Autofrettaged cylinders

As a final result of the explanations about strengthening measures the admissible static internal pressure for thick-walled cylinders is compared in Fig. 4.3-7 for different design strategies according to the equations (4.3-9), (4.3-10), (4.3-12) and (4.3-13) and the explained assumptions and optimisations. In the case of the monobloc (A), the two-piece shrink fit and the autofrettaged cylinders the maximum stress at the inner diameter stays within the elastic limit (00.2). Comparatively much larger is the admissible pressure when complete plastic yielding occurs as shown for the collapse pressure (pCOmpi pi. = Pcoii D). 

Figure 11.2a shows the stresses of an autofrettaged cylinder after removing the pressure the partial plastification causes significant compressive hoop stresses of the inner part of the tube indicated by high negative value of the tangential stress. 

The influence of the Bauschinger effect on the residual stresses of an autofrettaged cylinder was hardly investigated improvements of the analytical approach can be found, among others, in Refs. [25-28]. Finite element analysis was performed [29,30], and experimental verifications are reported by Underwood et al. [31]. 

Eig. 7. Pressure expansion curve of a thick-walled cylinder undergoing autofrettage (22). 

Fig. 18. Repeated pressure tests on autofrettaged EN25 steel cylinders, k = 2.25, with cross-bores (86,96). A, Autofrettaged at 324 MPa ...

A. E. Macrae, Overstrain of Metals and its Application to the Autofrettage Process of Cylinder and Gun Construction, HMSO, London, 1930. 

Autofrettage is also used for strengthening the walls of tubes, cylinders, pressure vessels, etc and it is claimed that it nearly doubles the elastic strength of the walls... 

Figure 1.4-10. Stresses in an autofrettaged thick-walled cylinder (plastification 50% through the wall)...

The so-called hypercompressors for the production of LDPE represent a special case. The ethylene is compressed in a primary piston compressor, with several stages up to around 200 to 300 bar the hypercompressor (or secondary compressor) brings the gas up from there to 3000 bar. The hypercompressors show pairwise-opposite-cylinders, and are built with up to fourteen cylinders in a multiplex arrangement. The components loaded by an internally pulsating pressure are either shrunk and/or autofrettage-treated in order to implement protective compressive residual stresses (Fig. 4.1-34). 

Easier to achieve than the autofrettage in order to increase the yield pressure is the construction of a compound cylinder. Here a second cylinder (or jacket)... 

The temperature limit for the outer heat-treated cylinder, as well as for the autofrettaged vessel, is limited to 500 C otherwise the temper is lost (Figure 2.1-1). 

Figure 2.1-1 Thick walled vessels (a) double walled cylinder with interference fit d (b) single walled, autofrettaged. (o = outer radius, i = inner radius, c = interference diameter).

Another common practice is to hydraulically expand the inner cylinder beyond the plastic range. This was the first application of autofrettage used with cannon barrels. In this case the inner cylinder or core is expanded beyond the yield range. When the pressure is released the inner core goes into compression. There are several positive effects that result from this process. First, the work hardening that results from the material stretch contributes to the strength of the part. Secondly, cylinders expanded in this fashion are more resistant to fatigue failures then those that are not. 

Autofrettage pressure is the pressure that causes initial overstrain of the bore to occur. It is that pressure that results in plastic deformation, beginning at the inside surface of the cylinder. This is initial autofrettage pressure or minimum autofrettage pressure . The maximum autofrettage pressure is the pressure resulting in plastic yielding across the entire wall thickness. 

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