Optimization of a cold-working process for increasing fatigue life

This paper introduces a method which allows the calculation of the optimal radial interference and the optimal mandrel shape on a cold-expanded bushing–hole connection commonly used in aerospace structures, in order to obtain the desired values of the residual stresses on the hole surface. This method has been developed by an extended campaign of FE analysis, planned and evaluated with statistical techniques on the basis of a previous presented closed-form method to determine the residual stress field. The method will give the possibility to reduce the zone of the hole surface subject to negligible residual radial stresses, obtaining also compressive residual circumferential stresses on the entire hole surface.An experimental axial fatigue test plan on aeronautical components with optimized cold-expanded bushing–hole connections and subjected to cyclic loads showed a substantial improvement in fatigue life.     

Stresses in a nonlinearly hardening pipe after expansion to large deformations,inelastic unloading,and removal of its inner layers

The problem of the numerical determination of residual stresses in an elastoplastic nonlinearly hardening pipe of V95ochT2 material in plane strain is examined.It is shown that due to secondary plastic deformations that develop during unloading, the greatest residual circumferential stresses reside at a certain distance from the inner surface of a thick-wall pipe.Removal of the internal layer of the pipe gives rise to a small change in the circumferential compressive stresses on its inner surface. In this case, the zone where the maximum residual circumferential compressive stresses reside is shifted deeper into the pipe with a simultaneous decrease in the distance from this zone to the new inner surface of the piper, which is formed after removal of a layer of metal.Translated from Problemy Prochnosti, No. 11, pp. 111–114, November, 1989.     

Producing bulk residual stresses in gas turbine blades

Inhomogeneous plastic strain has been used to produce a pattern of bulk compressive stresses that counteract macrodefect formation and growth in machine components, which increases the working life. Studies have been made on the residual stresses in a blade caused by uneven heating or compression. The stresses have been calculated by the finite-element method. The residual-stress pattern parameters have been related to the geometrical dimensions of the heat-or force-treated zone and the extent of the treatment.Translated from Problemy Prochnosti, No. 2, pp. 58–64, February, 1993.     

Wear of 3Y–TZP containing compressive residual stresses at the surface

Tetragonal zirconia polycrystals doped with 3 mol % yttria (3Y–TZP) was heat treated to stimulate the tetragonal to monoclinic phase transformation in order to induce compressive residual stresses at the surface. Pin-on-disc wear tests were performed at different sliding speeds to evaluate the wear behavior of treated and untreated materials. The friction coefficients were also measured. The conditions for mild and severe wear were defined. The material containing compressive residual stresses had lower wear resistance. A possible explanation for this behavior is discussed.     

The influence of roller burnishing on the fatigue crack propagation in notched round bars—Experimental observations under three-point bending

Roller burnishing involves a local plastic deformation on the surface that permits the fatigue strength of structures to be increased. Crack propagation is delayed by the introduction of compressive residual stresses. In this way, the process is particularly useful in the presence of stress concentrators, for example in the fillets of crankshafts or in notched shafts. Crack propagation in round bars has been widely investigated, experimentally and numerically. However, the aim of the present work is to study roller burnished notched shafts (whose groove approaches the fillets of crankshafts) loaded in three‐point bending. Experimentally, a sequence of destructive interrupted tests has been performed to obtain the crack kinetics and shapes during fatigue life. Thus, the beneficial influence of roller burnishing has been confirmed on both crack initiation and propagation. Some cracks were even stopped by compressive residual stresses. Otherwise, the crack shape observed was very original : cracks propagated first at the edge of the coupon and then, into the depth of the shaft. Multicracking and crack closure have been pointed out and appear to be of great importance.     

Correlation of residual stresses and fatigue limit with torsion under conditions of stress concentration Report 1. Substantiation of strength and longevity criteria

Results are given for a calculation and experimental study of the effect of residual stresses on fatigue limit with torsion for specimens with circular notches of semicircular and rectangular shape made of steels 40, 30KhGSA, EI961, and aluminum alloy V95. The possibility is demonstrated of a criterion for mean integral residual stresses through the thickness of a surface layer equal in depth to a nonpropagating fatigue crack in order to estimate the influence of residual compressive stresses on fatigue limit in the case of torsion with stress concentration.Translated from Probleray Prochnosti, No. 5, pp. 43–46, May, 1991.     

Messung von Festwalzeigenspannungsverteilungen an Bauteilen

Measurement of Deep-Rolling Residual Stress-Distributions on Components In many practical cases surface layers of components are the most heavily stressed material zones. In order to improve the fatigue strength and the wearability of components thermal, thermochemical and mechanical surface treatment methods like induction hardening, case hardening, nitriding, shot peening and deep-rolling are applied for several years. The effect of these methods depends on the increase of strength and on the production of compressive residual stresses in the heavily stressed surface layers. The mechanical processes also affect the surface roughness which may be reduced by choosing suitable process parameters. Due to the mechanical surface treatment by deep-rolling fatigue strength improvements up to 200% are possible [1], and the compensation of the notch effect was observed for notched specimen [2]. In the industry the deep-rolling is applied on e. g. crankshafts, screws, valve shafts and actuation shafts. The direct control of surface layer properties, like residual stresses and hardening, by x-ray measurements on axial sections of the components is presented here as a method for quality insurance of rolled parts. The measuring method may also be used to verify results of FEM-simulations. FEM-simulations are applied to estimate the modification of the residual stresses due to preparation.     

