Autofrettage

As autofrettage ( fr. : Self-ringed ) refers to a method for increasing the strength of pipelines for use in high and pulsating internal pressures . Here, the pipe is already exposed to an internal pressure above the later operating pressure and above the yield point , so that the areas on the inner wall plasticize . After releasing the pressure created in this sector compressive stresses that a cracking prevention during subsequent use and thus the fatigue strength up to the fatigue strength increase. With this treatment, the tubes can either be operated at a higher operating pressure and / or for a longer time than without autofrettage treatment.

history

The autofrettage has its origin in the development of weapons. The aim in the construction of gun barrels was to achieve ever higher gas pressures in order to be able to fire heavier projectiles or those with higher muzzle velocity. Initially, an attempt was made to reinforce homogeneous pipes with greater wall thicknesses, but this only has a limited effect, because at some point the material thickness of the pipe has hardly any influence on the load on the inner wall of the pipe. In the middle of the 19th century it was recognized that external pressure on the pipe counteracted the expansion pressure of the powder gases. Based on this principle, Daniel Treadwell developed the first multilayer pipes in the 1840s . Here steel jackets (sheath tube) were steel rings (annulus), or combinations of both (shroud tube) or steel wire or band ( wire tube ) to the core tube is shrunk . From 1847 Thomas Jackson Rodman chose a different path in parallel. It generated the internal stress during the casting process of the gun barrel. While the outer layer was heated, water cooling provided lower temperatures inside. In the early 1900s, the French artillery officer L. Jacob made experiments with gun barrels in which the residual stress was generated from the inside using high pressure. The process was described mathematically in 1909 by LB Turner at King's College . The autofrettage was first used in gun manufacture in France in 1923 and the method quickly found widespread use.

Autofrettage effect

a: tube in the homogeneous initial state
b: pulsating pressure is exerted on the inside until it is deformed
c: after the pressure has been reduced , internal stress remains in the material

The effect of the autofrettage is based on the mutual relationship of the plasticized inner and the elastically deformed outer zone. This is prevented by the inner plastically deformed zone from regaining its original shape; it remains stretched. This explains the tensile stresses occurring in the outer layer. The inner, plastically deformed layer is in turn compressed or also pressed together by the resilient outer layer. This explains the compressive stresses that occur here. One of the advantages of later operation with internal pressure is that the stress peaks that occur on the edge fiber (= innermost layer of the pipe) due to the internal pressure applied during operation are compensated for by the compressive stresses already present from the autofrettage.

Procedure

In the autofrettage process, the pipe is filled with a liquid, then the pipe ends are tightly sealed. This is usually done with high-strength screwed-on caps and special hydraulic fluids . Water is only used sporadically due to corrosion problems .

Now pumps usually apply the required pressure via pressure intensifiers . After a short holding time, the pressure can be relieved again. The process places high demands on the pump, pressure intensifier and measurement technology , since pressures of up to 15,000 bar must be reliably generated in individual cases. In addition, precise knowledge of the plastic behavior of the pipe material is necessary in order, on the one hand, to exhaust the desired effect, but, on the other hand, to prevent overstressing or even bursting of the line.

application areas

The autofrettage was used very early to increase the fatigue strength of guns , later, among others, by the Rheinmetall company for the 120 mm smoothbore gun of the Leopard 2 battle tank.

Further areas of application are:

Chemical plant construction
certain gas bottles
Common rail injection
Piping of water jet cutting systems
High pressure hydraulic lines
Barrel of firearms

Web links

Autofrettage problem description and solution approaches at www.dieckers.de

Individual evidence

↑ Uday S Dixit, Seikh Mustafa Kamal, Rajkumar Shufen: Autofrettage Processes: Technology and Modeling. CRC Press , 2019, ISBN 9780429757983 , pp. 15-16. [1]

[Autofrettage-1] Uday S Dixit, Seikh Mustafa Kamal, Rajkumar Shufen: Autofrettage Processes: Technology and Modeling. CRC Press , 2019, ISBN 9780429757983 , pp. 15-16. [1]