Thick adherend shear strength for polymer composites (compression)
Round-robin programme
A round-robin exercise undertaken in order to provide precision data to support the proposal of the thick adherend shear test (TAST) in compression as a proposed new work item for ISO standardisation.
The exercise was organised to principally assess the specimen manufacture stage of the tests, as past experience had shown this to be the most important part of the process. The exercise involved the manufacture of specimens by five participating organisations, using two different adhesives.
Each organisation was sent the materials needed to assemble and manufacture five specimens using each adhesive, along with a draft bonding and test procedure, and report sheets to fill in detailing the various stages in the specimen manufacture. Details of the manufacturing differences between the test laboratories are given below.
The round-robin supported findings from previous round-robin exercises that variability in the preparation of the specimens can have a marked effect on the failure load and the repeatability and reproducibility of the results. Results for repeatability and reproducibility of the specimens made using the two-part Adhesive A are comparable with those obtained in a previous round-robin for single-lap joints, the results for Adhesive B are significantly better.
|
Adhesive |
Mean stress (MPa) |
r |
Sr |
C.o.V. |
R |
SR |
C.o.V. |
|
A |
23.21 |
10.02 |
3.58 |
15.4 % |
20.81 |
7.43 |
32.0 % |
|
B |
42.65 |
8.83 |
3.15 |
7.4 % |
17.04 |
6.09 |
14.3 % |
Further tests were undertaken on a wider range of composites and metals. For Adhesive A it appears that the failure strength is largely independent of the adherend type and its thickness. This may be due to the adhesive failing in adhesion which would mean the surface preparation would be the critical factor in determining the failure strength. For Adhesive B it can be seen that the shear strength is not wholly dependent on the flexural stiffness and that there is an effect of adherend material, as the strength is highest for the stiffest adherend (steel) and lowest for the least stiff adherend (pultrusion).
Changing the thickness of the adherend appears to have had little effect on the failure strength of the specimens, suggesting that the maximum strength as obtained from the bulk adhesive has already been reached and that for a sufficiently stiff adherend there is no increase in the strength value.
