Autofrettage

As Autofrettage (French: Self- Ringed ), we describe a method for increasing the strength of pipelines for use in high pulsating internal pressures. The tube is exposed to after its manufacture to a later above the operating pressure and above the yield point pressure, so that the areas on the inner wall plasticize. After releasing arise in this area compressive stresses which prevent cracking in later use, thus increasing the fatigue strength up to the fatigue strength. The tubes may with this treatment either at a higher operating pressure and / or longer time be operated as a non- auto-frettage treatment.

Autofrettage effect

The effect of autofrettage is based on the mutual relationship of the plasticized inner and outer zone of the elastically deformed. This is prevented by the inner plastically deformed zone because retake its original shape, it remains stretched. This explains the tensile stresses in the outer layer. The inner layer is plastically deformed, in turn, is compressed by the spring-back or outer layer - pressed. This explains the compressive stresses occurring here. One of the advantages of later operation with internal pressure is now of that by the applied operating pressure at the outer fiber ( = innermost layer of the tube ) are glitches compensated by the compressive stresses already present from the autofrettage.

Implementation

Shall be carried out, in which the tube is filled with a liquid to minimize its compressibility and the tube ends are sealed. Most often this happens with high-strength bolted caps and special hydraulic fluids. Water is used only sporadically because of corrosion problems.

Well pumps usually bring about pressure intensifier the required pressure. After a short dwell time can be relieved again. The process places high demands on the pump, pressure booster and metrology because in individual cases pressures up to 15,000 bar must be implemented safely. Furthermore, a precise knowledge of the plastic behavior of the pipe material is required in order both to exhaust the desired effect, but on the other hand, to prevent overuse or even rupture of the line.

Areas of application

The autofrettage has been used very early in the time increasing the strength of guns, and later, among others by the company. Rheinmetall for the 120 -mm smoothbore gun of the Leopard 2 main battle tank

Other application areas are:

• Chemical plant
• Certain gas cylinders
• Common - rail injection
• Piping of water jet cutting systems
• High-pressure hydraulic lines
• Running of firearms