Environmental attack tests

Additionally, in order to investigate the synergistic effects of UV and moisture on the degradation, the programmed cube specimens were immersed in saturated saltwater and rainwater at room temperature for 90 days, respectively. They were prepared for the stress recovery test. All these immersed specimens were synehronously exposed to UV. The original foams without environmental attacks were also prepared as eontrol specimens for each group of programmed foams. 

Long-term durability of adhesively bonded joints may require resistance to a number of individual or combined degradation modes, including environmental attack, fatigue and time-dependent failures. Time-dependent failure mechanisms are often characterized nsing either a strength approach, involving creep and creep-rupture tests, or a fracture approach, in which debond rate is determined. In creep-rupture tests, adhesive joints are subjected to... 

From the above discussions, it may be concluded that to obtain significant water ingress into a typical adhesive joint at ambient temperature, and to observe any appreciable degree of environmental attack, may take many months, if not years, of exposing the joint to the aqueous environment. Accelerated-ageing tests are therefore a very essential tool in order to accelerate the kinetics of environmental attack, and so obtain durability data in a realistic time-scale for the development and selection of adhesive systems. As noted above, the lack of current knowledge on the details of the kinetics of the mechanisms of environmental attack certainly hinders the development of such tests. 

However, other approaches have also successfully been adopted. For example, Dickie and Ward [53] have studied single-lap joints exposed to a high humidity at moderately elevated temperatures but maintained a constant stress on the joints. Also, periodically the joints were removed from the high-humidity environment and exposed to a salt solution for a short time period. Using this accelerated-ageing test they were able to rank the durability performance of various adhesive systems in a comparatively short timescale. Further, they reported that not only were the kinetics of mechanisms of environmental attack accelerated, but also the exact details of the mechanisms were affected by the levels of the applied load. For example, for joints which consisted of bonded galvanised steel substrates, the effect of relatively high applied loads was to prevent the formation of an effective barrier of corrosion products, i.e. passivation of the substrate surface was prevented. This allowed the electrochemical corrosion process to proceed unimpeded, and hence at a faster rate than for similar, but unstressed, joints. Thus,... 

For essentially static loads, creep may be important and there are a variety of ASTM tests for assessing this (D1780-72, D 2293-69, D 2294-69, D 2918-71 and D 2919-71). Creep may be especially important where environmental attack or high temperatures cause softening of the adhesive. 

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