Sensitivity of operating conditions and material properties for thermoforming process

Abstract

The thermoforming process involves three stages: sheet reheat; forming; and solidification. A polymeric sheet is heated in an oven to the desired forming temperature distribution. The sheet is then deformed to take the shape of the mould cavity and subsequently solidified. The deformation of the sheet is assisted by the application of a pressure differential and/or the use of a moving plug.     

Piezoelectric response of porous ceramic and composite materials based on Pb(Zr,Ti)O3: experiment and modelling

Abstract

The present paper presents experimental and theoretical studies of the effective piezoelectric properties of porous Pb(Zr,Ti)O₃ based ceramics and composites. The experimental dependence of piezoelectric coefficients and dielectric permittivity on relative density is determined for porous materials containing different piezopassive components. The piezoelectric response of these materials in a wide range of volume concentrations of the piezopassive components are analysed in the framework of a model of a modified laminated composite with 2–2 and 1–3 connectivity elements. The role of these elements in forming different concentration dependences of the effective parameters of the porous piezoceramic and piezocomposite materials is discussed.     

Improved thermal modelling of SNF shipping cask drying process using analytical and statistical approaches

Abstract

Before being back filled with an inert gas and as preparation for shipment, a spent nuclear fuel shipping cask must usually be vacuum dried. This process results in an increase in the spent fuel temperature, due to the degradation of heat transport by the cover gas. The drying process is typically modelled by a thermal conduction set to zero in all the shipping cask free spaces. However, this approach does not take into account heat transfers that occur in a rarefied medium and, therefore, may be extremely conservative. A first analysis was performed in order to spot the cask areas whose thermal behaviour is modified by the drying process. This analysis involved the calculation of the Knudsen number, defined as the molecular mean free path to a representative length scale, for all the free spaces. The only area impacted by the drying process appeared to be the mechanical gap between the fuel basket and the shielding materials. During the drying process, the Knudsen number is actually large enough within the gap to consider the gas as a non-continuous medium. Results and methods coming from the microfluidics area were therefore used to develop a modelling, which is based on a double approach. First, an analytical approach was used. This approach consists in adding to the Fourier equation a new equation accounting for the thermodynamical non-equilibrium within the gap (Maxwell–Smoluchowski temperature jump). A thermal model, suitable to calculate heat transfers at pressures as low as 1 mbar, was developed. A second model, based on a statistical approach, was then developed. This model involves the Direct Simulation Monte Carlo method, a reference method used for microfluidics calculations. Computer simulations were performed and led to a good agreement with the results obtained by the analytical approach.     

Thermophysical properties of slags for process control

Abstract

Thermophysical properties have been shown to be important in a number of industrial processes. Unfortunately, for a number of properties there are scant data available and/or the data that are available are of dubious quality. In the Thermo-Physical Property Group at the National Physical Laboratory (NPL) the aim has been to develop measurement techniques capable of working in the difficult environments that are typical of industrial processes. This paper presents current developments at NPL in techniques to measure the thermal diffusivity of liquid slags using a laser flash and insights into the copper smelting process using interfacial tension techniques.     

Structure-property changes during hardening and tempering of new ultra high strength medium carbon low alloy steel

Abstract

A medium carbon low alloy steel, electroslag refined, modified AFNOR 15CDV6, has been developed for satellite launch vehicle and related applications. Conventionally processed (without electroslag refining) mostly bainitic AFNOR 15CDV6 (with 0·15 wt-% carbon and ~ 3·5 wt-% other alloying elements) has a yield strength of ~ 800 MPa. Electroslag refining, coupled with increased carbon (0·29 wt-% carbon, but no change in percentage of other alloying elements), increased the yield strength to about 1300-1400 MPa, without sacrificing ductility. The microstructure of the modified grade was martensitic. Martensite in the as hardened state was mostly in the form of laths, although ~20% plate martensite was also observed. Until 150°C tempering, no noticeable loss of tetragonality was observed, while the unit cell parameter c/a ratio dropped to almost 1 after 300°C tempering. The interesting observation at 150°C tempering was the predominant presence of fine rodlike ? carbide, which may also explain the increased yield strength. Tempering above 150°C converted the ? carbide to cementite, relatively thicker precipitates of similar morphology. At higher tempering temperatures, no evidence of spheroidisation of cementites was noted. The highest tempering temperatures of 500 and 600°C resulted in two marked changes in the microstructure: the appearance of M₂₃C₆ type (Cr, Fe and Mo bearing) carbides, and the appearance of, in some regions of the microstructure at least, a relatively 'recovered' lath structure. Misorientation among adjacent laths, nearly constant at 8-9° until 450°C tempering, increased noticeably, to 13 and 16°, after the respective tempering temperatures of 500 and 600°C.     

