Interrupted stress growth

Figure 3. The shear stress vs. time for an interrupted stress growth test. SGF PP data is the experimental data for a short glass fiber filled PP. PTT and Doi are the separate model predictions for the 7-mode PPT and the modified Doi theory respectively. The PTT+Doi is the addition of the three stress contributions. Prediction is for a shear rate of ls ...

The interrupted shear test described in Figure 3 may be useful in identifying the time for the orientation to relax. In this test a LCP is sheared until steady state stresses are obtained. The flow is then stopped and the fluid allowed to rest for various lengths of time before flow is started up again. The stress growth behavior after various periods of relaxation is then compared with the initial response. Some representative data for the 60 mole % PHB/PET system is presented in Figure 17. After three minutes of rest time in the melt we see that the first peak is not recovered. However, the second peak is nearly fully recovered. In fact we have observed that the second peak recovers in about 6 seconds. 

However, it is possible that the constant rate of AE activity is interrupted by local peaks of high rate of AE. This is due to the formation of local (internal) delaminations because of interlaminar stresses arising due to the presence of transverse cracks. This is more accentuated in less severe loading conditions. Under severe loading conditions = 80% CTu, R = 0.1) the rate of damage development (delamination growth) is so fast that leads to an overall high rate of AE emission. 

In the static crack growth experiments reported in this chapter, the time period for the application of the sustained load (before the interruption of the test for crack observations) was also typically smaller than the transition time, tT. This permits the stress intensity factor K to be used as a characterizing parameter for static crack growth. 

Two additional observations may provide clues as to the origin of the compressive stress. First of all, if the growth is interrupted while the stress magnitude is at its plateau value, the stress magnitude falls off rapidly. Then, upon resumption of the deposition flux, the falloff in stress magnitude is fully reversed and the same compressive stress plateau is eventually re-established (Shull and Spaepen (1996), Floro et al. (2001)). The second observation concerns the role of grain boundaries. It has been demonstrated that the stress in Pd films deposited on polycrystalline Pt substrates becomes compressive while the stress in otherwise identical films deposited onto single crystal Pt surfaces remains tensile (Ramaswamy et al. 2001). 

Friesen, C. and Thompson, C. V. (2002), Reversible stress relaxation during precoalescence interruptions of Volmer-Weber thin film growth, Physical Review Letters 89, 126103-4. 

---