Testing machines are precise, but not mind readers. Even microscopic edge flaws can make materials appear weaker than they really are, potentially skewing critical tensile data. Are you unknowingly compromising your material testing accuracy?
Universal testing machines are precise, but they are not mind readers. They measure the force applied to a specimen and how much it stretches, so microscopic edge flaws can make the material appear weaker than it really is. That’s why constant calibration is necessary when obtaining tensile data, especially in high-volume and strict regulatory environments.
Fortunately, there are numerous ways to prevent edge flaws that create stress risers in the test specimen. The key is consistent and accurate tensile testing sample preparation, which is reinforced by these strategies.
Adhere to Standardised Geometries
A single crooked edge, tiny scratch or unexpectedly sharp corner could disrupt the accuracy of testing machinery. Every sample undergoes tensile force, and if the lines are uneven or the spacing is inconsistent, the equipment compensates. Uneven geometry can concentrate stress or shift the failure point. Therefore, whether it is plastic or metal, the sample must follow standardised geometries to have the best chance of producing data that accurately represents the material.
The most common governing bodies for these standards are ISO and ASTM. These help labs determine how much of a material they need and what dimensions or shape are required for each type of test. They explain how stress concentrations affect tensile strength, as well as what pull, tear and bend tests are required. In high-stress industries such as medical device manufacturing and aviation, skewed data can lead to misunderstandings about the material’s capabilities.
Use Precision, Low-Impact Cutting Methods
During tensile testing sample preparation, manual cutting can be one of the most common sources of edge flaws. Even machinery that tends to produce rougher cuts could be to blame. Precision methods to create a clean specimen are crucial for repeatability and data clarity.
For example, CNC machining can be a powerful tool, especially for heat-sensitive plastics. A band saw may introduce many imperfections and even points of melting due to the heat it can generate at high speeds. Many CNC machines are now connected to the Industrial Internet of Things, using data like thermal readings and axis positioning to inform future cuts.
CNC-controlled cutting methods, such as waterjet cutting, can improve accuracy while reducing heat-related damage because waterjet cutting is a non-thermal process. Low-impact preparatory cutting could minimise the likelihood of premature snapping during tensile testing because the sample is purer.
Polish Longitudinal Edges
Some stamping machinery can incorporate stress risers into the process, because some equipment could unintentionally cause grooves on the sides. For materials such as metal, a post-polishing step before testing may be necessary, depending on the types of cutting and stamping machinery used at the facility.
This is particularly important for samples where the machine places marks that are perpendicular to the side the tensile machine is pulling. Polishing samples parallel to the pulling side with progressively higher grits will make them more uniform and durable for testing.
Maintain Sharp and Clean Dies
Die-cutting is used for countless materials, including elastomers and thin films. If the die is dull, it compromises the sample. The cut will not be clean or in compliance with regulatory standards, resulting in skewed data. Constant refinement is important, especially because die-cutting is most useful for high-throughput sample preparation. At a microscopic level, tears and other deformities could form when attempting to position the sample in the machine.
Therefore, maintenance on die-cutting equipment is critical for accurate tensile data. Sharpen or replace dies every 100,000 hits, as infrequent maintenance can lead to broken die components. If the equipment is trying to cut a material with tensile strength or thickness it cannot handle, it will cause the machine to be off-centre or lead to further damage.
Account for Material Anisotropy
The direction and positioning of the material matter significantly for universal testing equipment. Additionally, some materials, such as composites, can exhibit varying properties depending on their orientation. These differences in anisotropy are easy to ignore but can make tensile data incomplete or lead to premature failure during testing.
For example, a rolled material, such as aluminium, can exhibit different tensile properties depending on whether it is cut transversely or longitudinally. The test results may not be inaccurate, but they do not paint the full picture of the material without seeing it from all angles. Aligning the metal to its weakest side could yield lower tensile performance than it could handle.
Avoid Work-Hardening During Preparation
Overworking a material before it reaches the universal testing stage could cause premature deformation or make it more brittle. Harsh cuts and processing cause additional strain. While it may not be noticeable on the surface, the internal composition may be compromised and alter how tensile data looks.
Deburr All Edges
Sometimes, samples need to be prepped with cutting machines, and others require drilling. These abrasive machines can leave small burrs — shavings from the material that remain slightly attached to the main body. These cause irregularities and confusing results on stress tests. These slivers may be less than an inch, but they can still catch and redirect pressure to a specific point. Deburring tools are available to refine samples before they reach the testing stage.
Use Automation for Consistency
Human error and variability may be the source of many upsets in a testing lab, whether it be faulty positioning or a misaligned cut. These factors are why more facilities are incorporating smart or automated software and hardware to make output more consistent.
The improvements may manifest as an automated punching system that requires minimal to no manual adjusting. It could also be software that programs a cutter to manipulate a sheet of metal. Even machine learning and artificial intelligence are becoming essential tools for measuring the fracture toughness and tensile strength of concrete samples.
How Stress Concentrations Affect Tensile Strength
Testers may not be able to see the small imperfections in their materials with the naked eye. However, a failed tensile-strength test will yield countless insights into ways to improve sample preparation. These tests are the cornerstone of many industries, including electronics and automotive. If a plane’s wings are expected to bend correctly, and an internal heart monitor needs to resist biological stressors, then reducing the risk of inaccuracies before a tensile test can be lifesaving.