PSI - Issue 5

Abdelouahid El Amri et al. / Procedia Structural Integrity 5 (2017) 363–368 Abdelouahid El Amri/ Structural Integrity Procedia 00 (2017) 000 – 000

366 4

p eff  by scaling the quasistatic yield stress

s y  with the following equation:

plastic strain rate

1 p p eff               1 C      

(1)





  p

p p    eff eff ,

s  



y eff

C and p are fit-parameters, determined for several temperatures. In order to validate the material definition, results of tension test simulations are compared with experimental results from literature. The coupling of the mechanical and thermal models follows a sequential approach. The mechanical and the thermal part of the problem are solved independtly using different solvers. The mechanical part uses a dynamic explicit solutions scheme, whereas the thermal part uses an implicit conjugate gradient solver. The bottlenek in computation time is the explicit mechanical time step mech t  which is limited by l is the element length, c the sound velocity, E the Young’s modulus,  the density and  the Poisson’s ratio. A common technique to speed up the simulation is the application of both mass and time scaling. The default approach for mass scaling is to prescribe the mechanical time step for the entire simulation. In order to meet the desired time step, the mass of elements with a time step lower than the prescribed one is artificially increased. In hot forming processes the material is depend on temperature and strain-rate. Therefore all-time dependent material and process parameters like strain-rate sensitivity, thermal conductivity and heat transfer coefficients have to be scaled according to the increase of tool velocity. mech l Δt c  with   2 E c ρ 1 ν   (2)

4. Results and discussion

Figure 2: stress variation with time test

Figure 3 : Damage variation under Number of cycle

Fig.2 shows that the stress variation under time test at different elongation of steel, at a bigger elongation the deformation rate is high and the deformation mechanism is combined between the grain boundary sliding and dislocation motion resulting in damage features appearing at the interface of the inclusion particularly at the grain boundaries. The maximum elongation after oscillation is slightly above 5mm. From the start to the top displacement, a separation process of the initially vertical legs is performed. After the evaluation of these critical zones under a specific load cycle number damage initiation can be predicted with an application of a proper criterion. Small stresses or a temperature change cause elastic deformations that disappears when the stresses cease or the initial temperature is recovered, but large stresses or temperature variations, give way to inelastic deformations on either free-standing or