The strength of adhesive joints will degrade with time due to interactions with the environment. In new bond designs there is unlikely to be much longer term service data. It is important to estimate how the properties of the bond may degrade with time. This might be done theoretically based on ageing data on similar joints. A common alternative is the use of accelerated ageing tests. Such tests were applied for example in the DTI MTS programmes on bond durability.
Figure 1 Results from accelerated ageing study on Permabond F241 Joints. MTS durability study, Beevers 1995.
To simulate the effect of natural ageing and environment on integrity of adhesively bonded joints. To predict longer term service performance and residual life.
Typically, laboratory samples will be subject to higher temperature or more severe environmental interactions than occurs in service for a shorter duration. In some applications empirical time –temperature relationships have been developed.
Accelerated ageing testing was used in the MTS programme to look at the durability of adhesive bonding
The results from one series of accelerated ageing tests undertaken on Permabond F241 applied to mild steel adherends are shown in Figure 1 above. In this case samples were immersed in water at 60°C. The levels of adhesive degradation are similar to those actually reported in the MTS forensic study of stiffener joints on the doors of the Foden Truck.
After 10,000 hours the average residual strength in the accelerated ageing tests is approximately 35% of the original value, as compared with the values of between 5 and 75% achieved by the door samples. The accelerated tests were performed on laboratory samples, with fairly accurately controlled bond line thicknesses of 0.25 mm.
Assuming that, at least in part, the degradation of the adhesive properties is related to the exposed adhesive surface area, the thicker bond line of the actual joints might be expected to result in greater degradation. Great care must, however, be exercised in drawing these conclusions due to the different adherend materials used in the two series of tests. Also, the degree of scatter in the laboratory sample results is not recorded in the reference.
MTS Project 3 Report No 9 Forensic Studies of Adhesive Joints Part 3 – Foden Truck, NPL February 1996
Experimental Assessment of Durability Test Methods MTS Project 3 Report No 8
