern probes or are at least available from the probe specification sheets. A few manufactures also give some arbitrary coding detailing the amount of damping. This coding is not standardized between various manufacturers.
Construction of Angle Beam Probes :
Angle beam probes are used extensively in UT, probably in about 50% of all UT work, considering the amount of welds being tested by ultrasonics.
An angle beam probe essentially a normal beam probe mounted on the plastic wedge. This causes the beam to be incident on the job at an angle. The beam travels in to the job at an angle larger than the incident angle. The cause of this phenomenon (called refraction) and the fact that this beam within the workpiece is of a different character (shear waves).
In one common type of a probe, the crystal is directly bonded to the wedge on the one side and to the backing member on the other. The plastic of which the wedge is made is usually perspex (also called plexiglas or lucite) and it is a clear transparent material.
Not all of the beam enters the workpiece. Quite a good amount of energy is reflected obliquely inside the plastic wedge. This energy, if it reaches the crystal again, will produce disturbing echoes on the CRT. So, the entire thicker end of the wedge is bonded to a material which absorbs sound wall. The bonding surface is frequently serrated for better absorption of the sound and vulcanised rubber is a common material used for this purpose.
The entire assembly of crystal with damping body, wedge with the sound absorber and connector is enclosed in a steel case.
Also available are 'removable wedge type' angle beam probes. These permit crystals of different frequencies to be used inter-changeably with wedges of different angles.
Removable wedge type is convenient because it is comparatively cheap to make wedges for a new or non-standard angle or to match curved surfaces of the workpiece. The crystal assembly is clamped to the wedge by screws.
Crystals meant to be used with removable wedges should not be used as normal beam probes since they are not wear resistant and may perform ultrasonically well only on plastics. For the latter reason it is not advisable to use a normal beam probe on a wedge to convert it to an angle beam probe.
Note that the angle beam probe sends shear waves into the workpiece which travel with a velocity of 3280 m/sec. in steel. For angle beam probes, change in test material not only means a change in velocity but also a change in the (refracted) beam angle.
In addition to the size and frequency, angle beam probes are to be identified by an angle also. The angle marked on the probe is not the incident angle in the wedge but the refracted angle in steel. If the material is different from the steel the angle also will change and this can be calculated. Standard angles available are 30, 35, 45, 60, 70 and 80 degrees.
Due to the contact surface being of softer plastic, angle beam probes get more easily worn out. A uniform wear has no noticeable effect on probe performance; only the point at which the ultrasonic beam leaves the wedge (the 'beam exit point' or 'beam index' or 'probe index') will slightly change. But, non-uniform wear will change the wedge angle and hence beam angle will change in addition to the beam index. This has to be checked periodically. Testing on curved surfaces may make the wedge also curved and it may not make good contact on flat surfaces. In such case the wedge should be ground flat.
For reasons mentioned above it is necessary to check the beam index and the beam angle of a probe periodically. This has been discussed in an earlier lecture.
Skip and path distances :
When testing plates, or welds in plates, the angle beam get reflected between either surfaces of the plate and thus shows a zig-zag path.
In the above figure, the distance AC is called the (full) 'skip distance' and the distance ABC is called (full) path distance. The skip distance and path distance are determined by the thickness of the plate (t) and the beam angle (β)
Full skip distance = 2 x t x tan β Full path distance = (2 x t) /cos β Flaw location with angle beam probe :
If a flaw is encountered by the beam before the half skip as shown in figure 16, the projected distance (X) of the flaw from the beam index and the depth (Y) of the flaw from the surface are given by :
X = b sin β
Y = p cos β, where 'p' is the beam path as seen on the CRT.
If however, the defect is beyond the half skip, the formulae get a bit complicated. It is easier to draw to scale a beaming diagram and mark the beam path on it and thus read the defect position from it. See Fig. 16
Skip and path distances in pipes :
The axial (or longitudinal) scan on pipes is identical to the case of plates discussed earlier. However in the circumferential scanning, the skip and path distances are not given by simple formulae and they found out by drawing to scale a beaming diagram. See Fig.17 For circumferential scan, there is an angle at which the beam touches the ID tangentially. This angle is called the limiting angle (βL). If the angle used is more than this
limiting angle, the beam will miss a region near ID. Therefore angles less than limiting angle should be used during circumferential scan. Limiting angle is given by the formula
Sin βL = ID/OD = Ri/R0 βL=Sm-1 Ri/R0 Connectors :
We have seen that the probe cable is attached to the probe and the instrument by connectors. There are many types of connectors. Some common types are :
(i) Lemo 0 (ii) Lemo 00 (iii) BNC (big and small) (iv) Microdot (big & small) (v) tuchel (vi) UHF and (vii) subvis.
If for some probe or instrument connector, a suitable cable is not available, we should obtain suitable adaptors e.g. Lemo-to-BNC.
Cables :
Ordinary electrical wire would not work for UT probe cables because the ordinary wire will pick up electrical interference due to high frequency machines (such as welding, drilling etc.) working nearby. The special cable used as probe cable is called 'co-axial cable' in which the electrical voltage is applied to a central conductor which is surrounded by an insulation is surrounded by a conducting wire mesh which is earthed; this earthing wiremesh is further covered by an insulation.
Care of probes and cables :
- Plug in connectors without excessive force or twist; each connector has its own way of connecting.
- Cables should not be bent too much and should be cleaned to remove couplant after use. Avoid damage to cables from sharp edges of the material being tested. Do not pull the cable off from the equipment; instead, unconnect the cable by holding at the connector. Replace the cables when they fail.
- Probes are delicate; dropping a probe may be the end of it, especially if it is a high frequency probe. Avoid excess pressure or rubbing on the probe. Use the probe as it is intended to be; probes with protective membranes should not be used without the membrane; angle probes to be used with replaceable wedges should not be used without the wedges (as a normal beam probes). Immersion probes should not be used as contact probes. Probes should not be used on surfaces hotter than the temperature for which they are intended. Probes with a length of cable permanently attached should be handled with care since the cable cannot be easily replaced if damaged.
Chapter-11