How do the Most Interesting Cylindrical Parts Benefit from Tensile Sample Preparation?

The Tensile Test was designed for a near-universal application. What that means is that the test itself can be conducted upon any material regardless of shape, size, and other inherent qualities with little to no deviation from the standard sequence. 

This is even reflected in the concept of preparing a sample and operating a tensile test machine. All that the operator has to do is to prepare the sample according to the proper specifications, depending on the testing standard, input the proper test settings, and take note of any observable and machine-drawn findings throughout multiple test cycles and specimen samples.

That being said, is the test applicable for specific sample material profiles like that of a cylinder? Is there any beneficial element to conducting the test for cylindrical parts? The short answer to those questions is a yes. For a more detailed answer, one should determine a few important aspects first.

Sample Geometry

To determine the compatibility of cylinders for the tensile test, we must go through the basic geometric profile of a cylinder first. Essentially, cylinders are two parallel circular ends joined by a narrow shaft in the middle. Conventionally, this would result in a material that takes the shape of a tube, a bowl, or a cup, depending on the respective dimensions of both circular ends and the shaft itself.

This geometric profile will imply that the cylinder is not only compatible with tensile tests. Moreover, its shape means that it is designed specifically to fit the tensile test process itself.

The reason for this is that the universal tensile test process requires samples to conform to a particular profile to yield optimum results. This profile assumes the form of a dog bone or a stylized letter “I” wherein there are two ends with specific dimensions joined at the center by a long, thinner, shaft.

This unique shape helps testers establish the most predictable point of failure in a sample specimen. That region tends to be near the center of the shaft. As the sample bends, elongates, and deforms through the tensile test process, the center of the shaft is where stress and fractures tend to form before breaking.

Thus, for cylinders, there is no separate molding needed to get the sample material to conform to test specifications. However, that does not mean that cylinders are exempted from the sample preparation process.

Preparing Cylinder Samples

Even if the cylinder’s default profile is highly compatible with tensile testing, a tester has to make sure that the sample itself is properly prepared to achieve optimum results. When preparing a cylindrical sample, the operator must consider the following areas.

  • Specimen – To start things off, the tester must determine how compatible the specimen is for the test. There is no longer any extensive sample creation needed for cylindrical parts, but the tester has to determine if the sample itself has dimensions that fit the purpose and test method as well as the test parameters. For instance, if the cylinder is plastic, then it need not be threaded as that might prematurely break the sample. Instead, pinning the sample to the clamps will work as it retains sample integrity while ensuring a firm grip.
  • Gage Dimensions – If possible, the gage or the middle region should be narrower or smaller than the shoulders located at both ends of the sample. This is so that the point of failure can be localized in the middle.
  • Length – The sample should have a length that is long enough to cover a considerable distance of at least a few inches between both shoulders. This is to allow the tester to properly monitor the propagation of fractures and other indicators of structural failure through the test process.  A general rule here is that the gage length should be at least twice the size of the diameter of the gage. This way, the test will only have to account for tensile stress for the entire test.
  • The Shoulders – Aside from possessing the proper dimensions, the shoulders themselves should have a surface that allows for maximum gripping. This is because the machine will hold on to the sample at both shoulders. A less-than-secure grip can lead to inaccurate results or worse, complete test failure.

There are many ways to ensure a secure grip in the sample. The most straightforward method is to thread the sample to create a contoured surface or add wedges from which the sample will be held on to.

The point here is to ensure that that sample does not slip from the machine’s grip even as tension is added to both loading cells.

The Test Proper

Once the sample is ready, all that the tester has to do is to vertically place the material into the machine. This will involve placing the edges of the shoulders to the upper and lower clamps. At this phase, it is crucial for the tester to manually adjust the length of the clamps to avoid the sample turning sideways or having an unpredictable contortion in the middle of the test.

Also, the tester has to be mindful of the inherent durability of the sample material. Samples made from brittle material need to be handled carefully to prevent premature shattering.

Once properly loaded, the machine will do the rest of adding the proper force at both ends of the clamps. This will cause the material to deform until it breaks.

The change in length of the material is what is called the Strain, while Stress itself is measured from the time that the sample material starts to deform until it breaks. Depending on the test parameters, a single stress test will be completed between 5 and 10 minutes. Of course, the tester has to perform the same test on multiple duplicate specimens to ensure accuracy.

To Conclude

So, is there a point in preparing a cylindrical sample for the tensile test properly? Again, the answer is in the affirmative. What is important to remember here is that the Tensile Test, like any other material testing process, aims for accuracy, predictability, and repeatability. 

To ensure that, the samples themselves must be made to conform to the proper testing dimensions. Even if they already possess the right default sampling profile, cylindrical samples still need to undergo the proper preparation process. If done right, every cylindrical sample material will yield the most accurate information for the tester, which should aid the manufacturer requiring such a test in the first place.

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