'Unravelling the Strength of Polymers: A Guide to Polymer Tensile Testing' by Lauren Ingleson

Polymers, the versatile materials which encompass everything from the plastic in our daily products to the fibres in our clothing hold a central place in our lives. But what allows these materials to possess such varied properties and applications? The answer to this question lies in the understanding of how polymers respond to mechanical forces, and this is achieved through the tensile testing. In this Additive Manufacturing Solutions Ltd (AMS) article, we will delve into the world of tensile testing, exploring what it entails, why it is crucial, and how it shapes the way we use polymers in various industries.

What is polymer tensile testing?

Polymer tensile testing is a fundamental and widely used method to examine the mechanical properties of polymers through an applied axial load. Tensile testing of polymer involves applying a pulling force onto a specimen until it fails. By carrying out this type of test we can gain insight into the strength, ductility, elasticity, and overall behaviour of polymers under various conditions. Many engineering companies use this type of testing and have their own standard operating procedures (SOP) however all will be based on ASTM Additive Manufacturing Center of Excellence D638 (American Society for Testing and Materials). This is the standardised test method for testing tensile properties of polymers, this standard outlines test and specimen parameters to ensure the reproducibility and repeatability of this type of test. This is the case for us at AMS, we have our own SOP which outlines how we perform the test and the specimen design that we use to ensure that all tests are valid.

Significance of specimen selection

Specimen selection is a crucial initial step in any testing procedure, this is no different when working with polymers. The chosen specimen acts as a representative of the entire material and their properties determine the accuracy and reliability of the results. Specimen selection is significant for several different reasons, some of these can be seen below:

• Data accuracy – The properties of polymers can vary significantly, even within the same batch, so the correct specimen selection helps to ensure that the test results accurately reflect the material’s behaviour.
• Consistency – Standardised specimens help to provide consistent results, allowing for meaningful comparisons between different tests and materials.
• Reliability – Accurate specimen selection and preparations reduces the likelihood of errors and deviations during the testing process thus ensuring the reliability of the data collected.

With these aspects in consideration, we carried out some round robin tests with different specimen parameters to establish which would provide less scatter throughout the results and therefore more reliable data. There were three different coupon designs, ASTM D3039 no tabs, ASTM D3039 90-degree tabs and an ASTM D638 Type 1, each design was tested 10 times and the results compared. Through the analysis of the results, it was determined that the ASTM D638 Type 1 coupon was the most consistent, showed a minimal amount of scatter and broke within the gauge length the most. This can be seen in the dataset below, which shows the average yield, elongation, and ultimate tensile strength (UTS) for each coupon design that was tested.

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Overall, each coupon performed lower that the material datasheet specifications in terms of tensile stress, strain and UTS. This could be due to issues within the printing process which could have influenced the performance of the coupons in testing. All the coupons were printed in the same orientation (XY), some of the initial tests were carried out with coupons in various orientations however, the XY coupons seemed to perform the best throughout the tests. Overall, the ASTM D638 coupon performed much better than the ASTM D3039 coupons and was also the coupon which failed in the gauge length the most. The mechanical data collected from this coupon was much more consistent and had less scatter within each property than the other coupons.

See tested ASTM D638 coupons below.

When selecting the most ideal coupon for testing purposes, there are several other factors which need to be considered such as shape, material orientation, avoiding defects, and environmental conditions. These are all factors which can influence the overall quality and performance of the coupons when being tested.

Applications and Future implications

As additive manufacturing continues to shape the future of manufacturing, tensile testing remains the cornerstone of ensuring that the printed parts meet stringent requirements for safety and performance. The result of tensile testing paves the way for innovation and progress in additive manufacturing, pushing the boundaries of what can be achieved with this technology.

Conclusions

In conclusion, tensile testing of 3D printed parts is a pivotal tool in the realm of material science and additive manufacturing. It enables engineers to understand the mechanical properties of printed materials, aiding in quality control, material selection and research and development. As the world of additive manufacturing evolves, it will be interesting to see what the impact of tensile testing will have on it.

All testing was completed using Instron testing equipment.

Author and Additive Manufacturing Solutions Ltd. Test Engineer: Lauren Ingleson