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This article presents outcomes from a comparative analysis involving three static burnishing processes: slide burnishing (SB), roller burnishing (RB), and deep rolling (DR). The treated material was 41Cr4 steel. The investigative methods used were fully coupled thermal-stress finite element (FE) simulations and natural experiments. Using one and the same magnitudes for the governing factors, the basic difference among the compared processes was the type of contact between the deforming element and the surface being burnished—sliding friction for SB and rolling contact for RB and DR. SB was implemented with a spherical-ended polycrystalline diamond whereas RB and DR were conducted using a single toroidal roller with the same magnitude for the radius of the toroid surface as that for the radius of the deforming diamond. The objects of comparison were in themselves processes and considered to be alterations in the thermodynamic systems’ states, as were the obtained surface integrities (SIs) of the treated specimens and their fatigue behaviors. It was established that three-quarters of the external work in SB converts into heat in the “deforming element–workpiece” contact area, which leads to the so-called softening effect in the surface layers. The comparison of the energy balances of the investigated processes clearly demonstrates the thermo-mechanical nature of the SB process, whereas the deforming processes in the RB and DR can be assumed to be purely mechanical. On the other hand, SB provides less roughness, significantly greater micro-hardness, larger-in-absolute-values compressive residual stresses, a more refined microstructure and, as a result, greater fatigue strength compared with RB and DR.

The paper presents a new single-operation two-transition technology for processing holes in sliding bearing bushings made of CuAl8Fe3 bronze. The technology was realized on a vertical machine cente...

D16AT aircraft Al-alloy was evaluated. The friction stir hole expansion (FSHE), solid mandrel cold working and symmetric cold expansion (SCE) methods were compared. The results are summarized on the basis of fatigue tests, S–N curves, X-ray diffraction, and microstructural analysis. Under the high-cycle fatigue, SCE provides more than 66 times longer fatigue life as compared to the solid mandrel method and more than 82 times greater fatigue life in comparison with the FSHE method. Through X-ray diffraction analysis it was found that the higher efficiency of the SCE method was due to symmetric distribution (with respect to the plate middle plane) of residual hoop stresses around the hole. On the other hand, the solid mandrel cold working method causes a significant gradient of the residual stress distribution through the plate thickness, which is a precondition for nucleation and propagation of corner fatigue cracks. The FSHE method efficiency was established to be primarily dependent on generated heat and equivalent plastic strains. The combination of these factors determines the beneficial microeffect of the microstructure modifying in the vicinity of the hole and a useful macroeffect due to residual compressive stresses. It was concluded that SCE method should be used for prestressing of fastener holes in the most loaded components in D16AT aircraft structures – wings and fuselage, while the FSHE one can be applied to processing of fastener holes in less loaded aircraft components.

This review paper is devoted to the slide burnishing (SB) of metal components—state-of-the-art, achievements and perspectives. SB belongs to the group of static methods for mechanical surface treatment used largely in aerospace, automotive and other industries. By means of the plastic deformation of the surface layers, the surface integrity (SI) of the respective component is improved greatly in terms of minimum roughness, micro-hardness and introduced residual compressive stresses. As a result, fatigue crack resistance, crack corrosion resistance, wear resistance and corrosion resistance increase dramatically. The main feature of SB is the sliding friction contact between the deforming element and the surface being treated. Using the differential-morphological method, an integrated classification of the static methods is created and the area of SB is outlined. The proposed morphological matrix, which can be expanded and supplemented contains existing burnishing methods as well as combinations of elements and interactions that can be used to synthesize new burnishing methods and tools. In addition, a literature review of the publications devoted to SB has been conducted. Further, an analysis of the published studies on different criteria has been carried out and graphic visualizations of the statistical results have been made. On this basis, relevant conclusions have been made and the directions for future investigations of SB have been outlined.

Static mechanical surface treatment (MST) processes based on the severe plastic deformation of the surface and subsurface layers improve the surface integrity (SI) of a metal component dramatically and thus its operational properties. The finite element method (FEM) is a basic simulation method used in the numerical investigations of MST processes. Although FEM always requires experimental verification of the results so obtained and an experiment to establish an adequate material constitutive model, this method saves of the researcher significant time and resources. Based on an analysis of the published studies devoted to FE simulations of static MST processes, five basic conditions have been found to be essential in order to build an adequate FE model. The theoretical formulations are then illustrated by creating FE models of the slide diamond burnishing (SDB) process using different strategies to make a comparative analysis between them. SDB is a static MST process with a thermomechanical nature. The adequacy of each FE model, respectively, strategy, is then assessed by comparing the FE results for the residual stresses with the experimental results obtained via the X-ray diffraction technique. It has been shown that a fully coupled thermal-stress 3D FE analysis of an SDB process with nonlinear kinematic hardening should be carried out. When the burnishing velocity is relatively small, the thermal effect can be neglected.

