Shear tension is stress that trys to separate laterally. As the name implies, "shear" as in scissors, (shears)
Compressive stress, squeezing something
Tensile stress is as it implies, pulling apart
In a spar with sheer webbs, under positive G loading, the upper member of the spar is under compression, the lower under tension, the webbs are in shear, resisting the opposed lateral loads of the upper and lower spar members.
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