QUASISTATIC STRAIN RATES ’ EFFECT TO THE PROPERTIES OF ADVANCED STEELS FOR AUTOMOTIVE INDUSTRY

The paper presents the findings of the strain rate effect, described in constitutive material models by strain rate sensitivity index m, to the strength and deformation material properties. These were evaluated from stress-strain diagrams recorded at tensile test using PC controlled testing machine TiraTEST 2300 according to STN EN ISO 6892-1. The high strength low alloyed steel H220PD, dual phase steel DP 600 and Trip steel RAK 40/70 were investigated at strain rates  = 0.0021, 0.083 and 0.125 s. The results indicate major influence of the strain rate to the strength properties while only minor influence have been found when evaluate the deformation properties. The results also shown single phase materials are more sensitive to the strain rate than dual phase materials.


Introduction
Technic evolution in present is characterised by effort to increase the cars' safety and power as well as reduction of weight and emissions.This requires better understanding to the processes involved in materials when car impacts to the barrier or when components for deformation zones are produced; mainly at higher strain rates.The cars compatibility at accident depends mainly on each car weight, structure of deformation zones and deformation properties of materials used.When car frontally impacts deformable barrier at speed lower than 20 km/h, the impact energy have to be totally absorbed in controlled manner by car-body structure and materials used for components of deformation zones to secure the passengers' safetysee Fig. 1.Proper car-body structure eliminates both (colored components on Fig. 1), the extreme deceleration or human body overloading over critical biomechanical value at first and intrusion of solid parts into the cabin at second.Thus, material properties not only for static, but also dynamic strain rates have to be known for complex analysis of car-body deformation components.The strain rate raising, resistance to deformation increases as well and some materials have shown material micro-and sub-structure change [1,2], yield strength and deformation to fracture growth [3].DOI   Strength and deformation properties of car-body structural components when tested before production are predicted by numerical simulations.The simulation reliability depends mainly on the accuracy of input datamaterial properties, boundary conditions, speed, forces, etc.not only for static but also dynamic strain rates [6].The strain rate, when structural car-body components are deformed at crash, is derived from car speed at crashsee Table 1.For initial car speed 64 km/h, as defined by EuroNCAP test at frontal impact, the strain rate of car-body structural components is 155 s -1 [7].Fig. 3 The strain rate at frontal impact with 40 % overhang Different testing equipment has to be used for material testing at different strain rates: mechanic or hydraulic testing machines offer the strain rate < 1s -1 , special testing equipment (VHS 8800 by Instron) offers the strain rates within 1 to 10 3 s -1 and specialised tests and equipment (SHPBT -Split Hopkinson Pressure Bar Test or TAT -Taylor Anvil Test) offer strain rates within 10 3 to 10 4 s -1see Fig. 4.
Fig. 4 Tests offering different strain rates

Materials and methods
The experiments were built to find out the strength and deformation properties of high strength steelshigh strength low alloyed steel H220PD, Dual phase steel DP 600 and Trip steel RAK 40/70.Findings could be used for virtual crash and formability tests at different strain rates.The chemical compositions of materials used for experiment have been shown in [8].Microstructures of experimental materials are shown on Fig. 4 to Fig. 6.High strength low alloyed steel H220PD -Fig.5 shows fine grained ferritic microstructure with average grain size ≅ 3 µm.Secondary nitrides and carbides also have been found.Dual phase steel DP 600 -Fig.6 shows ferritic-martensitic microstructure with volume content of ferrite 70 to 75 % and martensite 25 to 30 %.Some irregular martensitic islands with diameter approx.≅ 5.8 µm have been found in ferritic matric with average grain size ≅ 1.7 µm.Martensite morphology and size wasn't possible to identify easily in many areas.Different structural components allow verifying not only effect of the strain rate but also structure to the deformation characteristics [8].
Most of the simulation software use constitutive material models by Hollomon, Ludwik, Swift, Voce, Krupkowski-Swift [9,10] to define material stress-strain behaviour when deformed at static strain rates conditions.For middle and higher strain rates following constitutive material models are used [11,12]: -Hollomon: ) where K0material constant at static strain rate (reference one) C, Dstrain rate coefficients nstrainhardening exponent mparameter involving sensitivity to strain rate φtrue strain 0   -reference strain rate   -strain rate Coefficients presenting the strain rate effect to the true stress (or deformation resistance) could be calculated from modified Hollomon constitutive equation: If the eq.( 4) is written for two different strain rates: ) and dividing eq. ( 5) to eq. ( 6) we reach: Then, the strain rate sensitivity index is calculated as follows: The strain rates effect to the mechanical propertiesyield strength Re, ultimate tensile strength Rm, elongation A80, material constant K, strainhardening exponent n and normal anisotropy ratio rhave been evaluated from stress-strain diagrams recorded at tensile test using PC controlled testing machine TiraTEST 2300 according to STN EN ISO 6892-1.The machine crosshead speed v = 10, 300 and 600 mm.min -1 was recalculated to the strain rate as follows: When deformation properties and deformation work (energy absorption ability) are predicted by Finite Elements Method, forming limit curves (FLC) are other material property necessary to define.FLC allow defining the deformation scheme effect to the limit strains distribution in the sheet plane (φ1krit, φ2) and thickness (φ3krit).The limit strains φ1krit for assumed strain φ2 have been measured by tensile test of notched specimens.The notch radii R = 5; 15; 17.5 and 25 mm have modelled the minor strain φ2 and limit major strains φ1krit have been evaluated.Tests have been performed on testing machine TiraTEST 2300 with crosshead speeds v = 10, 300 and 600 mm.min -1 .Thus, strains are localised in the area defined by notch length (L0 = 2.R) and it is lower than initial length 80 mm as for tensile test.Therefore, the strain rate for these specimens have been evaluated as average value of major strain φ1krit and time t, when the maximum load of the specimen have been reached.Limit strains in the notch root area (critical section) have been measured by 3D optical photogrammetric system Argus with grid of dots Ø 0.5 mm and dots' pitch 1 mm -Fig.8 to Fig. 10.Consequently, the strain rates for notched specimens were 0.047; 0.21 and 0.5 s -1 when crosshead speed v = 10; 300 and 600 mm.min -1 have been set.

