Compressive Residual Hoop Stresses Research Articles

Effect of aircraft rivet installation process and production variables on residual stress, clamping force and fatigue behaviour of thin sheet riveted lap joints

This paper presents research on the effect of riveting process parameters on residual stresses, clamping stresses and clamping force in thin sheet riveted joints, as well as the fractographic observations of riveted lap joints. The following variables were considered in the study: riveting force, sheet thickness, sheet material and rivet material, rivet hole tolerance and rivet diameter tolerance. The research was carried out for 2024-T3 aluminium alloy sheets with a thickness of 1.6 mm and 1 mm and universal head MS20470-AD (2117-T4 aluminium alloy) rivets with shank diameters of 3.96 mm and 3.18 mm. The results obtained from finite element (FE) analyses were linked with observations of the fatigue crack initiation site, crack path and fretting phenomenon in real three-row riveted lap joints. The results show that the crack initiation site depends on the zone of compressive residual hoop stress in the sheets and the clamping force. The location of the crack path can be related to the location of the largest favourable residual normal stress in the sheets (acting in the direction of the joint loading), the value of the clamping force and the range of the residual clamping stress zone. Large fretting wear regions on the contact surface of sheets for joints with more severely squeezed rivets are associated with a larger zone and a higher value of clamping stresses, which during fatigue loading through friction intensify the harmful phenomenon of fretting.

Cold Expansion Process with Multiple Balls-Numerical Simulation and Comparison with Single Ball and Tapered Mandrels.

Cold expansion technology is an extended method used in aeronautics to increase fatigue life of holes and hence extending inspection intervals. During the cold expansion process, a mechanical mandrel is forced to pass along the hole generating compressive residual hoop stresses. The most widely accepted geometry for this mandrel is the tapered one and simpler options like balls have generally been rejected based on the non-conforming residual hoop stresses derived from their use. In this investigation a novelty process using multiple balls with incremental interference, instead of a single one, was simulated. Experimental tests were performed to validate the finite element method (FEM) models and residual hoop stresses from multiple balls simulation were compared with one ball and tapered mandrel simulations. Results showed that the use of three incremental balls significantly reduced the magnitude of non-conforming residual hoop stresses and the extension of these detrimental zone.

Generalized Plane Strain Study of Rotational Autofrettage of Thick-Walled Cylinders—Part II: Numerical Evaluation

The theoretical modeling of the rotational autofrettage of a thick-walled cylinder based on the generalized plane strain assumption has been presented in part I of the paper. In order to access the potentiality of the proposed theoretical model, the numerical evaluation of the analytical solutions is important. This part of the paper presents numerical evaluation of the generalized plane strain model for typical thick-walled cylinders. The residual hoop stress generated in the rotational autofrettage of a typical gun barrel is compared with the residual hoop stresses in the conventional hydraulic and swage autofrettage processes. Comparison shows that the rotationally autofrettaged gun barrel is capable of producing the same level of compressive residual hoop stress at the inner surface as that of the hydraulic autofrettage. In order to corroborate the analytical solution, a three-dimensional finite element method (3D FEM) analysis of the rotational process is carried out in ANSYS finite element package and the results are compared with the theoretical results. The comparison shows a good matching of the results between the theoretical evaluation and the 3D FEM analysis. Finally, a short feasibility analysis of the rotational autofrettage process of typical cylinders is carried out for the practical realization of the process.

Study the Effect of Anisotropy of Elastic-Plastic Properties on Residual Stress Development in Autofrettage of Thick Cylinder

Thick-walled cylinders subjected to high internal pressure and/or elevated temperatures are widely used in the aircraft, defence, nuclear and chemical industries especially for military applications e.g. the gun barrel in a ballistic event. In the absence of residual stresses, cracks usually form at the bore where the hoop stress developed by the working pressure is highest. To prevent such failure and to increase the pressure-carrying capacity of the pressure vessel, autofrettage process is used. During autofrettage a plastic region within the thick wall of the pressure vessel is produced by loading the pressure vessel in the plastic domain. Upon unloading, a residual compressive hoop stress is established within the plastic zone. This residual compressive stress counters the tensile hoop stress introduced due to service loading, thereby reducing the overall tensile hoop stress at inner surface. Therefore, autofrettage is used to introduce advantageous favourable compressive residual hoop stress inside wall of a cylinder and result in an increase in the fatigue lifetime of the component. Several researchers have studied the autofrettaged cylinders, both analytically and FE simulation. In this paper, effect of orthotropic anisotropy in the material properties will be studied on the stress field generated in the autofrettaged cylinder. Anisotropy in the material properties may result due to manufacturing processes like extrusion or pilgering. Plastic deformation leads to the development of preferred orientation of grains in the components due to slip occurring only in preferred slip planes in a crystal especially in a HCP material like Zr 2.5%Nb. In this paper, a thick cylinder made from Zr 2.5% Nb is loaded till elasto plastic interface is achieved and then unloaded to study effect of anisotropy on the development of residual stresses. Two material models are considered in the analysis i.e. isotropic and anisotropic plastic behaviour. The effect of anisotropy on the residual stress is investigated.

