Let’s watch a crack grow. We’ve used paper here, because the
loads are low and we’ve applied a force to the paper sheet by
stretching it across a metal strip in bending, as shown in the
video. You can see that initially the paper is quite capable of
withstanding the stress applied to it by the loading jig. And
this is fine even when there is quite a big cut in it. But when
the cut is lengthened the crack grows quite suddenly. Is the
effect simply one of decreasing the intact section area, causing
an increase in stress on the intact section? Effect of cracks –
reducing the intact cross-section Strips of paper 200 mm long
and 70 mm wide were cut. Some had notches cut into one side
giving a reduction in the intact cross-section of 5.16 × 10−6
m2. These and strips without notches were stretched until they
broke in a tensile test machine. The average stress required to
break the strip without a notch was measured to be 19.3 MPa.
Where the intact cross-section was reduced by a factor of 1.14
the average breaking st