MODIFICATION OF FLD EVALUATION FOR ULTRA HIGH-STRENGTH STEELS AS A RESULT OF USING CONTACT-LESS SYSTEMS FOR MEASURING DEFORMATION

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1 MODIFICATION OF FLD EVALUATION FOR ULTRA HIGH-STRENGTH STEELS AS A RESULT OF USING CONTACT-LESS SYSTEMS FOR MEASURING DEFORMATION Abstract Jiří SOBOTKA a, Pavel DOUBEK a, Michaela KOLNEROVÁ a, Pavel SOLFRONK a a TECHNICAL UNIVERSITY OF LIBEREC, Studentská 2, Liberec 1, Česká republika, jiri.sobotka@tul.cz During last years there was a huge progress in contact-less systems for measuring deformation by means of photogrammetry which in its consequence leads to improve knowledge about material behavior under plastic deformation. This paper describes deformation behavior of two ultra high-strength materials (DOCOL 1200M and TWIP 1200) by means of FLD and just on these maps of plasticity shows possibilities to better understanding of deformation behavior for such materials by the help of photogrammetry here by means of system ARAMIS v And for these materials is clear that there is a certain necessity to modify common evaluation of FLD which will be enable to take into account completely different deformation behavior of these ultra-high strength steels. Keywords: Photogrammetry, FLD, Twinning Induced Plasticity 1. INTRODUCTION Sheets producers are still under quite large pressure namely from automotive industry. Because there is requirement for safety of passengers on the one hand and on the other hand quite large requirement for light weight materials. There are wide reductions in weight, in fuel consumption and in the emission of exhaust gases. And that is why during last years was developed wide spectrum of materials suitable for automotive industry (IF, BH, DP, CP, MSW, TRIP and TWIP steels etc). In this paper are compared two ultra highstrength steels DOCOL 1200M and TWIP 1200 from the deformation behavior point of view. Their mechanical properties are shown in fig. 1. Because of comparison reasons there is also curve for deepdrawing material (DX56) using namely for car-body outer parts. As commonly known ultra high-strength steels exhibit very high mechanical properties (yield strength and ultimate strength) and low ductility as DOCOL 1200M. But for TWIP 1200 (Twinning Induced Plasticity) there are excellent both mechanical properties and ductility. Fig. 1. Static tensile test of measured materials Obr. 1. Statická zkouška tahem měřených materiálů

2 2.. TWIP SHEETS TWIP steels (Twinning Induced Plasticity) are group of materials where the increasing elongation with decreasing temperature is attributed to strain-induced twinning: the TWIP effect. These sheets belong to socalled high-strength steels and are still mainly in the development process. Such kind of materials contains austenite stabilizing elements, e.g. Mn or Ni. The developed light weight high manganese steels exhibit an extremely large elongation in combination with quite large yield strength. It is due to reality that with increasing manganese content up to about 20 wt-% Mn, the stacking fault energy will be decreased and extensive mechanical twinning occurs and these steels exhibit extraordinary high plasticity. In table 1 are written mechanical properties of tested materials. Table 1 Mechanical properties of materials TWIP 1200 and DOCOL 1200M Tab. 1. Mechanické vlastnosti materiálů TWIP 1200 a DOCOL 1200M Material mechanical properties TWIP 1200 DOCOL 1200M Yield strength R p0,2 [MPa] 558, ,71 Ultimate strength R m [MPa] 1125, ,78 Ductility A 50mm [%] 58,14 7,55 Monotone hardening modulus C [MPa] 2366, ,24 Deformation hardening exponent n [-] 0,429 0,056 Extraordinary properties of TWIP materials can be easily observed with the help of contact-less systems for measuring deformation. Such systems are using pre-calibrated cameras working in certain calibration volume and scanning stages (via frames) during all test. In our department we are working with system ARAMIS v Material TWIP 1200 under static tensile test was already tested and showed quite unique deformation behavior of this material. Some real basic knowledge is given in the fig. 2. Here can be clearly seen that major strain distribution in the moment just before crack opening is still say about homogenous. It was also proved that due to the massive twinning during deformation there are waves of deformation which are moving through material thus use all material volume. All of that finally lead to reality that material is hardened almost everywhere. For more details look at [3]. Fig. 2. Graphical major strain ϕ 1 distribution and its distribution along section TWIP 1200 Obr. 2. Grafické rozložení hlavního přetvoření ϕ 1 a jeho rozložení podél řezu pro TWIP 1200

3 3. EXPERIMENTAL PART MEASUREMENT FLD For measurement FLD is in our department using method of stretching shaved (with different width) samples by semi-spherical punch. From tested materials (TWIP 1200 and DOCOL 1200M) were cut samples with different shape. Were chosen 5 different widths (30, 75, 105, 120 and 210 mm) where 210 mm means full blanks. With the help of system ARAMIS were scanned all stages of tested material during test. Limit stage of material represents stage just before crack opening. For each width were measured 5 samples. Because of paper space are shown only that ones with width 30 mm. In fig. 3 (TWIP 1200) and fig. 4 (DOCOL 1200M) are graphical major strain ϕ 1 distributions for stage just before crack opening and there is shown position of section along which are make curves in fig. 7. Fig. 3. Graphical major strain ϕ 1 distribution and position of section for TWIP 1200 Obr. 3. Grafické rozložení hlavního přetvoření ϕ 1 a pozice řezu pro TWIP 1200 Fig. 4. Graphical major strain ϕ 1 distribution and position of section for DOCOL 1200M Obr. 4. Grafické rozložení hlavního přetvoření ϕ 1 a pozice řezu pro DOCOL 1200M

