Frequently Asked Questions
Producing ultrasonic test systems since 1987
click on any of the questions to find out more information.
Q. What’s the smallest defect you can detect?
There are a great number of variables influencing the answer to this question that it is impossible to answer in a simple way. The question should perhaps be “What is the smallest defect in my product that I need to detect to make it safe?”. There is no point in trying to detect sub-millimetre defects when the product can safely operate with defects 5mm in size. All that does is to make your inspection very slow and very expensive, both in equipment cost and man-hours.
The permissible defect size should be defined by the product designer based on information about the expected service conditions, the material properties and the required life of the product. Once this information is available it is possible to decide on the best equipment and inspection parameters for the job. The operating method should be defined by the NDT Supervisor based on relevant specifications and his/her experience.
If you really want to know the smallest defect technically detectable then you need to know the material properties, the condition, the required depth of penetration (how thick is it?), the expected shape of the defects and several other factors. As an example, in a thin metal part with a fine grain it may be possible to detect inclusions and voids as small as 30 microns, but only up to a depth of a few millimetres. In a composite part however, it would be impossible to discriminate a defect as small as this.
Q. What’s the difference between cartesian and robotic scanning systems?
Cartesian scanning systems operate with linear and angular with each one moving in a specific direction and with a fixed relationship to one another, for example X, Y, Z, A and B. Each axis has its own resolution, speed, accuracy and repeatability and can move independently to create a simple scan path. Industrial robots have a combination of angular axes (usually six) so to create a simple scan path such as a straight line requires simultaneous motion of at least 3 axes.
In a cartesian system it is easy to visualise which axes need move to create a scanning path, but more difficult in the case of a system using robots. One of the main criteria in choosing which type of system is needed is the shape of the part to be scanned and the more complex the shape the more care is needed. For some shapes it may be much simpler to use a cartesian system than a robotic system – one example would be a part with a “C” shaped cross section which requires access of a probe inside the “C”. Access might be easy with a cartesian system, but in a robotic system the end effector might not be able to reach inside because one of the joints collides with the part.
Q. How close to the surface can you detect defects?
This is the “near surface resolution”. The answer depends on the material, the test frequency and the type of defect. All other things being equal, higher test frequencies will detect defects closer to the surface, but this will limit the penetration. Usually the frequency is selected according to the material thickness, so the near surface resolution is more or less pre-determined.
As an example, in pulse echo testing of metals such as steel and aluminium alloys most specifications demand a near surface resolution of 4 or 5mm, but 2 or 3 mms is usually possible. With laminar defects it is often possible to detect defects closer to the surface, because sound reverberates between the surface and the defect, resulting in repeat indications which are apparently deeper than the defect.
When testing composite materials using a through transmission technique, lamination defects very close to the surface (ie in the first ply) can be detected, because this prevents any transmission of sound to the receiver, creating maximum attenuation.
Q. What is the thickest material you can test?
In traditional pulse echo testing it is possible to penetrate homogenous materials more than 1 metre thick, but the sensitivity to small defects reduces as the penetration increases. As ever, there are compromises to be made between penetration, resolution and near surface resolution.
In the case of composite materials pulse echo testing is limited to around 150mm. In through transmission inspection, honeycomb up to around 400mm thick can be successfully tested.
Q. How often do I need to calibrate the system?
There are two different types of calibration – electronic / machine calibration and routine calibration. Electronic / machine calibration is usually carried out annually and is controlled by specifications: national, international or customer based. This is carried out by the equipment manufacturer or by qualified / approved third parties.
Routine calibration is done to confirm that equipment is performing as expected on a day-to-day basis and that there has been no deterioration. The best way to achieve this is to incorporate a reference calibration piece as part of each inspection, so that a permanent record is held with the scan result for the component. Where parts are held in a fixture for the test the calibration piece can be included in the fixture.
Alternatively, a calibration piece can be tested on a regular basis, for example before each shift. In this case the tested parts should be quarantined until the next calibration scan is done, in case there has been a malfunction during the shift which affects the test.
Q. Can I calibrate the system myself?
Yes, if permitted by the relevant specifications and by your customer / end user.
