Papers by Keyword: Incubation Time

Abstract: The results of experimental studies performed during the past decade at Saint-Petersburg State University in collaboration with the Mechanics and Materials Science Research Centre at Ningbo University (China) and Lobachevsky State University of Nizhny Novgorod (Russia) with the aim of investigating the basic regularities of the high rate straining of NiTi shape memory alloys are reviewed. The studies were concerned with the mechanical behaviour of these materials at high rate compression and tension, and the effect of high rate straining on the basic functional properties (shape memory effect and two-way shape memory). Special attention was given to the application of dynamic fracture theory to NiTi shock loading and to methods for obtaining experimental findings concerning the theoretical parameters involved in the criteria for determining the transition of these materials from an elastic to an inelastic state while high rate straining was applied. The effect of the quasi-equilibrium structure of NiTi on martensitic transformations and the role of this structure in the formation of more complicated effects than shape memory and superelasticity were studied. The results obtained are used to elaborate a method for the improvement of the functional properties of NiTi and a procedure for reversing two-way shape memory induction.

Abstract: Simulation of dynamic crack growth under quasistatic loading was performed using finite element method with embedded incubation time fracture criterion [. Experimental data, used for comparison was taken from [. ANSYS finite element software package was used in order to receive FEM solutions. The fracture criterion was implemented as an external procedure written in C++. The developed model is not using and trimming parameters. Only initial experimental conditions and material properties measured in separate experiments are used. Received dependencies for crack velocities as a function of time closely follow those observed in experiments by J.Finberg. Simulation results provide a possibility to conclude that the incubation time approach is an effective method to predict fracture initiation as well as crack propagation at various loading rates. Dependencies of an instant crack velocity on the current level of stress intensity factor received in this work for quasistatic loads and in [ for high-rate loads is discussed and compared to those experimentally observed by K. Ravi-Chandar and W.G. Knauss [ and J. Finberg [.

Abstract: The review on effect of a high strain rate on the properties of TiNi-shape memory alloys is presented. The study of thermo-mechanical and functional properties of SMA after high strain rate loading was carried out. The object of study was an equiatomic TiNi shape memory alloy. The samples were tensioned at a strain rate of about 10³ s^-1 at various temperatures in martensitic, austenitic, and two-phase state, using the Split Hopkinson Pressure Bar technique. Two-way shape memory effects were investigated. Two-way shape memory after high strain rate loading was less than after the quasi-static one for all cases, except for straining in martensitic state.

Abstract: In this work, a simulation of the growth kinetics of layers on AISI 1018 steel was done by means of a kinetic model. This model considers a solid diffusion of boron into a semi-infinite medium where the boron solubility in the Fe phase depends on the process temperature. An expression of the parabolic growth constant was then obtained through an application of the mass balance equation at the (/substrate) interface. The present model was validated by the experimental data available in the reference work (I. Campos-Silva et al: Kovove Mater. Vol.47 (2009), p.1-9). A good concordance was observed between the experimental parabolic growth constants and the predicted ones by the model for an upper limit of boron in the phase equal to 8.91 wt.% ( as a fitting parameter of the model). In addition, the generated weight gain was estimated at the surface of the borided AISI 1018 steel as a function of the upper limit of boron in the phase and the temperature.

Abstract: Avoiding recrystallization of austenite in hot strip rolling of steels is highly important for enhancing mechanical properties of hot rolled products. The present work focuses on computation of incubation time t_inc for static recrystallization using laboratory hot deformation data and on extrapolation the results to industrial conditions. The computations are done based on application of critical conditions for initiation of dynamic recrystallization to the static case. No-recrystallization temperature in hot strip rolling is determined by setting t_inc equal to interpass time. Simulations allow for prediction of the onset of austenite static recrystallization after individual rolling passes during industrial hot rolling and evaluation of the effects of strip thickness, rolling speeds, etc.

