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Amanda Hunt

Metrology

Preparing Tensile Specimens for the Highest Accuracy

CNC milling machines are considered the ideal solution for preparing tensile specimens

Published: Tuesday, July 10, 2018 - 12:01

Tensile testing of materials is critical to a wide array of industries, which means preparing specimens for testing is equally important. If a specimen is not prepared correctly, the test results will be inaccurate; this is costly if a material fails a test that it should have passed, and potentially catastrophic if it passes a test it should have failed.

The basics of specimen preparation

The specimen’s cut is critical to the quality and accuracy of its test results. The sample must not have any jagged edges or nicks. Even the slightest such deformity can impact its tensile strength, affect the ability to receive consistent tensile results from the specimen, and render its test results out of line with other similar specimens.

Once a specimen is correctly cut, it must also be handled with care. Dropping, bumping, or otherwise mishandling the specimen may result in nicks, with the same effect on testing as a bad cut.

The old method: blanking presses

In the past, high-speed eccentric presses were used for specimen preparation. However, this causes an unacceptable level of damage to specimen edges—the cold hardening effect of eccentric presses runs deep into the material, up to around 35 percent of the specimen’s thickness. This creates excess deformation and compression of the material. The material would then need to be strengthened through cold working.

Cold working, also known as plastic deformation, is the process of changing a metal’s shape without the use of heat in order to strengthen it. The metal is placed under mechanical stress, creating a permanent change in its crystalline structure.

As its name implies, cold working is conducted at relatively low temperatures. Specifically, it is done below the given metal’s recrystallization point. Relatively malleable metals such as copper and aluminum are most commonly subject to the cold working process.

However, when cold working is done, permanent defects are created in the metal’s crystalline structure. More precisely, crystals are less able to move within the metal structure, making the metal more resistant to deformation. The resulting product has a higher tensile strength and hardness than the original form of the material.

Why this alteration matters

At a glance, it may seem as though cold working is an improvement, given the higher tensile strength. However, finding a material with sufficient tensile strength for a given purpose is frequently not as difficult as finding one with sufficient elasticity.

During testing, any material will reach a point beyond which the tensile load creates a permanent deformation, also known as the elastic limit. The force required to push a material to this limit is its yield strength. If a highly brittle material is put under stress, such as ceramics or glass, it will break before it shows any elasticity, meaning it does not have a distinct yield point. By making a metal more brittle, its yield strength is reduced, making it less useful to many industries. If the metal is made too brittle, such that its yield point cannot be calculated, it is effectively useless.

CNC milling for tensile specimen preparation

CNC milling machines, like the TensileMill CNC, are now considered the ideal solution for preparing tensile specimens for one main reason: it is the only option, apart from large and expensive water jet cutters, that does not result in either a heat-affected zone or surface deformation/cold work issues.

For example, if a laser or plasma cutter is used to make a sample, they can (and very likely will, in some cases) burn or melt the edges of the specimen. Not only does that harm the sample for the reasons listed above, burning or melting can create chemical alterations that further disrupt any test attempt, and in some cases no burning can occur due to the threat of toxins being released into the atmosphere.

When using a CNC mill, only low amounts of heat are transferred to the material. Furthermore, the specimen has machine coolant applied to the working area throughout the cutting process. This eliminates any issue with burns, and makes CNC mills particularly useful for use with any material that can have its properties changed by heat. This includes several metals, as well as plastics, acrylics, composites, and laminates.

CNC coolant usage

As mentioned, one of the things that separates CNC mill sample preparation from other methods is its use of machine coolant. This coolant serves two main functions: lubrication and cooling.

Metal-removal techniques, such as cutting a sample from a larger piece of material, generate friction, and thus heat. External friction—metal-to-metal contact—creates about one-third of the heat in the cutting process, while the resistance of metal atoms to movement in the shear zone (internal friction) creates two-thirds.

By applying coolant, the chlorine, sulfur, and phosphorous atoms in the coolant penetrate the micro-fractures in metallic surfaces. This prevents metal atoms from re-bonding during the cut, reducing the power needed to form a chip. Furthermore, lubricating the chip/tool and tool-flank/cut-surface interfaces decreases the area of the shear plane. As that area decreases, the power required to form a chip decreases further, and along with it the generated heat. In this way, the lubrication reduces external and, to a lesser extent, internal friction.

The actual cooling effect of the coolant is required to remove the heat that is still generated from the tool, chip, and workpiece. The coolant thus extends tool life by keeping those tools below their critical temperature range. In addition, it is easier to control the size of a thermally stable part. The substantial number of machine-finished parts in use today make it all the more important that the machine coolant help keep the part dimensionally stable.

Machine coolant is secondarily used to flush chips and fine metal pieces from the tool/workpiece interface. This prevents the finished surface from becoming marred, and reduces the occurrence of built-up edges, which can change the geometry of the cutting tool.

Which coolant is correct depends on your purposes. Straight oils, water, and soluble chemicals all have different lubricating and cooling capabilities, and should be chosen with care.

Choosing the right CNC mill

One of the traditional downsides to CNC machining is that it requires far more training than other specimen preparation methods. The TensileMill CNC is suggested in part because it removes the training requirement from being able to effectively use the machine. Designed with an easy-to-use interface, the TensileMill gives anyone the ability to cut highly accurate tensile specimens in seconds.

Depending on your needs, the TensileMill CNC MINI may be an even better option. It’s our newest flat tensile specimen preparation machine. Whether you adhere to standards from ASTM International, the International Organization for Standardization (ISO), Deutsches Institut für Normung e.V. (DIN—German Institute for Standardization), Japanese Industrial Standards (JIS), or other industry standards, a mini CNC mill is a robust, capable machine that offers a smaller footprint in both space and energy usage. And TensileMill’s CNC MINI—just like its larger cousin—runs on the same easy-to-use Carbon software interface.

No matter your tensile sample needs, CNC milling is the modern way to go.

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About The Author

Amanda Hunt’s picture

Amanda Hunt

As well as writing for TensileMill CNC, Amanda Hunt was a manufacturing and quality manager at General Motors. She is experienced in the automotive and plastics industries and the in-depth analysis of their mechanical properties. Hunt is also experienced in supplier handling, CAD design, and manufacturing collaborations. She is knowledgeable about managing, instructing, and maintaining the high levels of standards in a quality control facility.