Daniel S. Balint

Undulation instability of a compressed elastic film on a nonlinear creeping substrate

Undulation instabilities in a tri-layer system subject to variations of temperature are studied for the purpose of revealing a mechanism leading to the failure of thermal barrier systems. A pre-compressed oxide film with a small initial non-planarity is attached to a metal bond coat layer that is, in turn, attached to a thick superalloy substrate. The oxide film and the superalloy substrate are elastic, while the bond coat undergoes power-law creep at high temperature. The bond coat and the oxide film are subject to equi-biaxial stress changes whenever the temperature changes due to thermal mismatch with the superalloy substrate. The mismatch can be very large for some bond coat materials, such as PtNiAl, as a result of a reversible phase transformation that occurs while the temperature is changing. In the bond coat layer, the stress decays due to creep during periods at high temperature. However, during the initial stages of the decay period, the bond coat is highly susceptible to transverse deformation due to the nonlinear character of power-law creep, enabling the compressed film to undergo significant undulation growth over hundreds of thermal cycles. These periods of susceptibility appear to be a primary mechanism for undulation growth in a film that is initially nearly planar, and they explain the observation that undulation growth under cyclic temperature histories far exceeds that under isothermal conditions for the same total time exposure at high temperature. Although the behavior of the bond coat is highly nonlinear, an approximation has been developed which permits accurate description of undulation development under quite general conditions. Possibilities for reducing the susceptibility to undulation growth are discussed, as are further avenues for research.

A dynamic discrete dislocation plasticity method for the simulation of plastic relaxation under shock loading

In this article, it is demonstrated that current methods of modelling plasticity as the collective motion of discrete dislocations, such as two-dimensional discrete dislocation plasticity (DDP), are unsuitable for the simulation of very high strain rate processes (106 s−1 or more) such as plastic relaxation during shock loading. Current DDP models treat dislocations quasi-statically, ignoring the time-dependent nature of the elastic fields of dislocations. It is shown that this assumption introduces unphysical artefacts into the system when simulating plasticity resulting from shock loading. This deficiency can be overcome only by formulating a fully time-dependent elastodynamic description of the elastic fields of discrete dislocations. Building on the work of Markenscoff & Clifton, the fundamental time-dependent solutions for the injection and non-uniform motion of straight edge dislocations are presented. The numerical implementation of these solutions for a single moving dislocation and for two annihilating dislocations in an infinite plane are presented. The application of these solutions in a two-dimensional model of time-dependent plasticity during shock loading is outlined here and will be presented in detail elsewhere.

The development of continuum damage mechanics-based theories for predicting forming limit diagrams for hot stamping applications:

This paper presents a novel plane-stress continuum damage mechanics (CDM) model for the prediction of the different shapes of forming limit diagrams (FLCs) for aluminium alloys under hot stamping conditions. Firstly, a set of uniaxial viscoplastic damage constitutive equations is determined from tensile experimental data of AA5754 at a temperature range of 350–550℃ and strain rates of 0.1, 1.0 and 10 s−1. The tests were carried out on Gleeble materials simulator (3800). Based on the analysis of features of FLCs for different materials forming at different temperatures, a plane-stress damage equation is proposed to take account the failure of materials at different stress-state sheet metal forming conditions. In this way, a set of multiaxial viscoplastic damage constitutive equations is formulated. The model is calibrated from the FLC data at temperature of 350℃ and strain rate of 1.0 s−1 for AA5754. A good agreement has been achieved between the experimental and numerical data. The effect of the maximum principal stress, effective stress and hydrostatic stress on the materials failure features and on the shape of FLCs is studied individually and in combination. Using the newly developed plane-stress unified viscoplastic damage constitutive equations, the FLC of materials can be predicted at different temperatures and strain rate forming conditions.

Controlled Poisson Voronoi tessellation for virtual grain structure generation: a statistical evaluation

A controlled Poisson Voronoi tessellation (CPVT) model has been developed for producing two-dimensional virtual grain structures that are statistically equivalent to metallographic observations of polycrystalline materials in terms of the tessellations regularity and grain size distribution. The descriptive fitting model, which is critical to link the grain size distribution parameter to the regularity parameter, has been improved in this work; previously, the descriptive model was poor for large values of the distribution parameter c. A set of four physical parameters is involved in uniquely determining the grain size distribution properties and configuring the CPVT system. Emphasis is devoted to examining the effectiveness and robustness of the CPVT system in generating virtual grain structures with specified properties. Two series of statistical tests are performed to validate the agreement between the prescribed regularity and that of the resultant tessellations, and to investigate the details of the overall grain size distribution. In order to explore sample size effects, three statistical tests were conducted for a range of regularity values. In addition, a materials modelling system for crystal plasticity finite element analysis is demonstrated, which implements the CPVT model for grain structure generation. Two real microscopic images with different grain size distribution features are employed to examine the capability of the system to generate virtual grain structures that match physical measurements.

