Predicting durability

In the case of structural joints, cohesive failure of the adhesive is often observed. Indeed, in many instances, the requirement will be to engineer failure within the bulk of the adhesive layer. Modelling of the adhesive response as a function of water uptake with temperature is carried out to represent the influence of environmental exposure. With fatigued structural joints, by combining the use of fracture mechanics and appropriate modelling tools, the resultant crack propagation rates and consequently durability levels can be predicted as a function of various environmental and mechanical test parameters, such as frequency. Fatigue threshold values can be determined, which are used to predict durability performance. " ... 

In summary, the study of bond durability and, in particular, the development of predictive models is now a mature topic. Most bond durability tests provide only comparative data. Given the large number of adherend-pre-treatment-adhesive/primer combinations available and the range of exposure conditions to which a structure might be exposed, it is difficult to predict durability performance in the general case. There are limitations in the usefulness of current models once failure is observed away from the bulk adhesive. 

It has been shown that an accelerated test method (the automatic boil test—ASTM D-3434) is convenient for evaluating adhesive performance and for predicting durability for exterior applications. Samples are subjected to 800 cycles, which corresponds to approximately 16 years of exterior exposure for a good wood adhesive. Figure 6 compares the perform-... 

Figure 27. Water degradation curve and predicted durability...

Mechanical engineers will design the joint and calculate the loads to be transferred and predict durability and strength through calculation. 

Testing of adhesives is necessary in order to determine the level of performance and/or predicted durability. Some of the tests provide information on the working properties of the adhesive, such as viscosity, which affects mixing, application, and spreadability as well as wetting and penetration ofthe substrate. Othertest methods measure the amount of resin present (solid content). This not only influences the viscosity of the adhesive, but also the performance of the bonded assembly.PI... 

Standard test procedures have been developed to assist this selection process and predict durability in the absence of heritage data to establish suitability for purpose. However, these standard test methods rely on acceleration techniques to produce deterioration of properties in reduced timescales, which may induce mechanisms and pathways that are not present under normal service conditions. So these tests need to be supported by determinations conducted under normal service conditions. 

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