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Weapon Design Practice
Published in Donald E. Carlucci, Sidney S. Jacobson, Ballistics, 2018
Donald E. Carlucci, Sidney S. Jacobson
Autofrettage is a method of prestressing a tube to improve its load-carrying capability as well as its fatigue life. The procedure consists of plastically deforming the interior of the gun tube toward the outside diameter. Regions of the interior wall will now exceed the yield point, but the exterior will not have yielded. When the load is removed, the outer layers of the material attempt to return to their unstressed state but cannot because of the plastically deformed portion of the wall. Thus, an equilibrium condition is attained where the outer wall regions remain in tension and the inner wall regions are in compression. The process is physically accomplished by either pressurizing the interior of the tube with water above its elastic limit or by pulling an oversized mandrel through the tube to force the yielding.
Some Elastic-Plastic Problems
Published in Abdel-Rahman Ragab, Salah Eldin Bayoumi, Engineering Solid Mechanics, 2018
Abdel-Rahman Ragab, Salah Eldin Bayoumi
The autofrettage process is often used in strengthening high-pressure cylinders, such as gun barrels and extrusion containers. A non-hardening material cannot be strengthened theoretically by more than a factor of 2. However, one should not consider the values of the residual stresses as totally reliable. In particular, all metals behave inelastically, and the Bauschinger effect is observed when the cylinder is unloaded. This results in a decrease in the beneficial effects of the residual stresses calculated on the assumption of elastic unloading.
Introduction to Autofrettage
Published in Uday S Dixit, Seikh Mustafa Kamal, Rajkumar Shufen, Autofrettage Processes, 2019
Uday S. Dixit, Seikh Mustafa Kamal, Rajkumar Shufen
Autofrettage is a metal-working process, where beneficial compressive residual stresses are induced in the vicinity of the inner wall of a thick-walled cylindrical or spherical vessel. In this process, the non-homogeneous plastic deformation of the inner wall of the vessel is deliberately produced by applying a uniformly distributed load at the inner surface. The load causing the plastic deformation of the inner wall is called the autofrettage load. Due to the application of autofrettage load during operation, the wall of the cylinder splits into two zones. An inner plastic zone extends from the inner surface to a certain intermediate radial position within the wall thickness, beyond which the material up to the outer surface experiences elastic deformation, creating an outer elastic zone. The intermediate radial position is the demarcating line between the two zones and is known as the radius of elastic–plastic interface. The autofrettage load is gradually applied during operation till the desired plastic deformation is achieved at the inner wall. Subsequently, the unloading of the autofrettage load is carried out. During unloading, the applied autofrettage load is gradually reduced to zero. As a consequence, the plastically deformed material at the inner plastic zone tries to remain in the deformed state, and the elastically deformed material at the outer elastic zone tries to retain its original position. This counteraction between the inner plastic zone and the outer elastic zone generates compressive residual stresses at the inner surface of the cylinder and some portion beneath it. Thus, autofrettage is accomplished in the thick-walled cylinder. Now, when the autofrettaged cylinder is put in service to carry a high magnitude of internal pressure or temperature gradient, the compressive residual stresses at the inner side offset the tensile stresses due to the working load. Thus, the load carrying capacity of the cylinder is increased. The probability of crack initiation at the inner wall is also reduced due to the compressive residual stresses, which slow down the growth of cracks (Stacey and Webster, 1988; Perl and Aroné, 1988). This enhances the fatigue life. The process also enhances the stress–corrosion resistance of the cylinder in corrosive environments.
Estimation of optimum rotational speed for rotational autofrettage of disks incorporating Bauschinger effect
Published in Mechanics Based Design of Structures and Machines, 2022
It is observed from Table 1 that when the in-service rotational speed (Ωs) of the ASTM A723 disk is increased from 0.37 to 0.39, the optimum autofrettage speed to minimize the hoop stress in service remains constant. When Ωs is increased beyond 0.39, e.g., from 0.40 to 0.43, the optimum speed for the rotational autofrettage of the disk increases. In case of the in-service internal pressure, the optimum autofrettage speed for the disk increases with the increase in in-service internal pressure (ps). An increasing trend of the optimum rotational speed with the variation of corresponding in-service loads is observed in Table 2 for the SS304 disk to achieve the optimum hoop stress at the radius of elastic–plastic interface. It is to be noted that the optimum autofrettage speeds obtained corresponding to different in-service rotational speeds will also minimize the equivalent stress in the disks.
Transient thermomechanical sensitivity analysis using a complex-variable finite element method
Published in Journal of Thermal Stresses, 2022
G. Aaron Rios, Juan Sebastián Rincón Tabares, Arturo Montoya, David Restrepo, Harry Millwater
ZFEM has been used to simulate thermo-elasticity [14], autofrettage in a pressure vessels [20], and crack propagation with mixed mode loading conditions [21] to obtain first-order stress sensitivities with respect to design parameters for sensitivity analysis. These sensitivities were obtained using a ZFEM element that contains a duplicate set of nodes containing both real and imaginary nodes shown in Figure 1. The ZFEM element stores first-order sensitivities as imaginary nodal displacements along with the conventional real nodal displacements in a finite element analysis. As a result, the displacement vector includes real and imaginary solutions that provides real responses and first-order sensitivities of the analysis. In transient thermomechanical cases, the temperature and stress response will have the form: