There are 3 basic forms of mechanical stress – compression, tension, and shear. A working knowledge of these 3 basic forms will give you a fundamental understanding of mechanical stresses and their characteristics. We encounter these stresses many, many times a day in some form or other, and may benefit from knowing more about them.
We might casually define this word as “to make smaller“, or, “to compact“. And this is correct. Compression is the force or stress that presses or “pushes” as opposed to “pulling“. Imagine a sand castle on the beach and what would happen to it if you were to try to stand directly on top of it. Well, as you no doubt figured, the poor sand castle will be flattened under your weight, failing in compression.
Tension is in simple terms, the opposite of compression, and is the pulling apart of a material as opposed to pushing in. A good example of tension would be a game of tug o’ war. 2 teams are each pulling on a rope in opposite directions. The rope is therefore in “tension” due to the opposing forces pulling the rope in opposite directions. If the rope were to break, it will have failed in tension.
Perhaps the least known of the 3, shear stress is applied parallel to the face of the material in question. Say there’s a deck of cards stacked neatly on a table (for this example we’ll be treating the deck as one object, as opposed to many individual cards). Now if you were to take your palm and with one stroke, “spread” the deck out across the table, your deck of cards will have just technically failed in shear. You applied a force in a direction parallel to the face of the deck, making it a shear force.
Another example of shear stress that we may encounter in our every-day lives is cutting. When we cut a piece of fruit in half for example, the fruit is overwhelmed by the shear force introduced by the knife, due to its high mechanical advantage. Here it might be helpful to think of the knife as a very thin and sharp wedge that simply used its mechanical advantage (inclined plane) to force the fruit apart, as opposed to “cutting” through it.
Shear stress contrasts with normal stress, with normal stress being one that is perpendicular to the material face as opposed to parallel. Shear stress is denoted τ, the Greek alphabetical letter “tau”.
Ok, congratulations, now we know what it means when they say for example, something “failed in shear“. Materials vary greatly in strength and are often engineered specifically to resist a certain stress or stresses. Our physical World seems to favor compression over tension by virtue of the law of gravity.
Although it is true that there are materials that represent each of the 3 modes of stress in terms of strength, it cannot be overlooked that gravity smiles kindly on materials strong in compression rather than that of tension. A lack of tensile strength in materials has always been a fundamental obstacle in the development of architecture and continues to be so. But Man will always be on the lookout to find or create materials that can bear more load, be produced for less, and last longer.