https://ww2.mathworks.cn/help/pde/ug/3-d-linear-elasticity-equations-in-toolbox-form.html;jsessionid=a46bb928c190a0047ab105c51f67?s_tid=gn_loc_drop#mw_a9edc25e-ff09-484e-858f-94be20c8054a
https://ww2.mathworks.cn/matlabcentral/fileexchange/31788-the-plane-stress-problem?focused=b9fb29d2-2251-2d93-3acc-92cb17d0609e&tab=function