Volume 3, Number 1 January 2004
FYI Practical Contact Ultrasonics - Straight Beam Testing
by Jim Houf* and Bill Svekric†
What do thickness testers, sonar and fish finders have in common? All use sound to detect foreign objects in a sound carrying medium.
While fish finders and sonar are used in water and the majority of NDT straight beam inspections are performed in steel, all three applications can be calibrated to the degree of accuracy necessary to perform the required tasks. All use a zero degree (through-thickness) longitudinal wave sound beam to interrogate the part being inspected (in the case of fish finders and sonar, the part being inspected is water). All rely on the same principle — sound traveling through a uniform medium will reflect from the interface of that medium with a material having different acoustic characteristics.
Typical Applications
For NDT, the two most common applications of straight beam inspection are thickness measurement and verification of material quality. In both applications, the transducer is placed on the surface of the part using a liquid or gel couplant to couple the transducer to the part. Couplant allows the sound beam to cross the gap between the transducer and part. (For purposes of this article, it is assumed proper use of couplant is understood.) The sound beam then travels through the test piece, reflecting from the backwall or any internal planar reflector and returns to the transducer. If equipment is properly calibrated, the distance that the sound has traveled to the backwall or reflector will be displayed either on a digital readout and/or oscilloscope or liquid crystal display screen. Newer instruments may use either single or dual element transducers.
For thickness testing performed using a digital thickness tester, the unit is calibrated so that the distance the sound beam travels to the backwall is displayed as a digital readout on a liquid crystal display. A basic digital thickness testing instrument consists of a small battery powered, handheld generating unit and a three to four foot coaxial cable with an integral dual element, delay line transducer that plugs into the unit. The generating unit has a digital liquid crystal display showing thicknesses in thousandths of an inch. Most units have a known thickness round steel block attached to the main unit that is used to standardize the system prior to use.
Dual Element, Delay Line Transducers
The dual element, delay line transducer has two crystals, one transmitter and one receiver set side-by-side at the back of the transducer assembly (Fig. 1). The sound carrying material between the front face and the crystals (the delay lines) is divided longitudinally into two sections so that the transmitted sound travels down one side of the divider and returns up the other side to the receiver. The distance between the sending crystal and the front face (delay line) permits the near field effect to occur internally instead of in the part and also eliminates entry surface noise, permitting the inspection of thin materials.
Data Logging Units
Many units have data logging features that allow the operator to retain thickness readings for downloading to a word processor after inspections are completed. This feature can speed the inspection process significantly. One point of caution should be noted — periodic downloading is strongly recommended. The greater the amount of stored information, the greater the loss should the unit fail to work properly. The operator should check manufacturer recommendations for battery life and replacement procedures.
Standardizing Equipment Settings
To operate a basic unit, the transducer cable is plugged into the unit and the unit is turned on. A drop of couplant is then placed on the built-in block on the unit and the transducer is coupled to the block to allow the unit system to standardize. Note this is not calibration, but is done to standardize the system. Since the thickness of the block is known, the unit software will adjust the digital readout to accommodate any changes in transmission characteristics caused by changes in the coaxial cable or transducer. Modern units may have a hidden transducer behind the block that lets the unit automatically adjust for transducer wear. If the reading from the block is not correct, remove the transducer, reapply couplant and try again. The reading must match the block thickness or the next step, calibration, cannot be done accurately.
Dried couplant or debris on the block surface can sometimes cause an inaccurate reading. In this case, clean the surface of the block and try again. Consult the operator manual for other options if this does not help.