Hybrid forming processes for production of lightweight high strength automotive panel parts

Abstract

The research concentrates on a heat treatable AA 6082 aluminium alloy. A set of unified constitutive equations has been developed and determined from experimental data. In addition to modelling viscoplastic flow of the material at different temperatures, the equations contain other two specific features. One is to predict the failure of the material under various deformation conditions based on continuum damage mechanics theories. The other is to model the precipitation formation and growth under straining and aging conditions; thus, the strength distribution of formed parts can be predicted via process modelling. The determined unified constitutive equations are then implemented into the commercial finite element code ABAQUS/Explicit via the user defined subroutine, VUMAT. A finite element process simulation model and numerical procedures are established for the modelling of a hot stamping and cold die quenching processes for a spherical part with a central hole. To validate the simulation results, a test programme is developed, a test rig has been designed and manufactured and tests have been carried out under different forming rates. It has been found that very close agreements between experimental and numerical process simulation results are obtained for the range of forming rates carried out.     

Image based modelling of stress–strain behaviour in carbon/carbon composites

Abstract

Numerical modelling is based on two fundamentals: first, developing a finite element (FE) mesh that represents faithfully the sample structure; and second, using reliable input data. In this paper, a methodology for creating image based FE meshes from X-ray microtomography (XMT) data sets of two 2D woven carbon–carbon composites (graphitised and ungraphitised) that are candidate materials for nuclear applications is described. Input data for the mechanical properties of the constituent phases are determined by nanoindentation. Compressive tests are also performed to generate experimental stress–strain curves to validate the predictions. The results showed good correlation between the experimental and numerically modelled stress–strain curves, therefore giving confidence in the approach. The stress distributions within the structure were also modelled. These showed the effect of pore shape on stress shielding. These predictions were also compared with simple analytical models for the structure. The image based models showed better agreement.     

Effect of process conditions on superplastic forming behaviour in Ti–6Al–4V friction stir welds

Abstract

Friction stir welding of Ti–6Al–4V was performed on 5 mm thickness sheet. Wide ranges of processing conditions were tested, and an experimentally determined process window was established for this given material, thickness and tooling configuration. Welds made within this process window were also superplastically formed. It was found that the weld parameters influence joint quality in terms of microstructure, penetration and void formation in addition to process traits such as loads and tool wear. Furthermore, since the microstructure of the weld could be altered with the welding parameters, the degree of superplasticity in the joints could also be controlled. Finally, experimental observations were compared to a proposed analytic process model in an attempt to establish trends between the weld characteristics and the welding conditions used. Reasonable correlations were observed between the analytic model and experimental results.     

Modelling of viscoelastic coextrusion flows in multi-manifold flat dies

Abstract

A recently proposed modification of the viscoelastic Leonov model is employed as a stress calculator in FEM analysis with a full u-v-p-t numerical scheme for coextrusion flow in multimanifold flat dies with 30° and 90° entrance angles. It is shown that the predicted stresses, interface location and streamline fields are in good agreement with the measurements. It is also shown that extensional viscosity has to be used in the modelling of the coextrusion flow to confirm experimental data.     

Rolling to control residual stress and distortion in friction stir welds

Abstract

Considerable residual stress and distortion can be produced by friction stir welding, impeding industrial implementation. Finite element analysis has been used to develop three innovative rolling methods that reduce residual stress and distortion in friction stir welds. Of the three methods, post-weld direct rolling where a single roller is applied to roll the top surface of the weld after the weld metal has cooled to room temperature proved the most effective. The residual stress predictions from the model compared favourably with residual stress measurements reported in an accompanying paper. Finally, the effectiveness of using post-weld direct rolling is illustrated with an industrial example of a large integrally stiffened panel, where the distortion was virtually eliminated.