Diamond burnishing is a surface modification method aimed at improvements in the surface integrity (SI) and operating behavior of metal components. A cost-effective optimization approach for increasing the fatigue strength of diamond-burnished steel components has been developed. The basic idea is that the fatigue strength can be controlled by controlling some of the SI characteristics (surface micro-hardness, hardened-layer depth and roughness) whose measurements are not time-consuming and expensive. Thus, a multi-objective optimization task was set and solved using the weight vector method. The governing factors were the diamond radius and burnishing force. The resulting fatigue limit differed from the maximum fatigue limit by a mere 0.44%, which proves the effectiveness of the proposed approach. The results obtained for the fatigue limit are explained by means of an X-ray analysis of the introduced residual stresses and an analysis of the microstructures of the surface and subsurface layers. It has been established that a greater depth of the affected zone coupled with a smaller gradient in the alteration of the microstructure in depth provides larger fatigue strength.

Mechanical surface treatment, based on static severe plastic deformation of the surface layer, is a cost-effective approach to improving surface integrity and thus the operating properties of metal components. The present work examines the possibility of using single toroidal roller burnishing (STRB) to implement the deep rolling concept. The effect of the STRB process parameters on the fatigue behavior of 2024-T3 Al alloy specimens is investigated. The specimens were manufactured using a newly developed STRB device. The conventional fatigue limit of 2024-T3 Al alloy is established using the S–N curve approach and tangent method. The STRB implementation as the deep rolling process for 2024-T3 Al alloy specimens provides a maximum conventional fatigue limit of 256 MPa. Compared to the reference condition, the fatigue limit has increased by 38.4%. Calculated at the number of cycles corresponding to the conventional fatigue limit, the fatigue life is enhanced more than 2000 times over. The increase in the number of passes enhances the fatigue strength: as a result the conventional fatigue limit of the treated 2024-T3 Al alloy increased. Increasing the number of passes has practical meaning for up to six passes, after which the effect is negligible.

This article presents the outcomes of finite element (FE) simulations and X-ray stress measurements of residual stresses in high-strength 2024-T3 Al alloy introduced via the single toroidal roller burnishing (STRB) process. In terms of the deforming toroidal roller geometry, STRB is particularly suitable for deep rolling. A 3D FE model was developed using the flow stress concept, and the actual STRB kinematics was simulated to evaluate both hoop and axial residual stresses. The FE model was validated through a comparison of FE and X-ray residual stress distributions. The effects of the burnishing force, feed rate, and number of passes on the residual hoop and axial stresses were studied. It was established that increasing the feed rate leads to a decrease in the residual hoop stresses and an increase in the residual axial stresses. The greater burnishing force increases the compressive zone depth and only slightly increases the surface residual stresses. The FE and X-ray stress analyses confirm the effectiveness of STRB of 2024-T3 Al alloy to introduce significant residual compressive axial and hoop stresses.

Slide burnishing (SB) is a simple and efficient method for finishing symmetric rotary metal parts. The thermo-mechanical nature of SB has not been studied in the literature and is, therefore, an object of investigation in this article. A combination of finite element method (FEM) analyses and experiments is used, and a very important part of building the FEM model is the constitutive model of the surface layer of the workpiece being slide burnished. A temperature-dependent, nonlinear kinematic/isotropic hardening material constitutive model of the surface layer of 37Cr4 steel has been modelled on the basis of temperature-dependent indentation tests and inverse FEM analyses. In order to illustrate the benefits of this thermo-mechanical constitutive model, 3D FEM simulations of the SB of holes have been conducted; doing so establishes the effect of the generated temperature on the residual hoop stress distribution around the processed hole. Fully coupled thermal stress, sequentially coupled, and temperature-independent FEM simulations have been conducted to determine the residual hoop stresses. Alternatively, these stresses are experimentally obtained through the modified “split ring” method. It has been proven that a sequentially coupled thermal stress FEM simulation with nonlinear kinematic/isotropic hardening predicts the residual hoop stresses in such a manner as to be in satisfactory agreement with the experimental outcomes. It has been established that the generated temperature reduces the beneficial effect from the introduced residual stresses, thus reducing the fatigue life of the corresponding structural component.

