Autofrettage and Fatigue
On the previous issue we have talked about fatigue and its impact to high-pressure component life. This article will introduce a technique that is used widely in this industry to fight back fatigue and improve component life. This technique is called Autofrettage. The word Autofrettage was derived from French with a meaning of “self-hooping”. French people used this technique to pre-stress cannon and gun barrels to reduce the stresses on the walls from high pressure and temperature variation during firing. These stresses cause fatigue and eventually cracking.
Autofrettage is done by applying a pressure that is much higher than normal operating pressure to the subject high-pressure component, usually in the shape of a tube. This pressure is so high that it causes the inside wall of the tube to expand and creates a permanent deformation to a certain depth into the wall thickness. After this pressure is removed, the outside wall, which was under elastic deformation, tends to collapse inward. However the inside wall, which has expanded and deformed permanently, will not allow it. As a result, the inside wall has residual compression stresses and the outside wall residual tensile stresses.
For a component without Autofrettage, the stress level under normal operating pressure is the highest at the inside wall and decreases gradually towards the outside. With Autofrettage, the stress level is balanced out with the inside wall stresses reduced significantly. This, in turn, extends the fatigue life of the component. The impact can be doubling the component life.
More detailed information on this subject can be found in these two references: (a) F.H. Trieb, J. Schedelmaier, and M. Poelzl, “Developments on optimized Autofrettage of high pressure components for waterjet cutting pumps”, Proceedings of 17th International Conference on Water Jetting, Mainz, Germany, September 7-9, 2004; (b) ASME (American Society of Mechanical Engineering) Code, Boiler and Vessel Code Section VIII-Division 3, KD 1998 Edition, New York.
Beyond High Pressure
· What is SUPER-WATER®? SUPER-WATER® is an additive used in abrasivejets or waterjets in order to focus the cutting stream, increase cutting speed, and reduce wear of high pressure components. Traditionally it has been used with great benefit for high-pressure cleaning and water only cutting applications. Its application has more recently been expanded to Abrasivejet Machining. For more information, go to www.waterjets.org/super-water.html
· Like to learn waterjet cutting techniques and get hands-on experience? Check out "Hands-On Waterjet Cutting Techniques Course" offered by RICHEL at www.richel.com.
· A book with a working title of HIGH PRESSURE PUMPS & SYSTEMS is being written by Mike Gracey in Houston, Texas. If you have some clear photographs showing high pressure pumps, ultra-high pressure pumps and water jetting project, please contact him at firstname.lastname@example.org
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