Elasticity and Tensile Strength

By Joan Whetzel

When was the last time you played with one of those spring toys called a Slinky®? These toys demonstrate both elasticity and tensile strength as they are stretched out to their limits and then released and allowed to return to their original coiled shape. Elasticity and tensile strength are like a pushing and pulling tension that balance each other.

Hooke’s Law of Elasticity

Robert Hooke uncovered his Hooke’s Law of elasticity in 1660, which describes elasticity in mathematical terms. According to Hooke’s Law, when some object such as a metal pole or a spring, is stretched a specified distance (x), the object exerts a force (F) strong enough to returns the object to its original state, and that force is proportional or equal. (Remember Newton’s Laws of Motion where any action has an equal and opposite reaction.) As a mathematical equation, Hooke’s Law looks like this: F = -xk. In this case, k is the proportionality constant when applying Hooke’s Law to springs, like the Slinky® mentioned above and certain metals, namely steel. Under a substantial range of conditions, these materials (steel rods, Slinky, springs) will always conform to Hooke’s Law.

Elasticity

What’s happening is that when these metal materials are gently pushed, pulled or stretched out of shape – temporarily – an elastic displacement of their atoms has been attained. Releasing the force that pushed, pulled or stretched the item allows it to return to its original size and shape. Most solid materials behave elastically, but there are limitations to the amount of force that can be applied to the pushing, pulling and stretching and still having the materials behave elastically. This elastic limit is the maximum force that can be applied to an item before it’s permanently bent out of shape or stretched out of shape. In other words, stretching these materials beyond that point causes them to eventually give out and lose their elasticity. Brittle materials, when stressed beyond their elasticity limit, will fracture.

Elasticity and Force

To preserve structural integrity, a balance must be maintained between tension forces (pulling) and compression forces (pushing). Pushing against both ends of a heavy-duty spring creates compression forces. Pulling the two ends of the spring apart, or stretching it, creates tension forces.