The article presents a new technology for joint bar holes processing. Joint bars are components of rail bolted joints, which are a critical place for nucleation and propagation of fatigue cracks caused by cyclic loading due to passing trains. The probability of corner cracks arising, starting from the internal edges of the joint bar holes, is proven using the finite element method (FEM). On this basis, the necessity for a new technology for the enhancement of crack resistance of joint bar holes is grounded. The technology comprises slide diamond burnishing (SDB) as finishing of these holes. New tool equipment is developed including a combined cutting tool and an SDB device with elastic beam in order to set the necessary burnishing force. The equipment is intended for milling machine tools and machining centers. The optimal parameters of both cutting and SDB processes are obtained through planned experiments, regression analyses, genetic algorithm, and FE analyses. The distribution of the introduced beneficial residual hoop stresses is found by conducting a FE analysis of the SDB process. These stresses delay the nucleation and growth of fatigue cracks initiating in the hole surface. Both microscope analysis and fatigue tests prove this technology’s advantage, expressed in the increased crack resistance of joint bar holes.

Slide burnishing (SB) is a static mechanical surface treatment based on the severe plastic deformation of the surface for which the contact between the deforming element and the surface being treated is sliding friction. SB improves the surface integrity of metal structural and machine components dramatically. This paper is devoted to improving the fatigue strength of 41Cr4 steel hourglass-shaped specimens subjected to SB with a spherical-ended deforming diamond via different combinations of basic governing parameters. Since the residual compressive stresses introduced play a significant role for the fatigue behavior of the burnished components, a comprehensive parametric study of the SB process was conducted using fully coupled thermal-stress finite element (FE) simulations. The FE model’s adequacy was proven via comparison of the FE results for the residual stresses with X-ray diffraction measurements. The results obtained show that the diamond radius and the burnishing force have the strongest effects on the residual stresses, which, in turn, have a significant influence on the fatigue strength, respectively, fatigue life. An extensive experimental investigation of the effect of the selected SB basic parameters on the fatigue limit of the slide burnished specimens was carried out using Locatti’s method. The latter is based on the Palmgren–Miner linear damage hypothesis, which is a particular case of a general cumulative damage theory. A planned experiment was carried out, with the governing factors changed among four levels. Regression analysis of the experimental results was carried out, and a model for predicting the fatigue limit was obtained. Based on the model obtained, a one-purpose optimization was carried out using a genetic algorithm. By means of the optimal basic parameters, the fatigue limit of the processed specimens was increased by 22.7%—from 440 to 540 MPa. The fatigue life increased more than 100 times over after SB with the optimal basic parameters.

It is well known that apart from compressive residual stresses, smooth surface and microstructuring of the surface and subsurface layers are beneficial for enhancement of fatigue strength and load-carrying capacity of structural and machine components. This complex of properties can be achieved using surface severe plastic deformation. For symmetric rotational components made of high-strength aluminum alloys, slide burnishing is appropriate because of its simplicity and easy realization. The effect of the process parameters on the surface roughness, micro-hardness, and residual stresses obtained in slide burnishing of D16T aircraft aluminum alloy has been analyzed. The optimal values of the basic governing factors, which ensure minimum roughness (up to $$0.05\,\upmu\,{\text{m}}$$ ), have been established on the basis of a one-factor-at-the-time method, followed by a planned experiment and additional experiments. With the established combination of optimal values, the effect of number of passes and lubricant-cooler on the roughness, micro-hardness, and residual stresses in the surface being treated has been studied for two working schemes. To establish residual stress–depth profiles depending on the tool radius and burnishing force, FEM analysis of the slide burnishing process has been conducted. Thus, an evaluation of the effectiveness of slide burnishing as “mixed burnishing” has been made.

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.