Numerical Modeling and Analytical Investigation of Autofrettage Process on the Fluid End Module of Fracture Pumps

One of the most important components in the hydraulic fracturing is a type of positive-displacement-reciprocating-pumps known as a fracture pump. The fluid end module of the pump is prone to failure due to unconventional drilling impacts of the fracking. The basis of the fluid end module can be attributed to cross bores. Stress concentration locations appear at the bores intersections and as a result of cyclic pressures failures occur. Autofrettage is one of the common technologies to enhance the fatigue resistance of the fluid end module through imposing the compressive residual stresses. However, evaluating the stress–strain evolution during the autofrettage and approximating the residual stresses are vital factors. Fluid end module geometry is complex and there is no straightforward analytical solution for prediction of the residual stresses induced by autofrettage. Finite element analysis (FEA) can be applied to simulate the autofrettage and investigate the stress–strain evolution and residual stress fields. Therefore, a nonlinear kinematic hardening material model was developed and calibrated to simulate the autofrettage process on a typical commercial triplex fluid end module. Moreover, the results were compared to a linear kinematic hardening model and a 6–12% difference between two models was observed for compressive residual hoop stress at different cross bore corners. However, implementing nonlinear FEA for solving the complicated problems is computationally expensive and time-consuming. Thus, the comparison between nonlinear FEA and a proposed analytical formula based on the notch strain analysis for a cross bore was performed and the accuracy of the analytical model was evaluated.

Design of two-pass swage autofrettage processes of thick-walled cylinders by computer modeling

An ever-increasing industrial demand for pressurized thick-walled cylindrical components drives research and practice to increase their strength–weight ratio, extend their fatigue life, or to increase their pressure-carrying capacity. This can be achieved through an energy-efficient and safe two-pass swage autofrettage process by generating a favorable compressive residual hoop stress field in the inner layer of the cylinder prior to use. In this paper, a two-pass swage autofrettage process of a thick-walled cylinder was systematically investigated based on finite element analysis. A 105 mm cannon barrel made of high-pressure vessel steel ASTM A723-1130 was taken as a case study. An elastic nonlinear-hardening plastic material model with the Bauschinger effect was adopted. The mandrel’s axial pushing forces during swage autofrettage processes were analyzed. A 30–35% reduction in mandrel’s pushing forces has been achieved in the two-pass process. The residual stresses in swage autofrettaged thick-walled cylinders were predicted. The results of computer modeling were in agreement with neutron diffraction measurements. A maximum 18% reduction in von Mises stress in the swage autofrettaged thick-walled cylinders under an elastic-limit working pressure was identified. A maximum 31% increase in pressure-carrying capacity for the swage autofrettaged thick-walled cylinders was revealed. The optimum radial interference was proposed. Results from the two-pass process were compared with those from the single-pass process.

A new approach for pre-stressing of rail-end-bolt holes

Bolted joint railroad is the subject matter of this paper. Rail joint elements are subjected to cyclic and impact loads as a result of the passage of trains, which causes the origination and growth of fatigue cracks occurring, in most cases, around the bolt holes. Fatigue failure around rail-end-bolt holes is particularly dangerous because it leads to derailment of trains and, consequently, to inevitable accidents. Moreover, the cracking at rail-ends, which starts from bolt hole surface, causes premature rails replacement. The presence of residual compressive hoop stresses around the bolted holes, which is achieved by prestressing of these holes, extends the fatigue life of bolted joint railroads. This article presents an innovative technology for pre-stressing of rail-end-bolt holes, implemented on a vertical machining centre of Revolver vertical (RV) type. Two consecutive operations are involved in the manufacturing technology process: formation of the hole by drilling, reaming and making of a chamfer through a new combined cutting tool; cold hole working by spherical motion cold working through a new tool equipment, which minimizes the axial force on the reverse stroke. The new technology introduces beneficial residual compressive stresses around the bolted holes thereby preventing the fatigue cracks growth and increasing the fatigue life of these openings.

