The tensile butt test is less commonly used in adhesives work than the other test configurations, perhaps due to the perceived difficulty in preparing the specimens. Nonetheless, experimental results indicate that the method provides good discrimination and consistent results. The interpretation of results is less straight forward, as edge effects render a simple load over area analysis inaccurate.
Figure 1 Various forms of Tensile Butt Test
To assess the tensile strength of an adhesive joint
The test configuration is shown in Figure 1 above. A standard tensile butt test is performed using bar material, accurately bonded together, using jigs. This loading configuration is generally avoided in practice, thus the limited application of this method, in its standard form, in the forensic study. A modified form has been used, however, with angle sections bonded to flat sheet, in a simple pull off test, where the adhesive is loaded in, more or less, pure tension.
Tensile butt testing was used in the MTS programme to look at the integrity of adhesive bonding of stiffeners to the doors of Foden trucks. In this design stiffeners are attached to the skin using a toughened, two part acrylic adhesive, originally Permabond F241. The tests were carried out on an Instron hydraulic test bed, using a Zwick extensometer, modified to give zero gauge length.
Figure 2 Tensile butt test specimen
This geometry primarily loads the adhesive in tension, as per a conventional butt tension test, although the off set load application will introduce an element of peel stress. This is however, minimised by the stiffness of the adherends. The results from these tests are shown in the Table, below.
|
ID (mm ) |
Bonded Area (N) |
Load (MPa) |
Tensile Strength |
|
3 |
340 |
340 |
1.00 |
|
5 |
193 |
165 |
0.85 |
|
17 |
217 |
134 |
0.62 |
|
18 |
290 |
260 |
0.90 |
|
19 |
290 |
205 |
0.71 |
|
20 |
290 |
165 |
0.57 |
|
21 |
290 |
220 |
0.76 |
|
22 |
290 |
225 |
0.78 |
|
Average residual tensile strength (MPa) |
0.77 |
This test configuration is not a standard configuration, thus direct comparison data was not available.
In order to obtain comparison data for un-aged adhesive, aluminium alloy from the central portion of the top-hat stiffener, A, was removed and bonded using fresh Permabond F241 adhesive. In order to provide a direct comparison between the aged samples and the newly made samples, the same simple surface preparation as used during the original assembly of the joints was used. Both surfaces were wiped with an acetone soaked tissue, before applying the adhesive, and pressing for six minutes. Prior to testing the samples were conditioned at 100°C for 30 minutes. The use of the spare material from the centre of the door, rather than a freshly made sample from the factory ensured that as far as possible the surfaces bonded together were identical to the original bonded surfaces.
|
Ref. |
Bonded Area (mm ) |
Load (N) |
Tensile Stress (MPa) |
|
6 7 8 |
250 275 240 |
560 640 500 |
2.24 2.33 2.08 |
|
Average original tensile stress (MPa) |
2.22 |
Comparison of the two average strength figures indicates that the residual retained strength is approximately 35% of the initial strength. This is in line with the NDT results, where it was shown that the bonding remained intact over 2.5 to 5 mm of the original 12 mm lap. This also accounts, to some extent, for the relatively high levels of scatter encountered in these, and previous mechanical tests.
Observation of the fracture surfaces showed that the naturally aged samples had all failed in what appeared to be an interfacial mode, whilst the un-aged samples failed cohesively, with adhesive remaining on both surfaces.
Figure 3 Butt Tension Specimen Fracture Surface. 22
MTS Project 3 Report No 9 Forensic Studies of Adhesive Joints Part 3 – Foden Truck, NPL February 1996