Abstract: Avoiding recrystallization of austenite in hot strip rolling of steels is highly important for enhancing mechanical properties of hot rolled products, as well as for the products undergoing cold rolling and annealing or coating. Recrystallization can only be avoided if its incubation time is longer that the time intervals characteristic for a particular hot rolling process. The present work focuses on computation of incubation time t_inc for static recrystallization using laboratory hot deformation data and on extrapolation the results to industrial conditions. The computations are done based on application of critical conditions for initiation of dynamic recrystallization to the static case. No-recrystallization temperature in hot strip rolling is determined by setting t_inc equal to interpass time. Simulations allow for prediction of the onset of austenite static or metadynamic recrystallization after individual rolling passes during industrial hot rolling and evaluation of the effects of strip thickness, rolling speeds, etc.

Abstract: Fracture of quasi-brittle heterogeneous materials is steered by processes at several different scale levels. These processes can progress independently or affect each other. In order to model fracture of such materials one should account for all rupture processes contributing to overall fracture process. This paper is presenting structural-temporal approach for analysis of multiscale nature of brittle fracture. Notion of spatial-temporal cell for different scale levels is introduced. Problem of experimental determination of a fixed scale level is discussed. Possible interconnections of this scale level with higher and lower scale levels are discussed. It is shown that this can give a possibility to predict fracture on a higher (real) scale level having experimental data obtained on a lower (laboratory) scale. This possibility is of extreme importance for many applications where the possibility to evaluate material strength properties on real structure scale level does not exist (ex. geological objects, big concrete structures, trunk pipelines, etc.).

Abstract: It is demonstrated that energy input for fracture in many industrial processes can be optimised so that the energy cost of the process is minimised. Using a simple example of central crack it is shown that for a certain shape of the load pulse energy transmitted to the sample in order to initiate the crack has a strongly marked minimum. Received results indicate a possibility to optimise energy consumption of different industrial processes connected with fracture. Possible applications include drilling or rock pounding where energy input often accounts for the largest part of the process cost. Using this approach it will be possible to predict optimal operational parameters for bores, grinding machines, etc. and hence significantly reduce the process cost. In the second part of the paper the behaviour of energy input for initiation of fracture in conditions of contact interaction is studied. It is considered that a spherical particle is impacting the half-space. Stress field created as a result of the interaction can be estimated using the Hertz solution. Threshold particle velocity (and, hence, threshold kinetic energy) corresponding to initiation of rupture in the half-space can be found once the fracture criterion is defined. It will be shown that the value of this energy does significantly depend on load duration and has a marked minimum. Existence of energetically optimal modes of dynamic impact is claimed.

Abstract: We have investigated athermal and isothermal martensitic transformations (typical displacive transformations) in Fe–Ni, Fe–Ni–Cr, and Ni-Co-Mn-In alloys under magnetic fields and hydrostatic pressures in order to understand the time-dependent nature of martensitic transformation, that is, the kinetics of martensitic transformation. We have confirmed that the two transformation processes are closely related to each other, that is, the athermal process changes to the isothermal process and the isothermal process changes to the athermal one under a hydrostatic pressure or a magnetic field. These findings can be explained by the phenomenological theory, which gives a unified explanation for the two transformation processes previously proposed by our group.

Abstract: The growth kinetics of Fe2B layers formed at the surface of AISI 1018 was simulated. The paste-boriding (with a paste thickness of 4 mm) was applied to produce the Fe2B phase at the material surface; considering four temperatures (1123, 1173, 1223 and 1273 K) for 2, 4, 5, 6 and 8 h. The suggested model was based on the mass balance equation at the (Fe2B /substrate) interface. As a fitting parameter of the model, the surface boron concentration (12.16 wt. %B) was obtained in order to predict with a good agreement the experimental parabolic growth constants at the (Fe2B /substrate) interface derived from the literature. An expression of the parabolic growth constant at the (Fe2B /substrate) interface was obtained as a function of the two parameters: and . In addition, a relationship of the Fe2B layer thickness was also deduced that showed a good concordance with the experimental results from the literature.