The stress distribution close to the tip of a crack which has a finite tip radius and which is being opened either by means of a remotely applied tension field σ y , 0 or by means of a concentrated force (e. g. a wedge driven into the crack) has been computed. It is shown that there exist tensile stresses (σ x ) parallel to the plane of the crack and ahead of the crack tip. The maximum value of the tension σ x is an approximately constant fraction (~ 1/5) of the peak stress concentration σ y , max . which usually causes crack propagation. Inside a brittle solid, if a plane of weakness or potential cleavage is present and is roughly normal to the plane of the original crack, this interface may break and produce a secondary crack in such a manner as to interfere with the progress of the primary crack. If the ratio of the adhesive strength of the interface to the general cohesive strength of the solid is in the right range large increases in the strength and toughness of otherwise brittle solids may result.

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New measurements of electrical conductivity along the b axis of tetrathiafulvalenium-tetracyanoquinodimethanide (TTF-TCNQ) are combined with published results to provide a comprehensive summary including approximately 600 samples studied at 18 different laboratories. The magnitudes of these measured conductivities do not necessitate the assumption of superconducting fluctuations or any other collective state in which the conductivity exceeds the limitations of single-particle scattering. Since an adequate theory of the limitations of single-particle scattering for TTF-TCNQ does not exist at present, experiment alone does not rule out the possibility that collective effects may somewhat enhance or suppress the conductivity.The dc electrical conductivity of tetrathiafulvaienium -tetracyanotiuinodimethanide (TTF-TCNQ) was first reported by Ferraris, Cowan, Walatka, and Perlstein' in 1972. Since that time a number of workers have carefully studied the conductivity in this organic charge-transfer salt. Although the results are not in complete agreement and measurements are continuing in some groups, we would like to summarize the substantial body of work that has been completed. The observed magnitude of the conductivity appears to be com-parable to that in common metals and thus, from the limited point of view of an effective mean free path, consistent with a possible contribution to the transport properties from single-particle electron scattering.The original results of Ferraris etal. indicated that TTF-TCNQ was the most highly conducting organic compound known. Its dc conductivity rose with decreasing temperature like a metal and was highly anisotropic. The observed anisotropy was consistent with the crystal structure of other 13 5105

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