The paper is devoted to fatigue life enhancement of S355 steel welded stiffened plate containing T-shaped stiffeners and girders with non-circular openings. Its rounded corners are natural stress concentrators and potential places for initiation and growth of first-mode fatigue cracks. To slow down this process, beneficial residual compressive stresses are introduced around the non-circular opening corners through mandrel cold working of preliminarily drilled round holes in the corners' zone. The creation through cutting of the non-circular openings and subsequent welding of the T-shaped girders generates residual stress redistribution. The latter is studied both experimentally and numerically. X-ray diffraction method has been employed for residual stress analysis. The mandrel cold working process and subsequent cutting to form the non-circular opening have been simulated through a nonlinear finite element method (FEM) analysis. In order to simulate the residual stress redistribution due to welding of the T-shaped girders with non-circular openings, a sequentially coupled thermal-stress FEM analysis has been carried out. In order to identify the optimal value of the mandrel cold working interference fit (the difference between the mandrel diameter and the initial hole diameter) and welding sequence, a multi-objective optimization task is set and solved. The optimization has been based on a planned numerical experiment. The optimal value of the interference fit and the welding sequence have been found, which ensure high intensity and homogeneity of the residual compressive field around the opening roundings.

In this paper constitutive models of 2024-T3 aluminium alloy have been developed at room and high temperatures in order to be used in finite element simulations of cold hole working process. The material behaviour in the plastic field has been described by the non-linear kinematic hardening on the basis of uniaxial tensile test (half cycle) at 8 different temperatures in the range between 25 °C and 200 °C. The dependence of the yield stress at zero plastic strain and the kinematic hardening modulus C on the temperature have been established when the material parameter γ is equal to 10. The latter determines the rate at which C decreases with increasing plastic deformation. The constitutive model authenticity has been proved by finite element simulations of the uniaxial tensile tests. The creep behaviour of the 2024-T3 aluminium alloy at high temperatures (150–200 °C) has been described by the power-law model. The model material parameters A, n and m have been determined as functions of the temperature on the basis of uniaxial creep test. The constitutive model authenticity has been proved experimentally and by finite element simulations of the creep test. The obtained constitutive models have been used in finite element simulations of residual stress relaxation around cold worked open holes due to thermal overloading.

Article history: Received June 6, 2014 Accepted 17 June 2014 Available online 19 July 2014 The fatigue failure around rail-end-bolt holes is particularly dangerous since it leads to derailment of trains and consequently to inevitable accidents. It is well-known that the fatigue life of structural holed components, subjected to cyclic load, can be increased by generating compressive hoop stresses around the holes. These beneficial residual compressive stresses significantly reduce the maximum values of the operating tensile stresses arising at the critical points of the components and thus impede the formation of first mode cracks. A new approach to enhancement of fatigue life of rail-end-bolt holes has been developed. The approach involves sequential drilling and reaming through a new combined tool and then slide diamond burnishing by a new device. The technology implementation was carried out on machine tool. The process of creating residual stresses has been studied both experimentally and numerically. The experimental study was conducted by means of a modified split ring method. A reliable finite element modeling approach to the slide diamond burnishing process was developed. On this basis, the process was optimized by means of a genetic algorithm. As a result, the optimal combination of the governing process parameters is established, which ensures both maximum depth of the compressive zone and maximum absolute values of the residual stresses. © 2014 Growing Science Ltd. All rights reserved.

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.

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.

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.

This article presents the outcomes from analytical and experimental modelling, the power parameters of new burnishing of external cylindrical surfaces called spherical motion burnishing. The axes of the workpiece and the ring-shaped tool having toroidal acting surface intersect at an angle of 2°. The tool motion is superposition from a spherical motion and a rectilinear translation with respect to the workpiece. The method is implemented on lathes. The tool transfer angular velocity is assigned by the lathe spindle, and the relative one is a consequence from unknown manufacturing resistance. Analytical models of the power parameters of the new burnishing process have been obtained. The unknown manufacturing intensities in these models have been determined experimentally. For proving the authenticity of the proposed semi-analytical models a comparison of the results obtained for the axial force with the experimental ones has been made. For that purpose, the axial force has been measured by means of a strain gauge system. The comparison shows a good agreement.

