如果物体的外形尺寸发生突变，则在突变处局部应力会显著增大，这种现象称为_______。
维氏硬度的压头是()
A. 球形
B. 正四棱锥形
C. 长四棱锥形
D. 正四方形
A dentist adjusts the shade ofa restoration using a complementary color．This procedure will result in()
A. increased value
B. decreased value
C. intensified color
D. increased translucency
Brittle materials have which of the following characteristics()
A. they undergo a lot of permanent deformation before fracturing
B. they tend to have a very ordered atomic structure
C. they respond poorly to tensile forces
D. A and B
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