Q. Can I test my part without a liquid couplant?
Some inspections can be carried out using “dry coupled” probes which use special synthetic rubber materials to couple the sound to the part. These are usually used in the form of tyres in a “wheel probe” with the active element(s) located in the wheel hub. Their use is limited to relatively low frequencies (Around 2.5MHz maximum) because of the attenuation of higher frequencies in the rubber material. Also, the inspection might be limited by the effect of multiple echoes inside the wheel probe.
The term “dry coupling” can be a misnomer, because to get uniform and consistent results a thin film of liquid in the form of a mist spray might be necessary
Air coupling is possible for through transmission inspections at low frequencies – less than 1MHz.
Q. I can’t use water as a couplant – what can I use instead?
Water is the ideal couplant because it is inexpensive, easily disposed of and creates minimal attenuation of sound. However, some parts cannot tolerate water couplant, for example materials used in spacecraft and micro-electronic components.
Mineral oils can be used, particularly in stand-alone testing machines where parts are handled automatically. One thing to bear in mind that immersion probes are designed on the basis of water coupling so the focus will be affected using a couplant with a different sound velocity. Also, the sound attenuation in the liquid will invariably be higher than in water.
Q. Can I do a pulse echo test at the same time as a through transmission test?
With USL systems the answer is Yes. USL’s electronics are designed specifically with this in mind with the appropriate system configuration. In fact, you can do several different tests at the same time – for example one through transmission test and a pulse echo test from both sides of the part.
Q. I can’t find a defect that I know is there – why?
When using interface echo triggered gates, you should check that the gate triggering has not been affected by a change in surface finish or air bubbles on the surface, for example. Also check that the interface echo stays within the interface gate throughout the test – you can set a wide interface gate in most situations.
Check that the probe you are using has a focal range which is wide enough to cover the desired thickness – also that the DAC (Distance amplitude correction) is correctly set and is switched on.
When C-scanning, make sure that the scan index is not set at too large a value. Ideally the scan index should be set so that a potential defect is crossed at least three times,
Q. Your system is finding defects that I couldn’t see before. Why?
This is very common when changing from a manual inspection method to an automated one. Also, when changing from an old analogue system to a digital system, because older electronics often need several successive defect triggers to reach the correct peak amplitude values, whereas a digital system needs only a single trigger.
Q. How long can you make an inspection system?
This depends on a number of factors, such as the type of machine. For dual robot machines employing base rails for the robots to run along, the length can be several tens of metres without any trouble at all.
Q. How long does it take to manufacture an inspection system?
When USL commits to build an inspection system for a customer it is with the intention of making that system exactly as our client wants it, within technical limits of course. As each machine is designed, manufactured and installed by USL the process can take some time. Typically, a machine build from receipt of a customer order is in the region of 6-9 months from start to finish. Some of our larger machines, or particularly complex machines can take longer, but it is worth the wait.
Q. Can USL sell systems anywhere in the world?
Virtually, yes. Providing there are no current trade restrictions in place for the geographic location or the end customer, USL can supply an inspection almost anywhere it may be required. Contact USL and we will either assist you directly, or, where we have a local partner in place we will put in touch with your local USL point of contact.
Q. How much does a system cost?
This is a very difficult question to answer as it depends on a huge number of factors and considerations, such as the size of the machine, the technologies that are employed on the system, the final destination of the system, any specific requirements of the system and this is to name but a few. As a general rule though, a small 3 axis immersion inspection system could eb supplied for less than £200,000, whereas some of our largest and most capable installations could cost in the region of £2million. USL will be happy to discuss your specific requirements and prepare a formal technical proposal and quotation for a machine that is specifically suited to your own requirements.
Q. Where can I get support for USLScanner software?
USLScanner software is a remarkably capable piece of inspection software that is responsible for all aspects of inspection when using a USL inspection system. Every aspect of inspection set-up, CAD data import and scan planning, ultrasonic setup and data acquisition through to data analysis and reporting is done using a single interface software. If you require assistance with USLScanner software then contact us directly and we will be happy to put you back on the right track.
Q. Can I get applications development support?
Yes, of course, please contact USL directly if you require any application development support, we will do all we can to assist you. For simple assistance during normal working hours this assistance is free of charge, if you require additional support on complex applications, then this may incur some charges, speak to us and let us advise you how we can help.