A reliable finite element modelling (FEM) approach to the spherical motion burnishing (SMB) process is developed with a view to gaining a fundamental understanding of the process and its optimization. SMB is a patented method for mechanical surface treatment of external cylindrical faces aimed at the enhancement of the fatigue life of the metallic component as well as its roughness, micro-hardness, depth hardening, wear, and corrosion resistance. A special feature of SMB is its kinematics: the tool motion is a superposition of a spherical movement and a rectilinear translation with respect to the workpiece. In order to decrease the FEM problem size, an approximated kinematic theory for the SMB is developed. In accordance with this theory the tool motion is approximated with a series of planar movements. As a result, a plane strain SMB FEM model is developed. The initial roughness is modelled in order to to achieve a more realistic representation of the workpiece geometry. To establish the flow stress and sliding friction coefficient, a combined approach is developed which contains mechanical tests, sensitivity FEM analysis, and inverse FE analysis of the corresponding pushing process. A planned numerical experiment is carried out on the basis of the created SMB FEM model. Six regression models of the treated layer characteristics are obtained and analysed on the basis of the FEM simulations. The FEM results are evaluated and compared to the experimental ones and their validity is proved. Finally, the regression models are used as objective functions in a multi-objective optimization problem formulation of the SMB process. As a result, the optimal combination of the governing SMB parameters is established.

This paper presents a new method of mechanical surface treatment of external cylindrical surfaces and it is called “Spherical motion burnishing” (SMB). The axes of the workpiece and ring-shaped tool having toroidal acting surface intersect at angle θ =2°. The tool motion is superposition from a spherical motion and rectilinear translation with respect to the workpiece. The contact between the surface treated and its enveloped-tool acting surface is one of sliding friction in the presence of a lubricant. The method can be implemented on conventional machine tools, in particular on lathes by means of a relatively simple device and tool. To evaluate the manufacturing potentials of the SMB method a designed experiment has been carried out. The quality of the treated surfaces of workpieces made of low- and medium-carbon steel has been investigated experimentally. Mathematical models of the roughness and the residual axial normal stresses have been obtained on the basis of a planned experiment. The authenticity of the models has been proved on the basis of four extra tests. The method can be classified as mixed burnishing because it combines the advantages of hardening, dimensional burnishing and smoothing burnishing.

In cases of cold expanded holes, taking residual compressive stresses into account in strength estimation, without considering their relaxation under cyclic load, leads to inaccurate mathematical models for the prediction of fatigue life of structural components. This article presents the outcomes from experimental and numerical investigations of residual stress relaxation around cold expanded holes in medium-carbon steel under cyclic tension. An approach to experimental modelling of the relaxation is proposed. This approach reduces to minimum the volume of the experiment and at the same time provides a relatively simplified mathematical model applicable to engineering practice. A generalized mathematical model of residual stress relaxation has been obtained and it predicts the relaxation as a function of the number of cycles, parameters of the cycle and hole expansion rate. A metallographic analysis has been made for clarifying the physical nature of the residual stress relaxation. On this basis two FE models have been developed for quantitative estimation of the errors from the basic formulations in the study as well as for estimation of the residual stress relaxation. The experimental and the numerical outcomes are in a good agreement.

The article presents outcomes from finite element and X-ray diffraction analyses of the cold hole expansion process fulfilled by means of solid mandrel gradually passing through the hole. Regardless of the manner of accomplishing – split sleeve; split mandrel; with a mating tapered split sleeve; spherical mandrelling – an axial gradient of the residual stresses is obtained due to the interaction between the layers caused by an axial force flow which passes through the mandrel, the workpiece and the support to forma closed circuit. To predict the fatigue life it is necessary to know the residual stresses on the entrance and the exit faces immediately to the hole periphery which are considered to be potential crack initiation sites. It has been proved that there are two key moments in the building of an adequate finite element model. The first is modeling of a realistic contact mandrel-workpiece with or without mediator and workpiece-support. The second is a suitable constitutive model of the workpiece material. Six hardening models of low-carbon steel obtained on the basis of a symmetric strain-controlled experiment and half-cycle test data from unidirectional tension experiment have been used consecutively in the finite element model of the process. On the basis of a comparison with experimental results obtained by means of an X-ray diffraction technique it has been established that the nonlinear kinematic hardening model obtained by a strain-controlled cyclic test to achieve a stabilized cycle secures finite element results close to the experimental ones. The chosen finite element model has been compared with simplified ones following the residual circumferential normal stress criterion and its advantages have been proved.

