Study on dynamic mechanical properties of high nitrogen steels

Cheng Mio,To Zhong,Hi-ling Wu,Li-hong Bi,Wei Yun,Lin Yng,Yn-li Wng,Lin-yng Guo

aNo.52 Institute of China Ordnance Industries,Yantai,264003,China

bNational Key Laboratory of Science and Technology on Materials Under Shock and Impact,Yantai,264003,China

cJiangsu Shuguang Opto-electronics Co.,LTD,Yangzhou,225009,China

Keywords:High nitrogen steels Hopkinson bar Dynamic compression Dynamic tension Dynamic shear

ABSTRACT Reliable dynamic mechanical properties of high nitrogen steels are necessary for the design and assessment of armor structures subject to impact and blast.A series of experiments,based on Hopkinson bar techniques,were conducted and described in this study.The dynamic compression,tensile and shear properties of high nitrogen steel had been tested,and the stress-strain curves under high strain rates were obtained.The results have been showed as follows:High nitrogen steel has a remarkable strain rate strengthening effect.Compared to the static curves,the constitutive curves of dynamic tension and compression move upper.The dynamic compressive yield strength of high nitrogen steel increases first and then decreases with the increase of strain rate,and the yield strength varies in the range of 1465-1549MPa within the range of 1147-2042 s-1strain rate;The tensile strength of high nitrogen steel increases with the increase of strain rate.When the strain rate is greater than 1341 s-1,the tensile strength will not increase and the curve tends to be gentle.The pure shear yield strength of the high nitrogen steel is above 800 MPa.

1.Introduction

Armor steel is the main structural material for the bodywork,turret and additional armor of tanks and armored vehicles,which has good strength,toughness,ballistic performance and good technological performance[1-3].For a long time,the medium carbon and low alloy series by increasing the carbon content and using the corresponding heat treatment methods to improve the ballistic performance are used in armor steels at home and abroad[4,5].High nitrogen steels have been studied and applied abroad since 1960s.And in 1990s,further research on high nitrogen steel has added a new way to the development of armor steel[6].The strength of high nitrogen steel under the impact of high strain rate is obviously improved.When the projectile penetrates,the target of high nitrogen steel has strong impact hardening,which improves the ballistic performance of high nitrogen steel[7,8].More than ten years of researches have been carried out in the field of high nitrogen steel abroad,which has already been applied in the field of national defense and military industry,especially in armor protective materials[9].Therefore,it is necessary to study the dynamic mechanical properties of high nitrogen steels.In this paper,a series of experiments,based on Hopkinson bar techniques,were conducted and the dynamic properties of high nitrogen steel had been obtained,which could support the design and assessment of armor structures.

2.Experiment

2.1.Experimental materials

The raw material used in the experiment was 20 mm thickness nitrogen austenitic steel plate.The chemical composition was shown in Table 1.The quasi-static compression and tension stressstrain curves of high nitrogen steel were shown in Figs.1 and 2.They showed that the compression yield stress and tension yield stress of this material under quasi-static testare about 900MPa and 1000 MPa respectively.

2.2.Experimental method

Dynamic tests of high nitrogen steel were carried out on theSplit-Hopkinson bar at room temperature.It consists of gas gun,incident bar,transmitted bar,striker,buffer bar,shock absorber,and strain gage circuits to measure strain signal in the bars.Figs.3 and 4 show the Schematic of SHPB and SHTB device.

Table 1 Chemical composition of high nitrogen steel(ω/%).

In this paper,the compression and tension tests were conducted on Hopkison bar which diameter is 16mm and material is high carbon chromium alloy steel,the shearing tests were conducted on Hopkison bar which diameter is 20 mm and material is LY12 superhard aluminum alloy.The test specimens were shown in Fig.5,in which the compression specimens were measured diameter 8mm×4 mm,the tension specimens were measured diameter 4mm-gage length 10 mm,the thickness of shearing specimens were 2mm.

In tension tests,the gas gun launches the tubular striker to impact the incident bar.The transfer flange transfers the incoming elastic compressive stress wave into the elastic tensile stress which travels through the incident bar toward the specimen.When the tensile stress wave propagates into the specimen,it reverberates within the specimen until a nominally homogeneous stress state is achieved.And part of the wave is transmitted through the transmitted bar as a tensile wave,the rest is reflected back to the incident bar as a compressive wave.The strain signals were transferred into electrical signals by high dynamic strain indicator,and were recorded by the Multi-channel transient digital recorder.Analogously,in compression and shearing tests,through recorder the stress wave travels in the bar to acquire test result.

The test technology of Hopkinson bar is based on two hypothesis[10]:the one dimensional stress hypothesis and the stress uniformity hypothesis.The stress and strain relationship is derived according to the one-dimensional stress wave theory.

The end surface of specimen connected with incident bar is set 1,and the other end surface connected to the transmission bar is set 2.The displacements of interface 1 and 2 are U1(t)and U2(t)respectively.It is assumed that the strain signal of tension is negative and compression is positive.So:

Where:

c0elastic wave velocity;

εi（τ）,εr（τ）,εt（τ）strain signal of incident wave,reflected wave,transmission wave.

