Connect, no.140, January - February 2006, p.8

Up, down, up, down - cyclic pressure testing at TWI

If you're pressurising a vessel you're also inflating it. And if you're inflating it you're straining it. And if you're repeating that process, perhaps several times a minute, you're also fatiguing it.

Pressure induced fatigue is a loading regime which is sometimes overlooked at the design stage even though it's dealt with in great detail in the pressure vessel codes.

Of course fatigue failure shouldn't happen if a vessel is designed correctly. But the rule books can create doubts about the accuracy of the predicted stresses and material properties.

Cyclic testing of the actual vessel can put the designer's mind at rest by providing further safety assurance, or producing direct experimental data for fatigue design. That's where TWI comes in.

It is little appreciated that TWI offers a cyclic pressure testing facility to Industrial Members. The set-up in Fig.1 shows one of the cyclic pressure tests recently carried out in TWI's Engineering Hall near Cambridge.

The vessel under test is mounted within a bulletproof glass chamber. Whether the client wants solely to assure themselves that the anticipated fatigue life can be exceeded, or indeed test right up to catastrophic failure, like the one in Fig.2, TWI can handle the job.

Vessels can also be tested at elevated temperatures. Why not call TWI's Structural Assessment Section to find out more about its pressure vessel testing or e-mail: str@twi.co.uk

Fig.1. Cyclic pressure testing for small vessels	Fig.2. Burst testing of a pipe