Jordan T. Maximov

Modelling of residual stress field in spherical mandrelling process

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.

Spherical mandrelling method implementation on conventional machine tools

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.

Forming of cross-profile holes by adding rotations around coplanar axes

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.

Optimization method for metal-forming processes

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.

Finite Element Method for Plates with Dynamic Loads

The aim of the study described here is to investigate the influence on changing a definite load for finite element approximation of plate bending problems. We consider and analyse cases of stiffened and unstiffened rectangular plates which are submited the same impact. In order to apply the method of normal shapes, we derive the first essential eigenpairs. Numerical examples that illustrate the determination of the dynamical stresses are presented.

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.

Thermodynamic optimisation of cold expansion of holes by means of spherical mandrelling

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.

Modelling of the elastic line for twist drill with straight shank fixed in three-jaw chuck

The aim of this study is to present a new approach for investigation of an important problem of mechanical engineering. Namely, the model of twist drill embeded in three-jaw chuck is discussed. This problem could be considered as a variant of a beam on the Wincklers base which is under the influence of a cross-force. Our principal aim is to deduce the general mathematical model for this type of constructions. In order to determine the dynamic stresses of the drill using any variational numerical methods we present the corresponding variational formulations. These presentations are characterized by mixed formulation. So, the mixed finite element method is convenient for this kind of problems. The possibility for symmetrization of the weak formulation of the model problem is also discussed.

Two Approaches to the Finite Element Analysis of the Stiffened Plates

The goal of this study is to investigate and to compare two finite element approaches for presenting the stiffeners of the rectangular bending plates. The first approach is when the stiffness and mass matrices are obtained by superpositioning the plate and the beam elements. The model of the second one is realized only by the plate finite elements, but we give an account of the different stiffness of the stiffeners by means of elements with different thickness. The plates are subjected to a transversal dynamic load. We consider the corresponding variational forms.