9.单向应力状态如图所示,设材料的弹性模量为E,泊松比为μ。aa,εa分别表示沿n方-|||-向的正应力和线应变,关于下列四个结论:-|||-①当 alpha =(0)^circ 时, (sigma )_(a)=sigma _(a)=sigma /E;-|||-②当 alpha =(45)^circ 时, (sigma )_(a)=sigma /2 _(a)=sigma /2E;-|||-③当 alpha =(90)^circ 时, (sigma )_(a)=0 _(a)=0;-|||-④当 alpha =(135)^circ 时, (sigma )_(a)=sigma /2, (varepsilon )_(alpha )=(1-mu )sigma /2E-|||-下列选择中正确的是 ()-|||-(A)①②正确 (B)①④正确 (C)②③正确 (D)①②③正确-|||-"-|||-a-|||-σ-|||-题9图