Shear testing is commonly used in adhesive studies. The single lap shear joint is the most commonly used test method of this type and generally in adhesive studies. The primary advantage of this configuration, from the point of view of the forensic studies programme, is that suitable samples can readily be obtained from most commercial applications.
Figure 1 Schematic illustrating the basis of the single lap shear test
Comparative test between samples normally using identical adherends to determine shear strength and moduli.
The general popularity of the lap shear configuration lies in the apparent simplicity of the test, and the ostensible ease of interpretation of the results. Although shear strengths and moduli .are generally quoted based on these tests, the numerical values obtained do not represent the true values for these parameters.
Where the joint under investigation is not of the required geometry for a simple lap shear test, as shown in Figure 1(a), it may be possible to obtain the required geometry by cutting the joint as shown in Figure 1(b).
The tests are essentially comparative between samples using identical adherends and simple interpretation can be misleading when comparing results from specimens of differing sizes and/ or adherend stiffness. Further problems in interpretation can occur when only a small number of replicate tests are performed. The test results often display a large amount of variability 4, with significant scatter between results from nominally identical specimens (sometimes as high as 25%). Because of this the test is not particularly discriminating when used to identify slight differences in performance.
Lap shear 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.The results of lap shear tests on the naturally aged door samples are shown in the following table:
|
ID |
Load |
Area |
Shear Strength |
Shear Modulus |
|
(N) |
(mm2) |
(MPa) |
(MPa) | |
|
1 |
1080 |
515 |
2.10 |
137.8 |
|
2 |
1250 |
625 |
2.00 |
160.3 |
|
10 |
1090 |
441 |
2.47 |
159.4 |
|
13 |
4750 |
638 |
7.45 |
197 |
|
14 |
3700 |
696 |
5.32 |
186.4 |
|
E Average shear strength / modulus |
3.87 |
168.2 |
Typical average shear strength figures [1] for the adhesive on a 5000 series aluminium alloy, suggest a shear strength of 17 MPa, based on specimens 1" x 1/2" x 3 mm, and a shear modulus of 240 MPa at room temperature (bondline thickness 0.1 mm).
These figures are, however, based on laboratory prepared specimens, with controlled glue line thicknesses. The actual results obtained from the doors clearly had variable bondline thicknesses, varying from 0.16 mm to 1.34 mm. Although the manufacturer's maximum recommended gap fill thickness for pre-mixed adhesive is 1.5 mm, the increased bondline will result in lower strength values, as shown in Figure 2 below (from manufacturers' data). Based on these figures, typical strengths, as built, would be of the order 8 MPa, thus after natural ageing the adhesive retains between 5 and 75% of its original shear strength coupled with a reduction in modulus of between 10 and 36%.
Figure 2 Manufacturers data on the effect of bondline thickness on joint strength. Courtesy Permabond, DTI MTS programme
More detail on the single lap shear test and example data can be found in the following reports from the DTI funded MTS and ADH programmes.
MTS Project 3 Report No 9 Forensic Studies of Adhesive Joints Part 3 – Foden Truck, NPL February 1996
MTS Project 3 Report No 9 Forensic Studies of Adhesive Joints Forensic Studies of Adhesive Joints MTS Part 1 - General Introduction and Conclusions February 1996
[1] Private communication with Permabond UK Ltd Woodside Road, Eastleigh, Hants.
