Notch Sign

Figure 15 summarises the results of these experiments. Values of G/c are given for temperatures up to 10 °C, above wdiich LEFM methods do not apply, and values of R are given instead. Below the Tg of the mbber at about —90 °C, fracture is completely brittle, with no sign of stress-whitening, and a very low G/c- Between —90° and 10°, stress-whitening occurs at the tip of the notch, and Gjc increases with terrperature as the extent of the yield zone increases. It is dear from the work of Parvin and Wflliams that fracture takes place under plane-stress rather than plane-strain conditions in the stress-whit ed specimens. Away from the whitened zone. 

In section II.A.2. we discussed how the rf pulse acts on the nuclear spins. We now discuss this topic further and apply the results to several examples having to do with selective excitation. There are many reasons for wanting to perform selective excitation in NMR. Some of them are to simplify complex spectra, to do selective population transfer in order to get the sign of spin-spin coupling, and to study cross relaxation. An important application of what we might call selective de-excitation is a notch in the irradiation pattern to suppress an unwanted resonance such as the solvent peak in a complex proton spectrum. Morris and Freeman (1978) have reviewed many aspects of such experiments. 

In lead Vg, no initial Q wave occurs. A tall, notched R wave, or a slurred one, appears as the impulse spreads from right to left. This initial positive deflection is a sign of LBBB. The T wave is negative. 

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