10 Contamination-Induced Degradation/Enhancement of Interfacial Toughness and Strength in Polymer-Matrix Composite Interfaces 75 Fig. 10.2 (a) A representative set of force-displacement curves and (b) the corresponding mode-II fracture energy release rate curves, obtained from ENF experiments, for the examined cases [9] are presented on Fig. 10.1b. Sudden jumps in the measured energy release rate of the control case clearly show the arrest and propagation of the crack, which can be attributed to the reestablishment and collapse of the plastic process zone within the bond line. To the contrary, crack growth along the contaminated surface is relatively stable. In fact, the stability of the crack growth tends to increase for increasing contamination level, which can be perceived as another evidence for the contamination-induced shielding of the plastic process zone within the adhesive layer. Second, ENF tests were employed to characterize mode-II fracture toughness for varying contaminant concentration. Figure 10.2a depicts a representative set of force-displacement curves for the tested cases by ENF. Similarly, the variation in the initial stiffness is attributed to different initial crack length among the samples. It can be argued that, unlike mode-I bond line strength, the mode-II bond line strength is not significantly affected by increasing level of contamination. Figure 10.2b shows the corresponding mode-II energy release rate curves derived by corrected beam theory [11]. The initiation mode-II fracture energy was measured to be about 2 kJ/m2 for all examined cases. As the crack starts to advance, the energy release rate curves exhibit a linear rise up to the same steady-state level at about 2.3 kJ/m2. Therefore, one can note that the examined contamination does not show a notable influence on the mode-II fracture toughness and strength of the examined adhesive/adherend material system. The steady-state level values of the energy release rate curves were considered to be the bond line toughness for the corresponding mode of fracture. Figure 10.3a, b summarizes the measured mode-I and mode-II bond line toughness as a function of contaminant surface concentration. As can be clearly seen, mode-I toughness exhibits a significant degradation with increasing concentration level, shown in Fig. 10.3a. A trace level of contamination (level C) can result in about 20– 25% reduction of bond line toughness, moreover level J can further reduce mode-I toughness to the level that is an order of magnitude lower than the reference level. The authors have previously shown a correlation between mode-I bond line toughness and contaminant concentration for a different contaminant [11]. The contaminant examined in this study also causes a degradation mechanism displaying a very similar correlation with the surface concentration. In addition, the inserted images on Fig. 10.3a are optical images of the representative fracture surfaces for the presented cases. In the control case, the fabric weave periodicity completely appears on the fracture surface. This is a clear indication of fibertear failure, which is generally associated with enhanced ductility of the adhesive layer. To the contrary, the 55 g/cm2 contaminated fracture surface displays smooth adhesive surface. Such surface morphology implies that the separation occurs at the adhesive/adherend interface, which is attributed to brittle interfacial (adhesive) failure. The intermediate contamination level of 3 g/cm2 case exhibits a combination of both failure modes. On the other hand, mode-II bond line fracture toughness varies within few percent range of 2.2 kJ/m2 for increasing contaminant concertation, as shown in Fig. 10.3b. It is found to be almost insensitive to the examined contamination. Finally, SLJ tests were performed to examine the influence of the contamination on the bond line shear strength. Figure 10.4 shows a representative set of force-displacement curves for the examined cases. All cases exhibit an initial linear force rise up, which is followed by a nonlinearity commencing almost at the same force level ( 4 kN) for all cases. Then the failure
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