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double cantilever beam test question

Submitted by dukkipat on

I have a beginner's question on DCB testing of polymers. The fracture strength during Mode 1 loading increases with crack length due to the compliance of the test specimen. In other words it becomes harder to induce fracture as the crack length increases.Is my understanding correct?

Thanks

Venkat

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Siva P V Nadimpalli

Dear Venkat,

The increase in the toughness with crack length in mode-I or mixed-mode DCB fracture is not due to compliance changes of specimen, but it is due to the evolution of a "process zone" or "damage zone" or "plastic zone" ahead of crack tip. The size of this zone increases with crack length causing larger energy dissipation and hence toughness.

You can find more detailed explanation in the following paper.

Siva P V Nadimpalli, Jan K Spelt, "R-curve behavior of Cu–Sn3.0Ag0.5Cu solder joints: Effect of mode ratio and microstructure" Material Science and Engineering A, v527 (2010) pp724-734.

Although this paper is about fracture of lead-free solder joint, the same explanation can be extended to adhesive joints.

I hope this helps.

Regards,

Siva

Sat, 05/08/2010 - 17:53 Permalink
dukkipat

Dear Siva,

Thanks for educating me. Nice work on your paper. In the paper you mentioned that once the crack has initiated you stopped applying loading. I have two questions

1. If the load was maintained would you expect the crack to continue in the solder

2. For a given crack length can you reduce the critical load by introducing stress concentration zones in the solder such as sharp corners.

Again thanks and appreciate your time 

Regards

Venkat

Mon, 05/10/2010 - 00:13 Permalink
Siva P V Nadimpalli

Dear Venkat,

Thanks for your kind words.

Regarding your questions,

1) Yes, if the load is maintained the crack will propagate.

2) As you can see from Fig.13 of our paper that the different starting conditions (or notches of different type) did not effect the crack initiation significantly. Hence, even if you manage to create a sharp corner or notch, the critical loads may not be effected significantly, at least not in the lead free solder that we have tested.

I am glad to share my knowledge, please let me know if you have further questions.

Siva

Mon, 05/10/2010 - 20:51 Permalink