Polyurethane Adhesive Bond Inspection

Testing of a single adhesive bond is not always straight forward task using a simple A-Scan.


Increasingly, due to their ease of use, adhesives are being utilised to join metals or composite structures. Testing of a single adhesive bond is not always straight forward task using a simple A-Scan. It can be challenging and if there are multiple layers then satisfactory reflections cannot be relied upon especially if the construction is complicated and/or a variety of materials is involved.

Unlike aluminium or other homogenised metals, adhesives are often not consistently applied and usually show a high signal attenuation. Testing of polyurethane bond material requires a much lower probe frequency since the material composition is not homogenised and its grain structure is significantly larger in comparison to aluminium. Beside the ultrasound transmission, the geometry (foam core for example), surface condition and probe access are also significant factors to be considered.

The best solution is the through transmission shadow technique, using ultrasound penetration through a pitch and catch probe, the limitation being that access is required to both sides of the material. A dual or single element probe could be utilised and, in all cases, Sonatest recommends selecting a low damped piezoelectric having narrow band response, due to its higher sensitivity.

Go/No Go Implementation and Performance; Sonatest WAVE has a specifically designed feature for bond testing. A simple application pre-set can be programmed in the instrument with short-cuts for a more efficient operation. Furthermore, utilising the powerful pulsing capability and the wide band receiver to inspect less homogenised material makes WAVE a versatile tool for difficult inspection scenarios.

Polyurethane

Figure 1: Overview of the inspection


Pitch-Catch probe technique

The key to this technique is the shadow effect as shown in figure 2. One transducer is transmitting and on the opposite side of the material another probe is receiving.

If the ultrasound encounters a highly attenuative bond resulting from say, poor adhesion then the signal will be absorbed or reflected and the signal caught by the receiving probe will be reduced thereby identifying the problem. The A-scan is typically fully rectified, and a negative alarm gate is used for a go/no-go call.

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Figure 2: Pitch-Catch Go/No Go identification On the left, the sound goes through (above its white reference A-Scan) and on the right, a very low sound transmission as the red alarm triggers

Pulse-Echo single probe technique

This is backwall echo attenuation monitoring combined with the bond line signal checking using a single element probe on the outer surface of the material. The unit should be set in RF mode, so the A-Scan shows the full polarity of the signal to spot the acoustic impedance changes. It is also possible to set a negative gate alarm, so the light indicates a defective zone when the signal is dropped. A typical flaw will indicate a low back wall percentage FSH and the bond line echo will increase in parallel.

 

Figure 3 is a series of identified flaws in a two layer aluminium assembly with polyurethane bonding.

Dual crystal single probe technique

For difficult applications where the first layer is creating undesired echoes, the dual crystal probe can help. This is especially useful when dealing with thinner layers (less than 2 mm or 0.079 inch), the interface amplitude will improve the signal signature in the adhesive (bond material) to make it an easier go/no-go decision. If the incoming farthest echo is strong enough, it indicates that the signal is going through all the layers and with good bonding between the layers, a fair reference A-Scan and the lack of returning waves will be spotted efficiently.

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Figure 3: Various A-Scans of Polyurethane adhesive bonding

a) Good bonding
b) Thin line
c) First layer is pulled off
d) Absence of glue

For further information or support, please contact the Sonatest Applications Team: applications@sonatest.com