Comparison of differently shaped testing specimens for simulation of thermomechanical cycles.

Behulova, Maria

1. INTRODUCTION

In the field of production and treatment of materials for automotive industry, the great attention is paid to relatively cheap low alloyed steels (Stankova et al, 2005, Masek et al, 2006). A special combination of microstructural, mechanical and utility material properties of these steels can be achieved by proper alloying and following suitable heat or thermomechanical treatment.

Newly designed processes of thermomechanical treatment (TMT) are tested using laboratory simulators. In this paper, five testing specimens for simulator of thermomechanical cycles with different shapes are analysed using FEM simulation to evaluate and compare the homogenity of temperature fields in their active parts and stability by applied clamping force.

2. EXPERIMENTAL MEASUREMENT

Laboratory simulator of TMT SMITWELD with module TTU 2002 installed at the TU Chemnitz (Fig. 1a) is designated for experimental investigation of material behavior during TMT with the maximum deformation rate of 2 mm. [s.sup.-1] and clamping force of 20 kN.

The specimen with the length of 81 mm and active cylindrical part with the diameter of 6 mm and the high of 11 mm is at this time used for experimental measurements (Fig. 1b). Before experiment, the Ni-CrNi thermocouple is welded on the surface in the central part of active cylindrical zone for very accurate temperature measurement (Fig. 1c). The surface in the active zone is covered by a protective antioxidant coating. Testing sample is then screwed in jaws of simulator and subjected to TMT. Specimen heating is realised by direct electrical heating, cooling can be assured by air, nitrogen or water. Process of TMT is controlled by computer using prescribed thermal, deformation and loading regimes.

[FIGURE 1 OMITTED]

3. SIMULATION MODEL

Five specimens with different shapes (Fig. 2) were analysed using the finite element code ANSYS 10.0. The length of all specimens is 81 mm. The active part of specimens has cylindrical shape with the diameter of 6 mm and the length of 11 mm. The middle carrying part of the specimen 1 (Fig. 2a) is comprised of conical and cylindrical parts finished by a notch. The specimen 2 (Fig. 2b) has extended middle part with the successive change in diameter from the active to the clamping part. The middle part of the specimen 3 (Fig. 2c) is cylindrical with the diameter larger then in the active part. The specimen 4 (Fig. 2d) is equipped in the middle cylindrical part by five notches and the specimen 5 (Fig. 2e) by five threads.

For the analysis of temperature fields in specimens during direct electrical heating as well as stability analysis, simulation models were prepared including development of geometrical model, FEM model, definition of nonlinear material properties, initial conditions, boundary conditions and loading for electric, thermal and static problems.

Analysis of electric fields was based on the solution of Laplace equation in the form (Novak, 2001)

div([sigma] gradV) = 0 (1)

where [sigma] is electric conductivity and V electric potential. Transient temperature field in solids can be described by Fourier-Kirchhoff partial differential equation (Incropera & DeWitt, 1996)

c[rho] [partial derivative]T/[partial derivative]t = div([lambda]gradT) + [q.sub.v], (2)

in which [rho] is the density, c--the specific heat capacity, [lambda]--thermal conductivity and [q.sub.v]--volume density of internal heat sources, i. e. the heat generated in unit volume per second. By resistance heating this term corresponds to the Joule heat computed by electric analysis.

[FIGURE 2 OMITTED]

Nonlinear buckling analysis with gradually increasing loads was applied to find the load level at which a specimen became unstable (Ansys, 2005).

Specimens with suggested shapes were numerically tested for the steels with the chemical composition of 0.1C-0.25Si-1.25Mn-0.3Ni-0.01N-0.03Al [wt. %]. Electric, thermal and mechanical properties were supposed to be temperature dependent. Specimens with the initial temperature of 20[degrees]C were loaded by the time variable voltage during heating period of 60 s. During the first 10 seconds, the voltage was 0.5 V with its following decrease to the value 0.47 V. Specimen cooling by mechanisms of free convection and radiation to the air with the temperature of 20[degrees]C was considered. For buckling analysis, specimens were constrained on the bottom surface and loaded by buckling axis force on the top surface. Developed simulation model and solution procedure were verified using experimental measurements at TU Chemnitz (Behulova et al, 2006).

4. RESULTS

During direct electric heating, specimens are heated due to the Joule heat generated in specimens by passing of electric current. For illustration, the distribution of current density j and internal heat sources (Joule heat) [q.sub.v] in the specimen 1 at the end of heating in the time of 60 second is shown in Fig. 3. Maximal values of current density and Joule heat were computed for the specimen 4.

Fig. 4 illustrates temperature fields in analysed specimens and their active zones in the time of 60 seconds. Temperatures in the clamping parts of specimens are approximately the same for all specimens attaining the values from 20[degrees]C to 200[degrees]C. Maximal temperature at the end of heating of 1105[degrees]C was reached in the specimen 1. Maximal temperatures of specimens 3, 4 and 5 are lower than maximal temperatures reached by heating of specimens 1 and 2 but the temperature differences in the active cylindrical parts are considerably lower. The most homogeneous temperature field in active part is in the specimen 4 with five notches in middle part.

[FIGURE 3 OMITTED]

[FIGURE 4 OMITTED]

[FIGURE 5 OMITTED]

This result can be confirmed also by the temperature distribution along the rotation axis of specimen active parts at the end of heating (Fig. 5). Maximal temperature differences approximately 330[degrees]C were computed for the specimen 1. In term of homogenity of temperature distribution, the specimen 4 seems to be the most favorable.

On the other hand, the results of buckling analysis (Table 1) revealed that this specimen can be loaded during experimental TMT only by smaller clamping force. The real behavior of the specimen 4 under higher deformation loading can be seen from figure in the Table 1.

5. CONCLUSIONS

Based on the numerical analysis of temperature fields by direct electric heating, the specimen 4 can be recommended for TMT testing of steels using the laboratory simulator of thermomechanical cycles. However, according results of buckling analysis, the specimen 4 can be loaded by smaller clamping force that currently used specimen 1.

6. REFERENCES

Ansys Theoretical Manual, Release 10.0, SAS IP, Inc., (2005).

Behulova, M., Stankova, H. & Masek, B. (2006). Analysis of temperature distribution in samples by direct electrical heating. Materials Science and Technology [online]. Vol. 7, No. 1. ISSN 1335-9053.

Incropera, F., P. &DeWitt, D. P. (1996). Fundamentals of Heat and Mass Transfer. New York, J. Wiley&Sons, ISBN 0-471-30460-3.

Masek, B.; Stankova, H.; Novy, Z. & Meyer, L. W. (2005). Development of new incremental forming strategies for low-alloyed TRIP steels, 8th Int. Conference on Technology of Plasticity, Verona, ISBN 88-87331-74-X

Novak, P. (2001). Zdklady elektrotepelnej techniky. (Fundamentals of electrothermal Technique). Kosice, Mercury-Smekal, 2001.

Stankova, H.; Masek, B. & Meyer, L. W. (2006). The Influence of the Incremental Deformation Intensity on the Microstructure Development, 7th Int. Conference on Production Engineering and Design for Development, PEDD 7, ISBN 12-7340-49-8, 2006, Cairo, Egypt.

7. ACKNOWLEDGEMENT

The research has been supported by the project VEGA MS and SAV of the Slovak Republic No. 1/0837/08 and DAAD A/07/01388.

Tab. 1. Computed buckling forces for
analysed specimens.

Specimen Buckling
 force [kN]

 1 20.1
 2 16.3
 3 15.9
 4 8.4
 5 9.8