4 From all samples were chosen stages just before crack opening and thus were found out coordinates of maximal major strain ϕ 1 and minor strain ϕ 2 (blue and red curves in fig. 5). A few years ago was proposed standard how to decrease these maximal values because to avoid already necking in the thickness direction in material. This standard using inverse parabola and its peaks (both for ϕ 1 and ϕ 2 ) are new coordinates for FLD. In the following figures are shown firstly maximal values of ϕ 1 and ϕ 2 and values calculated by standard ISO both for TWIP 1200 and DOCOL 1200M are shown samples with the width 30 mm. Final FLD are given in fig. 6. TWIP 1200 DOCOL 1200M Fig. 5. Maximal values (peaks) and standard ISO using inverse parabola (green curves) Obr. 5. Hodnoty maxim (vrcholy) a norma ISO používající inverzní parabolu (zelené křivky) TWIP 1200 width 120mm DOCOL 1200 M width 120mm Fig. 6. FLD for TWIP 1200 and DOCOL 1200M measured by maximal values and ISO Obr. 6. FLD pro TWIP 1200 a DOCOL 1200M zjištěné pomocí maximálních hodnot a ISO 12004

5 4. PROPOSAL MODIFICATION OF FLD EVALUATION From the figures above is evident that these two materials have totally different deformation behavior. However this reality is not clearly see from position of FLC (Final Limit Curves) in FLD. On the one hand there is difference between FLC (especially by using standard ISO 12004) for TWIP 1200 and DOCOL 1200M but on the other hand this gap is not so as huge as was brightly clear during measurement FLD. To catch up different deformation behavior and compared it by using measured values was taken into account distribution of major strain ϕ 1 along longitudinal direction (in the sample axis). Immediately was evident that there are huge differences between such curves namely from the homogenous distribution of ϕ 1 point of view. For material TWIP 1200 such curves were not only placed much higher than ones for DOCOL 1200M but also were much closer to the homogenous shape even in the case of narrowest samples (width 30 mm) - see fig. 7. Here is shown not only major strain ϕ 1 distribution along section (graphically shown in fig. 3, 4.) but also calculation of area under this curve marked like S FLD. In all samples for FLD (from width 30 mm to 210 mm) was this area calculated in the range ±40 mm from the centre. From fig. 7 for TWIP 1200 was S FLD = 25,29 and for DOCOL 1200M was S FLD = 3,85. Finally to cover all different strain states was make diagrams using calculated S FLD for y-axis and strain ratio m ϕ for x-axis. Results are given in the fig. 8. Fig. 7. Major strain ϕ 1 curve and calculation S FLD for TWIP 1200 (left) and DOCOL 1200M (right) Obr. 7. Křivka hlavního přetvoření ϕ 1 a výpočet S FLD pro TWIP 1200 (vlevo) a DOCOL 1200M (vpravo) Fig. 8. Diagram m ϕ -S FLD for TWIP 1200 (left) and DOCOL 1200M (right) Obr. 8. Diagram m ϕ -S FLD pro TWIP 1200 (vlevo) a DOCOL 1200M (vpravo)

6 5. CONCLUSION During experimental part were carried out FLD measurements for two materials which belong into ultra highstrength steels group: TWIP 1200 M and DOCOL 1200M. The main aim of this article was to propose modification for FLD evaluation because of different ultra high-strength steels deformation behavior. Common FLD evaluation (especially by means of maximal measured strain values) sometimes can lead to the bad interpretation and comparison of tested materials because namely ultra high-strength type of steel can exhibit quite huge maximal strain. However strain distributing along longitudinal direction clearly shows that such strain is just only in the necking area of sample. Such behavior is typical of common ultra highstrength steels (like DOCOL 1200M) but FLD based on these values can quite increase FLC. So there can be favouritism for such materials cause if there is another high-strength material (in this case TWIP 1200) with totally different deformation behavior (twinning) - from the maximal strain values point of view could not be take into account huge difference between these materials. During experimental part was proposed modification of FLD via using area under strain distribution S FLD in dependence of strain ration mϕ for all widths used for FLD. According such comparison (see fig. 9 - left) difference is 81,75%. It is namely due to twinning in TWIP 1200 which results into massive hardening in every part of material during forming. Such reality can be clearly seen in fig. 9 - right, where strain distribution is almost homogenous right before rupture. Finally should be notice that such problem really needs to make much more research and measurements for future. Fig. 9. Comparison of materials via m ϕ -S FLD diagram and strain distribution for TWIP 1200 (width 120) Obr. 9. Srovnání materiálů pomocí diagramu m ϕ -S FLD a rozložení ϕ 1 pro TWIP 1200 (šířka 120) ACKNOWLEDGEMENTS This paper was written in support of grant project GAČR 101/07/P113 and research project MSM LITERATURE [1] HERTZBERG R.W.: Deformation and fracture mechanics of engineering materials, John Wiley & Sons, New York, 1996, ISBN [2] TALBERT, S.H. AVITZUR, B.: Elementary Mechanics of Plastic Flow in Metal Forming, John Wiley & Sons, New York, 1996, ISBN [3] SOBOTKA, J.: Hodnocení mechanických vlastností nových vysokopevnostních materiálů, Disertační práce, TU v Liberci, Liberec 2008