Dances with Rocks
Join Date: Apr 2001
Location: Jefferson City, MO
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Does this help?
ALUMINUM OR STEEL?
If any part of your bike is steel, it's probably 4130. Easy to make and unbelievably cheap, it is a workhorse metal. For steels, the first two numbers indicate the principle alloying elements, a lot like the first number in aluminum alloys. 41 is the area code for chromium and molybdenum. You would probably never guess that all the 'chromoly' in your frame weighs less than the meat in a Big Mac. The last two numbers, 30, represent the carbon content of the steel in hundredths of a percent. So 4130 contains 0.30% carbon. This is a critical amount that represents a compromise between strength and ease of weldability. Steels with less carbon aren't as strong. Steels with more carbon don't join as well. Steel is usually used in the normalized condition. Normalizing increases strength a little and involves making the steel uniform by heating and allowing diffusion. Let's start by taking a look at a pile of numbers for aluminum and steel:
ALUMINUM Alloy Tensile Proof Elongation
Designation Strength (psi) Strength (psi) (%)
6061-0 18000 8000 25
6061-T6 45000 40000 12
7005-T6 51000 42000 13
7050-T6 83000 73000 11
7075-T6 83000 73000 11
STEEL Alloy Tensile Yield Elongation
Designation Strength (psi) Strength (psi) (%)
4130 annealed 81250 52250 28
normalized 97000 63250 25
high T temper 118000 102000 22
low T temper 236000 212000 10
Next to the alloy designation you'll see something called tensile strength. Tension is when you pull on both ends of something. It is the opposite of compression, where you push the ends of something together. Tensile strength is how much force per unit area you can hang on the metal. In this case, I'm telling it to you in psi, or pounds per square inch. There are some other units people use: a ksi is 1000 psi (k means a thousand); a ksi is also 6.89 MPa. MPa stands for Megapascal and is in the metric system, so it's the one scientists usually use. Take a look at Figure 1. It's a graph of a hunk of aluminum (6061-T6) and a hunk of steel (normalized 4130) that I keep hanging weight from. They're not to scale, but they give you a good idea what's going on. The y-axis (up and down) is stress, the
Figure 1 This is what happens when you start bending metal. To the left is aluminum. To the right is steel.
Stress is given in psi, so it's really not just a weight, it's weight divided by area - per square inch, get it? The x-axis (side to side) is strain, how much longer the metal gets. It's given as a percent. There are two things that happen to a metal when you bend them. They start off elastic. That's the first part of the graph. You bend something a little and let go and it returns to it's old shape. Steels are a lot better at this. The yield strength represents the transition from elastic to plastic behavior. Pull harder then the yield strength and you start to permanently bend (deform) the metal. See the kink in the steel? That's the yield strength - where the steel starts to bend permanently. If you haven't already guessed, that's bad. Aluminum doesn't break the same way. It doesn't take nearly as much to bend aluminum permanently as it does steel. So scientists invented something called proof strength. This is an important number, so pay attention. It is the strength at 0.2% strain, and you use it to tell which aluminum alloy is stronger. The proof strength is how much force per unit area you can load the metal with and have it stay 0.2% longer when you take the weight away. For 6061-T6, you can hang 40000 pounds from a one inch square rod and it would permanently stay 0.2% longer than it was when you started. See the difference in tensile strength and proof strength for 6061 alloy that hasn't been heat treated (0) as opposed to the peak aged (T6) alloy? The heat treated aluminum is a lot stronger, for reasons we talked about last time. You can hang 45000 pounds from a one inch square rod of 6061-T6 before it will break in two. Elongation is the science word that tells you how much the metal stretches before it breaks. It looks like our 6061-T6 gets 12% longer before it comes apart.
It's important to remember that the table gives actual strength values. To compare these values with a different material, let's say steel, would be like comparing apples and oranges. You need to take a look at the specific strength. That's the ratio between tensile strength and density. This accounts for the fact that although steel is a lot stronger than aluminum, it also weighs a lot more - 2.9 times more. This will give you a number that can be used to give you a rough comparison between vastly different metals. For 6061-T6, do the math and you get a score of 420600. Do it for a 100 ksi steel, and you get 363600.
So aluminum is better than steel, because it's lighter. Not so fast. If only it were so easy. We've yet to consider a couple of things: stiffness (scientists call this modulus to confuse people) and fatigue resistance are the two big ones. Remember - it doesn't take as much to bend aluminum into pretzels. And don't forget cost. So stick around, we'll cover them next time.
If everything seems under control, you're just not going fast enough...
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