9.3: Why do cracks weaken a material?
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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 stress was measured as 9.4 MPa.
So the breaking strength falls by a greater factor than the reduction in cross-sectional area.
This is why cracks are so dangerous – because they weaken the material more than you would expect from the reduction in intact cross-section. Why?