Non destructive testing (NDT) methods are routinely used in industry to look for cracks and other defects in components.
Ultrasonics is one of the most widely used NDT methods. In pulse-echo ultrasonics, a probe is used to inject a pulse of ultrasound into the sample. The reflected ultrasound is then monitored using the same or a separate probe.
Figure 1 Modern Microplus Compact digital ultrasonic pulse-echo system. Left, schematic.
Planar defects such as disbonding or delamination are good reflectors of ultrasound. This makes the method well suited to forensic studies on adhesive joints.
To detect disbonding, delamination, cracking or other damage in adhesive joints
The ultrasonic data can be presented in a number of ways, commonly referred to as A-Scans, B-Scans or C-Scans. The A-Scan plots signal against a timebase. In a B-Scan a series of A-scan images are recorded scanning the probe in a linear fashion and presented as a greyscale image giving a vertical slice through the sample.
The C-Scan method which represents an image from above is well suited to detection of delaminations or disbonding in aircraft structures and bonded composite structures. In all cases a strong reflected signal will arise where defects are present. The time taken gives an indication of the depth of the defect in the sample. An example of a C-Scan image is shown in Figure 1 below.
Six off cuts from the flange section of the old Comet were subjected to ultrasonic examination using a pulse-echo technique. The ultrasonic transducer probe was scanned transversely at a spacing of approximately 0.75mm over the specimen which was immersed in a bath of water.
The output signal was recorded by a plotter which gave a continuous line for a good bond but defects and flaws caused an interruption. The generated images thus show a coarse grey-lined background with white areas coinciding with 'defects'. A typical image from a Comet bonded flange joint is shown in Figure 2.
Unfortunately the resolution of the system is insufficient to define sharp edge details due to refraction effects and the outlines appear blurred. However from the images of all the test pieces, the following observations can be made.
Figure 2 Ultrasonic pulse echo scan of old Comet aircraft. From MTS Forensics Report
NDT inspection using ultrasonics can provide a useful insight into the bonding condition of adhesive joints. An example is given below on the stiffeners bonded to the doors of Foden trucks. Non-destructive testing of the aged joints indicated that the simple assembly procedures had resulted in intermittent bonding of the surfaces. However, given the contact areas of the bonded surfaces, the strength of the completed joint was more than adequate to meet the performance requirements specified for the door.
Two samples from the doors, one from each of the stiffener profiles, were submitted for ultrasonic inspection to ascertain the degree of bonding between the skin and stiffener. The inspection was carried out using a transmission technique, using a Krautkramer USIP flaw detector with 2.25 and 1 MHz, unfocussed, 12.5 mm diameter probes. The samples were supported in a water bath along one edge and the probes positioned above and below the sample as shown in Figure 3 below. As any water finding a way between the two bonded surfaces would give the same indication as that from a region of good bonding, the edges of the samples were sealed using silicone sealant to prevent water penetration.
Figure 3 UT Examination of Truck Door 11. Location of stiffeners A and B indicated. Scan direction into paper
Signals emitted from the transmitter are passed through the bonded region and picked up by the receiver. The intensity of the received signal is dependent on the nature of the material through which it has passed. The ultrasound passes relatively easily through denser materials such as the aluminium alloy and less easily through less dense materials such as the adhesive. Where there are regions of disbonding, or absence of adhesive, the signal is attenuated to such an extent that very little signal is received at the transducer. Due to the fact that the adhesive layer is very thin, it is not easily identified, and it is thus difficult to identify the edge of the bonded section if there is a good bond.
The received signal was passed to a printer set up such that any signal detected in a gate would create an image on the paper, the degree of bonding being shown by a variation in the density of the signal, eg the darker the image, the better the bond. Where there is no bonding, the transmitted signal would be relatively weak, and the image would show white.
The resulting traces are shown in Figure 4 and Figure 5 for stiffeners (A) and (B) respectively. The two bonded flanges of stiffener A were scanned separately to save time during the scanning operation, whilst sample B was scanned in two passes due to the size of the plate. As can be seen from the twin traces of Figure 3, the bonding of the top hat stiffener flanges appears to be fairly intermittent, with some relatively large unbonded areas, most of the joint strength being obtained from a strip 2.5 - 5 mm wide on the outside edge of both flanges. The NDT results obtained did not provide sufficient resolution to be able to distinguish between areas of disbond and areas where adhesive was simply not present.
Figure 4 Ultrasound scan of Joint A
The traces from stiffener B, Figure 5, also indicate large unbonded regions. In this case most of the adhesion appears to be along the upper edge of the plate. Close inspection indicates that there are some vertical lines of adhesion as well. As a result of the size of the probes used, however, these relatively thin strips do not show up on the traces all that clearly. It would appear from the traces that the bottom half of the plate is somewhat less well adhered.
Subsequent visual inspection of sections through this plate have confirmed these findings.
Figure 5 shows a section removed from the anti-flutter panel (B), where the left hand side of the picture corresponds to the top of the door. The upper section is the anti-flutter panel and the lower a portion of the outer skin material. From this picture the presence of adhesive in the top quarter of the plate is clearly visible, a vertical strip of adhesive then drops from this area to the lower portion of the plate. Also clearly visible in the picture is the relative lack of adhesive in the lower quarter of the plate.
Figure 5 Ultrasound scan of Joint B
Figure 6 Section removed from the Anti-flutter Panel
More information on the method and the example quoted can be found in the following MTS reports
MTS Project 3 Report No 9 Forensic Studies of Adhesive Joints Part 3 – Foden Truck, NPL February 1996
DTI MTS Project 3 Environmental Durability of Adhesive Bonds Report No 9,
Forensic Studies of Adhesive Joints. Part 2 - Bonded Aircraft Structure.
NDT system shown Microplus Compact. Image courtesy Veritec Sonomatic Ltd. www.vsonomatic.com . Schematic Courtesy HOIS2000 Interactive Knowledge Base IKB http://www.hois2000.com/
More information on NDT methods can be found on the BINDT website, via the National NDT Center NNDTC, for composite materials at http://www.mms15.com/ or http://www.netcomposites.com/ , on the HOIS Interactive Knowledge Base IKB accessible via the BINDT website http://www.bindt.org/