Modeling of residual stress distribution around fastener holes in thin plates after symmetric cold expansion

The fatigue failure around fastener holes in metal structures is particularly dangerous, since it leads to inevitable accidents. The presence of residual compressive hoop stresses around these holes, imparted by means of mandrel cold working methods closes the existing cracks and impedes the formation of new ones and thereby extends the fatigue life of corresponding component. Although this type of pre-stressing of fastener holes has been of great advantage over recent years it has some disadvantages: significant and nonsymmetrical with respect to the plate middle plane axial gradient of the residual hoop stresses due to axial force flow passing through the plate, considerable surface upset, etc. The article presents a patented method and a tool ensuring symmetric cold hole expansion—introducing near-uniform residual hoop stresses around the hole along its axis having minimized and symmetric gradient with respect to the plate middle plane. Finite element (FE) simulations of the new process have been carried out. The residual stresses and out-of-plane deformations (surface upset) have been analyzed and compared with those obtained by mandrel cold working method and thus the advantages of the patented method have been shown. The symmetric cold expansion process has been realized experimentally and modeling of residual stress distribution around cold expanded holes in carbon steel specimens has been made. The residual hoop stresses have been measured by means of X-ray diffraction technique. The obtained generalized empirical mathematical model of the residual hoop stress distribution is polynomial with respect to the distance from the hole edge and has “coefficients” that are functions of the governing factors. On the basis of the information obtained from FE simulations, suitable fractional factorial experimental design has been synthesized and used for obtaining the polynomial “coefficients”. The verifications of the generalized model prove its authenticity.

A Study of the Combined Effects of Erosions, Cracks and Partial Autofrettage on the Stress Intensity Factors of a Thick Walled Pressurized Cylinder

For the investigation of cracked problems in thick-walled pressurized cylindrical vessels, the displacement-based finite element method has become one of the main computational tools to extract stress intensity results for their fatigue life predictions. The process of autofrettage, practically from the partial autofrettage level of 30% to full autofrettage level of 100%, is known to introduce favorable compressive residual hoop stresses at the cylinder bore in order to increase its service life. In order to extract the fatigue life, stress intensity factors (SIFs) need to be obtained a priori. The necessity for determining SIFs and their practical importance are well understood. However, it is usually not a trivial task to obtain the SIFs required since the SIFs largely depend on not only the external loading scenarios, but also the geometrical configurations of the cylinder. Our recent work has shown that the Bauschinger effect (BE) may come into play and affect the effective SIFs significantly for an eroded fully autofrettaged thick-walled cylinder. In this study, we further investigate the SIFs for the Bauschinger effect dependent autofrettage (BEDA) and the Bauschinger effect independent autofrettage (BEIA) at various autofrettage levels. The crack is considered to emanate from the erosion's deepest point in a multiply eroded cylinder. The commercial finite element package, ANSYS v12, was employed to perform the necessary analysis. A two-dimensional model, analogous to the authors' previous studies, has been adopted for this investigation. The residual stress field of autofrettage process, based on von Mises yield criterion, is simulated by thermal loading. The combined SIFs are evaluated for a variety of relative crack lengths with cracks emanating from the tip of erosions with various geometrical configurations and span angles. The effective SIFs for relatively short cracks are found to be increased by the presence of the erosion and further increased due to the BE at the same autofrettage level, which may result in a significant decrease in the vessel's fatigue life. Deep cracks are found to be almost unaffected by the erosion, but may be considerably affected by BE as well as by the level of partial autofrettage.

A novel method and tool which enhance the fatigue life of structural components with fastener holes

The fatigue failure around fastener holes in metal structures is particularly dangerous, since it leads to inevitable accidents. The presence of residual compressive hoop stresses around these holes, imparted by means of mandrel coldworking methods (including split sleeve cold expansion and split mandrel coldworking) closes the existing cracks and impedes the formation of new ones and thereby extends the fatigue life of corresponding component. Although this type of pre-stressing of fastener holes has been of great advantage over recent years it has some disadvantages: significant and nonsymmetrical with respect to the plate middle plane axial gradient of the residual hoop stresses due to axial force flow passing through the plate, considerable surface upset, etc. This article presents a novel patented method and a tool ensuring “pure” radial cold hole expansion – introducing nearly uniform residual hoop stresses around the hole along its axis having a minimized and symmetric gradient with respect to the plate middle plane. Because of this advantage, the method has been called by the authors “symmetric cold expansion”. Another advantage of the method is that it ensures one-sided process like the split sleeve and split mandrel. The carried out finite element (FE) simulations proved that the residual stresses in the novel method are much more uniformly distributed in comparison with the mandrel coldworking methods. The fatigue life improvement of the novel method in comparison with mandrel coldworking methods has been proved experimentally through fatigue cyclic test.