Evaluation of residual stress in BaTiO3-based Ni-MLCCs with X7R characteristics

The residual stress in BaTiO₃-based multilayer ceramic capacitors (MLCCs) with X7R characteristics was estimated at three different margins using a micro-indentation technique. The hardness at the margins of the MLCCs was in the range 8–13.1 GPa, depending on the direction at each margin, and the hardness decreased with increasing distance from the electrode. At the l and w margins parallel to the electrode, the residual stresses were indicative of compressive stresses of –45±8 MPa and –2.7±18 MPa in the direction parallel to the electrode, and tensile stresses of 177±71 MPa and 281±177 MPa in the direction perpendicular to the electrode, respectively. Moreover, the other t margin perpendicular to the electrode showed the reverse tendency in the residual stresses, with tensile and compressive stresses of 118±113 MPa and –54±11 MPa in the directions parallel and perpendicular to the electrode, respectively. The residual stresses created on the MLCCs depended on the margin with the lamination and on the direction to the electrode.     

Damage accumulation in light alloys on programmed loading. Part 2. Effects of fretting corrosion

A study has been made on how fretting corrosion affects damage accumulation in programmed loading with two-step blocks for D16AT, VT14, and AMg6 alloys. The accumulation is dependent on the relation between the stresses in the steps and the yield point for the surface layer of material. The hardening in the high step is due to the residual compressive stresses, which cause plastic strain with loading beyond the yield point of the surface layer but elastic strain in the internal layers.Translated from Problemy Prochnosti, No. 12, pp. 31–37, December, 1992.     

Influence of hydrovibration treatment of thin walled shells on the stressed and strained condition

The results of an experimental investigation of the hydrovibration method for reducing residual stresses are given. A mathematical model of the influence of the maximum and minimum cycle pressure in hydrovibration treatment on reduction of the component of tensile and compressive residual stresses is constructed. It was established that hydrovibration treatment makes it possible to reduce the original residual tensile stresses by 86%. This method makes it possible to significantly increase the fatigue strength and service life of welded parts such as thin-walled shells and tubes as the result of creation in them of the most favorable stressed condition, which is characterized by a quite high component of compressive and a low component of tensile residual stresses.Translated from Problemy Prochnosti, No. 6, pp. 98–101, June, 1990.     

An experimental and analytical study of fatigue crack shape control by cold working

This paper presents an experimental and analytical study of crack shape evolution in steel specimens under cyclic loading. It is widely known that the introduction of compressive residual stresses by cold working the surface can be highly beneficial in improving the fatigue performance of structural components. Although it is recognised that relaxation of surface compressive residual stress can reduce the potential benefits, the effects of residual stress on crack shape evolution are often overlooked. A recently developed technique termed controlled stitch cold working, which applies differing intensities of compressive residual stress at specific regions in a structure, is shown in the paper to considerably influence fatigue crack propagation by containing crack propagation in one primary direction.     

喷丸起落架外筒焊接热影响区的残余应力测定

本文用X射线法测定了喷丸起落架外筒焊缝影响区的残余应力,并从残余应力角度考察了起落架外筒现行工艺路线的优劣。测定结果表明,在相同构件上虽然所测方向和部位不同,但残余应力的分布、大小及符号基本相同。与前起外筒相比,喷丸处理在主起外筒上形成的残余压应力值较低。分析认为消除焊接残余拉应力的合理工序,是两种外筒均应采用焊后热处理,但应防止表面脱碳。     

An Investigation of Machining-Induced Residual Stresses and Microstructure of Induction-Hardened AISI 4340 Steel

Excessive induction hardening treatment may result in deep-hardened layers, combined with tensile or low compressive residual stresses. This can be detrimental to the performance of mechanical parts. However, a judicious selection of the finishing process that possibly follows the surface treatment may overcome this inconvenience. In this paper, hard machining tests were performed to investigate the residual stresses and microstructure alteration induced by the machining of induction heat-treated AISI 4340 steel (58–60 HRC). The authors demonstrate the capacity of the machining process to enhance the surface integrity of induction heat-treated parts. It is shown how cutting conditions can affect the residual stress distribution and surface microstructure. On the one hand, when the cutting speed increases, the residual stresses tend to become tensile at the surface; and on the other hand, more compressive stresses are induced when the feed rate is increased. A microstructural analysis shows the formation of a thin white layer less than 2 µm and severe plastic deformations beneath the machined surface.     

Photo-oxidation of polystyrene under load

The effect of a tensile stress on the rate of photo-oxidation of polystyrene was investigated. Molecular-weight measurements showed that tensile stress accelerates molecular scission in injection-moulded bars exposed to ultraviolet irradiation. Changes in the residual-stress distribution were observed during the exposure but they were not sufficient to reverse the sense of the residual stresses, and the residual stress near the surface remained compressive. Depth profiling indicated that the degradation process was oxygen-diffusion limited and an approximate analysis of the kinetics gave an activation energy for degradation of 4.2 kJm^–2, which is also consistent with a diffusion process.     

