Numerical modeling of the thermal contact in metal forming processes

Heat flow across the interface of solid bodies in contact is an important aspect in several engineering applications. This work presents a finite element model for the analysis of thermal contact, which takes into account the effect of contact pressure and gap dimension in the heat flow across the interface between two bodies. Additionally, the frictional heat generation is also addressed, which is dictated by the contact forces predicted by the mechanical problem. The frictional contact problem and thermal problem are formulated in the frame of the finite element method. A new law is proposed to define the interfacial heat transfer coefficient (IHTC) as a function of the contact pressure and gap distance, enabling a smooth transition between two contact status (gap and contact). The staggered scheme used as coupling strategy to solve the thermomechanical problem is briefly presented. Four numerical examples are presented to validate the finite element model and highlight the importance of the proposed law on the predicted temperature.The authors gratefully acknowledge the financial support of the Portuguese Foundation for Science and Technology (FCT) under the project PTDC/EMS-TEC/1805/2012 and by FEDER funds through the program COMPETE Programa Operacional Factores de Competitividade, under the project CENTRO-07-0224- FEDER-002001 (MT4MOBI). The second author is also grateful to the FCT for the postdoctoral grant SFRH/BPD/101334/2014. The authors would like to thank Prof. A. Andrade-Campos for helpful contributions on the development of the finite element code presented in this work.info:eu-repo/semantics/publishedVersio

Porous materials for thermal management under extreme conditions

A brief analysis is presented of how heat transfer takes place in porous materials of various types. The emphasis is on materials able to withstand extremes of temperature, gas pressure, irradiation, etc., i.e. metals and ceramics, rather than polymers. A primary aim is commonly to maximize either the thermal resistance (i.e. provide insulation) or the rate of thermal equilibration between the material and a fluid passing through it (i.e. to facilitate heat exchange). The main structural characteristics concern porosity (void content), anisotropy, pore connectivity and scale. The effect of scale is complex, since the permeability decreases as the structure is refined, but the interfacial area for fluid-solid heat exchange is, thereby, raised. The durability of the pore structure may also be an issue, with a possible disadvantage of finer scale structures being poor microstructural stability under service conditions. Finally, good mechanical properties may be required, since the development of thermal gradients, high fluid fluxes, etc. can generate substantial levels of stress. There are, thus, some complex interplays between service conditions, pore architecture/scale, fluid permeation characteristics, convective heat flow, thermal conduction and radiative heat transfer. Such interplays are illustrated with reference to three examples: (i) a thermal barrier coating in a gas turbine engine; (ii) a Space Shuttle tile; and (iii) a Stirling engine heat exchanger. Highly porous, permeable materials are often made by bonding fibres together into a network structure and much of the analysis presented here is oriented towards such materials. © 2005 The Royal Society

A steady-state Bi-substrate technique for measurement of the thermal conductivity of ceramic coatings

This paper presents a steady-state, bi-substrate technique for measurement of the through-thickness thermal conductivity of ceramic coatings, with a range of specimen thickness and porosity content. The technique is based on establishing unidirectional steady-state heat flow through the sample, sandwiched between a pair of (metallic) substrates with known thermal properties. Comparison between the heat fluxes passing through the two substrates allows a check to be made about the accuracy of the assumption of unidirectional heat flow. The interfacial conductances must be known and these can be estimated by testing samples of different thickness. Measured conductivities are likely to be more accurate if the interfacial conductance is relatively high. This is assisted by the introduction of a thin interfacial layer of a viscous, thermally conductive compound, or thermal pads of some sort, and by maintaining a suitable pressure across the setup. However, if such compounds (pastes) are used, then care must be taken to ensure that it does not enter the specimen via surface-connected pores, since this could significantly affect the measured conductivity. The reliability of the technique has been confirmed by testing fused silica samples of known thermal conductivity. It has also been applied to sprayed zirconia and plasma electrolytic oxide (PEO) alumina coatings. The values obtained were 1.05 ± 0.10 W m- 1 K- 1 and 1.63 ± 0.35 W m- 1 K- 1, respectively. © 2006

Residual stress generation during laser cladding of steel with a particulate metal matrix composite

Laser cladding is used to coat and repair the surface of various components. A significant issue relating to optimisation of the process is the generation of residual stresses. These are affected by the high thermal gradients inherent in the process, and associated differential thermal contraction. These stresses can lead to various types of distortion. A customised 3-D finite element model has been developed, incorporating these effects, based on simulation of conductive, convective and radiative heat transfer, and assuming elastic-perfectly plastic deformation behaviour. Creep effects have been neglected and the cladding (particulate metal matrix composite) has been treated as a continuum. Comparisons are presented between measured and simulated thermal fields and specimen deflection histories. The results indicate that the main features of residual stress generation in this type of system have been captured in the model. Implications for process optimization and control are briefly discussed. © 2006 WILEY-VCH Verlag GmbH and Co. KGaA, Weinheim

Residual stress generation during laser cladding of steel with a particulate metal matrix composite

Laser cladding is used to coat and repair the surface of various components. A significant issue relating to optimisation of the process is the generation of residual stresses. These are affected by the high thermal gradients inherent in the process, and associated differential thermal contraction. These stresses can lead to various types of distortion. A customised 3-D finite element model has been developed, incorporating these effects, based on simulation of conductive, convective and radiative heat transfer, and assuming elastic-perfectly plastic deformation behaviour. Creep effects have been neglected and the cladding (particulate metal matrix composite) has been treated as a continuum. Comparisons are presented between measured and simulated thermal fields and specimen deflection histories. The results indicate that the main features of residual stress generation in this type of system have been captured in the model. Implications for process optimization and control are briefly discussed. © 2006 WILEY-VCH Verlag GmbH and Co. KGaA, Weinheim

Residual stress generation during laser cladding of steel with a particulate metal matrix composite

Laser cladding is used to coat and repair the surface of various components. A significant issue relating to optimisation of the process' is the generation of residual stresses. These are affected by the high thermal gradients inherent in the process; and associated differential thermal contraction. These stresses can lead to various types of distortion. A customised 3-D finite element model has been developed, incorporating these effects, based on simulation of conductive, convective and radiative heat transfer, and assuming elastic-perfectly plastic deformation behaviour. Creep effects have been neglected and the cladding (particulate metal matrix composite) has been treated as continuum. Comparisons are presented between measured and simulated thermal fields and specimen deflection histories. The results a indicate that the main features of residual stress generation in this type of system have been captured in the model. Implications for process optimization and control are briefly discussed.status: publishe

Numerical Simulation of Lightning Strike Damage to Wind Turbine Blades and Validation against Conducted Current Test Data

This paper presents a novel numerical approach to simulate lightning strike damage to equipotential bonding interfaces of wind turbine blades, and model validation based on high-current testing. Modern rotor blades are equipped with metal receptors to intercept the lightning leader and metal down conductors to conduct the lightning current, preventing the direct attachment to the CFRP spars. In such conditions, damage in the form of resin thermal degradation and sparks develop inside the blade at the equipotential bonding interfaces. Excellent correlation was found between the numerical predictions and test results in terms of current and temperature distributions. High temperatures were predicted at the sparking areas observed in the tests, which suggested that the damage is thermally activated. Thermogravimetric analysis data indicated that the epoxy pyrolysis process evolves in stages, and that sparking events are often initiated by release of gases and formation of small voids at temperatures lower than expected