Mode II Edge Delamination of Compressed Thin Films

Ceramic coatings deposited on metal substrates generally develop significant compressive stresses when cooled from the temperature at which they are processed as a result of thermal expansion mismatch. One of the main failure modes for these coatings is edge delamination. For an ideally brittle interface, the edge delamination of a compressed thin film involves mode II interface cracking. The crack faces are in contact with normal stress acting across the faces behind the advancing tip. Frictional shielding of the crack tip has been shown to increase the apparent fracture toughness. Roughness effects associated with the separating faces can also contribute to the apparent toughness. A model of mode II steady-state edge delamination that incorporates combined friction and roughness effects between the delaminated film and substrate is proposed and analyzed. This model is used to assess whether frictional shielding and surface roughness effects are sufficient to explain the large apparent mode II fracture toughness values observed in experiments.

Microstructure evolution in metal forming processes

Part 1 General principles: Understanding and controlling microstructural evolution in metal forming: an overview Techniques for modelling microstructure in metal forming processes Modelling techniques for optimising metal forming processes Recrystallisation and grain growth in hot working of steels Severe plastic deformation for grain refinement and enhancement of properties. Part 2 Microstructure evolution in the processing of steel: Modelling phase transformations in steel Determining unified constitutive equations for modelling hot forming of steel Modelling phase transformations in hot stamping and cold die quenching of steels Modelling microstructure evolution and work hardening in conventional and ultrafine-grained microalloyed steels. Part 3 Microstructure evolution in the processing of other metals: Microstructure control in creep-age forming of aluminium panels Microstructure control in processing nickel, titanium and other special alloys.

Experimental and Numerical Studies on the Formability of Materials in Hot Stamping and Cold Die Quenching Processes

Formability of steel and aluminium alloys in hot stamping and cold die quenching processes is studied in this research. Viscoplastic‐damage constitutive equations are developed and determined from experimental data for the prediction of viscoplastic flow and ductility of the materials. The determined unified constitutive equations are then implemented into the commercial Finite Element code Abaqus/Explicit via a user defined subroutine, VUMAT. An FE process simulation model and numerical procedures are established for the modeling of hot stamping processes for a spherical part with a central hole. Different failure modes (failure takes place either near the central hole or in the mid span of the part) are obtained. To validate the simulation results, a test programme is developed, a test die set has been designed and manufactured, and tests have been carried out for the materials with different forming rates. It has been found that very close agreements between experimental and numerical process simulatio...

Investigation of the effects of thermal gradients present in Gleeble high-temperature tensile tests on the strain state for free cutting steel

A common feature of uniaxial high-temperature tension tests, and to a certain extent compression tests, performed using a Gleeble thermomechanical testing system that employs direct resistance heating for the characterisation of the rheological behaviour of materials is longitudinal and radial thermal gradients. The aim of this article is to experimentally quantify the axial thermal gradients for a given tensile specimen geometry of free cutting steel during heating and deformation, and design a modelling methodology to simulate their influence on the strain distribution as compared to the assumption of isothermal heating and deformation. For this purpose, a feedback algorithm was developed to control the electric current input in a similar manner to that applied by the Gleeble testing system, which implemented via the UAMP user subroutine in ABAQUS for use in electro-thermal simulations of the direct resistance Joule heating used by the Gleeble testing system. The predicted temperature fields were compared with the temperature distributions recorded experimentally along the gauge section of the tensile specimens. Finite element simulations of Gleeble tensile tests were carried out under isothermal conditions and using the temperature distributions calculated by the feedback algorithm for a range of strain rates and temperatures in order to evaluate the difference in predicted stress state. The results show that an isothermal assumption should only be used conservatively in finite element simulation of the Gleeble thermomechanical test employing direct resistance heating to avoid significant errors.

Elastodynamic image forces on dislocations.

The elastodynamic image forces on edge and screw dislocations in the presence of a planar-free surface are derived. The explicit form of the elastodynamic fields of an injected, quiescent screw dislocation are also derived. The resulting image forces are affected by retardation effects: the dislocations experience no image force for a period of time defined by the arrival and reflection at the free surface of the dislocation fields. For the case of injected, stationary dislocations, it is shown that the elastodynamic image force tends asymptotically to the elastotatic prediction. For the case of injected, moving dislocations, it is shown that the elastodynamic image force on both the edge and the screw dislocations is magnified by inertial effects, and becomes increasingly divergent with time; this additional effect, missing in the elastostatic description, is shown to be substantial even for slow moving dislocations. Finally, it is shown that the elastodynamic image force of an edge dislocation moving towards the surface at the Rayleigh wave speed becomes repulsive, rather than attractive; this is suggestive of instabilities at the core of the dislocation, and likely resonances with the free surface.

The Role of Homogeneous Nucleation in Planar Dynamic Discrete Dislocation Plasticity

© 2015 by ASME. Homogeneous nucleation of dislocations is the dominant dislocation generation mechanism at strain rates above 10 8 s -1 ; at those rates, homogeneous nucleation dominates the plastic relaxation of shock waves in the same way that Frank-Read sources control the onset of plastic flow at low strain rates. This article describes the implementation of homogeneous nucleation in dynamic discrete dislocation plasticity (D3P), a planar method of discrete dislocation dynamics (DDD) that offers a complete elastodynamic treatment of plasticity. The implemented methodology is put to the test by studying four materials - Al, Fe, Ni, and Mo - that are shock loaded with the same intensity and a strain rate of 10 10 S -1 . It is found that, even for comparable dislocation densities, the lattice shear strength is fundamental in determining the amount of plastic relaxation a material displays when shock loaded.