The tensile strength of coal

Apparatus is described which has been developed for the measurement of the tensile strength of coal by bending thin strips. Measurements on four coals show that the strength depends on the type of coal and on the direction of loading relative to the bedding planes.

One coal has been studied in some detail and the tensile strength is shown to decrease with increasing size of specimen. The effect is attributed to the increased probability of finding in larger specimens a weakness which imparts a low strength. It is estimated that for a given size of specimen the tensile strength is of the order of one-tenth of the compressive strength.

Further, it has been found possible to rejoin broken strips of this coal so that up to ten successive tensile tests can be performed on a single strip, thus enabling the distribution of the weaknesses to be studied. In general, there is an increment of strength between successive breakages, and after ten breaks the tensile strength is roughly double the value obtained at the first break. This method of studying distribution of strength does not appear to have been used previously, although it is likely to have applications to many other brittle materials.