A new approach to enhancement of fatigue life of rail-end-bolt holes

The object of this paper is bolted joint railroads as the accent is put on the material behaviour around the bolted holes. The fatigue failure around rail-end-bolt holes is particularly dangerous, since it leads to derailment of trains and consequently, to inevitable accidents. Moreover, the cracking at rail-ends, which starts from bolt hole surface, causes premature rails replacement. It is well-known that the presence of residual compressive hoop stresses around the bolted holes closes the existing first mode cracks and impedes the formation of new ones and thereby extends the fatigue life of the holed components. In this article a new approach to enhancement of fatigue life of rail-end-bolt holes has been developed on the basis of a novel method and tool for cold expansion (CE) of holes, patented by the authors. The major advantage of the method is in imparting around the holes of beneficial residual compressive hoop stresses which are symmetric toward the middle longitudinal plane of the rail and thus the axial stress gradient is minimum. The developed approach consists in setting and solving a multi-objective optimization task of the CE of the rail-end-bolt holes. Because of the specificity of the studied problem, the optimal solution has been found by finite element (FE) simulations. For that purpose generalized FE model of the object has been developed. Through this model the critical point around the outside holes has been localized and the cycle of variation of hoop stresses has been determined taking into consideration assembly stresses. On the basis of adapting the generalized FE model of the rail joint to simulating the CE process, a comparison between the cases with and without CE of rail-end-bolt holes has been made. On this basis the optimal degree of CE and the successive hole treatment have been found. After simulating the CE process by the determined optimal degree of CE, the beneficial effect of implementing the new approach has been proved.

Enhancement of fatigue life of net section in fitted bolt connections

The basic idea of this paper underscores enhancement of fatigue life of the net section in bolted connections by means of the developed method whereby beneficial residual hoop compressive stresses, distributed almost uniformly along the hole axis, are created around the bolt hole. Since the method includes bolt hole cutting up to a precise size after preliminarily cold hole expansion, it is especially appropriate for fitted bolt connections. In the case of conventional cold hole expansion the residual compressive hoop stresses are characterized by significant axial gradient and a tensile field sometimes arises on the hole entrance face. The proposed method homogenizes the compressive field around the bolt hole in an axial direction by means of residual hoop stress redistribution. These stresses significantly reduce the operating tensile stresses at the net section points. Due to the tensile operating load, the resultant hoop normal stresses (superposition from residual compressive and operating tensile stresses) at the net section points are significantly smaller in comparison with the conventional case. The developed method has been studied both experimentally, by X-ray diffraction technique, and numerically, by finite element (FE) simulations. Four FE models have been developed for investigation and optimization of the proposed approach. Application of this approach enhances the fatigue life of the net section in bolted connections due to operating tensile stress reduction.

Bauschinger Effect’ Influence on the Componud Cylinder Containing an Autofrettaged Layer

Autofrettage is used to introduce advantageous residual stresses into cylinders. The Bauschinger effect can produce less compressive residual hoop stresses near the bore than are predicted “ideal” autofrettage solutions. A723 steel is used for compound cylinder. This paper extends the analysis to material the addition of pressure or of shrink-fitting to the cylinders, providing associated residual stress profiles following various amounts of further yielding due to a net external pressure. The Bauschinger effects for “realistic” – Bauschinger effect dependent autofrettage are obtained. The 2-D analysis is performed via the finite element method. The Bauschinger effect is found to significantly lower the beneficial stress due to autofrettage.

