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专业: 力学

[求助] lammps设置应力拉伸的时候，应变却一直是负数？

如题所示，我在用lammps进行y轴单轴加卸载的时候，明明施加的是拉伸力（4GPa），但是应变却一直负向增大到了-0.05是什么原因？
# 读取结构+势函数+能量最小化
clear
boundary    p p p
units       metal
atom_style    atomic
dimension    3
timestep    0.002
read_data    NiTi_100K.data
pair_style    meam
pair_coeff    * * library.meam Ni Ti NiTi.meam Ni Ti

neighbor    2.0 bin
neigh_modify every 10 delay 0 check yes

# ------------ 能量最小化 ---------------
fix          relax all box/relax iso 0.0 vmax 0.001
min_style cg
minimize       1e-25 1e-25 10000 10000
unfix          relax

# 弛豫过程
fix          1 all npt temp 100 100 0.2 iso 0.0 0.0 2.0 drag 2.0
thermo       1000
run          40000
unfix       1

# 设置加卸载应力
compute    atompe all pe/atom
compute    atomke all ke/atom
compute       atomcentro all centro/atom bcc
compute          cna all cna/atom 3.64

#加卸载过程应变
variable       tmp equal "ly"
variable       L0 equal ${tmp}
print          "Initial Length, L0: ${L0}"
variable       p1 equal "(ly-v_L0)/v_L0"
variable       p2 equal "-pxx/10000"
variable       p3 equal "-pyy/10000"
variable       p4 equal "-pzz/10000"
variable       pe equal "pe"
variable       temp equal "temp"

#输出
reset_timestep  0
fix             stress all ave/time 1 100 200 v_p3 file Stress5de9cf.txt
fix             strain all ave/time 1 100 200 v_p1 file Strain5de9cf.txt
fix          def1 all print 200 "${p1}    ${p3}    ${p2}    ${p4}" file NiTi_def1.txt screen no

# Display thermo
thermo          500
thermo_style custom step v_p1 temp lx ly lz vol density ke pe etotal enthalpy epair press v_p2 v_p3 v_p4

#----------------------------Loading-----------------------------------
variable       s_xx equal -3.0
print          "set on command line s_xx=${s_xx}" #x-force on rigid body

variable       stmp equal ${s_xx}*10000
fix       load all npt temp 100 100 0.2 y 0.0 ${stmp} 2.0 x 0.0 0.0 2.0 z 0.0 0.0 2.0 nreset 10
dump          1 all custom 500 dump_loading_110K.lammpstrj id type x y z fx fy fz c_atompe c_atomke
write_data       loading_110K.data

run             40000
unfix          load
undump          1
#----------------------------Unloading-----------------------------------
print          "stress at the end of loading, p3: ${p3}"
print          "unloading"
fix       unload all npt temp 100 100 0.2 y ${stmp} 0.0 2.0 x 0.0 0.0 2.0 z 0.0 0.0 2.0 nreset 10
dump          2 all custom 500 dump_unloading_110K.lammpstrj id type x y z fx fy fz c_atompe c_atomke
write_data    unloading_110K.data

run             40000
unfix          unload
undump          2

unfix          def1

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