A new 3D finite element (FE) model of the cold hole expansion process by means of spherical mandrelling (SM) has been developed. The model has the following characteristics: the spherical motion of the tool has been defined by means of decomposition along static axes; interactions (normal and tangential contact with friction) defined in the tool-workpiece-supports system; nonlinear kinematic hardening of the workpiece material, defined on the basis of symmetric test with strain-controlled deformation to achieve a stabilized cycle. The actual spherical motion of the tool and the optimal value of the angle of nutation have been found through numerical minimization of the lost available work functional, which is equivalent to the minimum generated entropy. The FE model authenticity has been proved by means of comparison of the results for axial force variation with those obtained by experiment. The optimized FE model has been used for investigation of the residual displacements and stresses around cold expanded holes by means of SM. The FE results obtained have been compared with those obtained from other FE models.

Cold expansion is a well-known approach for the enhancement of the fatigue life of fastener holes in structural components by introducing residual compressive circumferential stresses around them. In this work, the hardening of a medium-carbon structural steel is studied experimentally and numerically from the point of view of the material behaviour during the cold expansion process of holes in structural steel components. Six material hardening models obtained on the basis of symmetric strain-controlled experiments and half-cycle test data from unidirectional tension experiments have been used in a two-dimensional axisymmetric finite element model of the process. Parallel with this an x-ray diffraction analysis of the residual stresses at the entrance and exit faces of the structural component has been carried out to prove the finite element result’s authenticity. It is established that the nonlinear kinematic hardening model obtained by a strain-controlled cyclic test to achieve a stabilized cycle for this steel secures finite element results close to the experimental ones. The constitutive hardening model obtained can be applied to a corresponding finite element model for stress analysis of steel structural component with cold expanded holes, subjected to an external load.

Residual compressive circumferential normal stresses around fastener holes enhance the fatigue life of the service components in metal structures. Fastener holes are considered as potential crack initiation sites for structures that undergo cyclic fatigue loading because of the stress and strain concentration. However, the use of a cold holes expansion by means of spherical mandrelling process to introduce a compressive residual stress field in the material surrounding the hole can enhance the service life of stressed structural components. This paper presents the outcomes from finite element simulation of spherical mandrelling process of holes with cracks. Isotropic plastic hardening and Von Mises yielding criterion have been used in the analysis. Two cases of depth of the crack have been analyzed. A two-dimensional elastic–plastic coupled thermomechanical finite element analysis has been performed. A tensile stress is then applied to the cold-expanded pre-cracked hole to assess the benefit of the spherical mandrelling process.

A modelling of the mechanical systems with dissipative processes as thermodynamic systems has been performed in this article. On the basis of the Gouy–Stodola's theorem the entropy generation minimization method for thermodynamic optimization of these systems has been applied. The outcomes of a thermodynamic optimization of metal-forming processes and rotor brake systems have been shown. It has been proved that the generated entropy is a generalized optimization criterion for this kind of systems. The vector optimization criterion has been substituted by the generated entropy functional depending on the vector of controlling factors (process parameters). The generated entropy plays the role of a surrogate function of a feasible scalarized function, and therefore it is not necessary to scalarize the vector optimization criterion. The optimization is reduced to generated entropy minimization. The effectiveness of the latter approach has been proved.

The paper presents a method of machining hypocycloidal inner and outer polyhedrons on lathes and this significantly broadens the manufacturing capabilities of the latter. The method as well as the synthesis of hypocycloidal polygon shaft joints are based on a formulated and proved theorem specifying the conditions of forming a hypocycloidal n-“gon” while rotating a linear (n − 1)-gon around two parallel axes. A finite element (FE) simulation of the stressed and strained state of the synthesized hypocycloidal polygon shaft joints, affected by torque, has been performed. As a result of the numerical experiment carried out a rational variant has been selected and a convenient for engineering practice formula has been obtained, defining the carrying capacity of the synthesized polygon shaft joint.

The paper presents outcomes from experimental research and numerical simulations which prove the efficiency of spherical mandrelling process (SM) to enhance the fatigue life of steel structural elements with cylindrical holes. It has been proved that SM of cylindrical holes on conventional machine tools enhances the fatigue life of the respective structural element three times and more. The residual normal stress field around holes formed by SM has been determined by means of finite element analysis and the practical significance of these stresses has been proved. A finite element simulation of dynamic impact on a strip with a hole obtained by cutting only and another one formed by SM has been carried out. The comparison between the fields of stresses in both cases shows advantages of SM process on conventional machine tools as a new technology against fatigue failure.

In conventional manufacturing, the spherical mandrelling (SM) method is an alternative to the methods of cold expansion of holes: split mandrel and split sleeve. SM is performed on conventional machines, thereby broadening their manufacturing capabilities. Mathematical models of spherical mandrelling manufacturing resistance have been obtained analytically and experimentally in order to set rational operating conditions and for strength estimation of the system: machine–device–tool–workpiece.