Stress Intensity Factors for a Curved-Front Internal Crack in an Autofrettaged Tube With Bauschinger Effect

Pressure vessel steels exhibit the Bauschinger effect that significantly reduces post-autofrettage residual compressive hoop stresses in the near-bore region in comparison with ‘ideal’ (elastic-perfectly plastic) behavior. These reduced hoop stress profiles were calculated using Von Mises’ criterion via a nonlinear analysis for the case of open-end (engineering plane strain) autofrettage. These profiles were then used to obtain stress intensity factor solutions via the Boundary Integral Equation (BIE) method, commonly known as the Boundary Element Method (BEM). Results are presented for tubes of diameter ratio 2 and 2.5 with an internal semi-elliptical surface crack having a maximum depth/surface length ratio of 0.4 (i.e., an eccentricity of 0.8). Crack depths range from 20% to 80% of wall thickness and results are presented for seven locations on the crack front from maximum depth to free surface. For crack depths up to 20% of wall thickness there is a significant reduction in magnitude of autofrettage stress intensity factor due to Bauschinger effect. For typical overstrain levels this reduction is approximately 30% of “ideal” values. Such a reduction may, in turn, cause an order of magnitude reduction in the fatigue lifetime of the vessel.

Residual Stresses and Lifetimes of Tubes Subjected to Shrink Fit Prior to Autofrettage

There is increasing interest in the use of compound cylinders that combine shrink fit and autofrettage, taking account of Bauschinger effect. Previous work has analyzed material removal from a plain, autofrettaged cylinder and the application of an external pressure or shrink to a previously autofrettaged plain tube. In this paper a different design philosophy is examined, namely the shrink fitting of two tubes prior to autofrettage. Such a process is shown to be beneficial in inhibiting loss of near-bore compressive residual hoop stress due to Bauschinger effect and thereby increasing calculated fatigue lifetime. These effects are described in detail for a specific geometry, and summary data suitable for use by designers are presented for a large range of configurations.

Autofrettage of Open-End Tubes—Pressures, Stresses, Strains, and Code Comparisons

Autofrettage is used to introduce advantageous residual stresses into pressure vessels. The Bauschinger effect can produce less compressive residual hoop stresses near the bore than are predicted by “ideal” autofrettage solutions. A recently developed numerical analysis procedure is adopted and extended. The ratio of calculated autofrettage pressure (numerical)/ideal autofrettage pressure (Tresca criterion and plane stress) is calculated and verified against available solutions. The case of open-end conditions based upon von Mises and engineering plane strain (constant axial strain with zero net axial force) is examined in detail. The ratio in this case varies between unity and 2/3, but exhibits very significant variations from the plane stress case when the diameter ratio of the tube exceeds 1.8. Results are within 0.5 percent of available analytical, numerical, and experimental results. A simple numerical fit allows all autofrettage pressures to be replicated to within 0.5 percent. The true plane strain pressure ratio is examined and shown to be inappropriate in modeling engineering plane strain. A number of residual hoop and axial stress profiles is presented for radius ratio 2.0. Calculated pressures are used to determine residual hoop stress values for tube diameter ratios from 1.1 to 3.0 for the full range of percentage overstrain levels. These comparisons indicate that Bauschinger effect is evident when the ratio autofrettage radius/bore radius exceeds 1.2, irrespective of diameter ratio. To assist designers the important values of residual hoop stress at the bore are summarized in a composite plot and a numerical fit is provided. The accuracy of the current ASME code using pressure criteria is assessed. The code is shown to be generally and modestly conservative. A design procedure is proposed which appears capable of extending code validity beyond 40 percent overstrain (the limit of the current code) and of eliminating the small nonconservatism at very low overstrain. Hoop strain values are calculated at both the bore and outside diameter of a tube of radius ratio 2 at the peak of the autofrettage cycle using von Mises criterion with open-end, closed-end, and plane strain conditions. These are compared with available solutions; general agreement is demonstrated, with agreement within 2 percent with an accepted simple formula in the case of open ends. ASME code predictions of percentage overstrain based upon strains at the peak of the autofrettage cycle are generally within 6 percent of numerical predictions. This is in turn produces an agreement within approximately 3 percent in residual bore hoop stress calculation. This discrepancy is generally conservative, becoming non-conservative only at overstrain levels exceeding 80 percent. Strain during removal of autofrettage pressure, in the presence of Bauschinger effect, is also calculated. This shows that the difference in strain during the unloading phase is up to 8 percent (ID) and 6.3 percent (OD) compared with the predictions of elastic unloading. These latter results show similar agreement with the ASME code as in the peak-strain analysis and permit correction of estimates of percentage overstrain based upon permanent bore enlargement.

