Shear Planes for Total Dummies

Shear plane in deformed rock

Just yesterday, one of the Antisocial Network staffers heard a discussion of reading comprehension. The point was that when readers lack at least some background knowledge on a topic, their level of comprehension plummets. That, she suggested, is why liberal arts graduates so often do a lousy job of answering technical questions. You know, questions like "What Is a Shear Plane?" – a question James Doehring attempted to answer for WiseGEEK.com's niche site, InfoBloom (oops: they moved to AboutMechanics.com, even though it's not about mechanics. Go figure)...

Before we get into Doehring's botched attempt to define the term, we'll do it for him: a shear plane is nothing more than the plane along which a body fails through shear. Shear is a form of brittle failure in which two or more parts of the object slide past one another along a surface. In earth science, the most recognizable shear plane is the fault plane along which rock bodies fail.

That's not what Doehring said, though: the communications major was so flummoxed by all the technical stuff he read in a Quora.com post that he managed to completely botch his answer. According to Jim,

"A shear plane is the plane in which shear stress occurs..."

...which, to put it mildly, is utter bull. A shear plane is the locus of movement in response to shear stress. Shear stress does not occur in a plane; shear stress occurs in a direction. In an effort to sound more erudite than he actually was, however, Doehring continued by opining that,

"At any given point in a structure, there are many possible planes that can be defined to measure stress. Accordingly, whichever plane contains the area in question is the shear plane..."

...which is, again, utter bull. Here's where James went way out on a technical limb and then sawed it off behind him:

"...if someone held the bottom of [a metal] bar stationary while trying to move the top of the bar to the right. The resulting internal stress is called shear stress, because parts of the bar are trying to slide, or shear, past each other..."

Why Doehring went to metals is questionable, but we suspect it's because he had no idea what the Wikipedia article about mechanical shear was saying. That's about the only place we can find a reference to moving ends of bars around.
It's a shame that Doehring didn't just google "what is a shear plane," because even Merriam-Webster would have told him that it's a fault plane. Such, however, is the result of an education deficient in even the simplest science... the sort of education that seems to often lead to a Dumbass of the Day award.

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