Short-term shearing

The shear modulus of a material can be determined by a static torsion test or by a dynamic test employing a torsional pendulum or an oscillatory rheometer. The maximum short-term shear stress (strength) of a material can also be determined from a punch shear test. 

Thus, reinforced polyimide (PI) upon exposure to gamma rays had a decay of short term shear strength for a zero dose value of 87.1 to 79.3 MPa after cohalt 60 irradiation to 1,000 kGy... 

Phase Few FIC models distinguish between precursors and nuclei. One of these was developed by Steenbakkers and Peters [135], based on the hypothesis that flow accelerates the sporadic creation of precursors. These authors found that the increase of the number density of spherufites, after short-term shear flows with different rates and durations, was captured very well by a dependence of 7/ on the stretch A of the HMW fraction of the melt, calculated by the Rolie-Poly model [165]. A power law with exponent 4 was found, as shown in Table 14.1. This is a refinement of the work of Zuidema et al. [19], where 7/ depended on the second invariant of the deviatoric part of the elastic Finger tensor Both are related according to... 

Short term stress-strain properties In pure shear ... 

Stoodley, P., Z. Lewandowski, J. D. Boyle, and H. M. Lappin-Scott. 1999c. Structural deformation of bacterial biofilms caused by short-term fluctuations in fluid shear An in situ investigation of biofilm rheology. Biotechnology and Bioengineering 65 83—92. 

When the Ink Is allowed to rest In the Instrument for 15 minutes or more and then steady shear Is Initiated, there Is a significant stress overshoot (Figure 1). Subsequently, the stress level shows a significant time dependence for a period of time that depends on the experimental conditions but Is generally less than 10 seconds. After this Initial period the stress appears to level-off at what will be termed the short term steady flow value. If the steady shear Is maintained for long periods of time, however. It Is found that the stress Is not constant but shows a small and very slow decrease. For the range of conditions tested here, the stress, and therefore the viscosity, drops by about 15% In one hour (Figure 2). The decrease Is approximately linear In a log (n) vs log (time) plot. 

Steady Shear (Uncured Inks). The short term steady flow stress levels (average values achieved between 30 and 120 seconds) were determined as a function of shear rate and used to calculate viscosities. Values of these viscosities for samples of the two Ink formulations are plotted against shear rate In Figure 3. 

Figure 3. Short term steady flow viscosity vs shear rate for two samples of BK-62 X and two samples of BK-60 +.

Analysis of Cure. A simple analysis of the cure results for short term steady flow can be performed by noting that for a number of polymerization reactions, the early stages of cure can be described by a first order type equation (9,10). In the simplest case this would mean that log (n) would vary linearly with time. To examine this possibility the data for various shear rates were analyzed by plotting log (n) vs. time (Figures 13 and 14). If the Initial points (zero cure time data) are excluded, the data for each shear rate can be fit, to a first approximation, with a straight line. The fact that the zero cure time points do not fall near the lines suggests that the mechanical property results show an Initiation time Just as was found previously In thermal experiments (2). 

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