Bauschinger Effect Design Procedures for Compound Tubes Containing an Autofrettaged Layer

Autofrettage is used to introduce advantageous residual stresses into pressure vessels. The Bauschinger effect can produce less compressive residual hoop stresses near the bore than are predicted by “ideal” autofrettage solutions. A design procedure was recently proposed which models material removal from the bore or outside diameter of a single, plain autofrettaged tube in the presence of Bauschinger effect. This paper extends the procedure to model the addition of pressure or of material (via shrink-fit) to the tube, providing associated residual stress profiles following various amounts of further yielding due to a net external pressure. Simple criteria are developed for determining, and avoiding, further yielding in the autofrettaged tube when it is used as part of a compound assembly involving shrink-fitting; these criteria are based upon net pressure differential between the bore and outside diameter of the autofrettaged tube. An alternative criterion, based upon bore hoop stress, is shown to be erroneous.

Bauschinger Effect Design Procedures for Autofrettaged Tubes Including Material Removal and Sachs’ Method

Autofrettage is used to introduce advantageous residual stresses into pressure vessels and to enhance their fatigue lifetimes. The Bauschinger effect serves to reduce the yield strength in compression as a result of prior tensile plastic overload and can produce lower compressive residual hoop stresses near the bore than are predicted by “ideal” autofrettage solutions (elastic/perfectly plastic without Bauschinger effect). A complete analysis procedure is presented which encompasses representation of elastic-plastic uniaxial loading material behavior and of reverse-loading material behavior as a function of plastic strain during loading. Such data are then combined with some yield criterion to accurately predict elastic-plastic residual stress fields within an autofrettaged thick cylinder. Pressure for subsequent reyielding of the tube is calculated. The numerical procedure is further used to determine residual stress fields after removal of material from inside diameter (i.d.) and/or outside diameter (o.d.), including the effects of any further plasticity. A specific material removal sequence is recommended. It is shown that Sachs’ experimental method, which involves removing material from the i.d., may very significantly overestimate autofrettage residual stresses near the bore. Stress ranges and stress intensity factors for cracks within such stress fields are calculated together with the associated fatigue lifetimes as such cracks propagate under cyclic pressurization. The loss of fatigue lifetime resulting from the Bauschinger effect is shown to be extremely significant.

Origin of a magnetic easy axis in pipeline steel

Oil and gas pipelines are generally magnetically anisotropic, with a magnetic easy axis in the pipe axial direction. This is of interest because magnetic flux leakage tools are commonly used for the detection and sizing of defects. In the present study we investigate the origin of this magnetic easy axis, using an angular magnetic Barkhausen noise technique to characterize the magnetic anisotropy. The texture, microstructure, and residual stress are examined as possible causes of the easy axis, using x-ray pole figure analysis and microstructural examination along with high and low temperature annealing treatments. Our results indicate that plastic deformation and residual stress are responsible for the magnetic easy axis, since an elimination of the residual stresses through low temperature “stress relief” heat treatment produces a magnetically isotropic structure without altering the texture or microstructure. X-ray pole figure analysis supports the conclusion that magnetic anisotropy is not related to texture in these materials. We conclude that the axial magnetic easy axis is due to a compressive residual hoop stress resulting from the cold bending and cold expansion of the pipe during processing.

LOW‐TEMPERATURE AUTOFRETTAGE: AN IMPROVED TECHNIQUE TO ENHANCE THE FATIGUE RESISTANCE OF THICK‐WALLED TUBES AGAINST PULSATING INTERNAL PRESSURE

Abstract— It is shown that autofrettage at low temperatures is superior to autofrettage at room temperature in enhancing the fatigue resistance of thick‐walled tubes against pulsating internal pressure. The physical reason is based on the well‐known temperature dependence of the mechanical behaviour of metals and alloys which generally exhibit an enhancement of both the yield stress and strain hardening behaviour at lower temperatures. As a consequence, significantly larger compressive residual hoop stresses can be introduced during pressurization at low temperatures than at room temperature. Experimental data obtained on thick‐walled tubes of the metastable austenitic stainless steel AISI 304 L which were subjected to pulsating internal pressure at room temperature after autofrettage at temperatures between‐110°C and room temperature are presented. These data demonstrate convincingly the advantages offered by low‐temperature autofrettage in enhancing both the fatigue life in the finite‐life region and the fatigue endurance limit in comparison with autofrettage at room temperature. In conclusion, some specific materials requirements for optimum low‐temperature autofrettage performance are discussed.