Schematic of standard T-peel specimen dimensions (mm)
The primary purpose of this test method is to determine the relative peel resistance of adhesive bonds between flexible adherends by means of a T-type specimen. The term flexible refers to the ability of the adherend to bend through 90° without breaking or cracking. This test geometry has been adopted by most standards bodies and is widely used by industry to evaluate environmental durability of adhesively bonded systems [9–11]. The T-peel test is particularly suited to testing thin aluminium alloys. Specimens are typically 25 mm wide, have a minimum bonded length of 150 mm, and 50 mm long arms [10]. The recommended thickness is 0.5 mm for steel and 0.7 mm for aluminium. Adhesive layer thickness is not specified. The cost of producing T-peel specimens is relatively low.
Testing is straightforward and requires no special fixture. The specimen can be readily loaded using standard tension/compression mechanical test equipment. It is important to ensure that the bonded portion of the specimen remains perpendicular to the applied load. The specimen is bent backwards 180° and peeled. Tests are normally conducted at displacement rates of 100 mm/min for metals and 10 mm/min for other adherends. The average peeling load per unit length/width is used to define the peel strength. This is taken from the force-displacement curve after the initial peak reading. At least five to ten specimens should be tested from each batch of material, depending on the standard used.
Typical force-displacement curve for a T-Peel test
T-peel tests are only suitable for use with thin flexible adherends. Adherends are generally thick enough to detach without undergoing significant deformation (i.e. stretching and bending). The analysis should account for the possibility of adherend deformation. The test has been shown to discriminate between various combinations of surface treatments and adhesives, although coefficients of variation are typically 20 to 30%, or higher [12]. Correlation between service and laboratory data has proved to be reasonably reliable, although qualitative. Unstable behaviour can cause difficulties in interpretation of test data and in ranking of environmental resistance.
Definition of fillet size in T-peel joint (including fillet forming tool) [13]
Note: The fillet size is the most important parameter controlling T-peel static strength. As the fillet size increases, the strength of the joint also increases. It is recommended that adhesive fillet for the T-peel joint specimen should be 50%.
| Advantages | Disadvantages |
|
Yields peel strength Straightforward/economic Discriminates between surface treatments/adhesives ISO 8510: Part 2/ISO 11339/ASTM D 1876 Suitable for in-situ environmental testing (QA only) Suitable for fatigue testing |
Limited to thin flexible adherends Not suitable for generating design data Large uncertainties in measurements No allowance for large adherend deformation Sensitive to fillet dimensions |
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