Tensile Strength Test of Small Specimens can be Difficult
Measuring the tensile strength of fiber, thread, wire or nanotube can be difficult due to the tiny specimen size. How are you going to grip it? How are you going to find a tensile test machine with the steadiness and resolution to give you good tensile test data? Does this require a specialized test machine or specialized instrumentation? TestResources has the experience and the answers. Let’s take it apart and look at each challenge.
Tensile Test Grips Are an Important Choice
Fiber strength tests of tiny thread, or fiber, wire or tube pose the same type of hurdles. All of these specimens have uniform cross-section and none are necked down in the middle. This means if you pinch the samples to grip them, it creates stress concentrations which cause premature failure right at the edge of the grip. So what is the solution to tensile testing small wire, fiber and other small samples? Do not “grip” them, glue them. TestResources offers two different types of glue-based gripping systems for these tiny constant-cross-sectional-area specimens:
1. The G99-045 Single Small Fiber Test Jig with Scrapeable Gluing Surfaces:
Tensile test small wire, fiber, nanotube and more with the G99-045 Single Small Fiber Test Jig. This system consists of a base fixture for holding a lower gluing surface and an upper gluing surface. The upper gluing surface is very, very light to allow it to hang from a tiny load cell without affecting its linearity. This upper gluing surface is pinned to the holder while you glue the specimen. Glue one end of the specimen to the lower gluing surface that is part of the base fixture. Glue the other end of the specimen to the gluing surface. After it has cured, mount the holder into the tensile test machine. Then carefully connect the chain from the load cell to the hook on the upper gluing fixture. Then finally release the pins holding the upper gluing fixture to the holder. Now when the machine pulls upward, it will be pulling only on the thread test specimen without causing unnecessary breaks.
2. G99-046 Micro Fiber Test Jig with Cardboard Frame and Glue Gripping System:
Tensile strength of fiber will be easy to measure with the G99-046 Micro Fiber Test Jig. The fiber test specimen is glued at the top and bottom of a cardboard frame. Then the frame is gripped in the test machine. The lower grip is a basic manual vice grip. The upper grip is a special vice grip which is designed to be as light as possible to minimize the tare weight on the small-capacity load cell. Initially the upper grip is pinned to a structure which is attached to the lower grip. After the cardboard frame is mounted in the grips, then you cut both sides of the cardboard frame. Then the last thing you do is unpin the upper grip such that it is hanging from the tiny load cell. Now when the test machine pulls upward, it will be pulling only on the fiber test specimen.
This gripping strategy has the big advantage of enabling you to stage several test specimens in cardboard frames prior to testing so you don’t have to wait for glue to cure between each test. This grip is a quick solution to tensile testing small wire.
3. To avoid gluing, try a very light capstan grip:
Tensile strength of fiber and uniform cross-section specimens can be determined using another strategy: a lightweight capstan grip. Specimens should be long because you will need to wrap the specimen around a capstan cylinder before gripping it. The friction on the capstan cylinder reduces the tensile force such that the specimen will not break at the edge of the clamp. For small capacity load cells, be careful about the tare weight hanging from the load cell. Combined force of the tare weight and the tensile force will be well within the linear region of the load cell. We offer special low-weight capstan grips such as our G13MP which weighs only 102 grams.
Tensile Strength of Fiber and Force Resolution
For tensile testing small specimens, it is important to make sure that the tensile strength is not less than five percent of the force capacity of the chosen load cell. This capacity range will not work well with low quality load cells.
Testing tensile strength of fiber may lead you to a very small capacity load cell such as the TestResources 100 gram capacity load cell. When you use a load cell this small it is important to plan the tare weight of your test setup such that the sum of the tare weight plus the tensile force does not exceed the capacity of the cell. The tare weight is the weight of the upper grip and other items that are hanging from the load cell. It is important to note that small load cells have an overload capacity. If you exceed the capacity of your cell you may compromise linearity; and you will be outside the calibrated region of the cell. Your TestResources application engineer can work with you to design your load cell capacity and gripping strategy to address these issues and ensure your testing success.
Tiny Samples and Strain Resolution
Tensile testing small wire and fibers gives simple strain results. Strain is pretty easy to understand in a tensile test of a constant-cross-sectional-area test specimen. The gauge length of the specimen must be the entire length between the grips because it may elongate and fail anywhere between the grips. That means the displacement of the test machine is the elongation of the test specimen. A separate clip-on extensometer is not necessary. Whenever you use the displacement transducer in the test machine to measure elongation of the test specimen, you should be concerned about the deflection of the test machine itself as an error in this signal. TestResources engineers can help you quantify and minimize this error with our specialized software. Call your TestResources and our application engineers can help guide you on this subject.
Machines and Grips Configured Best to Your Application
Tensile tests of thread, wire, and very small specimens can be accurate and successful if you take into account the stated above issues. With the special specimen fixturing and a small-capacity load cell, it can be performed successfully on a basic tabletop universal test machine such as the TestResources model 100Q225. Call your TestResources application engineer to make sure you have your test machine best configured for success.
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