Calibration
To calibrate the system once it has been standardized, place the transducer on a step wedge of acoustically similar material that covers the thickness range of the part to be tested. For the following example, a step wedge with five steps from 0.100 - 0.500 in. in 0.100 in. increments is used. The transducer is first placed on the 0.500 in. step and the readout is adjusted to match that thickness. The transducer is next placed on the 0.100 in. step and the zero control knob is used to set the readout for that step to 0.100 in. (Control functions vary from unit to unit and the appropriate operator manual should be referenced for the correct adjustment procedure.) The operator should then return to the 0.500 in. step and repeat the process until both the 0.500 and 0.100 in. readings are accurate. As a last step, check the readings from the 0.200, 0.300 and 0.400 in. steps. If the readout for each step is accurate, the unit is properly calibrated and the fixed markers or graticules on the screen directly below the baseline can be used to read the thickness (Fig 2). A significant advantage of the liquid crystal display screen presentation on a digital thickness tester is that the waveform often shows small discontinuities not large enough to cause the digital readout to change or that may only cause the readout to flicker back and forth between thicknesses.
Digital thickness testers with wave presentations are calibrated in a similar way, but there will be both a digital readout and a waveform presentation on a small liquid crystal display similar to that found on a full size flaw detector. As with a flaw detector, the horizontal baseline represents the sound path to the reflecting surface, with the left side being near the transducer and the right side being farther away from the transducer. Permanent horizontal and vertical gridlines called major graticules are superimposed on the screen cover to allow the operator to determine the distance the sound has traveled (Fig. 2). The major graticules typically have five subdivisions or minor graticules to improve reading accuracy.
Performing Inspections
Once the unit is calibrated, inspection can begin. To take a thickness reading, place the transducer on the test surface at the first inspection point. This causes a thickness reading to appear on the digital readout screen. If the display remains constant, manually record the reading or press the save or store key to record that reading digitally. Move to the next inspection point and repeat the process. If the display reading flickers or fluctuates between thicknesses, the reading should not be recorded.
A reading may flicker for several reasons. If the transducer is not seated properly on the part due to surface roughness or a slightly curved surface, moving or rotating the transducer slightly to get a solid coupling to the part may correct the problem. On smaller diameter pipes or tubes, the transducer may rock side-to-side allowing the couplant under the edge of the transducer to cause higher or thicker readings than actually exist. This problem can often be corrected by making sure the divider in the transducer is oriented perpendicular to the axis of the pipe so that the major point of contact will include both sides of the transducer (Fig. 3).
Readings that flicker may be related to the part itself (rough or pitted backwall, multiple nonmetallic inclusions, or laminations in the part). If two reflectors such as the backwall and the edge of a lamination are sending approximately the same amount of sound back to the transducer, the display may alternate between two thicknesses creating an unstable reading. In these instances, the digital unit with a waveform presentation can easily demonstrate the situation that is occurring.
Doubling is a confusing condition that can occur when doing thickness testing of materials that range in thickness from 0.040 - 0.080 in. in thickness. The sound bounces up and down twice in the part before detection by the crystal. The reading is twice or double the actual thickness (Fig. 4). Doubling should be suspected when the operator gets consistently thin readings that abruptly become almost twice that of the adjoining area.
As an example, readings drop consistently to around 0.060 in. as the operator moves the transducer across the part. Abruptly, the readings double with the next reading taken as 0.115 in. and a succeeding reading at 0.110 in. This indicates that doubling may be occurring. At this point, the operator should substitute a transducer with crystals manufactured at a slight angle (roof angle) to help aim the sound beam more favorably.
When using a full flaw detector or scope, the operator must select the type of transducer (single or dual element), the diameter, and the frequency. Older scopes require the operator to match the frequency setting on the unit to the transducer frequency. Newer units may match the two automatically. When selecting transducers, keep in mind that some frequency/diameter combinations can result in near fields that are unacceptable for the proposed thickness range. If this is in question, the operator should check with his Level III. One advantage obtained by using a dual element transducer is the tendency of the unit to trigger from the nearest reflector, such as internal corrosion or pitting. Single element transducers tend to read the reflector with the largest surface area. For example, a deep farside pit with a small surface area may not be picked out from the larger backwall signal. As a result, single element transducers may result in thicker readings than what may actually exist at that point in the part or may miss small reflectors as described in the example.