Results and discussion
Materials have been analysed from the view of both, crash and formability virtual tests at quasistatic strain rates.The strain rate effect to the: -strength propertiesyield strength Re and ultimate tensile strength Rm , -deformation propertieselongation A80, uniform elongation Ag, normal anisotropy ratio r and limit strain FLD0, -parameters of modified Hollomon constitutive equationmaterial constant K0, strainhardening exponent n and strain rate sensitivity index m, have been analysed.Table 2 to Table 4 show all measured values at different strain rates for experimental materials used, except the limit strains FLD0 shown in Table 5. published papers [13,14].The strength properties, within the quasistatic strain rates interval, increase due to lattice resistance to dislocations' movement.It is assumed, when the strain rate increases, it is not enough time to slip dislocations in planes the most properly oriented to the external loading.Thus, dislocations may also slip in planes with higher critical shear stress and consequently the higher stress is necessary to deform material [15].The highest increase of yield strength and ultimate tensile strength has been found for low alloyed steel H220PD with fine grained microstructure created by single phase structure and precipitates.Otherwise, Trip steel RAK 40/70 shows the highest number of barriers to the dislocations' movement by slipping due to its structure created by three phases.It is assumed, the more barriers to the dislocations' movement are contained in material, the lower dependence on the strain rate material presents.The strain rate effect to the deformation characteristics is shown in Fig. 13 and parameters of modified Hollomon constitutive equation is shown in Fig. 14.It has been found only minor influence of the strain rate variance to these material properties.The value deviations from the mean value lay within the standard deviation calculated.The finding complies to [16,17,18].It is assumed, when the strain rate increases, the plastic deformation takes place in more slip planes, i.e. in direction with higher shear stress.Measured values of limit strains FLD0 for experimental materials show also the minor influence of the strain rate variance to the forming limit curve position.When the strain rate increases, strains are localised to the fracture area, because the more barriers to the to the dislocations' movement, the lower difference in strain concentration onto the fracture position is.Thus, if the strain rate increases towards the critical value, any major decrease of deformation characteristics is found, but, in some cases the minor increase may even appear -Fig.15.

Conclusion
In the paper there are presented results of the strain rate influence to the strength and deformation properties of high strength steels: low alloyed steel H220PD, dual phase steel DP 600 and Trip steel RAK40/70.The findings have shown the strength properties are more sensitive than deformation properties, within the quasistatic strain rate interval 0.0021 s -1 to 0.5 s -1 .The yield strength has increased more than ultimate tensile strength, but the total and uniform elongation, normal anisotropy ratio neither strainhardening exponent didn't decreased.True stress-strain curves necessary when simulating crash or formability tests by finite element method have been analysed for three quasistatic strain rates.The true stress-strain curves' sensitivity to the strain rate was described by the strain rate sensitivity index m.The highest value of index 'm' has been found for low alloyed steel H220PD with the fine grained microstructure created by ferrite and precipitates (single phase structure).Otherwise, trip steel RAK 40/70 involves the highest number of barriers to the dislocation movement due to its three phase structure.It is supposed, the more barriers to the dislocations' movement are contained in material, the lower dependence on the strain rate material presents.The finding has to be seriously considered, if such materials are processed in automotive industry and pressure to time-shortening in stamping operations permanently increases.
The other very important material data when simulating crash or formability tests by finite element method are forming limit curves, as dependence the major strain φ1krit to minor strain φ2, mainly the initial point FLD0, i.e. φ1krit, φ2 = 0.Only the minor influence of the strain rate to this material parameter has been found.The deviations laid within the standard deviation calculated and the finding complies with the results of previously published papers.It is supposed, when increase the strain rate, plastic deformation begins (dislocations' movement) also in directions with higher shear stress and it expands to other locations.

Fig. 11 2 Fig. 12
Fig. 11 Strain rate effect to the yield strength Rp0.2 Fig. 12 Strain rate effect to the ultimate tensile strength Rm

Table 2
Material properties of high strength low alloyed steel H220PD

Table 3
Material properties of dual phase steel DP 600 about 15 MPa, for dual phase steel DP 600 about 8 MPa and for Trip steel RAK 40/70 about 1 MPasee Fig. 11 and Fig. 12.It has been found, the yield strength have increased more intensively than ultimate tensile strength.The finding complies with the results previously DOI 10.12776/ams.v22i1.655p-ISSN 1335-1532 e-ISSN 1338-1156

Table 4
Material properties of Trip steel RAK 40/70

Table 5
Limit strains FLD0 when measured for different strain rates .),aspresented in previous acc.toeq.(9.).The highest value of m = 0.019 has been found for low alloyed steel H220PD and the lowest one (m = 0.01) for dual phase steel DP 600.The strain rate sensitivity index m = 0.011 has been found for Trip steel RAK 40/70.Values for each experimental material are compared in Fig.16.As it is shown, single phase materials are more sensitive to the strain rate as multiphase materials.