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# Effect of specimen geometry on impact strength

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## Effect of specimen geometry on impact strength

(OP)
What effect does length of an impact test bar have on the strength reported?
Replies continue below

### RE: Effect of specimen geometry on impact strength

There should not be much variance in the geometry of impact test bars, assuming you are testing to a known standard. Are you doing Izod or Gardner testing?

### RE: Effect of specimen geometry on impact strength

(OP)
We are doing IZOD, some specimens are cut from flex bars so the thickness and width is the same but the length varies.

### RE: Effect of specimen geometry on impact strength

Length shouldn't matter but thickness will. As the the specimen gets thicker, it will transition from plane stress to plain strain, which is to say from a bi-axial to tri-axial stress state. This will facilitate craze formation near the crack tip and there will be a significant drop in energy per unit thickness.

Rick Fischer
Principal Engineer
Argonne National Laboratory

### RE: Effect of specimen geometry on impact strength

if you are using CVN test, the lenght also is important, since duration of test could be shorter if the test piece is shorter too, hence the energy could be lower than real. At the opposite side, if the test specimen is larger, more friction over the anvil, more energy to fracture

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