With the baseline fatigue performance now measured, the effects of different temperature(s) and/or climate cycling can be assessed. In the example shown below, performed on structural adhesive systems used by the automotive industry under pure mode I loading, the following environmental test regime was employed:
0 – 522 kilocycles 40°C 50% RH (‘dry’)
c.500 kilocycles Continuous climate cycle (‘wet’):
15 minutes salt fog (5% NaCl, pH 7)
105 minutes drying at 40°C and 45% RH
22 hours at 40°C and RH 90%
By performing a two-stage exposure regime, more information on the durability of the system can be gathered. Firstly, the effects of elevated temperature only on the fatigue resistance of the adhesive system can be determined. Once the relationship between G and da/dN is established, in this example at 40°C, the effects of the ‘wet’ climate cycle can be determined.
Figures 8 and 9 show the fatigue resistance for two different adhesive systems respectively, one an aluminium-epoxy and the second a steel-epoxy system. Their baseline fatigue resistance was measured, previously, at 23°C 50% RH.
For the aluminium-epoxy system (Sys C) shown in Figure 13, the non-ambient environments have not affected the baseline fatigue resistance. Neither the elevated temperature data (shown as red triangles) nor the climate cycle data (blue diamonds) have shifted significantly away from the baseline data (black squares). On inspection of the fracture surfaces, cohesive failure, identical to that seen from surfaces tested at 23°C, can be observed throughout the entire fatigue test (0 to end). This adhesive system therefore has very good environmental durability, based on the chosen test regime.
Figure 13: Results for the aluminium-epoxy adhesive system
However, the results from the same fatigue test for the steel-epoxy system (Sys D) are very different. The crack growth resistance at 40°C dry is significantly lower than the baseline resistance, measured at 23°C. The fracture surfaces, however, still reveal cohesive failure within the adhesive layer. After 1.2 million cycles (a day after starting the climate cycle), interfacial failure is observed. Associated crack growth rates after 1.2 million cycles are much higher, with increased scatter, indicated on Figure 14 as blue diamonds. Clearly the environmental durability of the steel-epoxy system is very poor.
Figure 14: Results for the steel-epoxy adhesive system
Clearly this example shows the two extremes, however, the response of other adhesive systems can lie in between these results. In these cases the ‘wet’ threshold value of G can be determined, together with the Paris Law constants, and used for finite element modelling purposes.
The RDCB test piece is a robust, versatile design that allows testing under a number of different modes of loading. Geometry independent, quantitative results are measured from the fatigue tests that can be used for design analysis using FEA. In-situ environmental exposures can be introduced in order to measure the environmental durability of adhesive systems.
The pure mode I test method has been submitted as a draft CEN Standard and, if approved, will become a full Standard test method by the end of 2005.
Next: Fracture Mechanics Design – Example
