Experiment displacement-controlled

Based upon results obtained from monotonic tensile experiments conducted with 0° SCS-6 SiCf/HPSN, 0790° Q/SiC, and 0° Nicalon SiCf/CAS-II composites, Shuler and Holmes38 have recommended a loading rate of 20-100 MPa/s to minimize time-dependent deformation during room temperature and elevated temperature monotonic tensile or flexural testing. Equivalent times-to-failure should be used in displacement controlled tests. 

One of the biggest choices made in selecting a DMA is to decide whether to choose stress (force) or strain (displacement) control for applying the deforming load to the sample. Because most DMA experiments run at very low strains ( 0.5% maximum) to stay well within a polymers linear region, both analyzers give the same results. 

With this objective, flat plate specimens have been cycled at SS0°C and 600°C in bending at AEA using displacement control (SOUFLE tests), and thin cylinder - thick < linder Junctions have been cycled at 600 C to combined thermal and mechanical loads at CEA (SOFA experiments). Results of these experiments will be presented at the SMIRT 14 Conference. 

As explained in section 5.2.5, the crack-growth resistance curve is a plot of the stress intensity factor versus the crack length a. Experiments are usually displacement-controlled to enable measurement of the load-displacement curve after the maximum force has been exceeded. 

As unstable crack propagation causes an unloading of the material, the J integral must not be used during this stage because, according to section 5.3.2, the equations are not valid in this case, even when the experiment is stabilised by displacement control. 

Figure 4. SEM of a typical fracture surface of an as received specimen, fractured in a displacement controlled experiment (5 = 5Mm/min), T= 1200°C...

Perhaps the most significant complication in the interpretation of nanoscale adhesion and mechanical properties measurements is the fact that the contact sizes are below the optical limit ( 1 t,im). Macroscopic adhesion studies and mechanical property measurements often rely on optical observations of the contact, and many of the contact mechanics models are formulated around direct measurement of the contact area or radius as a function of experimentally controlled parameters, such as load or displacement. In studies of colloids, scanning electron microscopy (SEM) has been used to view particle/surface contact sizes from the side to measure contact radius [3]. However, such a configuration is not easily employed in AFM and nanoindentation studies, and undesirable surface interactions from charging or contamination may arise. For adhesion studies (e.g. Johnson-Kendall-Roberts (JKR) [4] and probe-tack tests [5,6]), the probe/sample contact area is monitored as a function of load or displacement. This allows evaluation of load/area or even stress/strain response [7] as well as comparison to and development of contact mechanics theories. Area measurements are also important in traditional indentation experiments, where hardness is determined by measuring the residual contact area of the deformation optically [8J. For micro- and nanoscale studies, the dimensions of both the contact and residual deformation (if any) are below the optical limit. 

Thus, in order to reproduce the effect of an experimentally existing activation barrier for the scission/recombination process, one may introduce into the MC simulation the notion of frequency , lo, with which, every so many MC steps, an attempt for scission and/or recombination is undertaken. Clearly, as uj is reduced to zero, the average lifetime of the chains, which is proportional by detailed balance to Tbreak) will grow to infinity until the limit of conventional dead polymers is reached. In a computer experiment Lo can be easily controlled and various transport properties such as mean-square displacements (MSQ) and diffusion constants, which essentially depend on Tbreak) can be studied. 

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