# Quiz Question

One of a continuing series

There was a previous Quiz Question involving stress concentration in structural members. That got me to thinking about strength of materials. Here is an interesting fact:

First, modulus of elasticity is a measure of the amount of stretch per cross-sectional area for a given load. For example, the modulus of elasticity of structural steel is in the order of 200 GPa. For specially-treated tool steel the modulus is about 207 GPa—not much better. Yet the tool steel will take ten times the load before rupturing. Does this make sense, and why?

Post your responses in the comments section below.

## Update and answer

Greg has the right answer. Hardened steel is nor more rigid than structural steel. It just stretches farther before yielding. It’s counter intuitive that something like a hardened tool bit is just as pliable as an untreated steel bar.

## 3 thoughts on “Quiz Question”

1. Gregory Aicklen says:

As I recall, Young’s Modulus allows us to compute how far a member will stretch for a given stress as long as we’re still in the elastic region. It doesn’t tell us when the member will yield (by which I mean leave the elastic region) or ultimately fail. I don’t know exactly what you mean by rupture (i.e. materials scientists might have a specific meaning for that term,) but assuming that your numbers for structural steel and tool steel are correct, I assume that either the yield point for tool steel or the breaking point (or both) comes at a far higher stress value.

This question seems too easy. I must be missing something.

• You are not missing much. The question is easy to anybody who has taken a course on properties of materials.

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