Abstract

A finite element model has been developed for the 3D simulation of the sintering of a single track during a selective laser sintering process (SLS). The model takes into account both the thermal and the sintering phenomena involved in the process. Owing to the continuous movement of the laser beam the model takes also into account the transient nature of the problem. This is transformed into a pseudo‐static one through a transformation of the coordinates system of the equations. Nevertheless, this transformation introduces a convective term into the heat equations that produces instabilities in the solution. These instabilities have been solved by using a stream upwind Petrov Galerkin (SUPG) strategy together with a shock capturing scheme. Finally, a fixed point strategy is used for the solution of the analysis. The model has been tested through the solution of some examples.

Abstract

Residual stresses and distortions are two technical obstacles for popularizing the additive manufacturing (AM) technology. The evolution of the stresses in AM components during the thermal cycles of the metal depositing process is not yet clear, and more accurate in situ measurements are necessary to calibrate and validate the numerical tools developed for its simulation. In this work a fully coupled thermo-mechanical analysis to simulate the laser solid forming (LSF) process is carried out. At the same time, an exhaustive experimental campaign is launched to measure the temperature evolution at different locations, as well as the distortions and both the stress and strain fields. The thermal and mechanical responses of single-wall coupons under different process parameters are recorded and compared with the numerical models. Good agreement between the numerical results and the experimental measurements is obtained. Sensitivity analysis demonstrates that the AM process is significantly affected by the laser power and the feeding rate, while poorly influenced by the scanning speed.

Abstract

The present work aims at advancing an innovative computational methodology
that simulates steel and composite material fracture under cyclic loading following
a phenomenological approach, with calibration from both small scale and
large scale testing. This work addresses fatigue processes ranging from High Cycle
to Ultra-Low-Cycle Fatigue. An assessment of the current state of the art is done
for all the di erent fatigue types. Following, for Ultra-Low Cycle Fatigue a new
constitutive law is proposed and validated with experimental results obtained
on small scale samples. Industrial applications are shown for a large diameter
straight pipe under monotonic loading conditions and for a bent pipe under cyclic
loading. Emphasis is made on the capacity of the model to represent di erent
failure modes depending on the loading conditions. The research regarding this
part has been used in the frame of the European Project: \Ultra low cycle fatigue
of steel under cyclic high-strain loading conditions" (ULCF).

Regarding High Cycle Fatigue, a classic damage model is presented in combination
with an automatic load advancing strategy that saves computational time
when dealing with load histories of millions of cycles. Numerical examples are
shown in order to demonstrate the capabilities of the advancing strategy and a
validation of the model is done on small scale samples.

A new constitutive model is presented for Low Cycle Fatigue that uses the classic
plasticity and damage theories and simultaneously integrates both processes
in the softening regime. The capabilities of the model are shown in numerical
examples.

Finally, the High Cycle Fatigue damage model is applied to the constituents of
a composite material and the structural behaviour is obtained by means of the
serial/parallel rule of mixtures. Validation of the constitutive formulation is done
on pultruded glass ber reinforced polymer pro les.

Abstract

We propose here a numerical model for a three-dimensional simulation of glass forming processes. Using the basic philosophy of the Particle Finite Element method (PFEM), we introduce several new features adapting the strategy to suit the problem of interest. A modified fractional step method for the solution of the flow equations is applied. This approach, on the one hand, inherits the computational efficiency of the original fractional step approach, and on the other hand shows better mass conservation features. These features are particularly attractive taking into account the importance of the correct prediction of the glass product's wall thickness. A smart mesh update strategy and a simple mechanical contact scheme are introduced. In order to account for temperature-dependent viscosity, the heat equation is coupled to the mechanical model. Viscosity is obtained from the temperature field via an empirical law. The model is validated and an example modeling the processes in the final blow mold of the bottle manufacturing process is proposed.

Abstract

This work includes the numerical simulation of different locations within one meter thick walls experimentally investigated by Mangold (1994). Mangold simulated the hardening process in different walls during the frist seven days with the help of a so called "cracking frame". Continuous measurings of deformations gave the basis to quantify the stresses and thus the zero-stress temperature in the studied locations.

The aim of the work is to verify the prediction of stress development of the finite element program COMET includind the constitutive model, developed by Cervera et al. (1999). The reference temperature used for the calculation of thermal strains in the model is discussed in relation to the zero-stress temperature given by Mangold (1994). Moreover the effect of viscosity and the chemical aspect of the model are studied. Starting from a given development of temperature from Mangold (1994), all parts of the cross-section could be simulated in one element. The predicted stress development is compared with the experimental results of Mangold (1994).

Abstract

In this work we present a numerical model for the estimation of atmospheric seeing in observation sites. The particularity of the method is that it is based on a Variational Multiscale turbulence model, its main feature being that the numerical mechanisms which are used to deal with stability issues (convection and the inf–sup condition for incompressible flows) do also take care of the modeling of turbulence. Based on this turbulence model, we develop the expressions for the viscous and thermal dissipations, ∊∊num and χnum, which are later used for evaluating the constant of structure of the refraction index Cn2 following the classical model developed by Tatarski. Numerical examples show the behavior of the proposed numerical scheme when applied to turbulent flow practical cases, which include a convective boundary layer, the flow inside a transfer optics room, and a telescope enclosure.