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What are your Strengths…”Specific” that is?

When your being interviewed for a job be prepared to answer a classic interview question regarding your strengths. This is an invitation to talk about your skills…perhaps its analytical thinking; or, maybe organization…wait-just-a-minute, wrong topic! The strength I am referring to matters when one is considering the strength of a material. So, do you know, or did you ever consider, "Specific Strength" when deciding what material to use in a design? No…well, you should have! You may have a good design; but, is it the best design…. If not, consider adding Specific Strength to your decision matrix before making that all too critical material choice.

So, what is Specific Strength? 
  • Specific strength is a ratio equal to the ultimate tensile strength of a material divided by the material's density. Another term that is used to describe specific strength is breaking length; also known as self-support length.
  • Specific strength can be visualized as the maximum length a vertical column of material can be (assuming a constant cross-section) that could be suspended under its own weight if fixed at one end only but will not break.

 Specific Strength calculations are straightforward. Below is an example calculation:

Let say you want to use steel in your design. First you need to determine the ultimate tensile strength of 4340 steel. This is a simple lookup operation using a reference such as MMPDS for example. Let's use a ultimate tensile strength equal to 60,900 psi and assign a density of 0.29 pounds per cubic inch.Simply divide 60,900 lb/in2 by 0.29 lb/in3 to get 210,000 inches.

Comparing the steel's specific strength to that of say 6061 Aluminum having a density of 0.098 lb/in2 and an ultimate tensile strength of 45,000 psi, the specific strength will be equal to 415,384 inches. One quickly senses the benefit of aluminum over steel assuming; of course, you have a weight-sensitive…not a strength critical issue.

 Note: The units are length…however…what is the area of the column? Well, area is already defined in the tensile strength value. For instance, in the Steel example, the column's length was 210,000 inches; but the fixed cross-section is equal to a "Unit Area" …or, simply put, an area equal to one inch by one-inch square.

Hey! What about Carbon fiber!? Ok…in this example will use Figure 2 below. According to the chart, High strength carbon epoxy has a Specific Strength of about 1,600,000 inches. A factor of 7.6 times the specific strength of steel…perhaps this is one of the more obvious reasons why composites are so popular in the auto and aerospace industry.

Figure 2: Specific Modulus verse Specific Strength

So how do you read the Breaking Length chart? Looking at Titanium, for example, in the chart below, we see that the Breaking Length for Titanium is 29.4 kilometers (29,400 meters). This means that if we were to construct a column that is 29,400 meters in length (go ahead, I dare you to make it...go ahead), you could suspend the column from one end and it would not break; however, if you were to add 29,401 meters the column would break.

Glass has a breaking length of 83 miles. Obviously, glass is very strong; however, if you were to simply tap on the suspended glass rod with a little teaspoon, you would shatter the glass.

That concludes this article and once again my hope is that you now have either discovered a new appreciation for specific strength or have been reminded how such a simple ratio like Specific Strength can have such a big impact on your design. Remember to never lose sight of what your specific strength is…because as an engineer, you should be interested in materials that have high strength-to-weight ratios that translate into greater weight savings and improve performance to yield significant gains in overall system efficiency.

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Thursday, 17 January 2019
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