Damage and Constraint

One of the first lap-shear samples ever made had a strange worm-like pattern on it after it was cured, a sample of which is presented in Figure 4.40. Initially, this was thought to be a result of poor glass cleaning or from contaminates. When further samples exhibited this same pattern, however, it was noted that this pattern was not random, but the result of high residual stress causing the interface to debond during cure. It was this realization which first formed the link between polymer cure properties, residual stress, and interfacial stress.

Figure 4.40 – Sample pattern observed following cure; disk diameter is 3mm for reference.

To better understand where the pattern was coming from, and how it was forming, a careful experiment was set up to record the cure process, as seen in Figure 4.41. Two hot plates were setup on either side of an optical microscope with an aluminum bar running between them. The temperature was set such that after reaching equilibrium, the center of the bar was 60°C. The heights of the hot plates were adjusted such that with the sample loaded in the center, the epoxy disk was in focus of the microscope. Images were set to be automatically captured every 30 seconds for the four hour cure duration.

Figure 4.41 – Experimental setup for in-situ cure monitoring.

Figure 4.42 – Selected time lapse images during cure depicting the formation of a debond pattern at various times.

Weights were used during the cure process to stabilize the system during cure (both thermal and mechanical stabilization). Selected images from the cure duration are presented in Figure 4.42 indicating the formation of the pattern. A first thing to note is that the pattern doesn’t start to appear until near the end of the cure cycle, well after the gel point was reached in the epoxy. Secondly, the pattern is appearing while the sample is at elevated temperature, not during the sample cooling, meaning this is a cure

phenomenon, not a cooling phenomenon. Thirdly, the pattern forms in such a way that it never self-intersects, touches any portion of the disk boundary, or contacts any of the porosity bubbles. Finally, and not shown here, is that the pattern only appears in thin samples, where the mask is less than 0.1 mm thick. These last two items suggest this pattern is a function of the system constraint. Figure 4.43 presents this idea of constraint as it relates to the debond pattern observed.

Figure 4.43 – Conceptual description of the effects of constraint on a shrinking polymer matrix.

Without constraint, the epoxy wants to shrink equally in all dimensions due to curing. With low constraint, as the epoxy bonds to the glass surface, a stress is built up as the polymer shrinks. However, with the greater volume of material, this residual stress is

gradually dispersed through the epoxy away from the interface. When the constraint becomes high enough, this residual stress is enough to reach the interfacial strength and cause debonding. This suggests that the interfacial strength is at least partially controlled by temperature and that the cure induced stresses are not negligible. From this analysis, it can be concluded that using high volume fractions of fibres within a composite can lead to premature interface failure due to this cure induced debonding. One author examined this exact effect on cure induced debonding within dental composites and noted a similar effect [116].

To further this understanding, another series of tests was carried out with the same sample mask thickness of 0.05 mm, but with differing weights placed on the samples to increase the pressure during cure, seen in Figure 4.44. As the amount of weight increases on the sample, so too is there an increase in the pattern density. Though the branches of the pattern decrease in width with the increased cure pressure, the overall pattern area is increasing with respect to the increase in applied weight. Since the actual pressure on the disk during cure could not be measured, no correlation was possible to relate the pattern with the cure pressure, though a relationship is anticipated.

Figure 4.44 – Debond patters generated by applying pressure during cure using a) 100g b) 200g c) 300g d) 400g weights.

This pattern, while interesting, does represent a loss of contact area between the glass and the epoxy, and is perhaps better thought of as initial damage. It should be pointed out that though this effect disappeared with the use of silane coupling agents, the residual stresses are still present even if the interface is not failing during cure. Also, though not utilized in a quantitative manner, the concept of cure induced interface failure is an important one. As industrial composites strive for high levels of reinforcement with faster and higher temperature cures to speed the composite fabrication process, this effect is important to understand to prevent composites from being fabricated with initial amounts of damage.