Interfacial Fatigue Test

The load and displacement data can be used in conjunction with the effective modulus of the system to determine the evolution of the contact radius during the test. This method is based on the use of the version of Eq. (3) given in Eq. (6). 

In this equation, AS and AP are the displacement and load amphtudes illustrated in Fig. 23.10. Use of Eq. (6) to calculate the contact radius can be automated, and reduces error that is likely to occur with visual measurement of the 

Equation (1) can be used to calculate the applied energy release rate, Q, for the maximum and minimum loads at each cycle. These values are plotted in Fig. 23.11b. The final step in the analysis is to obtain the incremental decrease in contact radius between cycles, and to plot this as a function of AQ, the difference in applied energy release rates between the maximum and minimum load conditions. The results of this analysis are shown in Fig. 23.12. The circular data points are associated with samples that were held at 80 °C for 5 min before cooling back to room temperature square data points were for samples held for 10 min, and triangular data points for those held for 15 min. The lowest values of AQ are obtained at the beginning of the experiment, and are determined by the load amplitude and by the contact radius that develops during the compressive portion of the experiment. For all three tests, a critical Q occurs at about 

5 J At this point, the incremental decrease in the contact radius during each cycle begins to increase substantially, leading to complete failure of the interface. The image included in Fig. 23.12 is an example of the elastomer surface after the test. The failure resulted in the removal of a layer that can be observed visually, much like the final removal of the layer seen in Fig. 23.10. Therefore, it 

The ability of PEO to wet TMPC, and the presumed similarity in the surface tensions for these two polymers, indicate that PEO and TMPC are either thermodynamically miscible or very nearly so. Eurther evidence for the miscibility 

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In the following sections, we describe the ability to ran tests sensitive to values of Q that are characteristic of processes confined to the interfacial region. First, the sample preparation and experimental procedure are described for two types of tests a straight pull-off test and a cyclic interfacial fatigue test. Then, the results of these tests are presented with data for the forces required for fracture of the samples, as well as the calculations for Q related to the cycHc fatigue test We conclude with a discussion of the significance of the results that have been obtained for our model system. 

Fig. 23.5 Time dependence of the (a) load, (b) displacement, and (c) temperature for the cyclic interfacial fatigue test.

Several samples were fabricated fiom as-received material to not only deteimine the as-received interfacial shear stress but also as an aid in practicing the sample fabrication process. After initial trials of both machining and fiber push out testing, it was decided to settle on a sample thickness of 0.35 mm. In addition to the as-received material, several samples were made available that had been tested at 1204"C with a hold stress of 165.6 MPa in air for periods of4, 250 and 1508 hours. (The test was either a creep or a 2 hour dwell fatigue test [12].) All samples were machined per the procedure outline above. All samples for this effort were the same thickness to eliminate any issue about residual stresses being a factor due to machining. 

Mechanical low-cycle fatigue tests were performed (Ref 27) on several ICA joints and measured the resistance changes with high-sensitive micro-ohm technique. The resistance was observed to increase apparently at the initial stage of the tests, while the force required for the same deformation amplitudes decreased gradually. The authors attributed this phenomenon to the formation of wear tracks from filler frictions. However, they insisted that the influence of filler motion is limited and the dominant failure mechanism is interfacial fracture of the joint. 

In many applications, materials with different properties are combined, which leads to thermomechanical stresses that cause fatigue or interfacial failures. Computational modeling is often used when there is not sufficient time or resources available to perform actual product test, but fast reliability assessments are needed. 

Another basic major advantage is that the cyclic-fatigue fracture-mechanics data may be gathered in a relatively short time-period, but may be applied to other designs of bonded joints and components, whose lifetime may then be predicted over a far longer time-span. Obviously, the fracture-mechanics tests need to be conducted under similar test conditions and environments as the joints, or components, whose service-life is to be predicted. This is important since the fracture-mechanics test specimens do need to exhibit a similar mechanism and locus of failure (e.g. cohesively through the adhesive layer, or interfacially between the adhesive and substrate, or through the oxide layer on the metallic substrate, etc.) as observed in the joints, or components, whose lifetime is to be ranked and predicted. 

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