It is assumed that the original length and cross section area of the specimen are lsand Asrespectively,so the average strain in the specimen is:

The formula(3)is used to derive the derivative of time,and the strain rate in the sample is obtained.

The force of the end face 1 and the end face 2 of the specimen is P1(t)and P2(t)respectively.There:

Where:

A0cross-sectional area of the bar;

Eelestic modulus of the bar.

Average stress in the specimen is:

The above formulas showed how to calculate strain rate,strain and stress in the specimen by incident,reflection and transmission wave.

3.Results and discussion

3.1.The results of dynamic compression experiment

The results of dynamic compression were analyzed by one dimensional elastic wave theory.At least 3 repeated tests were conducted under each strain rate,and the coherence of each repeated curve was very well.Four dynamic compressive stressstrain curves of high nitrogen steel under different strain rate were shown in Fig.6.It can be seen from the diagram that high nitrogen steel is very sensitive to the variation rate.When the strain rate is 2042 s-1,the yield strength is about 1.5 GPa,which is nearly one time higher than that of quasi-static compression test.In the range of 1147-2042 s-1strain rate,the yield strength of materials varies in the range of 1465-1549 MPa.On the stress-strain curve,the elastic segment and the plastic hardening section of the real stress-strain curve of each specimen are linearly fitted respectively.The corresponding longitudinal coordinates of the intersection point of the two straight lines are defined as the yield strength of the material.The slope of the straight line fitted by the plastic section is the strain strengthening modulus of the material.The yield strength,strain hardening modulus and loading strain rate of each specimen were obtained by analyzing and fitting the experimental data.It is shown in Figs.7-9 that the dynamic compressive yield strength of high nitrogen steel increases first and then decreases with the increase of strain rate.The strain hardening modulus and compressive strength increase with the increase of strain rate.

Fig.10 is the specimens after compression.It showed that under tests strain rate the specimens were not crushed after compression.As the increase of loading strain rate,specimens after compression were thinner,the diameter of which were larger.

3.2.The results of dynamic tensile experiment

In order to obtain the dynamic mechanical properties of high nitrogen steel accurately,multiple unidirectional dynamic tensile tests were carried out.The dynamic tensile stress-strain curves were shown in Fig.11.In the range of 510-1892 s-1strain rate,the yield strength of high nitrogen steel varies in the range of 1500-1700MPa,which is higher than the quasi-static tension strengh.Therefore,high nitrogen steel has a remarkable strain rate strengthening effect.When the strain rate is 510 s-1,976 s-1and 1341 s-1,the tensile specimens of high nitrogen steel do not show the macroscopic fracture,and when the strain rate is 1676 s-1and 1892 s-1,the specimens are broken obviously during the tensile loading.It is shown from the Figs.12-14 that the dynamic tensile yield strength of high nitrogen steel materials increases with the increase of strain rate.The dynamic compressive yield strength of high nitrogen steel first increases and then decreases with the increase of strain rate.The strain hardening modulus and compressive strength increase with the increase of strain rate.The strain strengthening modulus increases first and then decreases with the strain rate,and the tensile strength increases with the increase of strain rate.When the strain rate is greater than 1341 s-1,the tensile strength will not increase,and the curve tends to be gentle.

The specimens after tension were shown in Fig.15.As the increase of strain rate,the necking phenomenon appeared until they were broken.The critical strain rate of fracture is between 1341 s-1and 1676 s-1.

Table 2 The results of dynamic shear experiment.

3.3.The results of dynamic shear experiment

The true stress-strain and the original waveform curves of high nitrogen steels at 1500 s-1,2000 s-1,2500 s-1and 3000 s-1strain rates are shown in Figs.16-23.

The pure shear stress response curves of high nitrogen steel first entered the elastic phase,and then began to enter the shear strengthening stage after reaching the shear yield strength.Until the shear strength was reached,the shear fracture was formed.

The experimental results show that in the range of strain rate 1500 s-1to 3000 s-1,the sensitivity of the dynamic shear strength of high nitrogen steel to the variation rate is not strong.The pure shear yield strength of the high nitrogen steel is above 800 MPa,regardless of the engineering stress-strain curve or the real stressstrain curve.

The engineering and real stress-strain curves are shown in Figs.25 and 26.The engineering stress is the ratio of the applied load to the original cross-sectional area of the gauge interval,and the engineering strain is the ratio between the elongation of the extended gauge and the original length of the gauge interval.The results of dynamic shear experiment under four strain rates are shown in Table 2.

4.Summary

(1)High nitrogen steel has a remarkable strain rate strengthening effect.Compared to the static curve,the constitutive curve of dynamic tension and compression moved upper.

(2)The dynamic compressive yield strength of high nitrogen steel increases first and then decreases with the increase of strain rate,and the yield strength varies in the range of 1465-1549MPa within the range of 1147-2042 s-1strain rate.

(3)The tensile strength of high nitrogen steel increases with the increase of strain rate.When the strain rate is greater than 1341 s-1,the tensile strength will not increase and the curve tends to be gentle.

(4)The pure shear yield strength of the high nitrogen steel is above 800MPa.