What is the definite definition of strength?


by scoutfai
Tags: definite, definition, strength
scoutfai
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#1
Nov12-09, 12:19 AM
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I have access to definite definition to terminology like stress(Pa), strain(dimensionless), ultimate strength(Pa), yield strength(Pa), etc. But I often get confused when someone just write "strength".

I give an example. Consider the following sentence:

"Titanium has high strength and high stiffness."

What is the strength referring here? Is it a measurable variable, or a qualitative description? If it is measurable, what is its unit, and in a typical stress-strain graph of a ductile material, where is this variable located?
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Mapes
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Nov12-09, 06:51 AM
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You're not alone in being confused. The word "strength" alone is ambiguous; it could refer to the yield strength in uniaxial tension, the ultimate strength in uniaxial tension, the shear strength, the fatigue strength, etc.

As an everyday (non-precise) term, it probably corresponds to the highest point on the engineering stress-strain diagram (not marked on yours), also known as the "ultimate strength," the point where a metal will begin to neck.
stewartcs
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Nov12-09, 09:28 AM
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Quote Quote by scoutfai View Post
I have access to definite definition to terminology like stress(Pa), strain(dimensionless), ultimate strength(Pa), yield strength(Pa), etc. But I often get confused when someone just write "strength".

I give an example. Consider the following sentence:

"Titanium has high strength and high stiffness."

What is the strength referring here? Is it a measurable variable, or a qualitative description? If it is measurable, what is its unit, and in a typical stress-strain graph of a ductile material, where is this variable located?
Used in that context the word strength generally means the material strength. So if you were to compare Titanium to Steel, you would see a higher overall strength (both yield and ultimate) for Titanium.

But like Mapes said...it's important to clearly define what one is referring to by using a qualifier like yield or ultimate...

CS

Mech_Engineer
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Nov12-09, 10:29 AM
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What is the definite definition of strength?


As a general rule, in the context of Mechanical Engineering a material's "strength" is it's yield strength or fatigue strength. Ultimate strength might be used for worst-case failure analyses but usually isn't used for design purposes because once the yield strength is passed the part has been permanenetly damaged. If a part is designed to fail at a certain time or in a specific location, then ultimate strength would be used.

The statement "Titanium has high strength and high stiffness" is most likely referring to Titanium's yield strength and elastic modulus. As a general rule Titanium's yield strength is not all that impressive, but when alloyed properly (such as Ti 6AL-4V) it's yield strength gets very high, and more importantly it's yield strength to weight (density) ratio goes through the roof.

Overall Titanium's elastic modulus (stiffness) is only slightly higher than aluminum and lower than steel, but Ti is also half the density of steel and specific alloys have a yield strength that can surpass high alloy steels, far beyond anything Aluminum is capable of. So parts properly designed with high strength alloys of Titanium can be around 50%-75% lighter than steel componnets while being as strong or stronger (based on yield strength), and far stonger than Aluminum alloy components.
scoutfai
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#5
Nov14-09, 11:02 PM
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Thank you to Mech_Engineer, stewartcs, & Mapes. Yours explanations are indeed very helpful in clarifying my confusion.


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