Stress-Strain Characteristics of Aluminum

Compared to most metals, aluminum has superior strength to weight ratio, corrosion resistance, and ease of fabrication. As a result, aluminum is one of the most widely used metals and is employed in products ranging from soda cans to aerospace components.

The strength of pure aluminum is very low, but its mechanical properties can improve substantially with alloying and heat treatment. These processes enable its widespread application in mechanical and electrical materials. Because it is second only to steel as a structural material, obtaining a stress strain curve for aluminum is crucial for determining the predictable and safe limits of its use.

In this video, we will look at the stress strain behavior of a common type of aluminum using the standard uniaxial tensile test.

Aluminum is lightweight and has roughly 1/3 the density of steel. Its modulus of elasticity, often cited to be about 70 gigapascal, or 10,000 kilopounds per square inch, is also about 1/3 that of steel.

As with steel, aluminum's mechanical properties can improve substantially by alloying, principly with zinc, copper, manganese, silicon, and magnesium. Cooled working or strain hardening, where the material is rolled or drawn through dyes, can also increase strength.

The uniaxial tensile test is typically used to study the elastic behavior of metals such as aluminum. This test generates a stress strain curve which shows how the material elongates and then fails as the applied force increases.

The failure of aluminum, or any material, progresses through several steps. Necking, void nucleation, void growth and coalescence, crack propagation, and finally, fracture. 6061-T6 aluminum has good strength and stiffness and is easy to finish and anodize. It is commonly used in casings for many electronic products such as laptops and televisions.

This is the stress strain curve for 6061-T6 aluminum. Notice how its stress strain curve does not exhibit a sharp yield point, but rather a gradual decrease in modulus of elasticity. Although this aluminum in fact does fail, the process is gradual and it is difficult to define a clear failure point when looking at the stress strain curve.

To determine a yield point for engineering purposes ASTM and other organizations have adopted the 0.2% offset approach. This method requires determining a best fit line for the linear portion of the behavior and drawing a line with the same slow beginning at 0.2% strain. The second line intersects the stress strain curve at a point that is arbitrarily defined as the yield strength.

Now that we understand the properties of aluminum and how they can be engineered, let's look at how to measure the stress strain curve to determine its ductile and mechanical characteristics.

Obtain cylindrical test specimen for common aluminum, such as 6061-T6. Use a caliber to measure the diameter at several locations near the middle of the specimen. Make these measurements to the nearest 2000th of an inch.

Next, hold the specimen firmly and mark a gauge length of approximately two inches. Make sure the gauge length is clearly etched, but with a shallow scratch so it does not become a stress concentration that can lead to a fracture. Measure the actual marked gauge length to the nearest 2000th of an inch.

Finally, install a strain gauge. The specimen is now ready for testing.

For this experiment we will be using a universal testing machine, or UTM, to measure the tensile properties of the specimen. First, turn on the testing machine and initialize the software. Set up the graphing and data acquisition parameters. Next, select a test that is compatible with the ASTM E8 protocol. Note the strain rates for the elastic and inelastic range. Then, set any additional actions in the software, such as stopping the machine at 5% tensile strength.

Manually raise the crosshead so the full length of the specimen fits easily between the top and bottom grips. Carefully insert the specimen into the top grip to about 80% of the grip depth. Align the specimen inside the top grip and tighten slightly to prevent the specimen from falling.

Slowly lower the top crosshead. Once the specimen is within about 80% of the bottom grip depth start specimen alignment within the bottom grips. The specimen should float in the center of the bottom grip. Apply lateral pressure to the specimen through the grips to ensure that no slipping occurs during testing.

The tightening process introduces a small axial load on the specimen. Use the software to adjust and minimize this preload and record its value. Attach the electronic extensometer securely to the specimen according to the manufacturer's instructions. The blades of the extensometer should be approximately centered on the specimen.

Start the test by applying the tensile load to the specimen, and observe the live reading of applied load on the computer display. Confirm the specimen is not slipping through the grips by making sure the measured load is increasing. Some time before sample failure, the software automatically pauses the test. Leave the sample in the test machine and remove the extensometer. Resume applying the tensile load until failure. Upon reaching the maximum load the measured loads will begin to decrease. At this point, the specimen starts to neck. Final fracture should occur in this necked region through ductile tearing.

