FDM 3D Printing: How Does Strain Rate Change Tensile Results?

A change in test speed can change the mechanical properties measured for a 3D-printed part. That matters when tensile-test results are used to compare materials or assess a part for further development. A study by Adi Pandžić, PhD, examined how strain rate affects three FDM materials based on PLA.

Key findings

• The study tested PLA, Tough-PLA and carbon-fibre-reinforced PLA (CFPLA) at five test speeds: 0.5, 1, 5, 50 and 100 mm/min.
• Tensile strength increased with test speed for PLA and Tough-PLA; CFPLA did not show a consistent trend.
• The Young’s modulus of PLA and Tough-PLA increased with speed, while CFPLA showed both increases and decreases.
• Tough-PLA yield strength reached 38.5 MPa at 50 mm/min, then fell to 36.9 MPa at 100 mm/min.
• Raising the speed from 0.5 to 100 mm/min reduced test time by up to 99.58% for PLA, 99.87% for Tough-PLA and 99.41% for CFPLA.

Why test speed matters

Fused Deposition Modeling, or FDM, is also known as Fused Filament Fabrication (FFF). It is an additive manufacturing process in which a 3D printer deposits thermoplastic material layer by layer. Stereolithography (SLA), in contrast, is a liquid-based additive process. FDM is used for prototypes, individual parts and small-batch production; the paper notes applications in areas including medicine, aerospace, architecture, electronics and automotive manufacturing.

The mechanical properties of a printed part depend on several factors, including material and printing parameters. A test result is therefore not just a property of the filament: how the part is made and how it is tested also matter. The paper reports that, at the time of the study, there were no official standards defining a mechanical testing method specifically for FDM materials. The study used ISO 527-2 for the specimen and tensile test, then compared results at different speeds.

In the paper, strain rate is reported as test speed in mm/min. A tensile test loads a specimen by pulling it. Tensile strength describes the highest stress the material withstands during the test, while yield strength marks the onset of permanent deformation. Young’s modulus indicates stiffness: a higher value means more stress is needed to produce a given elastic deformation.

What the study tested

The study covered three materials: Ultimaker PLA, Ultimaker Tough-PLA and Proto-Pasta CFPLA. As described in the paper, CFPLA contains 15% chopped carbon fibres by weight. The paper reports that the fibres increase rigidity, but do not necessarily make the filament stronger than standard PLA.

Specimens were made to ISO 527-2 and printed with 100% infill. PLA and Tough-PLA were printed on an Ultimaker S5 with the standard 0.4 mm AA print core. CFPLA was printed on an Ultimaker 2+ using a 0.5 mm steel nozzle, as a more wear-resistant nozzle is recommended for fibre-filled material. Layer height was 0.15 mm for all three materials. PLA and Tough-PLA used Cura’s “Engineering – Normal 0.15 mm” profile, while some parameters were adjusted for CFPLA.

Testing was performed on a Shimadzu AGS-X Std tensile machine. The machine has a maximum capacity of 10 kN; a 5 kN load cell was used for this research. Three identical specimens were tested for each material at each speed, and the reported values are means. The five speeds were 0.5, 1, 5, 50 and 100 mm/min.

How the materials responded

For PLA, both tensile strength and yield strength increased as the test speed rose. Tough-PLA showed the same general increase in tensile strength. Its yield strength, however, did not keep rising across the entire range: it increased from 0.5 to 50 mm/min, then dropped from 38.5 MPa to 36.9 MPa between 50 and 100 mm/min.

CFPLA behaved differently. Its tensile strength and yield strength did not rise consistently with speed; the results fluctuated. According to the paper’s conclusion, the yield-strength change for CFPLA from 0.5 to 5 mm/min was not significant. At 50 mm/min, yield strength increased by 15%, followed by a slight decrease at 100 mm/min.

PLA’s Young’s modulus was 2.9 GPa at both 0.5 and 1 mm/min. At 5 mm/min it increased by 3.4%, to 3 GPa, and remained at that value up to 100 mm/min. Tough-PLA followed a similar pattern, with a modulus between 2.4 and 2.5 GPa. For CFPLA, the modulus fluctuated in a manner similar to its strength; the paper reports that it was up to 50% higher than for PLA and Tough-PLA.

The materials also differed in strain response. Tough-PLA showed a linear decrease in strain as test speed increased. PLA and CFPLA did not follow that linear pattern. This variation is a reminder that the behaviour of one PLA filament should not automatically be assumed for another, particularly when the material is a fibre-reinforced composite.

What the results mean in practice

For engineers, product designers and teams using 3D printing in product development, the key practical point is to report test speed when presenting or comparing tensile data. A value measured at one speed may not match a value obtained at another. This is especially relevant when selecting data for technical comparisons between FDM materials.

For students and researchers, the work illustrates how one test condition can be examined alongside several outcomes: tensile strength, yield strength, Young’s modulus, strain and test duration. A shorter test at a higher speed does not, by itself, make the resulting value more useful or more suitable. Results must be interpreted in the context of the material and test method.

The findings do not mean that every PLA filament or every FDM part will respond in the same way. The study covered three materials, one set of printing conditions and three specimens for each material-speed combination. Its results therefore apply to the conditions described in the paper, rather than providing universal values for all printers, geometries or applications.

Limitations and next steps

The paper recommends that future research use more than three specimens per test, examine a wider range of strain rates and include more materials. This could provide a fuller picture of how test speed affects the mechanical properties of FDM parts. For meaningful comparisons, it is also useful to report the material, printer, nozzle, layer height, infill and other relevant manufacturing conditions alongside the test speed.

Conclusion

Published in the Proceedings of the 31st DAAAM International Symposium on Intelligent Manufacturing and Automation, this study by Adi Pandžić, PhD, found that test speed affects the measured tensile properties of PLA, Tough-PLA and CFPLA specimens, but not in the same way for every material. The results support reporting test conditions consistently and taking them into account when comparing FDM materials.

Frequently asked questions

Which test speeds were used?

The study tested each of the three materials at 0.5, 1, 5, 50 and 100 mm/min.

Which FDM materials were tested?

The study tested Ultimaker PLA, Ultimaker Tough-PLA and Proto-Pasta CFPLA, a PLA material reinforced with carbon fibres.

Did a higher test speed always increase tensile strength?

No. PLA and Tough-PLA showed increasing tensile strength as speed increased, while CFPLA had variable results. Tough-PLA yield strength decreased between 50 and 100 mm/min.

Frequently asked questions

Author: Adi Pandžić, PhD in Mechanical Engineering

This article summarises a published research paper; please refer to and cite the original paper. About the author

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