Fatigue Crack Closure in Polycarbonate
A joint project with the University of Plymouth has been using photoelasticity to examine the growth of fatigue cracks in polycarbonate. Direct evidence has been found of crack closure taking place in the specimens and this closure has been attributed to both crack wake plasticity and fracture surface roughness. A light field circular polariscope was constructed around the testing machine and the fringe patterns developing in 2mm thick compact tension specimens were captured on videotape. Figure 1 shows the fringe pattern apparent in the specimen at minimum load and has various features of the fringe pattern labelled. The notch tip fringes are associated with plastic deformation that took place in the first one or two cycles and was responsible for closure that took place early in the test. Of the two sets of fringe loops that appear to emanate from the crack tip, only the set nearer the notch are produced by the crack tip, the other pair of loops are associated with the bending of the specimen about the crack tip and move away from the crack tip as the applied load is increased. The loops at the crack tip are symmetrical about the crack axis, indicating that the crack was subject to mode I loading and they lean towards the notch, indicating that the stress field in a direction parallel to the direction of crack growth is compressive. A colour video showing the development of the fringes during a single load cycle may be seen by clicking either here or on figure 1.
Figure 1. A photograph showing the fringes in a 6x8mm section of the specimen, with no load applied. Click this picture to see a colour video of a complete load cycle.
Figure 2. A photograph of the crack taken in a dark field circular reflection polariscope, with the plastic zone and the crack within it clearly visible. Click on the picture to see a larger version of it.
The crack wake, which is visible in figure 1, is shown at a higher level of magnification in figure 2, where it is seen under a reflection polariscope. The crack may be seen to wander significantly within the plastic zone, which remains approximately straight. The birefringent fringes apparent in the plastic zone show localised strain raisers, which are due to either plasticity induced closure, an example of which is shown in figure 3, or to sharp changes in the crack direction leaving mismatching fracture surfaces, an example of which is shown in figure 4. The closure induced fringes are only apparent in the plastically deformed material, showing that the elastic material has been shielded from the localised effects of the crack wake contact.