Raising the fatigue strength of a construction by local laser heating

A method is proposed for raising the fatigue strength of a construction by inducing residual stresses by local heating with a high-power pulsed laser. Fatigue failure can be slowed or eliminated, as can the formation of fatigue defects, if a pattern of compressive residual stresses is produced in the most readily damaged zones. Experiments have been done on how laser heating affects the fatigue strengths of planar specimens and fatigue crack growth. Finite-element calculations have been used to examine the effects of the heating parameters on the residual stresses.Translated from Problemy Prochnosti, No. 4, pp. 10–17, April, 1995.     

Residual stresses after thermomechanical treatment involving nonuniform plastic deformation

Summary It was shown that nonuniform plastic deformation during TMT may lead to the appearance of high residual stresses of the first kind.For equal geometrical parameters of deformation (i. e., for equal deformation gradients and degrees of deformation), the magnitude of residual stresses is determined primarily by the mechanical properties of the phases deformed, (in the present case G, G₁, and _S) and, consequently, substantially depends on the deformation temperature t_d. Residual stresses of considerable magnitude, which must be allowed for in calculating the load-carrying capacity of parts subjected to TMT, are produced at t_d below the recrystallization temperature. Consequently, TMT which involves plastic deformation at 400°–550° C may produce residual stresses comparable to those produced by cold working.Practically no residual stresses are produced by HTMT; this is because structural steels have a very low resistance to deformation at temperatures above .No substantial differences were observed between the residual stresses produced by deformation of supercooled austenite and products of non-martensite decomposition, when the treatment was carried out at the same t_d.Quenching after deformation prevents stress relaxation and so increases the level of residual stresses which, however, do not reach the level of stress that would be produced by corresponding cold deformation.It was shown on the example of TMT with deformation in torsion that residual stresses after this treatment may serve as an additional source of directional strengthening. Particularly promising with respect to increasing the fatigue strength of machine parts should be surface TMT, which produces residual compressive stresses in the surface layers.     

Analysis of the reliability of piecewise homogeneous shell glass structures

On the basis of the established relationship between the durability of the material (of glass specimens) and glass products, we develop an algorithm for the analysis of reliability of piecewise-homogeneous shell glass structures. This algorithm is applied to the evaluation of reliability of a composite circular cylindrical shell operating under atmospheric pressure in a medium with different temperatures in different parts of the shell. The surface of the glass structure is split into areas of the same material in quasi-homogeneous and simple stressed state. On the surface of the structure, we determine the fields of total principal stresses affecting its reliability. Technological (residual) and working stresses are obtained by using the combination of experimental and theoretical methods. Thermal stresses in the structure are computed by applying numerical and analytic methods. Reliability analysis is performed for five years of operation.Pidstryhach Institute for Applied Problems in Mechanics and Mathematics, Ukrainian Academy of Sciences, L'viv. Translated from Fiziko-Khimicheskaya Mekhanika Materialov, Vol. 31, No. 4, pp. 20–27, July – August, 1995.     

Effect of cutting speed and tool rake angle on residual stress distribution in machining 2024-T351 aluminium alloy — unlubricated conditions

The residual stress distribution in the machining of 2024-T351 aluminium alloy was measured using an electrolytic etching technique. Ring-shape specimens were machined under unlubricated orthogonal conditions with high-speed steel tools having rake angles of 10, 15, 20 and 25° at cutting speeds ranging between 0.5 and 1.25 m sec^–1. The results of the investigation show that the residual stresses are compressive at the machined surface and decrease with depth beneath the machined surface. The maximum (near-surface) residual stress and the depth of the severely stressed region increase with an increase in the cutting speed. There seems to be little change in the residual stress distribution due to a change in the rake angle. The results are interpreted in terms of the variations in the amount of surface-region deformation produced by changes in cutting conditions.     

On the relationship between the length of Palmqvist cracks and residual surface stresses in WC-Co

WC-Co cylinders were subjected to cyclic compressive stresses for varying numbers of cycles and maximum compressive stresses. The fractional changes in the length of Palmqvist cracks and in the residual surface stresses were measured after precompression. It is found that the relationship between Palmqvist crack length and residual surface stresses depends on the conditions of precompression, and thus Palmqvist crack lengths cannot provide quantitative information on residual surface stresses.     

Investigation of residual stress relaxation under cyclic load

Compressive residual stresses induced by mechanical surface treatment such as shot peening, autofretage, hole expansion, laser shock peening, and low-plasticity burnishing can be highly beneficial to fatigue resistance. Cyclic relaxation of compressive residual stress, however, reduces the benefit. An analytical model is proposed for estimation of residual stress relaxation. Parameters considered by the model include the magnitude and distribution of the residual stress, the degree of cold working required, the applied alternating and mean stresses, and the number of applied loading cycles. An elasto–plastic finite element model was used to demonstrate the model.