After the test has ended, raise the crosshead, loosen the top grip, and remove the broken piece of specimen from it. Then, loosen the bottom grip and remove the other half of the specimen. Record the value at the maximum tensile load. Save the recorded data and the stress strain curve. Carefully fit the ends of the fractured specimen together and measure the distance between the gauge marks to the nearest 2000th of an inch. Record the final gauge length.

Finally, measure the diameter of the specimen at the nearest cross section to the nearest 2000th of an inch.

Let us now look at how to analyze the data we just collected. First, calculate the percent elongation of the specimen knowing the final gauge length and the initial gauge length. Calculate the reduction of area for each specimen using the final diameter and the initial diameter of the specimen. Next, calculate other material parameters using the experimental stress strain curves.

This is a plot of the strain gauge data up to the yield point of about 0.3%. The slope of the stress strain curve in this region is Young's modulus and is about 9,998 kilopounds per square inch, which is close to the nominal value of 10,000 kilopounds per square inch. The R squared value of 0.999 indicates excellent linearity for this data.

This is the data from an extensometer up to a strain of 5%. The curve shows a bilinear character, with a long elastic portion followed by a yield plateau with a low slope. To find the yield point for a material that does not exhibit a clear yield point, like this specimen, we use the 0.2% offset method.

First, we draw a line along the initial linear part of the curve. Then duplicate it start at a strain of 0.2%. The second line intersects the curve that is arbitrarily defined as the yield point. In this case, it is about 44.2 kilopounds per square inch. This is above the nominal yield strength of this aluminum which is 40 kilopounds per square inch.

If we plot the data very close to the yield point, the proportional limit is the stress where the curve begins to deviate from the linearity, about 39.1 kilopounds per square inch for this specimen.

This is the complete stress strain curve with the data below a strain of about 5% from the extensometer and above a strain of 5% from the crosshead displacement. The maximum stress is about 46.1 kilopounds per square inch at a strain of about 6.5%. This ultimate strength is just above the nominal ultimate strength of 45 kilopounds per square inch. The stress at failure is about 33.5 kilopounds per square inch. Toughness is the area under the stress strain curve and can be calculated with the trapezoidal rule to be 2.2 kilopounds per square inch.

The measurements for the heat-treated specimen indicate this type of aluminum may have elongations in the range of 8 to 13%. It is important to note that the percent elongation is an average value for the length of material between the gauge marks. Almost all the deformation, however, occurs in a small volume around the necked region, so the local strain could be much higher than the average strain.

In general, failure progresses from necking, to void nucleation and growth, to crack propagation, and finally, fracture. The failure surface is consistent with this process. For aluminum, an elongation less than 5% may be considered brittle, while an elongation greater than 15% may be considered ductile. The percent elongation in this specimen is relatively large. How should we describe this material?

We can compare its failure surface with that of two different types of steel. The size of the ??? for the aluminum specimen is greater than for a brittle cold rolled steel, but less than for a ductile hot rolled steel, so this type of aluminum can be characterized as semi-ductile.

In addition, we can look at the stress strain curves for these three metals. The cold rolled C1018 steel has high strength, indicated by the low strain at high stress, but fails at about 10% elongation, showing its low ductility. In contrast, the more ductile hot rolled A36 steel has much greater elongation to a maximum of almost 25% at lower stress than the cold rolled steel. The 6061-T6 aluminum we just tested has lower strength as well as failure at a lesser elongation than either steel.

Let's now look at some of the common applications of the tensile testing of aluminum. The most important use of stress strain curves is in quality control during the manufacture of aluminum. ASTM standards require tests on representative samples of each heat of aluminum and the results must be traceable to established benchmarks. Manufacturers use standards such as ISO TS 16949 for quality control and quality assurance of materials for automotive and other industries.

Aluminum foil for the cooking industry has a desired pliability so it can be easily handled and folded. Similarly, the aluminum used in cans for soft drinks must be strong enough to retain its shape when held, but easily crushable when necessary. Tensile testing ensures that these thin sheets of aluminum have the specified mechanical qualities.

You've just watched JoVE's introduction to the stress strain characteristics of aluminum. You should now know about the ASTM E8 standards laboratory test for determining the tensile properties of metallic materials. You should also understand how to prepare a specimen for ASTM testing and obtain the stress strain curve for typical aluminum.

Thanks for watching!