Fatigue life of structural elements with bolt holes depends mainly on residual stress distribution law around these holes. Residual compressive circumferential normal stresses around the hole reduce operating stress magnitudes to minimum values for cyclic tension and this enhances fatigue life and load-carrying capacity of structures. The presence of residual stresses is a result of the manufacturing process. Residual stresses can also be induced deliberately around the holes by means of appropriate working with suitably chosen parameters. Quantitative knowledge of residual stresses is necessary to model the stress field after applying an external load to a structural element in order to assess static or dynamic strength, fatigue strength including. This paper presents a combined approach consisting of experimental and numerical modelling of residual circumferential normal stress distribution when forming holes in workpieces of medium carbon steel by spherical mandrelling (SM). Since the object of study is residual macrostresses (stresses of first type), a mechanical method of their determination has been employed. On the basis of experimental outcomes, a mathematical model has been built and it predicts mean integral value distribution of residual circumferential normal stresses. Since the range of the experimental technique employed is limited by the wall thickness of the bushing being worked, numerical modelling of residual stresses by means of FE simulation has been performed. The numerical results obtained allow this mathematical model to be applied to various wall thicknesses by introducing correction factors for the polynomial coefficients. At the same time, the adequacy of the proposed FE model can be evaluated only by the experimentally obtained mathematical model. The SM efficiency for enhancing the load-carrying capacity of structural elements with cylindrical holes subjected to tension has been proved by means of FE simulation.

The paper presents a new method of finishing holes by means of plastic deformation referred to as spherical mandrelling (SM). The tool motion with respect to the fixed workpiece is a superposition of spherical and translation motion. The surface plastic deformation can be carried out on lathes, drilling and milling machines and machining centres and considerably widens their manufacturing potentials. The mechanics of the SM method has been explained. Two model problems have been formulated and solved and their solutions show the SM kinematics when implemented on conventional machine tools. The solutions to the set variational problems are found by means of an approach called analogy to mass geometry. A simplified diagram of a device and tool for implementing the SM method on conventional machine tools has been shown. The power characteristics of SM process have been determined both theoretically and experimentally and rational operating conditions have been found. A new method of measuring small changes in the load of a.c. motors of machine tools has been presented. SM has been compared to conventional mandrelling and SM efficiency has been proved.

The paper presents a general-purpose optimization method for metal-forming processes by plastic deformation based on their modelling as thermodynamic processes with entropy generation minimization. To this purpose, equations have been formulated describing a thermodynamic system consisting of two subsystems: a tool and a workpiece which is a basic feature of each metal-forming process. A mathematical model of the thermodynamic system has been derived based on fundamental equations in continuum mechanics related to a deformable solid and the basic laws of thermodynamics. The mathematical model describes the phenomena related to the thermomechanical strength of the tool; the energy balance of the system; displacements and deformation in the tool–workpiece system due to mechanical and temperature factors, etc. On the basis of the second law of thermodynamics, entropy generation has been defined as a generalized optimization criterion and its functional has been synthesized containing the tensor functions of stresses and of strain velocities, the scalar functions of temperatures and the relative velocities of the two media. A summarized algorithm for entropy generation minimization has been proposed and illustrated by a numerical example.

This paper presents two methods of forming profile holes, previously drilled in rotational and casing blanks. The methods can be implemented on lathes, milling, drilling and centering machines, etc. Kinematic synthesis of the methods is shown based on the kinematics of a body rotating around coplanar axes. The rational field of application of the methods has been found. Principal diagrams of the devices and tools enabling implementation of the method are shown.

The paper presents the outcomes of optimisation in the spherical mandrelling process (SM) employed for cold expansion of holes in order to create a zone of residual compression stresses around the worked holes. The multi-objective optimisation of the process is performed in two approaches: scalarisation of the vector optimisation criterion, performed by the reference approach (optimistic approach) and by an alternative (thermodynamic) approach. The vector optimisation criterion has been substituted by the generated entropy functional depending on the vector of controlling factors (process parameters). The generated entropy plays the role of a surrogate function of a feasible scalarised function, and therefore it is not necessary to scalarise the vector optimisation criterion. Optimisation is reduced to generated entropy minimisation. The effectiveness of the latter approach has been proved.