An advantage of using single element transducers is that coating thicknesses may be eliminated from the readings. This is done by reading the thickness value between the first and second backwall reflections. The same result may be obtained between the second and third backwall reflections if the display is difficult to read. Modern instruments have electronic gates that may be set to automate this process. The fact that coatings do not need to be removed saves time and eliminates the cost of coating removal and reapplication.
When a full flaw detector or scope is used for straight beam testing, setting the screen width and calibrating the instrument are one and the same. To set screen width, the transducer is first placed on a piece of acoustically similar material of known thickness such as a step wedge. Using two reflectors at a known distance, the UT operator then uses the range and delay controls to set screen width. The delay control shifts the entire screen display right or left without changing the distance between vertical traces. The range control expands or contracts the distance between vertical traces. (Consult UT reference text for technical explanation of range and delay.) These two control functions interact and it is necessary to alternately adjust each to obtain the desired screen presentation. Check the operator manual to confirm which knobs or touch pad keys control the range and delay functions for your machine.
To set up the screen width using a single element transducer, the operator can get two backwall reflections from the 0.500 in. step on the screen and by spreading the screen with the range control and shifting the display to the left with the delay, can set the left trace (first backwall echo) on the number 5 graticule and the right trace (second backwall echo) on the number 10 graticule. Once the two traces are set this way, the full screen width is 1.0 in., with major graticules representing 0.1 in. and the scope calibrated for straight beam inspection from 0 - 1 in. (Fig. 5).
The operator should next use the gain control to set the amplitude or trace height from the backwall echo from the test part to 100 percent of full screen height. Full screen height is used most commonly for backwall amplitude but operators should check their NDT procedure or job specification to determine exact requirements for the specific job.
Once the scope is calibrated and the amplitude set, the inspection technique is the same as for digital thickness testers, with the thickness being read from the horizontal graticules. When checking for laminations or inclusions, the depth of these discontinuities can be determined in the same manner. However, the governing specification may require acceptance or rejection to be based on amplitude of the resulting trace, the loss of backwall amplitude (loss of back) method or a combination of these two.
Amplitude can be read directly from the liquid crystal display or cathode ray tube screen, while full loss of back means the backwall trace drops completely off the screen. The specification may also call for a combination of the two methods and require rejection only if a reflector has a certain amplitude and causes a certain percentage of loss of backwall amplitude. An example of this would be a 40 percent full screen height reflector with a 50 percent loss of back.
Mapping
When a rejectable lamination or inclusion is found, the customer may want the operator to map the rejectable area. The most common method for mapping is to slide the transducer across the edge of the area where the signal first appears until the screen presentation meets the rejection criteria. The plate or part is then marked beside the center of the transducer. The transducer is then moved laterally until the same condition occurs and the plate is again marked. By repeating this procedure, the area of the defect can be outlined and mapped on the inspection report and on the surface of the piece being tested.
Conclusion
While this article is intended to provide information for the beginning UT operator, it may not be sufficient in some situations. If additional clarification is required, operators should consult their Level III. TNT
*Jim Houf is Senior Manager of ASNT’s Technical Services Department and is responsible for administration of all ASNT certification programs. He’s been involved in NDT since 1972 and an ASNT Level NDT Level III since 1984. He currently holds ASNT NDT and ACCP Professional Level III certificates in four NDT methods. He’s also an AWS Senior Welding Inspector and an ASQ Certified Quality Auditor. (800) 222-2768 X212, (614) 274-6899 fax, <jhouf@asnt.org>.
†Bill Svekric is President of Welding Consultants, Inc., Columbus, Ohio. He earned a Bachelor's Degree in Welding Engineering (BWE) from the Ohio State University. He received his ASNT NDT Level III certificates in 1978 and currently holds ASNT NDT and ACCP certification in four NDT methods. He is an AWS CWI and is an ASNT Fellow. (614) 258-7018, (614) 258-1996 fax; <wciinc@aol.com>.
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